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ANATOMY AND HISTOLOGY 
 
 OF THE 
 
 MOUTH AND TEETH 
 
 BROOMELL AND FISCHELIS 
 
ANATOMY AND HISTOLOGY 
 
 OF THE 
 
 MOUTH AND TEETH 
 
 BY 
 
 * 
 
 I. NORMAN BROOMELL, D. D. S. 
 
 DEAN AND PROFESSOR OF PROSTHETIC DENTISTRY, DENTAL ANATOMY AND HISTOLOGY, DENTAL 
 DEPARTMENT, MEDICO-CHIRURGICAL COLLEGE OF PHILADELPHIA 
 
 AND 
 
 PHILIPP FISCHELIS, M. D. 
 
 ASSOCIATE PROFESSOR OF HISTOLOGY AND DEMONSTRATOR IN EMBRYOLOGY, 
 MEDICO-CHIRURGICAL COLLEGE OF PHILADELPHIA 
 
 FOURTH EDITION, REVISED, WITH 368 ILLUSTRATIONS 
 
 PHILADELPHIA 
 
 P. BLAKISTON'S SON' & CO. 
 
 1012 WALNUT STREET 
 1913 
 
Copyright, 1913, by P. Blakiston's Son & Co. 
 
 
 
 THE . MAPLE. PRESS- YORK- PA 
 
TO 
 
 C. N. PEIRCE, D. D. S. 
 
 AS A SOUVENIR OF A LONG AND VALUED FRIENDSHIP AND A 
 
 TESTIMONY OF ESTEEM FOR HIS PROFESSIONAL 
 
 AND PRIVATE WORTH 
 
 THIS VOLUME IS RESPECTFULLY DEDICATED 
 
 BY THE AUTHORS 
 
PREFACE TO THE FOURTH EDITION 
 
 In view of the fact that it has become necessary to issue a new edition 
 of this work in a comparatively short time, the authors have taken ad- 
 vantage of the opportunity thus offered, to still further revise and enlarge 
 the subject matter along the lines begun in the third edition. 
 
 In that edition it was suggested that "it has been justly brought 
 forward within recent years* that there is a lack of interest and enthu- 
 siasm for collateral reading, special studies, and original investigation 
 not only among students, but also among graduates in Dentistry. The 
 experience gained in teaching has suggested to the authors that this is due 
 to the fact that dental students use to a great extent text-books on Anatomy, 
 Histology, and Embryology, chiefly intended for medical students, and 
 they generally believe that most of the matter which is contained in these 
 books is not intended for them, having no practical application. Their 
 attention is therefore limited to those chapters which seem of importance 
 to them, and thus an intelligent understanding of the subject is not 
 obtained. Recognizing this fact, it seemed advisable to include in this 
 Dental Text-book a description pertaining to general Cytology, general 
 Embryology, and Histogenesis, and a further presentation of the signifi- 
 cance of the facts thus gained for an intelligent understanding of the 
 formation of the organs of the mouth." 
 
 In the present edition this plan has been further amplified by the 
 addition of 
 
 i. A comprehensive description of the present status of our knowledge 
 of blood and lymph. 
 
 2. A comprehensive description of the structure of the circulatory 
 apparatus. 
 
 3. A general consideration of the structure of various forms of glands. 
 In addition to these features the subject matter of the various chapters 
 
 has been rearranged to conform with the general plan of the work. 
 
 It has been the aim to present the subject in a concise and attractive 
 form, including some illustrations which never before appeared in a 
 
 * Proceedings of the Meeting of the Institute of Dental Pedagogics. 
 
 vii 
 
Vlll PREFACE TO THE FOURTH EDITION 
 
 work designed for dental students. It is the hope of the authors that 
 the additions made will help to awaken an interest for collateral reading 
 and thus elevate the standard of education among dental graduates. 
 
 The authors acknowledge their indebtedness to the publishers for 
 their kind assistance in adding new illustrations to further the usefulness 
 of the book. 
 Philadelphia. 
 
PREFACE TO THE FIRST EDITION. 
 
 In the preparation of this work it has been the aim of the author to 
 systematically describe those parts of human anatomy which come 
 directly under the care of the stomatologist. In the earlier chapters, 
 which are devoted to a gross description of the mouth and those tissues 
 which enter into its construction, there has been no attempt at originality 
 other than in the arrangement, which includes a complete description of 
 one part before another is taken up. 
 
 In the writing and classification of the succeeding chapters the writer 
 has attempted what others, though wiser and better qualified, appeared 
 unwilling to undertake, and it is from the works of such as these that the 
 foundation for the present work has been derived. 
 
 Within the last few years the progress in nearly every branch of 
 dental education has made a work of this character an imperative want. 
 Dental therapeutics and dental chemistry have been well-nigh recon- 
 structed, while the investigations of the microscopist and physiologist have 
 brought forth many valuable revelations. Next in importance has been 
 the advance in, or rather the introduction of, technic teaching. Con- 
 siderable space has, therefore, been devoted to the surface anatomy of the 
 individual teeth, with a hope that it may be of value in dental anatomy 
 technic. 
 
 While in one or two instances the writer has departed from the field 
 assigned as a text, the parts thus included are so closely associated with 
 the mouth, both in a constructive and in a functional manner, that the 
 work would be lacking in completeness if they were omitted. 
 
 The illustrations are, with but few exceptions, the original work of 
 the author, being reproduced by photograph from the actual subject. 
 In many instances dissections were required to reveal the parts, this 
 being particularly true of those illustrations included in the chapter on 
 the Development of the Teeth, about one hundred dissections being re- 
 quired to accomplish the purpose. In preparing the illustrations de- 
 scriptive of the various surfaces of the individual teeth, the progress of 
 the work was materially interfered with by the difficulty experienced in 
 securing normal teeth out of the mouth; may their number ever grow less. 
 
 ix 
 
X PREFACE TO THE FIRST EDITION 
 
 The author desires to thus publicly acknowledge obligations to the 
 works of Tomes, Black, Morris, Stohr, Klein, and Strieker. He is also 
 indebted to Prof. A. P. Brubaker and to Dr. C. P. Shoemaker for valuable 
 assistance rendered, and to P. Blakiston's Son & Co. for their many cour- 
 tesies during the preparation of the volume. 
 
 That there is a place for such a work as this purports to be the writer 
 has but little doubt; that the following pages will fill that demand is his 
 earnest desire, and it remains for the reader to ascertain how far these 
 demands have been met in the direction of its aim and endeavor. 
 
TABLE OF CONTENTS. 
 
 PART I.— ANATOMY. 
 
 CHAPTER I. 
 
 General Description of the Mouth. — The Buccal Orifice; The Lateral 
 Walls of the Mouth; The Hard Palate, or Dome of the Mouth; 
 The Soft Palate and Fauces; The Floor of the Mouth; The 
 Tongue and its Attached Muscles, i 
 
 CHAPTER II. 
 
 Muscular Tissues of the Mouth; of the Lips; of the Cheeks; of the 
 Soft Palate; of the Tongue, 34 
 
 CHAPTER III. 
 
 The Bones of the Mouth. — The Superior Maxillae; The Palate Bones; 
 
 The Inferior Maxilla, or Mandible, 37 
 
 CHAPTER IV. 
 
 The Temporomandibular Articulation. — The Muscles of Mastica- 
 tion, 64 
 
 CHAPTER V. 
 
 General Description of the Teeth; The Permanent Teeth; Classifi- 
 cation, Surfaces, etc.; The Roots of the Teeth; The Dental Arch, 75 
 
 CHAPTER VI. 
 
 Occlusion of the Teeth, 88 
 
 CHAPTER VII. 
 
 The Blood- and Nerve-supply to the Teeth, 94 
 
 CHAPTER VIII. 
 
 A Description of the Upper Teeth in Detail. — Calcification, Erup- 
 tion, and Average Measurements; Their Surfaces, Ridges, 
 
 Fossae, Grooves, etc., 102 
 
 xi 
 
Xll TABLE OF CONTENTS 
 
 CHAPTER IX. 
 
 A Description of the Lower Teeth in Detail. — Calcification, Erup- 
 tion, and Average Measurements; Their Surfaces, Ridges, 
 Fossae, Grooves, etc., 164 
 
 CHAPTER X. 
 
 The Pulp-cavities of the Teeth, 190 
 
 CHAPTER XI. 
 
 The Deciduous Teeth, Their Arrangement, Occlusion, etc.; Calci- 
 fication, Eruption, Decalcification, Shedding Process, and 
 Average Measurements; Their Surfaces, Grooves, Fossae, 
 Ridges, and Pulp-cavities, 210 
 
 PART II.— HISTOLOGY AND HISTOGENESIS. 
 
 CHAPTER I. 
 
 General Cytology; General Embryology and Histogenesis, .... 233 
 
 CHAPTER II. 
 
 Elementary Tissues. — Epithelial Tissue; Connective Tissue; Mus- 
 cular Tissue; Nervous Tissue; Blood and Lymph, 248 
 
 CHAPTER III. 
 Circulatory Organs, Glands, 274 
 
 CHAPTER IV. 
 
 The Mucous Membrane of the Mouth. — Of the Lips; Of the Cheeks; 
 Of the Gums; Of the Roof of the Mouth, Hard and Soft Palate; 
 Of the Floor of the Mouth and the Tongue, 283 
 
 CHAPTER V. 
 
 Other Structures Within the Mouth.— The Gums; The Mucous 
 
 Membrane; The Alveolodental Membrane, 294 
 
 CHAPTER VI. 
 
 Glands and Ducts of the Mouth. — Of the Lips; Of the Cheeks; 
 Of the Hard and Soft Palate; Of the Tongue; The Salivary 
 Glands, 305 
 
TABLE OF CONTENTS Xlll 
 
 CHAPTER VII. 
 
 Tissues of the Teeth. — Enamel; Dentin; Cementum; The Tooth- 
 pulp; The Alveolodental Membrane, 313 
 
 CHAPTER VIII. 
 
 Embryology of the Mouth and Teeth, 374 
 
 CHAPTER IX. 
 
 Development of the Teeth. — The Dental Germs, Enamel Organ, 
 and Dentin Organ; The Dental Follicle; Calcification, Erup- 
 tion, etc., 391 
 
 CHAPTER X. 
 
 Anomalies of the Teeth, 451 
 
 Index, 461 
 
ANATOMY AND HISTOLOGY 
 
 OF THE 
 
 MOUTH AND TEETH 
 
 PART I.-ANATOMY. 
 
 CHAPTER I. 
 General Description of the Mouth. — The Buccal Orifice (the Lips).— 
 The Lateral Walls of the Mouth (the Cheeks).— The Hard 
 Palate, Dome or Roof of the Mouth.— The Soft Palate and 
 Fauces. — The Floor of the Mouth.— The Tongue and its Attached 
 Muscles. 
 
 The mouth (Fig. i) {stoma, pi. stomata) is the entrance or gateway 
 to the alimentary canal, and is situated between the superior and inferior 
 maxillary bones and their attached tissues. It contains the active organs 
 of mastication, the teeth, the organs of taste, of which the tongue is chief, 
 together with some of the parts which assist in articulate speech. Anato- 
 mists usually divide this cavity into two compartments, the teeth serving 
 to separate one from the other, the inner space being called the mouth, 
 while that between the teeth and lips or cheeks is known as the vestibule 
 of the mouth. In this description all that space bounded anteriorly by the 
 lips, posteriorly by the pillars of the fauces, and laterally by the cheeks, 
 will be considered as a single cavity, and the organs and structures con- 
 tained therein, together with all parts directly interested in its formation, 
 will constitute a text for this work. The entrance to the cavity of the mouth 
 is formed by a freely movable transverse orifice or slit, the buccal orifice, 
 while communication is made with the pharynx posteriorly through the 
 fauces. Entering into the construction of the mouth and assisting in the 
 performance of its functions are bones, ligaments, muscles, blood-vessels, 
 nerves, glands, ducts, etc., each of which will be described in turn. 
 
2 ANATOMY 
 
 THE BUCCAL ORIFICE, 
 
 or entrance to the cavity of the mouth, is a transverse opening somewhat 
 variable in extent, the extremities of which are known as the corners or 
 angles of the mouth. The orifice is bounded by two fleshy folds, the 
 
 Fig. i. — Outer Wall of Nasal Fossa, with Mouth, Pharynx and Larynx in Vertical 
 
 Section. {Deaver.) 
 
 _ a, Superior meatus; b, superior turbinate body; c, middle turbinate; d, inferior turbinate; 
 e, inferior meatus; g, tongue; h, posterior pillar of fauces; i, geniohyoglossus muscle; j, genio- 
 hyoid muscle; k, hyoid bone; /, mylohyoid muscle; m, thyrohyoid membrane; n, ventricle 
 of larynx; o, thyroid cartilage; p, diaphragma sellae; q, cavum sellae; r, sphenoidal sinus; 
 s, middle meatus; t, rhino pharynx; u, Eustachian orifice; v, hard palate; w, soft palate; 
 x, uvula; y, anterior pillar of fauces; z, tonsillar fossa; aa, oropharynx; bb, epiglottis; cc, ary- 
 epiglottic fold; dd, laryngo pharynx; ee, suprarimal portion of larynx; ff, ventricular band; 
 gg, vocal band; hh, infrarimal portion of larynx; it, cricoid cartilage; jj, tracheal ring. 
 
 upper and lower lips (labia), the former usually being in the form of a 
 double curve, coming together at the median line and forming a small 
 teat or tubercle, while the latter is made up of a single curve extending 
 from angle to angle. While this general description applies to the 
 
THE BUCCAL ORIFICE 3 
 
 labial forms most frequently met with, this must not be taken for a con- 
 stant condition. In some instances the lips are thin, with straight paral- 
 lel margins, firmly set against the teeth, and seldom separated from 
 each other when at rest. In another class they are thick, full, and promi- 
 nent, with their margins strongly curved, resting lightly against the teeth, 
 and more or less separated from each other during rest. Accompanying 
 these extremes as well as the intervening conditions are various other 
 peculiarities, such as the color, the rigidity or flexibility of the muscular 
 structure, etc. The upper lip generally overhangs the lower, but in some 
 instances the lower lip is the most prominent. Externally the lips are 
 covered by the common integument, internally and over their contiguous 
 surfaces by a continuation of the integument, the mucous membrane. 
 Between the external and internal coverings and forming the substance 
 of these fleshy folds are muscular fibers in which are imbedded numerous 
 blood-vessels, nerves, and glands (labial glands). By the various muscles 
 which enter into their construction the lips are loosely attached to the 
 surfaces of the maxillary bones. 
 
 The integument, or external covering of the lips, is similar to the 
 skin covering other parts of the body. In the male it is subject to a 
 peculiar change and modification of its outer layer, resulting in the pro- 
 duction of a hairy growth. 
 
 The mucous membrane, or internal covering of the lips, the begin- 
 ning of which is strongly manifest by its bright-red color, is without moist- 
 ure on the contiguous surfaces, is extremely sensitive, and contains a 
 number of vascular papillae, many of which are accompanied by nerve 
 terminals. Mucous membranes are described as lining certain cavities or 
 tracts, as the digestive tract, the respiratory tract, and the genito-urinary 
 tract, and it is upon the contiguous surfaces of the lips that the digestive 
 tract begins. The line of junction between the integument and the mu- 
 cous membrane is quite variable in form, but usually corresponds to the 
 general curvature of the lips. Internally at the median line each lip is 
 provided with a pronounced fold of mucous membrane, which is attached 
 to the basal portion of the gum, the frenum of the lip (fraenum labium 
 superioris and inferioris), which, in a measure, check the movements 
 of the lips. 
 
 Muscles of the Lips. 
 
 The muscular fibers within the substance of the lips are principally 
 those of a single muscle, the orbicularis oris, but associated with it is a 
 portion of the elevator and depressor muscles of the lips, the levator 
 
4 ANATOMY 
 
 labii superior is alceque nasi, levator labii superior is, depressor labii infer i- 
 oris or quadratus mcnti, and the zygomaticus minor. 
 
 Orbicularis Oris. — This is the sphincter muscle which surrounds 
 and controls the buccal orifice. In form it is an oval sheet with the 
 long axis placed transversely, the fibers being continued from one lip to 
 the other by passing around the angles of the mouth. It is divided into 
 an interna] or labial portion, and an external or facial portion. The 
 labial portion forms the red part of the lips, and has no bony attachment 
 except through the medium of the adjacent muscles. The external or 
 facial portion forms the deeper layer and blends with the surrounding 
 muscles, works in conjunction with them, and is provided with the follow- 
 ing small bony attachments : The nasolabial slips are attached to the 
 septum of the nose, other fibers are attached to the incisive fossa of the 
 superior maxilla over the position of the lateral incisor tooth, and to the 
 incisive fossa of the inferior maxilla near the socket of the lateral incisor 
 or cuspid tooth. 
 
 Structure. — The muscle consists of three sets of fibers, one of which 
 runs transversely, one in a vertical, and one in an anteroposterior direc- 
 tion. The transverse set is continuous with the fibers of the buccinator 
 or cheek muscle, and forms the greater part of the muscle. The red or 
 labial portion of the muscle is also formed from the same fibers, while the 
 vertical fibers form the superficial part of the facial portion and are con- 
 tinuous with the fibers of the levator and depressor muscles. Some of 
 these latter fibers pass around the corners of the mouth, thus becoming 
 transverse, those from above passing to the lower lip, while those from 
 below pass to the upper lip. The anteroposterior fibers pass from before 
 backward between the transverse fibers, and unite the mucous mem- 
 brane to the skin. These are chiefly found in the labial portion of the 
 muscle. 
 
 Relations. — The inner margin of the superficial surface is closely 
 connected with the integument, while superimposed between this and the 
 outer portion is a layer of fatty tissue. Upon the deep surface lies the 
 mucous membrane of the mouth, separated from the muscular tissue by 
 blood-vessels, the mucous glands, and small salivary glands. 
 
 Action.- — To bring the lips together, to draw the upper lip downward, 
 and the lower lip upward; to draw together the corners of the mouth; 
 to throw both lips outward; to draw them back against the teeth, and to 
 oppose the action of all other muscles that blend into it and inclining to 
 draw it in various directions. 
 
 Levator Labii Superioris Alaeque Nasi. — As its name implies, this 
 
THE BUCCAL ORIFICE 
 
 muscle is an elevator of the upper lip and the wing of the nose. It is one 
 of the superficial facial muscles, is thin and triangular, and is situated 
 by the side of the nose, extending from the infra-orbital ridge to the 
 upper lip. 
 
 Galea apo- 
 neurotiea 
 
 Orbicularis oeuli 
 
 Procerus 
 Quadr. labii sup. 
 pars angularis 
 Nasalis, pars transversa 
 Dilator naris anterior- 
 Dilator naris posteriori 
 Quadr. labii sup. 
 caput infraorbitale 
 
 Caput zygomaticum 
 
 Caninus 
 
 Orbicularis oris 
 
 Quadratus labii inferioris 
 Triangularis 
 
 Aurieularis 
 superior 
 
 Aurieularis 
 anterior 
 
 Occipitalis 
 
 Aurieularis 
 posterior 
 
 Fig. 2. — The Superficial Muscles of the Head and Neck. (Morris.) 
 
 Origin. — From the nasal process of the superior maxilla near its 
 orbital margin. 
 
 Insertion. — From its origin it passes almost directly downward, 
 dividing into two portions, the smaller of which is inserted into the nasal 
 wing, while the larger portion is prolonged downward, blending into the 
 
6 ANATOMY 
 
 orbicularis oris and levator labii superioris, and forming a part of the 
 substance of the upper lip. 
 
 Relations. — Superficially, by the integument; deeply, by the levator 
 anguli oris and compressor narium. 
 
 Action. — By its smaller and shorter portion to raise the wing of the 
 nose and to dilate the nostril; by its larger and longer portion to elevate 
 the inner half of the upper lip. 
 
 Levator Labii Superioris. — This muscle belongs to the superficial 
 layer, and derives its name from its action. 
 
 Origin. — From the facial surface of the superior maxilla, at a point 
 between the orbital cavity and the infra-orbital foramen. Also by the 
 attachment of a few fibers to the malar bone. 
 
 Insertion. — Passing downward and inward, it is inserted into the orbic- 
 ularis oris and the integument of the upper lip. Near its lower third 
 it joins the levator labii superioris alaeque nasi, and acts in conjunction 
 with it. Occasionally it is reinforced by fibers from the orbicularis pal- 
 pebrarum, which it receives at its outer border. 
 
 Relations. — Superficially, by the orbicularis palpebrarum and the 
 integument; deeply, by the levator anguli oris; the compressor nasi at 
 its origin, and by the infra-orbital vessels and nerves. 
 
 Action. — To elevate the upper lip. 
 
 Levator Labii Inferioris. — This muscle, also known as levator 
 menti, lies immediately beneath the mucous membrane of the lower 
 lip, and can best be dissected by everting the lip and lifting off the 
 membrane. 
 
 Origin. — From the incisor fossa of the lower jaw at its upper border. 
 
 Insertion. — Into the integument of the skin. 
 
 Relations. — With the labial integument, the lower border of the orbic- 
 ularis oris muscle, and its superficial surface with the oral mucous mem- 
 brane. Deeply it is in close contact with the periosteum and the depres- 
 sor labii inferioris. 
 
 Action. — To raise and cause to protrude the integument of the chin. 
 
 Depressor Labii Superioris. — This small muscle with its fellow of 
 the opposite side is sometimes found within the mucous membrane form- 
 ing the frenum of the upper lip. 
 
 Origin. — It arises from the incisive fossa along its lower margin and 
 some of its fibers are attached to that part of the alveolar process closely 
 associated with the fossa. 
 
 Insertion. — From the point of origin the fibers pass upward and are 
 attached to the lower border of the nostrils and partition of the nose. 
 
THE BUCCAL ORIFICE 7 
 
 Some of the fibers of this muscle are also attached to the integument cover- 
 ing the wing of the nose. The balance pass downward and mingle with 
 the fibers of the muscular structure of the upper lip. 
 
 Relations. — At its point of origin the fibers are closely associated with 
 the mucous membrane forming the gums. Above this the muscular 
 structure of the lip overlies these fibers. Deeply it rests upon the surface 
 of the superior maxilla, and joins its fellow of the opposite side at the 
 median line. 
 
 Action. — To depress the upper lip. 
 
 Depressor Labii Inferioris, or Quadratus Menti. — The name 
 of this muscle is derived from its form and action. It belongs to the 
 second layer of facial muscles, is quadrilateral in shape, and consists of 
 parallel fibers which meet above in the median line. 
 
 Origin. — At the outer aspect of the lower border of the inferior maxilla, 
 from a point near the symphysis to the space beneath the first bicuspid 
 tooth. 
 
 Insertion. — Its fibers pass upward and inward, and after uniting 
 with its fellow of the opposite side, blend into the body of the orbicularis 
 oris of the lower lip. 
 
 Relations. — By its superficial surface with the integument and a 
 portion of the depressor anguli oris; deeply, with the mental nerve and 
 vessels, a portion of the orbicularis oris, the mucous membrane lining the 
 lower lip, and the labial glands. 
 
 Action. — To draw down and somewhat evert the lower lip. 
 
 Zygomaticus Minor. — An extremely slender muscle belonging to the 
 superficial set of facial muscles. It is closely associated with a larger 
 muscle, the zygomaticus major, belonging to the angular series, to be 
 described in connection with the muscles of the cheek. 
 
 Origin. — From the anterior inferior part of one of the facial bones 
 — the malar — close to its junction with the superior maxilla. 
 
 Insertion. — It passes downward and forward, its fibers becoming 
 lost in the special elevator muscle of the upper lip about midway between 
 the median line and the angle of the mouth. 
 
 Relations. — Superficially, by the integument, by its deep surface with 
 the levator anguli oris, facial portion of the orbicularis oris, and the infra- 
 orbital branch of the facial nerve. 
 
 Action. — To elevate and somewhat evert the upper part of the lip. 
 
 The Blood-supply to the Lips. 
 
 The blood-supply to the lips is principally through the superior and 
 inferior coronary arteries, both of which are branches of the facial artery. 
 
8 ANATOMY 
 
 In addition to these the inferior labial artery and the sub-mental, also 
 branches of the facial, and the mental branch of the inferior dental artery 
 supply a part of the lower lip. 
 
 The superior coronary artery courses along the inferior margin 
 of the upper lip, between the mucous membrane and the fibers of the or- 
 bicularis oris muscle. At the median line it anastomoses with its fellow 
 of the opposite side. 
 
 The interior coronary artery, somewhat smaller than the superior, 
 supplies the lower lip by coursing through its substance in a manner simi- 
 lar to the superior coronary and also anastomoses with its fellow of the 
 opposite side at the median line. 
 
 Course of the Blood from the Heart to the Lips. — From the 
 heart to the aorta, to the common carotid, to the external carotid, to the 
 facial, to the superior and inferior coronary and the inferior labial arteries. 
 After passing through the labial capillaries the blood is returned to the 
 heart through the superior and inferior coronary veins, and the larger veins 
 of which they are branches. 
 
 Nerves of the Lips. 
 
 The general nerve-supply to the lips is principally by small branches 
 of the infra-orbital nerve for the upper lip, and by branches of the mental 
 nerve for the lower lip. The buccal and superior maxillary branches of 
 the lower division of the facial nerve supply the orbicularis oris muscle; 
 the upper division of the facial nerve sends branches which supply the 
 levator labii superioris alseque nasi, as well as the levator labii superioris 
 and the zygomaticus minor, while the superior maxillary branch of the 
 lower division of the facial supplies the depressor labii inferioris. 
 
 THE LATERAL WALLS OF THE MOUTH. 
 
 The Cheeks {buccce). 
 
 The cheeks are continuous with and similar in structure to the lips, 
 being covered internally by mucous membrane and externally by the com- 
 mon integument. Immediately beneath the mucous membrane are a 
 number of transverse muscular fibers, covered externally by a layer 
 of subcutaneous fat, and lying between this and the integument other 
 muscular tissue, the fibers of which radiate in various directions, accord- 
 ing to the action of the muscle to which they belong. Besides muscular 
 and fatty tissue, there are imbedded within the substance of the cheek 
 blood-vessels, nerves, and glands. The fatty tissue spoken of as interven- 
 
THE LATERAL WALLS OF THE MOUTH 9 
 
 ing between the muscular fibers gives to the cheek its fullness and 
 rotundity. 
 
 The integument, or external covering of the cheek, is similar in 
 structure to the skin covering other parts of the body, and, like the lips 
 in the male, is productive of a hairy growth. 
 
 The mucous membrane, or internal covering of the cheek, is similar 
 to that of the lips, containing numerous glands {buccal glands) which are 
 almost identical to, but smaller than, the labial glands. In addition to the 
 buccal glands which are distributed over the entire membrane, there are 
 about five of larger size, which open into the mouth in the region of the 
 molar teeth, and are called molar glands (see Glands of the Mouth). 
 
 The Muscles of the Cheeks (See Fig. 3). 
 
 The transverse muscular fibers referred to as being immediately 
 beneath the mucous membrane are those of the buccinator, a muscle 
 named from its action, that of being the chief muscle employed by the 
 trumpeter. External to the buccinator is the masseter, one of the muscles 
 of mastication, the elevator and depressor muscles of the angle of the 
 mouth, the levator anguli oris, and the depressor anguli oris, and the 
 dermal muscles, zygomaticus major and zygomaticus minor, and the 
 risorius. 
 
 Buccinator. — This muscle forms the greater portion of the lateral 
 wall of the mouth. It is deep-seated in the cheek, being one of the third 
 stratum of facial muscles. 
 
 Origin. — The fibers are distinct in their origin from a part of the alve- 
 olar process of the superior maxillary bone, at a point immediately over 
 the second and third molar teeth, from the anterior border of the pterygo- 
 maxillary ligament, a narrow band of tendinous fibers or raphe extend- 
 ing from the pterygoid plate of the sphenoid bone to the mylohyoid ridge 
 of the inferior maxilla near the position of the third molar tooth. Some 
 of its fibers also arise from the outer wall of the alveolar process of the 
 inferior maxilla below the second and third molars. 
 
 Insertion. — The fibers pass forward and converge as they reach the 
 lateral margins of the orbicularis oris; here the fibers of the upper portion 
 pass downward and blend into the muscles of the lower lip, while the 
 lower fibers pass upward and blend into those of the upper lip. Those 
 fibers which arise from the inferior maxilla pass forward and also blend 
 into the lower lip. 
 
 Relations. — Superficially, by the skin and subcutaneous fat, the duct 
 of Stenson, the masseter muscle, a portion of the angular group, and the 
 
IO 
 
 ANATOMY 
 
 facial arterv and vein. Passing over it are branches of the facial and buc- 
 cal nerves, also a layer of deep fascia continuous with that which covers 
 the upper part of the pharynx. By its deep surface it is in relation with 
 the mucous membrane and buccal glands. 
 
 Corrugator 
 
 Quadr.labii sup. 
 caput augularis 
 
 Caput infra- 
 orbitale 
 
 Nasalis, pars 
 
 transversa / •^SaSl 
 
 Caninus * ■■ ■ » ■»■ ■ 
 Depressor_ ^/^^C ( f 
 se-pti n...-,: ^Jf. "'< i !. 
 
 Nasalis, par= — /Nfe? -""SI 1 1, 
 
 alaris r^*S^ 
 
 Orbicularis oris \^ -iv^ N \ 
 
 Buccinator 
 
 Triangularis 
 
 Quadratus la- 
 
 bii inferioris 
 
 Men talis 
 
 Mylo-hyoid 
 
 Anterior belly 
 
 of digajtric 
 
 .-j Temporal 
 
 Posterior belly 
 of digastric 
 
 Splenius capitis 
 
 Scalenus anterior 
 
 FlG- 3.— The Deeper Muscles of the Face and Neck. (Morris.) 
 
 Action— -To draw outward or backward the angles of the mouth, thus 
 enlarging the buccal orifice and pressing the lips tightly against the 
 teeth; to force the food between the occlusal surfaces of the molar and 
 bicuspid teeth during mastication; to diminish the concavity of the cheek, 
 compressing the air contained therein and forcing it forward. It be- 
 
THE LATERAL WALLS OF THE MOUTH II 
 
 comes an auxiliary in deglutition by shortening the cavity of the pharynx 
 from before backward, through its connection with the superior 
 constrictor. 
 
 Masseter. — This muscle is placed immediately external to the 
 buccinator, and is one of the principal muscles of mastication. It is 
 short, thick, and somewhat quadrate in form, and is composed of two 
 sets of fibers, superficial and deep. The fibers of the former are directed 
 obliquely downward and backward; those of the latter, which are much 
 shorter, pass almost vertically downward. 
 
 Origin. — The superficial layer, from the malar process of the supe- 
 rior maxilla, and from the anterior portion of the zygomatic arch of the 
 malar bone. The deep layer from the posterior third of the zygomatic 
 arch, as well as from the greater part of its inner surface. 
 
 Insertion. — The superficial fibers are inserted into the ramus and 
 angle of the inferior maxilla, and the deep fibers into the upper half 
 of the outer surface of the ramus. 
 
 Relations. — By its external surface with the zygomaticus major, 
 risorius, and platysma myoides muscles, the parotid gland and its duct; 
 by the transverse facial artery, the facial vein, and facial nerve, and by 
 the integument. By its internal surface with the ramus of the inferior 
 maxilla, a mass of fat which separates it from the buccinator, and with the 
 temporal muscle. Its posterior margin is in relation with the parotid 
 gland, and its anterior with the facial artery and vein. 
 
 Action. — The principal action of this muscle is to close the jaw and 
 to draw it slightly forward. (For further description, see Muscles of 
 Mastication, part I, chap. iv. 
 
 The Angular Series. — The remaining muscles of the cheek are 
 those of the angular series, or those muscles which are inserted into the 
 angle of the mouth, two coming obliquely from above — the levator anguli 
 oris and the zygomaticus major — one running almost horizontally for- 
 ward — the risorius — and one ascending from below — the depressor 
 anguli oris. 
 
 Levator Anguli Oris. — This muscle, which receives its name from 
 its action, belongs to the second layer of facial muscles. It is formed in 
 the shape of a triangular sheet. 
 
 Origin. — From the canine fossa of the superior maxilla, immediately 
 below the infra-orbital foramen. 
 
 Insertion. — Passing downward and outward it is inserted into the 
 angle of the mouth, its fibers blending with those of the orbicularis oris 
 and the other angular muscles. 
 
12 ANATOMY 
 
 Relations. — Superficially, with the levator labii superioris, the zygo- 
 matics minor, and the infra-orbital vessels and nerves; deeply, with the 
 facial portion of the orbicularis oris and buccinator muscles, and the 
 mucous membrane of the mouth. 
 
 Action. — Especially to elevate the angle of the mouth, and to assist in 
 drawing these angles inward, decreasing the size of the buccal orifice. 
 
 Zygomaticus Major. — This muscle, the companion of which has 
 been described in connection with the muscles of the lips, belongs to the 
 first facial layer. It is composed of a long, fleshy band of muscular fibers, 
 which run direct from their point of origin to their point of insertion. 
 
 Origin. — From the malar bone, in close proximity to the zygomatic 
 suture. 
 
 Insertion. — From its origin it passes obliquely downward to the 
 angle of the mouth, and blends into the fibers of the orbicularis oris and 
 depressor anguli oris. 
 
 Relations. — Superficially, with the skin and subcutaneous fat; deeply, 
 with the malar bone, the masseter, and buccinator muscles, the facial 
 and transverse facial arteries, the facial vein, and branches of the facial 
 nerve. 
 
 Action. — To draw upward and outward the angles of the mouth, 
 as in smiling or laughing. By contracting, it throws into prominence the 
 cheek tissues in front of the malar bone, and forces the lower eyelid up- 
 ward. When acting simultaneously with its fellow of the opposite side, 
 the buccal aperture is widened, and the upper lip is elevated, exposing 
 the superior teeth. 
 
 Risorius. — One of the superficial set of facial muscles, receiving its 
 name from its supposed action in laughter {rider e, to laugh). It is flat 
 and ribbon-shaped, and is frequently very small and poorly developed. 
 
 Origin. — From the deep fascia covering the masseter muscle and 
 parotid gland, some of its fibers occasionally arising from, the mastoid 
 process of the temporal bone. 
 
 Insertion. — Passing transversely forward and inward to the angle of 
 the mouth, its fibers blend with those of the orbicularis oris, and the 
 depressor anguli oris. 
 
 Relations. — Superficially, with the integument and subcutaneous fat; 
 deeply, with the masseter and buccinator muscles, the facial artery and 
 vein, and branches of the facial nerve. 
 
 Action. — To draw the angles of the mouth directly outward, thereby 
 increasing the width of the buccal orifice. 
 
 Depressor Anguli Oris. — Also one of the superficial layers of facial 
 
THE LATERAL WALLS OF THE MOUTH 1 3 
 
 muscles, deriving its name in accordance with its action upon the angle 
 of the mouth. It is a triangular-shaped muscle with its base below, be- 
 coming narrow as it ascends. 
 
 Origin. — From the lower border of the inferior maxilla, and from its 
 external oblique line below the cuspid, bicuspid, and first molar teeth. 
 
 Insertion. — Passing upward and inward it is inserted into the integu- 
 ment at the angle of the mouth, its fibers blending into those of the muscles 
 previously described as coming together at this point. 
 
 Relations. — Externally, with the integument; deeply or internally, 
 with the depressor labii inferioris, the buccinator, and the inferior coro- 
 nary artery. 
 
 Action. — To draw down the angle of the mouth and to slightly ex- 
 tend it. 
 
 Blood-supply to the Cheeks. 
 
 The blood-supply to the cheeks is principally through the facial 
 artery and its direct branches, the superior and inferior coronary, the 
 transverse facial, and branches from the internal maxillary. 
 
 The Facial Artery and Branches. — The facial artery, also called 
 the external maxillary, enters the cheek after passing over the body of 
 the inferior maxilla at the anterior edge of the masseter muscle. It 
 courses obliquely forward and upward through the substance of the cheek, 
 until it reaches the inner angle or canthus of the eye, where it joins the 
 nasal branch of the ophthalmic artery, and is called the angular artery. 
 Near the center of the cheek the inferior labial artery is given off, which 
 passes forward and downward to the lower lip, but supplies a portion of 
 the cheek in so doing. Midway between the center of the cheek and the 
 angle of the mouth the superior and inferior coronary arteries are given off, 
 supplying that part of the cheek immediately adjacent to the angle of the 
 mouth, after which they pass on to supply the upper and lower lips. The 
 masseteric branch is given off in the immediate center of the cheek, at a 
 point immediately below the inferior labial, passes directly upward over 
 the masseter muscle, and anastomoses with branches of the internal 
 maxillary and transverse facial. There are also given off from the 
 main trunk near the center of the cheek the buccal branches, which pass 
 upward over the buccinator muscle, and also anastomose with branches 
 of the internal maxillary and transverse facial arteries. 
 
 The Transverse Facial Artery. — This is the largest branch of the 
 temporal artery. It is at first deeply seated in the substance of the pa- 
 rotid gland, after leaving which it courses transversely over and supplies 
 
14 
 
 ANATOMY 
 
 the masseter muscle, sends off small branches which supply the integu- 
 ment of the cheek, and anastomoses with the buccal, infra-orbital, and the 
 
 Orbicularis oeuli muscle 
 
 Transverse facia! artery 
 
 Zygomaticus minor 
 muscle 
 
 Zygomaticus major 
 muscle 
 
 Buccinator muscle 
 Masseteric branch 
 
 Masseter 
 
 Stylo-pharyngeus 
 muscle 
 Stylo-glossus muscle 
 
 Ascending palatine 
 
 branch 
 Tonsillar branch 
 
 Externa! maxillary 
 
 External carotid 
 artery 
 Posterior belly of 
 digastric muscle 
 
 Lingual artery 
 
 Frontal branch of ophthalmic 
 
 artery 
 Nasal branch of ophl/talmic 
 
 artery 
 
 Angular nr'ery 
 Levator labii super- 
 
 ioris alaeque nasi 
 
 muscle 
 In/, aorbifal arten/ 
 Levator labii super- 
 
 ioris proprius 
 Lateral natal artery 
 Caninus muscle 
 Artery of septum 
 Svperwr labial 
 
 artery 
 
 - Risorius muscle 
 
 Inferior labial artery 
 
 _ Mental branch of inferior 
 alveolar arteru 
 Quadratus labii inferioris 
 
 muscle 
 Inferior labial artery 
 Triangularis muscle 
 
 Submental artery 
 Branches to submaxillary 
 gland 
 
 Anterior belly of digastric muscle 
 Mylo-hyoid muscle 
 
 Hyo-glossus muscle 
 
 HYPOGLOSSAL NERVE 
 
 Scheme of the Right External Maxillary Artery. {Morris after Walsham.) 
 
 facial arteries. Besides the arteries already named, the deeper portions 
 of the cheek receive blood from two branches of the internal maxillary 
 artery, the masseteric branch and the buccal branch. The former supplies 
 
THE INTERIOR OF THE MOUTH 1 5 
 
 the masseter muscle and anastomoses with the masseteric branch of the 
 facial, while the latter supplies the buccinator muscle and anastomoses 
 with the buccal branches of the facial. 
 
 Course of the Blood from the Heart to the Cheeks. — From the 
 heart to the aorta, to the common carotid, to the external carotid, to 
 the facial and its direct branches, or from the external carotid to the tem- 
 poral, to the transverse facial and branches. 
 
 From the cheeks the blood is returned to the heart principally through 
 the facial vein, a division of the anterior superficial vein. It enters 
 the cheek at a point midway between the lower eyelid and the wing 
 of the nose, passes obliquely downward, being in close contact with the 
 anterior edge of the masseter muscle over the body of the lower jaw, join- 
 ing the internal jugular vein in the neck. The transverse facial vein 
 which follows the course of the transverse facial artery, and the supe- 
 rior and inferior coronary veins also collect and convey a portion of the 
 blood from the cheeks to the larger veins and thence to the heart. 
 
 Nerves of the Cheeks. 
 
 The nerve-supply to the cheeks is principally from the seventh or 
 facial nerve and its branches, the buccal branch supplying a greater part 
 of their substance. There are also a few fibers of the infra-orbital branch 
 of the seventh nerve distributed to the labiobuccal region. The buccal 
 branch of the lower division of the facial, also the buccal branch of the 
 inferior maxillary division of the fifth nerve, supplies the buccinator 
 muscle. The infra-orbital branch of the upper division of the facial nerve 
 supplies the zygomaticus major and the levator anguli oris; the buccal 
 branch supplies the risorius, and the supramaxillary branch of the lower 
 division of the facial nerve supplies the depressor anguli oris. 
 
 THE INTERIOR OF THE MOUTH. 
 
 For convenience of description the mouth may be divided into two 
 parts — a superior portion and an inferior portion. In dissection this 
 division may be accomplished by an incision beginning at the angles of 
 the mouth and carried backward and slightly upward through the sub- 
 stance of the cheeks until the temporomandibular articulation is reached. 
 After disarticulating this joint, another incision is made, beginning at the 
 joint on either side, carried downward and forward, then obliquely across 
 the throat, until the two come together at the median line. This latter 
 incision must be deep enough to completely sever the tissues of the throat. 
 
 The superior portion of the mouth contains the hard palate, or roof 
 
1 6 ANATOMY 
 
 of the mouth, the soft palate, and the sixteen upper teeth, firmly set in the 
 bone and surrounded by a dense fibrous tissue — the gums. The inferior 
 portion contains the tongue and its attached muscles, forming the floor 
 of the mouth, the sixteen lower teeth and the gums surrounding them. 
 
 THE SUPERIOR PORTION OF THE MOUTH (Fig. 5). 
 
 The osseous framework, or base upon which this half of the mouth 
 is constructed, is composed of a part of four bones — the two superior 
 maxillary, or upper jaw bones, and the two palate bones (see Bones of 
 the Mouth, p. 37). 
 
 The Hard Palate, or Roof of the Mouth (Figs. 5 and 6). 
 
 This is formed by the union of the palatal processes of the superior 
 maxillary bones and the horizontal plates of the two palate bones at the 
 median line. It is limited in front and laterally by the margins of the 
 alveolar process, or that portion of the bone which gives support to the 
 teeth, and ends posteriorly in an irregular border, to which is attached a 
 muscular, membrane-like curtain — the soft palate. The hard palate is 
 covered throughout by a thick and firm mucous membrane, seldom so 
 highly colored as that lining the lips and cheeks. The mucous mem- 
 brane is closely adherent to the bone through its common covering, the 
 periosteum. In the center of the hard palate is a ridge or fold of mucous 
 membrane, which follows the median line from before backward; this 
 is called the palatal or median raphe, and indicates the line of union 
 formed during the development of the parts. Anteriorly, the raphe ends 
 in a small papilla, which marks the opening of a canal in the bone — the 
 anterior palatal canal. Posteriorly, the raphe usually diminishes, in size, 
 but occasionally is well marked through the whole extent of the hard 
 palate. Near the center of the hard palate it frequently separates into 
 two or more smaller ridges, which are proportionately diminished in size, 
 and are continued backward side by side. On either side of this central 
 ridge, anteriorly, the mucous membrane presents a number of fantas- 
 tically arranged folds, the palatal ruga (wrinkles). These folds are 
 usually quite numerous and prominent, but are occasionally but slightly 
 developed. The nature of these wrinkles is strongly indicative of the char- 
 acter or temperament of the individual; thus, in the four basal tempera- 
 ments they may be divided as follows: in the bilious, heavy and strong, 
 composed of angles rather than curves; in the nervous, few in number, 
 close together, not prominent, and composed of long curves; in the 
 sanguine, quite numerous, fairly prominent, well rounded and graceful 
 
THE INTERIOR OF THE MOUTH 
 
 17 
 
 in outline; in the lymphatic, few in number, flat, widely separated, 
 and but little curved. Accompanying these varying conditions in 
 the rugae will be found a corresponding variation in the raphe. The 
 anterior and lateral margins of the mucous membrane covering the 
 hard palate form the palatal portion of the gingiva (gums). 
 
 Fig. 5. — The Superior Portion or Roof of the Mouth. 
 
 In figure 6 the hard palate is shown with its mucoperiosteum re- 
 moved. It will be observed that the bony plates are perforated by nu- 
 merous small openings or foramina, through which the body of the bone 
 receives its nourishment, broken by depressions for the accommodation 
 of the various mucous glands, and traversed by longitudinal grooves 
 which give lodgment to blood-vessels and nerves. 
 
 The arch formed by the hard palate from side to side (palatal arch) 
 
l8 ANATOMY 
 
 varies greatly in form, imparting much knowledge in regard to the tem- 
 perament of the individual, and in a measure controlling the quality of the 
 voice. Thus, in the sanguine temperament the roof of the mouth presents 
 almost a perfect oval. In the bilious type it is comparatively high and 
 flat, as it passes from the base of one alveolar process to the other, from 
 which point it descends abruptly to the necks of the teeth. In the ner- 
 vous type the roof is high and semi-elliptical or parabolic in shape, and 
 in the lymphatic it is low and flat. 
 
 In the same illustration the union or suture between the four bones 
 may be seen at the median line. Near the anterior third of this central 
 suture is the opening of the incisive or anterior palatal canal, the anterior 
 palatal joramen, the location of which has been referred to in the descrip- 
 tion of the mucous membrane. Near the posterior border, and situated 
 within the suture which unites the superior maxillary bones with the palate 
 bones (the palatomaxillary suture), are two other foramina, the posterior 
 palatal; and immediately behind these, and separated by a thin ridge of 
 bone, are the accessory palatal foramina, these being in the tuberosity of 
 the palate bones. (For further description of these foramina, see Bones 
 of the Mouth, p. 37.) By the vessels and nerves which enter the hard 
 palate through these various foramina, the mucous membrane and 
 glands receive their blood- and nerve-supply. 
 
 Blood-supply to the Hard Palate. — This is principally derived 
 from the posterior or descending palatal branch of the internal maxillary 
 or deep facial artery, which passes downward in the posterior palatal 
 canal and emerges through the posterior palatal foramen. Immedi- 
 ately on reaching the palate it divides into an anterior and a posterior 
 branch, the former passing forward in a groove provided for it to the an- 
 terior palatal foramen, where it anastomoses with the nasopalatal artery. 
 The groove in which the artery lies in its passage forward is usually at 
 the base of the alveolar process, and in some instances is converted into 
 a canal for a part of its length. The posterior branches pass backward 
 and downward to supply the soft palate. In connection with supplying 
 the hard palate proper, this artery carries blood to the palatal alveolar 
 walls, to the mucous glands, the mucous membrane, and the gums. 
 
 Course of the Blood from the Heart to the Hard Palate.— 
 From the heart to the aorta, to the common carotid, to the external 
 carotid, to the internal maxillary, to the posterior or descending palatal 
 branch of the latter. From the hard palate the blood is returned to the 
 heart by the superior palatal and inferior or descending palatal veins, the 
 former following the course of the superior palatal artery, while the 
 
THE INTERIOR OF THE MOUTH 19 
 
 latter originates at a point near the junction of the hard and soft palates, 
 passes downward, and joins the facial vein below the body of the inferior 
 maxilla. 
 
 Nerves of the Hard Palate. — The nerves of the hard palate are 
 the anterior or large palatal and branches from the nasopalatal, both of 
 which are branches of the sphenopalatal (Meckel's) ganglion. The an- 
 terior palatal nerve arises from the inferior angle of the ganglion, passes 
 
 Fig. 6. — The Hard Palate, or Roof of the Mouth, with its Membranous 
 Covering Removed. 
 
 downward, accompanied by the descending palatal artery, through the 
 posterior palatal canal, from which it emerges at the posterior palatal 
 foramen. From this point it passes forward in a groove of the hard 
 palate, and joins the nasopalatal nerve as it emerges from the anterior 
 palatal foramen. Accompanying this nerve in its course through the 
 posterior palatal canal are other branches of the sphenopalatal ganglion, 
 which pass to the soft palate, and will be described in that connection. 
 The nerves of the hard palate are all sensory, and filaments are dis- 
 tributed to the mucous membrane and glands and to the palatal portion 
 of the gums. 
 
 THE SOFT PALATE (Figs. 6 and 7). 
 
 The soft palate is attached to the posterior border of the hard palate, 
 from which it is continued as a backward prolongation of its soft tissue. 
 
20 ANATOMY 
 
 Hanging downward, with its free borders inclining backward, it may be 
 considered as forming a part of the posterior boundary of the mouth. It 
 partially separates the mouth from the nasal cavity and from the pharynx. 
 It is attached laterally to the walls of the pharynx, while its lower border 
 is free. The substance of the soft palate is composed of a number of thin, 
 but dense, muscular fibers, blood-vessels, nerves, and mucous glands, the 
 latter being similar to those of the hard palate. The anterior surface 
 of this muscular curtain is concave, directed forward and downward, 
 and is traversed by a mecli an "raphe. The posterior surface is convex, 
 directed backward and upward, and is continuous with the nasal cavity. 
 Suspended from the center of its free border is a small rounded or conic 
 membranous appendix, the uvula, and passing outward from the base 
 of this at each side are two curved muscular folds which extend outward 
 and downward, and are known as the pillars of the fauces. From the 
 position which these folds occupy they are divided into the anterior and 
 the posterior pillars of the fauces. The anterior pillar is formed from 
 muscular fibers which extend from the soft palate to the side and base of 
 the tongue (palatoglossus muscle), and is somewhat prominent as it passes 
 downward, outward, and forward. The posterior pillar approaches 
 more closely to its fellow of the opposite side than does the anterior. 
 It is formed of muscular fibers which extend from the soft palate above 
 to the pharynx below (palatopharyngeus muscle). It is somewhat con- 
 cave in its downward and backward course, and while closely united to 
 the anterior pillar above, is separated from it below, leaving, a triangular 
 interval or niche in which is lodged a small, almond-shaped body, the 
 tonsil, the space being known as the tonsillar recess. The intervening 
 space — bounded by the margins of the soft palate above, the root of the 
 tongue below, and the pillars laterally — is called the isthmus of the fauces, 
 which establishes the communication between the mouth and the pharynx. 
 The free margins of the soft palate, assisted by the pillars of the fauces, 
 mark the posterior boundary of the mouth. The entire surface of the soft 
 palate and its prolongations, the pillars of the fauces, is covered with 
 mucous membrane, being continuous with that of the mouth on its an- 
 terior surface, and with that of the nasal cavity on its posterior surface. 
 Muscles of the Soft Palate. 
 
 On each side the muscles of the soft palate are the palatoglossus, 
 palatopharyngeus, levator palati, and tensor palati, together with the 
 azygos uvula. 
 
 Palatoglossus. — A small fasciculus of fibers, somewhat cylindric 
 in form, expanding at either end into a thin sheet. It is named from its 
 
THE INTERIOR OF THE MOUTH 
 
 21 
 
 attachment to the soft palate and to the tongue. It is the prominence of 
 this muscle, together with its covering of mucous membrane, that forms 
 the anterior pillar of the fauces. 
 
 Pharyngeal _ 
 aponeurosis 
 
 Levator veli 
 palatini 
 
 Tensor veli 
 palatini 
 
 AUDITORY OR 
 
 EUSTACHIAN TUBE 
 
 M. uvulae 
 Hamular process 
 
 Fharyngo- 
 palatinus 
 
 Pharyngo-palatinus 
 
 Constrictor pharyngis 
 superior 
 
 Thyreoid cartilage 
 
 Cricoid cartilage 
 
 Crico-arytaenoideus 
 posterior 
 
 FlG . y _view of Muscles of Soft Palate, as seen from within the Pharynx. 
 (Morris Modified from Bourgery.) 
 
 Origin.— By a thin muscular sheet from the under surface of the 
 aponeurosis of the soft palate near the median line, its fibers uniting 
 
22 ANATOMY 
 
 with those of the opposite side. It passes downward in front of the ton- 
 sil and against the pharyngeal wall. 
 
 Insertion. — Into the side and base of the tongue. 
 
 Relations. — Superficially, it is covered by the mucous membrane 
 of the soft palate and tongue; deeply, in contact with the aponeurosis 
 of the soft palate, the superior constrictor muscle of the pharynx, and 
 one of the muscles of the tongue — the hyoglossus. 
 
 Action. — The lateral walls of the soft palate are drawn down, and 
 the sides of the tongue are drawn upward and slightly backward. Acting 
 in conjunction with the palatopharyngeus, the opening of the fauces is 
 constricted. 
 
 Palatopharyngeus. — This muscle — also named from its attach- 
 ments — is broad above, where it forms the greater part of the lower 
 half of the soft palate. Near the median line a few of its fibers blend 
 with those of its fellow of the opposite side. 
 
 Origin. — By two heads from a point near the raphe or median line 
 of the soft palate, passing downward and slightly backward, forming, 
 with its covering of mucous membrane, the posterior pillar of the 
 fauces. 
 
 Insertion.— Into the posterior border of the thyroid cartilage, and to 
 the inner surface of the lower part of the pharynx. 
 
 Relations. — In the soft palate, superficially, with the mucous mem- 
 brane, both anteriorly and posteriorly; above, with the levator palati; 
 and beneath, by the mucous glands. In the posterior pillar it is sur- 
 rounded with mucous membrane, and in the pharynx by the constrictor 
 muscles of the pharynx and the mucous membrane. 
 
 Action. — To constrict the opening of the fauces, by bringing together 
 the posterior pillars, thus depressing the soft palate and elevating the 
 pharynx. It controls the position of the soft palate during respiration, 
 and elevates the pharynx during deglutition. 
 
 Levator Palati. — This is a moderately thick muscle, and derives 
 its name from its action upon the soft palate. 
 
 Origin. — By a short tendon from the under surface of the petrous 
 portion of the temporal bone, and from the posterior and inferior aspect 
 of the cartilage of the Eustachian tube. 
 
 Insertion. — After passing downward by the side of the posterior nares 
 it is inserted into the median line of the soft palate, where its fibers 
 unite with those of its fellow of the opposite side. 
 
 Relations. — Externally, with the tensor palati and superior con. 
 strictor muscles; internally and posteriorly, with the mucous membrane- 
 
THE INTERIOR OF THE MOUTH 23 
 
 Action. — To raise the soft palate, bringing it against the posterior 
 wall of the pharynx. 
 
 Tensor Palati. — This is a slender and flattened muscular sheet, 
 and receives its name from its action upon the soft palate. 
 
 Origin. — From the scaphoid fossa at the base of the internal ptery- 
 goid plate and the spinous process of the sphenoid bone; also from the 
 outer side of the anterior aspect of the Eustachian tube. 
 
 Insertion. — After descending between the internal pterygoid muscle 
 and the internal pterygoid plate, and winding around the hamular process 
 of the latter, it is inserted into the transverse ridge on the horizontal 
 portion of the palate bone, and at the median line of the soft palate 
 where it is continuous with the aponeurosis of the opposite side. 
 
 Action. — To tighten or spread the soft palate laterally, forming a 
 septum between the posterior nares and the pharynx. It also opens the 
 Eustachian tube during deglutition. 
 
 The azygos uvulae (so named because it was at one time supposed 
 to be a single muscle) is composed of a pair of small muscles which orig- 
 inate from the aponeurosis of the soft palate and the nasal spine of the 
 palate bone. They pass downward and form or are inserted into the 
 uvula. 
 
 Relations. — Anteriorly, with the levator palati, palatoglossi, and a 
 part of the palatopharyngei; posteriorly, to the mucous membrane. 
 
 Action. — To shorten or draw up the uvula. 
 Blood-supply to the Soft Palate. 
 
 The poslerior or descending palatal branch of the deep facial artery, 
 after emerging from the posterior palatal canal, sends its posterior divi- 
 sion backward and downward to the soft palate, in the substance of which 
 they anastomose with the ascending palatal artery. After passing over 
 the superior border of the pharynx, the ascending pharyngeal artery sends 
 off branches which are distributed to the soft palate. A few branches of 
 the superior palatal branch of the internal maxillary and a few twigs 
 from the lingual artery also convey blood to the parts. 
 
 Course of the Blood from Heart to the Soft Palate. — From the 
 heart to the aorta, to the common carotid, to the external carotid, to the 
 facial or lingual, to the various branches named above, to the soft palate. 
 The return of the blood to the heart is principally through the superior 
 and inferior or descending palatal veins, both of which closely follow the 
 course of the arteries of the same name. 
 
 Nerves of the Soft Palate. — The small, or posterior, palatal, the 
 external palatal (both of which are branches of Meckel's ganglion), 
 
24 
 
 ANATOMY 
 
 branches of the glossopharyngeal nerve, and the following nerves which 
 supply the various muscles: filaments from the pharyngeal plexus to the 
 palatoglossus and palatopharyngeus, branches of the Vidian to the leva- 
 tor palati and azygos uvulae, and from the mandibular division of the fifth 
 nerve to the tensor palati.* 
 
 THE INFERIOR PORTION OR FLOOR OF 
 THE MOUTH (Fig. 8). 
 
 This half of the cavity of the mouth contains the tongue and its 
 attached muscles, the sixteen lower teeth firmly implanted in the jaw, and 
 
 Fig. 8. — The Inferior Portion or Floor of the Mouth. 
 
 the gums covering the alveolar walls. The base or osseous framework 
 upon which this portion of the mouth is constructed is principally made 
 up of a single bone, the inferior maxillary, mandible, or lower jaw bone 
 
 * A description of the upper teeth will be found in another chapter. 
 
THE INTERIOR OF THE MOUTH 
 
 25 
 
 (see Bones of the Mouth, p. 37). The hyoid bone, situated between the 
 angles of the mandible in the upper part of the neck, and at the base of 
 the tongue, giving attachment to many of the muscles about the floor of 
 the mouth, may also be considered in this connection. The floor of the 
 mouth is bounded anteriorly by the lower lip, laterally by the cheeks, 
 and below by the muscles attached to the external and internal oblique 
 lines of the mandible. 
 
 THE TONGUE {Lingua) (Fig. 9). 
 The tongue is a freely movable, highly sensitive, muscular organ. It 
 
 True Vocal Cord Posterior Wall of the Pharynx Corniculum Laryngis 
 
 False Vocal Cord 
 
 Tonsil 
 
 Adenoid Tissue 
 at Base of Tongue 
 
 Foramen Caecurr. 
 
 Epiglottis 
 
 Median Glosso- 
 epiglottidean Fold 
 
 Fungiform Papillae Circumvallate Papillae 
 Fig. 9. — Superior Aperture of Larynx. (Deaver.) 
 
 assists in the function of speech, participating, also, in the special sense 
 
26 
 
 ANATOMY 
 
 of taste, and in mastication and deglutition. The organ is attached 
 posteriorly to a U-shaped bone, the hyoid bone, which of itself is movable, 
 and is placed in the neck between the angles of the mandible and the 
 thyroid cartilage. 
 
 The tongue is suspended and kept in its position in the mouth by 
 numerous muscles, some of which are attached to the base of the skull, 
 and others to the mandible and hyoid bone. 
 
 The size of the tongue bears little or no relation to the size of the indi- 
 vidual, but is proportioned to the capacity of the alveolar arch, which space 
 
 it completely fills when at rest. The 
 shape of the tongue is influenced by 
 the shape of the alveolar arch; thus, 
 when the arch is contracted, narrow, 
 and pointed, the margins of the 
 tongue, when at rest, will assume 
 that form (a, Fig. 10) ; but when the 
 arch is broad and rounded anteriorly, 
 the margins of the tongue will also 
 be broad and rounded (b, Fig. 10). 
 
 The substance of the tongue is 
 chiefly composed of muscular fibers, 
 which are arranged in a complicated 
 manner, crossing one another at various angles, thus making the 
 movements of the organ exceedingly varied and extensive. Fibrous, 
 areolar, and fatty and glandular tissue enter into its structure, and it is 
 freely supplied with blood-vessels and nerves. Its free surface is covered 
 by a specialized mucous membrane, and over its entire surface are 
 numerous mucous follicles and glands. 
 
 Before continuing the description of the tongue, it will be necessary to 
 name its parts. The upper surface, or that facing the roof of the mouth, 
 is the dorsum {dor sum Ungues); those portions directed toward the cheeks 
 are known as the margins of the tongue; the thin narrow portion directed 
 forward and against the inner surface of the lower front teeth, is the 
 apex or tip (apex lingua). That portion between the frenum and extend- 
 ing back to the pillars of the fauces is the base, while that part of the dor- 
 sum immediately posterior to the tip is the post-tip, that region which 
 lies between the post-tip and the base being the prebase. The dorsum, 
 sides, and tip are free, while the base is attached by muscles to the man- 
 dible and hyoid bone. 
 
 From the base to the epiglottis is a fold which serves to limit the 
 
 Fig. io. 
 
THE INTERIOR OF THE MOUTH 27 
 
 movement of the latter organ, and to the sides of the base the pillars of 
 the fauces are attached. Under the anterior free extremity in the median 
 line is a fibrous muscular lamina or ligament, the frenum (frenum linguae), 
 which connects this part of the organ with the lower jaw and marks the 
 anterior border of the base of the tongue. The tongue is divided through 
 its anterior two-thirds by a slight longitudinal furrow, the median raphe, 
 which ends posteriorly near a small foramen, not constant in the adult 
 tongue, but plainly observed in the fetus or infant, the foramen coecum. 
 This foramen represents the upper termination of the thyreoglossus duct. 
 
 Papillae of the Tongue. — Over the anterior two-thirds of the dor- 
 sum and the sides and tip of the tongue are a number of small, soft, 
 conic eminences, which are known as the papillae of the tongue. These 
 are most numerous over the anterior part of the dorsum, and at the back 
 they are covered and partly hidden by an epithelial coating. In general, 
 the papillae are quite similar to those of the integument, not being com- 
 pound organs in their vascular and nervous supply. In consequence of 
 their variation in form and arrangement the papillae are variously 
 named. The largest papillae, being arranged like the letter V, are called 
 the circumvallate or falciform; those of medium size, the ■fungiform, are 
 so named from their resemblance to a young mushroom; and the smallest 
 and most numerous are known as the conic or filiform papillae. Each 
 papilla presents a broad, free end, and is attached by a constricted base, 
 which rests in a small, cup-like concavity, about the margins of which is a 
 well-formed circular rim. Beneath the thick epithelium of these parts 
 are numerous secondary papillae, and about the base of each papilla 
 are the openings of one or more glands. 
 
 The circumvallate or falciform papillae (Fig. 9) form a V-shaped line 
 at the posterior boundary of the dorsum. They are few in number 
 (varying from six to twelve), but are largest in size, not infrequently meas- 
 uring 1/4 of an inch in diameter. These papillae are generally regarded 
 as being gustatory, or directly interested in the sense of taste. Each 
 papilla is capped with a small secondary papilla. 
 
 The fungiform papillae (Fig. 9), of medium size, varying from 1/20 
 to 1/50 of an inch in diameter, are scattered over the dorsum, sides, and 
 tip of the tongue at irregular intervals, and are much more highly colored 
 than the smaller papillae which surround them. They vary greatly in 
 number, and being principally gustatory, account in a great measure for 
 the diversity in the acuteness of the sense of taste in different individuals. 
 These papillae, like the circumvallate, are capped with smaller secondary 
 papillae. 
 
28 ANATOMY 
 
 The conic ox filiform papillae (Fig. 9) are the smallest and most numer- 
 ous, and are thickly scattered over the entire surface of the dorsum in 
 front of the circumvallate as well as over the sides and tip of the tongue. 
 They are placed closely together, and with such regularity that they 
 fairly ridge the tongue with delicate lines, which run parallel with the 
 circumvallate in that region, but as the tip is approached they become 
 transversely inclined. These papillae are generally regarded as being 
 tactile or directly interested in the sense of touch, and are concerned in 
 directing the movements of the food during mastication. They also 
 possess secondary papillae upon their surfaces. 
 
 Immediately posterior to the circumvallate papillae are two shallow 
 grooves which follow the V-shaped line of the papillae and unite at the 
 foramen caecum. These grooves serve to indicate the line of junction 
 between the anterior and posterior portions of the tongue. The latter not 
 being within the cavity of the mouth will not be described. 
 
 Muscles of the Tongue (Fig. n). 
 
 The muscles of the tongue are both extrinsic-outward or external, and 
 intrinsic-inherent, inward, or special. 
 
 The extrinsic muscles include those which have their origin from 
 the base of the skull, the hyoid bone, or the mandible, and are the hyo- 
 glossus, geniohyoglossus, styloglossus, palatoglossus, and a few fibers of 
 the superior constrictor of the pharynx. The intrinsic muscles which 
 make up the bulk of the tongue are two in number, the superior lingualis 
 and inferior lingualis. 
 
 Hyoglossus.— As its name implies, this muscle extends from the 
 hyoid bone to the tongue. Its fibers are so arranged that they form a 
 thin square sheet. 
 
 Origin. — It arises from the whole length of the upper border of the 
 great cornu, from the body, and by a few fibers from the lesser cornu of 
 the hyoid bone. At their point of origin the fibers are in the form of a 
 thin sheet, and ascend toward the tongue almost parallel to one another, 
 but before reaching the tongue the anterior fibers pass slightly forward, 
 and at the upper margin of the side of the tongue bend inward and join 
 the fibers of the superior lingualis. In their distribution to this part of 
 the tongue they form a kind of submucous covering to the organ. 
 
 Insertion. — Into the posterior half of the side of the tongue, between 
 the styloglossus and superior lingualis muscles. 
 
 Relations. — Externally, with the digastricus, styloglossus, stylohyoid, 
 and mylohyoid muscles, the lingual and hypoglossal nerves, Wharton's 
 
THE INTERIOR OF THE MOUTH 
 
 2 9 
 
 duct, and the sublingual gland. Internally, with the lingualis, geniohyo- 
 glossus, and middle constrictor of the pharynx muscles, the lingual artery, 
 and the glossopharyngeal nerve. 
 
 Action. — To extend the tongue and to draw it backward, also to 
 
 Stylo-glossus 
 
 DORSUM OF TONGUE 
 
 Genio-hyo-glossus 
 Genio-hyoid 
 
 GREATER COPNU OF HYOID BONE 
 
 r~ STYLOID PROCESS 
 Stylo-hyoid 
 
 POSTERIOR PORTION 
 OF TONGUE 
 
 Stylo-pharyngeus 
 
 Hyo-glossuB 
 
 Thyro-hyoid 
 ligament 
 
 CARTILAGO TRITICEA 
 
 Thyro-hyoid 
 membrane 
 
 THYROID CARTILAGE 
 
 Median portion of 
 erieo-thyroid 
 membrane 
 
 CRICOID CARTILAGE 
 FIRST RING OF TRACHEA 
 
 p' > 'r ? i " """ ,f r f M! 
 
 Fig. 11. — Side View of the Tongue, with its Muscles. (Mrrois.) 
 
 draw downward the sides of the tongue, making its dorsum more convex 
 transversely. 
 
 Geniohyoglossus (Fig. 11). — This muscle also receives its name 
 from its three points of attachment, the chin internally, the hyoid bone, 
 and the tongue. It is a triangular-shaped muscle, narrow and pointed 
 at its attachment to the mandible, and broad and fan-shaped on approach- 
 ing the tongue. Being near the median line, it is separated from its 
 
3° 
 
 ANATOMY 
 
 fellow of the opposite side by a thin layer of connective tissue, the septum 
 of the tongue. 
 
 Origin. — It arises by a short tendon from the upper genial tubercle 
 of the lower jaw, from which point its fleshy fibers diverge fan-like to its 
 extensive insertion. 
 
 Insertion. — To the whole length of the tongue from base to apex 
 
 Vertical and Trans- 
 
 Mucous Submucous Superior Lingua i is verse Muscular Fibers 
 
 Membrane l issue Muscle 
 
 Inferior Linguaiis Muscle 
 Vena Comes ^ 
 
 Ranine Arteries Vena Comes 
 Initrnsic Muscular Fibers 
 
 Fig. 12. — Transverse Section of One-half of Tongue. (Deaver.) 
 
 immediately external to the median line, into the body of the hyoid bone, 
 and by a few fibers into the side of the pharynx. 
 
 Relations. — By its inner surface, with the septum of the tongue and 
 its fellow of the opposite side; by its outer surface, with the hyoglossus, 
 mylohyoides, styloglossus, and linguaiis muscles, sublingual gland, lin- 
 gual artery, and hypoglossal nerve. Superiorly, with the mucous mem- 
 brane of the floor of the mouth; inferiorly, with the geniohyoid muscle. 
 
THE INTERIOR OF THE MOUTH 
 
 31 
 
 Action. — Its anterior fibers assist in drawing back the tip of the 
 tongue, its posterior fibers throwing forward and protruding the tongue. 
 This muscle also depresses the center of the dorsum longitudinally, 
 making it concave transversely, and some of the lower fibers which are 
 attached to the hyoid bone elevate the bone and assist in raising the tongue. 
 
 Styloglossus. — Also named from its attachment, is a long fan-shaped 
 muscle, somewhat compressed laterally. 
 
 FR/ENUM L1NGU/E 
 
 Iiingualis inferior 
 
 Hyo-glossus 
 
 Genio-hyoid 
 
 Mylo-hyoid, reflected 
 
 Sterno-hyoid 
 
 - Iiingualis inferior 
 
 Genie— hyo-glossus 
 
 Stylo-glossus 
 Hyo-glossus 
 BODY OF HYOID BONE 
 Genio-hyoid 
 
 THYROID CARTILAGE 
 
 Fig. 13. — Under Surface of the Tongue with Muscles. (Morris.) 
 
 Origin. — From its point of origin at the tip of the styloid process of 
 the temporal bone, and from a portion of the stylomaxillary ligament, it 
 passes with a long curve, forward, slightly downward, then upward and 
 inward to its place of insertion at the side of the tongue. 
 
 Insertion. — Upon reaching the side of the tongue it divides into two 
 portions, the fibers of one portion passing transversely inward, while the 
 others pass longitudinally along the side of the tongue. 
 
 Relations. — Externally, with the internal pterygoid muscle, parotid 
 
32 ANATOMY 
 
 and sublingual glands, lineal nerve, and the mucous membrane of the 
 mouth; internally, with the superior constrictor and hyoglossus muscles, 
 and with the tonsil. 
 
 Action. — To draw the tongue backward and to produce a transverse 
 concavity to its upper surface by elevating its sides. 
 
 Superior Lingualis. — This is one of the intrinsic muscles, and is 
 situated immediately beneath the mucous membrane, extending from the 
 base to the tip of the organ. 
 
 Inferior Lingualis (Fig. 13). — This muscle is placed near the under 
 surface of the tongue, and is composed of two bands which extend from 
 base to apex, some of its fibers being attached posteriorly to the hyoid 
 bone, and in passing forward are placed between the hyoglossus and genio- 
 hyoglossus. Anteriorly, its fibers blend with those of the styloglossus. 
 
 Many of the fibers of this muscle run transversely and are placed 
 between the two former intrinsic muscles. These, together with some 
 fatty tissue compose the greater part of the substance of the tongue. 
 The fibers are attached at the median line to the fibrocartilaginous sep- 
 tum of the tongue, and laterally to the mucous membrane. In connection 
 with the transverse fibers there are a few placed vertically, which pass by 
 long curves from the dorsum to the under surface of the tongue. 
 
 Blood-vessels of the Tongue (Fig. 14). 
 
 This organ receives its blood principally through the lingual, facial, 
 and ascending pharyngeal arteries. The lingual artery arises from the 
 front of the external carotid near the facial, and often as a common trunk 
 with it. From its point of origin to the tongue it is divided into three 
 portions, the first or oblique, the second or horizontal, and the third or 
 ascending, and it is this latter portion which directly supplies the tongue. 
 Ascending tortuously beneath the hyoglossus muscle, it reaches the under 
 surface of the tongue, and, lying between the lingualis and hyoglossus 
 muscles, it is continued to the under surface of the tip of the tongue, at 
 which point it is called the ranine artery. At a point about corresponding 
 with the posterior margin of the hyoglossus muscle a branch is given off 
 (the dorsalis linguce), which passes almost directly upward, and, after 
 dividing into two or more small branches, supplies the back part of the 
 dorsum of the tongue and the mucous membrane about the circumvallate 
 papillae. At the anterior border of the hyoglossus muscle another branch 
 is given off (the sublingual artery) supplying the anterior muscular struc- 
 ture of the floor of the mouth. The facial artery by one of its muscular 
 branches supplies the styloglossus muscle. 
 
THE INTERIOR OF THE MOUTH 
 
 33 
 
 Course of the Blood from the Heart to the Tongue. — From the 
 heart to the aorta, to the common carotid, to the external carotid, to the 
 lingual artery and its smaller branches to the tongue. From the tongue 
 the blood is returned to the heart principally through the lingual vein, 
 which begins at the ranine vein beneath the tip of the tongue, passes back- 
 ward under cover of the mucous membrane, following the course of the 
 lingual artery until the hyoglossus muscle is reached, beyond which point 
 
 Posterior br of 
 descending palatine A 
 
 Palatine br. of 
 a-scendm qph aiyngeal A 
 
 Ascending phary -ngealA*- 
 
 Ascendinq palatine br 
 J 'of facial A 
 Tonsillar br. 
 of facial A. 
 Stylo pharynqeus M- 
 
 Facial A. 
 
 Middle conslriclorM '.- 
 Dors a lis linguae A 
 
 Lingual A — 
 External carotidA 
 
 -Descending pal a line A . 
 
 -A n terior br. of descending pala tine A . 
 ,Stylo-glossusM. 
 
 ,Palato -glossusM. 
 
 , Tonsillar br dors a I is linguae A 
 
 Superior Jhyroid 
 
 Infra- hyoid br of ' s^p.thyroidT^j)^ qastllc }f 
 Supra- hvoid br "■> <■' 
 
 of lingual A 
 
 rtery of f men urn 
 Subnifntiil A. 
 Cenio-hyuid M 
 Genio -hvo -glossus M. 
 Stylo -hyoid M. ^Sublingual A . 
 
 Fig. 14. — Arteries of Tongue and Tonsil. (Deaver.) 
 
 the fibers of the muscle separate the artery from the vein. After receiving 
 the sublingual and dorsalis linguae veins, the course of which corresponds 
 to the arteries of the same name, the vein passes backward and down- 
 ward and empties into the internal jugular. 
 Nerves of the Tongue. 
 
 The mandibular division of the fifth nerve by its lingual branch sup- 
 plies the papillae of the anterior portion and sides of the tongue, while the 
 lingual branch of the glossopharyngeal supplies the circumvallate papillae, 
 the base, and posterior sides. A few branches of the superior laryngeal 
 are distributed to the back part of the root of the organ. The motor 
 nerve of the tongue is the hypoglossal or ninth, supplying both the extrinsic 
 and intrinsic muscles.* 
 
 * A description of the lower teeth will be found in another chapter. 
 3 
 
CHAPTER II. 
 
 Muscular Tissues of the Mouth; of the Lips; of the Cheeks; of 
 the Soft Palate ; of the Tongue. 
 
 MUSCULAR TISSUES OF THE MOUTH. 
 
 Muscular Tissues of the Lips. — The minute bundles forming the 
 fasciculi of the oral sphincter muscle — the orbicularis oris — are distributed 
 between the submucosa of the mucomembranous portion and the sub- 
 cutaneous tissue of the cutaneous portion of the lips. The muscular 
 fibers radiate in three principal directions upon either side of the median 
 line: from the angle of the mouth toward the median line, and from the 
 fleshy slips of the maxilla and mandible — the musculi incisivi. As the 
 fibers from the angle of the mouth pass to the substance of the lip, they 
 are arranged in a laminated manner. When the median line is reached, 
 one set of fibers terminates somewhat abruptly in the subcutaneous tissue, 
 another set is continued beyond the median line and attached to the cutis 
 of the opposite side, while a third set, without crossing the median line, 
 is attached to the incisive fossae of the maxilla and mandible. The numer- 
 ous muscular fibers of the internal labial or mucomembranous portion, 
 and the external, facial, or cuticular portion, penetrate the parts and 
 terminate in close proximity to the epithelium or to the base of the papillae. 
 Delicate, hair-like fibers which are continuous with the sarcolemma 
 slightly penetrate the cutis and membrana propria. A few of the fibers, 
 which may be classed with the terminals of the outrunning muscles 
 from the lips, are arranged in a number of fasciculi in the subcutaneous 
 portion, pass through the fasciculi of the orbicularis oris, reach the sub- 
 mucous tissue, where they cross and recross one another, and finally pass 
 into the membrana propria, where they end in fan-like terminals. The 
 fasciculi of the orbicularis differ somewhat in the upper and lower lips; 
 in the former the bundles are strongly developed toward the angle of 
 the mouth, while in the latter the median bundles are the strongest. 
 The labial muscular tissues are of the transversely striated variety. The 
 fibers are cylindric in form, having rounded or pointed extremities in 
 the interior, and broad or flattened ends where they come in contact 
 with the periosteum. When examined with a high power each fiber 
 
 34 
 
MUSCULAR TISSUES OF THE TUNGUE 35 
 
 shows alternately broad and narrow striae, the former being dim, while 
 the latter is bright in appearance. With a stronger power both the broad 
 and narrow striae are seen to be transversely striated. 
 
 Muscular Tissues of the Cheeks. — The muscles entering into the 
 construction of the lateral walls of the mouth have already been described 
 in part I, page 12, giving the relations existing between the individual 
 muscles, together with the general disposition of the various fasciculi. 
 Histologically considered, these muscles partake of all the characteristics 
 of striated or voluntary muscular tissue. In the body of the buccinator 
 and masseter muscles the fibers are cylindric and have definitely pointed 
 or rounded ends. Near their termini, particularly in the latter muscle, 
 the inner extremities of the terminal fibers are pointed, while the outer 
 ends, or those by which the attachment is formed, are broad and rather 
 flat. 
 
 Muscular Tissues of the Soft Palate. — The disposition of the 
 striated muscular tissue of the soft palate is extremely complicated. The 
 azygos uindce, the only true longitudinal muscle in the soft palate, has 
 its origin from aponeurosis of the soft palate and from the nasal spine of 
 the palate-bone, the fibers passing backward upon either side of the 
 median line. This is a double muscle, and near its point of origin the 
 two portions are distinct and separated by a definite space, but upon 
 reaching the base of the uvula they become closely associated. The 
 fasciculi do not continue to the apex of the uvula, but immediately beyond 
 the center of its length are thrown out fan-like toward the sides, terminat- 
 ing in a manner similar to the fibers of the lips. In passing from before 
 backward a number of small fasciculi are given off, which reach out 
 laterally and traverse the glandular lobes, completely surrounding them, 
 after which they again return to the principal fibers at the median line. 
 The palatopharyngeus muscle is divisible into two parts, the upper 
 extremities of which lie partly in front and partly behind the levator 
 muscles. The greater number of the fibers of one set, situated in front 
 of the levators, form a curved, flattened aponeurosis. The fibrous 
 border of the hard palate serves as an attachment for the convex border 
 of this portion, while the other border, which is concave, is directly 
 toward the arch of the levators. The fibers of the palatopharyngeus, 
 situated behind the levators, form a number of loose fasciculi inter- 
 spersed by fat-cells. In passing toward the free border of the soft palate 
 the fibers become much more delicate, and, separating, some course in 
 front and others behind this muscle. In this location the fibers become 
 closely associated with the glands, and either end here or are continued 
 
36 ANATOMY 
 
 to the submucosa, or even to the membrana propria of the mucous 
 membrane. The fibers of the palatopharyngeus unite with the fibers of 
 the levators, and an arch-like fasciculus is formed by this union which, 
 subdividing, passes in front of the azygos uvulae to the opposite side. 
 All of these fibers run outward and downward, and unite with the 
 extremities of the other palatal muscles, the fibers of which are some- 
 what more regularly distributed. Like the muscles of the lips and 
 cheeks, the several fasciculi of the palatal muscles form a delicate 
 plexus, and a quantity of fatty tissue is found between the various 
 fasciculi. 
 
 Muscular Tissues of the Tongue. — The tongue is divided into 
 two equal lateral portions by a median septum — the septum Ungues. 
 This central septum, composed of a vertical layer of compact, fibrous, 
 connective tissue, extends the entire length and depth of the lingual 
 median line. Beginning at the hyoid bone, it gradually increases in 
 prominence until the middle of the organ is reached, beyond which point 
 it becomes less pronounced and finally disappears near the tip. The 
 bundles of the muscular tissues are arranged longitudinally, transversely, 
 and vertically. The former lie immediately beneath the mucous mem- 
 brane, including the superior lingualis above and the inferior lingualis 
 below, together with the greater part of the styloglossus. The superior 
 lingualis extends from the base to the tip of the organ, and by short 
 fasciculi its fibers are attached to the overlying tissues. The fibers of 
 this muscle are placed between the hyo- and styloglossi muscles of the 
 opposite side, both of which overlap the fibers of the lingualis near the 
 base of the tongue. The inferior lingualis also gives off several small 
 fasciculi and fibers to the mucous membrane beneath, and is composed 
 of two bands which reach from the base to the apex, each being 
 placed between the hyoglossus and genio-hyoglossus muscles. Th 
 transverse fibers, which are placed between the superior and inferior 
 lingualis muscles, originate from the septum linguae, and form the bulk 
 of the organ. From their point of origin these fibers course outward and 
 upward to the sides of the tongue. Those fibers which are vertically 
 disposed decussate with the transverse fibers, and pass from the dorsum 
 toward the under surface of the tongue, the fibers curving gracefully with 
 their concavity directed toward the under surface. In most instances 
 the ascending vertical fibers, as well as the transverse fasciculi, pass 
 between those longitudinally disposed and connect with the submucosa. 
 
CHAPTER III. 
 
 The Bones of the Mouth: The Superior Maxillae, The Palate Bones, 
 The Inferior Maxilla or Mandible. 
 
 SUPERIOR MAXILLARY BONES. 
 
 The superior maxillary bones, two in number, one on each side of 
 the median line or center of the face, are irregular in shape, and may be 
 classed as the largest bones of the face, with the probable exception of 
 the mandible or inferior maxilla. From the central position which they 
 occupy they contribute largely to the bony framework of this portion of 
 the skull. They are not only instrumental in forming the major portion 
 of the roof of the mouth or hard palate, but assist in the formation of 
 the floor of the orbit, and the sides and base of the nasal chamber. They 
 furnish a solid and firm foundation for the sixteen upper teeth, and by 
 their variety in form contribute much to the character and quality of the 
 voice. The outer or facial surface of these bones provides attachment 
 for numerous muscles. Each superior maxillary bone presents for 
 examination a body, four surfaces, and four processes. The body may be 
 described as forming an irregular triangle, its general contour depending 
 much upon the temperament of the subject. Within the body of the bone 
 is an irregular cavity, the maxillary sinus or antrum of Highmore. 
 
 The four surfaces of the body of the bone are the superior or orbital, 
 the lateral or facial, the proximal or nasal, and the posterior or zygomatic. 
 
 The Superior or Orbital Surface, which assists in forming the 
 greater portion of the floor of the orbit, is slightly concave over its anterior 
 two-thirds, and somewhat convex over the remaining or posterior third. 
 The three borders of this surface form almost an equilateral triangle, and 
 are named, as indicated by their location, the anterior, the posterior, and 
 the mesial or proximal. The anterior border is convex from before back- 
 ward and slightly concave throughout its length. That portion which 
 forms a part of the lower border of the completed orbit is smooth, while 
 the remaining portion is roughened to form an articulation with the malar 
 bone. The posterior border extends from the center of the malar process 
 backward and inward to the orbital process of the palate bone, which 
 
 37 
 
38 ANATOMY 
 
 articulates with the superior maxillary at this point. A portion of this 
 border, together with the orbital process of the palate bone, is instru- 
 mental in forming the anterior boundary of the sphenomaxillary fissure. 
 
 The mesial or proximal border is marked by an irregular thin edge, 
 which articulates with a portion of two bones, the lacrimal anteriorly, 
 and the os planum of the ethmoid bone posteriorly. Only the posterior 
 two-thirds of this border presents an articulating edge, the remaining or 
 anterior third being smooth and forming the commencement of the lacri- 
 mal groove, which in the articulated skull becomes a canal, passing 
 downward and backward to communicate with the inferior meatus of 
 the nose. Beginning at the posterior border of this surface and running 
 forward will be found a deep groove — the infra-orbital groove. When 
 near the center of the surface, this groove dips down and is covered by a 
 layer of bone, from which point it passes forward as a canal — the infra- 
 orbital canal — making its exit at a point about £ of an inch below the 
 border of the orbit, near the center of the facial surface of the bone, 
 the foramen thus formed being the infra-orbital foramen. Near the 
 root of the nasal process, and immediately within the anterior border 
 of this surface, is a small depression which marks the origin of the inferior 
 oblique muscle of the eyeball. 
 
 The Lateral or Facial Surface (Fig. 15). — This surface is made 
 up of the anterior part of the bone; it is irregularly concave, and pre- 
 sents a greater variety in form than any other part of the bone, with the 
 single exception of the palatal process. It is bounded above by the infra- 
 orbital ridge, and the roughened surface of the malar process which artic- 
 ulates with the malar bone; below, by the border of the alveolar process; 
 anteriorly, by the frail concave border of the opening into the nasal cavity, 
 the anterior nasal spine, and the perpendicular margins of the bone be- 
 neath. Posteriorly, this surface is separated from the posterior or zygo- 
 matic surface by a strong projecting eminence, the malar process. 
 
 The canine fossa is a deep depression, situated almost in the center 
 of this surface, the bone at this point being extremely thin and closely 
 related to the floor of the antrum. The concave floor of this fossa is 
 frequently traversed by one or two smaller convex ridges, corresponding 
 to the roots of the bicuspid teeth. 
 
 The canine eminence is a prominent ridge running vertical to the 
 body of the bone immediately anterior to the canine fossa, and corre- 
 sponding in position to the root of the cuspid tooth, the size and type of 
 the tooth having much to do with its extent and prominence. This 
 ridge gives origin to one of the depressor muscles of the upper lip, and also 
 
SUPERIOR MAXILLARY BONES 
 
 39 
 
 to one of the depressor muscles of the wing of the nose. The incisive or 
 myrtiform fossa is a depression found between the canine eminence and 
 the inner margin of the bone. The depth of this fossa is in a measure 
 controlled by the position and size of the teeth, and by the amount of 
 prominence in the canine eminence. 
 
 The infra-orbital foramen, which transmits the infra-orbital nerves 
 and blood-vessels., is immediatelv below the center of the infra-orbital 
 
 Nasal Process 
 
 Infra-Orbital 
 Foramen 
 
 Canine Fossa 
 
 Nasal Spine 
 Facial Surface 
 
 Incisive Fossa 
 
 Canine Eminence 
 
 Orbital Surface 
 
 Malar Process 
 
 Posterior Dental 
 Canals 
 
 Tuberosity 
 
 Fig. 15. — Left Superior Maxilla, Outer or Facial Surface. 
 
 ridge, and near the upper margin of the canine fossa. It is oval in form, 
 and faces almost directly toward the median line. Between this foramen 
 and the infra-orbital ridge is the point of origin for the principal levator 
 muscle of the upper lip, the levator labii superioris proprius. The whole 
 extent of the facial surface may present a number of vertical ridges, or the 
 same space may be regular and smooth, the condition being controlled 
 by the size and shape of the tooth-roots and the thickness of the bone 
 covering them. One of the levator muscles of the angle of the mouth, 
 the levator anguli oris, is attached to this surface near the upper border 
 of the canine fossa. 
 
4 o 
 
 ANATOMY 
 
 The Proximal or Nasal Surface (Fig. 16). — Above, this surface 
 presents a large, irregular opening into the maxillary sinus, this opening 
 being nearly closed in the articulated skull by neighboring bones. In 
 front of the opening into the sinus, and standing perpendicular from the 
 body of the bone, is the strong ascending plate of the nasal process, 
 marked near its lower extremity by a rough, horizontal ridge, the inferior 
 
 Nasal Process 
 
 Ridge for Middle 
 Turbinal 
 
 Middle Meatus 
 Thin plate of 
 bone over Lac- 
 rymal Groove 
 
 Ridge for Infe- 
 rior Turbinal 
 
 Nasal Spine 
 
 Anterior Pala- 
 tine Groove 
 
 Inferior Meatus 
 
 Posterior Palatine 
 Groove 
 
 Palate Proces 
 
 Fig. 16. — Left Superior Maxilla, Internal, Proximal, or Nasal Surface. 
 
 turbinated crest, which gives attachment to the inferior turbinated bone. 
 The smooth, concave surface immediately above this ridge corresponds 
 to the middle meatus of the nose, and forms the external wall of that pas- 
 sage. Below the opening into the sinus and the nasal process, and occupy- 
 ing the anterior two-thirds of the middle of this surface, is a large semi- 
 circular space, forming the outer wall of the inferior meatus of the nose. 
 Below this space, and projecting inward from the body of the bone, is 
 the palatal process, which articulates with the corresponding process 
 
SUPERIOR MAXILLARY BONES 4 1 
 
 of the opposite bone. At the anterior superior angle of the nasal surface, 
 and passing downward just behind the nasal process, is the lacrimal 
 groove. In the articulated skull this groove becomes a canal, the lacri- 
 mal canal, the ethmoid and the inferior turbinated bones assisting in its 
 formation. The canal passes downward and slightly backward, and 
 opens into the inferior meatus of the nose. It is about 1/2 of an inch in 
 length, and gives passage to the lacrimonasal duct. 
 
 The lacrimal tubercle is a small prominence of bone formed at the 
 junction of the anterior border of this surface, with the external surface 
 of the nasal spine. The extended portion of the lacrimal duct, the 
 lacrimal sac, finds lodgment at this point. 
 
 The posterior palatine or palatomaxillary canal commences near the 
 middle of the posterior border of this surface, appearing in the disarticu- 
 lated bone as a groove, and, passing downward and forward, gives pas- 
 sage to the posterior palatine vessels and anterior palatine nerves. The 
 canal is made complete by the articulation of the superior maxillary with 
 the vertical plate of the palate bone. On the posterior portion of the nasal 
 surface, extending from the irregular opening into the antrum downward 
 to a point opposite the palatal process, is a roughened surface about 1/2 
 of an inch in width, which marks the extent of articulation with the 
 palate bone. 
 
 The proximal or nasal surface presents four borders — superior, in- 
 ferior, anterior, and posterior. The superior border is irregular, and 
 articulates with the lacrimal and ethmoid bones. The inferior border 
 projects inward, and forms a strong horizontal plate — the palatal process. 
 This process defines the border from before backward to the posterior 
 third, at which point it is marked by the lower border of the roughened 
 surface which articulates with the palate bone. The anterior border is 
 sharp, frail, and irregular in outline, and forms the free margin of the 
 opening into the nasal cavity. The posterior border is marked by the 
 inner margin of the zygomatic surface, being smooth upon its upper half, 
 and roughened upon its lower half, at which point it articulates with the 
 palate bone. 
 
 The Posterior or Zygomatic Surface (Fig. 16). — This surface is 
 partly convex and partly concave, and is bounded above by a well- 
 defined margin, which serves as the dividing-line between this and the' 
 superior or orbital surface. This border is also marked by a roughened 
 margin on the posterior portion of the malar process, the orbital portion 
 of the palate bone articulating at this point. The major portion of this 
 border is smooth and rounded, forming the lower border of the spheno- 
 
42 ANATOMY 
 
 maxillary fissure, and marked by a notch, the commencement of the infra- 
 orbital groove. The outer border of the surface is formed by the malar 
 process, and by a line drawn from this point directly downward to the 
 alveolar process. The inner border is smooth and somewhat irregular 
 above, while below it is roughened for articulation with the palate bone. 
 
 The tuberosity, which also forms a portion of the inferior border of 
 this surface, is a roughened and rounded eminence of bone, and is pene- 
 trated by a number of nutrient vessels, which enter the many small fora- 
 mina at this point. Between the tuberosity and the body of the zygomatic 
 surface are several large apertures leading into canals, which pass into, 
 and give nourishment to, the substance of the bone. These canals trans- 
 mit the posterior dental blood-vessels and nerves, one of which, after 
 passing over the outer wall of the maxillary sinus, unites with the anterior 
 dental canal. The tuberosity is posterior to, and above, the third molar 
 tooth, in some instances extending directly backward from this tooth for 
 the distance of half an inch or more, but usually the tooth penetrates the 
 base of the tuberosity, leaving but a thin layer of bone posterior to it. 
 
 The inferior border of the posterior or zygomatic surface is formed 
 by that portion of the alveolar process which supports the second and 
 third molar teeth. 
 
 The bone presents four processes for examination — the nasal, the 
 malar, the palatal, and the alveolar. 
 
 The nasal process is a strong, irregular piece of bone, standing ver- 
 tically above the body of the bone proper, and forming the lateral boundary 
 of the nose. This process is greatly increased in strength by the infra- 
 orbital ridge joining it at or near its base, and ascending its external 
 anterior surface to some extent. That portion of the process posterior 
 to its junction with the infra-orbital ridge assists in forming the inner 
 wall of the orbit. 
 
 The external or anterior surface of the nasal process is marked by a 
 number of shallow grooves, traces of the development of the bone. 
 Scattered over this surface are a number of small foramina, the entrances 
 to minute canals transmitting nutrient vessels to the body of the bone. 
 This surface gives origin to one of the lip muscles, the levator labii superi- 
 oris alaeque nasi. 
 
 The internal surface of the nasal process is usually described as in- 
 cluding all that portion between the superior border and the floor of the 
 anterior nares. The surface is marked by two concave portions and 
 two ridges. The two ridges divide the surface into three parts — the 
 superior meatus, the middle meatus, and the inferior meatus of the nose. 
 
SUPERIOR MAXILLARY BONES 43 
 
 The superior meatus is the smallest of the three, and occupies the slightly 
 concave space above the superior ridge. The middle meatus, partly 
 concave, and partly convex, includes the space between the superior 
 and the inferior ridges, and extends from the free margin of the bone in 
 front to the lacrimal groove behind. The inferior meatus, which is 
 much the largest, occupies all that concave surface between the inferior 
 ridge above and the palatal process below, and extends from the anterior 
 margin of the bone backward to the point of articulation with the palate 
 bone. The two ridges previously referred to are known as the superior 
 turbinated crest, which articulates with the middle turbinated bone, and 
 the inferior turbinated crest, which articulates with the inferior turbinated 
 bone. 
 
 The malar process is a large, irregular portion of bone situated 
 at the angle of separation between the facial and zygomatic surfaces, 
 and presents a triangular, roughened surface for articulation with the 
 malar bone. The superior boundary of this process is formed by the 
 orbital surface and the outer end of the infra-orbital ridge; the inferior 
 boundary may be marked by an irregular imaginary line running from 
 the upper margin of the canine fossa to a point between the first and second 
 molar teeth, while the posterior inferior boundary may be traced from 
 the outer superior angle of the zygomatic surface downward and forward 
 to the point above referred to. This process, as well as the nasal process, 
 is subject to much variety in form and general outline. The malar 
 process, assisting as it does in forming what is commonly called the cheek 
 bone, is particularly variable in size, and in certain types and races it is 
 so prominent as to become a controlling feature in the facial form. One 
 of the muscles of mastication — the masseter — has a portion of its origin 
 from the malar process. 
 
 The palatal process is more directly interested in the formation of 
 the cavity of the mouth than any other portion of the superior maxillary 
 bone. By articulating with its fellow of the opposite side, it forms about 
 three-fourths of the hard palate or roof of the mouth, the remaining fourth 
 being formed by a portion of the palate bones. It is thick and strong, and 
 projects horizontally inward from the inner surface of the body of the 
 bone. It presents two surfaces for examination — a superior or nasal 
 surface, and an inferior or oral surface. The superior or nasal surface 
 is smooth and more or less concave, and forms the floor of the nares. 
 The inferior or oral surface is also concave, but is much roughened by 
 numerous small projections, between which are lodged the mucous 
 glands. Upon the anterior portion of this surface are a number of 
 
44 ANATOMY 
 
 small foramina, which mark the entrance to numerous small canals 
 giving passage to nutrient vessels to supply the body of the bone. Near 
 the center of the posterior third are the anteroposterior grooves, which 
 accommodate the posterior palatine nerves and blood-vessels. This 
 process also presents for examination three borders and various other 
 points of interest. The three borders are the interior, posterior, and 
 mesial. The anterior border is thick and somewhat irregular; the posterior 
 border is thin and frail, and articulates with a portion of the palate 
 bone. The mesial border presents a wide articulating surface in front, 
 behind it is narrow, the whole extent of this border articulating with the 
 corresponding process of the opposite bone. 
 
 The Nasal Spine. — At the anterior superior angle of the palatal 
 process is a well-defined spine — the nasal spine — being formed by a pro- 
 longation of the process beyond the level of the facial surface of the bone. 
 This process, when articulated with its fellow of the opposite side, forms 
 the base of the nose. 
 
 The Nasal Crest. — Beginning at the base of the nasal spine, and ex- 
 tending backward along the median border of the bone, is a sharp, 
 irregular ledge of bone, the nasal crest. This portion of the process 
 articulates with the vomer. 
 
 The incisor crest is a continuation of the nasal crest anteriorly, pro- 
 jecting beyond the nasal spine in the form of a sharp, spear-like point. 
 
 The incisive foramen, or foramen of Stenson, is situated immediately 
 back of the incisor crest, and leads downward and forward from the nasal 
 chamber toward the mouth, entering that cavity just back of the central 
 incisor tooth. This passage in the single bone is a simple groove, but 
 in the articulated skull it becomes the anterior palatine canal, which, 
 after passing downward, opens on the nasal surface of the palatal process 
 by four foramina — the incisive foramina, and the foramina of Scarpa, or 
 the naso-palatine foramina. These foramina transmit the naso-palatine 
 nerves. 
 
 The palatal process of the superior maxilla is subject to a greater 
 variety in form than any other portion of the bone, this variation in the 
 articulated skull being the cause of the many different curves assumed 
 by the roof or dome of the mouth. 
 
 The Alveolar Process. — This process forms the lower margin of 
 the bone, and extends from the base of the tuberosity behind to the 
 median line in front, at which point it articulates with the same process 
 of the opposite bone. It has an outer and an inner margin corresponding 
 to the buccal and palatal surfaces of the roots of the teeth, which are 
 
SUPERIOR MAXILLARY BONES 45 
 
 firmly imbedded in it. Its general form from before backward is that of 
 a gradual curve, somewhat variable in different bones, the extent of this 
 variation depending on the type or race to which the bone belongs. The 
 body of the process is made up of an outer and an inner plate, which are 
 connected by numerous septa of cancellated bone. The outer plate of 
 the process is continuous with the facial and zygomatic surface of the body 
 of the bone, and assists in forming these surfaces. It is quite thin and 
 frail, and the position of the alveoli beneath are well shown by the numer- 
 ous vertical ridges upon it. The inferior margin of the outer plate is 
 reinforced by an additional thickness of bone, forming the border of the 
 alveolar sockets. The inner plate of the alveolar process is much 
 heavier and stronger than the outer plate, and extends from the margins 
 of the alveoli below to the palatal process above. The inferior margin 
 of this plate is not reinforced except in the region of the molars. The 
 construction of the inner plate is, to a great degree, controlled by the 
 shape and position of the palatal process. In the lymphatic tempera- 
 ment this process, when articulated with its fellow, forms a flat or 
 shallow dome to the oral cavity, and in so doing gradually curves into 
 the alveolar process, giving it additional thickness. The depth of the 
 process in this type is not great, and the roots of the teeth are short 
 and heavy in proportion. In the bilious temperament the inner al- 
 veolar plate is deep and abrupt, extending from the inferior margin 
 upward in almost a perpendicular direction to the palatal process which 
 joins it almost at right angles. The alveolar process gives origin to one 
 of the cheek muscles — the buccinator — which is attached to the outer 
 plate near its upper margin, and directly over the space occupied by the 
 second bicuspid and first molar teeth. 
 
 The Alveoli or Tooth Sockets. — These cavities, which are variable 
 in number, are formed by the outer and inner plate of the alveolar process, 
 and by numerous connecting septa of bone placed between the two plates. 
 The shape and depth of each cavity is regulated by the form and length 
 of the roots of the teeth which they support. The first socket, or that 
 next to the median line, gives support to the central incisor tooth. It 
 forms almost a perfect cone, and has an average depth of half an inch. 
 Its lower border is circular, and the anterior or labial portion describes 
 a larger circle than the posterior or palatal half. The mesial and distal 
 walls are somewhat flattened. The second cavity, proceeding backward 
 from the median line, supports the lateral incisor tooth. It is also conic, 
 but somewhat smaller than the preceding. It is seldom over 3/8 to 5/16 
 of an inch in depth. It is much flattened on its mesial and distal walls, 
 
46 ANATOMY 
 
 giving the appearance of an oblong, rather than a round, cavity in trans- 
 verse section. This socket, as well as that for the central incisor, occupies 
 an almost vertical position in the process. Very frequently the socket 
 for the lateral incisor presents a slight distal curve at its upper extremity. 
 The third socket, or that giving support to the cuspid tooth, is much 
 larger and deeper than either of those previously described.. It extends 
 upward, inward, and backward with an average depth of 5/8 to 3/4 of 
 an inch. In transverse section, its labial wall presents a much larger 
 circle than its palatal margin. The labial and distal walls are much 
 flattened and somewhat convex. The general direction of this socket is 
 slightly to the distal. The socket which supports the first bicuspid is 
 usually divided from mesial to distal by a thin septum of bone, thus form- 
 ing an outer or buccal socket, and an inner or palatal socket. This 
 division seldom exists to the full depth of the cavity, but usually begins 
 about midway of its length. The lower margin of this socket is oblong or 
 egg-shaped, its outer or buccal portion forming a larger curve than its 
 palatal. The lateral walls are slightly concave or flattened, until the point 
 of separation is reached, when they become more circular, the alveoli 
 above this point becoming cone-shaped. It is not uncommon for this 
 socket to be a single cavity, and when thus formed it resembles a 
 flattened cone, with the buccal and palatal margins rounded. The 
 next socket gives support to the second bicuspid tooth, in most in- 
 stances being a single cavity, but in rare instances it is divided near 
 its upper extremity. In general outline it resembles the socket for the 
 first bicuspid. 
 
 The socket for the first molar is much larger than any of those pre- 
 viously described; its inferior margin presents a circular outline on its 
 buccal and palatal portions, the former curve being larger than the latter. 
 The mesial and distal walls are flattened and slightly concave. The 
 upper three-fourths of this socket is divided into three separate compart- 
 ments, being so arranged that two are upon the buccal and one upon the 
 palatal side. The septa separating the two buccal cavities from the pala- 
 tal cavity are heavy and strong, while that placed between the two buccal 
 sockets is thin and frail. The two buccal cavities are usually flattened 
 upon their mesial and distal sides. The palatal socket is larger and 
 somewhat deeper than the buccal, the average depth of all being about 
 1/2 of an inch. The socket for the second molar is similar in most respects 
 to that for the first molar, except that it is somewhat smaller. The same 
 description might answer for the third molar socket, which in general 
 is similar to the alveoli for the other molars. It is smaller than the second 
 
SUPERIOR MAXILLARY BONES 47 
 
 molar socket, and may be a single cavity, or it may be divided into three 
 compartments. 
 
 Articulations. — The superior maxillary bone articulates with its 
 fellow of the opposite side, with the frontal, lacrimal, ethmoid, palate, 
 vomer, malar, and inferior turbinated bones. Occasionally it articulates 
 with the sphenoid bone. 
 
 Attachment of Muscles. — The muscles attached to this bone are 
 eleven in number, and are as follows : 
 
 Compressor nares, Internal pterygoid, 
 
 Orbicularis oris, Orbicularis palpebrarum, 
 
 Levator labii superioris alaeque Levator labii superioris proprius, 
 
 nasi, Inferior oblique, 
 
 Levator anguli oris, Buccinator, 
 
 Depressor alse nasi, Masseter. 
 
 Blood-supply. — The maxilla receives its vascular supply from 
 numerous large arteries. They are derived from the alveolar, infra- 
 orbital, nasopalatal, descending palatal, ethmoidal, nasal, frontal, and 
 facial branches. 
 
 Development. — The superior maxilla arises from four points of 
 ossification, which are deposited in membrane. These four centers 
 make their appearance as early as the eighth fetal week, this eariy begin- 
 ning making it somewhat difficult to accurately follow its growth. The 
 four centers are named, as located, premaxillary, maxillary, malar, and 
 prepalatal. The premaxillary nucleus gives rise to the incisive portion 
 of the bone, or that part supporting the incisor teeth. During early life 
 this division of the bone is separated from the body of the bone, and is 
 known as the premaxillary portion (Fig. 17). Union between the pre- 
 maxillary portion and the maxilla proper takes place about birth, and the 
 suture thus formed is visible on the facial surface until the sixth or seventh 
 year, and on the palatine surface until the adult period. The palatal 
 suture extends as far back as the posterior border of the anterior palatal 
 canal. This nucleus also sends a narrow process upward which forms 
 part of the outer boundary of the anterior narial aperture. On the 
 palatal aspect it furnishes a speculum which surrounds the anterior and 
 mesial walls of Stenson's canal. The posterior limit of the premaxillary 
 portion is indicated by the suture on the palatal surface. The maxillary 
 nucleus forms the greater portion of the body of the true maxilla and the 
 nasal process. The malar center gives origin to the malar process, 
 and all that portion external to the infra-orbital groove. The prepalatine 
 
4 8 
 
 ANATOMY 
 
 center gives rise to the nasal surface of the bone and that portion of the 
 palatal process posterior to Stenson's canal. 
 
 Development of the Alveolar Process. — This process is represented 
 at birth by the walls of a deep groove, in which are lodged the partly 
 calcified, deciduous teeth and the germs of most of the permanent teeth 
 (see Development of the Teeth). 
 
 The growth of the process continues with the growth of the teeth 
 until, finally, at about the seventh month after birth, the dental organs 
 
 Palatal Portion 
 
 Fig. 
 
 Malar Portion 
 
 -Left Superior Maxillary, about the Third Year, Enlarged. 
 
 are completely encased within its walls. With the decalcification of the 
 roots of the deciduous teeth comes the loss of the process surrounding 
 them, and, as the permanent teeth advance to take their place in the 
 arch, the process is again built up about their roots. 
 
 The Maxillary Sinus, or Antrum of Highmore* (Fig. 16).— This 
 is a large cavity situated within the body of the maxilla. Its general 
 shape is that of a pyramid, with its base directed toward the median line, 
 or nasal surface, its apex pointing toward and extending into the malar 
 process, and, in some instances, penetrating the malar bone. The size 
 of the cavity varies in different subjects and in the opposite bone of 
 
 * Described separately, in preference to including in general description of the bone. 
 
SUPERIOR MAXILLARY BONES 
 
 49 
 
 the same subject. The average capacity is about three fluidrams, but 
 this may be increased to six or eight fluidrams. The size of the bone and 
 the prominence of the malar process control, in a measure, the size of 
 the cavity; but not infrequently the largest bone will present the smallest 
 sinus. Sex also appears to exert a controlling influence over the capacity 
 of the cavity, it being greater in the male than in the female. In youth 
 the cavity is quite small, the walls being much thicker proportionately 
 than in the adult. The walls of the sinus in the matured subject are 
 quite thin and frail, and are four in number. The superior wall is formed 
 
 Fig. 18. — Developing Maxillary Bones about the Fifth Month after Birth. 
 
 by a thin plate of bone, the floor of the orbit. This surface is almost flat, 
 and serves as a roof to the cavity. Near the anterior margin of this sur- 
 face is a thick rib of bone which marks the course, and forms one of the 
 walls of the infra-orbital canal. 
 
 The inner wall, or that looking toward the nasal surface, is formed 
 by the thin bony layer separating this cavity from that of the nares. The 
 outer or lateral surface, formed by the facial and zygomatic surfaces of 
 the bone, is smooth, and convex from before backward. Near the center 
 of this surface the cavity may penetrate the malar process, and in the dis- 
 articulated skull would present an opening at this point. The inferior 
 wall is formed by the alveolar process, and is marked by a number of 
 irregular eminences corresponding to the roots of the neighboring teeth. 
 The teeth referred to are generally the first and second molars, and occa- 
 
 4 
 
50 ANATOMY 
 
 sionally the second bicuspid. It is not unusual for the roots of one or more 
 of these teeth to penetrate the floor of the sinus, in consequence of which 
 the lining membrane of the cavity may suffer disease generated in the 
 teeth. 
 
 The inferior wall is much the strongest of the four, and, besides the 
 unevenness of the surface produced by the tooth-roots, it frequently 
 supports a number of thin, bony partitions, which may completely or 
 partly divide the floor of the cavity into numerous small compartments. 
 
 The posterior portion of the lateral wall is marked by the posterior 
 dental canals, which give passage to the posterior nerves and blood-vessels. 
 In like manner the anterior portion of the lateral wall is grooved for the 
 reception of the anterior dental nerves and blood-vessels. Upon the inner 
 wall, or that forming the base of the pyramid, is an opening which commu- 
 nicates with the middle meatus of the nose. In the articulated skull this 
 opening is quite small, being from 1/8 to 1/4 of an inch in diameter. 
 The correct idea of this opening cannot be obtained by studying the 
 individual bone, as the numerous perforations then to be observed are 
 closed or partly closed by articulation with adjacent bones. The mucous 
 membrane lining the nasal cavity enters the sinus through the small 
 aperture above referred to, and forms a continuous lining over its entire 
 surface. Cryer has thrown much light upon the relations of the maxillary 
 sinus to the mouth and teeth, and he has demonstrated beyond a doubt 
 that the relationship existing between the parts is susceptible to extensive 
 variation. He has shown that in some instances the cavity upon one 
 side will be large, with its floor broken by the tooth-roots, while that upon 
 the opposite side will be extremely small and far removed from the root 
 apices. In fact, these researches have so revolutionized the subject 
 under consideration that the foregoing description is only reliable in so 
 far as it treats of the conditions most frequently met with. 
 
 THE PALATE BONE. 
 
 The palate banes (Fig. 19), two in number, are situated immediately 
 posterior to the two maxillae, and with them complete the hard palate 
 They also assist in forming the boundaries of the orbital and nasal cavities, 
 the sphenomaxillary, the sphenopalatine, and the pterygoid fossa, the 
 sphenomaxillary fissure, the posterior ethmoidal cells, and the maxillary 
 sinus. When in position in the skull, these bones are wedged between 
 the maxillae and the sphenoid bone. They are rectangular in outline, and 
 each bone presents for examination a horizontal and a vertical plate, a 
 tuberosity, and two processes, the orbital and the sphenoid. 
 
THE PALATE BOXE 
 
 51 
 
 The horizontal plate, smaller than the vertical, assists in forming 
 the hard palate, and corresponds to the palatal process of the maxilla. 
 In entering into the construction of the hard palate the form of this plate 
 varies to the same degree as the palatal plate of the maxilla. In general, 
 it is described as quadrilateral in shape, having two surfaces and four 
 borders. The superior surface, which is concave from side to side, forms 
 the posterior floor of the nasal chamber. The inferior surface completes 
 the hard palate posteriorly, and presents, near its posterior border, a 
 transverse ridge for the attachment of one of the muscles of the soft 
 palate (the tensor palati) ; the anterior border is serrated for articulation 
 
 e VTA<- Pfto c , 
 
 Fig. 19. 
 
 'serosity 
 
 I 
 
 PTERYGOID FOSSA 
 
 -The Two Palate Bones in their Natural Position, 
 Dorsal Mew. (Testul.) 
 
 with the palatal process of the maxilla. The posterior border is free, 
 curved, and sharp, and marks the posterior boundary of the hard palate. 
 At the median line this border terminates in a sharp point, which, when 
 articulated with the corresponding bone of the opposite side, forms the 
 posterior nasal spine; to this point the azygos uvulae muscle is attached. 
 The external border is situated just below the junction of the horizontal 
 and vertical plates. In this portion is a groove which assists in forming 
 a portion of the posterior palatal canal. The internal border is broad 
 and serrated for articulation with its fellow of the opposite side. When 
 the palate bones are in position in the skull, these borders form a ridge, 
 continuing the crest formed by the palatal process of the maxilla, this 
 crest receiving the inferior border of the vomer. 
 
 The vertical plate is thin and frail and extends from the floor of 
 the nasal chamber below to the upper extremity of the sphenopalatine 
 notch above. It has two surfaces and four borders. 
 
52 ANATOMY 
 
 The external surface is roughened for articulation with the maxilla, 
 excepting a small triangular surface near the upper extremity, which 
 forms a portion of the sphenomaxillary fossa, and a small portion near 
 the middle of the surface close to the anterior border, which forms a 
 portion of the wall of the maxillary sinus. Near the posterior boundary 
 of this surface is a vertical groove, which forms, when articulated with the 
 maxilla, the posterior palatal canal, transmitting the descending palatal 
 nerves and vessels. 
 
 The internal surface is divided into three shallow depressions by two 
 transverse ridges — the superior and, inferior turbinated crests. The 
 lower depression thus formed assists in the construction of a portion of 
 the interior meatus of the nose. The crest immediately above this depres- 
 sion articulates with the inferior turbinated bone. The central depres- 
 sion, the largest of the three, forms a portion of the middle meatus of the 
 nose, the crest above articulating with the middle turbinated bone. The 
 superior depression — much smaller but deeper than either of those pre- 
 viously described — forms a large part of the superior meatus. The an- 
 terior border of the vertical plate is thin and sharp, the inferior turbinated 
 crest protruding near the center of the border, and forming the maxillary 
 process. This process assists in closing the maxillary sinus by being 
 received into the maxillary fissure of the maxilla. At the upper extrem- 
 ity of this border is the orbital process, which presents for examination 
 live surfaces, three of which are articular. The anterior or maxillary 
 surface is directed outward, upward, and downward. It is oblong in 
 form and articulates with the posterior superior angle of the inner surface 
 of the maxilla. The posterior or sphenoidal surface is directed back- 
 ward, upward, and inward, and articulates with the vertical plate of the 
 ethmoid bone. The superior or orbital surface is triangular in form, 
 extending upward and outward, forming the posterior angle of the floor 
 of the orbit. The external or zygomatic surface is smooth, oblong, and 
 directed outward, backward, and downward, forming a portion of the 
 sphenomaxillary fossa. 
 
 The posterior border of the vertical plate is irregular and serrated, 
 and comes into relation with the internal pterygoid process, terminating 
 below in a prominent tuberosity. This presents three grooves or flutes. 
 The inner receives the internal pterygoid, the outer the external pterygoid 
 process, while the middle groove completes the pterygoid fossa, and 
 gives attachment to a portion of the internal pterygoid muscle. This 
 process also gives rise to the superior constrictor of the pharynx. 
 Passing through the tuberosity are a number of small canals, those on the 
 
THE PALATE BONE 53 
 
 nasal side being the accessory palatal canals. Near the junction of the 
 tuberosity with the horizontal plate is the opening of the posterior palatal 
 canal, and beyond this the small external palatal canals. 
 
 The sphenoidal process is at the superior end of the posterior border. 
 It is variable in shape and curves upward, backward, and inward. It 
 presents a superior, an external and an internal surface, and two borders 
 — an anterior and a posterior. 
 
 The superior surface, the smallest of the three, is marked by a groove, 
 which assists in forming the sphenopalatine canal. This surface articu- 
 lates with the horizontal portion of the sphenoidal turbinated bone. 
 The external surface assists in forming the sphenomaxillary fossa by its 
 anterior portion, while the posterior portion is rough for articulation 
 with the pterygoid plate of the ethmoid bone. The internal surface is in- 
 strumental in forming a portion of the outer wall of the posterior nafes, 
 and for this purpose is smooth and concave. The anterior border 
 forms the posterior margin of the sphenopalatine notch. The posterior 
 border is serrated, and articulates with the inner surface of the pterygoid 
 process. 
 
 The superior border of the vertical plate is divided by a deep notch or 
 foramen, which divides the orbital from the sphenoidal process. This 
 opening is the sphenopalatine notch or foramen, and transmits the spheno- 
 palatine vessels and nerves from the sphenopalatine fossa to the nasal 
 chamber. 
 
 The inferior border of the vertical plate joints the external border of 
 the horizontal plate. Extending downward and backward from the 
 inferior and posterior borders is the pyramidal process, the borders 
 of which are serrated for articulation with both pterygoid plates of the 
 sphenoid bone. 
 
 Articulations. — The palate bone articulates with the sphenoid, 
 superior maxilla, sphenoidal turbinated, inferior turbinated, ethmoid, 
 and with its fellow of the opposite side. 
 
 Attachment of Muscles. — The following muscles are attached to 
 the palate bone: 
 
 Tensor palati, Internal pterygoid, 
 
 Azygos uvulae, Superior constrictor of pharynx. 
 
 Blood-supply. — The arteries which supply this bone are derived 
 from branches of the descending palatine, the sphenopalatine, and 
 pterygopalatine. 
 
 Development. — The palate bone is developed from a single center 
 
54 
 
 ANATOMY 
 
 deposited in membrane. This center makes its appearance about the 
 eighth or ninth fetal week, near the line of junction between the horizon- 
 tal and vertical plates. At birth these plates are about the same length, 
 but soon after this period, when the nasal sinuses increase in height, 
 the vertical plate begins to lengthen, and continues to do so until it be- 
 comes nearly double the length of the horizontal plate. 
 
 INFERIOR MAXILLARY BONE. 
 
 The Inferior Maxillary, Mandible, or Lower Jaw Bone (Fig. 20). — 
 This bone, having no osseous union with the skull proper, may be con- 
 
 Fig. 20. — The Mandible or Inferior Maxilla. Right Side, External or Facial Surface. 
 a, Sigmoid Notch; b, Coronoid Process; c, Condyle; d, Neck of Condyle; e, Ramus; 
 /, External Oblique Line; g, Angle; h, Mental Foramen; i, Mental Protuberance; j, Body. 
 
 sidered as one of its appendicular elements. It is the heaviest and 
 strongest bone of the head, gives support to the sixteen lower teeth, and 
 serves as a framework for the lower half or floor of the mouth. It is 
 situated at the lower extremity of the face, and immediately below the 
 superior maxillary and malar bones, while its posterior extremity rests 
 
INFERIOR MAXILLARY BONE 55 
 
 against the glenoid fossa of the temporal bone, forming a movable articu- 
 lation with this cavity. In general, the bone is symmetric in outline, 
 and presents for examination a horizontal portion, or body, and two 
 vertical portions, or rami, which in the adult are almost perpendicular to, 
 or at right angles with, the body of the bone. 
 
 The body, or horizontal portion, consists of two identical halves, 
 which meet at the median line and form a slight vertical ridge, the sym- 
 physis. This line indicates the point of union between the two lateral 
 halves, which at birth are usually separated, but soon after this period 
 become firmly united. Each lateral half of the body presents two sur- 
 faces — an external and an internal; and two borders — a superior and an 
 inferior. 
 
 The external or facial surface (Fig. 20) is smooth and convex, 
 and furnishes a number of points for examination. Beginning at the 
 median line, the symphysis ends inferiorly in a prominent triangular 
 surface — the mental protuberance, or chin. 
 
 The Incisive Fossa. — Passing backward from the symphysis, and 
 immediately above the triangular ridge which forms the mental process, 
 is a decided but shallow depression — the incisive fossa. This fossa 
 gives origin to one of the elevator muscles of the chin — the levator menti. 
 Slightly posterior to and below this fossa, on a line corresponding to the 
 position of the cuspid tooth, is an oblong depression for the origin of the 
 depressor muscle of the lower lip — depressor labii inferioris. 
 
 The External Oblique Line. — Extending obliquely across the facial 
 surface from the mental process to the base of the vertical portion of the 
 bone, and continuous with its anterior margin, is a well-defined ridge — 
 the external oblique line. Xear the center of this ridge, or below the 
 position occupied by the bicuspid and first molar teeth, is the point of 
 attachment of the depressor muscle of the angle of the mouth — the de- 
 pressor anguli oris. Somewhat anterior to and above this point is the 
 origin of the depressor muscle of the lower lip — the depressor labii inferi- 
 oris. Between the line of origin of the depressor anguli oris and the in- 
 ferior border of the bone is a roughened surface for the attachment of 
 the platysma myoides muscle. This roughened surface divides the body 
 of the bone into an upper and a lower portion. That portion above is 
 known as the alveolar or mucous portion, while that below is called the 
 basilar or non-mucous portion. The attachment of the platysma myoides 
 muscle at this point marks the lower boundary or floor of the mouth. The 
 superior or alveolar portion of the bone is within the cavity of the mouth, 
 and is covered with mucous membrane and mucoperiosteum; while the 
 
56 ANATOMY 
 
 inferior or basilar portion is outside and below the cavity, and is covered 
 with periosteum similar to other bones. 
 
 The Mental, or Anterior Dental Foramen. — Midway between the 
 superior and inferior border of the body, and usually below the second 
 bicuspid tooth, is a large foramen — the mental or anterior dental fora- 
 men — giving passage to the mental branches of the inferior dental nerve 
 and accompanying blood-vessels. The position of this foramen is not 
 constant, but, as previously stated, it is usually below the second bicuspid, 
 or between this point and the first bicuspid. The buccinator muscle, 
 which forms a large portion of the lateral wall of the mouth, has its origin 
 from the facial surface of the mandible, being attached to the alveolar 
 portion immediately below the molar teeth. 
 
 The Internal Surface of the Body of the Bone (Fig. 21). — The 
 median line is marked by a slight vertical depression, representing the line 
 of union, and corresponding to the symphysis externally. 
 
 The Mylohyoid, or Internal Oblique Ridge. — The internal surface is 
 divided into two portions by a well-defined ridge — the mylohyoid, or 
 internal oblique ridge. It occupies a position closely corresponding to the 
 external oblique ridge on the facial surface. Beginning near the base 
 of the bone at the median line, it passes backward and upward, increasing 
 in prominence until the base of the vertical portion of the bone is reached, 
 into which it gradually disappears. This ridge gives origin to the mylo- 
 hyoid muscle, which forms the central portion of the floor of the mouth. 
 In correspondence to the facial surface of the bone, the attachment of the 
 mylohyoideus muscle forms the dividing line between the mucous mem- 
 brane and mucoperiosteum covering the upper portion of the body of 
 the bone, and the periosteum covering the inferior portion. 
 
 The Genial Tubercles. — Near the lower third, at the median line, is a 
 roughened eminence — the genial tubercles. Taken collectively, these are 
 in two pairs — a superior and an inferior. The superior pair (usually the 
 largest) give origin to the geniohyoglossus muscle, and the lower pair to 
 the geniohyoid muscle. 
 
 The Sublingual Fossa. — By the side of the genial tubercles, and 
 above the mylohyoid ridge, is a shallow, smooth depression — the sub- 
 lingual fossa. One of the salivary glands — the sublingual — is partially 
 supported in this fossa. 
 
 The Digastric Fossa. — Below the mylohyoid- ridge, and near the 
 median line, is a slight depression — the digastric fossa — which affords 
 attachment for the digastric muscle. 
 
 The Submaxillary Fossa. — In the center of the internal surface, ex- 
 
INFERIOR MAXILLARY BONE 
 
 57 
 
 tending from before backward, between the mylohyoid ridge and the 
 lower border of the bone, is an oblong depression — the submaxillary 
 fossa. In this fossa rests another of the salivary glands — the maxillary. 
 The Superior or Alveolar Border. — This border extends from 
 the junction of the body, with the vertical plate on one side, to the corre- 
 sponding point on the other. The construction of this border is similar 
 to the alveolar border of the superior maxilla. At the anterior portion 
 
 Fig. 21. — The Mandible or Inferior Maxilla. Right side, Internal Surface, a, Coronoid 
 Process; b. Sigmoid Notch; c, Cancellated Tissue; d. Condyle; e. Ramus;/, Inferior Dental 
 or Mandibular Foramen; g, Angle; h, Body; i, Internal Oblique Line. 
 
 it is narrow, but gradually increases in width as it proceeds backward — 
 in some instances following the line of the body of the bone; in others, 
 inclining inward, or to the lingual. Each lateral half is marked by eight 
 sockets, for the accommodation of the sixteen lower teeth. They are 
 smaller in proportion than the alveolar sockets in the superior maxilla. 
 The socket nearest the median line receives the central incisor tooth, 
 and is the smallest of the number. It has an average depth of 7/16 of 
 an inch, is conic from above downward, oblong in transverse section, 
 with its lateral walls flattened. The second socket gives support to 
 the lateral incisor; it is a trifle larger than 'the central incisor socket, 
 
58 ANATOMY 
 
 but in other respects is quite similar. The socket for the cuspid 
 is situated at the anterior angle of this border, and is much larger 
 and deeper than the incisor sockets. It has an average depth of 9/16 
 of an inch; its lateral walls are compressed, and sometimes slightly 
 concave. In transverse section the labial wall forms a larger curve than 
 the internal or lingual wall. Passing backward, the next two sockets are 
 for the support of the bicuspids; they are circular in outline, with an aver- 
 age depth of 1/2 of an inch. The cavity for the first bicuspid is usually 
 a little larger than that for the second. In rare instances one or the other 
 of these sockets will be divided for the accommodation of two roots. 
 The sockets for the first and second molars present a circular outline 
 upon their free margins, but below they divide into two flattened, cone- 
 shaped cavities — one anterior and one posterior. The flattened sides of 
 these cavities are concave in the center, and at their lower third curve 
 backward. The average depth of these sockets is 1/2 of an inch. The 
 socket for the third molar, like its superior fellow, is variable both in form 
 and position, frequently being crowded inside or outside of the tooth-line. 
 In some instances it is divided into two or more compartments. The 
 average depth is not over 3/8 of an inch. 
 
 The alveolar process, which composes the superior border of the 
 body of the mandible, differs from the same process in the superior max- 
 illa in one very important particular: instead of the outer plate being 
 thin and frail, it is equally as heavy as the inner or lingual plate. When 
 the tooth-line is inclined inward from the body of the bone, the posterior 
 outer wall is much heavier than the interior or lingual wall. 
 
 The Inferior Border of the Body of the Bone. — This border extends 
 from a slight depression, to be observed at the point of union between 
 the body and ramus, to the corresponding point upon the opposite side. 
 It is strong, rounded, and compact, and gives to the bone the greatest 
 portion of its strength. Near its junction with the ramus is the facial 
 notch, so named from the facial artery passing over this point. 
 
 The Ramus, or Vertical Portion of the Bone. — This vertical 
 plate is quadrilateral in outline, and presents two surfaces — extrenal 
 and internal; four borders — superior, inferior, anterior, and posterior; 
 and two processes — the condyloid and the coronoid. 
 
 The external surface is flat and smooth. Near the center it is slightly 
 concave and roughened for the attachment of one of the muscles of mas- 
 tication — the masseter. 
 
 The internal surface presents near the center an oblong opening — 
 the inferior dental or mandibular foramen — leading into the inferior dental 
 
INFERIOR MAXILLARY BONE 59 
 
 or mandibular canal. Surrounding this foramen, on its posterior inter- 
 nal margin, is the mandibular spine, to which is attached the spheno- 
 mandibular ligament. Running obliquely downward from the base of 
 the foramen, and beneath the spine, is a decided groove — the mylohyoid 
 groove — which accommodates the mylohyoid nerve, artery, and vein, 
 which pass forward to supply the floor of the mouth. Below and behind 
 this groove the surface is roughened for the attachment of another muscle 
 of mastication— the internal pterygoid. 
 
 The Inferior Dental or Mandibular Canal. — Beginning at the foramen 
 of the same name, this canal enters the body of the bone, passes downward 
 and forward horizontally, until it finds an exit at the mental foramen. 
 This canal lies immediately below the alveolar sockets, and from it are 
 given off smaller canals which open into the tooth-sockets through 
 minute foramina. Near the mental foramen the canal divides into a 
 number of smaller ones, which pass forward through the substance of the 
 bone to the sockets of the cuspid and incisor teeth. 
 
 The Superior Border of the Ramus. — This border is crescent-shaped, 
 and is otherwise known as the sigmoid notch. Arising from its anterior 
 portion is a flattened, cone-shaped process — the coronoid process. On 
 its posterior portion is a rounded or oblong eminence — the condyloid 
 process. The concave or crescent-shaped margin of this border is thin 
 and smooth in front, becoming wider and heavier as it approaches the 
 condyle. 
 
 The Coronoid Process. — The anterior margin of this process, being 
 a continuation of the external oblique line, is heavier at the base than at 
 the apex. The outer surface is smooth, and affords attachment to the 
 masseter, and a few fibers of the temporal muscle. The internal surface 
 is marked by a vertical ridge, which passes downward, increasing in size, 
 and finally joining the internal oblique line at a point posterior to the 
 third molar. The surface anterior to this ridge is grooved, and gives 
 attachment to a part of the temporal muscle above, and the buccinator 
 muscle below. The surface posterior to this ridge affords attachment 
 for the greater part of the temporal muscle. The posterior border of 
 this surface is thin, and forms the anterior margin of the sigmoid notch. 
 
 The Condyloid Process. — This may be described as the expanded 
 extremity of the posterior border of the ramus, and is quite variable in 
 form (see Occlusion of the Teeth). It is divided into a superior or 
 articular portion, and an inferior portion, or neck. 
 
 The articular portion of the condyle is more or less oblong, and is 
 convex above, fitting into the glenoid fossa of the temporal bone, and 
 
6o ANATOMY 
 
 forming, with the intcrarticular cartilage which lies between the two sur- 
 faces, the temporomaxillary articulation. 
 
 The neck is that constricted portion immediately below the articular 
 surface. It is flattened in front and presents a pit — the pterygoid fossa — 
 to which a portion of the pterygoid muscle is attached. Immediately 
 below the point of junction between the neck and the articular surface 
 externally is the condyloid tubercle, to which is attached the external 
 lateral ligament. 
 
 The Inferior Border of the Ramus. — This border is thick, rounded, and 
 continuous with the lower border of the body of the bone. At the point 
 of junction between this and the posterior border is the angle of the jaw. 
 The angle has a slight outward inclination, and is roughened for the 
 attachment of a part of the superficial portion of the masseter muscle. 
 
 The anterior border has been described in connection with the coro- 
 noid process. 
 
 The posterior border is smooth and rounded on its upper half, the 
 lower half being roughened for the attachment of the stylomaxillary 
 ligament. 
 
 Attachment of Muscles. — The following muscles are attached 
 to the mandible: 
 
 Buccinator, Superior constrictor of pharynx, 
 
 Depressor labii inferioris, Masseter, 
 
 Depressor anguli inferioris, Orbicularis oris, 
 
 Levator menti, Internal and external pterygoid, 
 
 Geniohyoglossus, Geniohyoid, 
 
 Platysma myoides, Mylohyoid, 
 
 Digastric, Temporal. 
 
 Development.* — On account of its early functional activity, the 
 mandible is among the first bones to ossify. Development takes place 
 from six centers for each lateral half, the nuclei being deposited as early 
 as the sixth or eighth fetal week, and after their establishment the develop- 
 mental process takes place very rapidly. The six centers of ossification 
 are principally named according to their position. The early preparation 
 for the development of the bone is found in the appearance of what is 
 known as the mandibular plates, which are thrown out from the sides 
 of the cranial base, and finally unite at the median line. Not long after 
 this period a cartilaginous band — Meckel's cartilage — is developed in the 
 substance of the mandibular plates, and is it about this cartilaginous 
 
 *See "Development of the Teeth." 
 
INFERIOR MAXILLARY BONE 
 
 6l 
 
 framework that ossification first takes place. The various centers are 
 distributed along the line of Meckel's cartilage, and are named as follows: 
 Mental, dentinary, coronoid, condyloid, angular, and splenic. The 
 mental center provides for the development of that portion of the bone 
 between the median line and the mental foramen. The dentinary center 
 
 Childhood 
 
 Adah 
 
 Senile 
 
 Fig. 22. — Chart Showing the Evolution and Degeneracy of the Mandible. 
 
 forms the lower border and outer plate, and provides for the establish- 
 ment of the crypts inclosing the developing tooth-follicles. 
 
 The coronoid and condyloid centers are both instrumental in con- 
 structing these processes, and the angular center provides for the angle of 
 the bone. The splenic center is somewhat later in making its appearance, 
 and from it the inner plate of the mandible is formed, the line of union 
 
62 ANATOMY 
 
 between it and the dentinary center being indicated by the mylohyoid 
 groove. While, as above stated, most of the centers of development are 
 along or near the line of Meckel's cartilage, the condyloid and coronoid 
 processes are developed from other cartilage. Soon after birth the two 
 lateral halves of the mandible begin to coalesce at the median line, this 
 union taking place from below upward; and by the eighth or tenth month 
 union is complete and the individual bone is established. The inferior 
 maxilla is subject to a continuous change in form, not only in regard to 
 its general contour, but also accommodating itself to the movements and 
 growth of the teeth, the former taking place at or about the angle, while 
 the latter occurs in the alveolar portion of the bone. 
 
 Figure 22 represents the changes which take place in the angle of 
 the mandible from youth to old age. It will be observed that the angle 
 formed in the adult bone, with the teeth in position, is almost a right 
 angle; and that in youth, with the deciduous teeth in the alveoli, the angle 
 is much more obtuse, which condition is again approached in old age. 
 
 THE HYOID BONE. 
 
 This is a U-shaped bone placed in the upper part of the neck at the 
 median line near the base of the tongue. It has no bony connection with 
 other bones; it is classed as a floating bone. It is made up of a body 
 and four processes. 
 
 The body, or central portion, is quadrilateral in outline, somewhat ob- 
 long from side to side, with its anterior aspect convex, and presents a 
 longitudinal ridge which divides it into a superior and an inferior portion. 
 It is also usually divided at the median line by a slight vertical ridge. 
 At the point of junction between the longitudinal and vertical ridges a 
 slight tubercle is formed. The anterior surface is given up to the attach- 
 ment of muscles. The posterior surface of the body of the bone is con- 
 cave and smooth, and is directed backward and downward. The superior 
 border gives attachment to the thyrohyoid membrane, while the inferior 
 border, which is somewhat thicker, gives attachment to the sternohyoid 
 and thyrohyoid muscles. 
 
 The processes known as the greater and lesser cornua are four in num- 
 ber, one of each kind on either side. 
 
 The greater cornua project backward and upward, and their lower 
 borders and anterior surfaces are occupied with muscles. The thyro- 
 hyoid ligament is attached to the posterior terminal corner. 
 
 The lesser cornua are short conical pieces of bone, and project upward 
 
THE HYOID BONE 63 
 
 and backward from the extremities of the body of the bone. They 
 give attachment to the stylohyoid ligaments. 
 
 Development. — Ossification takes place from five centers, one for 
 the body of the bone and one for each cornua. 
 
 Attachment of Muscles. — 
 
 . Geniohyoglossus, Sternohyoid, 
 
 Geniohyoid, Lingualis, 
 
 Thyrohyoid, Omohyoid, 
 
 Mylohyoid, Digastric, 
 
 Hyoglossus, Middle constrictor. 
 
 The thyrohyoid ligament, as well as the stylohyoid, and the thyrohyoid 
 membrane are also attached to this bone. 
 
CHAPTER IV. 
 
 The Temporomandibular Articulation. The Muscles of 
 
 Mastication. 
 
 TEMPOROMANDIBULAR ARTICULATION. 
 
 Although external to the cavity of the mouth, this articulation is so 
 closely associated with the masticatory function that it seems important 
 that a brief description of its construction and action should be given. 
 It receives its name from the two bones which enter into its formation— 
 the temporal and the mandible, or inferior maxillary. 
 
 Condyle 
 
 Ramus of 
 Mandible 
 
 Glenoid 
 •Fossa 
 
 Coronoid 
 Process 
 
 Fig. 23. — Temporomandibular Articulation. 
 
 This joint is the seat of motion in the mandible, and entering into its 
 construction are bones, ligaments, cartilage, and synovial membrane, 
 these being the tissues essential to all diarthrodial or movable articulations. 
 The various movable joints of the body are classified according to the 
 
 64 
 
TEMPOROMANDIBULAR ARTICULATION 65 
 
 nature of the movement, and correspond to the mechanical actions known 
 as hinge joint, ball-and-socket joint, gliding joint, pulley joint, etc. The 
 temporomandibular joint is of the diarthrodial class, and the movements 
 which it possesses are a combination of the gliding movement (arthrodia) 
 and of the hinge movement (ginglymus). The osseous parts entering 
 into the formation of the joint are the anterior portion of the glenoid 
 fossa of the temporal bone and the condyloid process of the mandible 
 
 (Fig- 23). 
 
 The glenoid fossa may be described as an oblong cavity, with its 
 base directed upward, being bounded anteriorly by a heavy bone ridge 
 (the anterior root of the zygoma), posteriorly by an irregular, flattened 
 portion of the bone (the tympanic plate of the petrous portion), internally 
 by a union of the anterior and posterior boundaries, and externally by the 
 middle root of the zygoma. The floor of the fossa is traversed by a well- 
 marked fissure — the glenoid fissure (fissure of Glaserius) — which divides 
 the fossa into two portions, an anterior and a posterior. The anterior 
 half is deeper and more concave than the posterior, and is the articulating 
 portion, being occupied by the condyle, while the posterior half gives 
 lodgment to the parotid gland. 
 
 The condyloid process of the mandible having been described 
 with that bone, in this connection reference will be made to the variety 
 of forms which it presents, and the influence which it exerts over the 
 nature of the tooth occlusion. This process, when narrow and oblong 
 (Fig. 23), closely resembles the ginglymus, or hinge joint, and will be 
 accompanied by teeth presenting deep, penetrating cusps, forming a 
 positive and well-locked occlusion (Fig. 24, B), with little or no lateral 
 motion. If the condyle presents the appearance shown in figure 25, 
 which resembles the enarthrodia, or ball-and-socket joint (although it 
 cannot be considered as such), the teeth associated with such a formation 
 will be provided with short, rounding cusps, and the occlusion will be 
 loose and wandering (Fig. 24, A). This difference in the form of the con- 
 dyle will be accompanied by a corresponding variation in the concavity 
 of the glenoid fossa. Not only does the osseous structure in the joint 
 partake of individual characteristics, but likewise the muscles and liga- 
 ments; their functions being to operate the articulation, they are developed 
 in accordance with the action required of them, which action is, in a 
 measure, dependent upon the conditions existing in the mouth. 
 
 Both the condyle and the glenoid fossa are covered with articular car- 
 tilage. In the latter this membrane extends over its anterior border, to 
 facilitate the play of the joint. The condyle is held in position in the fossa 
 5 
 
66 
 
 ANATOMY 
 
 by three ligaments — the capsular, the sphenomaxillary, and stylomaxillary. 
 The capsular ligament is divided into four portions — anterior and posterior, 
 external and internal. The anterior portion consists of a few fibers 
 
 Fig. 25. 
 
 connected with the anterior margin of the fibrocartilage, attached below 
 to the anterior margin of the condyle and above to the front of the glenoid 
 ridge. The posterior portion is attached above just in front of the glenoid 
 
TEMPOROMANDIBULAR ARTICULATION 
 
 67 
 
 fissure, and is inserted into the posterior margin of the ramus of the 
 maxilla just below the neck of the condyle. The external portion, 
 otherwise known as the external ligament, is the strongest portion of the 
 capsular ligament. It has a broad attachment above to the zygoma, 
 from which point it passes downward and backward, and is inserted into 
 the outer side of the neck of the condyle. The internal portion, or short 
 
 Styloid Process Capsular Ligament 
 
 Internal Lateral Ligament 
 
 Stylohyoid Ligament 
 
 Stylomaxillary Ligament 
 Fig. 26. — Temporomaxillary Articulation — -Internal Veiw. (Deaver.) 
 
 internal lateral ligament, is composed of well-defined fibers, having a 
 broad attachment above to the inner edge of the glenoid fossa and to the 
 alar spine of the sphenoid bone; below it is inserted into the inner side of 
 the neck of the condyle. 
 
 The sphenomaxillary, or long internal lateral ligament, is a thin, 
 loose band, situated some distance from the joint proper, and, as its name 
 implies, has its attachment above to the alar spine of the sphenoid bone, 
 
68 
 
 ANATOMY 
 
 and also to that portion of the temporal bone contiguous to it. It passes 
 downward and forward, and is inserted into the mandibular spine of the 
 maxilla. 
 
 The stylomaxillary ligament extends, from the styloid process of the 
 temporal bone, downward and forward, to be inserted into the poste- 
 
 External Lateral Ligament 
 
 Capsular Ligament 
 
 Styloid Process 
 Stylohyoid Ligament 
 Stylomaxillary Ligament 
 Fig. 27. — Temporomaxillary Articulation — External View. (Deaver.) 
 
 rior border of the ramus of the inferior maxilla, at a point between the 
 masseter and internal pterygoid muscles. 
 
 The inter articular fibrocartilage is an oval sheet placed between the 
 two articulating surfaces. It is thinnest at the center and becomes thicker 
 as the margins of the fossa are approached, at which point it is connected 
 with the fibers of the capsular ligament. Being placed immediately be- 
 
TEMPOROMANDIBULAR ARTICULATION 
 
 6 9 
 
 tween the two articular surfaces it divides the joint into two separate 
 synovial cavities. Each of these synovial cavities is occupied by a syno- 
 vial membrane, that occupying the upper compartment being the largest, 
 and passes from the margins of the glenoid fossa above to the upper 
 surface of the interarticular cartilage below. The membrane which 
 occupies the lower cavity is smaller, and passes from the under surface 
 of the interarticular cartilage above to the margins of the condyle below. 
 The blood-supply to this articulation is derived from the temporal, middle 
 meningeal, and ascending pharyngeal arteries. 
 
 The nerves are derived from the masseteric and auriculotemporal. 
 
 Fig. 28. — Showing Variation in the Shape of the Condyles. 
 
 The movements of this articulation present as great a range as any 
 other joint in the human body. While the chief movement is of the gingly- 
 moid or hinge character, brought into play in simple depression and 
 elevation of the mandible, it also has the power of extension and retrac- 
 tion, may be rotated from side to side, together with all the motions 
 intermediate between these. When the mandible is depressed, the con- 
 dyle moves on the fibrocartilage, and at the same time glides forward and 
 slightly downward until it rests on the anterior border of the glenoid 
 fossa; — this movement does not extend sufficiently to allow the condyle 
 to rest upon the extreme summit of the border, except in cases of excessive 
 movement, as in yawning, when the condyle may glide over the summit 
 
7 o 
 
 ANATOMY 
 
 and the joint become disarticulated. When the mandible is elevated, the 
 condyle slides backward and upward, and at the same time the fibro- 
 cartilage, which has extended with it, also retracts until the condyle is 
 settled in the fossa. The movement of extension and retraction is by a 
 horizontal gliding action, by which the mandible is thrust forward and 
 
 Temporal Muscle 
 
 Buccinator Muscles 
 
 Masseteric Nerve 
 I . . Masseteric Artery 
 Facial Artery 
 Facial Vein 
 
 Superficial Temporal 
 
 Artery 
 Facial Nerve 
 Masseter Muscle 
 
 Platysma Myoides 
 Muscle 
 
 Fig. 29. — Temporal Muscle. (Deaver.) 
 
 drawn back again. In this movement, as well as in the one previously 
 described, both condyles are similarly and simultaneously engaged. The 
 lateral or triturating movement is made in an oblique direction. This 
 consists in a rotation of the condyles within the fossae, the cartilage gliding 
 obliquely forward and outward on one side, and backward and inward 
 on the other, this action taking place alternately. This movement is 
 
TEMPOROMANDIBULAR ARTICULATION 7 1 
 
 more or less developed in accordance with the nature of the occlusion, 
 being favored by those teeth possessing but little cusp formation, with a 
 consequent loose and wandering occlusion; while in that type of tooth 
 associated with a long overbite and deep penetrating cusps, forming a 
 firm and well-locked occlusion, this movement will be but little developed. 
 If this movement be employed to throw the symphysis to one bide and 
 back again, and not from side to side, the condyle of that side rotates in 
 the glenoid fossa, while the condyle of the opposite side is drawn forward 
 and inward. 
 
 The Muscles of Mastication. 
 
 Occupying the back part of the side of the face, and forming an in- 
 dependent group, are four muscles, usually classed as the muscles of 
 mastication. While this is true to a great degree, they are not the only 
 muscles brought into action during this process. They are the masseter, 
 temporal, internal pterygoid, and external pterygoid. The masseter, 
 temporal, and internal pterygoid lift or close the lower jaw, the principal 
 function of the external pterygoid being to extend the lower jaw so that 
 the lower teeth pass beyond the upper. The muscles which open the 
 jaws, such as happens when the head is thrown backward, are the muscles 
 of the neck. 
 
 The masseter muscle is made up of a strong quadrate sheet, con- 
 sisting of two distinct layers extending from the zygomatic arch to the 
 mandible. The layers of which the muscle is composed differ somewhat 
 in size as well as in the directions which they take. 
 
 Origin. — The superficial layer, much the stronger and larger of the 
 two, arises from the lower border of the malar bone, and from the anterior 
 two-thirds of the zygomatic arch. The deep layer arises from the posterior 
 third of the lower border and from nearly all the internal surface of the 
 zygomatic arch. 
 
 Insertion. — After passing downward and backward it is inserted into 
 the outer surface of the ramus of the mandible. The deep layer, passing 
 downward and slightly forward, mingles with some of the fibers of the 
 superficial portion, and is finally inserted into the upper half of the ramus 
 of the mandible. 
 
 Action. — To draw slightly forward, and by its superficial layer to 
 close the jaw. "In closing the jaw it acts with less mechanical disad- 
 vantage than is usual with muscles. When the pressure to be overcome 
 is exerted upon the back teeth, the arm of the lever upon which the power 
 acts is almost as Ions; as that which intervenes between these teeth and the 
 
7 2 
 
 ANATOMY 
 
 fulcrum. This fulcrum is not at the temporomaxillary joint, but at a 
 point below the neck of the mandible, corresponding very nearly to the 
 lower attachment of the internal lateral ligament. Moreover, the result- 
 ant force of the muscle acting, as it does, upward and forward, is perpen- 
 dicular to the lever, which may roughly be described as a bar extending 
 downward and forward from the neck of the mandible to the point of the 
 chin." (Morris.) 
 
 Interarticular flbro-cartilage 
 
 External pterygoid 
 
 Internal pterygoid 
 
 Fig. 30. — The Pterygoid Muscles. (Morris.) 
 
 Relations. — It is covered superficially by the skin and fascia of the 
 platysma myoides, by the risorius and the masseter fascia, by the parotid 
 gland and ducts, facial veins, and portions of the facial nerve. Deeply, the 
 muscle lies in contact with the ramus of the jaw and the buccinator muscle, 
 being separated from the latter by a layer of fat. A small portion of the 
 temporal muscle also comes in relation to the deeper lying portion. 
 
 The temporal muscle is covered by a strong membrane, the temporal 
 fascia, which arises from the temporal ridge, and is inserted into both the 
 inner and outer portions of the upper border of the zygomatic arch. Near 
 this point it divides into two distinct layers, one passing to the inner, the 
 other to the outer margins of the zygoma. Below the fascia is continuous 
 
TEMPOROMANDIBULAR ARTICULATION 73 
 
 with the masseteric fascia. Passing downward from this to the inferior 
 borders of the ramus of the jaw, it envelops the masseter muscle. The 
 muscle itself is radiating and fan-shaped in form, located in the temporal 
 fossa, from which point it descends to the coronoid process of the 
 mandible. 
 
 Origin. — From nearly the entire surface of the temporal fossa, from 
 the temporal ridge, from the entire surface of the temporal fascia, down 
 to its lower attachment to the zygomatic process. 
 
 Insertion. — Into the coronoid process of the mandible. 
 
 Action. — To close the lower jaw, some of its fibers drawing the jaw 
 backward after the other muscles have protruded it. 
 
 Relations. — Its superficial portion is covered by the temporal fascia 
 which separates it from some of the auricular muscles; branches of the 
 facial nerve, the auriculotemporal nerve, and a portion of the epicranial 
 aponeurosis. The temporal fossa and the external pterygoid muscles 
 are in relation with it deeply. 
 
 The internal pterygoid muscle is a thick, quadrilateral, sheet- 
 like muscle, and receives its name from its origin and relative position. 
 
 Origin. — From the inner surface of the external pterygoid plate; the 
 tuberosity of the palate bone and a small portion of the maxilla. 
 
 Insertion.— Into the internal surface of the ramus of the mandible at 
 its lower and posterior borders and extending as high as the mandibular 
 foramen and mylohyoid. 
 
 Action. — To close the jaw and at the same time draw it backward and 
 throw it toward the opposite side. "The same remarks which were 
 made with respect to the very small loss of mechanical advantage in the 
 masseter muscle apply to this muscle. When closed it will draw the 
 jaw forward; and also it will help the external pterygoid in drawing the 
 ramus of its own side toward the middle line." (Morris.) 
 
 Relations. — Superficially, the internal maxillary vessels, the external 
 pterygoid muscle, the internal lateral ligament, the inferior dental and 
 lingual nerves. Deeply, the submaxillary glands; the tensor palati and 
 superior constrictor of the pharynx, as well as the stylohyoid and posterior 
 border of the digastric muscles. 
 
 The external pterygoid muscle is composed of two triangular sheets, 
 one passing in a horizontal and the other in a vertical direction. It re- 
 ceives its name from its attachment to the pterygoid process of the sphenoid 
 bone as well as its relation to its companion muscle, the internal pterygoid. 
 
 Origin. — It is composed of two distinct heads, an upper and a lower. 
 The upper head arises from the greater wing of the sphenoid bone, from 
 
74 ANATOMY 
 
 the internal pterygoid ridge, and external to the foramen ovale and fora- 
 men spinosum. The lower head arises from the outer surface of the ex- 
 ternal pterygoid plate. 
 
 Insertion. — The upper head is inserted into the inter-articular fibro- 
 cartilage, into the capsule of the joint as well as the neck of the condyle. 
 The lower head is inserted into the neck of the condyle. 
 
 Action. — To draw the condyle and inter- articular fibrocartilage for- 
 ward and inward. "The combination of these two movements produces 
 the oblique movement of the lower molar teeth of one side, forward and in- 
 ward with respect to the upper molars which are their opponents. It 
 should be observed also that this inward movement of one side is the 
 agent by which the ramus of the opposite side is moved outward. To 
 assist in opening the mouth by depression of the lower jaw. As the 
 transverse axis of this movement passes through the mandible at two 
 points situated below the necks of the rami, it follows that a forward 
 movement of the. condyles and necks will assist in the backward move- 
 ment of the angles and body which accompanies the depression of the 
 mandible." (Morris.) 
 
 Relations. — Superficially, some of the fibers of the internal pterygoid, 
 the temporal, and part of the masseter muscle. Deeply, the internal 
 pterygoid muscle, the middle meningeal and inferior dental vessels, in- 
 ternal maxillary vessels, and masseteric and posterior deep temporal 
 nerves passing behind or through the attachment to the upper head; the 
 inferior dental and lingual gustatory nerves beneath the lower head. 
 
CHAPTER V. 
 
 A General Description of the Teeth. — The Permanent Teeth 
 Classification, Surfaces, Etc. — The Roots of the Teeth.— 
 The Dental Arch. 
 
 A GENERAL DESCRIPTION OF THE TEETH. 
 
 A Tooth (Fig. 31). — One of the thirty-two specialized organs for the 
 seizure and mastication of food, placed at the entrance to the alimentary 
 canal (the mouth). The typical form of a tooth is a modified cone or 
 combination of cones, and is composed of 
 two fundamental parts — the crown and the 
 root or roots. The crown is that part which 
 projects beyond the gum and is visible in 
 the mouth; while the root is that part which 
 is implanted in the bone and covered by 
 the gum. Intervening between these two 
 extremities, and usually occupying a por- 
 tion of each, is a third division — the neck. 
 
 Completely covering the crown of a 
 tooth is a hard, vitreous-like substance, 
 enamel;* the root is covered by a hard, 
 bone-like substance, cementum;* while the Crown j 
 interior or body of the organ is composed 
 of a hard substance closely resembling bone, 
 the dentin* The neck of a tooth, which 
 serves to unite the crown to the root, and 
 which is usually formed at the expense of 
 each, is covered partly by enamel and partly 
 
 by cementum. This brief description in the singular number can best be 
 continued in the plural. Teeth are classified according to their form, which 
 is always in accordance with their function, into simple and complex. In 
 the simple class the single modified cone is the predominating form, the free 
 extremity of the crown serving as the base of the cone, while the apex is 
 formed bv the free end of the root. Included in this same classification 
 
 Aoe.- 
 
 Root 
 
 Neck 
 
 Fig. 
 
 *See Tissues of the Teeth, Part 1 1 . 
 
 75 
 
76 ANATOMY 
 
 are those teeth which are made up of a double cone, or a simple cone and 
 an inverted cone attached to each other at a common base (Fig. 31). 
 The purposes for which such teeth are adapted are those of grasping, 
 incising, and tearing and they are so arranged that the free extremities 
 of their crowns interlock or overhang the opposing teeth in the opposite 
 jaw. 
 
 In the complex class (Fig. 32) the external form of the tooth is pro- 
 duced by a combination of cones, some of which are simple, others inverted, 
 
 but all uniting at a common base — the neck 
 of the tooth. In this class the simple cones 
 form the roots of the tooth, while the crowns 
 are made up of a number of smaller cones, 
 much modified. Such teeth are adapted to 
 crushing and grinding, and are less inclined 
 to interlock during active service. 
 
 The teeth are divided into two grand 
 divisions — those of infancy and childhood, 
 called deciduous or temporary teeth, and those 
 of the adult period, known as permanent teeth. 
 The latter class being most important, will 
 first receive consideration. 
 
 The Permanent Teeth (Fig. 33).— The 
 permanent teeth, thirty- two in number, are divided into those of the 
 superior portion of the mouth, upper, and those of the inferior portion, 
 lower. In number they are equally divided, each jaw giving support to 
 sixteen. They are firmly imbedded in the alveolar sockets of three of 
 the bones of the mouth, the upper sixteen being attached to the two 
 superior maxillary or upper jaw-bones, and the lower sixteen to the 
 mandible, inferior maxillary, or lower jaw-bone. As above referred to, 
 the attachment of the teeth to the bones is by implantation in sockets, 
 the alveoli (see description, "Bones of the Mouth"). In this attachment 
 there is a special development of bone, closely modeled to the roots of 
 the teeth, and which is subservient to the ever-varying changes which 
 take place during the development of the organs. The joint thus 
 formed between the roots of the teeth and the alveoli is of the immovable 
 or synarthrodial class, and is styled gomphosis. Intervening between the 
 roots of the teeth and the walls of the alveoli is a delicate membrane — 
 the alveolodental membrane. 
 
 Before continuing the description of the teeth, a further classification, 
 which refers alike to the upper and lower organs, must be presented. 
 
A GENERAL DESCRIPTION OF THE TEETH 
 
 77 
 
 This classification is derived from the function and form of the teeth. 
 Figure 33 shows the thirty-two teeth removed from the jaws and placed 
 side by side in two straight lines. In the center is a perpendicular line, 
 which corresponds to the median line or center of the mouth, the teeth 
 at either extremity being those which occupy the back part of the mouth. 
 Without confining the description to either the upper or lower teeth, it will 
 be observed that the first two teeth upon either side of the median line are 
 similarly formed, and all four are called incisors (incidere, to cut); the 
 two larger incisors, being nearest the median line or center, are called 
 
 Lingual Surfaces 
 
 Labial and Buccal Surfaces 
 
 Upper 
 
 Lower 
 
 Lingual Surfaces Labial and Buccal Surfaces 
 
 Fig. 32>- — The Permanent Teeth. 
 
 central incisors; while the two smaller being placed at the side of the cen- 
 trals, are known as lateral incisors {lateralis, the side). The third tooth 
 from the median line upon either side is the cuspid (cuspis, a point), so 
 named from possessing a single cusp or point. Passing to the right or left 
 on the chart, or backward in the mouth, the fourth and fifth teeth from 
 the median line are the bicuspids (bi, two; cuspis, a point), having two 
 points or cusps. The bicuspid nearest the median line is the first bicus- 
 pid; that most distant from the median line is the second bicuspid. The 
 sixth, seventh, and eighth teeth from the median line upon either side 
 are those of another class, the molars (mola, a mill-stone), being named 
 according to their function, that of crushing or grinding the food. Pro- 
 
78 ANATOMY 
 
 cceding from before backward, the molars are denominated first molar, 
 second molar, and third molar. To sum up, the names and number 
 of the permanent teeth may be given by the dental formula, as follows: 
 
 Incisors, i Bicuspids, 2 
 
 Cuspids, I Molars, :! 
 
 The Surfaces of the Teeth. — The crown of each tooth presents 
 five surfaces, which are variously named, in accordance with the duty 
 which they are called upon to perform or suggestive of their location. 
 The outer surface of the incisors and cuspids, or that contiguous to the 
 lips (labia), is called the labial surface; the corresponding surface of the 
 bicuspids and molars, or that contiguous to the cheeks (bucca?), is the 
 buccal surface. That surface of both upper and lower teeth which faces 
 the palate or tongue is characterized as the lingual surface. 
 
 The proximate surfaces of the teeth are named with regard to their 
 relation to the median line, those surfaces nearest to this point being 
 called mesial, those most distant, distal. In addition to these four sur- 
 faces, which represent what might be termed the sides of the teeth, a fifth 
 surface is present, that which occludes with the teeth of the opposite jaw, 
 and is called the occlusal surface. 
 
 In the incisors and cuspids this surface is formed by the converging 
 of the labial and lingual surfaces, forming an edge to the free extremity of 
 the crown, named, from its action in mastication, the incisive or cutting- 
 edge. In the bicuspids and molars the various sides of the crowns re- 
 main nearly parallel to each other throughout their extent, thus provid- 
 ing an occlusal surface nearly equal to, or greater than, any of the others, 
 and one well adapted to the purposes for which it is intended — that of 
 grinding or crushing the food. 
 
 The Roots of the Teeth. — The upper incisors and cuspids are each 
 provided with one root; the upper first bicuspid may have one or two 
 roots, most frequently the latter; while in the second bicuspid a single root 
 is usually present. The upper first and second molars are each supported 
 in the jaw by three roots, and while in the upper third molar three roots 
 are most common, the number is quite variable, ranging from a single 
 cone-shaped root to three, four, five, or even six smaller branches given off 
 from a common base. 
 
 In the lower incisors, cuspids, and bicuspids, a single root is most 
 frequently met with, although the latter class, in rare instances, may be 
 provided with two. The lower first and second molars are each provided 
 with two roots, but in the third molar, like its upper fellow, the number 
 
A GENERAL DESCRIPTION OF THE TEETH 
 
 79 
 
 may be diminished or increased. In the upper molars, two of the three 
 roots are placed above the buccal half of the crown, and are called buccal 
 roots; the remaining root is placed above the lingual half of the crown, 
 and is designated as the lingual root. In the lower molars, one of the 
 two roots is placed below the anterior or mesial half of the crown, and is 
 named the mesial root, and the other below the posterior or distal half, 
 and is known as the distal root. In those teeth with a complicated root 
 formation, it would seem to be a 
 question whether they are pos- 
 sessed of a single root, with two 
 or more branches, or separate 
 and distinct roots throughout. 
 To determine this, some account 
 must be taken of the point at 
 which the bifurcation or trifurca- 
 tion takes place. If this separa- 
 tion be in close proximity to the 
 crown, the tooth should be con- 
 sidered as having more than one 
 root (Fig. 34, A); but, on the 
 other hand, if the point of sepa- 
 ration be some distance from the 
 crown, with a solid mass of root substance intervening, the tooth may 
 be said to possess a single root, with two or more branches (Fig. 34, B). 
 In the latter instance, that part of the tooth between the point of sepa- 
 ration and the crown is called the root or root base; while the prolonga- 
 tions beyond the point of separation are known as the branches of 
 the root. 
 
 The roots of the teeth are not only variable in number, but are also 
 subject to much variety in form. In the anterior teeth (the incisors and 
 cuspids) the roots are inclined to the form of the simple cone, which form, 
 however, is frequently more or less broken by a slight curvature near their 
 extremities, or by a slight compression of their lateral walls. In the poste- 
 rior teeth (the bicuspids and molars) the roots, root bases, or root branches 
 are all inclined to the conical form, but do not approach so nearly the 
 perfect cone as those of the anterior teeth. These roots are also more or 
 less crooked and flattened laterally. The free extremity of the roots 
 of the teeth, forming as they do the apex of these cone-like prolongations 
 of the crowns, are known as the apices or apical extremities. 
 
 The extent of the enamel covering to the crowns of the teeth is marked 
 
 Fig. 34. 
 
8o 
 
 ANATOMY 
 
 by a well-defined line, which completely encircles the neck of the tooth, 
 the cervical line. 
 
 The Dental Arch (Fig. 35). — The teeth are arranged in the jaws 
 in the form of two parabolic curves, the superior arch describing the 
 segment of a larger circle than the inferior, as a result of which the upper 
 teeth slightly overhang the lower. Figure 35 represents the sixteen 
 upper teeth in position in the bone, presenting their occlusal surfaces, a 
 part of their lingual surfaces also being visible. Viewed in this direction, 
 the gradual change in the crowns of the teeth from the simple incisors to 
 
 Incisors 
 
 
 Cusnid . — 
 
 — ■ — 
 
 
 
 
 
 HMfv 
 
 v4 
 
 m 
 
 A, 
 
 
 
 1 st Bicuspid 
 
 f j* : 
 
 >w. 
 
 - 
 
 ^ 
 
 
 2d Bicuspid 
 
 r iYw*- 
 
 
 
 y 
 
 
 1st Molai 
 
 
 , 
 
 
 
 - 
 
 2d Molar 
 
 L * 
 
 
 
 
 V 1 
 
 3d Molar 
 
 wT ^k B 1 '- 
 
 
 
 
 
 Fig. 35.— The Dental Arch. 
 
 the complex molars may be observed. An examination of the central 
 incisors will show how perfectly they are adapted to the process of cutting 
 or incising the food, the cutting-edge being sharp and the lingual surface 
 comparatively smooth and unbroken. In the lateral incisors the cutting 
 feature predominates, but the lingual surface is broken near the neck of 
 the tooth by a slight depression, surmounted by a more or less pronounced 
 fold of enamel, in many instances resembling a small cusp. The crown 
 of the cuspid furnishes the intermediate form between the simple and 
 the complex. This tooth, instead of being provided with a straight 
 cutting-edge, is surmounted at the center of its occlusal surface with a 
 well-defined point or cusp, descending from the summit of which are two 
 
A GENERAL DESCRIPTION OF THE TEETH 8 1 
 
 cutting-edges, one passing to the mesial and one to the distal. The 
 lingual surface of this tooth presents a marked contrast to the correspond- 
 ing surface of the incisors, being broad and full, and frequently provided 
 with a prominent ridge of enamel in the region of the neck, showing a 
 rapid approach to the complex form. In the bicuspids the buccal half 
 of the crown is quite similar to the crown of the cuspid, but in the lingual 
 half a complete revolution has taken place. The enamel fold — but slightly 
 apparent in the incisors, and somewhat increased in the cuspids — has 
 now become a fully developed cusp, resulting in the production of an 
 occlusal surface adapted to crushing or grinding, instead of incising or 
 tearing. In the molars, the increase in the size of the tooth-crown is ac- 
 companied with an occlusal surface much more complex than any of 
 the teeth previously described, and one well adapted to its function — 
 that of crushing and grinding the food. 
 
 Arrangements of the Teeth in the Dental Arch (Fig. 35). — Beginning 
 with the upper teeth, the central incisors are found occupying the center 
 of the arch, and are, therefore, slightly in advance of the laterals. These 
 teeth are so implanted in the alveoli that their crowns are not perpendicu- 
 lar, the cutting-edge being slightly more prominent than the neck of 
 the tooth. The roots are also somewhat inclined from the median line, 
 and as a result the crowns have slight mesial inclination, the mesial sur- 
 faces approximating each other at or near the cutting-edge, with a slight 
 space intervening at the necks. In certain typal forms — the bilious, for 
 example — when the front of the arch is flat, the labial surfaces of these 
 teeth form nearly .a direct line from side to side; while in those types in which 
 the arch is well rounded anteriorly, notably in the sanguine temperament, 
 the labial surfaces of these two teeth form a small segment of the arch, 
 so that the mesial extremity of the cutting-edge of each tooth-crown is 
 somewhat in advance of the distal. The lateral incisors are similarly 
 implanted in the alveoli, causing their cutting-edges to project. The 
 roots of the lateral incisors usually have a stronger distal inclination than 
 those of the centrals, and the crowns show a more marked mesial inclina- 
 tion. The mesial surfaces approximate the distal surfaces of the central in- 
 cisors at or near the cutting-edge. When the front of the arch is flattened, 
 these teeth are but little less prominent than the central incisors, but when, 
 the arch is well rounded they continue the segment begun by the centrals, 
 and are necessarily less prominent. While the occlusal surfaces of the 
 teeth are usually considered as forming a perfect plane (see Occlusion of 
 the Teeth), the lateral incisors are generally a trifle shorter than the 
 centrals. The cupids may be considered as occupying the corners or 
 
82 ANATOMY 
 
 turning-points of the arch. They are more prominently placed than the 
 adjoining teeth, this feature being increased by the bulging or general 
 convexity of their labial surfaces. The extremity of the occlusal surface 
 of the cuspids — i. e., the point of the cusp — is a trifle below the cutting- 
 edge of the laterals and about on a line with that of the centrals. While 
 the apical extremities of the roots of the cuspids are directed away from 
 the median line, the crowns assume almost a perpendicular, this con- 
 dition resulting from a bend in the tooth at the neck. Although the 
 perpendicular position is most commonly assumed by the crown, it is not 
 unusual to find either a mesial or distal inclination present. Reference 
 has been made to the cuspid teeth occupying a position which might be 
 termed the turning-points or corners of the arch, and in most instances 
 it may be thus considered; but in certain typal forms — the sanguine, for 
 example — the tooth-line is unbroken and passes over the cutting-edges 
 of the incisors, the summit of the cusps of the cuspids, and is continued 
 backward over the buccal cusps of the posterior teeth. The biscuspids 
 are placed nearly perpendicular in the arch, but occasionally deviate 
 from this by a slight mesial or buccal inclination. The length usually 
 corresponds to that of the central incisors, and their buccal surfaces are 
 slightly less prominent than the corresponding surfaces of the cuspids. 
 The increase in the buccolingual diameter of the crowns of the bicuspids 
 over that of the incisors and cuspids results in breaking the lingual line 
 of the occlusal surfaces. In the bilious and kindred types, the tooth- 
 line is carried directly backward from the cuspid to the first molar, mak- 
 ing the buccal face of the bicuspids equally prominent, but when the 
 arch is well rounded the second bicuspid is slightly more prominent than 
 the first. The first and second molars usually assume a perpendicular 
 position, but are occasionally inclined to the distal and buccal. The 
 relative prominence of the buccal as well as the lingual surfaces of these 
 teeth is also controlled by the form of the arch. The occlusal surfaces 
 are about on a level with those of the bicuspids and central incisors, 
 but generally the lack of development in the distal half of the crown 
 of the second molar results in the production of a slight upward curve 
 to the tooth-line level at this point (see Occlusion of the Teeth). On 
 account of the limited accommodations afforded it, the position of the 
 upper third molar is quite variable. It may be either to the buccal 
 or to the lingual of the tooth-line, and is usually strongly inclined to 
 the distal. In those cases in which there is a decided dip to the arch 
 (see Occlusion of the Teeth), this tooth is relatively shorter than those an- 
 
/ 
 
 A GENERAL DESCRIPTION OF THE TEETH 
 
 83 
 
 terior to it, but when the tooth-line level is a perfect plane, the length of 
 this tooth corresponds to the other molars and bicuspids. 
 
 Fig. 36. — The Tooth-line in the Lower Jaw. 
 
 The lower incisors are placed more nearly in a perpendicular position 
 than the upper, and a reverse condition exists, in the lateral incisors 
 
 Fig. 37. — The Tooth-line in the Lower Jaw. 
 
 being a trifle larger than the centrals. The lower cuspids are probably 
 more constant in their position than any class of teeth in the mouth, in 
 
8 4 
 
 ANATOMY 
 
 nearly all instances assuming a direct perpendicular. Like the upper cus- 
 pids, they may be said to establish the corners or turning-points of the 
 lower arch, and are somewhat more prominent in the tooth-line than 
 
 Fig. 38. — Sanguine. 
 
 neighboring teeth. All of the six anterior lower teeth may be slightly 
 inclined to the mesial. The lower biscupids and molars, instead of 
 having the buccal inclination possessed by the corresponding upper teeth, 
 
 Fig. 39.— Bilious. 
 
 are inclined to the lingual. The first molar seldom deviates either to the 
 mesial or the distal, the second molar is generally inclined to the mesial, 
 while the third molar is strongly inclined to the mesial. In the lower 
 
A GENERAL DESCRIPTION OF THE TEETH 
 
 85 
 
 arch the curve formed by the incisors and cuspids is the segment of a 
 smaller circle than the corresponding curve in the upper arch. This 
 curve may be continued over the buccal cusps of the bicuspids and molars, 
 
 Fig. 40. — Nervous. 
 
 or it may be broken at the cuspid and continued backward in a direct 
 line (Fig. 36). The teeth in the lower arch are placed directly over the 
 body of the bone as far back as the second bicuspids, while the molars 
 
 Fig. 41. — Lymphatic. 
 
 frequently overhang the body of the bone by an extension of the alveoli 
 inward (Fig. 37). 
 
 The curve described by the dental arch is quite variable, and this 
 
86 
 
 ANATOMY 
 
 variation is generally referred to in connection with the temperament. 
 Thus, in the sanguine temperament (Fig. 38), the arch is well rounded 
 anteriorly, the circle being continued backward to the region of the molars, 
 where the line is broken by slightly inclining to the lingual. In this 
 arch the distance in a straight line from the center of the second molar, 
 on one side, to the center of the corresponding tooth on the other, is about 
 equal to the distance from either of these points to the median line be- 
 tween the central incisors, forming a right-angle triangle. In the bilious 
 temperament (Fig. 39) the arch presents a broad front from cuspid to 
 
 Fig. 42. — Section of the Superior Maxilla Showing Tnterproximate Spaces. 
 
 cuspid, with but little curve; at these points it turns abruptly backward, 
 being continued almost in a direct line to its extremity. In this arch 
 the side of the triangle (represented by the line from molar to molar) 
 is much reduced in length. In the nervous temperament (Fig. 40) 
 the arch is Gothic in form, the segment formed by the anterior teeth 
 being that of a much smaller circle than either of the types previously 
 referred to. The distance from molar to molar is much less than the 
 distance from molar to median line. In the lymphatic temperament 
 (Fig. 41) the arch is well rounded and broad, the segment being that 
 of a much larger circle than any of the above, the side of the triangle 
 formed by the line from molar to molar being of the greatest length. 
 Interproximate Spaces. — In the mesiodistal direction the crowns 
 of the teeth, as a class, are broader at their occlusal surfaces or cutting- 
 edges than at their necks (Fig. 42). This bell-shaped form of the tooth- 
 crowns cause their proximate surfaces to touch at a point representing 
 
A GENERAL DESCRIPTION OF THE TEETH 87 
 
 their greatest mesiodistal diameter, which is usually near the cutting-edge 
 or occlusal surface. Between this point of contact and the cervical line 
 there exists a V-shaped space, called the interproximate space. These 
 spaces are largest in that class of teeth found in the nervous and bilious 
 types, where the necks of the teeth are much constricted, and the bell- 
 shaped crown strongly outlined. In teeth of this class the point of contact 
 is slight, and the interproximate spaces are only partially occupied by 
 the gum tissue, leaving a free passage between the point of contact and 
 the gingival margins. In the sanguine and lymphatic temperaments the 
 proximate surfaces of the teeth are nearer parallel with one another, thus 
 making the point of contact cover a greater extent of surface, and reducing 
 the size of the interproximate spaces. 
 
CHAPTER VI. 
 
 Occlusion of the Teeth. 
 
 As stated elsewhere, the teeth are arranged in the mouth in the form 
 of two parabolic curves, one of which occupies the upper half and the 
 other the lower half of the cavity. To properly perform their function it 
 is necessary for the upper and lower teeth to come into contact, which 
 
 Fig. 43. — The Teeth in Occlusion. 
 
 they are enabled to do by the movement of the lower jaw, and it is the 
 relation existing between the two when thus brought together that con- 
 stitutes the occlusion of the teeth. During mastication the teeth do not 
 only occlude, and remain stationary at a given point until the lower jaw 
 is again depressed, but, through the combined movements of the man- 
 dible, the lower teeth are made to move from side to side, thus grinding 
 or crushing any substance placed between the occlusal surfaces of the 
 bicuspids and molars. This gliding antagonism of the teeth is commonly 
 termed the articulation, and it is important that a distinction be made be- 
 
OCCLUSION OF THE TEETH 89 
 
 tween the terms "occlusion" and "articulation," the former referring to 
 the relations existing between the upper and lower teeth when brought 
 together normally and held firmly in that position, while the latter relates 
 to the various gliding movements of the lower teeth after being brought 
 into occlusion with the upper. In the majority of instances the segment 
 described by the upper arch is somewhat larger than that formed by the 
 lower, and the upper teeth project over and are partly outside of those in 
 the lower arch. Figure 43 presents a labial and buccal view of the teeth in 
 position in the alveoli, and also in occlusion. It will be observed that the 
 upper teeth are not directly antagonistic to those of the same name in the 
 lower arch. There are two reasons for the presence of this condition: 
 First, the mesiodistal diameter of the upper central incisors is much 
 greater than that of the corresponding lower teeth; second, the larger 
 circle present in the upper arch. This arrangement provides that 
 each tooth, instead of being antagonized by a single tooth of the opposite 
 jaw, is met in occlusion by a portion of two teeth. The upper central 
 incisor is met in occlusion by the entire cutting-edge of the lower central 
 incisor and the mesial third of the cutting-edge of the lower lateral in- 
 cisor. The upper lateral incisor is met in occlusion by the distal two-thirds 
 of the cutting-edge of the lower lateral incisor and by the mesial cut- 
 ting-edge of the lower cuspid. The upper cuspid is met in occlusion by 
 the distal cutting-edge of the lower cuspid and by the mesial two-thirds 
 of the buccal cusp of the lower first bicuspid. The upper first biscuspid 
 is met in occlusion by the remaining or distal third of the lower first bicus- 
 pid and by the mesial two-thirds of the buccal cusp of the lower second 
 bicuspid. The upper second bicuspid is met in occlusion by the re- 
 maining or distal third of the buccal cusp of the lower second bicuspid, 
 and by the mesial incline of the mesiobuccal cusp of the lower first 
 molar. The upper first molar is met in occlusion by the distal incline 
 of the mesiobuccal cusp of the lower first molar, by the entire distal 
 cusp of the same tooth, and by the mesial incline of the mesiobuccal cusp 
 of the lower second molar. The upper second molar is met in occlusion 
 by the distal incline of the mesiobuccal cusp of the lower second molar, 
 by the entire distobuccal cusp of the same tooth, and by the mesial incline 
 of the mesiobuccal cusp of the lower third molar. The upper third 
 molar is met in occlusion by the distal incline of the mesiobuccal cusp of 
 the lower third molar and by the entire distobuccal cusp of the same tooth, 
 thus being the only tooth in the upper arch with but a single opponent. 
 Likewise each lower tooth is met in occlusion by two in the superior arch, 
 with the single exception of the central incisor, which occludes with the 
 
go 
 
 ANATOMY 
 
 upper central alone. There are many variations from this so-called 
 typical occlusion, as above described, and any slight difference one way 
 or another should not be considered abnormal. In certain types the 
 segmental form of the upper arch is but little greater than that of the 
 lower, and the cutting-edges of the upper incissors occlude almost directly 
 upon the cutting-edges of the lower incisors. As a result, all of the upper 
 teeth are forced to the distal, and the relationship between the upper and 
 lower organs is much altered. When the upper teeth overhang the lower, 
 the lingual cusps of the upper bicuspids and molars penetrate the fossae 
 
 Fig. 44. 
 
 -The Mandible at the Adult Period, showing the Equilateral Triangle 
 described by the Dental Arch. 
 
 or sulci of the corresponding lower teeth, when in occlusion, and the 
 buccal cusps of the lower bicuspids and molars rest in the fossae of their 
 upper opponents. 
 
 To assist in the study of the occlusion of the teeth, some reference 
 must be made to the tooth-line lever, or plane of occlusion. For this 
 purpose the lines forming the facial angle are of value. These lines are 
 as follows: A fixed line representing the base of the angle may be drawn 
 from the center of the glenoid fossa, passing forward through the anterior 
 nasal spine or base of the nose (a, Fig. 45), the angle being completed by a 
 perpendicular line resting upon the labial surface of the upper incisors, 
 passing upward and touching the most prominent part of the forehead (b, 
 Fig. 45). The tooth-line level is approximately horizontal to this basal 
 line, but instead of a perfect plane we usually find the upper arch dipping 
 downward, while the lower arch will be provided with a corresponding 
 
OCCLUSION OF THE TEETH 
 
 91 
 
 depression. This dip to the arch which is known as the "compensating 
 curve," or the "curve of Spee," is greatest in the region of the bicuspisd, 
 and the extent to which it may exist varies with the type of tooth and the 
 consequent nature of the occlusion. 
 
 Fig. 45. — Lines showing Facial Angle, Caucasian or White Race. 
 
 Thus far no reference has been made to what is commonly termed 
 the overbite, and the cusp forms in the teeth. As these two factors exert 
 
 Fig. 46. — Lines showing Facial Angle, Negro or Mixed Races. 
 
 a dominating influence over the character of the occlusion, the effects which 
 they produce will be briefly described. The overbite is so named, from the 
 
92 ANATOMY 
 
 fact that the upper teeth project beyond, or overhang and partly cover the 
 labial and buccal surfaces of the lower teeth. This may be a pronounced 
 feature in the occlusion, or it may exist to a very slight degree. Al- 
 though the overbite is usually referred to as existing in the incisive 
 region alone, it is not confined to these teeth, but is also present in the 
 bicuspids and molars by the buccal cusps of the upper teeth overhanging 
 
 Fig. 47. — Lymphatic. 
 
 those of the lower. The extent of the overbite is gradually diminished 
 from before backward, the central incisors presenting the greatest amount 
 of overhanging surface, which condition is slightly decreased in the 
 laterals, and a corresponding reduction is continued until the third 
 molars are reached, at which point the overbite is scarcely observed. 
 Where the overbite is extensi/e, as shown in figure 43, the upper incisors 
 
 Fig. 48. — Sanguine. 
 
 overhanging and hiding from view about one-third of the labial surfaces 
 of the lower incisors, the cusps of the bicuspids and molars will be corre- 
 spondingly long and penetrating, the buccal cusps of the upper teeth ex- 
 tending well down over the buccal cusps of the corresponding lower 
 teeth. In an occlusion of this class, which is usually found in the nerv- 
 ous and bilious types, the dip to the arch will become a prominent 
 
OCCLUSION OF THE TEETH. 
 
 93 
 
 feature, the occlusion will be firm and well locked, and the lateral articular 
 movements will be slight during mastication. In the lymphatic and san- 
 guine temperaments the occlusion is loose and wandering, greater freedom 
 of movement being permitted by the short overbite and the corresponding 
 lack of cusp-formation. Figure 47 represents such an occlusion; the 
 
 Fig. 49. — Bilious. 
 
 superior arch is but little greater in its segmental outline than the inferior. 
 The cutting-edges of the upper incisors are somewhat more prominent 
 than those of the lower, but the former do not overlap the labial surfaces 
 of the latter. In an occlusion of this character the dip to the arch is not 
 so pronounced, and the articular movements are much more extensive. 
 
CHAPTER VII. 
 
 The Blood- and Nerve-Supply to the Teeth. 
 
 THE BLOOD-SUPPLY TO THE TEETH. 
 
 Briefly stated, the course of the blood from the heart to the teeth is 
 as follows: From the heart to the aorta, to the common carotid artery, 
 to the external carotid artery, to the internal maxillary artery, from the 
 various branches of which the teeth are supplied. 
 
 The Internal Maxillary Artery (Fig. 51). — This artery, otherwise 
 known as the deep facial, is the larger of the two terminal branches of 
 the external carotid. In addition to supplying the teeth, it is distributed 
 to the roof and floor of the mouth, to the maxillary sinus, and to other 
 parts of the face and head. It has its origin from the external carotid 
 artery opposite the condyle of the mandible within the substance of the 
 parotid gland, passes forward between the condyle of the jaw and the 
 sphenomaxillary ligament, from which point it passes obliquely upward 
 and forward between the external and internal pterygoid muscles until 
 it reaches the sphenomaxillary fossa, where its terminal branches are 
 given off. It is divided into three portions — the first or maxillary, 
 the second or pterygoid, and the third or sphenomaxillary. The teeth 
 are supplied from branches of the first and third divisions, the upper 
 teeth receiving their blood-supply from the alveolar or superior maxillary 
 and the infra-orbital branches of the third division, while the lower 
 teeth are supplied by the inferior dental or mandibular branch of the first 
 division. 
 
 The alveolar or superior maxillary branch arises, in common 
 with the infra-orbital branch, from the internal maxillary as it passes into 
 the sphenomaxillary fossa. It passes downward, in a tortuous manner, 
 in a groove provided for it in the back of the maxilla. In its downward 
 course it gives' off the following branches: The antral, to supply the an- 
 trum; the dental (known as the posterior dental arteries), which pass into 
 the substance of the bone through the posterior dental canals to supply 
 the molar and bicuspid teeth; the alveolar or gingival, to supply the gums; 
 and the buccal, to the lateral walls of the mouth. The anterior upper 
 teeth are supplied through the infra-orbital branch of the internal maxillary. 
 
 94 
 
Fig. 50. — Dissection Showing Blood-supply to the Teeth. 
 
 9S 
 
9 6 
 
 ANATOMY 
 
 This branch arises from the internal maxillary artery, generally, in 
 common with the posterior dental. It passes forward in company with 
 the maxillary division of the fifth nerve — first along the groove and 
 then in the canal on the orbital plate of the maxilla, and finally makes 
 its exit upon the face through the infra-orbital foramen. Besides giving 
 off branches to the orbital and nasal cavities, it supplies the incisor and 
 cuspid teeth through its anterior dental branch, which passes downward 
 through a groove in the anterior wall of the maxilla. 
 
 Infraorbital artery and nerve 
 
 Spheno-pqlaline branch 
 
 Posterior or descending palatine branch 
 Naso-palatine branch 
 
 Vidian branch 
 
 A nterior deep temporal artery 
 
 External pterygoid branch 
 
 Orbital branch 
 
 Nasal branch 
 
 A nterior 
 
 dental branch 
 
 Labial branch 
 
 Posterior dental 
 
 branch 
 
 Posterior deep/ temporal artery 
 
 Small meningeal 
 artery 
 
 Middle meningeal 
 artery 
 
 Temporal artery 
 Tympanic branch 
 
 Deep auricular 
 
 branch 
 A URICULO- TEM- 
 PORAL NERVE 
 
 — Masseteric branch 
 
 Incisive branch 
 Menial branch 
 
 Submental branch 
 
 External carotid 
 artery 
 
 Internal lateral or 
 Bpheno-mandibu- 
 lar ligament 
 
 Mandibular or 
 inferior dental 
 artery and nerve 
 
 Fig. 51. 
 
 Buccal branch with 
 portion of buccal nerve 
 
 Mylo-hyoidean branch 
 -Scheme of Internal Maxillary Artery. (Morris.) 
 
 Internal pterygoid branch 
 
 The Inferior Dental Artery (Fig. 51). — The lower teeth receive 
 their blood-supply through the inferior dental or mandibular artery. 
 This artery arises from the under part of the internal maxillary as it 
 passes downward and forward between the sphenomaxillary ligament 
 and the neck of the jaw and enters the inferior dental canal through the 
 inferior dental foramen. It passes forward in the canal accompanied 
 by the inferior dental nerve, and in so doing sends off twigs to supply 
 
Fig. 52. — Dissection Showing Nerve-supply to the Teeth. 
 97 
 
98 ANATOMY 
 
 the molar and bicuspid teeth. When the mental foramen is reached, 
 it divides into two branches, the incisive branch and the mental branch. 
 
 The incisive branch continues it course within the cancellated struc- 
 ture of the bone, sending off minute branches which supply the anterior 
 teeth, the terminal branches anastomosing with the artery of the opposite 
 side. 
 
 The mental branch passes out through the mental foramen accom- 
 panied by the mental branches of the inferior dental nerve, and supplies 
 the tissues of the chin and lower lip. 
 
 The Veins. — The blood, in returning from the teeth to the heart, 
 is first taken up by the posterior dental and inferior dental veins, which 
 in their course follow closely that of their corresponding arteries. These 
 veins, in conjunction with others which accompany branches of the inter- 
 nal maxillary artery, form the pterygoid plexus. At the posterior con- 
 fluence of this plexus the returning blood empties into the internal maxil- 
 lary vein. Accompanied by the internal maxillary artery it passes back- 
 ward and outward, enters the parotid gland, and finally empties into the 
 temporomaxillary vein midway between the zygoma and the angle of the 
 jaw. After leaving the substance of the parotid gland, the temporo- 
 maxillary vein passes downward until near the angle of the jaw, where 
 it divides into two branches, one of which passes downward and slightly 
 forward, uniting with the facial to form the common facial vein, and 
 the other, after passing downward and backward, empties into the 
 external jugular vein. The external jugular vein returns the principal 
 portion of the blood from the teeth, and from its point of beginning it 
 passes almost perpendicularly downward and empties into the sub- 
 clavian vein, which, by joining with the internal jugular vein, forms the 
 innominate vein, which, in turn, empties into the superior vena cava, 
 thus communicating with the heart. 
 
 THE NERVE-SUPPLY TO THE TEETH. 
 
 The nerves supplying the teeth are derived from branches of the 
 fifth cranial nerve, otherwise known as the trifacial or trigeminal nerve. 
 The fifth nerve is the largest of the cranial nerves, and consists of two parts, 
 a large root (sensory) and a small root (motor). The larger portion passes 
 into a ganglion (the Gasserian ganglion), frequently compared to the 
 ganglion on the posterior root of the spinal nerve. It arises, or makes its 
 appearance, at the surface of the brain, on the anterior part of the side 
 of the pons Varolii. The sensory, root, which, through its branches, 
 
THE BLOOD-SUPPLY TO THE TEETH 
 
 99 
 
 supplies the teeth, is composed of from 80 to 100 filaments, each inclosed 
 in a neurilemma, the entire bundle being bound together in a single nerve. 
 
 The fifth nerve is divided into three divisions: First, or ophthalmic; 
 second, or superior maxillary; and third, or inferior maxillary (mandibu- 
 lar) . The branches which supply the teeth are included in the second and 
 third divisions, the upper teeth being supplied by branches from the 
 superior maxillary nerve, and the lower teeth by branches from the in- 
 ferior maxillary nerve. 
 
 The Second Division, or Superior Maxillary Nerve (Fig. 53). — 
 This nerve, composed entirely of sensory fibers, is intermediate in size 
 
 ANTERIOR DENTAL MAXILLARY NERVE ORBITAL BRANCH 
 
 MA XILLAR Y NER VE 
 
 MECKEL'S GANGLION 
 
 POSTERIOR DENTAL 
 
 LOOP FORMED BY MIDDLE AND ANTERIOR DENTAL NERVES 
 Fig. $$. — The Maxillary Nerve, seen from Without. {Morris, after Beawris.) 
 
 between the inferior maxillary and the ophthalmic divisions. It passes 
 forward from the Gasserian ganglion and leaves the cranium through the 
 foramen rotundum. It traverses the upper part of the sphenomaxillary 
 fossa, and passes into the orbit through the sphenomaxillary fissure; then 
 passes forward along the infra-orbital groove, and enters the infra- 
 orbital canal, where it receives the name of the infra-orbital nerve. Pass- 
 ing through this canal, it emerges upon the face through the infra-orbital 
 foramen. The superior maxillary nerve, beside supplying the teeth, 
 sends off branches to the dura mater, to the orbit, and terminal branches 
 in three groups — labial, nasal, and palpebral. The branches given off 
 
TOO 
 
 ANATOMY 
 
 to the teeth are the posterior superior dental, the middle superior dental, 
 and the anterior superior dental. 
 
 The posterior superior dental arises from the second division of the 
 fifth nerve, by one or two roots, just before it passes into the infra-orbital 
 canal. It is divided into a superior and an inferior set; the former passes 
 forward and terminates in the canine fossa, while the latter, usually the 
 larger, enters the posterior dental canals, and, following the line of the 
 
 Fig. 54. — Dissection showing Mandibular (Third) Division of Fifth Nerve, a, Temporal 
 Bone; b, Mental Branches emerging through Mental Foramen; c, Lingual Nerve; d, Man- 
 dibular or Inferior Dental Nerve. 
 
 alveolar process through minute canals in the bone, sends off twigs to the 
 molar teeth, ending in a plexiform manner by communicating with the 
 middle superior dental nerve. This nerve is also distributed to the gums 
 and adjacent buccal mucous membrane. 
 
 Middle Superior Dental Nerve. — The infra-orbital nerve, soon after 
 entering its canal, gives off this branch, which passes outward, downward, 
 and forward over the outer wall of the maxillary sinus, and, after forming 
 
THE NERVE-SUPPLY TO THE TEETH IOI 
 
 plexuses with the posterior dental branches, gives off filaments to supply 
 the bicuspid teeth. 
 
 The anterior superior dental nerve, which is the largest of the dental 
 set, is given off from the infra-orbital nerve, enters a canal close to the 
 infra-orbital foramen, passes over the anterior wall of the maxillary 
 sinus, and, after communicating with the middle and posterior dental 
 nerves, divides into ascending and descending branches, the latter being 
 distributed to the incisor and cuspid teeth. 
 
 The Third Division, or Inferior Maxillary Nerve (Fig. 54). — 
 This is the largest of the three divisions of the fifth nerve, and is both 
 motor and sensory in its function. Besides being distributed to the 
 lower teeth, it sends filaments to the lower portion of the face, the muscles 
 of mastication, the tongue, and mandible. It arises from the Gasserian 
 ganglion, passes downward, and emerges from the skull through the fora- 
 men ovale, after which it divides into a small anterior (motor) branch 
 and a large posterior (sensory) branch. 
 
 The Inferior Dental Nerve. — This is the largest branch of the inferior 
 maxillary nerve. From its point of origin it passes downward internally to 
 the external pterygoid muscle, and, upon reaching a point between the 
 ramus of the mandible and the sphenomandibular ligament, it enters 
 the inferior dental canal through the posterior or inferior dental foramen. 
 Before entering the foramen, two branches are given off, a lingual and a 
 mylohyoid branch. The nerve is accompanied through the inferior 
 dental canal by the inferior dental artery, and, when the mental foramen 
 is reached, it terminates by dividing into an incisive and a mental branch. 
 Between the posterior dental foramen and the mental foramen the nerve 
 gives off a series of twigs to the bicuspid and molar teeth, and these, by 
 communicating with one another within the substance of the bone, form 
 a fine plexus. 
 
 The incisive branch follows the incisive arteries through the sub- 
 stance of that part of the bone between the mental foramen and the 
 symphysis, and supplies the incisor and bicuspid teeth, while the mental 
 branch passes forward to supply the chin and lower lip. 
 
CHAPTER VIII. 
 
 A Description of the Upper Teeth in Detail. — Calcification, Erup- 
 tion, and Average Measurements. — Their Surfaces, Ridges, 
 Fossae, Grooves, Sulci, Etc. 
 
 UPPER CENTRAL INCISOR. 
 
 ist 3d 6th 7th 8th 9th 10th nth 
 
 year year year year year year year year 
 
 Fig. 55. 
 Calcification Begins, from Three Centers, First Year after Birth. 
 Calcification Completed, Tenth to Eleventh Year. 
 Erupted, Seventh to Eighth Year. 
 Average Length of Crown, .39. 
 
 Average Length of Root, .49. 
 
 Average Length over All, .88. 
 
 During the first year after birth this tooth begins to calcify, this 
 process taking place along the future cutting-edge of the tooth in 
 three distinct lobes or plates, which afterward unite and form three 
 eminences or tubercles, the lines of this union being indicated upon the 
 completed crown by two more or less defined grooves — developmental 
 grooves. By the end of the third year the deposit of lime-salts has carried 
 the process of calcification to a point about midway between the cutting- 
 edge and the cervical line. By a continuation of this formative action 
 the calcification of the crown is completed between the fifth and sixth 
 year. At the beginning of the seventh year calcification has progressed 
 to such an extent that the neck of the tooth and base of the root are fully 
 outlined. Between the seventh and eighth year the cutting-edge of the 
 tooth begins to make its appearance through the gum at a point either to 
 the right or left of the median line, and, by a gradual absorption of the 
 
UPPER CENTRAL INCISOR 
 
 IO3 
 
 gum tissue, eruption takes place. During the following year about one- 
 eighth of an inch has been added to the length of the root. At the end of 
 the eighth year the root has become calcified to about one-half of its 
 completed length. During the ninth year, owing to a reduction in the 
 diameter of the root, the extent of growth has almost doubled that of 
 the. previous year, and a decided narrowing of the free root margins is 
 to be observed. At the eleventh year calcification is completed in the 
 outer root walls (Fig. 55). 
 
 The Crown of the Upper Central Incisor presents for examination 
 four surfaces — libial, lingual, mesial, and. distal; two angles — a mesial 
 and a distal; and a cutting-edge. The 
 general form of the crown is that of a 
 double inclined plane, or wedge-shape, 
 the cutting-edge representing the junc- 
 tion of the two sides of the incline, one 
 of which looks anteriorly (labial) and 
 the other posteriorly (lingual). The 
 labial side of the incline is convex, 
 while the lingual is concave from the 
 cutting-edge toward the root: but, 
 upon reaching its upper or cervical 
 third, it presents a slight general con- 
 vexity. The base of the wedge is 
 directed upward and partakes of the 
 contour of the neck of the tooth. 
 
 The Labial Surface of the Crown 
 (Fig. 56). — In general outline his surface resembles an imperfect quad- 
 rilateral. The margins of the surface are the mesial, the distal, the 
 cervical, and the incisive. The mesial margin begins at the lower border 
 or cutting-edge and passes upward, usually with a slight distal incli- 
 nation, gradually uniting with the cervical margin. The distal margin 
 begins at the cutting-edge and passes upward with a slight mesial 
 inclination, also joining the cervical margin. Both of these margins 
 possess more or less general convexity, and, at their junction with the 
 cutting-edge, form the mesial and distal angles of the crown. The 
 cervical margin is rounded and gradually passes into the two lateral 
 margins just described. The incisive margin is marked by the cutting- 
 edge, and extends from the mesial angle on one side to the distal 
 angle on the other. These four margins, which assist in giving to the 
 tooth its typal form, are quite variable. This difference particularly 
 
 Mesia 
 Angle 
 
 Distal. 
 Angle 
 
 Cutting Labial 
 Edge Grooves 
 
 Fig. 56. — Upper Central Incisor, 
 Labial Surface. 
 
104 
 
 ANATOMY 
 
 Cervical 
 
 Ridge 
 
 Distal 
 
 Marginal 
 
 Ridge 
 
 Distal 
 Angle 
 
 marked on the mesial and distal margins, where, in some cases, there 
 is a decided convergence in the direction of the root, forming what is 
 commonly termed the bell-shaped crown, while in others the same 
 margins will be nearly parallel with each other, making the width of the 
 crown almost as great at the cervical margin as at the cutting-edge. The 
 mesial angle is usually pointed and square, while the distal is much 
 rounded. This surface of the crown is slightly convex from above down- 
 ward, as well as from side to side, and in the majority of instances is of 
 greater vertical than transverse extent. Beginning at the incisive margin 
 
 are two slight longitudinal depressions 
 or grooves — the labial grooves — which 
 are resultant from the developmental 
 lobes previously composing the primitive 
 cutting-edge, and, for this reason, are 
 otherwise known as developmental 
 grooves. In many instances one or 
 more transverse ridges are found upon 
 the cervical portion, but these are 
 supplemental in character. 
 
 The Lingual Surface of the 
 Crown (Fig. 57). — This surface has its 
 borders formed by three marginal 
 ridges and the cutting-edge. The 
 marginal ridges are pronounced eleva- 
 tions of enamel, and surround the 
 surface upon three sides, the intervening space in many instances 
 being a decided concavity or fossa — the lingual fossa. The mesial 
 marginal ridge begins at the mesial angle of the crown, passes upward, 
 inward and backward, following the curvature of the mesial border. The 
 distal marginal ridge begins at the distal angle of the crown in a somewhat 
 less pronounced form, passes upward, backward, and inward, following 
 the curvature of the distal surface. Upon reaching the cervical portion 
 of the crown these two margins unite and form the cervicomar ginal ridge. 
 This ridge may be bold and prominent, or it may be but slightly developed. 
 Near its center it is frequently broken by a depression or pit — the lingual 
 pit. In some instances this pit is deeply penetrating; in others it assumes 
 the form of a fissure, and may completely sever the ridge. This border 
 is sometimes elevated into a sightly developed tubercle or cusp — the 
 cuspule. When this is present it has the appearance of being produced 
 by a fold of enamel, and encircling it is a well-marked fissure. The 
 
 ■ > 1 
 
 Mesial 
 Marginal 
 Ridge 
 
 Mesial 
 Angle 
 
 Lingual Fossa 
 
 Fig. 57. — Upper Central Incisor, 
 
 Lingual Surface. 
 
UPPER CENTRAL INCISOR 
 
 I05 
 
 lingual fossa is usually traversed by two longitudinal grooves, which 
 correspond to the developmental grooves of the labial surface. When 
 a capsule is present, the fissure which surrounds it frequently passes 
 into the fossa, or the fossa may be partly covered by the cuspule over- 
 hanging its cervical portion. When the cervicolingual fissure exists, it is 
 not unusual for it to bifurcate and throw a branch along the inner border 
 of each marginal ridge, or it may penetrate the fossa proper and divide 
 it into two parts. The lingual surface is somewhat less in extent than the 
 labial surface, this reduction being principally in a mediodistal direction, 
 the length of the two surfaces from the 
 cutting-edge to the cervical margin 
 being about equal. 
 
 The Mesial Surface of the Crown 
 (Fig. 58). — The outline of this surface 
 resembles an inverted cone or triangle, 
 the lines of which are more or less 
 broken, the apex of the cone terminat- 
 ing at the cutting-edge and the base 
 directed toward the root of the tooth. 
 The base of the cone is made concave 
 by the enamel margin or cervical line. 
 The margins of the mesial surface are 
 the labial, the lingual, and the cervical. 
 The labial margin is convex and 
 rounded throughout its entire extent, 
 
 from the cutting-edge to the cervical line. The contour of this margin 
 varies with the typal form of the crown, in some presenting a decided 
 and well-marked convexity, in others being but slightly curved. The 
 lingual margin is concave and rounded, but the line is much broken. 
 Beginning at the cutting-edge, it is decided and square, this feature 
 usually including the lower third. As it passes upward and the center 
 is approached, the line is more concave and rounded in a mesiolingual 
 direction, this latter feature increasing upon approaching the cervical 
 line. The cervical margin is that formed by the cervical line. It is 
 usually well defined, being concave or V-shaped, with the point of the V 
 more or less rounded, and with its free ends pointing one in a labial and 
 one in a lingual direction, the former being a trifle longer than the latter. 
 The surface between the borders presents a slight general convexity, 
 but with an inclination to flatness near the cervical portion, which is 
 occasionally developed into a slight concavity. Whatever deviations 
 
 Fig. 58. — Upper Central Incisor, 
 Mesial Surface. 
 
io6 
 
 ANATOMY 
 
 Lingual 
 
 
 Fossa 
 
 ■1 wk 
 
 Lingual 1 
 
 Wk jb 
 
 Ridge 
 
 
 Cervical Line 
 
 Cervical 
 
 Ridge 
 
 Fig. 59. — Upper Central Incisor, 
 Distal Surface. 
 
 may be present in the borders of this surface, from those assumed in the 
 
 description just given, their union at the cutting-edge will aways be in 
 
 a direct line with the long axis of 
 the tooth. 
 
 The Distal Surface of the 
 Crown (Fig. 59). — In a general way, 
 this surface resembles the mesial 
 surface just described. There are, 
 however, one or two minor points 
 of distinction: the borders are all 
 more rounded, the labial border 
 presenting a greater convexity, and 
 the lingual a more perfectly formed 
 Distal Angle concavity. The surface is quite 
 full in the center, from which it 
 slopes away in all directions, thus 
 producing a decided general con- 
 vexity. The cervical margin of the 
 
 surface is almost identical with the cervical margin of the mesial surface. 
 
 The distance in a direct line from the cervical border to the cutting-edge 
 
 is a trifle less than the corresponding measure- 
 ment on the mesial surface. The distal angle 
 
 is equally constant in its position, and, being 
 
 connected with the mesial angle in a direct 
 
 line by the cutting-edge, finds this latter margin 
 
 always in the labiolingual center of the crown. 
 The Cutting-edge of the Central Incisor. — 
 
 The cutting or incisive edge receives its name 
 
 from its function, that of cutting or incising 
 
 the food. It is formed by the junction of the 
 
 labial and lingual surfaces of the crown, and 
 
 extends almost in a direct line from the mesial 
 
 to the distal surface; at its union with the 
 
 mesial surface it assists in forming the mesial 
 
 angle of the crown, and serves the same pur- 
 pose by its union with the distal surface. In 
 
 the majority of instances it is an unbroken 
 
 line. In passing from the mesial to the distal 
 
 angle it converges slightly in the direction of the root, thus making the 
 
 crown a trifle shorter on the mesial than on the distal side. In the 
 
 Fig. 60. — A Young Upper 
 Central Incisor, Labial 
 Surface, showing Develop- 
 mental Grooves — a 
 
UPPER CENTRAL INCISOR 107 
 
 recently erupted tooth (Fig. 60) the line is broken by the developmental 
 grooves; these usually disappear by wear, but occasionally traces of their 
 existence remain, and thus permanently break the positive line that 
 would otherwise be present. As the cutting-edge approaches the distal 
 angle of the crown it is inclined to slope away, producing a less positive 
 angle than the corresponding mesial angle. In some instances the 
 cutting-edge is quite thin and inclined to sharpness, in others it is 
 blunt and dull, the former condition being present when there is a 
 decided overbite in occlusion, the latter occurring when this feature is 
 less pronounced. The cutting-edge is frequently referred to as the 
 occlusal surface, this term being employed to make the description more 
 uniform with the bicuspids and molars, and for this reason is permissi- 
 ble; but it is only in rare instances that the surfaces occlude directly with 
 the opposing teeth, the condition being most frequent in teeth of the 
 lymphatic type, and in cases of malocclusion. 
 
 The Cervical Margin. — This margin, which is distinctly outlined by 
 the free extremity of the enamel covering of the crown, also marks 
 the extent of the membrane covering the root. The margins formed by 
 this line are those of a double concavity and a double convexity. On 
 the labial and lingual portions it is concave rootward, while on the 
 mesial and distal sides it is convex in this direction. If a line be drawn 
 around the tooth at the extreme upper point of the enamel covering, 
 it will be found to touch only the labial and lingual prolongations, while 
 a space will exist between the line thus drawn and the cervical margins 
 of the proximate surfaces. The character of the cervical curvature 
 varies with the type of the tooth, being more or less pronounced as the 
 case may be. In a typical central incisor the cervical line of the labial 
 surface will usually form the segment of a larger circle than that of the 
 lingual, and, while the mesial convexity may be gracefully curved, the 
 distal may incline to angularity. 
 
 The Neck of the Tooth. — The neck of this tooth partakes of a form 
 between that of the crown and root which it joins. It is principally 
 formed by a sudden sloping of the enamel margin to meet the root. It 
 is broader on the labial than on the lingual surface, and is somewhat 
 flattened laterally, with an occasional depression or concavity on its 
 mesial portion. The neck of this tooth is seldom a decided anatomic 
 feature, being less pronounced than upon any other tooth. In the bicus- 
 pids and molars both the crowns and the roots assist in forming the neck 
 by a constriction of their adjacent parts, while in this tooth the crown 
 alone is instrumental in this direction. 
 
108 ANATOMY 
 
 The Root of the Upper Central Incisor. — The root of this tooth 
 is conic in form, its base directed downward, its apex upward. Viewed 
 in transverse sections its outline is that of a rounded triangle, one side 
 of which faces in a labial, one in a mesiolingual, and one in a distolingual 
 direction. The labial side is the most flattened, while the two remaining 
 sides are of equal length and oval in form. This triangular outline 
 usually continues throughout the entire length of the root, but in some 
 instances, near the apical end, may have a decided or slight distal curve, 
 included in which will be a more circular form. The taper of the root 
 from the base to the apex is very gradual upon the labial and lingual 
 surfaces, until the apical third is reached, when the two sides converge 
 more rapidly. The mesial and distal surfaces are somewhat flattened 
 and taper very gradually from the base to the apex. In a majority of 
 instances the root is much longer than the crown, but in rare cases its 
 length is barely equal to, or less than, that of the crown. 
 
 Bilious Type. — The crown of the upper central incisor in this 
 type is of greater longitudinal than transverse extent; large in size, abound- 
 ing in angles rather than curves. It possesses neither brilliancy nor 
 transparency of surface, but is slightly inclined to translucent . The 
 labial surface is flat, with more or less decided transverse ridges in the 
 cervical portion. The labial grooves are generally present in the form 
 of well-defined depressions. On account of the angular nature of the 
 tooth, this surface approaches closely to the quadrilateral form. The 
 mesial and distal surfaces are flat, with their margins bold and well 
 defined. The lingual surface also shows the angular nature of the crown 
 in having its marginal ridges squarely set and its developmental grooves 
 definitely outlined. The cutting-edge is rather thin, square, and sharp, 
 the line frequently being imperfectly formed. The mesial and distal 
 angles are both well produced, and the cervical margin, in keeping with 
 the rest of the parts, is inclined to angularity. 
 
 . Nervous Type. — The central incisor common to this temperament 
 is delicate and graceful in outline. The crown is of medium size, with 
 the length predominating over breadth. The enamel is inclined to trans- 
 parency, and is of a blue or bluish-gray color, presenting much brilliancy. 
 The labial surface is fairly well rounded, and the labial grooves are 
 present as slightly rounded depressions, which frequently extend well 
 toward the cervical margin, where they gradually disappear. In general 
 outline this surface partakes of the triangular form, the crown of the 
 tooth being broad at the cutting-edge and much constricted at the neck. 
 The mesial and distal surfaces show a convexity in every direction, 
 
UPPER CENTRAL INCISOR IO9 
 
 and the nature of the occlusion is manifest from the decided wedge- 
 shape appearance of the crown providing for a long overbite. Upon 
 the lingual surface but little in the way of detail is to be observed, the 
 entire surface from the cutting-edge to the cervical ridge being smooth 
 and concave. The cuspule previously referred to is occasionally present 
 in this type, breaking the general smoothness of the surface with its 
 prominence. The marginal ridges are poorly defined; the cutting-edge 
 is a sharp, unbroken line; the mesial and distal angles are present in 
 the form of long, graceful curves, rather than definite angles, this being 
 particularly true of the distal angle. The cervical line is decidedly 
 curved, the labial and lingual portions being deeply concave rootward, 
 while the mesial and distal are decidedly convex. 
 
 Sanguineous Type. — The crown of the central incisor is usually 
 above the average in size, but is well proportioned, abounding in curves 
 and rounded outlines. The enamel is inclined to translucency, particu- 
 larly near the cutting-edge. The labial surface is smooth and rounded; 
 the depressions formed by the labial grooves are slightly observable, 
 and extend but a short distance from the cutting-edge. The surface 
 is somewhat greater in longitudinal than in transverse extent, and ap- 
 proaches much nearer to a circular form than the corresponding tooth 
 of other types. The mesial and distal surfaces are well rounded, making 
 the point of contact with approximating teeth near the center of the 
 surface. The lingual surface abounds in heavy rounded lines; the 
 marginal and cervical ridges are particularly prominent, diminishing 
 the extent of the lingual fossa. A cuspule is frequently present in the 
 form of a well-rounded prominence. The cutting-edge is of moderate 
 thickness and slopes away from the center in either direction to assist 
 in forming the rounded mesial and distal angles. The cervical curvature 
 on the labial and lingual surfaces is an unbroken semicircle, while that 
 of the mesial and distal surfaces is less uniform. 
 
 Lymphatic Type. — In the central incisor of this typal form the 
 crown is large, but not shapely, and the breadth is equal to, or exceeds, 
 the length. The enamel coloring is muddy or brownish-yellow, and the 
 surface is lacking in brilliancy. The labial surface is flat and smooth, 
 with a faint sign of the labial grooves. The general outline of this 
 surface is that of a circular cone, with the cutting-edge for the base, and 
 the apex formed on the cervical margin. The mesial and distal aspects 
 present a striking contrast to the types previously described, by having 
 a labiolingual diameter greater than that represented between the cervical 
 line and the cutting-edge. These two surfaces are convex in a labio- 
 
I IO ANATOMY 
 
 lingual direction only, making the point of contact with approximating 
 teeth an extended surface rather than a single point. 
 
 The lingual surface is heavy and bulky, frequently to such a degree 
 as to produce a general convexity rather than a concavity, as found in 
 most typal forms. This surface is frequently broken by one or more 
 longitudinal grooves, but is seldom crossed by transverse lines of any 
 kind. The cutting-edge is barely deserving of the name. Although 
 formed by the free borders of the labial and lingual surfaces, these two 
 planes are so far separated at their incisive margins that the space between 
 them, instead of being an edge, becomes a more or less broadened surface, 
 and one upon which the lower incisors frequently occlude. The line 
 thus formed is straight and direct from the mesial to the distal angle 
 of the crown, both of which are well produced. The cervical curvature 
 is represented by the segment of a much larger circle than that found 
 upon teeth of other types, and the neck of the tooth is heavy and bulky, 
 showing but little constriction at this point. 
 
 UPPER LATERAL INCISOR. 
 
 4th 
 
 5th 
 
 7 th 
 
 9th 
 
 ioth 
 
 nth 
 
 year 
 
 year 
 
 year 
 
 year 
 Fig. 6i. 
 
 year 
 
 year 
 
 Calcification Begins, from Three Centers, First Year after Birth. 
 Calcification Completed, Tenth to Eleventh Year. 
 Erupted, Seventh to Eighth Year. 
 Average Length of Crown, .34. 
 
 Average Length of Root, .51. 
 
 Average Length over All, .85. 
 
 Like the central incisor, calcification in this tooth begins during 
 the first year after birth, the process taking place in the same manner, 
 from three centers, along the future cutting-edge, and gradually extend- 
 ing in the direction of the root. By the expiration of the third year the 
 cutting-edge and the angles of the crown are fully formed; the fourth 
 year finds the crown calcified to nearly one-half its completed length; 
 
UPPER LATERAL INCISOR 
 
 III 
 
 by the fifth year the cervical ridge is reached; while the sixth year usually 
 completes the process of calcification in the crown. At the close of the 
 seventh year the base of the root is fully outlined, during the following year 
 about one-eighth of an inch is added to its length, and still greater progress 
 is made during the ninth year, by which time fully three-fourths of the 
 root length has become calcified. During the tenth year the apical end 
 of the root begins to form by a sudden doubling-over of the free cal- 
 cifying margins, and by the eleventh year the surface of the root is com- 
 plete (Fig. 61). By the above description it will be observed that at the 
 time of eruption the root of this tooth is only calcified to about one-half 
 of its completed length, and the same may be said of the central incisor; 
 but so much time elapses between the beginning of the eruptive stage 
 and the period at which this phenomenon is completed that the calcifica- 
 tion of the root is usually finished by the 
 time the tooth assumes its permanent 
 position in the jaw. 
 
 The crown of the upper lateral 
 incisor, like that of the upper central 
 incisor, presents for examination four 
 surfaces — labial, lingual, mesial, and 
 distal — a cervical margin, a cutting-edge, 
 and a mesial and distal angle. The 
 general contour of the crown closely re- 
 sembles that of the upper central incisor, 
 except that it measures about one-third 
 less from mesial to distal, and is a trifle 
 shorter from the cutting-edge to the 
 cervical line. As in the central incisor, 
 the labial and distal surfaces form a 
 double incline plane, and unite below to 
 form the cutting-edge. The labial side of the incline is convex, while 
 the lingual is concave, but seldom so marked as that of the central incisor. 
 The base of the wedge, or double incline, formed by the cervical margin, 
 is correspondingly smaller than that of the crown of the central incisor. 
 
 The Labial Surface of the Crown (Fig. 62). — This surface of the 
 crown of the upper lateral incisor is more irregular in outline than the 
 corresponding surface of the central incisor. The margins of the surface 
 are the mesial distal, cervical, and incisive. The mesial margin begins 
 at the mesial angle and passes upward with a decided distal inclination 
 to meet the cervical margin. The distal margin is shorter and decidedly 
 
 Labial Grooves 
 
 Fig. 62. — Upper Lateral Incisor, 
 Labial Surface. 
 
112 
 
 ANATOMY 
 
 Cervical 
 Ridge 
 
 Distal Mar- 
 
 more convex than the mesial margin, this variation in outline being still 
 more marked when compared with the corresponding margin of the 
 central incisor. At the cutting-edge these two margins assist in forming 
 the mesial and distal angles of the crown, and by their continuation and 
 union above form the cervical margin. The incisive margin is formed 
 by the cutting-edge. Like the central incisor, the four margins of this 
 surface vary greatly in the different types; this is particularly true of 
 the two lateral margins, which at times are found to be in the form of a 
 
 direct line, or even slightly concave, 
 while in others they are both decid- 
 edly convex. This surface of the 
 crown shows a greater general con- 
 vexity than the labial surface of the 
 central incisor, the cervical portion 
 presenting a curve much more de- 
 cided than that near the cutting- 
 edge. The labial grooves are in all 
 respects similar to those described in 
 ge connection with the central incisor, 
 and extend from the cutting-edge to- 
 ward the center of the surface, where 
 they gradually disappear. Trans- 
 verse ridges are occasionally found 
 near the cervical portion of the 
 surface. 
 The Lingual Surface of the Crown (Fig. 63). — This surface of 
 the upper lateral incisor is subject to much variation in form, but presents 
 the same points for examination as the corresponding surface of the 
 central incisor. These consist of the marginal ridges, which are usually 
 more pronounced than those of the central, making the concavity or 
 fossa between them small and deep. In some instances the surface will 
 be smooth and flat, with an entire absence of ridges or fossae. The 
 distal marginal ridge is shorter and more bowed than the mesial, and the 
 cervical ridge is well marked and proportionately broader and stronger 
 than in the central incisor. In some instances the marginal ridges are 
 but slightly developed, with their cervical ends broadened and separated 
 by a deep fissure, giving the appearance of a terminal fold in the enamel. 
 The cervical ridge is frequently broken by a cuspule, which is usually 
 more pronounced than when found upon the central incisor. The 
 lingual fossa may be present as a smooth, unbroken concavity, or it may 
 
 Langupl Grooves 
 
 Fig. 63. — Upper Lateral Incisor, 
 Lingual Surface. 
 
UPPER LATERAL INCISOR 
 
 113 
 
 Fig. 64. — Upper 
 Lateral Incisor, 
 Mesial Surface. 
 
 be subdivided by a longitudinal ridge, which often exists to such an 
 extent as to force the remaining portions of the fossa well against the 
 marginal ridges, where they will be observed as slight depressions rather 
 than marked concavities. 
 
 The Mesial Surface of the Crown (Fig. 64). 
 — Viewing the crown from this aspect, the outline 
 is that of an inverted cone or triangle. The lingual 
 margin of the surface is well defined, and the angle 
 formed by the union of this surface with the lingual 
 surface is moderately acute. The labial margin is 
 well rounded, and passes into the labial surface 
 without a decided line of demarcation. The sur- 
 face on its upper or cervical third is usually flat- 
 tened and occasionally concave. At the center, and 
 continuing toward the cutting-edge, it is decidedly 
 convex in every direction, thus producing a promi- 
 nent point of contact with the distal surface of the 
 central incisor. 
 
 The Distal Surface of the Crown (Fig. 65). 
 — This surface also shows the characteristic wedge-shape of the crown, 
 and is principally different from the mesial surface in being convex 
 throughout. Near the center it is well rounded and full, providing a- 
 point of contact for the mesial surface of the cuspid. 
 The lingual margin, while being more decided in out- 
 line than the labial, is much more rounded than 
 the lingual margin of the mesial surface. From the 
 most prominent point near its center the surface slopes 
 away in every direction, the convexity being most 
 marked near the cutting-edge. 
 
 The Cutting-edge of the Lateral Incisor. — In the 
 young tooth the cutting-edge presents the three little 
 tubercles common to all incisors, the grooves which 
 divide them passing up over the labial and lingual 
 surfaces and forming the labial and lingual grooves. 
 These tubercles soon disappear by wear, leaving 
 the cutting-edge in the form of a direct lines and 
 connecting the two angles of the crown. Like the 
 central incisor, this margin of the crown may be thin and sharp, or it may 
 be thick and dull. 
 
 The Cervical Margin. — The line of demarcation between the crown 
 
 Fig. 65. — Upper 
 Lateral Incisor, 
 Distal Surface. 
 
114 ANATOMY 
 
 and the root of the tooth resembles so closely that described in connection 
 with the central incisor that it will only be necessary to mention one or 
 two characteristic differences. The lingual side of the line presents a 
 much smaller curve proportionately, and usually extends a little higher 
 in the direction of the root than that represented upon the labial portion. 
 The mesial and distal portions of the line dip well down, decreasing the 
 length of the crown on these surfaces; the margin on the former surface 
 is usually angular and V-shaped, while on the latter it is circular in form. 
 
 The Angles of the Crown. — The angles of the crown are the mesial 
 and the distal, and are formed in the same manner as the same angles of 
 the central incisor. The mesial angle is generally well produced, in most 
 instances being slightly acute; but when the cutting-edge is thin and 
 frail, the angle is frequently much obliterated by wear. That portion 
 of the crown of the upper lateral incisor which is usually referred to as 
 the distal angle is scarcely worthy of the name. It is usually present as 
 a long curve, which begins near the center of the cutting-edge and extends 
 well up on the distal surface. This characteristic outline is sometimes 
 so pronounced as to completely destroy the cutting-edge, the distal 
 surface being carried forward by a long curve ending in the mesial angle. 
 
 The Neck of the Tooth. — In this tooth the neck is usually marked 
 by a constriction much more pronounced than that found in the central 
 incisor. On the labial and lingual surfaces it is principally formed by 
 a sudden sloping of the enamel surface rootward, but on the two lateral 
 surfaces it is formed by a flattening or slight concavity of both the crown 
 and the root. 
 
 The Root of the Upper Lateral Incisor. — The root of this tooth 
 is conic in form, and is much more flattened from mesial to distal than 
 the root of the central incisor. At its junction with the crown it is circu- 
 lar in form, the labial portion forming the segment of a larger circle than 
 the lingual, this feature being observed throughout its entire length. The 
 flattening of the mesial and distal sides begins immediately above the 
 neck, and gradually increases as the center of the root-length is approached, 
 where it often develops into a slight longitudinal depression. As the 
 apex of the root is approached, this longitudinal depression gradually 
 disappears, and the root again becomes circular in form. The thickness 
 of the root is about one-third greater from labial to lingual than from 
 mesial to distal, and, while it is generally classed as a straight root, it is 
 frequently provided with a pronounced distal curve near the apical 
 extremity. In some instances it is found with a double mesiodistal 
 curve. 
 
UPPER LATERAL INCISOR 115 
 
 Bilious Type. — In this type the lateral incisor is frequently poorly 
 developed, the cutting-edge and distal angle are wanting, the crown 
 being in the form of a single conic cusp, the distal surface meeting the 
 mesial at a point near the mesial angle. This form of crown might 
 be classed as one of malformation, but the fact that it most frequently 
 occurs in this temperament would appear to indicate a normal condition. 
 When the crown takes the form common to incisors, it is of greater 
 longitudinal than transverse extent, the angles are well produced, and 
 the mesial and distal surfaces are flat and almost parallel with each other. 
 The labial surface is flat and is frequently broken by transverse ridges 
 near the cervical portion. The lingual surface presents well-marked 
 outlines and margins, a cuspule is seldom present, and the lingual fossa 
 is well marked, but not deep. The cutting-edge is thin and sharp, to 
 provide for the overbite, which is rather long. The cervical border is 
 square and angular. 
 
 Nervous Type. — In this typal form the neck of the tooth is a pro- 
 nounced feature. The crown is long and narrow, the constriction 
 forming the neck beginning well down on the crown and extending over 
 the cervical line to the surface of the root. The labial surface is convex 
 in every direction and the labial grooves fairly well defined. The mesial 
 surface is convex near the center and cutting-edge, but often shows a 
 slight concavity on its cervical portion. The distal surface is rounded 
 and smooth. The lingual surface presents a general concavity, a cuspule 
 being more frequently present than in other types.. The lingual fossa 
 is deep, and often extends beneath the cervicomarginal ridge in the form 
 of a circular fissure. The cutting-edge is thin and sharp, providing for 
 a long overbite; the mesial angle is pointed and well formed, while the 
 distal is usually much rounded. The cervical line is well arched, forming 
 the segment of a much smaller circle than that seen on the same tooth 
 of other temperaments. 
 
 Sanguineous Type. — The crown is well proportioned, with the 
 length slightly predominating over breadth, all the surfaces being more 
 or less rounded and smooth, showing the crown to be made up of curves 
 rather than angles. The labial surface presents a graceful convexity 
 throughout; the mesial and distal surfaces are both convex, with their 
 margins poorly defined. The lingual surface shows the rounded nature 
 of the crown in having its fossa and marginal ridges oval and blending 
 one into the other. The cutting-edge is moderately heavy and dull, 
 in keeping with the overbite, which is short. The cervical line is made 
 up of curves rather than angles. 
 
u6 
 
 ANATOMY 
 
 Lymphatic Type. — In this type the crown is generally of greater 
 transverse than longitudinal extent. The neck is poorly produced, the 
 crown and root uniting without any marked constriction of the parts. 
 The labial surface is much flattened from mesial to distal, and but slightly 
 convex in the direction of the long axis of the tooth. The mesial and 
 distal surfaces are but little rounded and are nearly parallel with each 
 other, so that the contact with adjoining teeth becomes an extent of 
 surface rather than a single point. The lingual surface is convex above, 
 but as the cutting-edge is approached it becomes flat, but seldom con- 
 cave. The marginal ridges are not well shown and the lingual fossa is 
 but a slight depression. The angles of the crown are well produced and 
 the cutting-edge thick and blunt, this marginal surface frequently occlud- 
 ing directly upon the opposing lower teeth. The curvature of the cer- 
 vical line is that of a long circle. 
 
 UPPER CUSPID. 
 
 5 th 
 
 6 th 
 
 7th 
 
 8th 
 
 8th 
 
 ioth 
 
 13th 
 
 year 
 
 year 
 
 year 
 
 year 
 Fig. 66. 
 
 year 
 
 year 
 
 year 
 
 Calcification Begins, from Three Centers, Third Year after Birth. 
 Calcification Completed, Twelfth to Thirteenth Year. 
 Erupted, Twelfth to Thirteenth Year. 
 Average Length of Crown, .37. 
 
 Average Length of Root, .68. 
 
 Average Length over All, 1.05. 
 
 About the third year after birth calcification begins in the central 
 lobe, which is gradually extended laterally, until, at the fourth year 
 it is met by the two lateral lobes, which are somewhat later in beginning, 
 and by the fifth year the three are united, the former eventually establish- 
 ing the single cusp of the tooth and the latter two the mesial and distal 
 
UPPER CUSPID 
 
 117 
 
 angles. About the sixth year two-thirds of the crown is formed, and by 
 the seventh year the constriction which marks the beginning of the neck 
 of the tooth commences to make its appearance. Between the seventh 
 and eighth year calcification in the crown is completed and the cervical 
 line established, during the following year nearly one-quarter of an inch 
 is added to the length of the root, and by the beginning of the tenth year 
 the root is formed for fully two-thirds of its entire length. Between the 
 twelfth and thirteenth years or at the time of eruption, calcification is 
 completed in the root so far at its surface is concerned (Fig. 66). In 
 this latter particular the cuspid differs from most of the other teeth, in 
 being completely calcined previous to, or about the time of, its eruption. 
 To reach its final position in the arch the tooth moves bodily downward, 
 the bone filling in behind; while in the incisors, bicuspids, and molars 
 the free calcifying root-extremi- 
 ties remain nearly stationary, the 
 crowns being forced downward as 
 the lime salts are deposited. 
 
 The crown of the upper 
 cuspid presents for examination 
 four surfaces — labial, lingual, 
 mesial, and distal — two margins 
 — the cervical margin and the 
 cutting-edge — and a mesial and 
 a distal angle. In general out- 
 line it is of the simplest form, 
 resembling the primitive cone- 
 shaped teeth of many fishes. 
 When viewed by looking directly 
 upon the mesial or distal surface, 
 the wedge-shape common to the 
 incisors is observed. The base 
 
 of the double incline is, however, much broader proportionately than the 
 corresponding measurement of the incisors. Looking at the crown from 
 a labial or lingual direction, its function, as both a penetrating and 
 incising organ, may be observed in the single cusp from which it derives 
 its name. The cusp, which is formed at the expense of the cutting- 
 edge, divides this latter margin into two distinct portions — the mesial 
 cutting-edge and the distal cutting-edge. 
 
 The Labial Surface of the Crown (Fig. 67).— The contour of 
 this surface is that of a broken circle more or less imperfectly drawn. It 
 
 Fig. 67. 
 
 Labial Ridge 
 -Upper Cuspid, Labial Surface. 
 
n8 
 
 ANATOMY 
 
 is bounded by five margins — mesial, distal, cervical, mesial-incisive, and 
 distal-incisive. The mesial margin is rounded from labial to mesial, and 
 slightly convex from the cutting-edge to the cervical line. The distal 
 margin is also rounded from labial to distal, presents a greater convexity, 
 and is somewhat shorter from the cutting-edge to the cervical line than 
 the mesial margin. By a continuation and final union of these two 
 lateral margins the cervical margin of the surface is formed, while by 
 their union with the cutting-edges the mesial and distal angles of the 
 crown are established. The mesial-incisive margin is usually slightly 
 
 concave near its center, although in 
 some instances it is convex. The 
 distal incisive margin responds to the 
 same description, although the con- 
 cavity, when present, is nearest the 
 point of the cusp. From the summit 
 of the cusp these two margins slope 
 away to join the mesial and distal 
 angles, the distal incline being about 
 one-fourth longer than the mesial. 
 This surface is generally of greater 
 longitudinal than transverse extent, 
 its greatest mesiodistal diameter being 
 from angle to angle, or at a point 
 immediately above them. The sur- 
 face is convex in every direction, and 
 is marked by a central longitudinal 
 ridge, usually well defined — the labial 
 ridge. Beginning at the summit of the cusp, this ridge is more or less 
 contracted laterally, but as it passes over the surface in the direction of 
 the root it becomes broadened and flattened, and gradually disappears 
 in the cervical portion. Upon either side of this ridge are the labial 
 grooves, well defined at their beginning, but which gradually blend into 
 the surface of the crown as they pass rootward. In some instances these 
 grooves are so strongly defined as to form a decided ridge upon the mesial 
 and distal margins of the surface — these are the labial marginal ridges. 
 
 The labial ridge and the two labial grooves mark the developmental 
 lines of the crown, the former resulting from the middle lobe, which 
 in this tooth is much the largest of the three, while the latter denotes 
 the line of junction between the middle and the lateral lobes. 
 
 The Lingual Surface of the Crown (Fig. 68). — This surface pre- 
 
 Lingual Ridge 
 Fig. 68. — Upper Cuspid, Lingual Surface. 
 
UPPER CUSPID 
 
 119 
 
 sents nearly the same general outlines as the labial, with the exception 
 of the cervical portion, which is more constricted, tending to produce an 
 oblong or egg-shape. It usually abounds in well-defined ridges and 
 depressions, giving to the tooth a rugged and strong appearance. There 
 is but little general concavity to the surface in passing from the point of 
 the cusp to the root. It may be flat, concave, or convex. As in the 
 incisors, the margins of this surface are formed by three marginal ridges 
 and by the cutting-edge. The mesiomarginal ridge is commonly a well- 
 defined fold of enamel, beginning at the mesial angle and passing upward 
 in the direction of the root, where it 
 unites with the cervicomarginal ridge. 
 It is sometimes quite narrow and rather 
 sharply outlined; at others, it extends 
 well toward the center of the surface in 
 the form of a well-rounded fold. The 
 distomarginal ridge, which is some- 
 what shorter than the mesial, begins at 
 the distal angle and passes rootward to 
 meet the cervical ridge. It is well 
 rounded in every direction, but seldom 
 so well produced as the mesial. The 
 cervicomarginal ridge, which is formed 
 by a continuation or union of the two 
 former, nearly always partakes of their 
 nature, except when broken by the 
 presence of a cuspule, which is fre- 
 quently found upon this tooth (Fig. 72). 
 
 This small cusp of enamel may be bounded on one or both sides by a 
 fissure, which often extends well under the cervicomarginal ridge, and 
 sometimes completely separates it from the two lateral ridges. Passing 
 through the center of the surface from the summit of the cusp to the base 
 of the cervicomarginal ridge is the lingual ridge, which corresponds to 
 the labial ridge of the labial surface. This ridge is usually well pro- 
 duced at or near the point of the cusp, and may continue so throughout, 
 but most frequently becomes reduced in size near the center of the sur- 
 face. Between this ridge and the mesio- and disto-marginal ridges are 
 two longitudinal depressions — the lingual grooves. 
 
 Mesial Surface of the Crown (Fig. 69). — In general outline this 
 surface resembles that of the central incisor, excepting that the wedge- 
 shape which it describes is more heavily set and blunt, with the surface 
 
 
 
 w 
 
 
 
 
 
 
 . kJ5 
 
 
 
 ''H 
 
 
 
 
 CeovicaJ 
 
 
 
 Ridge 
 
 
 
 Labia) 
 
 
 
 Groove 
 
 
 L M 
 
 Cervico- 
 lingual 
 Ridge 
 
 Lingual 
 Ridge 
 
 Fig. 69.- 
 
 -Right Upper Cuspid, Mesial 
 Surface. 
 
120 
 
 A \.\H> MY 
 
 extending beyond the base of the cone, in the direction of the root, to 
 the extent of about one-third of its entire length. In some cases the base 
 of the cone will be on a line with the cervical margin. The lower two- 
 thirds of the surface, or that nearest the mesial angle, is convex in every 
 direction; this convexity gradually disappears as the center is approached, 
 beyond which point it is much flattened, usually ending in a slight con- 
 cavity at the cervical margin. When looking directly upon this surface, 
 its margins will be found within the profile lines, these being represented 
 by the labial ridge anteriorly, and by the lingual and cervical ridges 
 
 posteriorly. The margins, 
 three in number, are the 
 labial, which is well rounded 
 and poorly denned; the lin- 
 gual, more or less distinctly 
 outlined and somewhat irregu- 
 lar; and the cervical, which 
 is represented by the extent of 
 the enamel covering of the 
 crown; this latter margin be- 
 
 |Cervicohn- ° 
 
 jguai Ridge m g concave in the direction of 
 the root. The most promi- 
 nent point of this surface 
 serves as a point of contact 
 for the distal surface of the 
 lateral incisor, the extent of 
 contact being much influenced 
 by the type of tooth, but in 
 the cuspid this is usually a 
 single point rather that an extent of surface. 
 
 The Distal Surface of the Crown (Fig. 70). — This surface in 
 many respects is similar to the mesial, particularly in its general outline. 
 The extent of surface is somewhat less and the convexity much more 
 marked than that of the mesial surface. The position of the distal 
 angle, which is the lower boundary of the surface, being much nearer 
 the cervical line, makes this surface about one-third shorter than the 
 mesial surface. The lateral margins of the surface, which are also within 
 the profile lines, differ from those of the mesial in being more clearly 
 defined. The cervical margin differs from that of the mesial surface 
 by having a concavity with much less depth. As stated above, the surface 
 is decidedly more convex than the mesial, the point of contact for the 
 
 Distal 
 Angle 
 
 Summit of 
 Cusp. 
 
 Fig. 70. — Left Upper Cuspid, Distal 
 Surface. 
 
UPPER CUSPID 
 
 121 
 
 mesial surface of the first bicuspid being almost in the center. Near 
 the cervical margin the surface is inclined to flatness, and frequently 
 concave. 
 
 The Cutting-edge, or Cusp. — As inferred in the beginning of this 
 description, the cuspid is both an incising and a penetrating organ, 
 the latter function being provided for by the presence of the single cusp, 
 which divides the cutting-edge into an anterior or mesial portion and a 
 posterior or distal portion. The mesial cutting-edge begins at the summit 
 of the cusp and slopes away to meet the mesial angle, which it assists 
 in forming. The outline of this edge is 
 usually gracefully curved and unbroken 
 unless permanently crossed by the labial 
 groove. The distal cutting-edge is gen- 
 erally somewhat longer than the mesial. 
 Immediately after leaving the summit of 
 the cusp it may be slightly concave; but 
 beyond this point it is well rounded, 
 until it reaches the distal angle, into cervicaj| 
 which it gradually disappears. This 
 edge is also frequently broken by the ^£^1 
 labial groove. In its entirety the cut- 
 ting-edge is subject 'to the same varia- 
 tions as those of the incisors — i. e., it may 
 be thin and sharp, or it may be thick 
 and blunt. 
 
 The Cusp. — The single cusp from which this tooth derives its name 
 is formed by the union of the labial ridge, the lingual ridge, and the 
 mesial and distal cutting-edges. The summit of the cusp is constant 
 in its position, always being in a direct line with the long axis of the 
 tooth, whether it be viewed from a mesial or a lingual direction. 
 
 The Cervical Line. — To describe this fully would be to repeat what 
 has already been said in connection with the incisors. This enamel 
 margin differs in one particular only from that of the incisors, and that 
 variation is not a constant one — the lingual portion is frequently extended 
 in the direction of the root, producing a short, positive curve at that 
 point. 
 
 The Angles of the Crown. — Owing to the rounded nature of the 
 majority of cuspid crowns, the term angle, as applied to its free extremities, 
 is almost a misnomer, and can only be considered as assisting in descrip- 
 tion. The mesial angle, which is formed by the union of the marginal 
 
 Marginal Ridges 
 Fig. 71. — Upper Cuspid, Lingual 
 Surface, Strongly Developed. 
 
122 
 
 ANATOMY 
 
 ridges of the labial and lingual surfaces with the mesial cutting-edge, is 
 seldom a well-produced angle, usually being rounded in every direction. 
 The distal angle, which is formed in a manner similar to the mesial, is 
 somewhat more deserving of the name, both the labio- and linguo-margi- 
 nal ridges frequently presenting angularity. The position of the distal 
 angle is usually well toward the center of the crown, and occasionally 
 above this point, and, although it may descend, it is seldom found on 
 a line with the mesial angle. 
 
 The Neck of the Upper Cuspid. — This may or may not be a dis- 
 tinctive feature, although when viewed from a labial aspect, the lateral 
 flare or bulging of the crown gives the appearance of a decided con- 
 striction between the crown and the root; but, when examined from 
 the mesial surface, this constricted appearance is absent, the contour 
 of the crown passing into that of the root, with the cervical line alone 
 marking the extent of each. The tooth at this point is well rounded 
 anteriorly, flattened laterally, and again rounded 
 posteriorly, the latter forming the segment of a 
 smaller circle than that of the labial surface. 
 
 The Root. — This tooth possesses the largest 
 and longest root of any of the teeth, in the latter 
 respect usually exceeding the central incisor by 
 about one-third, and the lateral incisor by one- 
 fourth or more. Like the base of the crown, it is 
 rounded on the labial and lingual surfaces, and is 
 flattened laterally, this form usually being continued 
 throughout its entire length. It gradually diminishes 
 in size from the neck to the apex, and in its entirety 
 forms a perfect cone. On the mesial and distal 
 sides it is not only much flattened, but is frequently 
 provided with a longitudinal depression, which is 
 most marked near the center of its length. In 
 some instances this root is possessed of a slight distal 
 curve, which may be gradual from the base to the apex, or it may exist 
 in a more positive way by a sudden distal curve near its apical extremity. 
 Bilious Type (Fig. 72). — The rounded outlines common to the 
 cuspid are less pronounced in this type than in any other, and instead 
 of curves, angles are present. The crown is above the average size, 
 length predominating over breadth, the cusp well formed, and the 
 angles strong. The labial surface is often crossed by a number of trans- 
 verse ridges near the cervical portion, the labial ridge is bold, as are also 
 
 Fig. 72. — Bilious Type, 
 Distal Surface. 
 
UPPER CUSPID 123 
 
 the labiomarginal ridges. The mesial and distal surfaces possess no 
 distinguishing features, but the lingual, like the labial, shows the angular 
 nature of the crown in having its margins and ridges squarely set. 
 
 A cuspule is more frequently found in this type than any other, 
 and sometimes reaches down to a point corresponding to the transverse 
 center of the crown. The neck is moderately well produced, and the 
 cervical line decidedly V-shaped on its lateral portions, while on the labial 
 and lingual it takes the form of a broken circle. The cutting-edges are 
 rather heavy and square, and are nearly of equal 
 length. In this temperament the cuspid tooth 
 often partakes of the form described in connection 
 with the lateral incisor of the same type — i. e., the 
 absence of the cutting-edge and one or both angles, 
 making the crown a perfect cone. 
 
 Nervous Type (Fig. 73). — The crown is of 
 much greater longitudinal than transverse extent, 
 the outlines oval and gracefully formed, and the 
 neck is much constricted from mesial to distal, 
 being made so by the lateral flare of the body of 
 the crown, which is a distinctive feature of this 
 type. The labial ridge is well formed near the 
 summit of the cusp, but usually disappears near 
 the center of the surface. The labiomarginal *ig. 73.— Nervous 
 
 • j u 1 ,1 r Type, Labial Surface, 
 
 ridges are seldom present, and the surface in 
 
 general is convex and smooth. The mesial and distal surfaces show a 
 pronounced convexity near the angles, and often a slight concavity 
 between this point and the cervical line. The lingual surface, while 
 generally showing all the descriptive lines, may be considered smooth; 
 it is convex from mesial to distal, and slightly concave in the direction 
 of the long axis of the tooth. The cusp is long and penetrating, the 
 distal cutting-edge is much longer than the mesial, and both are in- 
 clined to sharpness. The cervical line on the labial and lingual surfaces 
 is deeply arched, frequently giving to the gingival margin a receded 
 appearance. 
 
 Sanguineous Type (Fig. 74). — The crown in this type abounds in 
 long curves, the longitudinal and transverse extents are nearly equal, 
 the angles, owing to their circular form, are barely deserving the name 
 while the cusp and cutting-edges are outlined by one long, oval sweep. 
 The labial surface is prominent and convex, and the developmental 
 grooves are fairly well shown. The mesial and distal surfaces show a 
 
124 
 
 ANATOMY 
 
 moderate general convexity, while the lingual abounds in well-rounded 
 ridges and borders. The constriction forming the neck of the tooth 
 is moderate. The cervical line is in the form of perfectly arched curves, 
 
 forming on the labial surface the segment of 
 a circle corresponding to the circumference of 
 the crown of the tooth. 
 
 Lymphatic Type. — In this type the 
 crown is usually greater in its transverse than 
 in its longitudinal measurement; it is lacking 
 in graceful outline, and may best be described 
 as being short, thick, and heavy set. None 
 of the surfaces abound in descriptive lines, 
 although transverse ridges are sometimes 
 present on the labial surface near the cervix. 
 Both the labial and lingual surfaces are con- 
 vex in every direction, while the mesial and 
 distal are inclined to flatness. The cusp is 
 heavy and blunt, and the cutting-edges, which 
 are nearly of equal length, are thick and 
 dull. The mesial and distal angles are well 
 produced. The cervical line is almost a 
 direct line encircling the neck of the tooth, 
 the segmental form on the labial surface 
 being that of a much larger circle than any of those previously described. 
 The neck is thick and heavy, and the roots generally short. 
 
 Fig. 74. — Sanguineous 
 type, Distal Surface. 
 
UPPER FIRST BICUSPID 12 5 
 
 UPPER FIRST BICUSPID. 
 
 7th year 8th year 9th year 10th year nth year 12th year 
 
 Fig. 75. 
 
 Calcification Begins, from Four Centers, about the Fourth Year. 
 Calcification Completed, Eleventh to Twelfth Year. 
 Erupted, Tenth to Eleventh Year. 
 Average Length of Crown, .32. 
 
 Average Length of Root, .48. 
 
 Average Length over All, .80. 
 
 This tooth, although presenting a crown of vastly different contour, 
 is developed by a process almost identical with that of the incisors and 
 cuspids. As the name implies, it is made up of two cusps, one forming 
 the buccal and the other the lingual half of the crown. Calcification 
 in the buccal cusp is from three centers and begins about the fourth year, 
 the central lobe first receiving the lime salts. During the following 
 year the two lateral lobes begin to calcify, soon followed by a union of 
 the three, thus completing the margins and summit of the cusp. Unlike 
 the incisors, but corresponding to the cuspid, the middle lobe is much the 
 largest of the three, frequently forcing the developmental (buccal) grooves 
 well toward the angles of the crown. The development of the lingual 
 cusp corresponds to the development of the cervical ridge on the incisors 
 and cuspids, except that it has a separate center of calcification, and a 
 cusp almost as large as the buccal results. Between the fifth and sixth 
 year union between the two cusps takes place, the line of confluence 
 being permanently recorded by a well-defined groove, which traverses 
 the crown from mesial to distal. This groove, although forced to occupy 
 a different position, corresponds to the lingual groove of the incisors 
 and cuspids. After the union of the cusps the process of calcification 
 is continued into the body of the crown, and by the seventh year it is 
 
126 
 
 ANATOMY 
 
 Buccal Triangular Ridge 
 
 more than half completed. The eighth year usually finds the crown 
 fully formed and the base of the root or roots outlined. As this tooth 
 is generally provided with two roots, the first indication of bifurcation 
 will be observed between the eighth and ninth years by a filling-in near 
 the center of the mesial and distal walls, which finally become united 
 by a thin septum of dentine or cementum. After this period the roots 
 calcify separately, and by the middle of the ninth year about one-third 
 of their length is established. During the following year, or at about 
 the time of eruption, the development of the roots has extended to about 
 three-fourths of their complete length, and between the eleventh and 
 
 twelfth years, or at a time corres- 
 ponding to that of the crown assum- 
 ing its final position in the arch, calci- 
 :us P fication is completed (Fig. 75). 
 roove The Crown of the Upper First 
 
 pistai Bicuspid presents for examination 
 
 iroove r r 
 
 •istomar- five surfaces — buccal, lingual, mesial, 
 
 ginal ' ° ' ' 
 
 Ridge distal, and occlusal. In general, the 
 contour of the crown is irregularly 
 quadrilateral, being about one-third 
 greater in its buccolingual measure- 
 ment than from mesial to distal. It is somewhat flattened from mesial 
 to distal, but rounded on its buccal and lingual surfaces. 
 
 The Occlusal Surface of the Crown (Fig. 76). — The contour 
 of the crown is best observed by a view of this surface, which may be 
 described as trapezoidal or irregularly quadrilateral in form. The 
 four margins of the surface are those which represent the four lateral 
 surfaces — the buccal, lingual, mesial, and distal — the latter two being 
 in the form of well-defined ridges — the mesio- and disto-mar ginal ridges. 
 The buccal margin is formed by the mesial and distal inclines of the 
 buccal cusp; it has a slight buccal convexity, and at its union with the 
 proximate surfaces assists in forming the mesial and distal angles of the 
 crown. The distal half of this margin is usually somewhat longer than 
 the mesial, and the distal angle is less pronounced than the mesial. The 
 lingual margin presents a much greater convexity than the buccal, but 
 the curve formed is the segment of a much smaller circle. As in the 
 buccal margin, the distal half of this margin is the longest, but unlike 
 the former, its free extremities pass into the mesial and distal margins 
 without producing angles. The mesio- and disto-marginal ridges are 
 strong folds of enamel which arise from the mesial and distal angles and 
 
 Lingual Cusp 
 Fig. 76. — Upper First Bicuspid, 
 Occlusal Surface. 
 
UPPER FIRST BICUSPID 127 
 
 converge slightly as they pass to the lingual, where they are gradually 
 lost in the lingual margin. 
 
 The Cusps (Fig. 76). — These are two in number, and are named, 
 in accordance with their location, buccal and lingual. The buccal cusp 
 is the larger and longer of the two. From the summit of this cusp four 
 ridges descend — one in a mesial direction, forming the mesial cutting- 
 edge of the crown; one in a distal direction, forming the distal cutting- 
 edge; one to the buccal surface, the buccal ridge; and a fourth, the buccal 
 triangular ridge, descends the central incline. The mesial and distal 
 ridges enter into the formation of the mesial and distal angles at their 
 extremities; the latter is slightly longer than the former, and both are 
 frequently broken near the center by the grooves of development — the 
 buccal grooves. The buccal ridge may be well developed and extend 
 almost to the cervical line, or it may be slight and disappear near the 
 center of the surface. The buccal triangular ridge usually ends some- 
 what abruptly in the central groove, but in some instances it is con- 
 tinued and joins a similar ridge from the lingual cusp, this union form- 
 ing the transverse ridge. The triangular ridge often bifurcates near 
 the center "of" its incline, and is continued in two distinct but smaller 
 ridges. The lingual cusp is much less angular than the buccal; the apex 
 is usually rounded, while the descending ridges are generally three in 
 number instead of four. The mesial and distal ridges are nearly of the 
 same length, and pass without interruption into the mesio- and disto- 
 marginal ridges. The triangular ridge is less .clearly defined than its 
 fellow of the buccal cusp, and it is not unusual for it to be entirely 
 wanting. The lingual aspect of the cusp is smooth and rounded, 
 presenting nothing in the form of a ridge in correspondence with the 
 buccal ridge of the buccal cusp. Like the incisors and cuspids, the 
 summits of these cusps are usually in a direct line with the long axis 
 of the tooth. 
 
 The developmental grooves, all of which are observed upon the occlusal 
 surface, are the central, mesial, distal, two triangular, and two buccal. 
 The central groove is the most marked, is deeply sulcate, and extends 
 through the center of the surface from mesial to distal, ending just 
 within the two marginal ridges in two irregularly formed depressions 
 or pits — the mesial and distal pits. This groove marks the line of union 
 between the buccal and lingual lobes. The mesial and distal grooves 
 are not always well denned, but may usually be observed as fine lines 
 passing over the central portion of the mesio- and disto-marginal ridges. 
 The mesio- and disto-triangular grooves becin in the mesial and distal 
 
128 
 
 ANATOMY 
 
 
 HFt^^^H 
 
 Buccal 
 
 BF i ^H 
 
 Ridge 
 
 ^K ^^^J 
 
 [Mesial 
 
 Br . ^H 
 
 Angle 
 
 KwBK 
 
 Distal 
 Angle 
 
 Distal cut- 
 ting-edge 
 
 Buccal Grooves 
 Fig. 77. — Upper First Bicuspid, 
 Buccal Surface. 
 
 pits, and pass in the direction of the mesial and distal angles, where they 
 are either lost, or may be traced as slight depressions passing over the 
 buccal ridges near the angles, and they may further continue over the 
 
 buccal surface in the direction of the 
 root. These two grooves, together 
 with the mesial and distal above re- 
 ferred to, form the outlines of the 
 mesiobuccal and distobuccal develop- 
 mental lobes. The buccal grooves 
 will be described in connection with 
 the buccal surface. Supplemental 
 grooves are seldom found in connec- 
 tion with the buccal half of the oc- 
 clusal surface, but are occasionally 
 present on the central incline of the 
 lingual cusp. 
 
 The Buccal Surface of the Crown 
 (Fig. 77).- — In many respects this surface 
 resembles the corresponding or labial surface of the cuspid. It is 
 bounded by four margins — occlusal, mesial, distal, and cervical. The 
 occlusal half of the surface is formed of the buccal cusp, and is cone- 
 shaped, while the cervical half is irregu- 
 larly quadrilateral in form. The extent 
 of surface from the cervical line to the 
 point of the cusp is usually about one- 
 third greater than the greatest mesio- 
 distal diameter, which is represented by 
 a line drawn from the mesial to the 
 distal angle. The form shown is that 
 of a general convexity, the summit of 
 which is surmounted by a longitudinal 
 ridge — the buccal ridge. This ridge, 
 which is formed from the central de- 
 velopmental lobe, is most pronounced 
 near the occlusal margin, and gradually 
 disappears near the center of the surface. 
 Upon either side of the buccal ridge are 
 
 two grooves — the buccal grooves — which denote the line of union between 
 the central and the two lateral lobes, and beyond these are the angles of 
 the crown. The buccal ridge springs from the buccal cusp, the summit of 
 
 Summit of Lingual Cusp 
 
 Fig. 78.- — Upper First Bicuspid, 
 
 Lingual Surface. 
 
UPPER FIRST BICUSPID 1 29 
 
 which is generally in a direct line with the long axis of the tooth; when 
 there is a deviation from this the summit is usually thrown a little to 
 the mesial, resulting in a reduction of the length of the mesial cutting-edge. 
 
 As previously stated, the greatest mesiodistal diameter of the surface 
 is on a line with the angles of the crown; this measurement is much reduced 
 at the cervical line, so that the point of contact with adjoining teeth is 
 thrown near the occlusal margins of the crown. This variation in the 
 transverse measurement also results in what is commonly referred to as 
 the "bell shape" of the crown. The cervical margin of the surface is 
 fairly well arched, but seldom to such a degree is the corresponding 
 margin upon the incisors and cuspids. 
 
 The Lingual Surface of the Crown (Fig. 78). — This surface is 
 smooth and decidedly convex, the absence of strongly developed grooves 
 and ridges contributing to the former fact. Like the buccal surface, 
 its greatest transverse measure- 
 ment is at the base of the cusp in 
 which it terminates. The extent 
 of the surface is about one-third 
 less than that of the buccal, and 
 its occluding and cervical margins 
 are alone well defined, the mesio- 
 distal convexity passing so gradu- 
 ally into these respective surfaces 
 that a positive line of distinction 
 can scarcely be recognized. In 
 passing from the cervical line to 
 the occlusal margin, the surface is FlG 79 ._ Upper First Bicuspid, Mesial 
 rapidly carried toward the center Surface. 
 
 of the crown. The cervical margin of this surface is usually in the form 
 of a direct line encircling the neck, but occasionally presents a slight 
 concavity in the direction of the root. 
 
 The Mesial Surface of the Crown (Fig. 79). — This surface of the 
 crown has three of its borders well defined; these are the buccal, the 
 occlusal, and the cervical, the remaining or lingual margin passing so 
 gradually into the lingual surface that no positive line of demarcation 
 can be given. The buccal margin extends from the mesial angle to the 
 cervical line, and invariably presents a slight buccal inclination, thus 
 increasing the width of the surface on its cervical portion. The occlusal 
 margin is formed by the mesiomarginal ridge, and by a portion of the 
 ridge descending from the lingual cusp. It is irregularly V-shaped, 
 9 
 
130 
 
 ANATOMY 
 
 Buccal 
 Ridge 
 
 and in many instances is broken in the center by the mesial groove. The 
 cervical margin differs from those of the incisors and cuspids, nearly 
 always being in the form of a straight line from buccal to lingual. The 
 surface in general is flattened, but shows a slight general convexity near 
 the occlusal margin, and frequently a slight concavity immediately below 
 the cervical line, this form placing the point of contact with the distal 
 surface of the cuspid near the occlusal margin. The surface is occa- 
 sionally divided into a buccal and a lingual portion by the mesial groove, 
 which may extend to the cervical line, but which generally disappears 
 
 near the center of the surface. 
 The buccal half of the surface 
 which is formed from the mesial 
 developmental lobe is inclined to 
 Buccal Root angularity, while the lingual half 
 is decidedly rounded, particularly 
 in the direction of the occlusal 
 margin. 
 
 The Distal Surface of the 
 Crown (Fig. 80). — In general, 
 this surface resembles the mesial, 
 being flattened and bounded by 
 three more or less distinct mar- 
 gins. The slight buccolingual 
 convexity is not confined to the 
 occlusal portion of the surface, 
 but is inclined to extend to the 
 cervical margin, in this particular being at variance with the mesial 
 surface. This surface passes into the lingual by a much longer curve 
 than that shown on the mesial surface. 
 
 The Angles of the Crown. — These are two in number and, as in 
 the teeth previously described, are named, according to their location, 
 mesial and distal. The mesial angle is formed by the union of the mesio- 
 marginal ridge and mesial cutting-edge. It is primarily the product of 
 the mesial developmental lobe, and is usually well produced. The 
 distal angle, which is formed in a like manner, is inclined to be more 
 rounded. 
 
 The Neck of the Tooth. — In most typal forms the neck of the 
 upper first bicuspid is well defined, particularly upon the mesial and 
 distal surfaces. Viewing the tooth from a buccal aspect, the neck is 
 a distinctive feature, but when studied from the mesial or distal sides, 
 
 Triangular Ridge, 
 Occlusal Surface 
 
 Fig. 80. — Upper First Bicuspid, Distal 
 Surface. 
 
UPPER FIRST BISCUPID 131 
 
 the constriction is scarcely observed, this being particularly the case if 
 the tooth has but a single root. In general, the neck partakes of the 
 contour of the crown, being convex on the buccal and lingual, and flat- 
 tened and frequently slightly concave on the mesial and distal. 
 
 The Roots of the Upper First Bicuspid. — This tooth is usually 
 developed with two roots, sometimes with only one, and in rare instances 
 it may have three. When two roots are present, one is above the buccal 
 and the other above the lingual half of the crown, and are named, accord- 
 
 Fig. 81. — Types of Bicuspids. 
 
 ing to their location, as buccal and lingual. In general form the tfwo 
 roots are quite similar, but the buccal is usually a trifle longer than the 
 lingual. They taper off to a slender apex, and are inclined to curve in 
 various directions near their extremities. The point of bifurcation is 
 frequently some distance above the neck, so that the tooth may be said 
 to possess a single root with two branches. Below the bifurcation the 
 root assumes the form of the neck or cervical portion of the crown, but 
 as the bifurcation is approached, the mesial and distal sides present a 
 longitudinal groove, which gradually increases in depth until the single 
 root becomes separated. The curves of the root-branches above referred 
 to are, in the buccal branch, first to the buccal and then to the lingual; 
 while the lingual branch first shows a slight lingual inclination immedi- 
 ately above the point of separation, followed by a gentle buccal curve 
 as the apical end is reached. In some cases the bifurcation begins 
 
132 
 
 ANATOMY 
 
 immediately above the neck of the tooth; in others it may occur in the 
 apical third; while a third class is represented by the two roots being 
 united throughout their entire length by a thin septum of dentin and 
 cementum, or, as occasionally happens, by a layer of cementum alone. 
 In this latter instance each root is provided with a distinct canal. When 
 the tooth has but a single root, it is much flattened from mesial to distal, 
 the flatness being slightly broken by an inclination to convexity. The 
 four surfaces usually converge toward the apical end, which is oblong from 
 buccal to lingual, and generally provided with a slight distal curve. The 
 
 presence of three roots is so rare that 
 the condition might be classed as a 
 malformation; but when they do exist, 
 two are usually attached to the buccal 
 and one to the lingual half of the 
 crown, with the point of separation 
 near the neck of the tooth. 
 
 Bilious Type (Fig. 82).— The 
 upper first bicuspid of this tempera- 
 mental type is marked by a crown of 
 moderate length, the neck well pro- 
 nounced, and the cusps and angles 
 marked by angular outlines. The 
 buccal ridge is strongly defined, and 
 the buccal grooves, which extend well 
 up on the buccal surface, cross the mesial and distal cutting-edges, sepa- 
 rating them into two distinct parts. The cusps are long and penetrating, 
 and are nearly of equal length, assisting to form the firm and well-locked 
 occlusion common to this type. The mesial and distal surfaces are nearly 
 parallel with each other; they are seldom convex, so that the approximating 
 teeth are in contact over an extent of surface rather than a single point. 
 The cervical line is but little curved. 
 
 Nervous Type (Fig. 83). — In this temperament the bell-shaped 
 crown is strongly observed, the crown being long and much con- 
 stricted at its neck. The extreme length of the buccal cusp, and the 
 marked cervical constriction, produce an appearance in the buccal 
 surface resembling the labial surface of the cuspid tooth. The develop- 
 mental grooves are finely outlined, and the cusps long and penetrating, 
 usually being more pronounced than in any other class. The cutting- 
 edges are sharp and inclined to angularity; the mesial and distal surfaces 
 
 Fig. 82. — Bilious Type, Distal 
 Surface. 
 
UPPER FIRST BICUSPID 
 
 1 33 
 
 are convex near their occlusal margins, but near the cervical line a 
 pronounced concavity is observed, which is continued upon the correspond- 
 ing root-surfaces. This formation forces the point of contact with 
 adjoining teeth well toward the occlusal surface, and results in an exten- 
 sive V-shaped interproximate space. The cervical line is sharply and 
 gracefully formed, the curvature being well arched. 
 
 Sanguineous Type. — The typical upper first bicuspid of this class 
 is provided with a crown well proportioned, its length being somewhat 
 greater than its breadth, but about equal to its buccolingual measure- 
 ment. The buccal surface is seldom broken by the 
 buccal grooves, and is strongly convex in every direc- 
 tion. The lingual surface is much more rounded 
 than the same surface of other types. The mesial 
 and distal surfaces are usually smoothly convex, with 
 an occasional slight concavity immediately below the 
 cervical line. Upon the occlusal surface the grooves 
 are rounded and obscure, rather than sharp and well 
 defined, and the cusps, much less pronounced than in 
 either of the types previously described, are rounded 
 and smooth; this latter fact is particularly true of the 
 lingual cusp, which is usually much smaller than the 
 buccal. The cutting-edges of the buccal cusp are 
 scarcely deserving of the name, being broad and 
 rounded throughout. The form of the mesial and 
 distal surfaces above described provides for a point of contact near the 
 center of each surface, leaving a slight interproximate space both above 
 and below this point. 
 
 Lymphatic Type. — An examination of the upper first bicuspid of 
 the lymphatic type results in finding a tooth vastly different from any 
 of those previously described. The length of the crown from the cervical 
 line to the point of the cusp is less than either the mesiodistal or bucco- 
 lingual measurements. In general appearance it is lacking in symmetry, 
 or poorly proportioned. The buccal surface presents a gradual convexity 
 from mesial to distal, and seldom has the buccal ridge well developed. 
 The lingual surface is smoothly convex and passes off into the lingual 
 root without the interposition of a decided neck. The mesial and distal 
 surfaces are flattened and sparingly convex, and are nearly parallel with 
 each other, so that the contact with adjoining teeth is inclined to be 
 distributed over the entire surface, leaving little or no interproximate 
 space. The cusps are short, flat, and rounded, and the occlusal surface 
 
 Fig. 83. — Nerv- 
 ous Type, Buccal 
 
 Surface. 
 
134 ANATOMY 
 
 much flattened in general, corresponding with the nature of the occlusion, 
 which is loose and wandering. The developmental grooves and ridges 
 are fairly well shown, while the cutting-edges and angles of the crown 
 are smooth and rounded. The neck is less pronounced in this type than 
 in any other, the curvature of the cervical line is very slight, the root is 
 short and heavy set, frequently passing well up toward the apex before 
 bifurcating. 
 
UPPER SECOND BICUSPID 135 
 
 UPPER SECOND BICUSPID. 
 
 7th year 8th year 9th year 10th year nth year 12th year 
 
 FIG. 84. 
 
 Calcification Begins, from Four Centers, about the Fifth Year. 
 Calcification Completed, Eleventh to Twelfth Year. 
 Erupted, Eleventh to Twelfth Year. 
 Average Length of Crown, .29. 
 
 Average Length of Root, .55. 
 
 Average Length over All, .84. 
 
 The process of development in this tooth is identical with that of 
 the first bicuspid, calcification in the buccal half of the crown taking 
 place in one central and two lateral lobes, while the lingual half is developed 
 from a single center. The calcifying process is about one year later 
 than that in the first bicuspid, the summit of the buccal cusp receiving 
 its lime salts about the beginning of the fifth year. During the following 
 six months calcification begins in the lateral lobes, and also in the lingual 
 lobe. By the sixth year the occlusal surface and a portion of the crown 
 are completed by a union of the various lobes, and at seven years the 
 crown is calcified for more than two-thirds of its completed length. 
 Between the eighth and ninth year the contour of the crown is established, 
 and the neck of the tooth and outline of the root-base formed. At the 
 tenth year about one-third of the root-length is formed, and during the 
 following year about 1/8 of an inch is added to it. By the eleventh or 
 twelfth year calcification is completed (Fig. 84). 
 
 This tooth so closely resembles the first bicuspid that a description 
 in detail will be unnecessary; there are, however, a few minor points 
 which are at variance and must be described in order to distinguish one 
 from the other. In general, the tooth is a trifle smaller than the first 
 bicuspid, the cusps are somewhat shorter, and the various ridges less 
 
136 ANATOMY 
 
 distinct. A distinguishing feature of the occlusal surface is found in the 
 diminished length of the central groove (Fig. 88). This groove, as 
 observed in the first bicuspid, extends from mesial to distal for fully 
 three-fourths of the entire width of the surface; but in the second bicuspid 
 it is diminished by one-third, being thus reduced by the broadened mar- 
 ginal ridges, which force the mesial and 
 distal pits well toward the center. It is not 
 uncommon to find the triangular grooves 
 joining the central groove directly in the 
 Trianguiarcenter of the surface, forming a central pit 
 
 Grooves 
 
 from which may radiate numerous small 
 supplemental grooves and ridges. The 
 summits of both the buccal and lingual 
 Fig. 85 — Second Upper Bicuspid cusps are nearer to the mesial than to the 
 
 Occlusal Surface. , . . r , . . , , , . 
 
 distal surface, thus increasing the length of 
 the ridges which descend from them in a distal direction, and decreasing 
 those which pass to the mesial. The buccal surface presents a greater 
 convexity than that of the first bicuspid; the buccal grooves are usually 
 shallow depressions and are frequently entirely wanting, thus giving the 
 buccal ridge the appearance of extending its margins to the angles of 
 the crown. Unlike the first bicuspid, 
 the mesiodistal diameter of the mesial 
 surface is but little more than the 
 same measurements on the lingual 
 surface. The neck of the tooth is 
 not quite so pronounced as that of 
 the first bicuspid, thus giving less of 
 the bell shape to the crown. One 
 very important difference between 
 this tooth and the first bicuspid is in 
 the root-formation. We have seen that 
 
 the first bicuspid is generally pro- MIMI Klll):i ^^ ^Lingual 
 
 vided with two roots, while in the 
 second bicuspid a single root is usually 
 
 nresent T ikp the first himeniH FlG - 86 -— u PPer Second Bicuspid, Me- 
 presenr. ±,lKe tne nrst bicuspid, s ; a l Surface. Most common form. 
 
 there are exceptions to this, the tooth 
 
 sometimes being provided with two, and in rare instances with three, 
 roots. When the single root is present, it partakes of the form of 
 the crown at its base, being well rounded on the buccal and lingual 
 portions and much flattened on the mesial and distal. The mesiodistal 
 
UPPER SECOND BICUSPID 
 
 137 
 
 diameter of the rcot at its base is only about one-third that of the bucco- 
 lingual measurement, this proportionate size continuing throughout 
 its entire length. In passing from the base to the apex the root is gradu- 
 ally diminished in size, finally ending somewhat 
 abruptly in an oblong extremity. In some in- 
 stances the apical end is round and pointed, 
 resembling the apex of the incisors and cuspids, 
 but when thus formed the root is usually curved 
 near its apical third and somewhat extended in 
 length. The mesial and distal surfaces are pro- 
 vided with a well-defined longitudinal concavity, 
 extending from the cervical margin to the apex and 
 dividing the root into a buccal and a lingual por- 
 tion. This depresson is often so decided that the 
 contour of the single root is almost lost, and in its 
 place the appearance is that of two roots similar to 
 those described in the first bicuspid. The length 
 of the root is usually a little greater than that of 
 the first bicuspid, but the crown being a trifle 
 shorter, results in producing a tooth the entire length of which is about 
 equal to that of the first bicuspid. 
 
 Fig. 87. — Young Up, 
 per Second Bicuspid - 
 Linguaf Surface. 
 
ANATOMY 
 
 UPPER FIRST MOLAR. 
 
 2d year 
 
 3d year 5th year 
 
 6th year 
 
 Fig. 8i 
 
 7th year 
 
 10th year nth year 
 
 Calcification Begins, from Four Centers, about One Month Before Birth. 
 Calcification Completed, Ninth to Tenth Year. 
 Erupted, Sixth to Seventh Year. 
 Average Length of Crown, .30. 
 
 Average Length of Root, .51. 
 
 Average Length over All, .81. 
 
 This tooth being the first of the permanent organs to erupt, pre- 
 cedes all others in the process of calcification, beginning to receive its 
 lime salts as early as the eighth fetal month. The form of the crown 
 being so entirely different from those previously described, embodies 
 a developmental process which is also different, four distinct lobes being 
 present, one for each cusp, these making their appearance during the 
 first year after birth, closely followed by a completion of the occlusal 
 surface by the union of the free calcifying margins, these lines of union 
 being finally represented by the developmental grooves of the occlusal 
 surface. After the completion of this surface, calcification proceeds in 
 the direction of the base of the crown, and at the beginning of the third 
 year about two-thirds of the crown is formed. During the fifth year 
 the contour of the crown is completed, and at the beginning of the eruptive 
 period, or about the sixth year, the developmental process has extended 
 to the base of the roots, and an effort at trifurcation begun. At seven 
 years the three roots with which the tooth is provided are branching out, 
 each into its own socket, subsequent development in each being con- 
 tinued as a separate and distinct process. The tenth year more than 
 half completes the calcifying process in the roots, and at the beginning 
 of the eleventh year the root apices are formed. Like the first bicuspid, 
 the crown of this tooth presents for examination five surfaces — occlusal, 
 buccal, lingual, mesial, and distal. In general contour it is irregularly 
 
UPPER FIRST MOLAR 1 39 
 
 quadrilateral, with the angles of the crown more or less rounded, two 
 of its sides convex and two flattened or slightly concave. The length 
 of the crown from the cervical line to the summits of the cusps is about 
 equal to, or slightly less than, its mesiodistal diameter, while the bucco- 
 lingual measurement is usually a trifle greater than the mesiodistal. 
 The Occlusal Surface of the Crown (Fig. 89). — The coronal 
 outline of this tooth is best studied when looking directly upon this 
 surface, which shows the two convex sides above referred to, represented 
 
 Mesiobuccal Cusp 
 
 Buccal Groove 
 
 Central Fossa 
 
 Oblique Ridge 
 
 Mesiolingual Cusp 
 The Fifth Cusp 
 
 Distolingual Distolingual 
 Groove Cusp 
 
 Fig. 89. Upper First Molar, Occlusal Surface. 
 
 by the buccal and lingual margins, with the mesial and distal margins 
 more or less flattened. The surface is bounded by these four margins, 
 which are nearly of equal length, the angles formed by their union being 
 more or less rounded, two of which, the mesiobuccal and the distolingual, 
 are acute angles, while the mesiolingual and distobuccal are obtuse angles. 
 The surface is divided into four developmental portions — the mesiobuccal, 
 distobuccal, mesiolingual, and distolingual. Each one of these parts 
 is surmounted by a well-defined point or cusp, which likewise is named 
 in accordance with its location. These various parts are separated from 
 one another by four developmental grooves — the mesial, the buccal, 
 the distal, and the distolingual. In the center of the triangle formed 
 by the central incline of the mesiobuccal, distobuccal, and mesiolingual 
 cusps is a deep depression — the central fossa — while near the distal 
 margin is a somewhat similar depression — the distal fossa. Traversing 
 the surface in various directions are a number of ridges and supplemental 
 grooves, each of which will be described in turn. 
 
 The Marginal Ridges of the Occlusal Surface. — These are four in 
 number — the mesial, distal, buccal, and lingual. The mesiomarginal 
 ridge is a well-pronounced elevation of enamel which passes from the 
 mesiobuccal angle to the mesiolingual angle. It is slightly concave in 
 
140 ANATOMY 
 
 the direction of the root, and is broken near the center of its concavity 
 by the mesial groove, upon either side of which are frequently found one 
 or two small points or tubercles, which are formed either by a division 
 of the mesial developmental groove, or by one or more supplemental 
 grooves. These grooves pass over the ridge and are continued for a short 
 distance on the mesial surface. Descending from the mesiobuccai cusp, 
 the ridge passes in a lingual direction to meet the mesiolingual cusp, and 
 in so doing has a slight distal inclination until the mesial groove is reached, 
 after passing which it makes a sweeping distal curve and is lost in the 
 lingual margin. This ridge marks the line of junction between the 
 occlusal surface and the mesial surface. The distomarginal ridge in 
 some respects resembles the mesial just described, being concave and 
 ascending in a buccal and lingual direction, with a somewhat rounded 
 outline, to the summits of the distobuccal and distolingual cusps. The 
 depth of the concavity is usually greater than that of the mesial margin, 
 and is frequently crossed near the center by the distolingual groove, 
 frequently so marked as to produce a V-shape to the center of the margin. 
 There are occasionally found upon either side of this central groove one 
 or more small tubercles, corresponding to those of the mesial ridge, but 
 they are less frequent and less pronounced. This ridge forms the line 
 of demarcation between the occlusal surface and the distal surface. The 
 buccomarginal ridge begins at the mesiobuccai angle, and gradually 
 ascends to the summit of the mesiobuccai cusp, from which it afterward 
 descends in a distal direction to the buccal groove; continuing, it again 
 ascends the distobuccal cusp, after descending from which it ends in the 
 distobuccal angle. The nature of this ridge is a series of cutting-edges, 
 giving to the cusps their angular nature. Besides the buccal groove, 
 which makes a decided break in the center of its course, the ridge is 
 frequently crossed by numerous small supplemental grooves occurring 
 in various locations and forming a series of minute tubercles; this latter 
 condition is most frequently present in young teeth, and is soon obliterated 
 by wear. The course of this ridge is not that of a direct line from mesial 
 to distal, but in its ascent of the mesiobuccai cusp it is inclined to the 
 buccal; in its descent it presents a corresponding return to the lingual, 
 and the same variations are observed in passing over the distal cusp. 
 The linguomarginal ridge begins at the mesiolingual angle of the crown 
 and passes distally to the distolingual angle, differing from the three 
 previously described by being heavy and rounded in its nature, more 
 irregular in outline, and divided nearest to its distal extremity instead 
 of in the center of its length. From the point of beginning it makes a 
 
UPPER FIRST MOLAR 141 
 
 curved ascent to the summit of the mesiolingual cusp; descending from 
 this in a distobuccal direction, it divides, one portion passing to join the 
 triangular ridge of the distobuccal cusp, the two uniting to form the 
 oblique ridge, the other portion continuing in the direction of the disto- 
 lingual cusp, before reaching the base of which it is broken by the disto- 
 lingual groove. From this groove the ridge makes a sudden and direct 
 ascent to the summit of the distolingual cusp, after passing which it 
 gradually descends in a long curve to join the distomarginal ridge. Like 
 the buccal 'ridge, it is frequently crossed by numerous supplemental 
 grooves. The ridge forms the lingual margin of the occlusal surface 
 and gives to the cusps their angularity. 
 
 The Cusps (Fig. 89). — These are four in number — the mesiobuccal, 
 distobuccal, mesiolingual, and distolingual. 
 
 The Mesiobuccal Cusp (Fig. 89). — In extent of surface this is usually 
 the largest cusp, although it is sometimes exceeded by the mesiolingual. 
 From the summit of the cusp three ridges descend — the buccal ridge to 
 the buccal surface, the buccomarginal ridge making a double descent, 
 and the mesiobuccal triangular ridge, the latter descending the central 
 incline and ending in the central fossa. The mesial base of the cusp is 
 frequently crossed by one or more supplemental grooves, which begin at 
 the mesial margin and pass in the direction of the central fossa. The 
 central slope of the cusp contributes to the formation of the central fossa, 
 its extent in this direction being controlled by the mesial and buccal 
 grooves, which together form the mesiobuccal triangular groove. 
 
 The Distobuccal Cusp (Fig. 89). — This cusp is frequently the smallest 
 in extent of surface, but is usually longer and more pointed than the others. 
 Like the mesiobuccal cusp, three ridges descend from it — the buccal 
 ridge to the buccal surface two which spring from the buccomarginal 
 ridge, and the distobuccal triangular ridge, which descends obliquely 
 toward the distal center of the surface and joins a similar ridge (pre- 
 viously described) from the mesiolingual cusp, the two forming the 
 oblique ridge. The mesial portion of the base of this cusp assists in 
 forming the central fossa, while a portion of the distal contributes to the 
 formation of the distal fossa. The inner boundary of the cusp is formed 
 by the buccal groove, the distal groove, and by a portion of the disto- 
 lingual groove. 
 
 The Mesiolingual Cusp (Fig. 89). — As above stated, this cusp is 
 frequently the largest in extent of surface, and is somewhat rounded, 
 with its summit poorly defined. The ridges which descend from it 
 correspond in name and number to those of the buccal cusps, the linguo- 
 
142 ANATOMY 
 
 marginal ridge making a double descent, the mesiolingual ridge descend- 
 ing to the lingual surface, while the central incline is marked by the 
 mesiolingual triangular ridge, which ends in the central fossa. Toward 
 the mesial portion of the cusp one or more small ridges are frequently 
 present, extending from the marginal ridge to the mesial groove. The 
 distal descent of the marginal ridge is bifurcated, one portion making 
 a sweeping curve and joining the transverse ridge from the buccal cusp, 
 forming the oblique ridge previously referred to, the other portion pass- 
 ing in a distal direction and ending at the distolingual groove. The 
 central incline of this cusp forms the lingual side of the central fossa, 
 and its boundaries are outlined by the mesial, distal, and distolingual 
 grooves. 
 
 The Distolingual Cusp (Fig. 89). — This cusp is usually the smallest 
 of the four; it is triangular in outline, with the summit nearest the mesio- 
 lingual portion. The ridges which descend from this cusp are only two 
 in number, one passing in a mesial direction and forming a portion of the 
 linguomarginal ridge, the other passing to the distal, with a gradual 
 buccal curve, to join the distomarginal ridge. Of the two remaining 
 inclines, one looks in a distolingual direction, presenting a surface which 
 is smooth and rounded; the other, sloping by a broad expanse in a mesio- 
 buccal direction, ending in the distolingual groove, and also assisting to 
 form the distal fossa. This latter incline is often crossed by small sup- 
 plemental grooves, which take a winding course from the base to the 
 summit of the incline. The inner margin or outline of the cusp is formed 
 by the distolingual groove. 
 
 The Fifth Cusp (Fig. 90). — Although usually referred to as possessing 
 but four cusps, this tooth is frequently developed with five, the additional 
 lobe being situated on the lingual side of the mesiolingual cusp, about 
 midway between its summit and the neck of the tooth. When present, 
 it is distinctly separate from the main cusp by a well-developed groove — 
 the mesiolingual groove. Both the cusp and the groove may be more or 
 less developed, the former in some instances assuming dimensions cor- 
 responding to that of the distolingual cusp, and the latter sometimes 
 being as well marked as the distolingual groove. When thus pronounced, 
 the groove begins near the center of the mesial surface, and passes ob- 
 liquely toward the summit of the mesiolingual cusp, before reaching which 
 it makes an abrupt turn rootward, and joins the lingual terminal of the 
 distolingual groove, this union frequently resulting in a well-defined pit — 
 the lingual pit. This cusp, as usually found, is small and apparently 
 without function. When occurring on the tooth of one side, it is usually 
 
UPPER FIRST MOLAR 
 
 143 
 
 present on the corresponding tooth of the opposite side. It is seldom 
 present on any but the upper first molar. 
 
 The Fossae and Grooves of the Occlusal Surface (Figs. 89 and 
 go). — The fossae are two in number — central and distal. The central 
 fossa occupies a position near the center of the surface, and is formed by 
 the central incline of the mesiobuccal, distobuccal, and mesiolingual 
 cusp, which usually give it a three-sided form. Connecting the three 
 sides of the fossa, and in a measure assisting in its construction, is the 
 
 Buccal Groove 
 
 Distal Groove 
 
 Mesiobuccal 
 
 Cusp 
 
 Central Fossa 
 
 Mesiomar- 
 
 ginal Ridge 
 
 The Fifth 
 Cusp 
 
 Mesiolingual Oblique 
 Cusp Ridge 
 
 Fig. 90. — Upper First Molar, Occlusal Surface, Strongly Developed, showing 
 Presence of Fifth Cusp. 
 
 mesiomarginal ridge and the oblique ridge. The depth of this fossa, 
 as well as that of the distal, is of course regulated by the length of the 
 cusps, which in turn is much influenced by the temperamental type of the 
 tooth. The floor of the fossa is deeply marked by two of the grooves 
 of development — the mesial groove and the buccal groove. The former 
 begins on the mesial surface, passes over the mesiomarginal ridge, and 
 continues in an irregular line to the floor of the fossa; the latter, begin- 
 ning near the center of the buccal surface, enters the fossa by crossing 
 the buccomarginal ridge near the center of its length, and also ends in 
 the central pit of the central fossa. As previously referred to, the union 
 of these two grooves forms the mesiobuccal triangular groove. From 
 the central pit of this fossa another groove is given off — the distal groove. 
 It is usually well defined at its beginning, but as it passes over the oblique 
 ridge it is generally partly obliterated, although occasionally being so 
 marked as to divide this ridge. The distal fossa is much smaller than 
 the central, and is of an entirely different form. Its walls are principally 
 formed by the distolingual incline of the oblique ridge, and the mesio- 
 buccal incline of the distolingual cusp; a portion of the distomarginal 
 
144 
 
 ANATOMY 
 
 Mesiobuccal 
 Root 
 
 ridge and the distal incline of the distobuccal cusp also assist in its forma- 
 tion. Like the central fossa, its sides are more or less irregular, from 
 the presence of various grooves and ridges in its vicinity. The greatest 
 length of the fossa is in a distolingual direction, and it is traversed by 
 a deep developmental groove — the distolingual groove. When the distal 
 groove crosses the oblique ridge, it usually extends to the floor of this 
 fossa. 
 
 The Buccal Surface of the Crown (Fig. 91). — This surface, which 
 is the result of a union between the mesial and distal developmental 
 
 lobes, may be divided into a 
 mesial and a distal half. These 
 two portions are quite similar 
 in outline, and are separated 
 Lingual Root from each other by the buccal 
 groove, which usually ends near 
 |the center or about half-way to 
 the cervical line in a decided 
 pit — the buccal pit. In some 
 instances this groove is con- 
 tinued to the cervical line, or 
 even beyond this to the bifur- 
 cation of the roots. Both the 
 mesial and distal half are pro- 
 vided with a longitudinal ridge 
 (the buccal ridges) — one the 
 mesiobuccal ridge and the other 
 the distobuccal ridge. These 
 are similarly formed and descend from the summits of the respective cusps, 
 at which point they are usually well defined, but gradually disappear as 
 they pass toward the cervical line. The location of the buccal groove being 
 a little to the distal of the center of the surface, gives to. the mesial portion 
 a somewhat greater extent than the distal. The margins of the surface, 
 which form an irregular quadrilateral, are the mesial, distal, occlusal, and 
 cervical. The mesial and distal margins are rounded, and gradually con- 
 verge as they pass rootward, making the average diameter of the surface 
 about one-fourth less at the cervical line than at the base of the cusps. 
 In some instances these margins are slightly concave over their cervical 
 portion, and convex on approaching the occlusal margins; or the mesial 
 may be concave and the distal convex throughout their entire length; 
 in some types they appear as straight lines and are parallel with each 
 
 Distobucca 
 Cusp 
 
 Mesiobuccal 
 Cusp 
 
 Fig. 91.- 
 
 Buccal Groove 
 
 -Upper First Molar, Buccal 
 Surface. 
 
UPPER FIRST MOLAR 
 
 145 
 
 other. The occlusal margin is formed by the marginal ridges as they 
 pass over the two buccal cusps, being in the form of the letter W. The 
 cervical margin is usually a direct line drawn around the circumference 
 of the tooth, but in some instances deviating slightly from this. Immedi- 
 ately below the cervical line, and conforming to its general direction, is 
 a rounded fold of enamel — the cervical ridge. 
 
 Lingual Surface of the Crown* (Fig. 92).— Like the buccal sur- 
 face, this surface is developed from two lobes — the mesio- and disto- 
 lingual lobes — the line of union between the two being recorded by a well- 
 defined groove — the lingual 
 groove. This groove, which 
 is a continuation of the dis- 
 tolingual groove of the occlusal 
 surface, usually ends near the 
 center of the lingual surface in 
 a well-defined pit — the lingual 
 pit — or it may continue root- 
 ward and gradually disappear. 
 It is located a little to the 
 distal of the center of the sur- 
 face, thus making the mesial 
 a trifle larger than the distal 
 portion. The mesial half of 
 the surface is smooth and 
 convex; the lingual incline of 
 
 the mesiolingual cusp is seldom provided with a well-defined ridge, 
 although usually referred to as the mesiolingual ridge. The distal half 
 of the surface is also smooth and rounded, with the mesiodistal convexity 
 much more marked than that of the mesial lobe. The cervical ridge is 
 seldom so pronouced as that of the buccal surface, but the enamel 
 frequently makes a sudden dip at this point to meet the cementum of 
 the root. That portion of the surface immediately below the cervical 
 ridge is smooth and unbroken, slightly convex in the direction of the 
 long axis of the tooth, and flattened or slightly convex from mesial to 
 distal. The margins cf the surface are the mesial, distal, occlusal, and 
 cervical. The surface passes so gradually into the mesial and distal 
 that it is somewhat difficult to define these margins. In general, the 
 margins converge slightly in the direction of the root. Both the occlusal 
 
 Distolingual 
 Angle 
 
 Fig. 92.- 
 
 Cervical 
 Line 
 
 Mesial 
 Groove 
 
 Lingual Ridge 
 Mesiolingual Cusp 
 
 -Upper First Molar, Lingual 
 Surface. 
 
 * When the fifth cusp is present, the anatomy of the mesial half of this surface is some- 
 what more complex. 
 
146 ANATOMY 
 
 and cervical margins are similar to the corresponding margins of the 
 buccal surface. 
 
 The Mesial Surface of the Crown (Fig. 93). — This surface is 
 almost an unbroken plane, being smooth and flat. In some instances 
 it is crossed near the center of its occlusal margin by a continuation of 
 the mesial groove, but this is seldom so pronounced as to divide the 
 surface. The occlusal third of the surface is inclined to a slight general 
 convexity, providing a point of contact for the distal surface of the second 
 
 bicuspid, but between this 
 
 and the cervical line there is 
 
 often a slight concavity. The 
 
 , Mesiobuc- margins of the surface are the 
 
 Lingual H «■ caljRoot 
 
 Root occlusal, buccal, lingual, and 
 
 cervical. The first named is 
 
 formed by the mesiomarginal 
 
 cervical I B ridge of the occlusal surface, 
 
 and is concave in the direc- 
 tion of the root. The buccal 
 
 Mesiolingual <■ I Mesiobuc- j v 1 
 
 Angle iL-^^^^^M I caf Angle and lingual margins are 
 
 rounded, and, unlike the lat- 
 
 Fig. 93.-Upper First Molar, Mesial era J mar gi ns f the buccal 
 
 • Surface. ° 
 
 and lingual surfaces, diverge 
 in the direction of the roots. The cervical margin is slightly concave in 
 the direction of the occlusal surface, and its length is much greater than 
 that of any other margin of the crown. This surface is more extensive 
 than either the buccal, lingual, or distal, and is about equal to that of 
 the occlusal. When the fifth cusp is present it alters the form of the 
 lingual margin of this surface by crossing it near the center, and ex- 
 tending for some little distance on the face of the surface. 
 
 The Distal Surface of the Crown (Fig. 94). — Taken in its entirety, 
 this surface usually presents a general convexity. The lingual half of 
 the surface is usually somewhat more prominent than the buccal, the 
 latter being flattened and frequently slightly concave, particularly near 
 the cervical portion. In some instances the surface is traversed by a 
 continuation of the distolingual groove, which, after passing over the 
 distomarginal ridge, is continued in a longitudinal direction, dividing 
 the surface into two equal parts. Not infrequently this groove, instead 
 of existing as such, is represented as a shallow depression, often extend- 
 ing to the bifurcation of the roots. The margins of the surface are four 
 
UPPER FIRST MOLAR 
 
 J 47 
 
 Lingual 
 Root 
 
 Mesiobuc- 
 cal Root 
 
 Distobuc- 
 cal Root 
 
 in number: the occlusal, which closely resembles the corresponding 
 margin of the mesial surface; the buccal, which is not well defined; the 
 lingual, somewhat angular; and the cervical, formed by the cervical line. 
 
 The Neck of the Tooth. — When looking upon the buccal surface, 
 the constricted portion forming the neck is greatest at a point immediately 
 above the cervical line. Viewed in this direction the crown is usually 
 bell-shaped, and both the crown and the base of the roots assist in pro- 
 ducing the neck. Viewed from a lingual direction, the neck is a dis- 
 tinctive feature, but is seldom so marked as when examined from 
 the opposite side. When studied 
 from either a mesial or a distal 
 aspect, the neck appears above 
 the cervical line, the prominent 
 fold of enamel immediately adja- 
 cent to this line forcing the neck 
 rootward. 
 
 The Roots of the Upper First 
 Molar. — The roots are three in 
 number, two of which are on 
 the buccal side, and are, there- 
 fore, called mesiobuccal and disto- 
 buccal roots, and one on the lingual 
 side, known as the lingual root. 
 These three roots are given off 
 from a common base, which is 
 
 sometimes referred to as the root, while those parts above the point of 
 trifurcation are considered as root-branches. The number, location, 
 and form of the roots of this tooth are, perhaps, more constant than those 
 found in connection with any other cuspidate tooth. The common base 
 from which the roots are given off is similar in contour to the crown of 
 the tooth, excepting in those cases in which the form of the root is carried 
 over this base to meet the neck of the tooth. 
 
 The mesiobuccal root (Fig. 93) is flattened from mesial to distal, broad 
 at its base from buccal to lingual, from which point it gradually tapers 
 to the apex. At the base the mesiodistal measurement is less than one- 
 third that of the buccolingual. In its course it is first inclined to the 
 mesial, but after reaching the center of its length it makes a decided 
 distal curve, which looks almost directly to the distal. The mesial side 
 of this root is decidedly flattened at its base, but as the center of the 
 surface is reached a shallow longitudinal groove is present, which is 
 
 Disto- 
 lingual 
 Cusp 
 
 Fig. 
 
 s 
 
 Distobuc- 
 cal Cusp 
 
 91. 
 
 Distolingual Groove 
 
 -Upper First Molar, Distal 
 Surface. 
 
I48 ANATOMY 
 
 continued to the region of the apex. The distal side is also possessed 
 of a similar groove, which extends throughout its entire length. Both 
 the buccal and lingual sides of the root are smoothly convex, the latter 
 being only about half the width of the former. 
 
 The distobuccal root (Fig. 94) is much the smallest of the three, and, 
 while inclined to flatness on its mesial and distal sides, it is much more 
 rounded than the mesial root. The mesial side is provided with a slight 
 longitudinal groove, and in rare instances a similar groove exists on the 
 distal side. The buccal and lingual sides are similar to those of the 
 mesial root. The root is generally straight, and tapers gradually from 
 base to apex, ending in a rounded point. 
 
 The lingual root (Fig. 94) is usually the largest and longest of the 
 three, and is more rounded in form than either of the buccal roots. The 
 lingual surface is inclined to flatness near its base, and is provided with 
 a well-defined longitudinal groove, which is sometimes independently 
 formed, while at others it is present as a continuation of the lingual 
 groove. This root being the only one given off from the lingual side of 
 the tooth, is constructed with a mesiodistal measurement about equal 
 to that of the base of the crown at this point. From its place of beginning 
 it passes first in a lingual and then in a buccal direction, forming a long 
 curve and ending in a sharp-pointed apex. 
 
 Bilious Type. — The upper first molar of this temperament is mani- 
 fest by a crown with angles well produced, the marginal ridges and 
 cutting-edges of the cusps bold and well marked. The cusps are of 
 medium length, with summits angular and pointed. The developmental 
 grooves are deep and often sulcate, and numerous supplemental grooves 
 are found upon the occlusal surface. The longitudinal and transverse 
 measurements of the crown are about equal, and when viewed upon the 
 occlusal surface, the angular nature of its anatomy is noted as a distinc- 
 tive feature. The neck is fairly well developed, giving a slight bell- 
 shape to the crown. The cervical line is made up of angles rather than 
 curves, and the roots are long and straight. 
 
 Nervous Type. — Like the teeth previously described under this 
 class, the crown of this tooth is of greater longitudinal than transverse 
 extent; the neck is especially well formed, producing a decided bell- 
 shape to the crown. The cusps are long and penetrating, the marginal 
 ridges sharply defined, as are also those ridges upon the central incline 
 of the cusps. The grooves of development are decided and frequently 
 sulcate. The buccal surface is rounded and smooth, with the buccal 
 groove extending well toward the cervical line. The lingual surface 
 
UPPER FIRST MOLAR 149 
 
 also presents a general convexity, and is usually divided by the lingual 
 groove. The mesial surface is convex over its cervical third, and the 
 occlusal margin is a decided convex ridge, serving as a point of contact 
 for the adjoining tooth, and thus forming the characteristic V-shape 
 common to this temperament. It is in this type that the fifth cusp is 
 most frequently present. The cervical line is much curved, and the 
 roots are slim and frail. 
 
 Sanguineous Type. — The crown of the upper first molar of this 
 type usually presents a slightly greater longitudinal than transverse 
 extent. The angles of the crown are poorly formed, being rounded and 
 smooth. The cusps are of moderate length, and are rounded in their 
 nature; the marginal ridges, as well as those ridges of the central incline 
 of the cusps, are less distinct than either of the forms previously described. 
 The buccal and lingual surfaces are convex and seldom broken by grooves; 
 the mesial and distal surfaces are convex in every direction, throwing 
 the point of contact with adjoining teeth near the center of the surface. 
 The roots are inclined to be large and oval in form, while the cervical 
 line is a series of long curves. 
 
 Lymphatic Type. — In this temperament the crown is much less 
 in its longitudinal than transverse measurement. The neck of the tooth 
 is poorly defined, the crown passing into the root-base without a marked 
 constriction. The mesial and distal surfaces are flattened and nearly 
 parallel with each other, providing a broad contact surface. The buccal 
 and lingual surfaces each present a marked general convexity, the latter 
 being frequently broken by the distolingual groove. The tooth is pro- 
 vided with cusps which are short and heavy-set; the marginal ridges, 
 as well as all the ridges common to the occlusal surfaces, are poorly defined. 
 The developmental grooves are shallow and terminate abruptly. There 
 is but little curvature to the cervical line, and the roots are short, heavy- 
 set, and inclined to cluster together. 
 
IS© 
 
 ANATOMY 
 
 UPPER SECOND MOLAR. 
 
 e*~ t& ft F f 
 
 7th year 
 
 8th year 9th year 
 
 10th year 
 
 Fig. 95. 
 
 nth year 12th year 16th year 
 
 Calcification Begins, from Four Centers, about the Fifth Year. 
 Calcification Completed, Sixteenth to Eighteenth Year. 
 Erupted, Twelfth to Fourteenth Year. 
 Average Length of Crown, .28. 
 
 Average Length of Root, .51. 
 
 Average Length Over All, .79. 
 
 Calcification in this tooth takes place in precisely the same manner 
 as that of the first molar, but the formative process is much later in begin- 
 ning, the lime-salts commencing to accumulate in the four separate lobes 
 about the fifth year. At the beginning of the sixth year the formation 
 of the cusps is completed, soon after which they coalesce and the occlusal 
 surface of the crown is established. At the beginning of the eighth 
 year fully two-thirds of the crown is calcified, and the following year 
 the crown and neck are completed and the root-base outlined. By the 
 tenth year the beginning of separate root-development is observed; at 
 the twelfth year, or at the time of eruption, the roots are formed to about 
 one-half of their completed length, the process continuing until the six- 
 teenth or seventeenth year, when calcification is completed and the root 
 apices formed (Fig. 95). In many respects this tooth closely resembles 
 the first molar previously described, the crown presenting the same 
 number of surfaces similarly named, and also being provided with 
 the same number of roots. Notwithstanding this fact, there are a 
 number of ways in which they are at variance. The crown of the sec- 
 ond molar is smaller than that of the first, and the quadrilateral out- 
 line common to the first molar is much compressed and broken in the 
 second. The distal cusps are much smaller proportionately than the 
 mesial cusps, this being particularly true of the distolingual cusp. This 
 
UPPER SECOND MOLAR 
 
 151 
 
 reduction in size of the distal cusps gives to that portion of the occlusal 
 surface a slight distal incline. 
 
 Occlusal Surface of the Crown (Fig. 96). — The general contour 
 of the crown is best studied by viewing it directly upon the occlusal 
 surface; this aspect also shows to best advantage the difference in form 
 between this and the first molar, as shown in figure 90. The mesial 
 and lingual outlines closely resemble the corresponding outlines on 
 the first molar, but the buccal and distal are much at variance, the 
 former passing into the latter without a distinct line of demarcation 
 existing between the two, this gradual blending of one into the other 
 being at the expense of the distobuccal angle of the crown, which is poorly 
 
 Buccal Groove 
 
 Distobuccal Cusp 
 
 Distal Fossa 
 
 Distolingual Groove 
 
 Mesiobuccal 
 Triangular Ridge 
 
 Mesial Groove 
 Mesiomarginal Ridge 
 
 Mesiolingual Cusp 
 
 Distal Incline of 
 Mesiolingual Cusp 
 
 Fig. 96. — Upper Second Molar, Occlusal Surface. 
 
 developed. The crown is much compressed in a distobuccal-mesio- 
 lingual direction, making this measurement of the occlusal surface about 
 one-third less than the mesiobuccal-distolingual measurement. The 
 cusps are much inclined to cluster toward the center of the surface, this 
 being especially true of those on the lingual half. 
 
 Marginal Ridges of the Occlusal Surface (Fig. 96). — Like the occlusal 
 surface of the upper first molar, this surface of the second molar is 
 bounded by four marginal ridges — the mesial, distal, buccal, and lingual. 
 They are usually less marked than those found on the first molar, and 
 are much more variable in their individual anatomy. The mesiomarginal 
 ridge extends from the summit of the mesiobuccal cusp to the summit 
 of the mesiolingual cusp. It is concave in the direction of the body of 
 the crown, and is broken near its central portion by the mesial groove. 
 In some instances one or more small supplemental grooves are found 
 
152 ANATOMY 
 
 to cross it. Compared with the mesiomarginal ridge of the first molar, 
 its length is much less and the convexity not so pronounced. The 
 distomarginal ridge, owing to the variation in form and size of the distal 
 cusps, is difficult to describe definitely; suffice it to say that it extends 
 from the summit of the distobuccal to the summit of the distolingual 
 cusp. The concavity is V-shaped, and is usually crossed near the center 
 by the distolingual groove. In some instances the distolingual cusp 
 is almost wanting, in others the distobuccal is but little developed; when 
 either of these conditions is present, the marginal ridge is extended 
 either to the buccal or to the lingual, in a measure taking the place of 
 the missing cusp. The mesial half of the buccomar ginql ridge closely 
 resembles the corresponding margin of the first molar; beginning at 
 the mesiobuccal angle it ascends to the summit of the mesiobuccal 
 cusp, after which it descends by a longer incline to the buccal groove. 
 The distal half of the ridge, unlike that of the first molar, presents much 
 variety, its form being controlled by the character and position of the 
 distobuccal cusp, usually small. As most frequently observed, it ascends 
 to the summit of the cusp, and in so doing it presents a decided lingual 
 inclination. In passing down the distal incline the lingual inclination 
 is increased and gradually passes into the distomarginal ridge. Branch- 
 ing off from the mesiomarginal ridge, the linguomar ginal ridge ascends 
 to the summit of the mesiolingual cusp and descends by a much shorter 
 incline to the distolingual groove. This portion of the margin is thrown 
 well toward the center of the surface, the location of the cusp carrying 
 it to that point. Like the distal half of the buccal margin, the outline 
 of the distal half of this margin is controlled by the position and form of 
 the distolingual cusp. In the majority of cases, when the cusp is moder- 
 ately strong, the ascent from the distobuccal groove to the summit of the 
 cusp is short and abrupt, the descent being somewhat more gradual, 
 and with a decided buccal inclination it passes into the distomarginal 
 ridge, or ends abruptly at the distal end of the distolingual groove. 
 
 The Cusps and Ridges (Fig. 96). — This tooth is provided with 
 four lobes or cusps, two of which are located on the buccal, and two on 
 the lingual side. They are usually smaller and less angular than the 
 cusps of the first molar. This is particularly true of both the distal 
 cusps, and especially of the distolingual cusp, which is often quite diminu- 
 tive and occasionally entirely wanting. When this latter condition 
 exists, the lingual half of the surface is for the most part occupied by 
 what would otherwise be the mesiolingual cusp, the absence of the 
 distal cusp permitting the distolingual groove to occupy a position near 
 
UPPER SECOND MOLAR 1 53 
 
 the extreme distolingual angle, that portion of the surface which is 
 distal to the groove being a portion of the distomarginal ridge. 
 
 The Mesiobuccal Cusp (Fig. 96). — Like the corresponding cusp of 
 the first molar, this cusp is usually the longest of the four, and in many 
 instances covers a greater extent of surface than any of the others. Its 
 base is outlined by the buccal and mesial grooves, the two together form- 
 ing the mesiobuccal triangular groove. Descending from its summit 
 to the buccal surface is the mesiobuccal ridge; the marginal ridge makes 
 a double descent, one in a mesial and one in a distal direction, while 
 sloping toward the central fossa is the mesiobuccal triangular ridge. 
 The cusp is seldom traversed by supplemental grooves such as are found 
 on the corresponding cusp of the first molar. 
 
 The Distobuccal Cusp (Fig. 96). — As previously stated, this cusp is 
 not constant in its form; in some instances it is bold and well produced, 
 corresponding closely to the mesiobuccal cusp just described. When 
 thus pronounced it is possessed of ridges, and bounded by grooves which 
 are similar to those described in connection with the first molar. More 
 frequently the cusp is much rounded, its summit being carried well 
 toward the center of the surface. When this formation exists, the buccal 
 ridge is absent, the marginal ridges short and rounded; the distobuccal 
 triangular ridge which descends from it toward the center of the crown 
 is short and heavy set. 
 
 The Mesiolingual Cusp (Fig. 96). — In the majority of instances this is 
 the largest cusp, particularly when there is a degenerate tendency in the 
 distolingual cusp. Descending from its summit are a number of ridges, 
 the marginal ridges being given off as already described, the mesiolingual 
 triangular ridge descending the central incline to the central fossa, and 
 when the cusp has an additional mesiodistal extent by the presence of a 
 diminutive distal cusp, other ridges descend in the same direction. The 
 lingual descent of the cusp is smooth and more rounded than the corre- 
 sponding surface of the first molar, and is seldom elevated in the form of a 
 definite ridge. The central outline of this cusp is marked by the mesial, 
 the distal, and the distolingual grooves. 
 
 The Distolingual Cusp (Fig. 96). — In no other cusp do we find such 
 a diversity of form as in the distolingual cusp of the upper second molar. 
 In some instances it is fully as prominent as its neighbor just described, 
 in others appearing as a mere fold of enamel, and it is not uncommon 
 to find it entirely wanting, the distomarginal ridge extending to occupy 
 a portion of the space which it should claim. Deductions might be 
 drawn from an average between these two extremes, wherein the existing 
 
x 54 
 
 ANATOMY 
 
 cusp would be much smaller than any of the others, the summit rounded 
 rather than sharp, but with a decided inclination to occupy the extreme 
 distolingual angle of the surface, in this latter respect differing from the 
 distobuccal cusp. The mesial and buccal outlines of the cusp are formed 
 by the distolingual groove, and its mesiolingual incline contributes to 
 the formation of the distal fossa. 
 
 The Fossae and Grooves of the Occlusal Surface (Fig. 96). — 
 These in name, number, and general form are similar to those of the 
 first molar. The central fossa is never, strictly speaking, in the center 
 of the surface, and is formed by the central incline of the mesiobuccal, 
 distobuccal, and mesiolingual cusps. It is seldom so deep as the central 
 fossa of the first molar. The distal fossa is more or less pronounced, 
 its size and position being controlled by the extent of development in 
 the distolingual cusp. The distolingual groove, which usually crosses 
 the lingual surface of the first molar near its center, is not constant in 
 its location on this tooth, in some cases being near the center, in others 
 near the distolingual angle of the crown. The 
 buccal groove is never constant in its location, 
 usually crossing the buccomarginal ridge and pass- 
 ing over the buccal surface near its mesiodistal 
 center, but it is not uncommon to find it forced to 
 the distal by a diminution in the size of the distal 
 cusp. 
 
 Buccal Surface of the Crown (Fig. 97). — 
 The most constant difference between this and the 
 corresponding surface of the upper first molar is 
 the wandering location of the buccal groove. 
 While in the majority of instances it may be found 
 near the mesiodistal center of the surface, it is not 
 uncommon to find it passing over the distal third, 
 or even as far posterior as the distobuccal angle. 
 In general, the surface is somewhat more convex and necessarily less 
 extensive than the buccal surface of the first molar. The buccal ridges 
 which descend from the summit of the two buccal cusps are seldom so 
 marked as those on the first molar, and in many instances the distal 
 ridge is wanting. The distal half of the surface frequently passes into 
 the distal surface, by a long gradual sweep, there being no line of demar- 
 cation between the two. 
 
 The Lingual Surface of the Crown (Fig. 98). — In keeping with 
 the other surfaces just described, the lingual surface differs from the 
 
 Fig. 97. — Upper 
 Second Molar, Buc- 
 cal Surface. 
 
UPPER SECOND MOLAR 
 
 J 55 
 
 Cervical 
 Line 
 
 Disiomar- 
 ginal?Ridge 
 
 Lingual Groove, ending 
 in Lingual Pit 
 
 Fig. 98. — Upper Second Molar, Lingual 
 Surface. 
 
 corresponding surface of the first molar in that it presents a greater 
 general convexity. This is particularly true in passing from the cervical 
 line to the occlusal surface. The lingual groove is also less constant 
 in its location. In most in- 
 stances it is to be found a 
 little to the distal of the cen- 
 ter, in others being as far 
 posterior as the extreme distal 
 third of the surface, and in 
 rare instances it is entirely 
 wanting. The general char- 
 acter of this surface, which is 
 smooth and convex, is seldom 
 broken by the presence of 
 well-defined ridges, such as 
 are usually found descending 
 from the lingual cusps of the 
 first molar. The mesial, dis- 
 tal, and buccal surfaces, as well as the surfaces under consideration, are 
 proportionately smaller than those of the first molar, and, while this refers 
 to both the transverse and longitudinal measurements, it is particularly 
 applicable to the latter. 
 
 The Mesial Surface of the 
 Crown (Fig. 99). — Aside from 
 this surface being of less extent 
 Mesiobuc- than the corresponding surface of 
 the first molar, there are no other 
 differences of importance. In 
 many instances, however, there 
 is a decided tendency for the 
 surface to be concave from 
 buccal to lingual, the convex 
 distal surface of the first molar 
 closely fitting into this concavity. 
 Another variation which is fre- 
 quently observed is that of the 
 longer and more gradual sweep which it takes in passing into the lingual 
 surface. 
 
 The Distal Surface of the Crown (Fig. 100). — This differs from 
 the distal surface of the first molar principally in its more pronounced 
 
 Mesiolingual 
 Angle 
 
 Mesiobuc- 
 cal Angle 
 
 Fig. 99. — Upper Second Molar, Mesial 
 Surface. 
 
156 
 
 AN \ ni\i\ 
 
 convexity. Its general form is also much influenced by the nature of 
 the two distal cusps. If one or the other of these is sparingly developed, 
 either the buccal or lingual hall" of the surface, as- the case may be, is 
 quickly rounded off to pass into the deficient lobe. 
 
 The Angles of the Crown. — The increased inclination for the 
 crown of this tooth to general convexity dispels, in a measure, the presence 
 of angles, as such, in correspondence to the four corners of the first 
 molar. In some instances the crown is represented as a fairly well- 
 formed quadrilateral, in which case the angles are well defined, but 
 
 usually this outline is so much 
 broken by a mesiodistal com- 
 pression that the angular form 
 of the crown is entirely abolished. 
 But, whatever the form of the 
 crown may be, it is well to ad- 
 here to the commonly accepted 
 term, and speak of that point at 
 which the sides of the crown 
 unite as the angles, each being 
 named in accordance with its 
 location. 
 
 The Neck of the Tooth.— 
 The principal variation between 
 the neck of this tooth and that 
 of the first molar, is that produced by the greater general convexity of 
 the crown, which contributes to the production of a neck much more 
 constricted. There is also a greater variety in the contour of the neck, 
 incident to the variation in the general outline of the crown. 
 
 The Roots of the Upper Second Molar. — These are the same 
 in name and number as those of the first molar — two buccal and one 
 lingual. In many respects they differ from the roots of the first molar. 
 They are much smaller, frequently inclined to cluster together, and are 
 often fused, in some instances, all three being united, in others the union 
 existing between but two. When isolated, each root usually presents a 
 decided distal curve near its apical third. When the crown is flattened 
 from mesial to distal, as before described, the distobuccal root is forced 
 to occupy a position much more to the lingual than that assumed by 
 the mesiobuccal root. The lingual groove seldom passes over the lingual 
 root, as observed on the first molar. 
 
 Fig. ioo. 
 
 -Upper Second Molar, Distal 
 Surface. 
 
UPPER THIRD MOLAR 1 57 
 
 UPPER THIRD MOLAR. 
 
 m w* 
 
 10th year nth year 12th year 14th year 1 8th year 
 
 Fig. ioi. 
 
 Calcification Begins, Ninth Year. 
 Calcification Completed, Eighteenth to Twentieth Year. 
 Erupted, Seventeenth to Twentieth Year. 
 Average Length of Crown, .24. 
 
 Average Length of Root, .44. 
 
 Average Length over All, .68. 
 
 Calcification of this tooth takes place in precisely the same manner 
 as that in the first and second molar, with the exception of the number 
 of lobes, which are sometimes three and sometimes four. The lime- 
 salts begin to accumulate between the eighth and ninth year, and con- 
 tinue with somewhat more activity than that of the first and second molar. 
 Between the ninth and tenth year the three or four cusps, of which the 
 future tooth is to be composed, have coalesced, and by the eleventh year 
 calcification in the crown of the tooth is completed; at the end of the 
 following year the roots, which are variable in number, have made 
 considerable progress; at the fourteenth year they are calcified to about 
 half their length, while at a period between the eighteenth and nineteenth 
 year the formative process is completed (Fig. ioi). This tooth, like 
 the cuspid, is usually fully formed before eruption takes place. 
 
 This tooth is subject to a greater variety of form than any other; 
 in rare instances it is similar in general outline and cusp formation to 
 the first molar, but in a vast majority of cases it is dissimilar, the most 
 constant deviation being its size, which on the average is about one- 
 third less. In the accompanying illustration (Fig. 102) the forms most 
 frequently met with are shown. It will be observed that the contour 
 of the tooth in general is much more rounded than either the first or second 
 molar. The buccal angles of the crown are alone well marked, the 
 
158 
 
 ANATOMY 
 
 mesial and distal surfaces passing into the lingual surface by a long, 
 gradual sweep, and thus obliterating the lingual angles. In many 
 instances the tooth is tritubercular, and is usually made so by the absence 
 or diminutive size of the distolingual cusp. Just as this cusp was inclined 
 to degenerate in the second molar, so we find this retrograde develop- 
 mental tendency increased in the third molar. With this change in the 
 construction of the occlusal surface, there is a corresponding variation 
 in the grooves, ridges, and fossae. 
 
 Fig. 102. — Various Types of Upper Third Molar. 
 
 Mesial Surface of the Crown (Fig. 103). — In many particulars 
 this surface corresponds in form and outline to the mesial surface of the 
 first molar; it is, however, usually much more convex, seldom presenting 
 a concavity or even a positive flatness. The surface is not only rounded 
 from buccal to lingual, but also from the cervical line to its occlusal 
 margin. Thus formed, a point of contact is provided near the center 
 of the surface. The occlusal margin, the buccal margin, and the cervical 
 margin are almost identical to those of the first molar, but in most in- 
 stances the lingual margin is wanting, the surface gradually passing 
 into the lingual without a decided line of demarcation. 
 
 Distal Surface of the Crown (Fig. 104).— This surface is much 
 less extensive in comparison to the size of the crown than the correspond- 
 ing surface of either the first or second molars. It is decidedly rounded 
 
UPPER THIRD MOLAR 
 
 159 
 
 Buccal 
 Root 
 
 Mesiobuc- 
 cal Cusp 
 
 Mesial Groove 
 Fig. 103. — Upper Third Molar, Mesial 
 Surface. 
 
 in every direction and is frequently crossed by the distal developmental 
 groove, and sometimes by one or more supplemental grooves. The 
 general form of the surface is much influenced by the presence or absence 
 of the distolingual cusp; with the 
 former, the surface is more exten- 
 sive, presenting less convexity and 
 resembling more closely the distal 
 surface of the first and second 
 molars; with the latter, the extent Mesiobuc- 
 
 ' ' cal Root 
 
 of the surface is decreased and 
 the convexity increased. 
 
 Buccal Surface of the 
 Crown (Fig. 105). — The mesial 
 portion of this surface is in no 
 way at variance with the mesial 
 portion of the buccal surface of 
 the first or second molar, but 
 much variety of form exists in 
 
 the distal portion. The buccal groove which serves to separate these 
 two portions is located well toward the distal third of the surface, thus 
 reducing the size of the distal portion to about one-third that of the 
 mesial portion. In general, the surface is but little more convex than 
 
 the corresponding surface of the 
 first and second molar. Its mesial 
 border is definitely outlined, as 
 are also the cervical and occlusal 
 margins, but the distal margin 
 cannot be definitely located, the 
 surface tending to pass gradually 
 into the distal surface. Like the 
 distal, the extent of this surface 
 is much regulated by the size and 
 shape of the distobuccal cusp, 
 which, like the distolingual cusp, 
 is inclined to degenerate. 
 Lingual Surface of the Crown (Fig. 106). — Like the distal surface 
 previously described, the form of this surface is much influenced by the 
 presence or absence of the distolingual cusp. When this cusp is wanting 
 or but little developed, the surface presented is decidedly convex and 
 smooth; in many instances the mesiodistal curvature described is almost 
 
 Buccal 
 Groove 
 
 Fig. 104.- 
 
 Mesiolin- 
 gual Cusp 
 
 -Upper Third Molar, Distal 
 Surface. 
 
i6o 
 
 ANATOMY 
 
 a perfect semicircle, and in passing from the cervical line to the occlusal 
 margin the surface is carried well toward the center of the crown by 
 a long gradual sweep toward the lingual. The lingual groove is usually 
 absent. The change in form produced by the presence of the disto- 
 
 lingual cusp is principally 
 noticeable in a less pro- 
 nounced convexity and the 
 presence of the lingual 
 groove, which may be 
 noticed as a slight depres- 
 sion or as a well-defined 
 groove. This groove, when 
 present, is always located 
 near what would represent 
 the distolingual angle of the 
 
 Mesiobuc- 
 cal Root 
 
 Mesiobuc- 
 cal Cusp 
 
 Lingual Root 
 
 Distobuccal 
 Root 
 
 Distal 
 Groove 
 Distal buccal 
 Cusp 
 
 Fig. 105. — Upper Third Molar, Buccal 
 Surface. 
 
 crown; the distolingual cusp 
 seldom if ever being of suf- 
 ficient size to force its location near the center of the surface as in the 
 first molar. In some instances this groove is shown upon the lingual 
 surface when the cusp is not present; in this case the distomarginal 
 ridge represents in a manner the cusp by its bold, heavy development. 
 Occlusal Surface of the Crown (Fig. 107). — When looking directly 
 upon this surface, an opportunity is presented 
 to study the general contour of the crown; the 
 most noticeable difference in this respect be- 
 tween this tooth and the first molar being ob- 
 served in its smaller size, and the absence of 
 well-marked angles. It will be noted that the 
 mesial and buccal outlines in a measure resem- 
 ble the corresponding outlines of the first and 
 second molars, but there is scarcely any similarity 
 existing when comparing the distal and lingual 
 outlines. In some instances the crown is tri- 
 angular (Fig. 103) ; in others the mesiobuccal and 
 bucco-marginal outlines form an obtuse angle, 
 the free ends of which are joined together by a 
 
 long semicircle, the latter constituting the distal and lingual outlines (Fig. 
 103). Again, almost the reverse of this last-mentioned form is seen, the 
 buccal and distal outlines constructing the angle, while the semicircular 
 connection between the two is made up of the mesial and lingual outlines. 
 
 Fig. 106. — Upper 
 Third Molar, Lingual 
 Surface. 
 
UPPER THIRD MOLAR 
 
 161 
 
 The Marginal Ridges. — The mesiomar ginal ridge is usually well 
 defined, and in most instances is crossed near its center by the mesial 
 groove, and frequently by two or more supplemental grooves. This 
 marginal ridge is probably the most constant in form, the numerous 
 variations to which the surface is liable seldom making any material 
 alteration in it. Unlike the ridge above described, the distomar ginal 
 ridge is most variable in its construction, nearly all of the forms charac- 
 teristic of the occlusal surface exerting a controlling influence over it. 
 In rare instances the ridge resembles that of the first and second molars, 
 but this form is most frequently interfered with by the absence or diminu- 
 tive size of the distolingual cusp, the ridge itself frequently supplying the 
 place of the cusp. In many 
 cases the ridge is elevated near 
 its central part by being rein- 
 forced by a portion of the 
 oblique ridge. When the dis- 
 tolingual cusp is wanting, this 
 ridge not infrequently de- 
 scends from the summit of 
 the distobuccal cusp to the 
 distal groove and from this 
 point ascends obliquely to the 
 summit of the lingual cusp. 
 
 The bucco -mar ginal ridge may be described as similar in most respects to 
 the corresponding margin on the first and second molars, the principal vari- 
 ation being in the distal half, which is much shorter and less pronounced. 
 In the linguomar ginal ridge, again, much variety in outline is noticeable. 
 In nearly all instances the ridge is thrown much nearer the center of the 
 body of the crown, and, when the tooth is bicuspid in form, it simply 
 makes a mesial ascent of the lingual cusp, followed by a gradual incline, 
 and passes into the distal ridge, as above noted. When the distolingual 
 cusp is present, the ridge is similar to that upon the first and second 
 molars, with the exception of the distal portion, which is less clearly 
 marked. 
 
 The Cusps (Fig. 107). — As previously stated, the form most frequently 
 met with is tritubercular, two of the cusps being upon the buccal and 
 one upon the lingual half of the surface. 
 
 The Mesiobuccal Cusp (Fig. 107). — This cusp corresponds in nearly 
 every particular to the mesiobuccal cusp of the first and second molars; 
 it is the most constant in size and form of the three. Its summit is usually 
 
 Fig. 107. — Upper Third Molar, Occlusal 
 Surface. 
 
1 62 ANATOMY 
 
 angular, and the numerous ridges which descend from it are well defined 
 and similar in name and number to those of the first molar. 
 
 Distobuccal Cusp (Fig. 107). — The constant inclination to degeneracy 
 in the distal portion of the crown of the tooth is noticeable in this cusp, 
 which is much smaller than the mesiobuccal and scarcely half as large 
 as the corresponding cusp of the first and second molars. In some 
 instances, however, it is inferior only in size, retaining its angularity, 
 being possessed of small but well-defined ridges. 
 
 The Lingual Cusp (Fig. 107). — When the three cusps alone are pres- 
 ent, this one is much the largest, the extent of the surface covered 
 
 being all of the lingual half of the crown. The 
 summit of the cusp, which is thrown well toward 
 the center of the body of the crown, is prominent, 
 but seldom angular. Only in rare instances will 
 there be found a lingual ridge descending there- 
 from, but the central incline is usually marked 
 by a number of wrinkles or folds of enamel re- 
 sembling minute ridges. The central boundary 
 of this cusp is marked by the mesial and distal 
 Fig. 108.— Upper Third developmental grooves. 
 
 Molar, Occlusal Surface, . _^ . ,. 7 „ „. . T . , 
 
 with Distoiingual Cusp The Distoiingual Cusp (Fig. 107).— It is the 
 
 and Distal Fossa Poorly presence or absence of this cusp that contributes 
 
 Denned. * 
 
 most to the variations present in the crown. 
 When present, it is usually diminutive in size, and is without definite 
 form. In many instances nature is apparently attempting to cast it off 
 in precisely the same manner in which she is attempting to add to the 
 first molar by a development of the "fifth cusp," the distobuccal cusp 
 appearing to hang to the distoiingual angle of the crown in a manner 
 very similar to the "fifth cusp." When thus situated, it is separated 
 from the body of the crown by a groove, which cannot be considered as 
 being upon the occlusal surface. When located in its normal position, 
 it has for its inner boundary the distoiingual groove. 
 
 The Fossae and Grooves of the Occlusal Surface. — The great 
 variety and form common to this surface exerts a controlling influence 
 over the size, number, and position of the grooves and fossae. In the 
 tritubercular class the central fossa alone is present. The developmental 
 grooves, with the exception of the buccal, are not definitely outlined, 
 but, descending toward the fossae from the central incline, are numerous 
 small ridges divided from each other by a like number of diminutive 
 supplemental grooves. The distal groove is sometimes well defined, 
 
UPPER THIRD MOLAR 1 63 
 
 and crosses over the oblique ridge, which in this type becomes the disto- 
 marginal ridge. When the distolingual cusp is present, all of the ridges 
 and grooves are more pronounced. In this case the central fossa cor- 
 responds more closely to the central fossa of the other molar teeth, this 
 resemblance increasing just in proportion as the size of the distolingual 
 cusp increases. The distal fossa, in a vast majority of instances, is 
 present as a mere pit; the size of this fossa is likewise much controlled 
 by the extent of development in the distolingual cusp. Where the disto- 
 marginal ridge is supplementary to the distolingual cusp, the distolingual 
 groove lies between the former and oblique ridge. Another peculiarity 
 found only upon the occlusal surface of this tooth is, what appears 
 to be an effort upon the part of the cusps to cluster toward the center. 
 This is common only to those teeth possessing three cusps, and accompany- 
 ing this form the central fossa shows a number of fantastically arranged 
 grooves and ridges which ascend the cusps, passing over the marginal 
 ridges and breaking them into a number of small tubercles. 
 
 Temperamental Types. — The third molar is probably less influenced 
 by the character and habits of the individual than any other tooth in 
 the mouth. The inclination to a general degeneracy is no doubt favored 
 by civilization. With a constant decline in the functional activity, 
 brought about by the present culinary methods common to civilization, 
 this tooth in a measure becomes useless, and nature is gradually mak- 
 ing an effort to cast it off. While there are undoubtedly many indi- 
 viduals possessed of the highest mental attainments with the third 
 molar as fully developed as either the first or the second, this condition 
 is usually confined to those possessed of little intellectuality. If, in 
 general, the temperament of the subject be taken into consideration, 
 the cusp-formation on this tooth will correspond in a relative degree 
 to that on the bicuspids and molars. 
 
CHAPTER IX. 
 
 A Description of the Lower Teeth in Detail. — Calcification, Erup- 
 tion and Average Measurements. — Their Surfaces, Ridges, 
 Fossae, Grooves, Sulci, etc. 
 
 THE LOWER TEETH. 
 
 In most respects the anatomy of the lower teeth is similar to that 
 of the upper, but in each class we find a slight variation existing between 
 the two sets. As compared to the upper incisors, the crowns of the 
 lower incisors are more slender and somewhat more angular in outline. 
 The roots are more slender, proportionately longer, more flattened 
 laterally, and seldom crooked. The crowns of the lower incisors are 
 probably more constant in form than those of any other teeth, seldom 
 varying except in size. The mesiodistal measurement of the crown 
 of the lateral incisor is a trifle greater than that of the central, a con- 
 dition exactly the reverse to that of the upper incisors. The labial 
 and the lingual surfaces of these teeth are smooth, and, with the exception 
 of young teeth, show but little trace of the developmental process by 
 the presence of grooves, fissures, etc. The outline of the lower cuspids 
 is almost identical to that of the corresponding teeth in the superior arch, 
 excepting that they are in every way more slender. The bicuspids are 
 proportionately smaller in every direction than those of the upper jaw, 
 their cusps are smaller, and they are seldom found with more than one 
 root. The crowns of the lower molars are somewhat larger than those 
 of the upper, and are provided with five cusps instead of four,* and they 
 are attached to the alveolus with two, instead of three, roots. In the 
 incisors, cuspids, and bicuspids the process of development is the same 
 as in the corresponding upper teeth, calcification taking place from the 
 same number of centers along the coronal extremities. In the molars, 
 however, development may proceed from five centers instead of four, 
 as in the upper molars. The manner of development, and the period 
 at which this action takes place, so nearly corresponds with that of the 
 upper teeth that the process will not be repeated. 
 
 * The lower first molar has five cusps in ninety per cent, of cases, while in the second 
 five cusps are present in about fifty per cent. 
 
 164 
 
LOWER CENTRAL INCISOR 1 65 
 
 LOWER CENTRAL INCISOR. 
 
 Calcification Begins, First Year after Birth. 
 
 Calcification Completed, about the Tenth Year. 
 Erupted, Seventh to Eighth Year. 
 Average Length of Crown, .34. 
 
 Average Length of Root, .47. 
 
 Average Length over All, .81. 
 
 Like the upper central incisor, this tooth presents for examination 
 four surfaces, a cutting-edge, and various angles, margins, etc. By the 
 union of the labial and lingual surfaces at the cutting-edge the incisive 
 feature is established and the double incline plane common to incisors 
 produced. 
 
 The Labial Surface of the Crown (Fig. 109). — This surface is 
 smooth and convex, its general outline resembling an inverted cone, 
 the base of which is formed by the cutting-edge and the apex by the 
 cervical line. The margins of the surface 
 are, with the exception of the cutting-edge, 
 not so well defined as those of the upper 
 central. Near the cutting-edge the mesial 
 and distal margins pass somewhat abruptly 
 into the respective lateral surfaces, but as 
 the neck of the tooth is approached they are 
 much rounded. The incisive margin 
 squarely cut, and is nearly at right angles 
 with the long axis of the tooth. The cervical 
 margin is fairly well defined, and is deeply 
 concave in the direction of the root. Except 
 in very young teeth, this surface is seldom 
 much broken by the labial grooves; but in 
 certain tvpes one or more transverse ridges 
 
 . . ..... Fig. 109. — Lower Incisor, Right 
 
 may be found occupying the cervical third. side, Labial Surface. 
 
 The mesiodista! diameter at the cutting- 
 edge is about one-third greater than at the cervical line, and, while 
 these measurements are likely to vary in accordance with the tempera- 
 ment of the subject, this variation is not so pronounced as in the upper 
 incisor. The surface is frequently inclined to flatness near the incisive 
 margin, the general convexity becoming more marked as the cervical 
 line is approached. 
 
 The Lingual Surface of the Crown (Fig. no). — In general out- 
 line this surface resembles the labial, with the exception of the cervical 
 
1 66 
 
 ANATOMY 
 
 Lingual Grooves 
 
 Mesiomar- 
 ginal Ridge 
 
 Lingual 
 Fossa 
 
 Fig. iio. — Lower Incisor, Right Side, 
 Lingual Surface, Strongly Developed. 
 
 margin, the lines of which are somewhat more acute. The surface 
 presents a marked concavity from the cutting-edge to the cervical ridge, 
 and also a slight transverse concavity near the incisive margin. All of 
 
 the margins are more definite than 
 those of the labial surface. The 
 mesial and distal margins are formed 
 by the marginal ridges common to 
 these borders, but these ridges are not 
 so well defined as those of the upper 
 incisors. The cervicomarginal ridge 
 is present as a well-rounded band of 
 enamel, but is never a well-defined 
 cingulum, or cuspule. The depres- 
 sion between these marginal ridges is 
 so slight that it can scarcely be re- 
 ferred to as a fossa, although usually 
 characterized as the lingual fossa. 
 The lingual grooves are generally more pronounced than the corres- 
 ponding developmental grooves of the labial surface, but end more 
 or less abruptly before reaching the cervical ridge. The mesiodistal 
 measurements of the surface are a trifle less than the corresponding 
 measurements of the labial surface. 
 
 The Mesial Surface of the Crown (Fig. 
 in). — The outline of this surface is exactly 
 the reverse of the labial and lingual just de- 
 scribed, being a cone, with its base directed 
 downward or in the direction of the root, 
 while its apex is formed by the mesial ex- 
 tremity of the cutting-edge. The cervical 
 margin of the surface, or that represented by 
 the base of the cone, is concave; the labial 
 and lingual margins are rounded over the 
 cervical third and inclined to angularity near 
 the cutting-edge. There is a slight convexity 
 over the entire surface, which is most marked 
 near the center. The lingual half of the 
 cervical portion slopes away to the distal, 
 passing gradually into the lingual surface. By the union of this surface 
 with the cutting-edge and the labial and lingual surfaces, the mesial 
 angle of the crown is formed. This angle is well outlined and reaches 
 
 Cutting-edge 
 
 Cervical 
 Line 
 
 Cervical 
 Ridge 
 
LOWER CENTRAL INCISOR 
 
 167 
 
 Labial 
 Ridge 
 
 Cervical 
 Line 
 
 Fig. 112.- 
 
 -Lower Incisor, Distal 
 Surface. 
 
 out toward the median line, giving to this portion of the crown a promi- 
 nent appearance. Near the cutting-edge the surface presents a slightly 
 rounded prominence, which provides a 
 point of contact with the corresponding 
 tooth of the opposite side. 
 
 The Distal Surface of the Crown 
 (Fig. 112). — In a general way this sur- 
 face closely resembles that of the opposite 
 or mesial side of the crown. Near the 
 cutting-edge the surface is usually more 
 prominent and presents a more marked 
 convexity, and near the cervical margin 
 it is flattened and sometimes slightly con- 
 cave. The union of this surface with the 
 cutting-edge and the labial and lingual 
 surfaces forms the distal angle of the 
 crown, which, like the mesial angle, is 
 square and well defined. The margins of the surface are in no way 
 different from those of the mesial surface. 
 
 The Cutting-edge. — In the young tooth this incisive margin is 
 _ , , „ thin and generally divided into three distinct parts 
 
 Developmental Grooves ° J x 
 
 (Fig. 113) by the developmental grooves, but these 
 disappear so early that they can scarcely be considered 
 in connection with a description of the fully developed 
 tooth; in fact, the cutting-edge of this, as well as that 
 of all the lower incisors, is so susceptible to change by 
 mechanical abrasion that a normal condition is of but 
 short duration. After the disappearance of the 
 primitive cusps, and before further abrasion has taken 
 place, the edge is fairly sharp and placed nearly at 
 right angles with the crown. As in the upper incisors, 
 the cutting-edge is in a line with the long axis of the 
 tooth. The labial margin of the edge is slightly con- 
 vex, while the lingual is irregularly concave to the 
 same extent. 
 
 The Cervical Margin. — This marginal line, 
 which is marked by the extent of the enamel cap, 
 corresponds closely to that of the upper incisors, 
 dipping down with a graceful concavity on the labial and lingual surfaces 
 with a corresponding convexity on the mesial and distal surfaces. The 
 
 Fig. 113. — Young 
 Lower Incisor, with 
 Cutting-edge, show- 
 ing the Lines of 
 Development. 
 
l68 ANATOMY 
 
 prominence of this enamel margin, together with the nature of the 
 curvature, is much influenced by the tooth type. 
 
 The Neck of the Tooth. — A distinctive feature of this tooth is 
 found in the convergence of its mesial and distal surfaces in passing 
 from the cutting-edge rootward, thus producing a neck much constricted 
 from mesial to distal. When examined from either the mesial or distal 
 surface, this feature is scarcely noted, the crown passing into the root 
 with little more than the cervical line as a mark of separation. The 
 labial and lingual portions of the neck are rounded and narrow, while 
 the two lateral sides are flat and broad. 
 
 The Root. — The root of this tooth is usually smaller than that of 
 any other tooth in the mouth. It is much flattened from mesial to distal, 
 while the labial and lingual aspects are rounded and narrow. Besides 
 being flattened and broad, the mesial and distal sides are usually found 
 with a longitudinal depression extending from a point near the base of 
 the root almost to its apex. These surfaces gradually taper from the 
 base to the apex, while the labial and lingual first widen from the base 
 and then gradually taper to the apex. The contour of the root-base 
 is generally reduced at the apical extremity, although in some instances 
 the latter is a rounded point. While the root of the tooth is usually 
 straight, there is sometimes a tendency for the apical third to have a 
 slight distal inclination. 
 
 LOWER LATERAL INCISOR. 
 
 Calcification Begins, First Year after Birth. 
 
 Calcification Completed, Tenth to Eleventh Year. 
 Erupts, Eighth to Ninth Year. 
 Average Length of Root, .35. 
 
 Average Length of Root, .50. 
 
 Average Length over All, .85. 
 
 The crown of this tooth differs from the central incisor in being 
 broader from mesial to distal at the cutting-edge, resulting in a crown 
 more strongly bell-shaped. The cutting-edge, instead of being at right 
 angles to the long axis of the tooth, slopes to the distal at the expense 
 of the distal angle, which is much rounded, while the mesial angle closely 
 resembles the corresponding angle of the central incisors. The labial 
 and mesial surfaces do not differ materially from the corresponding 
 surfaces of the central incisor, excepting that the lingual more frequently 
 shows the lines of development, and the distal is at variance in having 
 that portion which contributes to the formation of the distal angle ex- 
 tended and prominent. The marginal ridges of the lingual surface are 
 
LOWER CUSPID 
 
 169 
 
 probably more definitely outlined than those of the central incisor, and 
 the crown in general presents a stronger appearance. The neck of the 
 tooth is similar to the neck of the central incisor, as is also the root, with 
 the exception of a slight addition to its length. 
 
 LOWER CUSPID. 
 
 Calcification Begins, Third Yeah after Birth. 
 
 Calcification Completed, Twelfth to Thirteenth Year. 
 Erupts, Twelfth to Thirteenth Year. 
 Average Length of Crowx, .40. 
 
 Average Length of Root, .60. 
 
 Average Length over All, i.oo. 
 
 There is probably a greater similarity existing between the upper 
 and lower cuspid than in any other class of teeth in the mouth. Oc- 
 cupying as they do a prominent position in the dental arch, and being 
 called upon to perform the double function of incising and tearing the 
 food, their crowns are strong and heavy-set, and their roots long and 
 firmly anchored in the alveoli. Like the upper cuspid, the crown of the 
 lower is surmounted by a single Labial Grooves 
 
 cusp, from the summit of which 
 descend a mesial and a distal 
 cutting-edge. There is also a 
 labial, lingual, mesial, and distal 
 surface presented for examination. 
 
 The Labial Surface of the 
 Crown (Fig. 114). — The crown of 
 the tooth, being a little longer than 
 that of the upper cuspid, gives to 
 this surface the appearance of be- 
 ing more slender, when in reality 
 there is but little difference in the 
 width of the two teeth. This sur- 
 face is smooth and convex, and, 
 while the labial grooves are usually 
 
 Fig. 114. — Lower CuspiJ, Labial Surface. 
 
 present, they are not so marked as those found upon the corresponding 
 upper tooth. A pronounced feature of the surface is the labial ridge, 
 which extends from the summit of the cusp to the cervical line, provid- 
 ing additional strength to the crown. Aside from this ridge and the labial 
 grooves, the surface is occasionally broken by one or more transverse 
 ridges over the cervical portion. The margins of the surface closely 
 resemble those of the upper cuspid, the incisive and mesial being definite 
 
170 
 
 ANATOMY 
 
 Lingual Grooves 
 
 in character, while the distal is made equally indefinite by the passing 
 
 of the labial into the distal surface by a gentle curve. 
 
 The Lingual Surface of the 
 Crown (Fig. 115). — The ridges and 
 grooves of this surface are far less 
 bold in their character than those 
 of the upper cuspid. The lingual 
 ridge, which divides the surface 
 
 Distal Angle 
 
 'FDistomar- 
 ginal Ridge 
 
 Ridge 
 
 Fig. 11=;. 
 
 -Lower Cuspid, Lingual 
 Surface. 
 
 Lingual 
 Ridge 
 Mesiomar- 
 ginal Ridge 
 
 cervical into two equal parts, extends from 
 the summit of the cusp to the base 
 of the cervical ridge, while the 
 marginal ridges pass rootward from 
 the angles of the crown, and, unit- 
 ing, form the cervical ridge. The 
 slight depressions between the 
 lingual ridge and the marginal 
 ridges correspond to the palatal 
 grooves of the upper cuspid, but 
 in this tooth partake more of the 
 
 nature of fossae. 
 
 The Mesial Surface of the Crown (Fig. 116). — A peculiarity 
 
 found in connection with this surface is the general plane existing between 
 
 the crown and root-surface. In all 
 
 other teeth the mesial and distal 
 
 surfaces are found to bulge some- 
 what beyond the corresponding 
 
 surface of the root, but this surface 
 
 of the lower cuspid is not only 
 
 usually in a direct line with the 
 
 mesial surface of the root, but is 
 
 occasionally inclined to the distal, 
 
 resulting in a crooked or bent ap- 
 pearance to the tooth. In addition 
 
 to this individual peculiarity, the 
 
 surface is flat and passes by a long 
 
 curve to meet the lingual surface. 
 
 The Distal Surface of the Fig. n6. 
 
 Crown (Fig. 117). — This surface is 
 
 somewhat less in extent than the mesial surface, and, in place of being 
 
 flat and in line with the root-surface, that portion near the angle of 
 
 Labial 
 Ridge 
 
 Cervical 
 Ridge 
 
 Lingual 
 Ridge 
 
 Mesiomar- 
 ginal Ridge 
 
 Cervical 
 Line 
 
 -Lower Cuspid, Mesial 
 Surface. 
 
LOWER CUSPID 
 
 171 
 
 Lingual 
 Ridge 
 
 Cervicolin- 
 gual Ridge 
 
 Mesial 
 Groove 
 
 the crown presents a marked convexity, while that near the cervical line 
 is frequently slightly concave. This general form of the surface further 
 assists in producing the distal crook previously referred to. The lingual 
 margin is well defined and somewhat angular, while the surface passes 
 so gradually into the labial that a positive line of demarcation can scarcely 
 be said to exist. 
 
 The Cusp and Cutting-edges. — In most respects these are similar 
 to the corresponding parts of the upper cuspid. The length of the 
 mesial cutting-edge is usually somewhat less than that of the distal, but 
 this difference is seldom so marked 
 as that found in the upper cuspid. 
 The mesial and distal angles of the 
 crown are equally as pronounced 
 as those of the corresponding upper 
 tooth. 
 
 The Neck of the Tooth — 
 This is shown by a fairly well- 
 marked constriction, but the pass- 
 ing of the mesial surface of the 
 crown into the mesial surface of 
 the root is not broken by this cir- 
 cular depression. On account of 
 this latter feature the neck of this 
 tooth is somewhat less pronounced 
 than that of the upper. 
 
 The Root of the Lower Cuspid. — The root of this tooth is some- 
 what shorter and more flattened on its mesial and distal sides than that 
 of the upper cuspid, this lateral flatness frequently amounting to a 
 decided longitudinal depression or groove. As referred to in the descrip- 
 tion of the crown, the mesial side of the root is continuous in a direct 
 line with this surface of the crown, but as the apical third of the root is 
 approached, there is frequently found a slight distal inclination which 
 affects alike both the mesial and distal sides. The labial and lingual 
 surfaces of the root are abruptly convex and taper very gradually from 
 the cervical line to the apex, while the mesial and distal surfaces taper 
 much more rapidly, the four ending in a slender apex usually flattened 
 from mesial to distal. 
 
 Fig. 117. — Lower Cuspid, Distal Surface. 
 
172 
 
 ANATOMY 
 
 THE LOWER BICUSPIDS. 
 
 In many respects these teeth are similar to the bicuspids of the up- 
 per jaw, the chief differences being that they are somewhat shorter and 
 smaller in every respect. Their crowns are much more rounded and 
 the cusps are never so strongly developed. Unlike the upper bicuspids 
 the buccal and lingual cusps are connected by a transverse ridge. The 
 roots are much less flattened from mesial to distal, and are seldom 
 bifurcated. 
 
 Summit of Buccal Cusp 
 
 LOWER FIRST BICUSPID. 
 
 Calcification Begins, about the Fourth Year. 
 
 Calcification Completed, Eleventh to Twelfth Year. 
 Erupted, Tenth to Eleventh Year. 
 Average Length of Root, .30. 
 
 Average Length of Crown, .54. 
 
 Average Length over All, .84. 
 
 In general, the crown of this tooth is much more rounded and 
 smaller in all its measurements than that of the upper bicuspid. The 
 buccal surface presents a much greater convexity, which results in 
 forcing the summit of the buccal cusp well toward the center of the long 
 axis of the tooth. The mesiodistal and buccolingual measurements of 
 
 the crown are nearly equal, and 
 about correspond to the maxi- 
 mum length of the crown. As 
 Buccal Groove [ n t h e U pp er bicuspids, the de- 
 
 Mesial Pit rr r 
 
 velopment of this tooth is similar 
 to that of the incisors and cus- 
 pids, the buccal cusp being de- 
 rived from three lobes, while 
 the lingual results from a single 
 center. 
 
 The Occlusal Surface of the Crown (Fig. 118).— This surface 
 is so unlike that of the corresponding surface of the upper first bicuspid 
 that a separate description without further comparative reference is 
 required. In general outline the form of a rounded triangle is approached, 
 the buccal margin serving as one side of the triangle, while, by the union 
 of the mesial and distal margins to form the lingual, the remaining sides 
 are established. 
 
 The Buccal Cusp. — As previously stated, the summit of this cusp 
 is thrown well toward the center of the surface. Descending from it 
 
 Buccal Ridge 
 
 Distal Pit 
 
 Triangular 
 Ridge 
 
 Lingual Ridge 
 Fig. 118. — Lower First Bicuspid, 
 Occlusal Surface. 
 
THE LOWER BICUSPIDS 
 
 173 
 
 
 Buccal Ridge 
 
 
 Distobuccal 
 Angle 
 
 ■ 1 
 
 Buccal 
 Groove 
 
 Cervical 
 Ridge 
 
 ■ J 
 
 Cervical 
 Line 
 
 Fig. 
 
 119. — Right Lower First 
 cuspid, Buccal Surface. 
 
 Bi- 
 
 are four well-defined ridges — the buccal ridge to the buccal surface, 
 
 the mesial cutting-edge, the distal cutting-edge, and the triangular 
 
 ridge, the latter descending in a lingual 
 
 direction to meet the lingual ridge or 
 
 cusp. This ridge divides the surface 
 
 into two parts, the center of each being 
 
 marked by a well-defined pit — the mesial 
 
 and distal pits. The mesial and distal 
 
 cutting-edges are frequently crossed by 
 
 the buccal grooves, and mark the line 
 
 of union between the central and two 
 
 lateral lobes of the buccal cusp. The 
 
 marginal ridges, one of which begins at 
 
 the mesial angle and the other at the 
 
 distal angle, pass to the lingual, where 
 
 they unite to form the lingual ridge or 
 
 cusp. 
 
 The Lingual Cusp. — This cusp is 
 
 seldom well developed, and corresponds to the cervical ridge of the 
 
 incisors and cuspids. The extent of development in the lobe is ex- 
 tremely variable, in some instances amounting to little more than a 
 
 continuation of the mesio- and disto- 
 marginal ridges, while in others there is 
 a building-up of the enamel in the form 
 of a small tubercle. When this latter 
 condition is present, the triangular ridge 
 of the buccal cusp contributes to its 
 formation. The triangular ridge fre- 
 quently divides into two or more 
 smaller ridges, which usually end in the 
 mesial pit, but in some instances they 
 continue to the lingual, and divide the 
 lingual ridge into two or more smaller 
 tubercles. 
 
 The Buccal Surface of the Crown 
 (Fig. 119). — This surface is smooth and 
 
 Buccal Cusp Triangular 
 Ridge 
 
 Distal Pit 
 
 Fig. 120. — Left Lower First Bi- 
 cuspid, Lingual Surface. 
 
 convex in all directions, and in general 
 outline there is but little variation be- 
 tween it and the corresponding surface of the upper first bicuspid. It 
 is traversed from the point of the cusp to the cervical line with a 
 
174 
 
 ANATOMY 
 
 Lingual 
 
 Cusp or 
 
 Ridge 
 
 Buccal 
 Ridge 
 
 Fig. 
 
 121. — Left Lower First Bicus- 
 pid, Mesial Surface. 
 
 rounded ridge, the buccal ridge, upon either side of which are the 
 buccal grooves. 
 
 Lingual Surface of the Crown (Fig. 120). — This surface is more 
 
 or less extensive in accordance with 
 the character of the lingual lobe. In 
 most instances the measurement from 
 the summit of the cusp or ridge to the 
 cervical line is about one-half that of 
 the same measurements on the buccal 
 surface. From mesial to distal a well- 
 rounded convexity is present, while 
 from the occlusal margin to the 
 cervical line it is straight or only 
 slightly convex. The surface passes 
 so gradually into the mesial and distal 
 surfaces that no definite lateral mar- 
 gins exist. 
 
 The Mesial Surface of the 
 Crown (Fig. 121). — In the region of 
 the occlusal margin this surface is prominent, with a marked convexity 
 from buccal to lingual; but as the cervical margin is approached, the 
 surface recedes to the distal, and is 
 flattened or is possessed of a slight 
 general convexity. The occlusal and 
 cervical margins alone are well de- 
 fined, the buccal being gracefully 
 rounded, while the surface passes to 
 the lingual with a long curve. 
 
 The Distal Surface of the 
 Crown (Fig. 122). — There is but 
 little difference between this and the 
 mesial surface; the occlusal portion of 
 the surface is somewhat less promi- 
 nent, resulting in less of the bell- 
 shaped appearance to this side of the 
 crown. 
 
 The Neck of the Tooth.— The 
 neck of this tooth is marked by a well-defined constriction, the enamel 
 of the crown suddenly folding in to meet the cementum of the root at 
 the cervical line, forming a band or ridge which completely encircles the 
 
 Triangular Ridge 
 
 Distal Pit 
 
 Lingual 
 Ridge 
 
 Cervical 
 Ridge 
 
 Fig, 
 
 122. — Right Lower First Bi- 
 cuspid, Distal Surface. 
 
THE LOWER BICUSPIDS 
 
 J 75 
 
 tooth. The amount of constriction appears to be evenly distributed 
 between the various parts, so that, viewed in all directions, the neck 
 becomes a distinctive feature of the tooth. 
 
 The Root of the Lower First Bicuspid.— The root of this tooth 
 is usually straight and tapers gradually from base to apex. In rare 
 instances it is bifurcated, and when thus formed, those portions beyond 
 the point of separation are more or less crooked. In the single root the 
 apical third often curves slightly to the distal. The buccal and lingual 
 sides are convex throughout their entire length, while the mesial and 
 distal may be slightly convex, flattened, or provided with a slight longi- 
 tudinal concavity. In passing from buccal to lingual the mesial and 
 distal sides converge, thus resulting in a narrowing of the lingual side 
 of the root. 
 
 Buccal Cusp 
 
 LOWER SECOND BICUSPID. 
 
 Calcification Begins, Between the Fourth and Fifth Year. 
 Calcification Completed, Eleventh to Twelfth Year. 
 Erupts, Eleventh to Twelfth Year. 
 Average Length of Crown, .31. 
 
 Average Length of Root, .56. 
 
 Average Length Over All, .87. 
 
 In general contour this tooth is similar to the lower first bicuspid, 
 excepting that the crown is somewhat more rounded and the lingual 
 cusp more fully developed, this latter feature causing it to closely resemble 
 the upper bicuspids. The crown is frequently a trifle shorter than that 
 of the lower first bicuspid, but the length 
 of the root generally exceeds that of the 
 latter, making this the longer tooth of Triangular 
 the two. Rldge 
 
 The Occlusal Surface of the 
 Crown (Fig. 123). — The occlusal sur- Distal Pit 
 face of this tooth presents a greater 
 variety in form than any other tooth of 
 its class. The general outline of the 
 surface is that of a broken circle, in 
 most instances the mesial and distal 
 margins showing almost as much of a 
 convexity as that of the buccal and lingual. The summit of the 
 buccal cusp usually extends well toward the center of the surface, 
 but it is sometimes forced toward the buccal by an increased develop- 
 ment in the lingual cusp. The buccal grooves, which cross the mesial 
 
 Lingual Cusp 
 
 Fig. 123. — Lower Second Bicuspid, 
 Occlusal Surface. 
 
176 
 
 ANATOMY 
 
 as those 
 
 of the first bicuspid 
 
 
 Buccal Ridge 
 
 
 Distobuccal 
 Angle 
 
 H M 
 
 Groove 
 
 Cervical 
 Ridge 
 
 m jm 
 
 Cervical 
 Line 
 
 Fig. 
 
 124. — Right Lower Second Bi- 
 cuspid, Buccal Surface. 
 
 and distal cutting-edges of the buccal cusp, are seldom so well defined 
 
 but they occasionally pass over these 
 marginal ridges and form well-marked 
 grooves, which end in the mesial and 
 distal pits. The triangular ridge of the 
 buccal cusp is usually more prominent 
 than in the first bicuspid, and divides 
 the surface into two portions, which are 
 about equal in extent, the center of each 
 portion being provided with a small pit 
 — the mesial and distal pits. As in the 
 first bicuspid, the mesio- and disto- 
 marginal ridges begin at the mesial and 
 distal angles of the crown, pass to the 
 lingual, and, uniting, form the lingual 
 ridge or cusp. The lingual cusp, while 
 generally well developed, is never so 
 prominent as the buccal. The lingual 
 
 lobe is sometimes divided by a groove which passes from buccal to 
 
 lingual, thus forming three cusps upon the surface. When this latter 
 
 condition is present, the mesial and distal grooves are fully outlined 
 
 from the mesio- and disto-marginal Buccal Grooves 
 
 ridges to the center of the surface, where ^^^^^m- 
 
 they unite with the groove previously 
 
 referred to and form a central pit or °^f Rfj£; 
 
 fossa. Another form frequently met 
 
 with is one in which the surface closely 
 
 resembles that of the upper bicuspids, 
 
 two well-defined cusps being present, 
 
 separated from each other by a central 
 
 groove, which passes from mesial to 
 
 distal and joins the triangular grooves 
 
 at these joints. The resemblance to the 
 
 upper bicuspids is further increased by 
 
 the presence of two small pits, one on 
 
 the mesial and one on the distal half of 
 
 the surface. 
 
 The Buccal Surface of the Crown (Fig. 124).— The principal 
 
 variation between this and the buccal surface of the lower first bicuspid 
 
 is that it is less extensive and the buccal grooves somewhat less defined. 
 
 Summit 
 of Lin- 
 gual Cusp 
 
 Fig. 125. — Lower Second Bicuspid, 
 Lingual Surface. 
 
THE LOWER BISUCPIDS 
 
 177 
 
 Lingual Cusp 
 
 Buccal Cusp 
 Buccal Ridge 
 
 FlG. 126. 
 
 — Lower Second Bicuspid, 
 Mesial Surface. 
 
 It presents a general convexity, which is most pronounced near the 
 center, between which point and the occlusal margins it is slightly inclined 
 to flatness. The summit of the buccal cusp is usually to the mesial of 
 the center of the occlusal mar- Triangular Ridge 
 
 gin, so that the mesial cutting- 
 edge is considerably longer than 
 the distal, this fact also resulting 
 in forcing the buccal ridge to the 
 mesial of the center of the sur- 
 face. The mesial angle of the 
 crown, as observed when looking 
 directly upon the buccal surface, 
 is in a direct line with the mesial 
 side of the root, while the distal 
 angle extends beyond this cor- 
 responding line, and gives a 
 prominent or bulging appearance 
 to this section of the crown. 
 
 The Lingual Surface of the Crown (Fig. 125). — Proportionately, 
 this surface is more extensive than the corresponding surface of the first 
 bicuspid, this increase being produced by the additional development 
 of the lingual cusp. It is well rounded from mesial to distal, and passes 
 
 into these surfaces without the existence 
 of a positive line of separation. From 
 the cervical line to the occlusal margin a 
 slight convexity is present. The general 
 outline of the surface is much influenced 
 by the conditions present upon the oc- 
 clusal surface. 
 
 The Mesial Surface of the Crown 
 (Fig. 126). — In the region of the occlusal 
 margins this surface is decidedly convex 
 from buccal to lingual, but in passing 
 toward the cervical line a gradual flat- 
 ness is apparent, which, however, seldom 
 amounts to a perfect plane. While 
 there is a gradual convergence of this 
 and the distal surface toward the root, it is not so marked as that of 
 the first bicuspid, resulting in less of the bell-shaped appearance to 
 the crown. 
 
 Buccal Groove 
 
 Lingual 
 Cusp 
 
 FlG. 127. 
 
 — Lower Second Bicuspid, 
 Distal Surface. 
 
178 ANATOMY 
 
 The Distal Surface of the Crown (Fig. 127). — The description 
 given of the mesial surface applies equally well to this, there being but 
 slight variation existing between the two. Occasionally this surface 
 will present a greater convexity in the region of the occlusal margin, but 
 this is not a constant feature. 
 
 The Neck of the Tooth. — The crown of the tooth being some- 
 what smaller, and the root proportionately larger and longer than that 
 of the first bicuspid, results in diminishing the amount of constriction 
 at the neck, and for that reason this feature is less definite. 
 
 The Root of the Tooth. — As previously stated, the root of this 
 tooth is larger and longer than that of the first bicuspid. The mesial 
 and distal sides are flattened and frequently provided with a longitudinal 
 groove. In some instances it is rather blunt, ending in a heavy, rounded 
 apex; in others it tapers very gradually from the base to the apex, ending 
 in a slim, pointed extremity. 
 
 THE LOWER MOLARS. 
 
 THE LOWER FIRST MOLAR. 
 
 Calcification Begins, about One Month before Birth. 
 Calcification Completed, Ninth to Tenth Year. 
 Erupts, Sixth to Seventh Year. 
 Average Length of Crown, .30. 
 
 Average Length of Root, .52. 
 
 Average Length over All, .82. 
 
 The process of development in this tooth corresponds to that of 
 the upper first molar, calcification beginning upon the various cusps 
 as early as the eighth fetal month, the crown being completely calcified 
 by the fifth year, the roots formed and the root apices established by 
 the eleventh year. There is, however, one important difference between 
 the development of this tooth and the corresponding upper tooth; that 
 of calcification taking place usually from five centers instead of four, and, 
 as a result, we find the occlusal surface provided with five well-developed 
 cusps separated from one another by five developmental grooves. When 
 compared with the upper first molar, the crown of this tooth is found to 
 be somewhat less in size; in general outline it is subject to a greater varia- 
 tion and is much more angular in its nature. The mediodistal measure- 
 ment of the crown is nearly always greater than the buccolingual, and 
 the length of the crown from the occlusal margins to the cervical line is 
 proportionately less than that of the corresponding upper molar. 
 
 The Occlusal Surface of the Crown (Fig. 128). — The general 
 outline of the crown is best studied when looking directly upon this 
 
THE LOWER MOLARS 
 
 179 
 
 Buccal 
 Cusp 
 
 Distobuc- 
 cal Cusp 
 
 Distal 
 Groove 
 
 surface. Two principal varieties exist: one in which the sides or mar- 
 gins of the surface appear to be flattened or straightened out, and the 
 other when these same margins are gracefully rounded. In either 
 form the buccal line is the longest, so that the mesial and distal lines 
 converge to meet the lingual. This common form gives to the buccal 
 angles an acute character, while the lingual angles are about equally 
 obtuse. The surface is divided into five distinct or developmental 
 portions, each of which is surmounted by a cusp, named, as their location 
 indicates, mesiobuccal, buccal, distobuccal, mesiolingual, and disto- 
 lingual. Separating these parts are 
 five developmental grooves — the 
 mesial, the distal, the buccal, the 
 lingual, and the distobuccal. The 
 four former cross the marginal 
 ridges from the various surfaces 
 and end in the central fossa, while DlstalPlt j 
 the latter passes from the disto- 
 buccal angle and joins the distal 
 groove, their union being marked 
 by a slight depression or pit — the 
 distal pit. Branching off from the 
 various grooves are a number of supplemental grooves, the presence of 
 which results in the production of a number of smaller ridges. 
 
 The Marginal Ridges of the Occlusal Surface. — Properly speaking 
 these are only two in number, the mesiomarginal ridge and the disto- 
 marginal ridge. Those margins which correspond to the buccal and 
 lingual ridges of the upper molars are so broken by the various cusps 
 and developmental grooves that a definite marginal ridge scarcely exists, 
 as will be observed by the description of these parts. 
 
 The mesiomarginal ridge is strongly outlined, passing from the 
 mesiobuccal to the mesiolingual angle of the crown in the form of a bold 
 angular ridge. In some instances it is broken near the center by the 
 mesial groove passing over it to reach the mesial surface, in others being 
 further divided by numerous small supplemental grooves. 
 
 The distomar ginal ridge is much shorter and less decided than the 
 mesial, and extends from the distobuccal to the distolingual angle. 
 In nearly every instance it is broken by the distal groove, which crosses 
 it to reach the distal surface. 
 
 The buccomar ginal ridge is formed by the various ridges which 
 descend in a mesial or distal direction from the three buccal cusps. 
 
 Disto- Lingual Mesio- 
 lingual Groove lingual 
 Cusp Cusp 
 
 Fig. 128. — Lower First Molar, Occlusal 
 Surface. 
 
l8o ANATOMY 
 
 Near the center the margin is broken by the buccal groove, and it is again 
 broken at its distal third by the distobuccal groove, both of which pass 
 over it to reach the buccal surface of the crown. This is much the 
 longest margin of the surface, and in its entirety presents a gradual 
 buccal convexity. 
 
 The linguomarginal ridge is principally made up of the distal incline 
 from the mesial cusp, and by the mesial incline from the distal cusp. 
 Near the center it is broken by the lingual groove, which passes over it 
 to reach the lingual surface. This margin, unlike the buccal, is not 
 always convex, but in many instances is almost a straight line, extending 
 from the mesial to the distal angle. 
 
 The Cusps (Fig. 128). The Mesiobuccal Cusp (Fig. 128). — This 
 is usually the largest, though not always the longest, cusp of the group. 
 It is bounded by the mesial and buccal surfaces and by the mesial and 
 buccal grooves, which together form the mesiobuccal triangular groove. 
 Descending from the summit of this cusp to the distal is a well-defined 
 ridge — a part of the buccomarginal ridge — while in a mesial and lingual 
 direction the descending ridges contribute to both the bucco- and mesio- 
 marginal ridges. Descending toward the center of the surface and 
 ending in the central fossa is the mesiobuccal triangular ridge. 
 
 The Buccal Cusp (Fig. 128). — This cusp, which is placed a little 
 to the distal of the center of the buccal surface, is separated from the 
 mesiobuccal cusp by the buccal groove, and from the distobuccal cusp 
 by the distobuccal groove. It is about one-half the size of the mesio- 
 buccal cusp, and a trifle less in length. Descending from it are two ridges, 
 one in a mesial and one in a distal direction, which form a portion of the 
 buccomarginal ridge; descending to the buccal surface is the buccal 
 ridge, while the central incline gives place to a fourth ridge — the bucco- 
 triangular ridge. 
 
 The Distobuccal Cusp (Fig. 128). — This cusp is much the smallest 
 of the five, and is located at the distobuccal portion of the crown, in 
 some instances being nearest the buccal surface, in others forced to the 
 distal by an increase in the size of the buccal cusp. It is separated from 
 the buccal cusp by the distobuccal groove, and from the distolihgual cusp 
 by the distal groove. The ridges which descend from it contribute to 
 both the bucco- and disto-marginal ridges, and descending toward the 
 distal pit is the distobuccal triangular ridge. 
 
 The Mesiolingual Cusp (Fig. 128). — This cusp is second in size, 
 and frequently the longest and most pointed. It has for its boundaries 
 the mesial and lingual surfaces, and the mesial and lingual grooves. 
 
THE LOWER MOLARS 
 
 I8l 
 
 The ridge which descends from it in a mesiobuccal direction assists in 
 forming the mesiomarginal ridge, while that which passes to the distal 
 forms a part of the linguomarginal ridge. In the direction of the central 
 fossa a pronounced ridge is present — the mesiolingual triangular ridge 
 — which is often supplemented by one or more smaller ridges running in 
 the same direction. 
 
 The Distolingual Cusp (Fig. 128). — This cusp usually occupies the 
 distolingual portion of the crown, although sometimes being forced well 
 
 toward the lingual by the Buccal Groove 
 
 distobuccal cusp. It is 
 separated from the mesio- 
 lingual cusp by the lingual 
 groove, and from the dis- 
 tobuccal by the distal 
 groove. Two of the ridges 
 which descend from it as- 
 sist in fo r ming the linguo- 
 and disto-marginal ridges, 
 while the one which de- 
 scends the central incline is 
 the distolingual triangular 
 ridge. The central inclines 
 of the mesiobuccal, buccal, 
 mesiolingual, and disto- 
 lingual cusps contribute to 
 
 the formation of the central fossa, while the buccal, distobuccal, and 
 distolingual central inclines assist in forming the distal pit or fossa. 
 
 The Buccal Surface of the Crown (Fig. 129). — This is the most 
 extensive of the lateral surfaces of the crown. It is convex from mesial 
 to distal, and also from the occlusal margin to the cervical line. The 
 width of the crown from the mesial to the distal angle is always somewhat 
 greater than that at the cervical line, the difference being governed! by 
 the typal form of the tooth. A little to the mesial of the center of the 
 surface is the buccal groove, which, after crossing the buccomarginal 
 ridge, is usually quite deep; but as it proceeds in the direction of the 
 root it gradually disappears, or it may end abruptly in a well-defined pit 
 — the buccal pit. The distobuccal groove enters the surface near the 
 distobuccal angle, and gradually becomes less pronounced as it passes 
 rootward. It is seldom so well defined as the buccal groove, and usually 
 ends when about half-way to the cervical line. The occlusal margin 
 
 Fig. 129. — Lower First Molar, Buccal 
 Surface. 
 
182 
 
 ANATOMY 
 
 Distoliiigual Mesiolingual 
 Cusp Cusp 
 
 Fig. 130. — Lower First Molar, 
 Lingual Surface. 
 
 is made irregular by the presence of the three buccal cusps; the cervical 
 margin is nearly straight from mesial to distal, and is surmounted through- 
 out by a strong enamel fold, the cervicobuccal ridge. The mesial margin 
 
 is longer than the distal, but neither of 
 them is well defined. 
 
 The Lingual Surface of the Crown 
 (Fig. 130). — This surface is smooth and 
 convex in every direction. It is gener- 
 ally divided into two portions, a mesial 
 and a distal, which are nearly equal in 
 extent. This separation is formed by 
 the lingual groove, which is sometimes 
 deep and sulcate, at others shallow, and 
 not infrequently entirely wanting. The 
 surface is nearly one-third less in extent 
 than the buccal, the convergence of the 
 mesial and distal surfaces in passing to 
 the lingual accounting for this difference. 
 The occlusal margin is formed by the 
 double incline of the two lingual cusps; the cervical margin is either 
 straight or slightly concave in the direction of the occlusal surface, 
 while the mesial and distal margins are rounded and poorly defined. 
 
 The Mesial Surface of the Crown (Fig. 131). — This surface is 
 inclined to flatness, with a slight 
 bulging near the center, which marks 
 the point of contact with the approxi- M esiobuc- 1 
 mate tooth. It is usually smooth, and 
 unbroken by developmental or other 
 grooves, although the mesial groove 
 occasionally traverses it after crossing 
 the marginal ridge from the occlusal 
 surface. Near the center of the cer- 
 vical third a slight concavity is often 
 present. The margins of the surface 
 are somewhat irregular, the occlusal 
 margin being made irregularly concave 
 by the ridges which descend from the 
 two mesial cusps; the cervical margin is slightly concave in the direction of 
 the occlusal surface, and, while the buccal margin inclines to the lingual as 
 the occlusal surface is approached, the lingual is almost perpendicular. 
 
 Central Fossa 
 
 cal Cusp 
 Cervico- 
 buccal 
 Ridge 
 
 Fig. 131.- 
 
 -Lower First Molar, Mesial 
 Surface. 
 
THE LOWER MOLARS 
 
 183 
 
 Fig. 
 
 The Distal Surface of the Crown (Fig. 132). — Unlike the mesial, 
 this surface is possessed of a decided convexity in every direction. It 
 is surmounted by a portion of the distobuccal and distolingual cusps,, 
 and is frequently broken by the distal groove, which reaches it after 
 crossing the marginal ridge from the occlusal surface. The occlusal 
 margin is irregularly formed of the marginal ridges which descend from 
 the distobuccal and distolingual 
 cusps; the cervical margin is 
 usually straight, while the buc- 
 cal and lingual are rounded and 
 indefinite. 
 
 The Neck of the Tooth.— 
 One characteristic feature of 
 this tooth is the greater circum- 
 ference of the crown at the oc- 
 clusal margin over that at the 
 cervical line, giving a flaring 
 appearance to the crown, and 
 resulting in the production of a 
 neck which is much constricted. 
 This is particularly noticeable 
 when looking upon the buccal surface of the tooth; but when looking 
 upon the mesial or the distal surface, this feature is not so pronounced, 
 although the rather heavy fold of enamel which surmounts the cervical 
 line contributes much to the formation of the neck from these aspects. 
 
 The Roots of the Tooth. —The roots of this tooth are two in num- 
 ber — one of which is placed beneath the mesial, and the other beneath 
 the distal half of the crown — and are named the mesial root and the 
 distal root. The fact that the point of bifurcation is constantly in close 
 proximity to the neck or crown of the tooth is a sufficient reason for 
 the statement that two roots exist, rather than a single root with two 
 branches. The roots are both much flattened from mesial to distal, 
 and broad at the base from buccal to lingual. 
 
 The mesial root is usually the larger and longer of the two. x\fter 
 leaving its base it generally inclines to the mesial, but beyond the center 
 of its length it is provided with a distal turn, which in some instances 
 amounts to a decided crook. The center of the mesial side is occupied 
 by a longitudinal depression, as is also the distal side, making this part 
 of the root thin, giving the appearance of an effort to bifurcate, which 
 condition is occasionally present. The buccal and lingual sides of the 
 
 -Lower First Molar, Distal 
 Surface. 
 
1 84 
 
 ANATOMY 
 
 root are rounded and smooth, and taper gradually to the apex, which 
 is somewhat broadened from buccal to lingual. 
 
 The distal root is usually straight, with a more gradual taper through- 
 out, ending in an apical extremity more pointed than that of the mesial 
 root. A longitudinal depression is also present upon both the mesial 
 and distal sides, but is never so pronounced as that upon the mesial root. 
 The buccal and lingual sides are convex and smooth. The root possesses 
 little or no inclination to bifurcate. 
 
 LOWER SECOND MOLAR. 
 
 Calcification Begins, about the Fifth Year. 
 
 Calcification Completed, Sixteenth to Seventeenth Year. 
 Erupts, Twelfth to Sixteenih Year. 
 Average Length of Crown, .27. 
 
 Average Length of Root, .50. 
 
 Average Length over All, .78. 
 
 This molar differs in so many particulars from the lower first molar 
 that a separate description will be called for. The principal variation 
 is usually found in the absence of the fifth lobe or cusp,* resulting in the 
 production of an occlusal surface much less complicated. 
 
 The Occlusal Surface of the Crown (Fig. 133). — When the crown 
 is studied by looking directly upon this surface, the variations between 
 
 this and the first molar are 
 readily noted. Four equally 
 proportioned cusps are observed, 
 separated from each other by 
 four developmental grooves. A 
 single pit or fossa is present, the 
 four grooves arising from this 
 one point. In general outline 
 two principal varieties exist: one 
 in which the opposite sides of 
 the crown are nearly of the same 
 length, and parallel with each 
 other, with the angles rounded; 
 the other, in which either the 
 buccal or lingual margin is the longest, with the mesial and distal margins 
 converging one way or the other, as the case may be. The marginal 
 ridges are formed in a manner similar to those of the first molar, with 
 the exception of the distal portion of the buccal ridge, which is not 
 
 15 £ 6 rtJK 
 
 Fig. 133. — Lower Second Molar, Occlusal 
 Surface. 
 
 * When five cusps are present, the anatomy of this surface does not differ from that of the 
 first molar. 
 
THE LOWER MOLARS 
 
 185 
 
 Distobuccal Buccal Mesiobuccal 
 Cusp Groove Cusp 
 
 Fig. 134. — Lower Second Molar, 
 Buccal Surface. 
 
 broken by a developmental groove. Each marginal ridge is divided 
 near its center by one of the grooves of development, the mesial groove 
 crossing the mesiomaiginal ridge, the buccal groove crossing the bucco- 
 marginal ridge, the lingual groove 
 crossing the lingual ridge, and the 
 distal groove passing over the distal 
 ridge. In many instances numerous 
 supplemental grooves are present, 
 which in turn form a number of 
 smaller ridges. The four cusps are 
 the mesiobuccal, distobuccal, mesio- 
 lingual, and distolingual. In a gen- 
 eral way they are similar to the cusps 
 of the first lower molar, excepting 
 that they are somewhat larger and 
 probably less pointed and less angu- 
 lar. Each cusp is provided with a 
 number of ridges, which descend from 
 
 the summit to the base, two of these contributing to the formation of 
 the marginal ridges, one passing to the buccal or lingual, and one, the 
 d triangular ridge, descending the cen- 
 
 tral incline of each cusp. The names 
 given to these various ridges are iden- 
 tical with those of the first molar. 
 
 The Buccal Surface of the 
 Crown (Fig. 134). — The principal 
 difference between this and the cor- 
 responding surface of the first molar 
 is that produced by the absence of the 
 fifth cusp, the surface being divided 
 into two parts instead of three. The 
 single division is caused by the buccal 
 groove, which reaches the surface after 
 crossing the buccomarginal ridge 
 from the occlusal surface. The posi- 
 tion of this groove is usually a little 
 to the mesial of the center of the 
 surface. Like the buccal groove of the first molar, it may disappear 
 gradually as it passes toward the cervical line, or it may end in a well- 
 marked pit — the buccal pit. 
 
 00 O. EL, ac O. 
 
 c 3 - .E 3 
 
 Fig. 135. — Lower Second 
 Molar, Lingual Surface. 
 
i86 
 
 ANATOMY 
 
 The Lingual Surface of the Crown (Fig. 135). — This surface 
 so closely resembles the corresponding surface of the first molar that 
 it is somewhat difficult to distinguish one from the other. The occlusal 
 
 
 Mesiobuc- 
 cal Angle 
 
 Lingual 
 Groove 
 
 Fig. 136. — Lower Second Molar, Mesial Surface. 
 
 margin may be a trifle less irregular, and in some instances more ex- 
 tensive, than the buccal surface, this latter feature seldom occurring in 
 the first molar. 
 
 Mesial Groove 
 
 Buccal 
 Groove 
 
 Fig. 137. — Lower Second Molar, Distal Surface. 
 
 The Mesial Surface of the Crown (Fig. 136). — This surface cor- 
 responds to the mesial surface of the first molar, being flattened or 
 slightly convex from buccal to lingual, with an inclination to a slight 
 depression or concavity near the cervical margin. 
 
THE LOWER MOLARS 
 
 l8 7 
 
 The Distal Surface of the Crown (Fig. 137). — On account of 
 the absence of the fifth cusp, this surface is less complex than that of 
 the first molar. It is convex in all directions; in most instances smooth, 
 in others broken by the distal groove, which reaches it after crossing 
 the distomarginal ridge from the occlusal surface. 
 
 The Roots of the Tooth. — Like the first molar, these are two in 
 number, a mesial and a distal. They are much less constant in form, 
 are often nearer together, and in some instances united. When the 
 two roots exist — which may be considered the normal condition — they 
 are less flattened upon their mesial and distal sides, with the longitudinal 
 depression wanting or but slightly apparent. These roots, therefore, 
 are more rounded in general, taper more gradually from neck to apex, 
 and end in a rounded apex, this often being provided with a slight distal 
 curve. 
 
 LOWER THIRD MOLAR. 
 
 Calcification Begins, Eighth to Ninth Year. 
 
 Calcification Completed, Eighteenth Year. 
 Erupts, Sixteenth to Twentieth Year. 
 Average Length of Crown, .26. 
 
 Average Length of Roots, .36. 
 
 Average Length over All, .62. 
 
 This tooth is probably subject to a greater variety in form than 
 any other. There are, however, two varieties which are most frequently 
 
 met with. In one the crown of the 
 tooth is similar to the lower second 
 molar, being provided with four 
 cusps, which are separated from one 
 another by four developmental 
 grooves (Fig. 138). The other is 
 to the lower first molar, 
 
 Disto- 
 
 linguai similar 
 
 Groove 
 
 Bucco- having five cusps and five develop- 
 
 mar- 
 
 ginai mental grooves. 
 
 Ridge ° 
 
 While these two forms are those 
 most commonly met with, the oc- 
 clusal surface may be so broken by 
 numerous supplemental and devel- 
 opmental grooves that even six or 
 eight well-defined cusps may be 
 present. Whatever complications may exist upon the occlusal surface, 
 a central fossa is usually present, from which radiate the various develop- 
 
 Disto- 
 mar- 
 ginal 
 Ridge 
 
 Mesio- 
 mar- 
 ginal 
 
 Ridge 
 
 mo 
 Fig. 138. — Lower Third Molar, 
 Occlusal Surface. 
 
158 ANATOMY 
 
 mental grooves. When the central fossa is absent, the space which it 
 should occupy is usually taken up by a rounded cusp, by the interference 
 of which the grooves are prevented from uniting, and their course is 
 much distorted. Along with these variations, the tooth is subject to 
 much variety in size. In some instances the crown is one-third less 
 in circumference than that of either the first or second lower molars, 
 while in others it is a trifle greater. The increase in the size of the crown 
 is generally accompanied by an increase in the number of cusps. One 
 feature very common to the crown is its inclination to the circular form, 
 
 Disto- Disto- Mesio- 
 
 lingual buccal buccal 
 
 Cusp Cusp Cusp 
 
 Buccal 
 Groove 
 
 Fig. 139. — Lower Third Molar, 
 Buccal Surface. 
 
 Fig. 140. — Lower Third Molar, 
 Lingual Surface. 
 
 almost resulting in the absence of the angles common to molars in general. 
 The marginal ridges are, of course, subject to the ever-varying con- 
 ditions to be found upon the occlusal surface; in general, they are poorly 
 defined, and are frequently crossed by numerous small supplemental 
 grooves, dividing them into many minute tubercles. The latter are 
 smooth and strongly convex, with their general outlines much influenced 
 by the number of cusps. ^ 
 
 The Roots of the Tooth. — While this tooth is strongly inclined 
 to be two-rooted, like the other lower molars, this condition is by no 
 means the common one. Like the crown, the roots are probably more 
 variable than those of any other tooth. A single conic root may be 
 
THE LOWER MOLARS 189 
 
 present, or a mesial and a distal root may exist; again, the mesial root 
 may bifurcate, thus resulting in three. In some instances, four, or even 
 five, branches may be given off from a common base. When more 
 
 Fig. 141.— Types of Lower Third Molars. 
 
 than two roots are present, they are usually much twisted or crooked, 
 and, while generally inclined to the distal, are liable to branch in various 
 directions. 
 
 A better idea of the variations in this tooth may be had from the 
 accompanying illustration (Fig. 141). 
 
CHAPTER X. 
 
 The Pulp-cavities of the Teeth. 
 
 In the preceding chapters the study of the teeth has been confined 
 to their external forms; it will now be necessary to learn something of 
 their internal anatomy, and for this purpose various dissections of each 
 individual tooth must be made. 
 
 Dissections. — First, a longitudinal dissection of each tooth should 
 be made by sawing or filing from labial to lingual in the anterior teeth, 
 or from buccal to lingual in the posterior teeth. Second, a longitudinal 
 dissection by sawing from mesial to distal. Third, numerous transverse 
 dissections by sawing through the crown or root at various points. 
 
 These dissections will expose to view a central cavity with outlines 
 closely corresponding to those of the tooth itself. This is called the 
 pulp-cavity, and in the vital tooth contains the formative and life-sustain- 
 ing substance of the dentin, the dental-pulp. The pulp-cavity is divided 
 into two principal parts, that portion within the crown of the tooth being 
 the pulp-chamber, while that traversing the root is the pulp-canal. At 
 the apex of the root the canal ends in a small foramen, the apical fora- 
 men* which transmits the blood-vessels and nerves to the pulp. The 
 pulp-chamber occupies the center of the crown and is always a single 
 cavity; the pulp-canals are prolongations from this central cavity, and 
 are usually one for each root, although in some instances two or more 
 canals are present in a single root. The form of the pulp-chamber 
 varies with the shape of the crown, the outline of the cutting-edge in the 
 incisor teeth being leproduced in that part of the chamber nearest to the 
 cutting-edge, while in the bicuspids and molars the occlusal surface is 
 reproduced on the wall of the pulp-chamber, immediately beneath it, 
 the lateral walls corresponding to the various sides of the tooth. In 
 the incisors and cuspids the pulp-chamber passes so gradually into the 
 pulp-canal that a positive line of demarcation between the two is not 
 observed. In the bicuspids and molars the canals may be readily 
 distinguished by a sudden constriction and branching out of the cavity 
 into the various roots, which prolongations gradually decrease in size 
 
 * In many instances there is more than one foramen. 
 
 190 
 
THE PULP-CAVITIES OF THE UPPER TEETH 191 
 
 until the apical foramen is reached. The size of the pulp-cavity is much 
 influenced by the age of the tooth, its functional activity, character of 
 the occlusion, etc. The tooth-pulp, as the formative organ of the dentin, 
 gradually decreases in size as the tooth develops (see Development of 
 the Teeth), and as a result of this action the youngest teeth are provided 
 with the largest pulp-cavities. At the time of eruption of a tooth, the 
 diameter of the pulp-cavity is about equal to one-half the diameter of 
 the crown, while the length of the canal must, of necessity, accord with 
 the extent of root-calcification. As the growth of the tooth proceeds, 
 the diameter of both the chamber and canal is gradually diminished; 
 this gradual reduction in size is continued during the life of the tooth, 
 and if permitted to proceed until old age, the chamber and canal may 
 become almost or entirely obliterated. It must be remembered that 
 while the diameter of the root-canal is diminished with the growth of 
 the tooth, its length increases, continuing to do so until the time of com- 
 plete root-calcification. During the period of root-development the 
 diameter of the root-canal is greatest at the free or apical end of the root, 
 at -which point it presents a funnel-shaped opening (Fig. 142). As the 
 root continues to calcify, this funnel-shaped extremity of the canal 
 advances in the direction of calcification, and finally, as the formative 
 process nears completion, the mouth of the funnel gradually disappears, 
 and the apical foramen is established. The various lobes of the teeth 
 are penetrated by a prolongation of the pulp-cavity, these being called 
 the horns of the pulp-chamber. The depth to which the horn penetrates 
 the lobe varies in accordance with the form of the latter. If the tooth 
 is one provided with long, penetrating cusps, the horns of the pulp- 
 chamber will also be long, but if the cusps be poorly formed, the horns 
 of the chamber will be short. In the anterior teeth, when the lobal 
 construction is outlined by well-marked developmental grooves, the 
 horns of the pulp-chamber will be three in number and directed toward 
 the cutting-edge. These are most marked in young teeth, and gradually 
 disappear as age advances. The functional activity of the teeth also 
 serves to materially reduce the size of the pulp-chamber. Thus, when 
 opposing teeth occlude squarely and firmly against each other, with more 
 or less rubbing or sliding during mastication, the external surface is 
 prone to rapid abrasion, and, as a direct result of this, external change, 
 the pulp-chamber undergoes a corresponding alteration by a growth 
 of secondary dentin about its walls. 
 
I92 ANATOMY 
 
 THE PULP-CAVITIES OF THE UPPER TEETH. 
 
 Upper Central Incisor. — Figure 142 represents a number of la- 
 biolingual sections presenting the relative size and shape of the pulp- 
 cavity in the upper central incisor at various ages. In No. 1 the con- 
 dition existing at about the sixth year, or at a time immediately prior 
 to the eruption of the tooth, is shown. The tooth-crown is fully formed 
 and calcified; the cervical line may be observed, as well as a small portion 
 of the root-wall. The pulp-chamber, which is represented by the dark 
 
 Fig. 142. — The Pulp-cavity in the Upper Central Incisor, from the Sixth to 
 the Tenth Year. 
 
 portion of the cut, occupies about one-third of the diameter of the crown 
 at its greatest width. The pulp-chamber at this age, when viewed 
 in this direction, forms almost a perfect cone, the base of which is directed 
 upward or toward the future extremity of the root, and its apex down- 
 ward in the direction of the cutting-edge of the tooth. The apex of the 
 cone may end somewhat abruptly, or it may be lengthened into a slender, 
 horn-like projection, extending well toward the cutting-edge. No. 2 
 represents the same tooth about the seventh year, or at a time shortly 
 after its eruption. The pulp-chamber has become slightly reduced in 
 its basal diameter, while but little change has taken place in the apex. 
 That portion of the pulp-cavity above the cervical line represents a part 
 of the future pulp-canal. At this age the canal is a direct continuation 
 of the conic pulp-chamber, ending above in a broad, funnel-shaped 
 extremity. No. 3 shows the condition of the cavity about the eighth year. 
 The diameter of the pulp-chamber is considerably diminished, the apex 
 has slightly receded, and the horn-like projection has partly disappeared. 
 The increase in the length of the canal is about 3/16 of an inch over its 
 length at seven years. The two parallel sides of the canal have lengthened 
 
THE PULP-CAVITIES OF THE UPPER TEETH 1 93 
 
 proportionately, and the funnel-shaped extremity is reduced in diameter 
 owing to the gradual narrowing of the roots-wall. No. 4 gives the 
 relative size of the pulp-chamber and canal at the ninth year, or at a 
 time when root-calcification is nearing completion. The decrease in 
 the capacity of the chamber is readily apparent; the horn-like projection 
 has disappeared and the parallel sides of the canal are partly extended 
 into the chamber, thus reducing the length of the cone. In the canal a 
 greater reduction has taken place in its diameter, while its length has 
 increased about 1/4 of an inch over that at eight years, and the diameter 
 of the funnel-shaped opening is but little greater than that of the body 
 
 Fig. 143. — Pulp-cavity in the Upper Central Incisor. 
 
 of the canal. No. 5, which represents a section of the tooth about the tenth 
 year, shows calcification in the root completed, and the apical foramen 
 established. A glance at the illustration will show the gradual decrease 
 in the capacity of the pulp-cavity and the completion of its growth in an 
 apical direction. At this stage of development the fan-shaped extremity 
 of the canal gradually disappears, and for the first time in the life of the 
 tooth the canal partakes of the external root form throughout its entire 
 extent. 
 
 Figure 143 A, represents the size and form of the average pulp- 
 cavity in the adult upper central incisor. In its entirety it represents a 
 double cone, with a common base near the cervical line, the pulp-cavity 
 forming one cone and the pulp-canal the other. At this common base 
 the cavity assumes its largest diameter, which measurement is approxi- 
 mately equal to one-fourth the labiolingual diameter of the tooth. The 
 extent and "form of the lower cone, or that represented by the pulp- 
 chamber, varies in the adult tooth with the tooth type. Thus, in the 
 nervous type the cone is long and narrow, with the apex ending in a hair- 
 
194 ANATOMY 
 
 like projection. In the tooth of the lymphatic temperament the cone 
 is prone to be wide, with its apex ending abruptly. In the sanguine and 
 bilious types the form and extent of the cone do not partake of either 
 of the foregoing extremes, but, in keeping with the outline of the crowns, 
 are intermediate between them. Figure 143 B, represents the average 
 condition of the pulp-cavity in the central incisor in advanced age, and 
 shows a general reduction in the size of both chamber and canal. A 
 further study of the pulp-chamber and canal may be made by a mesio- 
 distal section made through the long axis of the tooth (Fig. 143 C). 
 The outline of the cavity, viewed in this way, closely follows the outline 
 of the crown and root of the tooth. There is no distinct division between 
 the chamber and the canal, the former gradually blending into the latter. 
 The outline of the entire cavity is that of a single cone, with its base 
 directed toward the cutting-edge and its apex in the direction of the 
 apical extremity of the root. The lower margin of the pulp-chamber, 
 or that nearest the cutting-edge of the crown, is broad from mesial to 
 distal and thin from labial to lingual. This margin in the average adult 
 tooth is about on a line with the center of the labial surface of the crown, 
 and the lateral walls of the cavity as they pass upward converge slightly, 
 and finally blend into the walls of the canal, at a point somewhat beyond 
 
 Fig. 144. — Transverse Sections, Root of Upper Central Incisors, Slightly 
 
 Enlarged. 
 
 the cervical line. During the early life of the tooth the margin of the 
 chamber nearest the cutting-edge presents three well-defined horns, 
 corresponding to the three rudimentary lobes found upon the cutting- 
 edge at this period. These horns rapidly disappear, and are seldom 
 found after the fifteenth year. In certain tooth types, however, the 
 mesial and distal horns may continue present until adult age, and even 
 into mipMle life, but when this occurs it is not the result of the temporary 
 tooth form, but is occasioned by the permanent angular outline of the 
 crown. 
 
 Figure 144 represents a number of transverse sections of an upper 
 central incisor, showing the outline and relative size of the pulp-cavity 
 
THE PULP-CAVITIES OF THE UPPER TEETH 195 
 
 in passing from the base of the crown toward the apex of the root. No. i 
 shows the outline of the cavity at the cervical line; No. 2 represents the 
 condition 1/8 of an inch nearer the apex of the root; No. 3 is from the 
 center of the root length, while No. 4 is from the region of the apex. 
 
 Upper Lateral Incisor. — The pulp-cavity in the upper lateral 
 incisor is so nearly identical with that of the central that it will only be 
 necessary to call attention to one or two points which are at variance. 
 Figure 145 shows the five stages as represented by the growth of the 
 tooth. In general it will be observed that the cavity is much smaller 
 than that of the central incisor, but this difference is to be accounted for 
 
 Fig. 145. — Pulp-cavity in the Upper Lateral Incisor, from the Sixth to the 
 
 Tenth Year. 
 
 in the smaller proportions of the tooth. No. 1 shows the condition of 
 the crown and pulp-cavity about the sixth year, the pulp-cavity occupying 
 a large portion of the partly clacified tooth-crown. No. 2 represents 
 the conditions present at the seventh year, or about the time of the 
 eruption of the tooth. The pulp-chamber at this age resembles a perfect 
 cone, the base of which reaches to the root-walls, and faintly outlines 
 the beginning of the future pulp-canal. In No. 3, at eight years, the 
 length of the root has increased about 3/16 of an inch, and the parallel 
 sides of the walls of the pulp-canal have made their appearance. In No. 
 4, at nine years, by the growth of the root the canal has considerably 
 increased in length and at the same time much decreased in diameter, 
 while in No. 5, at ten years, the root is completely formed, the apical 
 foramen established, and the maximum size of the entire pulp-cavity 
 in the fully formed tooth shown. 
 
 Figure 146, A, shows the average condition of the pulp-cavity in 
 the upper lateral incisor at adult age, while figure 146, B, represents 
 the same tooth in old age. In a mesiodistal section — figure 146, C — a 
 very close resemblance to the pulp-cavity in the central incisor will be 
 
196 ANATOMY 
 
 noticed. While the pulp-cavity is smaller than that of the central 
 incisor, it is usually a trifle larger in proportion to the size of the tooth. 
 Owing to the marked constriction at the neck of this tooth, there is 
 occasionally found a slight line of distinction between the pulp-chamber 
 and canal, but in the majority of instances this is not to be observed. 
 The horns of the pulp-chamber are in every respect similar to those 
 of the central incisor, excepting when they exist permanently, in which 
 case the mesial horn is usually the longest. By the transverse sections 
 shown in figure 146, the gradual decrease in size and change in form in 
 the root-canal are presented, the sections being similar to those made 
 in the root of the central incisor. 
 
 Fig. 146. 
 
 Upper Cuspid. — The pulp-cavity of this tooth is in general similar 
 to that of the incisors, excepting that the coronal extremity of the chamber 
 is conic and inclined to a horn-like projection which penetrates the single 
 cusp of the tooth-crown in the direction of its summit. Figure 147 
 represents a number of labiopalatal sections. No. 1 shows the condi- 
 tion of the pulp-cavity about the seventh year, or fully five years before 
 the eruption of the tooth. The pulp-chamber partakes of the cone shape 
 previously referred to, but the margins, instead of being straight lines, 
 are somewhat bowed or concave, thus conforming more closely to the 
 outline of the crown. The central horn of the chamber is proportionately 
 
THE PULP-CAVITIES OF THE UPPER TEETH 197 
 
 longer than that of the incisors, in correspondence with the cusp of the 
 tooth. At this age the formative process has barely extended to the 
 root-walls; therefore, the width of the cavity is about equal to its length. 
 In No. 2, at eight years, an increase in the capacity of the chamber over 
 that of the incisors is shown, this being the result of the greater bulk in 
 the. tooth-crown. The cone-like outline of the chamber is somewhat 
 broken by an effort of its margins to follow the outline of the crown. 
 In No. 3, at nine years, the principal change has taken place in the 
 canal, which has lengthened fully 3/16 of an inch, and the funnel-shaped 
 extremity, instead of joining with the pulp-chamber direct, is continued 
 below by two parallel walls to the true beginning of this cavity. At ten 
 
 ■ V? .^fl ■ i UF ■ l» ■■■':- 
 
 
 
 
 B \m YWLM IJr 
 
 
 
 mk A B jtm WW A 
 
 W ■ \^i 
 
 HfJ 
 
 
 B\ JB 
 
 Hk JR 
 
 Fig. 147. — Pulp-cavity in the Upper Cuspid, from the Seventh to the Twelfth 
 
 Year. 
 
 years, No. 4, a more marked transformation has taken place in both 
 portions of the cavity. The diameter of the chamber at the cervical line 
 has diminished, as has also the length of the cone. The increase in the 
 length of the root, which has'been proportionately greater than that of 
 the preceding year, has extended the length of the canal about 3/8 of 
 an inch. The walls of the canal are no longer parallel with each other, 
 but are inclined to follow the root-outlines. The funnel-shaped opening 
 is much reduced both in length and breadth. No. 5 represents the 
 condition at the time of the eruption of the tooth, or about the twelfth 
 year. The general outline of the pulp-cavity is that of a double cone, 
 with a common base at a point nearly corresponding to the cervical line. 
 The diameter in both the chamber and canal has considerably decreased, 
 while the central horn in the former has further receded. The calcifica- 
 tion of the root externally is about complete and the foramen formed. 
 In this particular the cuspid tooth differs from the incisors, and in fact 
 from all other teeth, in having its root-calcification about completed and 
 
198 
 
 ANATOMY 
 
 the apical foramen established at or soon after the time of its eruption. 
 Figure 148, A, gives an idea of the capacity of the pulp-cavity in the upper 
 cuspid at maturity, while figure 148, B, shows the condition in advanced 
 age. Figure 148, C, is a mediodistal section of a matured upper cuspid. 
 The coronal extremity of the pulp-chamber is square, and but little 
 inclined to follow the outline of the mesial and distal cutting-edges. 
 The chamber passes into the canal without a mark of separation, and 
 the latter gradually diminishes in diameter as the apex of the root is 
 
 D 
 
 G 
 
 Fig. it 
 
 approached. At its point of beginning the canal is sometimes inclined 
 to flatness from mesial to distal, but in passing toward the apex this 
 tendency disappears, and it becomes more circular in outline. In figure 
 148, D, a transverse section through the tooth at the cervical line is 
 shown, giving an idea of the proportionate size and form of the canal in 
 the adult tooth, while E, F, and G represent transverse sections through 
 the root of the same tooth at various points between the cervical line 
 and the apex of the root. 
 
 Upper First Bicuspid. — The study of the pulp-cavity in this tooth 
 differs in many particulars from that of the incisors and cuspids. First, 
 the line of distinction between pulp-chamber and the root-canal or 
 
THE PULP-CAVITIES OF THE UPPER TEETH 1 99 
 
 canals is, in most instances, definitely marked by the bifurcation of 
 the roots and a corresponding branching of the pulp-cavity into two 
 fine canals, one of which occupies the center of each root. This division 
 of the cavity brings, the center of the pulp-chamber almost on a level 
 with the cervical line. In figure 149, No. 1 shows the partly calcified 
 crown of the upper first bicuspid at the seventh year. A portion of the 
 pulp-chamber alone may be studied at this period, and this is found to 
 be somewhat irregular in outline, with a broadened, funnel-shaped open- 
 ing above, and two small, cone-like projections below, pointing into 
 either cusp of the crown. These latter projections are the horns of the 
 pulp-chamber, and are named in accordance with the cusp which they 
 
 Fie 149. — Pulp-cavities of the Upper First Bicuspid, from the Seventh to the 
 
 Twelfth Year. 
 
 occupy. In very young teeth it is not unusual to find these horns pene- 
 trating the dentin almost to the enamel-wall. No. 2, at eight years, 
 shows the crown fully calcined and the outline of the base of the roots 
 established. The horns of the pulp-chamber have slightly receded, and 
 the branching of the canals is made manifest by the central deposit of 
 dentin. In No. 3, at nine years, the capacity of the pulp-chamber is 
 much decreased, and appears to have receded bodily rootward. The 
 roots are calcified to about one-third their full length, and the canals 
 which traverse them are each provided with the funnel-shaped opening 
 at their free calcifying extremities. In No. 4, at ten years, the decrease 
 in the size of the pulp-chamber is not only caused by the deposit of 
 dentin upon the occlusal and lateral walls, but from the direction of the 
 roots as well. The diameter of the root canals is much less than at nine 
 years, but the walls are as yet parallel. No. 5 shows the roots fully 
 formed and the apical foramina established, which condition occurs 
 about the twelfth year. The horns of the pulp-chamber have receded 
 
200 
 
 ANATOMY 
 
 somewhat, and the center of this cavity is now almost on a level with 
 the cervical line. The canals have assumed the form of the roots them- 
 selves, and their diameter is much diminished. The illustration shows 
 the proportionate maximum size of the chamber and canals in this tooth 
 after completion of surface calcification. It will be observed that the 
 foramina are proportionately smaller than those of the incisors and 
 cuspids at a corresponding period, this condition resulting from the 
 lesser diameter of the roots. 
 
 Figure 150, A, illustrates the approximate size and form of the pulp- 
 
 i) 
 
 E F G 
 
 Fig. 150. — Pulp-cavities of the Upper First Bicuspid, Enlarged about One-third. 
 
 chamber and canals at adult age, and attention is called to the appear- 
 ance of the horns of the pulp-chamber. It will be observed that the 
 horn which penetrates the buccal cusp is larger and more pointed than 
 that directed toward the lingual cusp; this condition is fully explained 
 by the buccal cusp being proportionately larger and longer than the 
 lingual. In the same figure, B represents the pulp-cavity in the first 
 upper bicuspid at advanced age. The foregoing description applies 
 only to the two-rooted bicuspids, but as many of these teeth have but 
 one root, an additional description will be necessary. When a single 
 root is present, many varieties in the outline of the pulp-cavity will 
 be presented; this variation, however, seldom affects the capacity or 
 
THE PULP-CAVITIES OF THE UPPER TEETH 201 
 
 form of the pulp-chamber. Two distinct canals may exist in the single 
 root (Fig. 150, C), branching off from the chamber, one from the buccal and 
 one from the lingual portion. These canals gradually taper in the direc- 
 tion of the apex of the root, and may end in a single foramen, or in distinct 
 foramina. Occasionally the canals will unite before reaching the root- 
 apex and continue as a single canal ending in a single foramen, or they may 
 communicate at one point and again diverge and finally end in separate 
 foramina. In some instances the pulp-canal appears to be a direct 
 continuation of the pulp-chamber, extending throughout the length of 
 the root in the form of a flattened canal, with its greatest diameter from 
 buccal to lingual (Fig. 150, D). When two separate canals exist in the 
 single root, the outward appearance of the root indicates a near approach 
 to two roots; when the single flattened canal is present, the root is also 
 flattened and shows no sign of bifurcation. Reference has been made 
 to the horns of the pulp-chamber, and in this connection it will be well 
 to speak of the extent to which they may exist. In that type of tooth 
 provided with long penetrating cusps the horns will dip well down into 
 the cusp occasionally to the full depth of the dentin, and in rare instances 
 may penetrate the enamel. In those teeth lacking in cusp-formation 
 the length of the horns will be correspondingly reduced, and may be 
 entirely wanting. In the two-rooted upper first bicuspid the floor of 
 the pulp-chamber, or that part of the cavity directed rootward, is promi- 
 nent and rounded in the center, from which point it gradually slopes 
 toward the entrances to the canals, one of which arises from the extreme 
 buccal margin, and the other from the extreme lingual margin. Figure 
 150, E, represents a transverse section of the two roots immediately 
 below the point of bifurcation. F represents a section of the two roots 
 midway between the cervical line and the apical extremity, while G is 
 a transverse section of D at the cervical line. 
 
 Upper Second Bicuspid. — The pulp-cavity of this tooth is in 
 many respects similar to that of the first bicuspid, the principal variations 
 being in the horns of the chamber, which are proportionately smaller 
 in correspondence with the diminution in cusp-formation. There is 
 usually no positive line of demarcation between the chamber and canal, 
 the latter being quite large, and broad from buccal to lingual. The 
 extent of the pulp-chamber is sometimes well defined by the presence of 
 two root-canals, similar to those described in connection with the first 
 bicuspid. In rare instances the tooth may possess two roots, each of 
 which would be traversed by a canal. Figure 151 represents the various 
 stages of the development of the pulp-cavity, as shown by a longitudinal 
 
202 
 
 \\ \ lu\n 
 
 section from buccal to lingual. No. i shows the condition at seven 
 years, or at a time when a portion of the crown only is calcified, in con- 
 sequence of which the pulp-chamber alone can be studied at this period. 
 
 Fig. 151. — Pulp-cavities in the Upper Second Bicuspid, from the Seventh to 
 the Twelfth Year. 
 
 The buccal and lingual horns of the chamber may be observed penetrat- 
 ing the dentin in the direction of their respective cusps. No. 2 shows 
 the advance made in the formative process by the eighth year, or at a 
 
 Fig. 
 
 152. 
 
 A B 
 
 -Section of Upper Second Bicuspid, Slightly Enlarged. 
 
 time immediately prior to the eruption of the tooth; the outline of the 
 chamber is completed and the walls of the future pulp-canal faintly 
 outlined. At this period there has been but little change in the horns 
 
THE PULP-CAVITIES OF THE UPPER TEETH 203 
 
 of the pulp-cavity. No. 3 shows the condition of the tooth at the ninth 
 year, or at the beginning of its eruptive period. The diameter of the 
 chamber has somewhat decreased, the horns have slightly receded, the 
 funnel-shaped extremity of the cavity has advanced beyond the cervical 
 line, and is now confined to the canal alone. No. 4 represents the 
 condition of the pulp-cavity about the tenth year. A gradual decrease 
 in the diameter of both the chamber and canal is observed, and the horns 
 of the pulp-cavity are growing less prominent. The length of the root 
 having increased nearly one-quarter of an inch, we find a corresponding 
 addition to the length of the canal. No. 5 shows the maximum size of the 
 pulp-cavity in the upper second bicuspid, which condition accompanies 
 the completion of the external calcification at the twelfth year. As 
 previously stated, the cavity, in its entirety, presents no line of separation 
 between the chamber and canal, but gradually tapers from its broadened 
 base in the crown to its ending at the apex of the root. In this tooth, 
 as well as in all those previously described, the apical foramen at the 
 time of completion of root-calcification is comparatively large, and 
 readily penetrated during operations upon it. Figure 152 illustrates 
 a number of sections of an upper second bicuspid at maturity. The 
 same figure also represents two transverse sections, A being at the cervical 
 line, B midway between the cervical line and apex of the root. 
 
 PULP-CAVITIES OF THE UPPER MOLARS. 
 
 The inner anatomy of the molar teeth being much more complicated 
 than any of those previously described, it will be found necessary to make 
 a number of dissections in various directions in order to obtain a com- 
 prehensive idea of the location and form of the different parts of the 
 pulp-cavity. The line of demarcation between the pulp-chamber and 
 canals is always definite, the former occupying a central position in the 
 crown and seldom extending beyond the cervical line, while the latter 
 are given off from the floor of the chamber and penetrate the various 
 roots, their entrances being marked by small funnel-shaped openings in 
 the floor of the chamber. In the matured tooth the form of the chamber 
 usually corresponds to that of the crown of the tooth. The lateral walls 
 of the chamber are four in number, and are named according to their 
 location — mesial, distal, buccal, and lingual. The average thickness 
 of these walls at maturity is about equal to the diameter of the pulp- 
 chamber. In that type of tooth common to the lymphatic temperament 
 where there is but little constriction at the neck, resulting in the various 
 
204 
 
 ANATOMY 
 
 sides of the tooth-crown being nearly parallel with each other, the pulp- 
 chamber is nearly quadrilateral in form; but in those teeth marked by a 
 decided constriction at the neck, most marked in the nervous tempera- 
 ment, the extent of surface covered by the floor of the chamber is much 
 less than that occupied by the occlusal portion. In the former class, 
 the entrances to the various canals are much farther apart than in the 
 latter. The occluding wall is usually much thicker than the lateral 
 walls, and is penetrated by the horns of the pulp-chamber, one of which 
 extends into each cusp. As in the bicuspids, the extent to which the 
 horns penetrate the cusps is controlled by the prominence of the latter. 
 
 The floor of the pulp-chamber is irregu- 
 c larly rounded, being high in the corner 
 and gradually falling away in the direc- 
 tion of the canals. The entrances to the 
 B root-canals, three in number, are placed 
 in the form of an irregular triangle, called 
 the molar triangle. The mesial side of 
 the triangle is usually the longest, the distal 
 next in length, and the buccal the shortest. 
 In young teeth the entrances to the canals 
 are usually in the from of funnel-shaped 
 openings, are comparatively easy of access, 
 bat after maturity may disappear and be but little larger than the canals 
 themselves. To properly study the position occupied by the entrance 
 to the canals on the floor of the pulp-chamber, a transverse section of 
 the tooth should be made at a point somewhat above the cervical line, 
 at the same time preserving both the crown and the roots of the tooth 
 for comparison. The entrance to the lingual canal, which is usually 
 the largest and most readily accessible, may be located by a line drawn 
 through the center of the occlusal surface of the crown (Fig. 153) from 
 buccal to lingual, A, and by another line drawn from mesial to distal 
 almost parallel with the linguomarginal ridge, passing through the 
 summits of the mesiolingual and distolingual cusps, B; the point at which 
 these two lines intersect will mark the approximate location of the lingual 
 canal. The entrance to the mesiobuccal canal may be located by a line 
 drawn from the inner side of the mesiobuccal angle to a corresponding 
 position near the distobuccal angle, C. This should be intersected 
 by a line drawn from the summit of the mesiobuccal cusp to the summit 
 of the mesiolingual cusp, D, the point at which these two lines cross 
 marking the entrance to the mesiobuccal canal. The location of the 
 
 Fig. 
 
THE PULP-CAVITIES OF THE UPPER TEETH 205 
 
 entrance to the distobuccal canal is found by the line, C, which is inter- 
 sected by another line, E, drawn from the summit of the distobuccal 
 cusp to a corresponding point on the distolingual cusp. The nearer 
 the tooth-crown approaches to the quadrilateral, the nearer will the 
 molar triangle approach the equilateral. 
 
 Upper First Molar. — In the dissection of this tooth, the pulp-cham- 
 ber and two of the root-canals only can be shown, but these will be 
 sufficient to pursue the study with intelligence. Figure 154 shows a 
 number of longitudinal sections, made in such a manner as to expose 
 the Ungual canal, usually the largest, and the mesiobuccal canal. No. 1 
 illustrates the approximate size and form of the pulp-chamber at the 
 
 Fig. 154. — Pulp-cavity of Upper First Molar, from the Fifth to the Ninth 
 Year. Lingual and Mesiobuccal Canals. 
 
 fifth year. At this period the chamber occupies a large proportion of 
 the center of the tooth-crown. Two of the four horns are seen, one of 
 which penetrates the mesiobuccal cusp, and one the mesiolingual cusp. 
 In many instances the horns of the molar teeth are quite slender, penetrat- 
 ing the dentin to a greater depth than shown in the illustration, in the 
 form of minute hair-like projections, which in some instances reach 
 almost to the enamel walls. 
 
 No. 2 illustrates the condition of the pulp-cavity at the sixth year, 
 or at the time of eruption. The outline of the pulp-chamber is completed, 
 and the floor has begun to make its appearance by a central deposit 
 of dentin. It will be observed that the lateral walls of the chamber are 
 somewhat less in thickness than the occluding wall, a condition which 
 will become more pronounced as the tooth develops. With the beginning 
 of the formative process in the floor of the chamber we find the trifurca- 
 tion of the roots established, and the beginning of the canals outlined. 
 The canals at this period are quite similar to those of the bicuspid, being 
 provided with a funnel-shaped extremity, which extends from the free 
 
206 ANATOMY 
 
 calcifying margins of the roots to the floor of the chamber. No. 3 shows 
 the change which has taken place at the seventh year. While the pulp- 
 chamber is somewhat reduced in size, but little change is noticeable 
 in its outline. By this time the floor of the chamber has become an 
 important factor in the tooth development. By the constant lateral 
 extension of this central deposit of dentin the floor of the chamber is 
 gradually spread out, this alteration being at the expense of the entrances 
 to the root-canals, which become reduced in diameter as the floor is 
 extended. The horns of the chamber are slightly less prominent, but this 
 part of the cavity has the appearance of having receded bodily root- 
 ward. The roots have advanced somewhat beyond the point of trifurca- 
 tion, and a definite outline has been given to the canals. At this period 
 the diameter of the root-wall is about equal to the diameter of the pulp- 
 canal. Along with the gradual decrease in the diameter of the roots, 
 there is observed a corresponding decrease in the width of the funnel- 
 shaped extremities of the canals. 
 
 At the eighth year, No. 4, a gradual reduction in the capacity of both 
 the chamber and canal is noted. Accompanying the above condition 
 there is found a corresponding increase in the thickness of the surround- 
 ing walls. The horns of the pulp-chamber are much reduced in size, 
 and the form of the chamber more closely resembles that of the general 
 contour of the tooth-crown. The increase in the length of the roots 
 is proportionately greater than that of previous years, in consequence 
 of which the length of the canals is increased to a greater degree. In 
 No. 5 the maximum size of the chamber and canals is apparent, which 
 condition takes place about the ninth year, or at a time when calcification 
 of the tooth is completed externally. In some instances, owing to the 
 additional length of the lingual root, the apical foramen may not be 
 established before the tenth year. At this latter period it is safe to assume 
 that all three canals have completed their longitudinal extent, and the 
 foramina, although proportionately large, have been established, so 
 that a more definite description of each canal may be given. The lingual 
 canal (Fig. 155, A) is usually the largest, and branches off from the floor 
 of the chamber, near the mesiodistal center of tl j extreme lingual margin, 
 the entrance in the average tooth being well defined by a circular, funnel- 
 shaped opening. The direction of this canal is usually upward and 
 slightly inward, until the apical extremity is approached, at which point 
 it is inclined to the buccal. The circular form presented at the beginning 
 of the canal is generally continued throughout its entire length, in this 
 repect differing from the two buccal canals. The average length of 
 
THE PULP-CAVITIES OF THE UPPER TEETH 207 
 
 the lingual canal is about 1/2 of an inch. The mesiobuccal canal (Fig. 
 155, B) branches off from the floor of the chamber, at its extreme mesio- 
 buccal angle, and the entrance, instead of being funnel-shaped and easy 
 of access, is flattened from mesial to distal, and frequently difficult to 
 enter. This flattened form continues throughout its course, which for 
 the distance of 1/8 of an inch is in a buccal and mesial direction; beyond 
 this point it is usually inclined to the buccal, until the upper third of the 
 root is reached, where it turns rather abruptly to the distal. This canal is 
 generally a trifle shorter than the lingual, averaging about 3/8 to 7/16 of 
 
 D e F 
 
 Fig. 155. — Pulp-cavities of Upper First Molar, Slightly Enlarged. 
 
 an inch. The distobuccal canal (Fig. 155, C) branches off from the floor 
 of the chamber at the extreme distobuccal angle. In those teeth which 
 most nearly approach the quadrilateral form, the entrance to this canal 
 will be farther from the center of the tooth, the molar triangle in this 
 instance being almost an equilateral. It sometimes happens that the 
 entrance to this canal is directly in the floor of the pulp-chamber, near to, 
 but not against, its buccodistal angle. The entrance is usually abrupt, 
 seldom being funnel-shaped, making it by far the most difficult of access. 
 It is inclined to be circular in form, and more or less tortuous in its course. 
 Immediately above the point of beginning it is inclined toward the buccal 
 and distal; near its center it may incline slightly to the mesial; and finally, 
 
208 ANATOMY 
 
 at its upper third, turns somewhat abruptly in a distobuccal direction. 
 This canal is usually the shortest of the three, its average length being 
 about 3/8 of an inch. 
 
 Figure 155 also illustrates a number of transverse sections of this 
 tooth, D being made at the cervical line, looking toward the crown, E 
 looking toward the roots, while F represents a transverse section at a 
 point immediately above the floor of the pulp-chamber. 
 
 Upper Second Molar. — In many respects the pulp-chamber of 
 this tooth is similar to that of the first molar, but there are a few variations 
 which must be briefly described. First, the outline of the tooth-crown 
 
 9th year nth year 13th year 15th year 18th year 
 
 Fig. 156. — Pulp-cavities in the Upper Second Molar, from the Ninth to the 
 Eighteenth Year. 
 
 being much more flattened from mesial to distal, a corresponding varia- 
 tion is noted in the form of the pulp-chamber, increasing the length of 
 the mesial side of the molar triangle, and decreasing the length of the 
 buccal and distal sides. The chamber is more or less flattened from 
 mesial to distal, making it somewhat oblong from buccal to lingual. 
 Second, on account of a reduction in the prominence of the cusps, the 
 horns of the cavity are usually somewhat less pronounced than those of 
 the first molar. Third, the floor of the cavity is less convex, and slopes 
 more gradually toward the entrances of the various canals. In a general 
 way, the rules given for ascertaining the approximate location of the 
 entrances to the canals in the first molar apply to this tooth. The com- 
 parative size and form of the pulp-chamber and canals during the develop- 
 ment of the tooth are shown in figure 156, extending from the ninth to 
 the sixteenth or eighteenth year, at which latter period the crown and 
 roots of the tooth are fully calcified externally. 
 
 Upper Third Molar. — In this tooth the conditions are so variable 
 that a description of the pulp-cavity taken from a single tooth would be in- 
 sufficient. In the majority of instances the outline of the tooth-crown 
 
THE PULP-CAVITIES OF THE UPPER TEETH 200. 
 
 approaches the triangular form, and in consequence the pulp-chamber 
 is triangular rather than quadrilateral or oblong. The mesial border 
 of the chamber is the longest, the distal next in length, and the buccal 
 the shortest of the three. The horns are generally less in number and 
 much less pronounced than those of either the first or second molars. 
 The floor of the chamber may be broken by irregularities similar to 
 those previously described, or it may be entirely absent, this latter con- 
 dition occurring when the tooth has but a single root accompanied by a 
 single canal. The various stages of development having been given in 
 connection with the general description of the tooth, no attempt will 
 
 Fig. 157. — Longitudinal Sections, Upper Third Molar, Slightly Enlarged. 
 
 be made to describe this by longitudinal sections, the complications in 
 root-form making such a proceeding impracticable. Instead of so doing, 
 the space will be devoted to a brief description of the variety of pulp- 
 canals found in this tooth. Probably the most frequent condition is 
 that which resembles the first and second molars — i.e., three canals 
 branching off from the chamber in as many different roots, two to the 
 buccal and one to the lingual. When the three canals exist, the entrances 
 to them will be well beyond the cervical line, where they will be found 
 clustered much closer together than those of the first and second molars, 
 this difference in their location being so marked that the diagram pre- 
 viously given cannot be depended upon in an attempt to locate them. 
 The usual course of these canals is first slightly mesial, then distal, and 
 finally in a distolingual direction. On account of the pulp-chamber 
 extending well beyond the cervical line, the canals are much shorter than 
 those of the first or second molars, their average length being less than 
 1/2 of an inch. Another form frequently met with is that of the flattened 
 single canal, occurring when the tooth has but a single root, which shows 
 14 
 
2IO ANATOMY 
 
 no signs of trifucating (Fig. 157, A). In this instance the pulp-chamber 
 gradually passes into the canal, and the chamber is without a floor. Such 
 a canal is shaped like the chamber at its point of beginning; but as it 
 passes toward the apex it becomes flattened in the direction of the smallest 
 diameter of the root. But little difficulty is experienced in entering 
 such a canal, and usually it is readily followed to its apex. Another 
 condition frequently met with in the single-rooted third molar is that 
 of one or more canals branching off from the floor of the chamber, their 
 course through the root-substance being without regard to the external 
 contour of the root (Fig. 157, B). These canals, which may exist to 
 the number of five or six, are usually very minute, and in some instances 
 may pass from the floor of the chamber to the apex of the root almost 
 in a direct line, and end in distinct foramina, or they may take a tortuous 
 course, and when near the apex unite, ending in a single foramen. When 
 the tooth is provided with four, five, or even six small roots, as sometimes 
 occurs, each root will be traversed by a minute canal, the entrances to 
 these being variously placed about the floor and lateral margins of the 
 pulp-chamber (Fig. 157, C). In all operations upon this tooth it must 
 be recalled that it is the last to be calcified, and consequently the canals 
 and foramina are proportionately larger than in the other teeth; at the 
 same time, it possesses one advantage over the others — i.e., (with the 
 single exception of the cuspid), being fully calcified at or about the time 
 of its eruption. 
 
THE PULP-CAVITIES OF THE LOWER TEETH 211 
 
 PULP-CAVITIES OF THE LOWER TEETH. 
 
 C B A 
 
 Fig. 158. — Pulp-cavities of the Lower Incisor. 
 
 The outline of the pulp-cavities of the lower teeth, like those of the 
 upper, corresponds to the general tooth contour. The comparative size 
 of the cavity at various stages of tooth development will not be repeated 
 in this description, the conditions being similar to those in the upper 
 teeth (see also Development of the Teeth). 
 
 Lower Incisors. — The pulp-cavities of the lower central and lateral 
 incisors are so nearly alike that a single description will answer for 
 both. Figure 158, A, represents a labiolingual section of a lower incisor, 
 showing the most frequent form of the pulp-cavity. The tooth from 
 which the section was prepared was one about middle life, the cavity 
 in younger teeth being proportionately larger, while a gradual decrease 
 in diameter would be noted with advancing age. There is no mark 
 of distinction between the pulp-chamber and canal, so that an imaginary 
 separation would have to be made at the cervical line, or slightly below 
 that point. Taken in its entirety, the cavity presents the form of a 
 double cone, the common base of which is slightly to rootward of the 
 cervical line. The chamber penetrates the crown fully half-way to 
 the cutting-edge, at which point it ends in a thin, fan-like margin (best 
 observed in mesiodistal section), while the canal gradually decreases 
 in size until the apical foramen is reached. Although this is the most 
 common form of the pulp-cavity in the lower incisors, it is by no means 
 the constant condition. The tooth is not infrequently provided with a 
 medium-sized pulp-chamber, which extends somewhat below the cervical 
 
212 
 
 ANATOMY 
 
 line beyond which point it branches into fine canals, which are continued 
 separately until the apical third of the roots is approached, when they 
 again unite, and finally end in a single foramen. Figure 158, C, repre- 
 sents a mesiodistal section of a young lower incisor, in which the three 
 small horns of the pulp-chamber are apparent. At this period the fan- 
 shaped extremity of the pulp-chamber occupies about one-half of the 
 mesiodistal diameter of the crown, and the horn-like projections extend 
 well toward the enamel cap. Figure 158, B, shows the average size and 
 form of the pulp-cavity at maturity, by a mesiodistal section through the 
 long axis of the tooth. In this it will be observed that the horns of the 
 pulp-cavity have disappeared, and that the capacity of the cavity in 
 general is much reduced. 
 
 Fig. 159. — Pulp-cavities of the Lower Cuspids. 
 
 Lower Cuspids. — The pulp-chamber and canal in this tooth, while 
 usually conforming to the general contour of the tooth, are frequently 
 found to vary greatly, both in outline and in size. The most common 
 form, however, is that shown in Fig. 159, A, a labiolingual section of an 
 adult tooth. The chamber and canal have no line of demarcation, and 
 unite at a common base considerably below the cervical line, the former 
 penetrating the crown of the tooth to a point about midway between the 
 cervical line and the summit of the cusp, at which point it ends in a sharp, 
 hair-like projection. Accompanying this common form there is much 
 
THE PULP-CAVITIES OF THE LOWER TEETH 
 
 213 
 
 variation in size, even in teeth of the same age. Fig. 159, B, shows 
 another labiolingual section of an adult lower cuspid, in which the pulp- 
 cavity fails to accurately follow the outline of the tooth, and its capacity 
 is much less than that shown at A. The root of this tooth is in most 
 instances circular, in which case the canal will be similarly formed; but 
 occasionally the root will be much flattened from mesial to distal, and 
 as a result of this the canal will also be much flattened. The canal of 
 this tooth is seldom divided. In Fig. 159, C, a mesiodistal section of a 
 lower cuspid is shown, and it will be observed that the fan-shaped ex- 
 tremity of the chamber common to the incisors is absent, the cavity end- 
 ing rather abruptly, or by a fine line near the center of the crown. The 
 same illustration also shows a number of transverse sections, which will 
 give an idea of the form of the cavity at various parts of the tooth. 
 
 Lower Bicuspids. — The pulp-cavities of these teeth may be best 
 described collectively, thus affording an opportunity for comparison. 
 Unlike the upper bicuspids, it is seldom that the canals are definitely 
 
 Fig. 160. — Sections of Lower Bicuspids. 
 
 separated from the chambers. That part of the cavity within the crown, 
 however, is usually quite wide from buccal to lingual, and unites with the 
 canal by a long, funnel-shaped constriction. The center of the pulp- 
 chamber may be considered as being about on a level with the cervical line. 
 In the first bicuspid the pulp-cavity is provided with a single horn, which 
 extends with more or less prominence in the direction of the buccal cusp. 
 That part of the chamber facing the lingual cusp is usually rounded off. 
 
214 ANATOMY 
 
 In the second bicuspid the occlusal wall of the pulp-chamber generally 
 presents a different form; two well-defined horns are usually present, of 
 which the buccal is the longest; or the chamber may be prominently 
 rounded at these points. The pulp-chamber of the second bicuspid is 
 generally larger than that of the first. The canals of these teeth are 
 usually circular throughout, and are readily penetrated until the apical 
 third is reached, beyond which point they are extremely small. In some 
 instances the canal divides near the center of the root, and is continued 
 as two canals, ending in distinct foramina, or, after separating, they may 
 again unite, and end in a single foramen. In Fig. 160 the average size 
 and form of the canal in these teeth is shown by a number of mesiodistal 
 and transverse sections. 
 
 Fig. 161. — Sections of Lower Molars, Enlarged about One-third. 
 
 Lower Molars. — The form of the pulp-chambers of the lower molars 
 corresponds to the general outline of the crown, and the form of the root- 
 canals is similar to the general contour of the roots. The pulp-chambers 
 approach the quadrilateral form; the buccal and lingual sides are some- 
 what the longest, the mesial next in length, and the distal, usually slightly 
 rounded, is the shortest. The occluding wall is convex rootward, sloping 
 i^ the direction of the various cusps, each of which is penetrated by a horn, 
 Like the horns of these pulp-chambers in general, the extent to which these 
 
PULP-CAVITIES OF THE LOWER TEETH 215 
 
 penetrate the cusps is influenced by the age, type, and functional activity of 
 the organ. The floor of the cavity is convex in the direction of the occlusal 
 surface, but this convexity is principally from mesial to distal. From the 
 summit of this convexity the floor slopes to the entrances of the canals, the 
 opening into which is inclined to be funnel-shaped rather than abrupt. 
 The lateral walls of the chamber are much inclined to follow the general 
 contour of the crown. The horns of the pulp-chamber are usually more pro- 
 nounced in the first than in the second molar, and still less clearly defined 
 in the third than in the second. The roots of the first molar being some- 
 what further apart than those of the second, the floor of the chamber in 
 the former is slightly more extensive than in the latter. To study the 
 pulp-cavities of these teeth a longitudinal section should be made through 
 the center of the tooth from mesial to distal. Fig. 161, A, shows such a 
 dissection through the first molar, and illustrates the average size and 
 form of the chamber and canals at adult age. The canals join the 
 chamber by a funnel-shaped opening, and but little difficulty will be found 
 in effecting an entrance, but to follow them to their apices will be more 
 perplexing. The roots of this tooth being much flattened from mesial to 
 distal, the canals are also flattened in this direction, but broad from buccal 
 to lingual. The entrances of these canals may be found at the extreme 
 mesial and distal margins of the pulp-chamber, and usually extend from 
 the buccal to the lingual walls of the cavity. It is not uncommon for the 
 mesial canal to divide soon after leaving the chamber, and continue as 
 two canals, ending in separate foramina (Fig. 161, B). This condition 
 is seldom present in the distal canal, which is usually straight from its 
 mouth to the apical foramen. The capacity of the pulp-chamber is 
 usually a trifle less than that of the first molar, and the entrances to the 
 canals are somewhat nearer together. In other respects the cavity is simi- 
 lar to that of the first molar. Fig. 161 also shows a number of transverse 
 sections through a lower molar, and gives an idea of the size of the canals 
 at various parts of the roots. In some instances the roots of the second 
 molar coalesce, in which case a single root-canal may be present. In 
 the third molar the most common form of the pulp-cavity is one similar 
 to that of the first, but both the chamber and canals are smaller. Unlike 
 the pulp-cavity of the corresponding upper tooth, this tooth is not sub- 
 ject to so much variation, although it is sometimes found with a single 
 root traversed by a single canal, which may be accompanied by a rather 
 large pulp-chamber. 
 
CHAPTER XL 
 
 The Deciduous Teeth, Their Arrangement, Occlusion, Etc. ; Their 
 Calcification, Eruption, Decalcification, Shedding Process, 
 and Average Measurements; Their Surfaces, Grooves, Fossae, 
 Ridges, Sulci, and Pulp-Cavities. 
 
 THE DECIDUOUS TEETH. 
 
 Central Incisor 
 
 Lateral Incisor 
 
 Cuspid 
 
 First Molar 
 
 Second Molar 
 
 Uppei 
 
 Lower 
 
 Central Incisor Lateral Incisor Cuspid First Molar Second Molar 
 
 Fig. 162. — The Deciduous Teeth, Upper and Lower, from the Left Side of the Mouth. 
 
 As implied by the word deciduous, these teeth are temporary in their 
 nature, and, after subserving the purposes of early childhood, are thrown 
 off by an operation of the economy to give place to the permanent organs. 
 The shedding process takes place in the incisors between the seventh 
 and eighth years, in the molars from the tenth to the eleventh years, and 
 in the cuspids about the twelfth year. This shedding process, however, 
 
 216 
 
THE DECIDUOUS TEETH 217 
 
 does not indicate the period at which the degeneracy of the tooth begins, 
 for, in a year or two after the root is completely formed and the apical 
 foramen established, decalcification begins at the apical extremity and 
 continues in the direction of the crown until absorption of the entire root 
 has taken place and the crown is lost from lack of support. Decalcifi- 
 cation in the incisors begins between the fourth and fifth years, in the 
 molars from the seventh to the eighth years, and in the cuspids about 
 the ninth year. 
 
 The deciduous teeth are twenty in number, ten in each jaw, and may 
 be classified as follows: Four incisors, two cuspids, and four molars. 
 The incisors, central and lateral, occupy the central portion of the arch, 
 are placed two upon each side of the median line, and are succeeded by 
 the four permanent incisors, which finally occupy the same position. 
 The cuspids are located immediately to the distal of the lateral incisors, 
 and are displaced by the permanent cuspids. The first and second molars 
 come next in the arch, but, unlike the anterior teeth, are followed by per- 
 manent successors of another class, the first and second bicuspids, the 
 permanent molars erupting posteriorly to these as the jaw increases in 
 length. 
 
 In general the deciduous teeth resemble their permanent successors, 
 yet there are a number of minor differences which will require a compara- 
 tive description. Both the crowns and the roots are much smaller in 
 every direction than those of the permanent teeth, but the diameter of 
 the crowns is proportionately greater than that of the roots, while the 
 roots are proportionately longer. The fact that the roots are smaller in 
 proportion than the crowns is productive of a neck much more constricted. 
 The roots of the deciduous teeth are the same in number as those of the 
 corresponding permanent teeth, the incisors and cuspids being provided 
 with one, the upper molars with three, and the lower molars with two. 
 
2l8 
 
 ANATOMY 
 
 THE OCCLUSION OF THE DECIDUOUS TEETH. 
 
 Cuspid Incisors 
 
 First Molar 
 Second Molar 
 
 Permanent 
 First Molar 
 
 Fig. 163.— The upper Dental Arch about the Seventh Year. 
 
 The arrangement of the deciduous teeth in the jaws is similar to that 
 of the permanent organs, the upper teeth describing the segment of a 
 larger circle than the lower, in consequence of which the upper teeth 
 close over or outside of the lower. The character of the occlusion in the 
 deciduous teeth is not subject to so much variation as that found in con- 
 nection with the permanent set, this being accounted for by the more 
 constant form in the crowns of the former. The relations existing between 
 the upper and lower deciduous teeth when in contact is such that each 
 tooth, with the exception of the lower central incisor and the upper second 
 molar, occludes with two teeth of the opposite jaw, the upper central 
 incisor being opposed by the entire cutting-edge of the lower central and 
 the mesial third of the lower lateral; the upper lateral coming in contact 
 with the remaining two-thirds of the lower lateral and a portion of the 
 mesial half of the lower cuspid, this arrangement continuing throughout 
 the series. The foregoing description of the occlusion of the deciduous 
 teeth is applicable to but a small part of their transitory existence. Soon 
 after they are fully erupted and have assumed their respective posi- 
 tions in the arch, the increase in the size of the bone is sufficient to create 
 a slight space between the teeth, which condition is soon followed by a 
 greater separation through the protrusion of the anterior teeth, caused by 
 the growth and approach of the permanent teeth from behind. 
 
 The calcification of the deciduous teeth is similar to that of the per- 
 manent, the process in the incisors and cuspids beginning along the cut- 
 
THE DECIDUOUS TEETH IN DETAIL 
 
 219 
 
 ting-edges in three distinct lobes, while in the molars a center of calcifi- 
 cation is provided for each cusp (see Development of the Teeth). 
 
 THE DECIDUOUS TEETH IN DETAIL. 
 
 UPPER CENTRAL INCISOR 
 
 Eighteenth Month 
 after Birth 
 
 Sixth Month after 
 Birth 
 
 Fortieth Week 
 
 Twentieth Week 
 
 Fourth Year 
 
 Fifth Year 
 
 Seventh Year 
 
 Fig. 164. 
 
 Calcification Begins, about the Fourth Fetal Month. 
 
 Calcification Completed, Seventeenth to Eighteenth Month after Birth. 
 Erupts, Sixth to Eighth Month after Birth. 
 Decalcification Begins, about the Fourth]Year. 
 
 Shedding Process Takes Place, about the Seventh Year. 
 Average Length of Crown, .23. 
 
 Average Length of Root, .39. 
 
 Average Length over All, .62. 
 
 This tooth, as well as all of the deciduous teeth, presents for examina- 
 tion numerous surfaces, margins, and angles, these being the same in 
 name and location as those of the permanent teeth. 
 
 The Labial Surface of the Crown (Fig. 164). — This surface is smooth 
 and generally convex, but with an inclination to flatness near the incisive 
 margin. The mesial margin is slightly convex in the direction of the 
 length of the tooth, and rounded from labial to lingual. The distal mar- 
 gin is decidedly convex from the cutting-edge to the cervical line, in many 
 instances forming almost a complete semicircle, which is usually at the 
 expense of the distal angle of the crown. The cervical margin is deeply 
 concave in the direction of the root, and the incisive margin is straight over 
 its central portion and rounded or angular at its extremities. The labial 
 grooves are seldom so well defined at those upon the permanent incisors. 
 
 The Lingual Surface of the Crown. — In some instances this 
 surface is smooth and concave near the cutting-edge and convex over the 
 cervical portion, with the marginal ridges well defined. In other cases it is 
 concave from the cutting-edge to the cervical ridge, being provided with a 
 longitudinal ridge in the center, a slight depression upon either side, and 
 
2 20 
 
 ANATOMY 
 
 marginal ridges poorly defined. In the former instance the lingual 
 fossa is present; in the latter it is absent. The mesial and distal sur- 
 faces of the crown are both smooth and convex, the 
 former being inclined to flatness over its cervical 
 third — a condition which is seldom present in the 
 latter. The mesial angle is alone well defined, the 
 cutting-edge passing into the distal surface with a 
 long, gradual sweep, thus in a measure destroying 
 the distal angle. The neck of the tooth is marked 
 by a decided constriction, which is principally pro- 
 duced at the expense of the crown alone. The 
 root of the tooth, when compared with the root of 
 the permanent central incisor, is much longer in 
 proportion to the length of the crown. In some 
 instances it is flattened from mesial to distal, 
 these two sides converging as they pass to the lingual; in others it is flat- 
 tened from labial to lingual. Generally speaking, it is a single root, 
 but is occasionally provided with a slight mesial curve near its apical 
 third, and it is sometimes curved slightly from labial to lingual. 
 
 Fig. 165. 
 
 UPPER LATERAL INCISOR. 
 
 Sixteenth Month 
 after Birth 
 
 Fortieth Week 
 
 Twentieth Week 
 
 Fifth Year 
 
 Seventh Year 
 
 Eighth Year. j(* 
 
 Fig. 166. 
 
 Calcification Begins, about the Fourth Fetal Month. 
 
 Calcification Completed, Fourteenth to Sixteenth Month after Birth. 
 Erupts, Seventh to Ninth Month after Birth. 
 Decalcification Begins, about the Fifth Year. 
 
 Shedding Process Takes Place about the Eighth Year. 
 Average Length of Crown, .25. 
 
 Average Length of Root, .45. 
 
 Average Length over All, .70. 
 
 The various surfaces of this tooth so closely resemble those of the cen- 
 tral incisor that a separate description will be unnecessary; in a general 
 
THE DECIDUOUS TEETH IN DETAIL 
 
 221 
 
 way, however, there are a few minor points of 
 difference. The tooth is smaller in every direc- 
 tion excepting in its length, which is generally 
 equal to and frequently greater than that of the 
 central incisor. The diameter of the root is but 
 little less than that of the central, while the 
 mesiodistal measurement of the crown is about 
 one-third less, in consequence of which the neck 
 of the tooth is not so well defined. The angles of 
 the crown are more rounded than those of the 
 central incisors. 
 
 Fig. 167. — Upper Lateral 
 Incisor, Labial Surface. 
 
 UPPER CUSPID. 
 
 Second Year 
 
 Sixth Month 
 after Birth 
 
 Ninth Year 
 
 Tenth Year 
 
 Twelfth Year 
 
 Birth 
 
 Thirtieth Week 
 Embryo 
 
 Fig. 168. 
 
 Calcification Begins, about the Fifth Fetal Month. 
 
 Calcification Completed, about Two Years after Birth. 
 
 Erupts, Seventeenth to Eighteenth Month after Birth 
 Decalcification Begins, about the Ninth Year. 
 
 Shedding Process Takes Place, about the Twelfth Year. 
 Average Length of Crown, .25. 
 
 Average Length of Root, .53. 
 
 Average Length over All, .78. 
 
 Like the permanent cuspid, the general contour of this tooth is that of 
 a double cone, the lines of which are somewhat broken. The greatest 
 mesiodistal extent of the crown is from angle to angle, and this measure- 
 ment about corresponds with the width of the crown of the central incisor. 
 
 The Labial Surface of the Crown (Fig. 168). — This surface is 
 strongly convex from mesial to distal, and slightly so from the cutting-edge 
 to the cervical line. It is bounded by five margins: mesial, distal, cervical, 
 
222 
 
 AN \h>MY 
 
 mesio-incisive, and disto-incisive. The mesial and distal margins are 
 rounded and smooth, the cervical well outlined by the cervical line and 
 base of the cervical ridge, while the two incisive margins are formed by 
 the mesial and distal cutting-edges. The labial grooves are thrown well 
 toward the lateral margins, and are usually more distinct than those 
 upon the incisors. The labial ridge is prominent. 
 
 The Lingual Surface of the Crown. — This 
 surface is generally divided into two portions by 
 the lingual ridge, which extends from the sum- 
 mit of the cusp to the base of the cervical ridge. 
 On either side of this ridge are the lingual 
 grooves, but which appear more in the form of 
 small fossae. The marginal ridges are fairly well 
 defined. 
 
 The Mesial and Distal Surfaces of the 
 Crown. — The extent of these two surfaces is fre- 
 quently much interfered with by the slope of the 
 mesial and distal cutting-edges, which may be so 
 long that the angles of the crown are forced 
 well toward the cervical line, in some instances almost obliterating 
 these two surfaces. When the cutting-edges are shorter, these sur- 
 faces present a marked general convexity. While the summit of the cusp 
 will always be found to be in a direct line with the long axis of the tooth, 
 there is in nearly every instance a difference in the length of the cutting- 
 edges, and. unlike the cutting-edges of the permanent cuspid, the mesial 
 is usually the longer. The neck of the tooth is much constricted and 
 the root straight and conic. 
 
 Fig. 169. — Upper Cuspid. 
 Labial Surface. 
 
THE DECIDUOUS TEETH IX DETAIL 
 
 THE ITPER MOLARS. 
 
 223 
 
 Upper First Molar. 
 
 . wenty-two 
 
 Months Old 
 
 Or.e Year Old 
 
 Forty Wee- 
 
 Fig. 170. 
 Calcification Begins, about the Fifth Fetal Month. 
 
 calcification completed, eighteenth to twentieth month after blrth. 
 Erupts. Fourteenth to Fifteexth Moxth after Birth. 
 Decalcification Begixs. Sixth to Se\"exth Ye.ar. 
 
 Sheddlxg Process Takes Place, .about the Texth Ye.ar. 
 A\"erage Lexgth of Crowx. .20. 
 
 Average Length of Root, .39. 
 
 Average Lexgth over All. .59. 
 
 The contour and lobate construction of the crown of this tooth is 
 peculiar to itself, being dissimilar to any other class of teeth in the mouth. 
 Calcification takes place from three centers, two for the buccal and one 
 for the lingual half of the crown. The general form of the crown may 
 best be studied by an examination of the occlusal surface. 
 
 The Occlusal Surface of the Crown. — The outlines represented are 
 those of an irregular quadrilateral, of which the buccal and mesial sides 
 are the longest. The angles of the quadrilateral are somewhat variable. 
 the mesiobuccal being acute, the mesiolingual obtuse, while the two 
 distal angles are rounded right angles. The surface is surmounted by 
 three cusps, a mesiobuccal. a distobuccal, and a lingual. These various 
 cusps are separated from one another by three developmental grooves — the 
 mesial, the distal, and the buccal. The marginal ridges are sharp and 
 well defined, this being particularly true of the buccal and lingual, which 
 resemble cutting-edges. The mesio-marginal ridge begins at the 
 mesiobuccal angle, and. after making a Long distal curve, ends in the 
 mesial incline of the lingual cusp. The center of the surface is deeply 
 and irregularly concave, producing the central fossa, and descending 
 from the various ridges and cusps surrounding it are numerous supple- 
 mental grooves and ridges. The various developmental grooves are not 
 inclined to cross the marginal ridges, although in some instances one or 
 two mav be found to do so 
 
224 
 
 ANATOMY 
 
 The Buccal Surface of the Crown (Fig. 170). — This surface is 
 generally smooth and convex, with an excessively developed cervical ridge, 
 which is particularly prominent at its mesial extremity. The buccal 
 groove is in the form of a slight depression, and the buccal ridges, common 
 to all molars, are scarcely to be observed. The mesial, occlusal, and cer- 
 vical margins are distinctly outlined, while the distal margin is obliterated 
 by the gradual passing of this surface into the distal surface. 
 
 The Lingual Surface of the Crown. — This surface is circular in 
 outline, decidedly convex and smooth, and is seldom broken by grooves 
 
 and ridges. It is most prominent 
 near the center, from which point 
 it slopes in every direction. The 
 cervical ridge is not so pronounced 
 as that of the buccal surface, but 
 there is a sudden rounding of the 
 surface in a cervical direction to 
 meet the lingual root. 
 
 The Mesial Surface of the 
 Crown. — This surface is probably 
 more extensive than any of the 
 others; it is inclined to flatness, 
 with a slight conic convexity over 
 its occlusal third, and a slight con- 
 cavity near the cervix. The buc- 
 colingual measurement of the sur- 
 face is nearly twice as great as that from the occlusal margin to the 
 cervical line. It is much more prominent near the occlusal margin, so 
 that a V-shaped space usually exists between it and the distal surface of 
 the cuspid. 
 
 The Distal Surface of the Crown. — The extent of this surface 
 is much less than that of the mesial; it presents a general convexity, and 
 is seldom broken by grooves or ridges, although occasionally the distal 
 groove crosses its occlusal margin. Like the deciduous teeth previously 
 described, the neck of the tooth is marked by a decided and abrupt con- 
 striction, this form appearing to arise from the heavy enamel folds which 
 are present near the cervical line, rather than from any marked con- 
 striction in the base of the roots themselves. 
 
 The roots of the tooth are three in number — a mesiobuccal, a disto- 
 buccal, and a lingual; of these, the latter is usually the largest and longest. 
 The two buccal roots are much flattened from mesial to distal, while the 
 
 Fig. 171. — Occlusal Surfaces of the 
 Deciduous Molars. 
 
THE DECIDUOUS TEETH IN DETAIL 
 
 225 
 
 lingual is compressed in the opposite direction. The apical ends of the 
 roots are much separated from one another, the triangle which these 
 points form being almost twice the size of the triangle formed by the base 
 of the roots. The apical ends are usually provided with a central curve. 
 
 Upper Second Molar. 
 
 One Year Old 
 
 Eight Years m 
 
 Ten Years 
 
 Eleven Years 
 
 Fig. 172. 
 
 Calcification- Begins, between the Fifth and Sixth Fetal Months. 
 
 Calcification Completed, Twentieth to Twenty-second Month after Birth. 
 Erupted; Eighteenth to Twenty-fourth Month after Birth. 
 Decalcification Begins, Seventh to Eighth Year. 
 
 Shedding Process Takes Place, Eleventh to Twelfth Year. 
 Average Length of Crown, .22. 
 
 Average Length of Root, .46. 
 
 Average Length over All, .68. 
 
 The most remarkable feature about the crown of this tooth is its close 
 resemblance to the crown of the upper permanent first molar. The 
 various surfaces are almost identical, the de- 
 velopmental process, and consequently the cusp- 
 formation, is the same, the marginal and other 
 ridges common to the occlusal surface corre- 
 spond, and both the central and d,istal fossae are 
 present, together with the various developmental 
 grooves. A description of the crown will, there- 
 fore, be unnecessary; suffice it to say that it is 
 much smaller in every direction and is some- 
 what more constricted at the neck. The roots 
 are the same in name and number as those of 
 the first permanent molar, but they are more 
 widely separated at their apical extremities. In general form they are 
 smaller than those of the upper first deciduous molar. 
 15 
 
 Fig. 173. — Upper Sec- 
 ond Molar, Buccal 
 Surface. 
 
226 ANATOMY 
 
 THE LOWER DECIDUOUS TEETH. 
 
 A description in detail of the lower incisors and cuspids would practi- 
 cally be a repetition of that given of the corresponding upper teeth, and 
 for that reason will be passed with a limited reference to each. The 
 lower molars being in many respects unlike the upper, they will require 
 a separate description. 
 
 Lower Central Incisor (Fig. 174). 
 
 Calcification Begins, about the Fourth Fetal Month. 
 
 Calcification Completed, Sixteenth to Eighteenth Month after Birth. 
 Erupted, Sixth to Eighth Month after Birth. 
 Decalcification Begins, about the Fourth Year. 
 
 Shedding Process Takes Place, about the Seventh Year. 
 Average Length of Crown, .19. 
 
 Average Length of Root, .35. 
 
 Average Length over All, .54. 
 
 This is the smallest of the lower teeth, in this 
 respect being at variance to the upper central, which 
 is larger than the lateral. The mesiodistal diameter 
 of the crown is but little less than that from the 
 cutting-edge to the cervical line. The mesial and 
 distal angles are similar, both being pointed and 
 square. The cervical ridge is quite pronounced and 
 the neck much constricted. 
 
 The root is usually straight and tapers gradually 
 Fig. 174-— from base to apex. It is broader on the labial than 
 
 .Lower Central 1 
 
 incisor, Labial on the lingual side, and the mesial and distal sides 
 are but little flattened. 
 
 Surface. 
 
 Lower Lateral Incisor (Fig. 175). 
 
 Calcification Begins, about the Fourth Fetal Month. 
 
 Calcification Completed, Twelfth to Fourteenth Month after Birth. 
 Erupted, Seventh to Ninth Month after Birth. 
 Decalcification Begins, about the Fifth Year. 
 
 Shedding Process Takes Place about the Eighth Year. 
 Average Length of Crown, .19. 
 
 Average Length of Root, .39. 
 
 Average Length over All, .58. 
 
 This tooth is larger than the central incisor, and closely resembles the 
 upper lateral both in size and form. The crown is more rounded in its 
 
THE DECIDUOUS TEETH IN DETAIL 
 
 227 
 
 nature than that of the central, forming a greater general convexity to 
 the labial surface, and less concavity to the lingual. The mesial angle of 
 the crown is fairly well denned, while the distal is 
 usually much rounded by a long, circular sweep of 
 the cutting-edge to meet the distal surface. 
 
 The mesial surface of the crown is flattened and 
 somewhat prominent at the angle, while the distal 
 surface is strongly convex. The labial grooves are 
 but slightly visible, while the corresponding lingual 
 grooves are quite pronounced. The neck of the tooth 
 is even more marked than that of the lower central 
 incisor.. The root is long and tapering, slightly flat- 
 tened from mesial to distal, with a decided longitudinal 
 groove on both the mesial and distal sides. The 
 labial and lingual sides are rounded, and there is an 
 inclination to crookedness, which is usually from mesial to distal 
 
 Fig. 175 — 
 Lower Lateral 
 Incisor, Labial 
 Surface. 
 
 Lower Cuspid (Fig. 176). 
 
 Calcification Begins, about the Fifth Fetal Month. 
 
 Calcification Completed, about Two Years after Birth. 
 
 Erupted, Seventeenth to Eighteenth Month after Birth. 
 Decalcification Begins, about the Ninth Year. 
 
 Shedding Process Takes Place, about the Twelfth Year. 
 Average Length of Crown, .23. 
 
 Average Length of Root, .45. 
 
 Average Length over All, .68. 
 
 Fig. 176. — 
 Lower Cuspid, 
 Labial Surface. 
 
 The principal variations between this tooth and 
 the upper cuspid are observed in the diminished 
 mesiodistal measurement of the crown, together with 
 it being somewhat less angular in outline. The 
 ridges and grooves common to the various surfaces 
 are not so marked as those of the upper cuspid, re- 
 sulting in a smoothly formed crown throughout. The 
 root is larger in proportion to the size of the crown 
 than that of its upper opponent, thus producing a 
 neck much less constricted. It is usually straight, or 
 provided with a slight distal inclination near its apical 
 extremity, and much flattened from mesial to distal, 
 these two sides converging to the lingual, forming a 
 rounded triangular outline. 
 
2 28 ANATOMY 
 
 Lower First Molar (Fig. 177). 
 
 Calcification Begins, about the Fifth Fetal Month. 
 
 Calcification Completed, Eighteenth to Twentieth Month after Birth. 
 Erupted, Fourteenth to Fifteenth Month after Birth. 
 Decalcification Begins, Sixth to Seventh Year. 
 
 Shedding Process Begins, about the Tenth Year. 
 Average Length of Crown, .24. 
 
 Average Length of Root, .38. 
 
 Average Length over All, .62. 
 
 Upon making an examination of the occlusal surface of this tooth it 
 will be observed that the crown is made up of four irregularly formed lobes, 
 separated from one another by four well-defined grooves. Each lobe is 
 
 provided with a cusp, more or less prominently 
 developed. Between the various cusps are two 
 fossae — one occupying the distal two-thirds of the 
 surface (the distal fossa) and the other the re- 
 maining or mesial third (the mesial fossa). 
 The outline of this surface, which represents 
 the contour of the crown in general, is that of 
 an oblong square, with its angles more or less 
 Fig. 177.— -Lower First r0 unded, and having a slight variation in its 
 
 Molar, Buccal Surface. _ ° ° 
 
 parallel lines. Each lobe denotes a separate 
 center of calcification, and the four giooves the lines of union between 
 the various parts. 
 
 The mesiobuccal lobe is somewhat irregular in contour and is fre- 
 quently the largest of the four. It assists in forming the mesiobuccal 
 angle of the crown and the greater part of the mesial fossa. Descending 
 from this cusp to the lingual is a pronounced triangular ridge, which is 
 made continuous by uniting with a similar ridge from the corresponding 
 lingual cusp. By this union a transverse ridge is established, separating 
 the mesial from the distal fossa. The central boundary of this lobe 
 is formed by the mesial groove, which arises from the distal fossa, passes 
 over the transverse ridge to the mesial fossa, from which it continues to 
 the lingual, and by the buccal groove, which branches off from the 
 mesial somewhat to the distal of the transverse ridge, passing over the 
 buccomarginal ridge to the buccal surface. 
 
 The Distobuccal Lobe. — This cusp is generally smaller than the 
 mesiobuccal, and is more pointed and more regular in outline. It assists 
 in forming the distobuccal angle of the crown, and by its central incline 
 forms about one-third of the distal fossa. Its boundaries are formed by 
 the buccal, mesial and distal grooves, the latter beginning in the distal 
 fossa, and passing over the distomarginal ridge to the distal surface. 
 
THE DECIDUOUS TEETH IN DETAIL 229 
 
 The Mesiolingual Lobe. — In the recently erupted tooth the summit 
 of this cusp is long and pointed, and frequently remains the most pro- 
 nounced of the four. It is triangular in outline, and, as above referred to, 
 furnished a triangular ridge, which, by uniting with a like ridge from the 
 mesiobuccal cusp, forms the transverse ridge. By its central incline it 
 assists in forming the mesial fossa. Its boundaries are formed by the 
 mesial groove and the lingual groove, the latter arising near the center of the 
 distal fossa, passing to, and sometimes crossing, the linguomarginal ridge. 
 
 The Distolingual Lobe. — This is usually the smallest of the four. 
 It is inclined to be rounded, rather than angular, and in some instances 
 is poorly developed. It assists in forming the distolingual angle of the 
 crown, as well as a portion of the distal fossa. Its central boundaries are 
 formed by the lingual and distal grooves. 
 
 The marginal ridges of the surface are abruptly but irregularly 
 formed, ascending and descending the various cusps in a manner similar 
 to those previously described. 
 
 The Buccal Surface of the Crown (Fig. 177). — This surface is 
 smooth and generally convex, with a mesiodistal measurement about 
 twice as great as that from the cervical line to the occlusal margin. 
 The surface is most prominent over its cervical third, forming a well- 
 rounded and bold cervical ridge, a feature strongly characteristic of this 
 tooth. The distal center of the surface is broken by the buccal groove, 
 which usually ends near the center in a shallow depression or pit. 
 
 The Lingual Surface of the Crown. — This surface is much less 
 extensive than the buccal. It is smooth and convex throughout, 
 and is broken near its distal center by the lingual groove, which gradually 
 disappears as it passes rootward. The cervical ridge is not so prominent 
 as that of the buccal surface. 
 
 The Mesial and Distal Surfaces of the Crown. — These are slightly 
 convex in every direction, the former passing, by a gradual sweep, into 
 the lingual surface, destroying the angularity of the crown at this 
 point, while the latter passes more abruptly, forming an acute angle. 
 These surfaces are both prominent near the occlusal margin, making the 
 point of contact with adjoining teeth near the surface. The bold cer- 
 vical ridge of the buccal surface is discontinued or greatly diminished 
 upon these surfaces, both of which are inclined to pass very gradually 
 into the base of the roots. 
 
 The roots of this tooth are the same in name, position, and number 
 as those of the lower permanent molars. They are much flattened from 
 mesial to distal, the center of their flattened sides being further com- 
 
23O ANATOMY 
 
 pressed by a deep longitudinal groove, which extends from the base to 
 the apex of each root. In passing from the base of the roots to their 
 apices they become more widely separated, until these extremities are 
 much wider apart, proportionately, than those of the permanent molars. 
 
 Lower Second Molar (Fig. 178). 
 
 Calcification Begins, between the Fifth and Sixth Fetal Months. 
 
 Calcification Completed, Twentieth to Twenty-second Month after Birth. 
 Erupted, Eighteenth to Twenty-fourth Month after Birth. 
 Decalcification Begins, Seventh to Eighth Year. 
 
 Shedding Process Takes Place, Eleventh to Twelfth Year. 
 Average Length of Crown, .21. 
 
 Average Length of Root, .44. 
 
 Average Length over All, .65. 
 
 The anatomy of this tooth being almost identical to that of the 
 lower first permanent molar, it will be unnecessary to enter into a descrip- 
 tion in detail. The lobes, and consequently the 
 grooves, are the same in position, name, and 
 number, and a similar developmental process is 
 recorded. The tooth is not characterized by a 
 prominent cervical ridge, such as is found upon 
 the lower first molar, the crown passing very 
 gradually into the root-base with a neck moder- 
 ately constricted. 
 
 THE PULP-CHAMBERS AND CANALS OF 
 THE DECIDUOUS TEETH. 
 
 tig. 178. — Lower Second 
 
 Molar, Buccal Surface. ^ few g enera } remarks in reference to these 
 
 cavities, in connection with the information to be derived from the 
 accompanying chart and its annexed description, will sufficiently instruct 
 the reader, without the necessity of treating each tooth individually. 
 The pulp-chambers and canals, like those of the permanent organs; 
 assume the form of the external contour of the tooth, the crown of the 
 tooth being provided with a central cavity, the pulp-chamber, partaking 
 of outlines closely resembling those of the crown, while the root is 
 traversed by the pulp-canal, likewise conforming to the shape of the 
 root. One very important distinction between the pulp-chambers and 
 canals of the deciduous teeth and those of the permanent organs is that 
 the former are proportionately larger. It must also be noted that the 
 apical foramina in these teeth are so transitory in their nature that there 
 remains but a very brief period during which the canals may be said 
 to be fully formed. It will be recalled that in a very short time after 
 the roots have become completely calcified, decalcification begins, and 
 
THE DECIDUOUS TEETH IN DETAIL 
 
 23I 
 
 this process of degeneracy, beginning at the apical extremities of the 
 roots, very early destroy the foramina, which have in a measure served as 
 a protection to the surrounding parts during operations upon the canals. 
 With the canals proportionately larger than those which occupy the 
 roots of the permanent teeth, the foramina during their very limited 
 existence are also much larger, and much more readily penetrated. 
 With these ever-changing conditions in the pulp-canals of the deciduous 
 teeth, it is of importance that a definite knowledge of what takes place 
 should be acquired, and it is for this purpose that the accompanying 
 chart has been prepared (Fig. 179). 
 
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PART II.-HISTOLOGY AND HISTOGENESIS. 
 
 CHAPTER I. 
 
 General Cytology; General Embryology; and Histogenesis. 
 
 The foregoing pages have been devoted to the description of the face 
 and oral cavity, so far as they can be studied with the naked eye, either 
 by close inspection, superfically as it were, or after their component parts 
 have been made manifest by dissection. That part constitutes what is 
 commonly known as gross or macroscopic anatomy. 
 
 Similar to all other regions of the body, however, the functions of 
 the individual parts composing the face and the oral cavity, in health, 
 and the pathological processes which take place in them when they are 
 diseased, are dependent on the normal or pathological condition of the 
 minute structures or tissues, of which these parts are composed. These 
 minute structures cannot be seen with the naked eye, they must be exam- 
 ined with the aid of a microscope, and this part of study is therefore 
 known as microscopical anatomy or histology — the science of tissues. 
 
 The minute structures of the face and oral cavity, generally speaking, 
 in no way differ from the structures of other parts of the body, but here, 
 more than in any other region of the body, the anatomy as well as the 
 histology of the parts cannot be well understood, unless their gradual for- 
 mation or development — embryology — is studied. To enable the student 
 of dentistry to understand more intelligently the details of the parts in 
 which he is most interested, it is therefore essential to have a more or less 
 thorough knowledge of the broad principles underlying, not only the 
 mature structural arrangements of the human body, but also the develop- 
 ment of the organs — organogenesis — as well as the development of the 
 tissues of which these organs are composed — histogenesis. 
 
 In submitting parts of different organs of the body to a more close 
 examination with the microscope, we find revealed a great variety of 
 more or less complex textural arrangements, each characteristic of the 
 part examined, and adapted to the function which the given part has to 
 fulfill. Some tissues we find consisting of either very thin or more or 
 
 233 
 
234 HISTOLOGY 
 
 less coarse fibers running parallel to each other in a compact fashion and 
 forming firm or elastic bands; others we find showing the same kind 
 of fibers loosely arranged and interlacing with each other, thus forming 
 more or less typical networks. In some instances we find similarly 
 formed elements, so-called cells, joined together in a mosaic-like fashion 
 and forming either coverings of surfaces or various tubes or sacs. The 
 majority of organs consist of fibers and cells, modified and combined 
 together in either a simple or more or less complex manner. We also 
 observe that the substances filling the spaces between the fibers and 
 cells, the so-called ground substances, vary in their consistency; it may be 
 a fluid or a semifluid, it may be of a more or less firm nature, or it may 
 be very hard. Notwithstanding this great variety, however, all the 
 tissues of which the animal body is composed can be grouped into five 
 distinct classes, each one of which has its very well defined character- 
 istics of structure, which makes it specially adapted to the various func- 
 tions which it has to fulfill in the animal economy. These classes com- 
 prise the elementary tissues, which by their various modifications and 
 combinations form all the organs of the body. They are the following: 
 i. Epithelial tissue. 2. Connective tissue. 3'. Muscular tissue. 4. Nerv- 
 ous tissue. 5. Blood and lymph. 
 
 Before we attempt to describe the individual elementary tissues, 
 it is necessary to become familiar with some features which are common 
 to all of them. This can be best accomplished by giving a brief history 
 of the beginning and gradual development of our knowledge on the subject. 
 
 The principal features in the minute structure of animals are the 
 same as of plants, and it may be justly stated that animal histology 
 took its origin from the histology of plants. In 1667 Robert Hooke 
 published the results of his microscopical investigations of a piece of cork. 
 He found that it consisted of a number of small boxes or cells, and thus 
 the name cell was introduced into histologic nomenclature. In 1838 
 the botanist Schleiden published the results of his elaborate studies of 
 plant structures, and established the fact that, no matter how widely 
 individual plants and parts of plants differ from one another in their 
 general appearance, in their minute structure they reveal themselves as 
 constituting aggregations of cells, or modifications of such. He however 
 laid particular stress upon the fact that each cell contains a substance, 
 semifluid in nature, which constitutes the essential, life-carrying part 
 of the cell, which by later investigators was named protoplasm. Further- 
 more, he found that within the protoplasm there can always be distin- 
 guished a kernel-like body, which was named nucleus. 
 
GENERAL CYTOLOGY; EMBRYOLOGY; AND HISTOGENESIS 
 
 2 35 
 
 Inspired by personal contact with Schleiden, the anatomist Schwann 
 undertook extensive examinations of various tissues of animal bodies. 
 In 1839 he published the results of his investigations and established the 
 fact, that no matter how widely various animals and organs of animals 
 differ in regard to their general appearance, in their minute structure 
 they always present aggregations of cells, or modifications of such. 
 
 This conformity in the results of the investigations of Schleiden and 
 Schwann has revealed the most interesting as well as the most important 
 fact, that just as all chemical compounds have as their ultimate units 
 
 Fig. 180. — Cells from bulb of a fresh onion forming a membrane. M. cell membrane; N , nu- 
 cleus seen from the surface; Nn, nucleolus; V, vacuoles, X240. (After Stirling.) 
 
 the atoms or the molecules, so all living tissues have as their ultimate 
 units the cells. This forms the foundation for all considerations of 
 plant and animal histology up to the present time, notwithstanding 
 the recent advancement in our knowledge of the structure and life of the 
 cell itself. 
 
 According to the views of Schleiden, Schwann and their contempo- 
 raries, a cell consists of: (1) a cell-membrane; {?.) a substance contained 
 within — a protoplasm; and (3) a small kernel-like body — a nucleus. 
 
 Continuous extensive studies of the structure of cells in various 
 animal tissues have, however, very soon revealed the fact that while in 
 all cells there can always be demonstrated a protoplasm and a nucleus, 
 the presence of a cell-membrane is an exceedingly rare occurrence, and 
 that this part therefore cannot be considered as an essential one in the 
 structure of a cell. It was also found that there may be a number of 
 
236 HISTOLOGY 
 
 other structural elements present in one or the other kind of cells, but by 
 their absence the individuality of the cell is by no means lost. The 
 establishment of these facts naturally led to a change in the conception of 
 the cell-structure, and in 1861 a distinguished investigator, Max Schullze, 
 narrowed down the definition of a cell to a mass of protoplasm containing 
 a nucleus u &nd this definition is generally accepted as the most appropriate 
 one up to the present time. 
 
 Another very important point pertaining to the conception of the 
 cell has attracted the attention of all well-known investigators for a long 
 time, namely, the origin of cells in general, and t e formation of new cells 
 in the different tissues for substituting old ones when they are worn out 
 or lost through injury or disease. It was at one time believed that cells 
 may originate spontaneously by transformation from organic or inorganic 
 chemical compounds. By persistent investigations it was however 
 gradually determined that the so-called spontaneous generation is much 
 less frequent than had at first been supposed, and at present the belief 
 in the formation of cells in that way is scarcely held by anybody. It is 
 definitely established that all varieties of animal cells, no matter to which 
 class of tissues they may belong, always originate from preexisting cells 
 of the same kind. Furthermore, it was ascertained that while the 
 different tissues of the animal body can be easily distinguished from one 
 another in their mature state, there are great similarities between them 
 in the early periods of the development of the animal. These similari- 
 ties become the more pronounced the further back we trace them to 
 their starting-point, and find that all tissues collectively take their origin 
 from the same source, namely, from the ovum or egg, which, according 
 to our knowledge at present time, is also nothing else but a single cell. 
 
 The polymorphism of cells, or the great variety of cell-forms, which 
 was observed in the animal body, has naturally suggested the assumption 
 that the structure of cells with their essential and accessory parts is by no 
 means a simple one, and further investigations have proved this to 
 be correct. The study of cells and tissues has been greatly facilitated 
 by the discovery of various substances, animal as well as vegetable, and 
 particularly chemical compounds, which, when brought in contact with 
 the tissues, show a distinct chemical affinity to the different parts of it 
 and possess the ability of staining them in a distinct characteristic manner. 
 Some of these substances stain the protoplasm of the cells and are 
 therefore called protoplasmic stains, others stain the nucleus and are there- 
 fore called nuclear stains. By application of these staining materials it 
 was made possible to reveal the details in the structure of the various 
 
GENERAL CYTOLOGY; EMBRYOLOGY; AND HISTOGENESIS. 
 
 2 37 
 
 cells, and in the following we will give a description of the minute struc- 
 ture of the cells, as it is generally accepted at present time, and may be 
 observed in one or the other kind of physiologically differentiated cells 
 during the various stages in their life-history. 
 
 According to the statement made above, the essential parts of a 
 cell are the protoplasm and the nucleus. The term protoplasm is how- 
 ever used in modern histology to designate the substance of the whole 
 cell, while the substance of the cell-body surrounding the nucleus is 
 termed cytoplasm; the substance of the nucleus is sometimes termed 
 nucleoplasm, but most frequently simply nucleus. We will also use these 
 terms in our description. 
 
 Nuclear membrane 
 
 Achromatic 
 substances 
 of the nu- 
 cleus 
 
 Linin 
 
 { Nuclear sap 
 
 Cuticular stratum 1 
 
 Filar-mass 
 
 Interfilar-mass -> 
 
 '-Inclusions 
 
 Netknots 
 
 V Nucleolus 
 
 Microsomes 
 
 Chromatic 
 substances 
 of the nu- 
 cleus 
 
 • Cell membrane (pellicula) 
 
 Fig. 181. — Scheme of'a Cell. Microsomes and filar-mass only partly sketched. (Stohr.) 
 
 Cytoplasm. — The structure of the cytoplasm has been described 
 differently by different investigators, and several theories have been 
 advanced in that respect. Some have described it to be of an homo- 
 geneous nature; others found it to be reticular; again, it was claimed to 
 be granular; by some it is supposed to be foam-like. Thanks to experi- 
 mental research-work carried out within recent years, it is positively 
 known, at present, that the most important of all vital manifestations of 
 the cells — metabolism — takes place in the cytoplasm, and this being the 
 case the various stages of the process naturally manifest themselves in the 
 structure of the cytoplasm. In this, and in the diversity of the methods 
 used in preparing tissues for examination, must be sought the cause for 
 
238 HISTOLOGY 
 
 the difference in the results obtained. There is a general agreement 
 among histologists, however, that even when fresh, unstained tissues are 
 examined there can be recognized in the cytoplasm two different parts, 
 one forming a reticulum or network and called spongioplasm; the other, 
 a substance more homogeneous in character, which fills out the spaces 
 of the network and is called hyaloplasm. The reticulum of the spongio- 
 plasm may at times show a bead-like structure, thus presenting a granular 
 appearance. The hyaloplasm may also be not entirely homogeneous, 
 but contain very fine granules which are known as microsomes. There 
 may be also found in the cytoplasm various foreign inclosures, such as 
 particles of pigment, droplets of oil, some apparently empty spaces known 
 as vacuoles, etc. All such inclosures are collectively known as meta-, 
 para- or dentoplasm. There is also very often seen a condensation of the 
 peripheral part of the cytoplasm, constituting what is called an ecto- or 
 exo plasm, which without any sharp lines of demarcation passes into the 
 more central part, called endoplasm. A cell-wall or -membrane must be 
 considered as a highly specialized part of the exoplasm. 
 
 Nucleus. — In the great majority of instances, the nucleus is a spheri- 
 cal or oval body, situated usually either in the center of the cell or nearer 
 to one pole of it than to the other, and varies in size generally in propor- 
 tion with the size of the cell. In general its structure may be said to be 
 somewhat similar to that of the cytoplasm, as it also consists of a reticu- 
 lum or network, and a substance, more fluid in nature, filling the meshes 
 of it. The network of the nucleus is however more complicated in its 
 structure. By means of staining the specimens with so-called nuclear 
 stains it has been revealed that the nuclear network consists of two 
 different substances, first, of an exceedingly delicate, non-stainable reticu- 
 lum, apparently very similar to the one of the cytoplasm, and called 
 linin; secondly, of a substance which has the form of threads, or of a 
 network, or of a mass of granules, and has the power of absorbing very 
 actively certain dyes. This latter substance has therefore received the 
 name chromatin substance in contradistinction to all other substances 
 of the nucleus, which are not affected by the dyes, and therefore known 
 as oxhro matin substance. 
 
 The nucleus is separated from the cytoplasm by a distinct line of 
 demarcation, which is known as nuclear membrane. This may however 
 disappear at times, thus making possible an interchange of the substance 
 of the nucleus with that of the cytoplasm. It generally takes place 
 during the process of the so-called cell-division, of which we will speak 
 later. 
 
VITAL MANIFESTATIONS OF CELLS 239 
 
 Nucleolus. — This is a small body found in the center of the nucleus 
 in the great majority of cells. In some instances, as for example in nerve 
 cells, it becomes very conspicuous. The significance of it in the cell- 
 life is however not as yet well established. 
 
 Centrosome. — This is a minute body observed in cells in close relation 
 to the nucleus and generally situated just outside of the nuclear membrane. 
 It consists of a more homogeneous substance and contains in its center a 
 small dot, the centriole. The centrosome can not always be seen, but it 
 becomes very conspicuous during a certain period of activity of the cell, 
 namely, during cell-multiplication, when it plays a very significant role 
 
 Other constituents of cells have been mentioned already. 
 
 VITAL MANIFESTATIONS OF CELLS. 
 
 In animals consisting of but one cell, the so-called unicellular organ- 
 ism, all life functions are naturally exerted by this one cell. The results 
 of microscopic and experimental investigations of the last half century 
 have however led to the conviction that the life activities of all the familiar 
 higher animals, in health as well as in disease, are dependent on the 
 activities of the cells of which they consist. It is for that reason that, 
 while the study of the details of this subject belong to the domain of 
 physiology, some of the more important points of it are also generally 
 considered in connection with the description of cell-structure. The 
 most conspicuous vital characteristics of cells are the following: 
 
 Metabolism. — Of all the vital activities of cells, this is the most essen- 
 tial one. It represents the sum total of all those physico-chemical proc- 
 esses taking place in cells which have as their ultimate result the main- 
 tainance of the life and individuality of the cells. It therefore embraces 
 the process of taking into the body material suitable for its nutrition; 
 the process of converting some of it into a part of its own substance to 
 replace that which became worn out; transformation of another part 
 into various kinds of energy; finally, the elimination of subtsances, not 
 suitable for further use, as waste-products. 
 
 Growth. — Newly-formed cells are generally not of the same size as 
 mature cells, therefore a part of the substances assimilated by young cells 
 is used up by them for the increase of their size. This goes hand in hand 
 with the shaping of the cell until it becomes identical with the one from 
 which it originated. 
 
 Irritability. — This is a faculty of living cells to respond to various 
 influences: mechanical, chemical, thermical, etc. It generally results in 
 
240 HISTOLOGY 
 
 those peculiar manifestations of the cells to which they are specially 
 adapted, for example, secretion, motion, etc. 
 
 Conductivity. — This is the ability of cells to convey and transmit 
 impulses from one part of the cell to another and from one cell to another. 
 
 Motion. — This is the property of cells to change their relative posi- 
 tion to the surrounding media. It occurs in three different forms: 
 
 (1) Amoeboid Motion. — Similarly to the amoeba, cells may send out thin 
 or thick projections from their body, called pseudopodia, and if such pro- 
 jections become attached to some foreign object and the rest of the body 
 is drawn after it, there results a creeping motion. Cells endowed with 
 such motion are known as wandering cells. In some instances such cells 
 may, by means of their processes, surround foreign bodies and either 
 incorporate them by assimilation into their own body-substance, or 
 deposit them by expulsion into various other localities. Such wander- 
 ing cells are known as phagocytes. 
 
 (2) Ciliary Motion. — Some cells, usually those columnar in shape, 
 have one of their free surfaces beset with a number of delicate, transparent, 
 hair-like processes called cilia, which are constantly lashing forward 
 and backward (reminding of the blades of grass in a field, when acted 
 upon by a strong wind) producing a strong current, thus sweeping along 
 various small substances. 
 
 (3) Contractile Motion. — This is generally observed in fiber-shaped 
 cells, such as are found in muscles. It consists of a shortening of the fibers 
 in the direction of its long axis. The result is an approximation of the two 
 ends of the fiber and consequently an approximation of the parts to 
 which the two ends are attached. 
 
 Reproduction. — While all the just mentioned cell-properties either 
 serve to maintain their own individuality, or are the manifestations of 
 their function in the animal economy, reproduction is the property of 
 cells to form new cells similar to their own, a kind of rejuvenation, as it 
 were. The object of this is either to substitute new cells for the ones 
 which have become worn out and cast off, or, by adding new ones to their 
 own ranks, increase their functional ability. 
 
 The studies of the structural changes which take place in cells 
 during cell-reproduction, have been pursued with great zeal and enthu- 
 siasm during the last four or five decades, because it was through these 
 studies that the true and complex structure of cells was revealed. We 
 will therefore consider this phenomenon in its details at this point. 
 
 The cell-structure, as described on page 238, is considered as the 
 one which presents the cell in its so-called resting state, and the changes 
 
VITAL MANIFESTATIONS OF CELLS 
 
 241 
 
 which have been observed in it during the process of reproduction are 
 the following: 
 
 In the first place, the centrosome, whether it has previously been visible 
 or not, becomes very distinctly defined, and divides in two, which grad- 
 ually separate from one another, and ultimately pass to opposite poles 
 
 
 /--■chr. ', ^ 
 
 Y.-sp. 
 
 Fig. 182. — Diagram of indirect or mitotic cell- division; A, cell with nucleus in resting stage; 
 B, cell with nucleus in skein stage and with centrosomes separating; C, formation of chromo- 
 somes; D, longitudinal splitting of the chromosomes; E, separation of the chromosomes; 
 F, diaster stage; G, H, formation of the two daughter cells; c, centrosome; cl, chromatin thread; 
 chr., chromosomes; nu, nucleus; n, nucleolus; sp., nuclear spindle; w, cell wall. (After 
 Galloway.) 
 
 of the cell; they remain connected with each other however by means 
 of finely drawn-out non-stainable fibers, known as the achromatic spindles. 
 In the meantime the nuclear membrane disappears and the chroma- 
 tin substance of the nucleus, whatever be its form during the resting 
 stage, assumes the form of a continuous, densely coiled-up thread, which 
 16 
 
242 HISTOLOGY 
 
 is known as the close skein. This coil gradually becomes in a measure 
 entangled, and is then known as the loose skein. 
 
 The next step is one of the most significant ones in the whole process. 
 The whole chromatin thread becomes broken up into a number of seg- 
 ments known as chromosomes, and it is a very important fact that, while 
 the number of chromosomes varies in cells of animals of different species, 
 there is always an equal number of them in cells of animals of the same 
 species. The chromosomes become arranged in the equatorial plane 
 of the spindle in a somewhat star-like fashion, and this is then spoken of 
 as the monaster stage. Next, a splitting of each individual chromosome 
 in its longitudinal direction takes place, and in this way the number of 
 chromosomes becomes doubled, each one consisting now of a pair, or 
 presenting a twin-chromosome, as it were. The two chromosomes of 
 each pair gradually separate from one another and one of each pair ulti- 
 mately passes to opposite poles of the spindle, near the centrosome. Here 
 they also assume a star-like arrangement, and we have there what is 
 known as the Master -stage. The chromosomes are generally regarded at 
 the present time as the bearers of heredity. 
 
 The changes, which take place in each one of the stars, from now on 
 present simply the reverse of those described above for the formation of 
 the monaster, i.e., the individual chromosomes become united with one 
 another, thus forming two skeins. At first the skeins are loose, then a 
 condensation takes place and two close skeins are formed. Finally, 
 around each one of them a membrane is formed, and there appear two 
 nuclei similar to the one from which they originated. 
 
 While this final shaping of the nuclei is going on, the cytoplasm of 
 the cell becomes constricted half-way between the two nuclei, and when 
 this is completed a division has thus taken place, and we have two cells 
 instead of the original one. 
 
 The various investigators have laid particular stress on one or the 
 other stage during the process of cell-multiplication. Accordingly, various 
 names have been given to the same process, they all have their justifica- 
 tion and are in use as synonyms. Taking into consideration the ultimate 
 result of the process, it is called cell- division. In view of the fact that 
 the changes are most conspicuously going on in the nucleus, it has been 
 spoken of as karyokynesis. Because the chromatin substance of the 
 nucleus assumes the form of a thread, the process is called mitosis. Finally, 
 as this form of cell-division is accomplished in a rather complicated 
 round-about way, it is also known as indirect cell-division. 
 
 While the process of cell-division just described is the most frequently 
 
ORGANS AND TISSUES 
 
 243 
 
 met with, there can be observed a more simple one, which leads to the 
 same end. Without any preliminary rearrangement in its structure, the 
 nucleus of the cell becomes elongated, then it assumes a dumb-bell shape, 
 finally the connecting neck becomes broken across and thus two so-called 
 daughter nuclei are formed which gradually separate from one another; 
 in the meantime a constriction of the cytoplasm takes place, and as a 
 result there appear two cells instead of one. For the reason that in 
 this form of cell-division the nuclear substance does not assume any 
 thread-like arrangement, and the end is accomplished in a rather simple 
 
 Fig. 183. — Direct cell division (Amceba). — A, active specimen with pseudopodia;l becom- 
 ing spherical preliminary to division; C, beginning of elongation and constriction; D, later 
 stage; E, daughter cells forming pseudopodia; ec, exoplasm; en, endoplasm;/, food particle; 
 n, nucleus; ps, pseudopodium; v, vacuole. (After Galloway.) 
 
 manner, this process is called amitosis or direct cell-division. This 
 process has been observed in the white corpuscles of the blood, occasion- 
 ally in the liver, and not infrequently in pathologic conditions; it is assumed 
 that this mostly occurs in cells having a lowered vitality, there is however 
 much to be learned yet in regard to this question. 
 
 ORGANS AND TISSUES. 
 
 All through the animal kingdom, with the exception of the unicellular 
 organisms, we observe, among the various elements of which they consist, 
 the manifestation of a phenomenon commonly known as division of 
 labor. We find groups of cells united together in various fashions to 
 perform a certain function, and such aggregations of cells, specialized to 
 fulfill well-defined duties in the economy of the organism, are called organs. 
 Only among the lower animals, however, do we find that the organs consist 
 
244 HISTOLOGY 
 
 of only one kind of cell. Higher in the scale of animal organization, we 
 find the organs exhibiting more and more complexity in their make-up. 
 They consist of groups of various kinds of cells, and the individual groups 
 present very little similarity to each other, sometimes becoming modified 
 to such an extent that it is very difficult to recognize the cellular nature 
 of them. While the name organ conveys the idea of a physiological unit, 
 the texture of it is known as tissue. If the tissue presents an aggregation 
 of elements, similar in character, we speak of simple tissues; and where 
 we have an aggregation of elements of various kinds, we speak of complex 
 tissues. 
 
 To gain a proper understanding of the characteristic appearance of 
 the various simple tissues, their various modifications, combinations and 
 transformations in forming that multitude of structures, which is ob- 
 served in a mature organism, it is necessary to trace them to their first 
 beginning, or histogenesis. 
 
 We have stated elsewhere that an animal body takes its origin from 
 the egg, ovum, which is nothing else but a single cell. While ova of 
 various types of animals differ in size, they all have relatively the same 
 structure and are all specialized for the same purpose — to produce a new 
 individual. The changes which an ovum undergoes in course of its 
 development into a new individual are in all cases principally the same, 
 and therefore the knowledge gained from the study of one form serves us, 
 in a general way, to understand all others. The study of these consecu- 
 tive changes constitutes the subject of General Embryology and 
 Histogenesis. 
 
 General Embryology and Histogenesis. 
 
 In all animals with sexual mode of reproduction, the cell from 
 which the whole organism ultimately develops is itself a product of 
 the union of two highly differentiated and specialized cells; one, supplied 
 by the male individual and formed in the testicle — the spermatozoon; the 
 other, supplied by the female individual and formed in the ovary — the 
 ovum. These two so-called elements of reproduction are derived by trans- 
 formation of special cells in the body, the so-called germinal cells, which 
 are set aside, as it were, for the purpose to be eliminated from the body 
 at certain periods and used as a foundation for the propagation of the 
 species. The process of entrance of the spermatozoon into the ovum, 
 and the ultimate union and fusion of the two elements is known as 
 the fertilization. Before this fertilization can take place, however, the 
 
GENERAL EMBRYOLOGY AND HISTOGENESIS 
 
 245 
 
 ovum has to undergo a preliminary process called maturation, which 
 consists in the elimination of half of the quantity of its nuclear substance 
 in form of two small bodies, expulsed at one of the poles of the ovum, 
 and therefore called polar bodies. 1 The fertilized ovum is that cell from 
 which a new being gradually develops; and while the mode of develop- 
 ment varies somewhat in its details in different species of animals, the 
 general principles of the process always remain the same. In the follow- 
 ing we endeavor to give a short general account of the essential points 
 
 -s*. 
 
 **.- 
 
 Fig. 184. — Four stages in the maturation and fertilization of the ovum (partly diagram- 
 matic). A, formation of the polar bodies and entrance of the spermatozoon; B, the male and 
 female pronuclei; C, nuclei coming together; D, pronuclei uniting to -form segmentation 
 nucleus; e. «., egg nucleus; p. b., polar bodies; s, spermatozoon; s. c, sperm centrosome; 
 s. n., sperm nucleus; 5. e., segmentation nucleus produced by the union. (After Galloway.) 
 
 in the development of the ovum, but for the details of it we must refer 
 to the various text-books on embryology. 
 
 After a short period of rest, the fertilized ovum begins to undergo 
 the so-called process cleavage or segmentation. By means of karyo- 
 kinesis it divides into two halves, producing two cells. Each of these cells 
 in turn again divides, giving rise to four cells, and this is succeeded 
 by another division, forming eight cells, and by repeated division of this 
 kind there arises a solid mass of smaller cells called morula or mulberry 
 mass, from its resemblance to a berry. As the cells increase in number, 
 the mass also increases in size by the absorption of nutriment, and a 
 
 1 By expulsion of the polar bodies the number of chromosomes in the ovum is reduced 
 to one-half of it; the fusion with the spermatozoon, which is also supplied onlv with half the 
 number of chromosomes, the normal quantity is restored and the fertilized ovum thus contains 
 heredity bearing substance from both its progenitors. 
 
246 
 
 HISTOLOGY 
 
 gradual arrangement of the cells in a definite fashion takes place, varying 
 however with the character of the ovum. They may arrange themselves 
 
 Neph 
 
 Fig. 185. — Diagrams showing the development of the germ layers. {After van Beneden 
 andStohr.) A, Two-celled stage; B, four-celled stage; C, morula stage; D to I, transverse 
 section of various stages of the blastoderm. 
 
 in the form of a layer of cells spherically surrounding the so-called seg- 
 mentation cavity; or they may be spherically arranged around a mass of 
 yolk; or they may form a disk-like arrangement, floating as it were on 
 
GENERAL EMBRYOLOGY AND HISTOGENESIS 247 
 
 a spherical mass of yolk. In anv case we observe the cells to be cylin- 
 drical in shape and, lying side by side in regular fashion, form a some- 
 what skin-like arrangement. This is therefore called the blastoderm, 
 which means a germinal skin, and the individual cells are named blasto- 
 meres. The continuous multiplication of the cells of the blastoderm 
 results in either an infolding (invagination) or a splitting off (delami- 
 nation) of some of the cells, which by their own gradual multiplication 
 form a distinct continuous layer of cells beneath the first one. It is 
 then generally spoken of as a two-layered blastoderm, of which the 
 outer layer is called ectoderm or epiblast, and then the inner layer entoderm 
 or hypoblast. These two layers are called the primary germinal 
 layers, and the cells of which they consist show great similarities in 
 many respects. Very soon, however, some of the cells, on the surfaces 
 of the two primary layers which face each other, detach themselves 
 therefrom by the process of splitting off or delamination and lodge in 
 the gradually widening space between the layers. As the number of 
 these cells increases also through their own multiplication, it very soon 
 becomes possible to distinctly recognize a third layer of cells, which, on 
 account of its being situated between the other two, has received the 
 name of middle layer or mesoderm or mesoblast. With the formation 
 of the mesoderm, the blastoderm is said to consist of three germinal layers, 
 and with this the foundation for the development of the various tissues 
 and organs is finally established. The resemblance of the cells consti- 
 tuting the different layers gradually becomes more and more lost, and 
 differentiation then begins to take place. Through all the consecutive 
 changes, the derivatives of the ectoderm and entoderm retain the charac- 
 teristic tendency to be arranged in groups of cells lying closely side by 
 side in a simple or stratified fashion. The derivatives of the mesoderm 
 are, on the contrary, characterized by a looser arrangement of their cells, 
 some of which remain connected with one another by means of proto- 
 plasmic prolongations or processes, while others retain the faculty of 
 wandering away and intermingling with the derivatives of the other two 
 layers. Each of the three layers gives rise to well-defined structures 
 adapted to distinct physiological functions, but none of them can in this 
 respect be substituted by another, without producing abnormal conditions 
 in the organism. 
 
CHAPTER II. 
 
 Elementary Tissues: Epithelial Tissue, Connective Tissue, 
 Muscular Tissue, Nervous Tissue, Blood and Lymph. 
 
 The differentiation of the three layers of the blastoderm into various 
 tissues goes on parallel with the development of the body in general, and 
 with the growing complexity of organization along the scale of the animal 
 
 -yn£ ^ 
 
 Fig. 186. — A, Diagram of a longitudinal section through the body of a Hydra; it presents 
 the structure of an animal, in which the walls consist of only two layers of cells specialized to 
 perform all the function. B, a small portion of the wall more highly magnified. (After 
 Galloway.) 
 
 kingdom, the number of varieties of tissues as well as their complexity 
 also increases. On page 234 we have already indicated, however, that 
 we can resolve all the various tissues into five well-characterized typical 
 groups, which are known as elementary tissues. These are: (1) epithelial 
 
 248 
 
EPITHELIAL TISSUE 249 
 
 tissue, (2) connective tissue, (3) muscular tissue, (4) nervous tissue, 
 (5) blood and lymph. 
 
 Among the lower animals there are some which resemble to a con- 
 siderable extent the early embryonic stages of higher animals. Their 
 walls consist of two layers of cells, which resemble one another very 
 much, and are transformations respectively from the ectoderm and 
 entoderm. They are endowed with the ability to perform all functions 
 which characterize living bodies and represent that typical group of cells 
 known as epithelial tissue. Accordingly, in the higher animals, also, the 
 principal functionating parts of the various organs, including the nervous 
 system, are by various transformations derived from the ectoderm or 
 entoderm, and are either epithelial tissue proper or highly specialized 
 modifications of it. 
 
 The mesoderm begins to develop when the other two layers are 
 already well differentiated, and, accordingly, the tissues which take 
 their origin from that layer are found very sparingly, or not at all, in 
 lower animals, but become more and more conspicuous as we ascend in 
 the complexity of animal organization. It gives rise mainly to tissues 
 which, while not constituting the principal parts in the texture of various 
 organs, are nevertheless of very gerat importance to them and form indis- 
 pensable auxiliary parts of them. These are the connective tissues, 
 muscular tissue and blood and lymph. Some epithelial formations take 
 their origin in the mesoderm also, but their number is very limited. 
 
 We will now consider the individual tissues in detail. 
 
 I. EPITHELIAL TISSUE. 
 
 Epithelial tissue is found as coverings of all surfaces of the body, 
 those directly exposed to the air as well as those which form variously 
 shaped cavities and communicate with the air indirectly through narrow 
 or wide openings. The cells composing the epithelial tissue are known as 
 epithelial cells. These are definite in outline, show very clearly a cyto- 
 plasm and a nucleus, and lie side by side in a regular fashion. There is 
 just enough substance — so-called intercellular substance — between the indi 
 vidual cells to hold the cells together. Three principal forms of cells are 
 generally met with: (1) the flattened or squamous, (2) the cylindrical or 
 columnar, and (3) the many-sided or polyhedral. If a single layer of cells 
 is present, it is known as simple epithelium; if there are two or more super- 
 posed layers of cells present, it is spoken of as stratified epithelium. In 
 simple squamous epithelium the cells are flattened or scaly and the 
 
250 HISTOLOGY 
 
 nuclei are round and also flattened. In the stratified squamous variety, 
 which is the one most frequently met with, only the superficial layers are 
 squamous, while the deeper ones are more irregular, and may gradually 
 become columnar. 
 
 In columnar epithelium there is quite a variety in the outline of the 
 cells. In simple columnar epithelium the cells may be either long — 
 high columnar — or of a medium size — cuboidal — -or very short — low colum- 
 nar. They may be beset on their free surfaces with numerous minute 
 hair-like processes, which are constantly vibrating during life and are 
 known then as ciliated epithelium. In the stratified variety also the 
 superficial layer only may be typically columnar, while the- others may be 
 of a different shape. Among a continuous superficial layer of columnar 
 epithelium, there may be found scattered here and there cells which have 
 somewhat the shape of a conical cup and contain in their cytoplasm mucus 
 in various states of formation. From time to time a contraction of their 
 cell-body takes place, and their content, which is of a mucous character, 
 is poured out upon the surface. Owing to their shape, these cells are 
 called goblet cells. 
 
 The polyhedral epithelium may be found in various localities in the 
 body. If the body of the cells contains some pigment granules, it is 
 known as pigmented epithelium. When polyhedral or columnar epithe- 
 lium is forming the constituting part of the so-called glandular organs, it 
 is known as glandular epithelium. A very conspicuous variety of epithe- 
 lial cells of various shapes is represented in the so-called neuro-epithelium, 
 which is specialized for the creation and perception of the special senses. 
 
 Epithelial tissue never contains blood-vessels; their nutrition takes 
 place by the absorption of nutritive juices through the clefts between 
 the cells, or the cement-substance. It is obvious that when a stratified 
 epithelium consists of a large number of layers, the superficial ones may 
 receive very little nourishment or none at all, which accounts for the 
 constant exfoliation of cells from the surface skin and other parts. 
 
 II. CONNECTIVE TISSUE. 
 
 Connective tissue is the most widely distributed tissue in the animal 
 body. It holds the individual parts of other tissues together; it connects 
 the various tissues with one another, and at the same time keeps them 
 separated from one another; it gives firmness to the body as a whole, and 
 a support for the various organs within it; it forms variously constructed 
 channels for the distribution of the nutritive material to the various parts 
 
CONNECTIVE TISSUE 
 
 25 1 
 
 of the body. With the variety of functions just enumerated there is a 
 corresponding variety of forms in which connective tissue is found repre- 
 sented in the body. 
 
 We have stated above that connective tissue takes its origin from 
 the mesoderm. With the advance in the differentiation of the ectoderm 
 and entoderm the derivatives of these two layers gradually become 
 separated from one another, and the changes of the mesoderm follow 
 closely in their steps. It becomes split into two secondary layers, one of 
 which attaches itself to the derivatives of the ectoderm, ready to serve 
 them with the above-enumerated functions, and is called parietal meso- 
 derm, while the other plays the same role in regard to the derivatives of 
 
 n.c. ; 
 
 —ect. 
 
 md. 
 
 Fig. 187. — Diagrams of transverse sections through the body of an embryo of a vertebrate. 
 It shows the relation of the three germinal layers, ect., Ectoderm; ent., entoderm; mes., 
 mesoderm; coe., coelom or body cavity; sk., beginning of skeleton; «., beginning of spinal 
 cord; g. lumen of the gut. (After Gallcnvay.) 
 
 the entoderm, and is called visceral mesoderm. The gap, which remains 
 between these two secondary layers, is known as the body-cavity or 
 coelom. The characteristic feature of the mesoderm tissue is that the 
 cells do not lie side by side and form continuous layers, but are separated 
 from one another, sometimes quite considerably, and the spaces between 
 the cells are occupied by a substance somewhat gelatinous in consistency 
 which is called intercellular substance and is obviously a product of the 
 cells themselves. The cells are stellated or spindle-shaped, and their 
 thinned-out processes unite and interlace with one another, forming a 
 network. The formation of the various forms of connective tissue from 
 the mesoderm in course of development is due mainly to the differentia- 
 
252 HISTOLOGY 
 
 tion and various chemical changes which that intercellular substance 
 or matrix undergoes. In view of the fact that the intercellular substance 
 constitutes the predominating part in connective tissue, it is the fea- 
 ture upon which the classification of this tissue is based, and we can dis- 
 tinguish five well-defined characteristic groups: (1) mucous or embryonic 
 connective tissue, (2) ordinary or fibrous connective tissue, (3) cartilage, 
 (4) bone, (5) dentin. 
 
 Mucous or Embryonic Connective Tissue. — This tissue closely 
 resembles in its structure the mesoderm tissue in earliest stages of the 
 embryo, as described above. It consists of a semi-gelatinous, mucoid 
 matrix, and within it are scattered stellate or spindle-shaped cells, and 
 here and there thin fibers. The latter present mainly the elongated and 
 anastomosing processes of the cells, some of them however are un- 
 doubtedly products of the matrix itself, by a process that reminds some- 
 what of coagulation. This tissue is found at birth in the umbilical cord 
 as the so-called jelly of Wharton; in the adult human body it is found in 
 the pulp of the teeth and in the vitreous humor of the eye. 
 
 Ordinary or Fibrous Connective Tissue. — This tissue is present in 
 the skin and mucous membrane, in the intermuscular tissues, in tendons, 
 in fascia and aponeuroses, and in the tissues connecting various organs. 
 It is composed of a meshwork of fine fibers of two kinds. The first, which 
 makes up the greater part of the tissue, is formed of very fine, white, 
 structureless fibers arranged closely in bundles and bands crossing 
 and intersecting in a'l directions. The second variety, or the yellow, 
 elastic fiber, has a much sharper and darker outline, not arranged in 
 bundles, but is intimately mingled with the white fibers by twisting 
 around and among its filaments. These are known as the elementary 
 connective-tissue fibers. The size of the connective-tissue bundles 
 depends upon the number of elementary fibers present, and by a variation 
 in the arrangement of the bundles variety in the character of the fibro- 
 connective tissue is produced in different localities. When the fibrous 
 connective tissue is formed into an unbroken mass, as in mucous mem- 
 brane, the minute bundles are collected into smaller or larger groups 
 (the trabecidce), and these are in turn associated into groups. In the skin 
 and mucous and serous membranes, the trabecular of the connective- 
 tissue bundles are separated, and, by crossing and recrossing one another, 
 form a dense, fan-like structure. In other tissues, as the tendons and 
 fascia, the bundles are arranged in parallel layers. In the submucous 
 tissues the connective-tissue fibers are loosely woven, the fibers crossing 
 and intermingling, with the intervening spaces unusually large, resulting 
 
CARTILAGE 253 
 
 in a loose, flabby tissue. Two varieties of fibrous connective tissue are 
 distinguished — namely, (a) Compact or fibrillar connective tissue, form- 
 ing bands of either white fibers, or yellow elastic fibers, or mixed fibers; 
 and (b) loose or areolar connective, forming a network or reticulum. 
 Modifications of the loose variety are found represented in adipose tissue 
 and lymphoid tissue. 
 
 The fibrous connective-tissue cells are few in number, of several 
 varieties, and variously shaped, being flattened, stellate, or apparently 
 distorted by pressure from surrounding cells or fibrous bundles. In 
 the mucous membrane the cells are oblong and somewhat flattened, 
 having many branches which reach out and, uniting with like processes 
 from neighboring cells, form a network. Other connective-tissue cells 
 are comparatively larger, oval or rounded in form, granular in appearance, 
 rich in protoplasm, and are known as plasma-cells. The body of connect- 
 ive-tissue cells, besides containing a nucleus, frequently contains pigment- 
 granules; these are known as pigment-cells. These are seldom found in 
 mucous or serous membranes, being principally confined to the integu- 
 ment. Fat-globules may also be found in fibrous connective tissue, and 
 when of considerable size unite and form a rounded cell, called a fat-cell. 
 Numerous fat-cells uniting, and well supplied with blood-vessels and 
 nerves, form adipose tissue, or fat. Fat-cells are frequently found in 
 areolar tissue as well. When fibrous connective tissue is immediately 
 contiguous to epithelium, it becomes somewhat modified and a new 
 membrane is formed, called the basement membrane, or membrana propria. 
 This membrane is a thin, transparent, structureless layer, and, when in 
 connection with those mucous membranes provided with a layer of vas- 
 cular fibrocellular tissue, may appear as the formative substance out of 
 which successive layers of epithelial cells are generated. In the ducts 
 and glands — for example, the salivary glands — the basement membrane 
 forms the proper walls of the tubes, and the cells here generated, and 
 corresponding to the epithelial cells of the coarser mucous membranes, 
 are known as gland-cells, rather than epithelial cells. This, however, 
 is a distinction without a perceptible difference, the location and function 
 as secreting cells being alike in each. 
 
 Cartilage. — Cartilage is a semi-opaque, non-vascular tissue, white 
 in color, and composed of a matrix containing nucleated cells. The 
 matrix is somewhat elastic and rather dense. The cells are simple in 
 form, being spheric or slightly inclined to angularity. The variation in 
 the character of cartilage is due rather to the difference in the character 
 of the matrix than to the cellular structure, the principal variation in the 
 
254 HISTOLOGY 
 
 cells being in their size. The cells lie in the spaces or lacunae of the 
 matrix, which they completely fill. Investing the free surface of most 
 cartilaginous tissue (articular cartilage excepted) is a thin but tough 
 and firm fibrous membrane — the perichondrium. This membrane is well 
 supplied with blood-vessels and nerves, and is essential to the growth and 
 maintenance of the cartilage. There are three varieties of cartilage — 
 namely, hyaline cartilage, elastic cartilage, and fibrocartilage. 
 
 Hyaline cartilage is of a faint pearly-blue color, slightly transparent, 
 and is found investing the articular ends of the bones — for example, 
 the condyles of the mandible; also forming the costal and nasal carti- 
 lages, as well as those of the trachea, bronchi, and a part of the larynx. 
 Hyaline cartilage is distinguished by a granular or homogeneous matrix. 
 The cells, which contain a nucleus with nucleoli, are usually grouped 
 together in patches, and are somewhat irregular in outline, appearing 
 flattened near the free surface of the tissue in which they are placed, and 
 inclined to be perpendicular to the surface in the more deeply seated 
 portions. The matrix is dimly granular in appearance, resembling 
 ground glass, and receiving its name from this fact. That part of the 
 cartilage close to the perichondrium is supplied with cells much smaller 
 than those occupying the lacunae in the substance of the mass, and the 
 growth of the cartilage is most active in this part. Lining each lacuna 
 is a delicate membrane (the capsule), which primarily is but partly filled 
 out, but as the cell or cells increase in size, this membrane is carried to 
 the walls of the lacuna. Articular hyaline cartilage is non-vascular, 
 being nourished by the blood-vessels of the bone beneath. 
 
 Elastic cartilage is of a dull-yellow color, and is sometimes called yellow 
 cartilage. It is not present in the mouth, but occurs in the external 
 ear, in the epiglottis, and in part of the larynx. Its structural compo- 
 sition is quite similar to hyaline cartilage, but may be distinguished 
 from it by a network of fine elastic fibers which penetrate the matrix. 
 The cells are rounded or oval, containing nuclei and nucleoli. 
 
 Fibrocartilage is yellowish or milky white in color, and is much more 
 widely distributed throughout the body than the elastic variety. It is 
 present in the temporomandibular articulation. Like those previously 
 described, it is composed of cells and a matrix, the latter being made up 
 of fibrous connective tissue arranged in bundles, and for this reason it 
 is scarcely deserving the name of cartilage, only that in other portions 
 continuous with it cartilage-cells may be found in abundance. Between 
 the strata of the fibrous bundles are numerous nucleated cells, which are 
 oval and more or less flattened, and each enveloped in a delicate capsule. 
 
BONE 
 
 255 
 
 Cartilage is further classified into two divisions — temporary and 
 permanent — the former term being applied to that kind of cartilage which 
 in the fetus and in youth is destined to be converted into bone (for example, 
 Meckel's cartilage); the latter class including all those cartilages which 
 are generated as such, and continue to serve in that capacity. Temporary 
 cartilage closely resembles the hyaline variety, being formed of a matrix 
 in the lacunae of which the cells are located. These cells, however, are not 
 grouped together as in hyaline cartilage, but are more uniformly distrib- 
 uted throughout the matrix. 
 
 Bone. — Bone is mainly composed of tricalcium phosphate and cartil- 
 age. The matrix of osseous tissue has a distinguishing feature produced 
 
 Calcified 
 Matrix 
 
 Center~of Calcifica- 
 tion 
 
 Fig. 18S. — Developing Bone. X 40. 
 
 by the blending of organic and inorganic substances, resulting in hard- 
 ness, solidity, and elasticity. The combination of organic and inor- 
 ganic elements in bone is of such a nature that either part may be removed 
 without destroying the other. The matrix is composed of the salts 
 of lime, especially calcium phosphate, and of slender fibrils united 
 by a cement-substance into bundles of various sizes. The cement-sub- 
 stance is chiefly composed of insoluble lime-salts, principally carbonates 
 and phosphates. These two kinds of structure are found to be present 
 in different parts of the same bone, forming a dense or compact, and a 
 
256 HISTOLOGY 
 
 spongy or cancellated tissue. The former occur in the shaft of long 
 bones and in the outer layer of flat or irregularly formed bones. Cancel- 
 lated bone-substance occurs in the extremities of the long bones and in the 
 interior of flat and irregular bones. The irregularly formed maxillary 
 bones give place to both kinds of bony structure; the external layer of 
 the superior maxillae and the body and rami of the inferior maxilla are 
 composed of compact tissue, while the interior of these bones and the 
 condyloid processes of the mandible are spongy or cancellated in their 
 nature. When examined by the microscope the bony substance is found 
 
 :*3. \ 
 
 
 -;. £> I ->r>x 
 
 .». » . ■ *^ '■ ■ f o. .- - — -*■ <s 1 *■* 
 
 Concentric JHHH \v" "'* r-' . *V .' -■ " 
 
 Lamella j -j^-..—'""" » -■: -Jl' ~~ ."- " r - 
 
 . ' ■ - *• - * - - <• , «• • ~> ->. a 
 
 , r „ ^"* / - V - "*- H^j - ££ - „ 'x ^ 
 
 I 1 . 1 ■■'■'; 1 !." _ . ^- .'-•-. V > ""* ' - * v » ■* 
 
 Canal ^' s ""- — "3* t » ' V. . .; ' _- 1 . ■ j : , v. Haversian 
 
 Canal 
 
 
 Fig. 189. — Transverse Section though Shaft of Long Bone. X 30. 
 
 occupied by numerous little spindle-shaped spaces — lacuna. Branching 
 out from these in various directions are minute canals — canaliculi — 
 which anastomose with similar canals from neighboring lacunae. In the 
 maxillary bones no other canals than these may be visible, but if a trans- 
 verse section be cut through one of the long bones, an additional space 
 makes its appearance (Fig. 189) 
 
 These spaces are known as the Haversian canals. They are circular 
 in outline and appear as a center for a small, circular district mapped out 
 by concentric layers, the lacunas and canaliculi following the same con- 
 centric plan, and through each other communicating with the Haver- 
 sian canals. The general direction of the Haversian canals is longitu- 
 
BONE 257 
 
 dinal with the long axis of the long bones, and in the flat or irregular- 
 shaped bones they are somewhat irregular in formation and ramify in 
 various directions. In the osseous matrix each lacuna contains a bone- 
 cell. These are nucleated, protoplasmic cells. In developing bone, 
 these cells, which do not completely fill the lacunae, are connected by 
 numerous branches or processes passing through the canaliculi; in older 
 bone very few processes are observed. 
 
 There are two processes by which bone may be prepared for histo- 
 logical examination, by one method which results in the destruction of 
 
 
 Lacuna 
 
 Haversian 
 Canal 
 
 : . A ! ' 
 
 Fig. 190. — Longitudinal Section of Long Bone. X 30. 
 
 the organic elements, or by another which removes the inorganic ele- 
 ments. In the former process the organic matter is removed by simply 
 drying the structures, after which thin sections may be prepared and care- 
 fully examined under the microscope, when the Haversian canals, lacunae, 
 and canaliculi will be seen forming a complete concentric network. In 
 the latter method the inorganic substance is removed by immersing a 
 fresh bone in dilute picric acid, C 6 H 2 (N0 2 ) 3 OH, which readily decalcifies 
 it, and when properly prepared sections are placed under the microscope 
 the organic contents of the lacunae and canaliculi alone are visible. 
 
 The concentric laminae of bone is riveted together by numerous 
 delicate rods of processes named Sharpey's fibers, these delicate fibers 
 passing through the 'aminae to perform this office. 
 
258 HISTOLOGY 
 
 Periosteum and Bone-marrow. — The interstices of spongy bone 
 are filled with a soft mass — the bone-marrow — and the external surface 
 of the bone is covered by a fibrous membrane — the periosteum. This 
 membrane is absent where bones are joined to each other by ligament 
 or cartilage, and over articular surfaces. The periosteum is a compact 
 connective-tissue membrane. It consists of two layers: an outer, fibrous 
 layer rich in blood-vessels, which forms the connection with adjacent 
 structures; an inner or osteogenetic layer containing few blood-vessels, 
 loose in texture, but rich in elastic fibers and spheric connective-tissue 
 cells, with oval nuclei. These are the formative cells of bone and are 
 called osteoblasts. These cells appear in the lower strata of the inner 
 layer, or the layer in contact with the bone, and are especially numerous 
 during the period of development. Through the blood-vessels of the 
 bone the marrow, internally, is placed in communication with the perios- 
 teum externally; small branches given off from the numerous arteries and 
 veins of the periosteum enter the Haversian canals, upon which they pass 
 to the canaliculi, thus communicating with the blood-vessels of the 
 marrow. In like manner numerous nerves enter the substance of the 
 bone, first passing into the Haversian canals, after which they become 
 closely associated with the minute blood-vessels and are distributed 
 to the periosteum and bone-marrow. The bone-marrow, besides fill- 
 ing the interstices of the spongy substance, is also found occupying 
 the central cavity of long bones, and in the larger Haversian canals. 
 The marrow is of two varieties, distinguished by its color, being either 
 red or yellow. Red marrow is found in the flat bones (including the 
 maxillae), the vertebras, and ribs, while yellow marrow occurs in the 
 long bones of the extremities. Red marrow is composed of a delicate 
 connective-tissue network supporting, besides the marrow-cells, a few 
 fat-cells and giant-cells. In the long bones the yellow marrow is sur- 
 rounded by a connective-tissue membrane lining the medullary canals. 
 Marrow-cells and giant-cells are present in abundance. Marrow is very 
 vascular and contains many osteoblasts. 
 
 Dentin. — This structure, as well as cementum, which in many 
 particulars closely resembles bone, will be fully considered in connection 
 with the histology of the tissues of the teeth. 
 
 III. MUSCULAR TISSUE. 
 
 Muscular tissue consists of elongated or fiber-cells and according to 
 the structure of these fibers it is divided into three classes — non-striated, 
 striated and cardiac. 
 
MUSCULAR TISSUE 
 
 2 59 
 
 a 
 
 
 * ' - F 
 
 fff 
 
 -A 
 
 
 «/ 
 
 r 
 
 
 1 
 
 ' F 
 
 Fig. 191. — A. — Longitudinal section of smooth muscle fibers — a. muscle fiber; b. nucleus; 
 c. fibrous tissue between fibers. B. — Cross-section of smooth muscle fibers — a. perimysial 
 connective tissue; b. blood-vessel; c. nucleated fiber; d. nonnucleated fiber. C. — Longitudinal 
 section of voluntary muscle fibers. — a. sarcolemma; b. nucleus; c. end of muscle fiber; d. dark 
 bands; e. intermediate disc;/, nucleus; g. lateral disc. D. — Diagrammatic section of cross 
 and long striations — a. dark disc; b. lateral discs; c. intermediate disc. E. — Cross-section of 
 voluntary muscle — a. perimysium; b. endomysium; c, nucleus of perimysium; d. fibrillre; e. 
 nucleus of muscle; /, sarcolemma. F. — Longitudinal section of cardiac muscle fibers — 
 a. muscle fiber; b. nucleus; c. branch. G. — Cross-section of cardiac muscle fibers — a. peri- 
 mysial sheath; b. nucleus of sheath; c. muscle fiber; d. nucleus; e. radial plates of fibrilke. 
 (Radasch.) 
 
20O HISTOLOGY 
 
 Non-striated, Smooth, or Involuntary Muscular Tissue. — This 
 tissue consists of contractile fiber-cells which are elongated, spindle- 
 shaped, and cylindric, with exceedingly elongated extremities, which 
 become shorter and thicker through contraction. They are quite variable 
 in length (i/io to 1/450 of an inch), and are composed of a pale, homo- 
 geneous-looking protoplasm, each inclosing an elongated or rod-shaped 
 nucleus, which is flattened if the cell is so formed. The muscular fibers 
 are firmly bound together by a cement-substance, forming fasciculi, 
 which in turn are collected into strata or membranes, which may be dis- 
 posed parallel, or crossing and recrossing, forming an intricate network. 
 The connective-tissue septa provide a passageway for the larger blood- 
 vessels, while the capillaries penetrate the fasciculi forming a compli- 
 cated network with oblong meshes. Involuntary muscular tissue is not 
 found in the mouth except in the ducts of the salivary glands. They form 
 the main constituent part of the middle layer or coat of blood-vessels 
 and are particularly abundant in arteries. 
 
 Striated or Voluntary Muscular Tissue. — Striated muscular 
 tissue is composed of long, cylindric fibers, which are regularly transversely 
 striated. In most instances their extremities are attached to bones by 
 means of tendons, as, for example, the cheek- and lip-muscles. The 
 fibers are grouped together by fibrous connective tissue into various sized 
 bundles, forming fasciculi. There is much variation in the length of the 
 fibers composing the fasciculi in different muscles. In most instances 
 the fasciculi which serve to make up the bundles of a single muscle con- 
 tinue parallel with one another throughout their length. Surrounding the 
 whole muscle is a layer of connective tissue called epimysium; this pene 
 trates between the individual bundles and forms a covering for each one 
 of them, which is called the perimysium, and passing from this into the 
 substance of the bundle is a stilt more delicate connective tissue, the 
 endomysium, which separates the individual fibers from one another. 
 The former structure carries the larger blood-vessels and nerve-fibers, 
 while the latter supports the capillaries. 
 
 Each muscular bundle may again be divided into smaller bundles, 
 which in turn are ensheathed in a similar manner and further divisible, 
 so continuing until the primitive fasciculi, or so-called muscular fiber, 
 is reached. Striped muscular fiber consists of a structureless, elastic 
 sheath, the sarcolemma, which structure represents the cell-membrane, 
 and closely invests a number of filaments or fibrils. Besides the fibrillae, 
 there is contained within this fine, structureless, transparent membrane 
 the sarcoplasm, a faintly granular substance resembling protoplasm, 
 
NERVOUS TISSUES 261 
 
 but not identical with it. This substance serves in the capacity of a 
 matrix for the fibrillar. The nucleus is found beneath the sarcolemma. 
 The fibrillae are arranged parallel to one another, being supported by 
 the sarcoplasm. It will thus be seen that each fiber of a striated muscle 
 comprises the sarcolemma, the muscle-nuclei, the fibrillae, and, finally, 
 the sarcoplasm, filling all the interstices, first between the fibrillae of 
 each muscle-column, between the columns of each group, and between 
 the groups themselves. The disposition of the sarcoplasm may be most 
 favorably studied by a cross-section through the fibers, appearing as 
 a delicate but clear network, within the meshes of which are the muscle- 
 columns. Striated muscular fibers are usually tapering off and becom- 
 ing thinner toward their extremities. In rare instances they are branched 
 at their ends. This condition is present in the tongue, the extremities of 
 the fibers passing transversely into the oral mucous membrane, where 
 they become further subdivided. The striated or voluntary muscles 
 make up the muscular tissues of the lips, cheeks, tongue, and soft palate. 
 Cardiac or Involuntary and Striated Muscular Tissue. — This 
 tissue is found only in the heart. The fibers are short, striated and have 
 the nucleus in the center. 
 
 IV. NERVOUS TISSUES. 
 
 Until within recent times it has been stated that the nervous tissue 
 consists of two histologic elements known as nerve-cell and nerve-fiber ; 
 that these two elements differed not only in their mode of origin, but in 
 their structure and physiologic endowments. At the present time it 
 is believed that the entire nervous system consists of an infinite number 
 of definite independent morphologic units, which, through having a com- 
 mon origin and a similarity of structure, have, nevertheless, different 
 functions in different parts of the body. This neurologic unit has been 
 termed the neuron, and, as represented schematically in figure 192, may be 
 said to consist of: First, the nerve-cell, or neurocyte; second, nerve- 
 process, or axon; third, the end-tufts, or terminal branches. Each of 
 these three main portions of the neuron presents a variety of secondary 
 features which are related to their functional activities. 
 
 The Nerve-cells, or Neurocyte. — The nerve-cells are found in the 
 cortex of the brain, in the interior of the spinal cord, in the various 
 ganglia of the cerebrospinal and sympathetic nervous systems, and in the 
 organs of special sense. All neurocytes are the modified descendants of 
 
262 
 
 HISTOLOGY 
 
 independent oval or pear-shaped cell 
 
 16 
 
 ''■ Dendrites 
 
 — Collateral Branch 
 
 Fig. 192. — Diagram 
 Stbhr's ' 
 
 .Terminal Branches 
 
 of a Neuron. (From 
 Histology.") 
 
 s (the neuroblasts) , originating from 
 the epithelial cells which form the 
 medullary tube. The neurocyte is 
 at first smooth, devoid of processes, 
 and endowed with ameboid move- 
 ment. In the course of develop- 
 ment the cells project a greater or 
 less number of processes and as- 
 sume a variety of shapes and sizes, 
 in accordance with variations in 
 functions; thus, the cells may be 
 spheroid, pyramidal, spindle- 
 shaped, stellate, etc. The body of 
 the cell consists of a protoplasmic 
 basis, more or less granular, con- 
 taining a well-defined nucleus and 
 nucleolus. A centrosoma has also 
 been found in the nerve-cell in 
 many situations. There is no evi- 
 dence, however, of the existence of 
 a cell-membrane. From the body 
 of the neurocyte there arises one 
 or more protoplasmic processes, 
 which, passing outward in various 
 directions, divide and subdivide 
 into a greater or less number of 
 branches, which are collectively 
 known as dendrites or dendrons. 
 The ultimate subdivisions and 
 terminations of a dendrite, though 
 forming an intricafe feltwork, 
 always end free, never anastomos- 
 ing with one another. Arising from 
 the cell-body, the dendrites resemble 
 in appearance and structure the 
 cell-protoplasm, or cytoplasm. In 
 the cortex of the cerebrum and in 
 the cortex 0} the cerebellum the 
 dendrites are characterized by 
 short, lateral projections known 
 
NERVOUS TISSUES 263 
 
 as lateral buds or gemmules, which impart to the dendrite a feathery 
 appearance. 
 
 The Axon, or Nerve-process. — The axon is the first outgrowth of the 
 protoplasm of the neuroblast, but with the development of the neurocyte 
 it becomes so differentiated from the dendrites that it can be readily 
 distinguished from them. It usually arises from a cone-shaped projection 
 of the cell-body, though occasionally it arises from a dendrite itself. It is 
 characterized by a short, regular outline and a hyaline appearance. The 
 majority of the cells, especially in the mammalia, possess but one axon, 
 though in the developing ganglion-cells of the spinal nerves two distinct 
 axons are present. In their subsequent development the two axons appear 
 to blend together to form but a single axon, which, at a short distance from 
 the cell, again divides into two branches, which pursue opposite directions, 
 one passing directly into the spinal cord, the other toward the periphery. 
 The axon may continue as an individual structure for an indefinite dis- 
 tance, varying from a few 'millimeters to too cm. In the former instance 
 the axon, at a distance of a few millimeters from the cell, breaks up into 
 a number of branches, which form an intricate feltwork in the neighbor- 
 hood of the cell. This type of cell is not widely distributed, being 
 confined largely to the cerebellum. In its course the axon, more espe- 
 cially in the central nervous system, gives off a number of side-branches or 
 collaterals, which do not differ from the axon itself, either in structure or 
 appearance. The axon of the peripheral nerves, especially the spinal 
 nerves, are devoid of collaterals throughout their extent, except, perhaps, 
 in the immediate neighborhood of the cell. The more or less elongated 
 axon becomes inclosed at a short distance from the cell with a thick 
 layer of fatty material, forming a medulla or myelin, inclosed by a deli- 
 cate cellular sheath (the neurilemma), and thus constitutes what is com- 
 monly known as a medullated nerve-fiber: In the central nervous sys- 
 tem the neurilemma is frequently wanting. In the sympathetic system 
 the myelin is wanting, though the axon is inclosed by a delicate sheath 
 resembling the neurilemma, thus constituting a non-medullated nerve- 
 fiber. The collateral branches are provided with similar investments. 
 
 The End Tufts, or Arborizations. — Each axon, as it approaches 
 its final termination, breaks up into a number of branches, which vary 
 in complexity and appearance in different regions. They are always 
 free from any medullary investment, and appear to be formed by the 
 splitting of the axon into a number of fine filaments, which remain inde- 
 pendent of one another. In peripheral organs, as muscles, glands, and 
 blood-vessels, the tufts are in direct organic connection. In the cen- 
 
264 
 
 HISTOLOGY 
 
 tral nervous system the end-tufts are in more or less intimate relation 
 with the dendrites of other neurons. 
 
 Nerves. — Nerves are to be regarded, therefore, as groups of axons, 
 
 Fig. 193. — A. Multipolar cell from cerebral cortex; B. multipolar cell from spinal cord; 
 C. pyramidal cell from cerebral cortex; D. unipolar cell; E. bipolar cell; F. cell of Purkinje, 
 antler cell; G. mossy cell; H. spider cell; I. cell from spinal cord of an ox, showing pigment 
 granules; K. ganglion; L. sympathetic or amyelinated fibers; M. longitudinal section of 
 myelinated nerve fiber — a. neurilemma; b. myelin sheath; c. axis cylinder; d. node of Ranvier; 
 e. nucleus; N. cross-section of osmicated nerve fibers; O. myelinated nerve fiber of a guinea- 
 pig showing; the reticulum P. myelinated nerve fibers of a toad, showing reticulum (neuro- 
 keratin); R. motor neuron, showing nerve cell, dendrites, axis cylinder and ending of latter 
 in a muscle, S. cross-section of nerve trunk. (Radasch.) 
 
 with their medullary investments connecting the peripheral organ with 
 the central nervous system. 
 
 The nerves are arranged in two great systems — the cerebrospinal 
 
NERVES 
 
 265 
 
 and the sympathetic. In the cerebrospinal nerves the conducting 
 media — the nerve fibers — are arranged in parallel or interlacing bundles, 
 and these are further grouped into nerve-branches or nerve-trunks. 
 The bundles are connected by intervening fibrous connective tissue 
 (the epineurium) , and through this tissue the principal blood-vessels 
 ramify to supply the nerve-trunks, together with a plexus of lymphatics 
 and numerous fat-cells and plasma-cells. 
 
 The size of the nerve-bundles, or funiculi, is regulated according 
 to the size and number of nerve-fibers which they contain. Investing 
 each funiculus, or primary bundle of nerve-fibers, is a connective-tissue 
 sheath — the perineurium. The fibers composing this sheath are arranged 
 in lamellae, being separated from one another by lymph-spaces vari- 
 able in size, through which communication is afforded the lymphatics of 
 the epineurium. Within the bundles the nerve-fibers are held together 
 by fibrous connective-tissue — the endoneurium. The epineurium holds 
 
 Epineuriurr 
 
 Perineurium 
 
 Endoneurium 
 
 Perineurium 
 
 Fig. 194. — Transverse Section, Bundles of Nerve-fibers, Human Median Nerve. X30. 
 
 together and envelops the several funiculi of the nerve-trunk, the peri- 
 neurium investing each funiculus, or primary bundle of nerve-fibers, 
 and the endoneurium extending among and around the individual fibers. 
 Nerve-fibers are divided into two classes — which classification is depen- 
 dent upon the presence or absence of a medullary sheath or covering — 
 into the medullated or white, and the non-medidlaled or gray. The 
 medullary sheath, or white substance of Schwann, is a bright, fatty sub- 
 stance (the myelin) surrounding the axon, or axis-cylinder, the conducting 
 
266 HISTOLOGY 
 
 or central part of a nerve-fiber. Between the medullary sheath and the 
 axis-cylinder there is present a small amount of albuminous fluid. Closely 
 surrounding the medullary sheath, and forming the outer boundary 
 of the nerve-fiber, is the neurilemma, or sheath of Schwann. Between 
 this delicate, structureless membrane and the medulla there are placed 
 at intervals oblong nuclei, surrounded by protoplasm; these are the 
 nerve-corpuscles. Besides the division of nerve-fiber into medullated 
 and non-medullated, each division is susceptible of further subdivision, 
 dependent upon the presence or absence of the neurilemma. Non- 
 medullated nerve-fibers without a neurilemma are composed of an axis- 
 cylinder only; they are cylindric or band-like in form, transparent, and 
 show faint, longitudinal striations. Non-medullated nerve-fibers with a 
 neurilemma are composed of an axis-cylinder surrounded by a neurilemma, 
 and are homogeneous throughout their extent. 
 
 Medullated nerve-fibers are those which are partly, but never entirely, 
 invested by a medullary sheath. They may or may not possess a neuril- 
 emma; in the former instance they consist of an axis-cylinder and a 
 medullary sheath only. The axis-c>linder, or essential part of the nerve- 
 fiber, is cylindric or band-like, occasionally exhibiting a delicate, longitu- 
 dinal striation, which appearance is due to its being composed of a primi- 
 tive fibrillae. 
 
 The nerve-cells or ganglia-cells are found in the ganglia as well as 
 along the course of the nerves. They are composed of granular or 
 faintly striated protoplasm, inclosing a characteristic nucleus within which 
 is a nucleolus. They differ greatly in form as well as in size, the spheric, 
 spindle-shaped, and irregularly stellate forms being the most common. 
 In the latter numerous processes are given off, forming the stellate out- 
 lines. The cells are variously named, according to the number of proc- 
 esses. If one process is present, the cell is termed a unipolar cell; if 
 two, a bipolar; and if a number of processes exist, they are named multi- 
 polar. The processes are of two varieties — the axis-cylinder process 
 and the branched protoplasmic process. The various forms are most 
 readily distinguished in the multipolar cells. The axis-cylinder process 
 is readily characterized by its hyaline appearance and unbroken outline. 
 The protoplasmic processes are thicker, granular, and striated. 
 
 V. BLOOD AND LYMPH. 
 
 It is rather difficult to conceive that blood and lymph may be spoken of 
 as representing one of the elementary animal tissues. The conception of 
 an elemetary tissue, as exemplified in the four tissues just considered, 
 
BLOOD AND LYMPH 267 
 
 carries with it the idea of some well-defined stationary structure. Blood 
 and lymph on the contrary, present fluids, which uninterruptedly stream 
 along within closed channels — the blood-vessels and lymph-vessels. A 
 closer study of the development and structure of blood and lymph, how- 
 ever, reveals a very close relation and great similarity of their cellular 
 elements to those of the other elementary tissues. On the other hand, 
 we must take into consideration that blood and lymph constitute the 
 medium by means of which nutrient and building material is carried to 
 all remotest, as well as minutest parts of the body, and in exchange re- 
 ceives and carries away the waste products. Now, it is quite obvious 
 that the distribution of blood and lymph throughout the body can only 
 take place when the free motion of its constituent parts is not handicapped 
 by a tenacious or solid intercellular substance. It can only be accom- 
 plished because the intercellular substance is a fluid. If a quantity of 
 blood is drawn and exposed to the air it coagulates, which means, that the 
 fluid intercellular substance becomes fibrous, and as the density gradually 
 increases its real similarity to other elementary tissues becomes very 
 evident. While, thus, blood and lymph present conspicuous peculiarities, 
 it is nevertheless justifiable to consider them as a distinct elementary 
 tissue. We may define it as: "a tissue like all other tissues, consisting 
 of cells and intercellular substance, but in which the intercellular substance 
 is a fluid." 
 
 Development. — Blood and lymph, as well as the channels in which 
 they originate in the mesoderm, from groups of cells especially prede- 
 termined for that purpose. Some of the cells become joined together by 
 means of their protoplasmic processes in a chain-like fashion, and grad- 
 ually becoming flattened, form continuous epithelium-like layers known 
 as endothelium or mesoihelium, which ultimately become arranged into 
 the shape of tubes. Other cells of the same group do not exhibit any pro- 
 cesses of their cytoplasm, they are more or less spherical in outline, and 
 while the formation of the tubes is going on, these cells remain separated 
 from one another and become imprisoned, as it were, within the latter. 
 The gradually developing tubes are the future blood- and lymph-vessels, 
 and the cells inclosed within the latter are the future blood- and lymph- 
 cells or corpuscles. Between the individual corpuscles there gradually 
 forms an intercellular substance, which becomes differentiated into a 
 fluid known as blood- and lymph plasm and thus a possibility for the cells 
 to stream along within closed channels and fulfill the duties assigned 
 to them in the economy of the body, becomes established. 
 
 We also observe, however, that many of the cells belonging to the same 
 
268 
 
 HISTOLOGY 
 
 group do not meet the fate of the cells just described but retain an in- 
 dependent individuality. Some of them become endowed with a vital 
 characteristic peculiar to one of the lowest unicellular organisms — the 
 ameba, which is known as ameboid motion. Owing to this faculty, 
 these cells are constantly changing their place and travel along between 
 the cells in the variously differentiated tissues, and are therefore known 
 as wandering or migratory cells. Some of the migratory cells are able 
 
 M.T 
 
 Fig. 195. — Section from the Head of a Rabbit Embryo of Ten and One-half Days, 4.4 mm., 
 to show Mesenchyma. (Slohr.) Epi. and M.T., Ectodermal epithelium of the epidermis and 
 medullary tube, respectively. N., Nucleus; P., protoplasm; and I.S., intercellular substance 
 of a mesenchymal cell. Two of these cells show mitotic figures. B.V., Blood-vessel, lined by 
 endothelium. One of the blood-vessels contains an embryonic red blood corpuscle. 
 
 to incorporate into their cytoplasm various substances of inorganic or 
 organic nature, as well as whole living micro-organisms. They may 
 either digest them or in traveling along, carry these substances and in- 
 cidently deposit them in other localities, sometimes quite remote from the 
 place where they have been picked up. These cells are known as phago- 
 cytes, and are of great significance in the body economy. 
 
 Some of these migratory cells are identical with the cellular elements 
 found in the lymph, and are therefore known as lymphoid cells. These 
 cells are found distributed throughout the body and, in various localities 
 
BLOOD AND LYMPH 269 
 
 they aggregate into larger masses and form the so-called lymphoid tissue 
 and lymphoid organs, of which we will speak later. 
 
 Structure. — We have to keep in mind that the ultimate role of blood 
 and lymph is to keep up the interchange of substances in the body, which 
 in a general way takes place as follows: The lymph absorbs the ultimate 
 products of digestion, and delivers them, through the thoracic duct, to 
 the blood. Laden with these substances, the blood passes through the 
 lungs and here it gives off the deleterious C0 2 to the atmosphere and 
 receives in exchange a corresponding amount of O. It then passes to 
 the heart, and through the activity of that organ it becomes distributed 
 throughout the body. Now, just as the function of the blood differs from 
 the function of the lymph so these two tissues differ in their structure. 
 The active elements in the process of absorption taking place in the 
 lymph are the above- described migratory cells. They constitute the 
 cellular elements of the lymph and are therefore known under the common 
 name of lymphocytes. The active elements in the process of exchange of 
 gases taking place in the blood are cells containing a chemically active 
 substance hemoglobin, which is the cause of the red color of the blood, 
 and these cells are therefore known as the red corpuscles or erythrocytes. 
 In view of the fact however, that the lymph, as stated above, ultimately 
 enters and becomes a component part of the blood it is obvious that the 
 blood must contain the active elements of both, and we actually find in 
 the blood two kinds of cellular elements, which are correspondingly 
 called: (a) colored or red blood corpuscles or erythrocytes and (b) colorless 
 or white blood-corpuscles or leucocytes. The average number of these 
 per cubic millimeter is 5,000,000 red corpuscles to 8,000-10,000 white 
 corpuscles, making a ratio of 500 :i. 
 
 Erythrocytes. — The red blood-corpuscles are usually described as 
 colored, non-nucleated, biconcave, circular discs, of 7.5-8 micr. or 
 1/3200 inch, in diameter. When blood is at rest, the corpuscles show 
 a decided tendency to stick together in clumps or be piled up in columns 
 known as rouleaux. The main characteristics of the erythrocites have 
 always been, and still continue to be, a matter of investigation and dis- 
 cussion, and the following points must be noted. 
 
 The color is due to the iron-containing chemical compound, which is 
 known as Hemoglobin. When examined singly, the corpuscles have a 
 rather greenish-yellow tint, but when aggregated in larger quantities, and 
 suspended in their natural medium, the blood-plasm, they appear red. 
 
 In regard to shape, there is hardly any doubt that the form of circular 
 biconcave disks is greatly predominating. Some, however, present a 
 
270 
 
 HISTOLOGY 
 
 cup-shape; we also see some of a spherical or globular shape, and a very 
 conspicuous form is the one known as crenated. This variety of forms of 
 the red corpuscles can be explained without difficulty if we take into 
 consideration the condition of the fluid in which they are suspended. It 
 is obvious that when the density of the plasm is decreased the corpuscles 
 will become more permeated with the fluid and will expand. This ex- 
 pansion may cause a bulging out only on one side of the disc and will 
 produce a cup-shape, or it may cause a bulging out on both sides and the 
 globular form will be the result. On the other hand, if the density of the 
 
 Fig. 196. — Red Corpuscles in Various Conditions. (Stohr.) A, Biconcave disc as seen 
 with objective lowered and C, with objective raised B, Seen on edge. D, Cup-shaped form. 
 E, Irregular contractions and distortions. F, Corpuscles from which the hemoglobin is re- 
 moved. (Shadows.) G, Crenated form. H, Extrusions from the corpuscle. 
 
 plasm is increased, by evaporation for example, the surface tension of the 
 corpuscles will be decreased, an irregular shrinkage will take place and 
 the crenated form will result. 
 
 In regard to the nucleus, we have to state that, while the colored 
 blood-corpuscles of lower animals are nucleated, those of human blood 
 and blood of other mammals are, when examined in the adult, always 
 non-nucleated. This fact made it rather doubtful, whether these cor- 
 puscles can be considered as cells, as it is not compatable with our defini- 
 tion of a cell as: a mass of protoplasm containing a nucleus. The study 
 of the development of the blood has however entirely cleared up that 
 point. We have stated above, that blood and lymph, as well as the 
 channels in which it flows, originate in the mesoderm, from cells specially 
 destined for that purpose, which are therefore known as angioblasts. 
 Those of the angioblasts, which later become the cellular elements of 
 the blood are known as hematoblasts and are like the rest of the mesoderm 
 cells, nucleated. A number of these hematoblasts become gradually 
 colored through formation of hemoglobin within the cells and are then 
 known as erythroblasts. These cells still contain a nucleus, but gradually 
 the latter disappears and they become moulded into the well-known 
 erythrocytes. The exact process of the disappearing of the nucleus is not 
 
BLOOD AND LYMPH 27 I 
 
 positively established as yet; some investigators are of the opinion that 
 the nucleus becomes extruded from the cell and may be found as a dis- 
 tinct small corpuscle within the blood, others think that the nucleus 
 becomes entirely dissolved and no traces of it are left. Be that as it 
 may, tjje fact established beyond doubt is, that all red corpuscles possess a 
 nucleus in the early stages of their life-history and therefore must be con- 
 sidered as true cells, which have undergone important modification. 
 Deprived of the nucleus, the red blood-corpuscles have lost their ability 
 to produce new cells, and consequently new corpuscles can only be sup- 
 plied by gradual transformation from undifferentiated hematoblasts. 
 This process takes place throughout the entire life of the organism, 
 mainly in the bone-marrow, and it is in this locality where we can observe 
 and study the erythrocytes in all stages of their development. 
 
 Leucocytes. — The white blood-corpuscles are colorless spherical bodies, 
 which are constantly present in blood, but owing to the fact that they are 
 colorless and their number is very much smaller than the number of the red, 
 they are not so easily detected. These corpuscles present all attributes 
 of true cells and have always been considered as such. We can distin- 
 guish in them a cytoplasm and a nucleus, and a great majority of them 
 manifest ameboid movement. Due to the latter faculty they can make 
 their way out or in through the thin walls of the fine blood channels and 
 creep about as wandering cells among other cells of the tissues. During 
 their excursions some may incorporate various particles which they are 
 liable to meet with and also extrude them here and there on the route 
 which they cover, thus acting as phagocytes. 
 
 Within the last few decades, the migratory activity of the leucocytes 
 and their phagocytic action became to be considered as one of the most 
 important factors in carrying and transmitting disease-producing germs. 
 They are also considered as factors of great importance in curing, as well 
 as preventing, diseases, by holding up these germs, destroying them and 
 removing the debris from the body. It is due to this fact mainly, that 
 the white blood-corpuscles have attracted the attention of many distin- 
 guished investigators, and many interesting and important facts have been 
 revealed. It was in these studies particularly that the great importance of 
 using various dyes for microscopic examinations has been very clearly 
 demonstrated, since it was the application of the various stains which led 
 to the most important discoveries in this domain. We know at present, 
 that the cells of the blood which are known collectively as leucocytes, are 
 not all of the same kind. They differ in size as well as in structure and 
 the following classifications and nomenclature is in vogue at present. 
 
272 HISTOLOGY 
 
 (1) One classification is based upon the size of the cells and the 
 character of the nucleus, and there can be distinguished four varieties 
 of cells from this point of view. 
 
 (a) Small Mononuclear Leucocytes. — As the name indicates, these cells 
 contain only one nucleus and are of small size. They are also known as 
 small lymphocytes, and constitute about 20 per cent, of all leucocytes in 
 the blood. 
 
 (b) Large Mononuclear Leucocytes. — They are also known as large 
 lymphocytes, and their number is only about 1 per cent. 
 
 (c) Polynuclear or Polymorphonuclear Leucocytes. — As the name indi- 
 cates, these cells have several nuclei, and the shape of the latter varies. 
 This is the predominating form among the leucocytes of the blood, and 
 constitutes about 70 per cent. 
 
 (d) Transitional Leucocytes. — Under this heading are grouped leuco- 
 cytes with a structure somewhat midway between the others. They con- 
 stitute about 7 per cent. 
 
 (2) Another classification is based upon the presence or absence of 
 various granules in the cytoplasm. These granules can be distinguished 
 only when various stains are applied and manifest a difference in their 
 affinity to the various dyes. This method of studying blood for leuco- 
 cytes constitutes at present every-day work, we might say, of a clinical 
 examination. There have been many forms of leucocytes described and 
 quite a number of names are used. It is beyond the scope of this book 
 to describe them all, but we think that it is necessary that the readers 
 should be familiar with the principal points of this subject. To under- 
 stand the classification given below it is necessary to keep in mind the 
 following point. The anilin dyes used for microscopic examination are 
 procured in form of powders, which present chemical compounds known 
 as salts and are soluble in water or alcohol or in both. Like other 
 salts, they consist of a base and an acid and it has been found that in 
 some dyes the basic principle has the staining qualities, in others the acid 
 principle has the staining qualities, while in a third group the staining 
 qualities are due to both. We therefore distinguish three kinds of stains: 
 (a) basic stains, (b) acid stains, (c) neutral stains. On the other hand it 
 has been conclusively demonstrated that some of the granules in the cy- 
 toplasm are stainable with basic stains, and these are therefore called 
 basophilic granules, others are stainable with acid stains and are there- 
 fore known as acidophilic granules, and still others are stainable with 
 both and are known as neutrophilic granules. With these facts kept in 
 mind, the following classification will be conceived without difficulties. 
 
© 
 
 6 
 
 
 '','•'• *••*•• "*X'''"'iv °* 
 
 o 
 
 
 
 12 
 
 > til 
 
 MP 
 
 Fig. 197. — -Chief varieties of cells encountered in health and disease (Wright's stain). 1. 
 Normal red cell. 2. Common form of polymorphonuclear leucocyte. 3. Lesser lympho- 
 cyte. 4. Eosinophilic myelocyte. 5. Eosinophilic leucocyte. 6-6. Neutrophilic leucocytes: 
 upper left, transitional form, on right neutrophilic myelocytes. 7-7. Large lymphocytes. 
 8. Normoblast (angioblasts) . 8. Normoblast showing division of nucleus, o. Normo- 
 blast nucleus. 10-11. Basophilic leucocytes. 12. Megaloblast. (Greene's Medical Diag- 
 nosis.) 
 
BLOOD AND LYMPH 273 
 
 (a) Acidophiles. — Leucocytes with granules stainable with acid 
 stains. They are also known as Eosinophiles, because the stain used 
 when they were first described was eosin. 
 
 (b) Basophiles. (Coarse). — They are also called "mast cells." 
 
 (c) Basophiles. (Fine). — They are identical with the large mononu- 
 clear leucocytes. 
 
 (d) Neutrophils. — They are identical with the polymorphonuclear 
 leucocytes. 
 
 It is now a very well-established fact that various diseases are char- 
 acterised by the predominance in the blood of the patients, of one or the 
 other of the various forms of leucocytes. It has also been found that in 
 the same disease the character of the leucocytes as a predominating 
 feature may differ during the various stages of the disease. These facts 
 are undoubtedly of great significance, but the causes underlying them are 
 still a mystery. 
 
 Blood-platelets.— Besides the erythrocytes and leucocytes, there has 
 been observed in the blood a third form of corpuscles, which are known 
 as blood-platelets. These are very minute circular discs, which are con- 
 sidered to be of importance as factors in blood coagulation, and are there- 
 fore also called thrombocytes. In regard to their structure the statements 
 of the various investigators differ very materially, and also in regard to 
 their origin many views have been expressed. While some investigators 
 consider the platelets as distinct independent cellular elements, others 
 look upon them as disintegrated fragments of erythrocytes or of leuco- 
 cytes. Some see in them the expelled nuclei of erythrocytes, others con- 
 sider them as constricted and discarded protoplasmic parts of leucocytes. 
 We must say, that notwithstanding the great amount of research work 
 done and quite considerable literature accumulated, the mystery surround- 
 ing these corpuscles still remains unsolved. 
 
CHAPTER III. 
 
 Circulatory Organs, Glands. 
 
 In the previous chapters we have described the elementary tissues 
 and have pointed out that each one of these tissues is differentiated to per- 
 form a well-defined function in the economy of the organism as a whole. 
 The functions of the individual tissues however become manifest only when 
 two or more of the elementary tissues are associated to form more or 
 less complex units known as organs. There are a number of organs in 
 which dentists are more or less interested, as these organs either form 
 the wall of the mouth cavity, or are situated within the mouth cavity, 
 or in close proximity to it. As regards their gross anatomical relations, 
 these organs have been considered in the first part of this book and we 
 are now ready to study their minute anatomy. 
 
 ORGANS OF CIRCULATION. 
 
 The organs of circulation consist of a system of tubes of various sizes 
 known as blood-vessels, and a central apparatus which furnishes the 
 motive power for propelling the blood within these tubes — the Heart. 
 
 If we consider, for example, the blood circulation in the mandible, 
 we find that the blood starting from the heart takes its course first to the 
 aortic arch. Here it is distributed among vessels of smaller caliber, 
 one of these being the common carotid artery. From this channel, the 
 blood is again distributed into arteries of still smaller diameter, and a 
 part of it enters the inferior dental artery which also branches off into a 
 number of still smaller vessels which conduct the blood to the pulps of 
 the individual teeth. Here we find the blood circulating in a network 
 of fine vessels known as capillaries. The walls of these capillaries are 
 very thin and therefore a close contact of the blood with the tissues takes 
 place. Becoming laden with the waste materials of the tissues, the blood 
 continues its journey through blood-vessels known as veins, which grad- 
 ually increase in size. Finally it enters the vena cava inferior, and is 
 thus brought back to the heart. 
 
 Of the three kinds of blood-vessels mentioned; the first, 
 
 274 
 
ORGANS OF CIRCULATION 
 
 275 
 
 i. Arteries carry the blood to the tissues; the second, 
 
 2. Veins carry the blood back from the tissues, while the third, 
 
 3. Capillaries distribute the blood among the tissues and thus become 
 instrumental in performing the function assigned to the blood in the 
 economy of the body. 
 
 Fig. 198. — Blood-vessels from a Rabbit Embryo of Thirteen Days, developing as Endo- 
 thelial Sprouts (en) from pre-existing vessels (bv) ; b.c, blood corpuscle within a vessel. (Stohr.) 
 
 Conforming to their functions, the three kinds of blood-vessels show 
 marked differences in their structure, and the understanding of this 
 subject may be facilitated by referring to the mode of development. 
 
 We have stated before (p. 267) that some cells in the mesoderm become 
 arranged in a simple squamous epithelium-like fashion, known as endo- 
 
 Fig. 199. — Small Arteries of Man. (Stohr.) Nuclei of endothelial cells; m, nuclei of 
 circular muscle fibers, at -m' seen in optical cross-section; a, nuclei of connective tissue. In 
 A, since the endothelium is out of focus, its nuclei are not seen. X240. 
 
 (helium, and form tubes consisting of a single layer of endothelium or 
 mesothelium. The walls of the capillaries consist practically only of a 
 single layer of endothelial cells, but as the caliber of the tubes increases 
 the walls become strengthened with a few strands of connective tissue. 
 The inner lining of all parts of the circulatory apparatus show the same 
 
276 
 
 HISTOLOGY 
 
 kind of an endothelial lining, and the differences which are manifest, are 
 found only in the tissues which surround this lining and give the tube 
 more strength, firmness and elasticity, depending on the size of the blood- 
 vessel, the amount of blood it contains and the degree of pressure which 
 it exercises upon its walls. If we take as typical, a medium sized artery, 
 we generally recognize three coats: 
 
 Endothelium 
 
 Intima 
 
 Media 
 
 Externa 
 
 Smooth 
 muscle fibers 
 
 Fig. 200. — Portion of a Cross-section of the Brachial Artery of Man. Xioo. (Stohr.) 
 
 i. The inner coat (intima) which consists of a single layer of endo- 
 thelial cells resting upon a subendothelial connective tissue and a strand 
 of elastic tissue known as the inner elastic lamina. 
 
 2. The middle coat (media) which consists of several layers of smooth, 
 short, circularly arranged muscle fibers interwoven with connective- tissue 
 fibers, mostly of the elastic variety. 
 
 3. The outer coat (adventitia) which consists of connective tissue which 
 ultimately blends with the connective tissue surrounding the artery as a 
 whole. 
 
 Veins have, broadlv speaking, the same structure as arteries but 
 differences are shown in the relative quantity of one or the other elementary 
 
ORGANS OF CIRCULATION 
 
 277 
 
 
 2, 
 
 
 Endothelium 
 
 Inner elastic 
 membrane 
 
 Connective 
 tissue 
 
 Nuclei of 
 
 smooth 
 
 muscle 
 
 fibers 
 
 Connective 
 tissue 
 
 Fig. 201. — Part of a Cross-section of a Vein from a Human Limb. X230. (Slohr.} 
 The elastic elements are shown very black. 1, Intima; 2, media; 3, externa. (The middle 
 of the three objects labled nuclei of smooth muscle is apparently an elastic fiber.) 
 
 mm/ 
 
 Fig. 202. — Section of the Heart. (Stohr.) ca, Capillaries; en, endothelium; La., left 
 atrium; I. v., left ventricle; mes., mesothelium (of the epicardium, or visceral pericardium); 
 p.c, pericardial cavity; p.p., parietal pericardium; r.a., right atrium; r.v., right ventricle; 
 si., sinusoids; v.b., bicuspid valve; v.t., tricuspid valve; v.v.s., valve of the venous sinus. 
 
278 HISTOLOGY 
 
 tissues. In a medium-sized vein, we generally find the walls thinner, 
 and the inner elastic lamina of the intima is much thinner or is entirely 
 wanting. The media shows less muscle fiber and more connective tissue. 
 The adventitia shows the same structure as that of the arteries, but is 
 considerably thicker. 
 
 While arteries and veins constantly have blood passing through them, 
 their tissues are not nourished from this source, but receive the blood 
 supply needed for their metabolism by means of special blood-vessels 
 situated within the outer connective-tissue 'layer {adventitia) which are 
 known as vasa vasorum. 
 
 If we now examine the structure of the heart, we find the following: 
 
 1. . The inner lining as a continuation of the inner lining of the blood- 
 vessels, consisting of a layer of endothelial cells and subendothelial con- 
 nective tissue corresponding to the intima and known as Endocardinm. 
 
 2. The middle coat (myocardium) which forms the bulk of the heart, 
 and consists of many layers of muscular tissue running in various direc- 
 tions and showing a special structure of the so-called cardiac muscle. 
 • 3. The outer coat (epicardium) shows some peculiarities, inasmuch as 
 it consists of a layer of endothelial cells and some subendothelial con- 
 nective tissue forming the so-called Epicardium. This layer when it 
 reaches the roots of blood-vessels, turns back again and forms the so- 
 called Pericardium. In the heart and in a number of veins, the endo- 
 cardium bulges out in the form of folds and forms the valves. In regard 
 to the nourishment of the heart, we repeat the same that was said 
 in regard to the blood-vessels, namely that while all the blood passes 
 through the heart, the supply of blood for the metabolism of the heart 
 tissue itself, is carried on by special vessels, the so-called coronary arteries 
 and veins, which therefore correspond to the vasa vasorum of the blood- 
 vessels and are located in a position corresponding with the latter, namely, 
 the connective tissue of the outer layer, the epicardium, beneath the 
 endothelium. 
 
 GLANDS. 
 
 A group of organs distributed in various localities throughout the 
 body and which are of interest to the dental student are the so-called 
 glands. Many of them are considered in the appropriate chapters in this 
 book, here only a few points of a more general character should be 
 mentioned. 
 
 Broadly speaking, glands are organs consisting of one or more ele- 
 
GLANDS 
 
 279 
 
 Epitheimm l V: ^si W^^L f 
 
 Tunica 
 propria 
 
 1 1 
 
 Submucosa 
 
 Muscle fibers 
 
 Fig. 203. — Vertical Section through the Mucous Membrane of the Lip of an adult Man. 
 X30. (Stohr.) 1, Papilla; 2, excretory duct; the lumen is cut at only one point; 3, accessor}' 
 gland; 4, a branch of the excretory duct in transverse section; 5, gland bodies grouped into 
 lobules by connective tissue; 6, a gland tubule in transverse section. 
 
 Part of an Excretory Duct ^^&j^^^|S|l 
 
 A Crescent Consisting 
 of Eight Serous Cells 
 
 £*W* 
 
 >. 
 
 3> -^F?|r^\n J^W 
 
 Tangential 
 Section of 
 Serous Cells 
 
 Cross Section 
 with Mucous 
 Cells and (left) 
 thick Men- 
 brana Propria 
 
 Lumen 
 
 Fig. 204. — Section of a Human Sublingual Gland. X252. (Stohr.) 
 
280 
 
 HISTOLOGY 
 
 mentary tissues but in which the epithelial tissue takes the leading part 
 in performing the function of the given organ. The function of glands 
 is to produce various substances of great importance to the economy of 
 the organism; and the epithelial cells in the glands are the elements which 
 are specialized and adapted to do the work of secreting these substances. 
 In lower animals we find distributed among other epithelial cells indi- 
 vidual secreting cells which are known as unicellular glands. In higher 
 
 Excretory duct 
 
 Traces of secre- 
 tory ducts 
 
 End pieces 
 
 Fig. 205.- — Diagram of the Human Sublingual Gland. {Stohr.) 
 
 animals we also find in various localities individual cells which are 
 endowed with the faculty of secretion, and these cells are known as 
 goblet cells. As a rule, however, glands consist of groups of cells united 
 together for the common function of secretion. The secreting cells are 
 arranged in the form of tubes or sacs and the substances which they pro- 
 duce are conveyed to the surface through outlets known as ducts. Owing 
 to the fact that the secreting cells in glands are arranged in a variety of 
 shapes, the following classification is in vogue: 
 
 I. Tubular Glands. 
 
 a. Simple; b. compound. 
 II. Saccular or alveolar Glands or racemous glands, 
 a. Simple; b. compound. 
 
 III. TUBULO-ALVEOLAR GLANDS. 
 
 Glands are situated either within the surface coverings themselves as 
 for example in the mucous membrane of the digestive tract, or they are 
 
GLANDS 
 
 28l 
 
 imbedded in connective tissue beneath the mucous membrane known as 
 submucosa as for example in the skin or in the mucous membrane of the 
 lip. A number of glands are located as separate bodies imbedded 
 among other important organs, as for example the various salivary glands, 
 and communicate with the surface upon which they pour their products 
 by means of long ducts. A description of the various glands in detail 
 will be found in the following chapters. 
 
 Beside the type of glands just described, there are a number of glands 
 which produce substances of very great importance in the economy of 
 the body, but these substances are not conveyed to the surface and there- 
 
 x- 
 
 > 
 
 #.* 
 
 -~ J>, —Qfc •* V *? - i 
 
 mm 
 
 . > v : 
 
 *■* i 
 
 
 n 
 
 Fig. 206. — Longitudinal Section of a Lymph Node. (Radasch.) 
 a, Hilus; b, arteriole; c, venous sinuses; d, adipose tissue; e, secondary nodule of cortex; 
 /, vein of medulla; g, subcapsular lymph sinus; h, germinal center of secondary nodule; 
 i, i, trabecular; k, capsule; /, lymph sinus; m, medullary cord. 
 
 fore these glands have no direct outlets in the form of ducts and are known 
 as ductless or internally secreting glands. The substances produced by these 
 glands come in close contact with the blood through the walls of the capil- 
 lary blood-vessels distributed throughout the texture of these organs and 
 are taken into the blood and thus become distributed throughout the body. 
 The glands belonging to this group are the thyroid bodies, the parathyroids, 
 the adrenal and the hypophysis. The significance of these organs and 
 their products is still a subject of considerable discussion. 
 
 There is also a third group of organs, which have long been classi- 
 fied as glands and even at present are occasionally considered under 
 that heading, the so-called lymph glands. It was stated on page 267 
 that the so-called lymphoid cells aggregate in larger quantities within 
 
282 HISTOLOGY 
 
 the meshes of areolar tissue and form the lymphoid tissue. In some 
 localities we find more dense aggregations of lymphoid cells which are 
 known as lymph follicles. In the center of the follicle, the cells are 
 more loosely arranged and quite a number of mitotic figures can be 
 observed. This indicates that multiplication of cells is taking place 
 and therefore the lymph follicles are considered as the main localities 
 where lymph cells originate in the adult. In various regions of the 
 body are found interposed in the lymphatic circulation a number of 
 lymph follicles grouped together and surrounded by a connective tissue 
 capsule, and these are known as lymph nodes or incorrectly lymph 
 glands. As examples of such nodes may be mentioned: cervical nodes, 
 parotid nodes, lingual nodes, submaxillary nodes, etc. If we recall, 
 however, that under the name gland we understand an epithelial organ 
 the cells of which are endowed with the power of secretion it is obvious 
 that, when this name is applied to an organ consisting of lymphoid 
 cells which never secrete, it is a misnomer. 
 
CHAPTER IV. 
 
 The Mucous Membrane of the Mouth; of the Lips; of the Cheeks; 
 of the Gums ; of the Roof of the Mouth, Hard and Soft Palate ; 
 of the Floor of the Mouth; the Tongue. 
 
 HISTOLOGY OF THE TISSUES OF THE MOUTH. 
 
 Mucous Membrane of the Mouth. — The mucous membrane lining 
 the cavity of the mouth consists of two parts — the epithelium and the 
 tunica porpria; beneath the latter, and forming the deeper part of the 
 mucous membrane, is the submucosa. 
 
 Older layer 
 of Cells 
 
 Infant Lay- 
 er of Cells 
 
 Connective 
 Tissue 
 
 Fig. 207. — Vertical Section, Mucous Membrane of the Mouth, Human Embrvo. X150. 
 
 The epithelium of the mouth is a thick, stratified, squamous epithe- 
 lium, the most superficial cells being scale-like or horn-like. The cells 
 are arranged similar to those in the epiderm, the lower layers are 
 columnar in form, and contain very little pigment. 
 
 283 
 
284 HISTOLOGY 
 
 The tunica propria is a somewhat dense feltwork of interlacing connect- 
 ive-tissue bundles, interspersed with elastic fibers. The tunica propiia 
 penetrates the epithelium in the form of cylindric or conic papillae, which 
 differ in length with the variation in the thickness of the epithelium. 
 As the mucosa is usually thickest in the lips, gums, soft palate, and uvula, 
 accordingly the papillae are of the greatest length in these parts. The 
 tunica propria passes into the submucosa so gradually that a positive line 
 of demarcation cannot be established. 
 
 The submucosa consists of a bundle of fibrous connective tissue with 
 but few elastic fibers. This structure is somewhat loose in texture and 
 is loosely attached to the underlying periosteum. Over the major portion 
 of the gums and the entire hard palate the submucosa is attached to the 
 bones of the mouth through the medium of their periosteal covering. 
 It is in this loosely constructed tissue that the glands of the mucous mem- 
 brane are situated. These are for the most part branched, tubular, mucous 
 glands. Besides adipose tissue in the form of groups of fat-cells, striped 
 muscular tissue is present in the submucosa. In some parts of the mouth 
 this tissue forms a conspicuous portion— namely, in the sphincter muscle 
 of the lips (orbicularis oris) ; also in the soft palate, uvula, and pillars of 
 the fauces. 
 
 The blood-supply to the mucous membrane of the mouth is principally 
 distributed in two systems, the larger vessels to the submucosa and the 
 capillaries to the tunica propria. The larger vessels break up and send 
 a dense network of capillaries through its substance and to the numerous 
 papillae which extend into the epithelium. Numerous veins ramify 
 through the superficial part of the tunica propria. The lymphatics form 
 two networks, the submucosa giving place to the coarser vessels, while 
 the fine parts are distributed to the tunica propria. 
 
 Nerve-supply to the Mucous Membrane of the Mouth. — In the sub- 
 mucosa the medullated nerve-fibers form a wide-meshed reticulum, from 
 which numerous primitive fibrillae pass to the tunica propria, where they 
 terminate or continue as non-medullated nerve-fibers, and penetrate the 
 papillae of the epithelium, forming networks. 
 
 Mucous Membrane of the Lips. — Beginning as a direct continuation 
 of the integument or external covering of the lips, the labial covering, 
 including the integument, may be divided into three parts — namely, a 
 cutaneous portion (best described in this connection), a transitory portion, 
 and a mucomembranous portion. 
 
 The cutaneous portion, covered by a thin epidermis, consists of a 
 double layer of somewhat flattened epithelium. Immediately beneath 
 
MUCOUS MEMBRANE OF THE LIPS 285 
 
 this is a thin, cellular, mucous layer, the cells composing it being spheroid 
 in form, and containing nuclei which are proportionately large. Sub- 
 jacent to this is the cutis, composed of fasciculi of fibers intersecting 
 and closely woven together, the principal fibers passing toward the free 
 border of the mucous membrane covering the contiguous surface of the lip. 
 These fibers are for the most part connective-tissue fibers, intermingled 
 with elastic-tissue fibers. Numerous small, vascular papillae are found 
 upon the surface of the cutis; these are cylindric or conic in form, and 
 project for some distance into the rete mucosa — the lower layers of 
 living cells of the epidermis. Equally distributed at various depths in 
 this tissue are numerous hair- and sebaceous follicles. The general 
 direction of the hair-follicles in the upper lip is downward, while those 
 occupying the lower lip are turned upward. Other than the distinction 
 noted by the difference in color of the parts, the cutaneous portion may 
 be distinguished from the transitional mucous membrane by the absence 
 of hair-follicles and sebaceous glands in the latter. 
 
 The transitional portion of the mucous membrane of the lips is out- 
 lined externally by the outer border of the red portion of the lips, and 
 internally by that prominent part of the labial convexity which comes in 
 contact with the opposing labial fold, leaving the transitional portion 
 exposed to view when the lips are in occlusion. The epithelial layer 
 of this surface does not begin where the hair-follicles cease to exist, a 
 slight interspace appearing upon the cutaneous portion which is devoid 
 of these follicles. At its line of beginning the transitional portion of the 
 labial mucous membrane is quite thin, but rapidly increases in thickness 
 in passing toward the mucomembranous portion. Superficially the 
 cells are much flattened, closely associated with one another, and devoid of 
 nuclei. The cells of the middle and deeper layers are oblong or spheric, 
 and provided with irregularly shaped nuclei. The chief fibrous tissues 
 of the transitional portion, which are thinnest at the point where the 
 hair-follicles cease to exist, are united into flexiform fasciculi, which are 
 separated at various points to give passage to numerous minute blood- 
 vessels. The fibrous tissues increase in thickness as the mucomembran- 
 ous portion is approached. Numerous thin and somewhat elongated 
 papillae are distributed over the surface of the transitional portion. 
 
 The mucomembranous portion of the mucous membrane of the lips 
 includes all that portion covering the labial folds within the mouth, 
 beginning at the line of occlusion on the contiguous surface and extend- 
 ing to the gums. The epithelium is much thicker than that previously 
 described and presents the characteristic layers common to stratified, 
 
286 
 
 HISTOLOGY 
 
 squamous epithelium. Superficially the cells are flattened and tubular, 
 provided with nuclei of similar form. In the middle layer the cells are 
 flattened and oblong, followed in the deeper layer by irregulary formed 
 nucleated cells. A variety of fibers make up the structure — one class 
 fine in texture and united into fasciculi, intermingled with elastic fibers, 
 together with another set of coarse, strongly looped fibers. Whenever 
 the fibers of the tunica propria assume a definite general direction, they 
 are horizontal, passing from right to left and encircling the oral aperture. 
 The tunica propria is beset with numerous conic papillae which project 
 
 Embryonal 
 
 Mucous 
 Membrane 
 
 Buccal Cavity 
 
 Fig. 208. — Vertical Transverse Section through Head of Human Embryo, about the Sixth 
 Week, showing Single Buccal Cavity. X30. 
 
 into the epithelium; these are longest where the epithelium is thickest. 
 The mucous membrane forming the labial frena is covered by an epithelial 
 layer which is much thinner than that distributed to other parts of the lips. 
 The fibers in these are irregularly distributed, and the papillae are small 
 and not so numerous. The coronary arteries and their accompanying 
 veins course through the lips near the junction of the transitional with 
 the mucomembranous portion of the mucous membrane. 
 
 Mucous Membrane of the Cheeks. — The mucous membrane of 
 the cheeks presents but little variation in its structure from that of the 
 mucomembranous portion of the labial mucous membrane. The buccal 
 epithelium is the same in structure and thickness as that of the lips, 
 
MUCOUS MEMBRANE OF THE GUMS 
 
 287 
 
 excepting the disposition of cells in the middle layer, where they are greater 
 in number and more closely associated, being somewhat distorted by con- 
 tact. The papillae, which project from the mucosa into the epithelium, 
 are somewhat broad at their base, with elongated extremities, the height 
 of which is quite variable, in some instances penetrating well into the 
 epithelium, at others merely entering its deeper layer. At the anterior 
 portion of the cheek, or that in the region of the angle of the mouth, 
 
 Fig. 20Q. — Section through the Mucous Membrane of the Cheek, showing the Papillae of 
 the Mucosa in Transverse Section. 
 
 the mucous membrane, by its submucous portion, is in immediate contact 
 with the fibers of the buccinator muscle, and throughout the entire surface 
 of the cheek it is closely associated with this muscle. The membrana 
 propria is dense immediately beneath the epithelium, but as the buccinator 
 is approached it becomes much less so. 
 
 Mucous Membrane of the Gums. — The mucous membrane cover- 
 ing these parts is, on account of the numerous tendinous fasciculi which 
 enter into its construction, extremely dense and tough, these character- 
 istics being more strongly manifest here than in any other portion of the 
 
258 HISTOLOGY 
 
 oral mucous membrane. These qualities arc especially pronounced 
 about the gingival margins and over the major portion of the 
 alveolar walls, being closely bound down to the bone by direct pro- 
 longations of the tendinous fasciculi of the periosteum which penetrate 
 the membrane. As the gingival mucous membrane passes into that of 
 the lips and cheeks it gradually becomes less dense. The epithelium 
 of the mucous membrane of the gums is composed of lamina of tessellated 
 
 Epithelium 
 
 Papilla 
 
 Fig. 210. — Section through the Gums, showing Epithelium and Basement Membrane. 
 
 and ribbed cells. The superficial cells are the flattened cells of pave- 
 ment epithelium; subjacent to this they become thicker and deeply ribbed, 
 while the deepest cells are conic or cylindric with conic extremities. The 
 tissue composing the tunica propria is made up of flattened fasciculi of 
 connective tissue, the fibers of which run parallel with one another. 
 Numerous elastic fibers are also present. Three sets of fibers are to be 
 distinguished in the mucous membrane of the gums — those which run 
 vertically, those which pass in a horizontal direction, and those which 
 radiate or are distributed fan-like. Of the first-named, the fibers extend 
 
MUCOUS MEMBRANE OF THE ROOF OF THE MOUTH 289 
 
 from above downward; in the second class they pass from right to left 
 parallel with the surface; the third class, including those fibers which are 
 reflected from the alveolodental membrane, are distributed in fasciculi 
 about the margins of the alveoli. 
 
 Mucous Membrane of the Roof of the Mouth. Hard Palate. — 
 The mucous membrane covering the hard palate is, in very many respects, 
 dissimilar to that surrounding the necks of the teeth and forming the 
 palatogingival margins. Like the mucous membrane of the gums, that 
 overlying the hard palate is dense and tough. The papillae of the tunica 
 propria, which penetrate the epithelium, are not so numerous as those 
 upon the gums. In the posterior third of the hard palate they are some- 
 what more numerous and generally a little more prominent than those in 
 the anterior portion. In the median raphe and over the rugae, the papillae 
 are especially sparingly distributed. The epithelium is of the pavement 
 variety, somewhat thinner in front than behind, the cells being more 
 freely distributed at some points than at others. The mucous membrane 
 of the hard palate is less in thickness anteriorly than posteriorly. The 
 distribution of the fibers is such that they radiate from the alveolar borders 
 toward the center of the palate, the anterior fibers passing obliquely 
 backward, while those from the lateral walls pass parallel with one 
 another to the median line. For the most part the fibers are broad and 
 form a plexus between the epithelium and the submucous tissue. The 
 submucous tissue is sparingly distributed over the central portion of the 
 hard palate, but laterally is somewhat more abundant, containing a few 
 fat-cells. 
 
 Soft Palate, Uvula, and Fauces.— Passing backward from the posterior 
 margin of the hard palate, the mucous membrane overlies the fibrous 
 aponeurosis of the soft palate and its median and lateral prolongations — 
 the uvula and pillars of the fauces. The epithelium is of the laminated 
 pavement variety, with the deeper cells larger than those placed super- 
 ficially. The substance of the mucous membrane is composed of fasciculi 
 of connective tissue, intermingled with a plexus of elastic fibers. The 
 fibers are distributed in three principal directions — from side to side, 
 or horizontally, longitudinally, and obliquely. The oblique fibers are 
 instrumental in forming the submucous tissue of both the soft palate and 
 uvula. Numerous conic papillae project from the tunica propria into the 
 epithelium; these are larger and more numerous on the uvula than on the 
 soft palate. The tunica propria is somewhat variable in thickness to 
 accommodate the glands, which are more or less numerous, and present 
 in greater numbers in one instance than in another. In general the 
 19 
 
290 
 
 HISTOLOGY 
 
 membrane as a whole is thinnest along the margin of the hard palate, 
 gradually increasing in thickness as the free border is approached. The 
 folds of mucous membrane forming the pillars of the fauces present no 
 peculiarity differing from that of the soft palate, save a more generous 
 supply of elastic fibers. 
 
 Mucous Membrane of the Floor of the Mouth. 
 
 The Tongue. — The entire unattached surface of the tongue is covered 
 by a reflection of the mucous membrane of the floor of the mouth. In this 
 organ the general structure of the mucous membrane does not vary from 
 that of other oral mucous membrane, being composed of an epithelium, 
 a tunica propria, and a submucosa. The mucous membrane covering 
 
 Epithelium 
 
 Fig. 211. — Longitudinal Section through Mucous Membrane of the Human Tongue. X 20. 
 
 the dorsum of the tongue presents special characteristics, which differ 
 from that of the under surface and the floor of the mouth in general. 
 In the former location the papillary elevations of the tunica propria are 
 conspicuously developed, and with their covering of stratified, scaly 
 epithelium cause the peculiar furred appearance. Three classes of 
 papillae are distinguished, named, in accordance with their form, filiform 
 papillae, fungiform papillae, and circumvallate papillae. 
 
 The filiform papilla, which are very numerous over the entire dorsum 
 and sides of the tongue, are conic and frequently prolonged into numerous 
 horn-like processes, known as secondary papillae. As elevations from 
 the tunica propria they are composed of well-defined fibrillated tissue, 
 
MUCOUS MEMBRANE OF THE FLOOR OF THE MOUTH 
 
 >QI 
 
 intermingled with numerous elastic fibers. The pavement epithelial cells 
 are found overlapping one another, and provided with processes which 
 project beyond the papillae. 
 
 The fungiform papilla are also distributed over the entire dorsum 
 and sides of the tongue, but are somewhat less numerous than the filiform 
 variety. They appear as well-defined elevations, and are connected 
 with the tunica propria by a constricted portion or neck. The entire 
 free or rounded surface of these papillae is beset with secondary papillae. 
 The epithelium is slightly thinner than that over the filiform papillae, this 
 being the principal distinguishing feature. The numerous capillaries 
 produce a rich red color, plainly observable through the transparent 
 
 Fig. 212. 
 
 -Longitudinal Section of the Mucous Membrane of the Human Tongue, showing 
 the Fungiform and Secondary Papillae. X 80. 
 
 epithelium. Connective-tissue bundles make up the bulk of these 
 papillae, few elastic fibers being present. 
 
 The circumvallate papilla are placed on the posterior portion of the 
 dorsum of the tongue, and are few in number (eight to sixteen). They are 
 much larger than those already described, and in general resemble modified 
 fungiform papillae. They are flattened and broad, and differ from the 
 fungiform by having a circular furrow or wall surrounding them. Second- 
 ary papillae are present on the free surface only, the sides, and in some 
 instances the walls surrounding them, being occupied by the end organs 
 of the special sense of taste — the taste-buds. Other taste-buds are found 
 upon the lateral margins of the tongue posteriorly, nestled in a group of 
 
292 
 
 HISTOLOGY 
 
 parallel folds of mucous membrane — the papillae foliata. The connective 
 tissue within these papillae is similar to that in the fungiform papillae. 
 On other parts of the tongue, or those portions not occupied by these 
 specially constructed papillae, the epithelium is similar to that in other 
 parts of the mouth. The tunica propria is less in thickness in and about 
 the tip of the tongue, and is intimately connected with the subjacent 
 muscular structure. As the root of the organ is approached, the tunica 
 propria becomes thicker and more dense. The submucosa is especially 
 
 Fig. 213. — Section through Epithelium, Near the Tip of the Tongue. X40. 
 
 intimately connected with the underlying parts at the margins and tip 
 of the tongue. The extreme portion of the root of the tongue has its 
 mucous membrane particularly modified by a special aggregation of 
 adenoid tissue — developed lymph-nodules. These are large and readily 
 perceptible to the naked eye. They are provided with a central opening, 
 which dips down into a well-defined vault or crypt, which is lined by a 
 reflection of the stratified oral epithelium. 
 
 Blood-supply to the Mucous Membrane of the Mouth. — The oral 
 mucous membrane derives its supply of blood from numerous branches 
 of the external carotid artery — namely, the superior and inferior coro- 
 nary, buccal, lingual, transverse facial, pterygopalatine, and the alveolar. 
 Entering the submucosa, the minute terminal branches of these arteries 
 are distributed parallel to the surface, and by anastomosis form plexuses 
 from which other minute branches are given off to supply the papillae of 
 the tunica propria. After coursing through the papillae the blood is 
 discharged into a similar venous plexus, and thus conveyed from the 
 
BLOOD-SUPPLY TO THE MUCOUS MEMBRANE OF THE MOUTH 293 
 
 parts. In a like manner the mucous membrane and papillae of the 
 tongue are supplied, branches of the lingual artery conveying the blood 
 to the parts. The dorsalis linguae supplies the mucous membrane of the 
 dorsum of the tongue and pillars of the fauces, while the ranine artery 
 by its minute branches supplies the remaining mucous membrane. Each 
 papilla is entered by two or more arterial terminals, which divide, anas- 
 tomose, and finally send off capillary branches to the secondary papillae. 
 Nerve-supply to the Mucous Membrane of the Mouth. — The 
 distribution of the nerve-fibers to the oral mucous membrane is approxi- 
 mately similar in all parts. The fibers, which are of the medullated 
 variety, are first distributed to the submucosa, forming a wide-meshed 
 reticulum. From this fibers are given off to the tunica propria, terminat- 
 ing in end-bulbs, or, after losing their medullary sheath, are distributed 
 to the epithelium, where their free extremities lie between the epithelial 
 cells. The nerves of the mucous membrane of the tongue (the glosso- 
 pharyngeal and lingual branch of the fifth) may have their endings similar 
 to those in other parts of the mouth, or they may be intimately associated 
 with the taste-buds. 
 
 Literature. 
 
 Legros and Magitot, 1880. 
 
 Klein, "Structure of the Oral Lips," 1868. 
 
 Sebastian, "Anatomy and Physiology of the Labial Glands," 1842. 
 
 Kolliker, " Mikroskopische Anatomic" 
 
 Kirke, "Physiology. " 
 
 Strieker, "Human and Comparative Histology." 
 
 Stohr, "Text-book of Histology," 1896. 
 
CHAPTER V. 
 
 Other Structures Within the Mouth. — The Gums. — The Mucous 
 
 Membrane. — The Alveolodental Membrane. — 
 
 Glands, Ducts, Etc. 
 
 The Gums (Gingiva). — The gums are formed by a layer of tough 
 fibrous vascular tissue, covering the alveoli, closely attached to their 
 periosteum, and provided with a free margin (gingival margin), which 
 
 Epithelium 
 
 Alveolus 
 
 Fig. 214. — Section through Gingivae and Alveolus. 
 
 is closely molded to the necks of the teeth. They are covered on both 
 aspects by the general mucous membrane of the mouth, that overlying 
 the labial and buccal surfaces being reflected from the lips and cheeks, 
 
 294 
 
THE GUMS 295 
 
 the palatal surface being continuous with that of the hard palate, and 
 the lingual surface reflected from the under surface of the tongue and 
 floor of the mouth. In the immediate region of the necks of the teeth the 
 gums are especially thin and hard, being closely adherent to the peri- 
 osteum and alveolodental membrane in this region. In passing toward 
 the base of the alveolar walls the tissue becomes less firmly attached to 
 the underlying structure, and, when finally passing into the mucous 
 membrane of the cheeks and lips, is quite loose and flabby. This condi- 
 tion also prevails on the lingual aspect, but the palatal surface remains 
 firm throughout, the entire mucous membrane overlying the roof of the 
 mouth being similar in structure and attachment to that portion imme- 
 diately surrounding the necks of the teeth. In various situations about 
 the labial, buccal, and lingual surfaces of the gums small slender folds 
 of mucous membrane are, found extending to the surrounding tissues. 
 These folds, which act as a bridle or curb to the adjacent movable parts, 
 are known as the frena of the mouth. The principal frena are found at 
 the median line, and are three in number — the frenum labium superioris, 
 frenum labium inferioris, and the frenum linguae. The two former 
 extend from the inner surface of the lips to the gums, to which their extent 
 of attachment is somewhat variable. The frenum labium superioris is 
 usually much larger than the frenum labium inferioris, and its attachment 
 to the gum frequently extends almost to the gingival border. The frenum 
 lingua? extends from the under surface of the tip of the tongue to the lin- 
 gual surface of the inferior gums. This is a much stronger fold than 
 those connected with the lips. Similar bridles are found in the buccal 
 region, usually near the bicuspid teeth, but they are much smaller than 
 those at the median line The gingival margins, or that portion of the 
 gums embracing the necks of the teeth, present much variety in outline. 
 Instead of encircling the neck of the tooth in a direct line, the margins 
 are made up of a series of semicircles. Using the incisive region for 
 reference, the labial and palatal margins are concave rootward, while the 
 interproximate spaces are partly or completely filled by gum tissue, 
 having the outline reversed or convex in the direction of the crowns of 
 the teeth. The gingival margin is also termed the "free margin of the 
 gum," this name better describing its extent. As previously stated, the 
 gums are attached to the periosteum and peridental membrane, but in 
 most instances, and particularly before the adult period, the free margins 
 of the gums extend beyond the alveolodental membrane, the limit of which 
 is formed by the cervical line. That portion of the gum margin beyond 
 the cervical line is in close contact with the neck of the tooth, but is not 
 
296 HISTOLOGY 
 
 adherent to it, the connecting medium, the alveolodental membrane, not 
 being present to form the attachment. The curvature of the gingival 
 margins, and the nature of the tissues which enter into their construction, 
 are usually considered as strongly indicative of the temperament of the 
 individual. Thus, in the bilious temperament the margins are inclined 
 to angularity and the tissues rather thick and firm. In the sanguine type 
 the outline formed is almost a perfect semicircle, and the tissues are of 
 moderate thickness and firmness. In the nervous type the curvature 
 strongly parabolic, and the tissues firm and delicate. In the lymphatic 
 the tissues are loose and thick, and the curvature is long and poorly 
 defined. In some instances the interproximate spaces are completely 
 filled by the gingivae; in others the space is only partly occupied by these 
 tissues. The former condition is present when the proximate surfaces of 
 the teeth are nearly or quite parallel with each other, thus reducing the 
 capacity of the space. The latter condition is present when the crowns 
 of the teeth are bell-shaped and the interproximate spaces extensive. The 
 labial and buccal surfaces of the gums are more or less broken by numer- 
 ous prominences and depressions, all of which accord with the variations 
 upon the surface of the bone beneath. 
 
 Some distinction should be made between the gingival line and the 
 cervical line, the former referring to the free margin of the gum, and not a 
 fixed line, while the latter refers to that positive line established on the 
 tooth by the union of the enamel and cementum. 
 
 Mucous Membrane of the Mouth. — The term "membrane" in a 
 general sense is one applied to thin layers of tissue, somewhat elastic 
 and of a whitish or reddish color. Such tissues are found lining either 
 closed cavities or canals which open externally, absorbing or secreting 
 fluids, and enveloping various organs. The simple membranes are of 
 three varieties, being either mucous, serous, or fibrous. The mucous 
 membranes are so called from the clear viscid fluid (mucus) which they 
 secrete. They line the various cavities or tracts of the body which com- 
 municate with the exterior. The three grand divisions of mucous mem- 
 brane are those lining the digestive, respiratory, and genito-urinary pas- 
 sages. Lining the entire cavity of the mouth we find the beginning of the 
 digestive tract, being continuous with, in many respects similar to, the skin 
 on the exterior and performing similar functions within. It is soft, smooth, 
 and velvety, of a bright red color, and quite vascular; it is covered on the 
 exterior by a layer of epithelial cells overlying the vascular parts. Imme- 
 diately beneath this is a network of fibrous connective tissue forming the 
 proper mucous membrane, and still deeper is a third layer, somewhat 
 
THE ALVEOLODENTAL PERIOSTEUM OR ROOT MEMBRANE 297 
 
 loose in texture, but composed of fibrous connective tissue, the submucous 
 membrane. The oral mucous membrane, at its point of beginning on the 
 contiguous surface of the lips, is endowed with keen sensibility; it is dry, 
 bright red in color, and plentifully supplied with vascular papillae, in 
 many of which are sensory nerve terminals. Distributed along the line 
 of junction between the integument and the mucous membrane are 
 numerous sebaceous follicles, which, however, are devoid of hair-bulbs. 
 The characteristic dryness of this surface gradually becomes changed to a 
 mucus-secreting one, as that part of the membrane lining the interior of 
 the lips is approached. Distributed over the surface of the labial mucous 
 membrane are a number of minute openings, the mouths of the labial 
 glands, which lie immediately beneath the membrane. The buccal mu- 
 cous membrane, or that lining the cheeks, is similar to that covering the 
 internal surface of the lips. It is penetrated at various points by the 
 mouths of the buccal glands, which, in general, are smaller and less 
 numerous than the labial glands. In the region of the second molar 
 teeth the membrane is broken by four or five openings of larger size, 
 which communicate with the molar glands. The mucous membrane 
 covering the hard palate is thick and firm, less brilliant in color than that 
 covering the cheeks and lips, and firmly bound down to the periosteum. 
 Running from before backward at the median line, the membrane is 
 formed into a slight fold, the median raphe, while near the anterior portion 
 of the palate are a number of fantastically arranged folds, the ruga (see 
 General Description of the Mouth). The thin but rather dense fibrous 
 aponeurosis forming the soft palate is covered anteriorly by the oral mu- 
 cous membrane. Suspended from the center of the free margin of the soft 
 palate is the uvula, which is likewise covered by mucous membrane, and 
 from the base of this, on either side, are two muscular folds, which extend 
 outward and downward, forming the anterior and posterior pillars of the 
 fauces. The mucous membrane covering the tongue has already been 
 described in connection with that organ. From the mouth the digestive 
 mucous membrane passes through the fauces, pharynx, and esophagus 
 to the stomach, and is so continued throughout the whole digestive 
 tract. Other prolongations also pass into the ducts of the salivary 
 glands. 
 
 The Alveolodental or Peridental Membrane. 
 
 This membrane invests the roots of the teeth, and at the same time 
 lines the wall of the alveoli. Being reflected from the periosteum cover- 
 
 * For a minute description of the mucous membrane of the mouth see Part II. 
 
298 
 
 HISTOLOGY 
 
 ing the outer alveolar walls, it enters the alveolar sockets as a single 
 membrane, affording nourishment to the bone on one side and to the 
 cementum of the tooth on the other. It is a connective tissue of moderate 
 density, and is rich in its nerve- and blood-supply. The general direction 
 of its fibers is transverse, being attached at one extremity to the alveolar 
 
 wall and at the other to the 
 *** cementum of the root. The 
 
 connective-tissue fibers are not 
 merely attached to the surface 
 of the calcified structure, but the 
 strength of this attachment is 
 greatly increased by the passage 
 of the fibers into the substance of 
 the bone at one extremity and 
 into the lamella? of the cementum 
 at the other. In general, the 
 membrane is more closely ad- 
 herent to the cementum than to 
 the bone, usually clinging to the 
 former when removed from its 
 socket. The nature of the ar- 
 ticulation between the tooth-root 
 and the alveolar socket, to the 
 production of which this mem- 
 brane so largely contributes, is 
 one peculiar to itself. While 
 there is no marked mobility, 
 there is, nevertheless, sufficient 
 elasticity in the intervening mem- 
 brane to provide against the 
 severe concussions and lateral 
 strains incident to mastication, the former being provided for by the 
 general elasticity, while the latter is cared for by specially distributed fibers, 
 which serve to return the tooth to its normal position when slightly rotated 
 or laterally displaced. This elasticity is greatest in youth and up to the 
 meridian of life, after which time it gradually becomes less pronounced. 
 The membrane is thickest about the apical ends of the roots and in the 
 cervical region, and the distribution of the fibers at these points is some- 
 what different from those about the body of the root. In the former 
 location they are spread out fan-like from the apex of the root to attach 
 
 Fig. 215. — Root and Membrane of Tooth. 
 p, p, Peridental membrane; ap, apical 
 space; a, artery; al, al, alveolar process 
 /, /, dental ligament. 
 
THE NERVE-SUPPLY TO THE ALVEOLODENTAL MEMBRANE 299 
 
 themselves to the surrounding alveolar wall, while in the latter they 
 pass longitudinally over the alveolar margins to unite with the periosteum 
 of the parts. In conjunction with the functions already mentioned, the 
 peridental membrane is the median by which all forces applied to the 
 tooth-surface are taken up and conveyed to the brain, making it the organ 
 of the sense of touch to the tooth. In a normal tooth sensations of pain 
 alone are conveyed through the nerves of the pulp. The nerves of the 
 membrane act in precisely the same manner as do other sensory-nerve 
 terminals, being influenced by the slightest touch applied to the surface 
 of the tooth-crown.* In certain conditions of defective hearing the alveo- 
 lodental membrane may be made to assist this function by the use of an 
 instrument made for the purpose known as a dentiphone. 
 
 Blood-supply to the Alveolodental Membrane. — A very clear 
 idea of the blood-supply to this membrane may be obtained from 
 figure 215. 
 
 After entering the alveolar socket by one or more arterial branches, 
 the thickest portion of the membrane is gained where a number of smaller 
 twigs are given off, one or more of which enter the pulp-canal of the tooth- 
 root through the apical foramina supplying the pulp, which in turn 
 supplies the tooth-structure within, while the others ramify through the 
 substance of the alveolodental membrane, through its capillaries, supply- 
 ing the cementum from without; while passing through the membrane 
 further minute branches are given off which penetrate the walls of the 
 alveolus and anastomose with the arteries which supply the oral mucous 
 membrane, in this manner providing a generous blood-supply to the parts. 
 Further on we shall see that the tooth-pulp and the alveolodental mem- 
 brane spring from the same source (see Development of the Teeth), and 
 the blood-supply to the parts during the saccular stage of development 
 is alike distributed to the base of the pulp and to the follicular walls. 
 After the completion of the developmental process this distribution is but 
 little changed, the blood-vessels accommodating themselves to the altera- 
 tions incident to the generation of the parts. 
 
 The Nerve-supply to the Alveolodental Membrane. — The nerve- 
 supply to this membrane is distributed in a manner similar to the blood- 
 supply, being derived from filaments given off from the dental nerve and 
 entering the tooth-socket by the side of the blood-vessels, and by numer- 
 ous filaments which reach the structure by passing through the many 
 minute canals in the substance of the alveolar walls and the intervening 
 septa. 
 
300 HISTOLOGY 
 
 GLANDS OF THE MOUTH. 
 
 The glands of the mouth are of two kinds, being either serous or 
 mucous, and, as they differ in the character of their secretions, so they 
 differ in structure. The mucous glands are the most numerous, and are 
 found beneath the mucous membrane of the lips, in the same membrane 
 lining the cheeks, the hard and soft palate, the tonsils, and at the back of 
 the tongue. These glands are quite variable in size, but are all of macro- 
 scopic proportions, appearing when examined in this manner as minute 
 whitish bodies. The secretions from the glands are poured into the 
 mouth through small ducts which pass in various directions through the 
 substance of the mucous membrane. Beginning as a single duct for each 
 gland, they pass to the submucous tissue, here branching into two or 
 more smaller ducts terminating in alveoli, the number and size of these 
 depending upon the size of the gland with which they are connected. 
 The glands are variously named according to their location, those occupy- 
 ing the lips being known as labial glands; those of the cheeks, the buccal 
 glands; those of the palate, the palatal glands, etc. The mucous glands, 
 although differing in size, are similar when histologically considered. 
 
 The Labial Glands. — These are among the largest mucous glands 
 of the mouth, and are more numerous in the upper than in the lower lip. 
 The form, size, and location of the labial glands may best be studied 
 by dissection, which may readily be accomplished by first removing 
 the integument and muscular tissues from the parts, when they will be 
 brought into view. The glands are irregularly arranged, and are most 
 numerous near the median line. The body of each gland is surrounded 
 and held in position by connective tissue, as well as by the duct connect- 
 ing it with the interior of the mouth. 
 
 Besides the mucous glands of the lips, there are present numerous 
 sebaceous glands. These are somewhat smaller, and are situated beneath 
 the mucous membrane covering the contiguous surfaces of the lips, the 
 numerous ducts leading from them opening upon these parts. 
 
 The Buccal Glands. — The glands of the cheek, otherwise known 
 as buccal glands, are similar to, but smaller than those of the lips, and are 
 placed between the submucous tissue and the buccinator muscle. These 
 glands also pour their secretions into the mouth through numerous ducts 
 which pass through the buccal mucotis membrane. 
 
 In the region of the third molar teeth another set of mucous glands 
 open into the mouth, known as the molar glands. They are placed between 
 the buccinator and masseter muscles, are similar in construction, and 
 
GLANDS OF THE MOUTH 3OI 
 
 secrete a like fluid to those previously described, being larger than the 
 buccal and smaller than the labial glands. 
 
 Palatal Glands. — Situated between the mucous membrane of the 
 hard palate and the periosteum are numerous mucous glands similar to 
 those previously described. Provision is made for the accommodation 
 of these glands by many small depressions in the bony plates (see Fig. 6). 
 They are irregularly distributed over the surface of each lateral half of 
 the hard palate, but are absent at the median raphe and immediately 
 beneath the rugae. The mucous membrane of the hard palate is tense 
 and hard, in consequence of which it is not so thick as the buccal and 
 lingual membranes, and the ducts from the numerous glands are there- 
 fore not so long. The glands of the soft palate, uvulae, and fauces are 
 situated beneath the deep layer of mucous membrane covering these 
 parts, in the former structure opening on both the oral and nasal surfaces. 
 
 Lingual Glands. — The glands of the tongue are of two kinds — 
 mucous and serous. The former are chiefly found at the back part of the 
 tongue, but a few of smaller size are present near the tip. The serous 
 variety are to be found only at the back of the organ, and are closely asso- 
 ciated with the taste organs in this region. These glands are assisted in 
 performing their function by being placed between bundles of striped mus- 
 cular tissue, the activity of which forces the secretions to the surface 
 by compressing the glands. While the majority of the lingual glands are 
 present in the circumvallate region, a number are found distributed 
 beneath the mucous membrane of the borders and tip of the organ. 
 
 The Salivary Glands (Fig. 216). — These glands, while outside the 
 mouth, are so closely associated with its functions that a brief description 
 will be presented. The chief salivary glands are six in number, three on 
 each side. They are named parotid, submaxillary, and sublingual; the 
 former, secreting true, thin, watery saliva is a true salivary gland, while 
 the latter two are known as mixed, or mucosalivary glands, secreting 
 both mucus and saliva. 
 
 The Parotid Gland. — This gland is the largest of the three, and is 
 placed a little below and in front of the ear, having the following bounda- 
 ries: Anteriorly, by the ramus of the mandible; posteriorly, by the styloid 
 and mastoid processes of the temporal bone; above, by the root of the 
 zygoma, and below, by a line drawn backward from the angle of the jaw. 
 While the extent and outline of the gland is somewhat variable, its posi- 
 tion is approximately that outlined in figure 216. Its superficial surface, 
 somewhat lobulated, is in close relation to the skin and fascia, while 
 deeply it penetrates well into the neck by two processes, one of which 
 
3° 2 
 
 HISTOLOGY 
 
 passes behind the styloid process and beneath the mastoid process of the 
 temporal bone and the sternomastoid muscle, while the other passes in 
 front of the styloid process. Given off from the body of the gland and 
 extending in various directions are a number of processes or lobes, one 
 extending forward between the two pterygoid muscles, and known as the 
 pterygoid lobe; another passing into the glenoid cavity, the glenoid lobe; 
 while a third passes deeply between the carotid vessels, and is called 
 
 SOCIA PAROTiDIS 
 
 DUCT OF SOCIA 
 PAROTIDIS 
 
 DUCT OF PAROTID 
 
 Bristle inserted 
 into duet 
 
 Fraenum linguae 
 
 DUCT OF RIVINUS 
 
 SUBLINGUAL GLAND 
 
 PAROTID GLAND 
 
 Masseter muscle 
 
 DUCT OF SUBMAXILLARY 
 GLAND 
 Mylo-hyoid muscle 
 
 Anterior belly of 
 digastric muscle 
 
 Sterno-mastoid 
 muscle 
 
 Posterior belly of 
 digastric muscle 
 
 SUBMAXILLARY GLAND, 
 DRAWN BACKWARDS 
 
 Loop of fascia 
 
 DEEP PORTION OF SUBMAXILLARY GLAND 
 
 Fig. 2 i 6. — The Salivary Glands. (Morris.) 
 
 the carotid lobe. In many instances there is an additional lobe, which 
 is detached from the body of the gland, known as the socia parotids. 
 When present, this lobe is placed over the parotid duct and empties into it. 
 Passing through the substance of the gland are a number of arteries and 
 veins, principal among which are the external carotid, transverse facial, 
 and internal maxillary, the gland receiving its blood-supply by branches 
 from these. The internal carotid artery and internal jugular vein lie 
 close to its internal surface. The facial nerve and its branches and the 
 great auricular nerve pass through the gland from before backward, and 
 supply its substance with nerve-force. The weight of this gland is from 
 one-half to one ounce. 
 
GLANDS OF THE MOUTH 303 
 
 The Parotid Duct. — Leading from the gland to the mouth is the 
 parotid or Stenson's duct. After passing through the fat of the cheek 
 and the fibers of the buccinator muscle, the duct comes in contact with the 
 deep layer of the oral mucous membrane. After passing between this 
 structure and the cheek tissues for a short distance it enters the mouth 
 opposite the crown of the upper second molar tooth, the orifice of the 
 duct appearing on the surface of the mucous membrane in the form of a 
 small papilla, which may be readily observed with the naked eye. When 
 first given off from the gland a number of small ducts are present, but 
 these soon unite and form a single canal. The parotid duct is quite 
 dense, of considerable thickness, and is lined by a reflexion of the buccal 
 mucous membrane. 
 
 The Submaxillary Gland. — This gland, which receives its name from 
 occupying a position below the maxillary bone, is somewhat smaller 
 than the parotid. It is situated beneath the mylohyoid ridge, and occu- 
 pies the anterior part of the submaxillary triangle, extending upward to 
 occupy the submaxillary fossa on the lower border of the maxilla. Super- 
 ficially, it is covered by the skin and a few muscular fibers and the deep 
 fascia. The facial vein and branches of the facial nerve pass over its 
 superficial surface. Deeply, it is in relation with the mylohyoid and 
 hyoglossus muscles, and also with the mylohyoid artery and nerve. 
 The gland receives its blood- and nerve-supply from the arteries and 
 nerves which penetrate it. This gland is separated from the sublingual 
 gland by the mylohyoid muscle, and weighs about two drams. 
 
 The Submaxillary Duct. — The submaxillary duct, otherwise 
 known as Wharton's duct, passes forward and inward, opening into the 
 cavity of the mouth on the summit of a small papilla near the frenum 
 linguae. The duct, which is nearly two inches in length, first passes 
 through the adjacent muscular tissue, and finally beneath the oral mucous 
 membrane to its outlet. Like the parotid duct, it is lined by a reflexion 
 of the oral mucous membrane. 
 
 The Sublingual Gland.— This is the smallest of the salivary glands, 
 and is also named from its location beneath the tongue. Its position is 
 beneath the tip of the tongue and the mucous membrane covering this 
 part of the floor of the mouth. It rests in the submaxillary fossa of the 
 maxilla, meeting with its fellow at the median line, and extending as 
 far back as the mylohyoid muscle, which separates it from the sub- 
 maxillary gland. The gland is supplied with blood from the sublingual 
 and submental arteries, and with nerves from the gustatory. The weight 
 of this gland is about one dram. 
 
3°4 
 
 HISTOLOGY 
 
 The Sublingual Ducts. — This gland communicates with the mouth 
 by one large duct — the duct of Rivini — which springs from the main 
 portion of the gland, and by a number of smaller ducts, eight to twenty 
 in number, which open on the floor of the mouth. The duct of Rivini 
 follows the submaxillary duct, and opens with it at the same papilla. 
 The smaller ducts are "given off from a number of little lobes which cluster 
 about the fore part of the gland. 
 
CHAPTER VI. 
 
 Glands and Ducts of the Mouth; of the Lips; of the Cheeks; of 
 the Hard and Soft Palates; of the Tongue. — The Salivary 
 Glands. 
 
 GLANDS AND DUCTS. 
 
 Glands of the Mouth. — The glands of the mouth, like the glands 
 of other parts of the body, are composed almost entirely of epithelium, 
 and may, therefore, be classed with the epithelial tissues. Glands exist 
 in two principal forms — tubular and saccular (alveolar). The former 
 occur either singly or in groups, and are further subdivided into simple 
 tubular and compound tubular glands. A like condition is present in 
 the saccular glands and similar terms are employed to qualify them — 
 simple saccular glands and compound saccular glands. 
 
 A simple tubular gland is one composed mainly of a simple tube- 
 like structure; a compound tubular gland is one composed of a number of 
 smaller tubes emptying into a single duct. 
 
 A simple saccidar gland is one formed by a sacculation of serous or 
 mucous membrane into a single, simple sac, or by branched saccules 
 having an excretory duct (alveolar system); a compound saccular gland is 
 composed of a combination of branched saccules. 
 
 In the larger glands a sheath is formed by the surrounding connective 
 tissue, from which numerous septa are given off to the interior of the 
 gland, dividing it into compartments varying in size. These are known 
 as gland-lobules. The connective-tissue walls of the gland-lobules carry 
 the larger blood-vessels and nerves. Most glands are divided into two 
 essential parts — the gland-follicle and the excretory duct — the former 
 being specialized for the secretory function, while the latter, by com- 
 municating with the surface, conveys the secreted substance to that point. 
 
 The gland-follicles are composed of a layer of gland-cells, usually 
 simple in character, surrounding the follicular walls. External to these 
 is a specially modified connective tissue, forming the basement membrane, 
 or membrana propria. The appearance of the gland-cells and their 
 nuclei is continually changing, being thus influenced by their functional 
 activity. 
 
 20 3°5 
 
3°<5 
 
 HISTOLOGY 
 
 The excretory ducts consist of a wall of connective tissue and elastic 
 fibers, lined by a columnar epithelium, either simple or stratified. In 
 some instances the arrangement of the excretory ducts is much compli- 
 cated, being divided into secretory tubes, which in turn are subdivided 
 into smaller tubules — intercalated tubes. 
 
 Fie 217. — Section through the Glandular Tissue of the Tongue. X40. 
 
 The Glands of the Lips {Labial Glands). — The glands of the lips are 
 situated in the submucosa, and are first observed immediately within the 
 line of labial occlusion, at which point the thickness of the epithelium 
 becomes somewhat definite and general. These glands are variable in 
 size, but all are sufficiently large to be observed without the aid of the 
 microscope. They are of the compound tubular variety, and communi- 
 cate with the surface through an excretory duct, which throughout the 
 greater part of its extent is lined with stratified, scaly epithelium. In 
 passing from the surface toward the gland-follicle, the main duct takes a 
 spiral course obliquely through the tunica propria, and upon reaching 
 the submucosa gives off numerous branches and twigs which terminate 
 in the individual acini. The larger branches from the main duct are 
 
GLANDS OF THE CHEEKS 307 
 
 lined with stratified squamous epithelium, while the smaller twigs are 
 provided with columnar epithelium. In many instances the main 
 excretory duct, in its passage through the tunica propria, receives the 
 principal duct from small accessory ducts. The framework of the labial 
 glands is formed by the flexiform tissue composed of fasciculi of the fine 
 connective-tissue fibers belonging to the submucous layer, together with 
 delicate, coiled elastic fibers. This framework gives support to a minute 
 system of capillaries and small nerve-fibers supplying the acini. The 
 acini are so arranged that those belonging to a large duct are united 
 into b lobule by the submucous connective-tissue fasciculi, and these in 
 turn are formed into lobes. By a continuation of the same fasciculi and 
 fibers which limit a lobe, and in the meshes of which the acini are situated, 
 a sheath to the excretory duct is formed. Besides the branched, tubular, 
 mucous glands of the lips, there are occasionally found, at the edges of 
 the lips, sebaceous glands. 
 
 The Glands of the Cheeks {Buccal Glands, Molar Glands). — The 
 glands of the cheeks are also situated in the submucous layer of the mucous 
 membrane. They, like the labial glands, are of the compound tubular 
 variety, and when microscopically examined are found to be similar in 
 structure. They are somewhat larger than the labial glands and pro- 
 portionately less numerous. The chief duct from each of these glands 
 usually opens with a narrow mouth on the surface of the oral mucous 
 membrane, and in its passage through the tunica propria takes a vertical 
 or oblique direction. In the submucosa the chief duct branches into 
 two or more smaller ducts, taking up alveoli. As the buccal glands are 
 somewhat larger than the glands of the lips, they are composed of a greater 
 number of ducts and alveoli. 
 
 The Glands of the Hard and Soft Palate (Palatal Glands). — The 
 mucous glands of the hard palate are situated in the submucosa and 
 closely associated with the periosteum. They are compound tubular 
 glands, and in all essential particulars are similar to the labial and buccal 
 glands. They are quite numerous (200 to 300), isolated in the anterior 
 portion, but are grouped into a single row or into two rows posteriorly. 
 The glands are freely distributed in each lateral half, but are absent at 
 the median line. 
 
 In the soft palate the glands are of the same character, somewhat 
 variable in size, the largest being found in the uvula. The excretory 
 ducts from these glands vary in diameter, in the nature of their fibrous 
 structure, and in the direction taken in passing to the surface. Over 
 the surface of the soft palate the mouths of these ducts are represented 
 
3°8 
 
 HISTOLOGY 
 
 by minute orifices slightly smaller than the body of the duct, but in the 
 uvula the opposite condition is present, the mouth of the duct being wider 
 than the body. The course taken by the excretory duct is seldom a 
 direct one, but after receiving all tributary branches passes obliquely 
 through the tunica propria, and before entering the epithelium turns 
 at an abrupt angle, and so continues until the surface is reached. The 
 ducts are lined by a simple columnar epithelium, which in some instances 
 is ciliated; the walls of the tubes consist of gland-cells and a structure- 
 less membrana propria. In some instances the surface epithelium may 
 be reflected for a short distance and partly serve in the capacity of a lining 
 to the tubular walls. 
 
 The Glands of the Tongue {Lingual Glands). — In this organ two 
 varieties of glands are found, occurring both in the mucous mem- 
 
 Jp§ ! 
 
 
 Fig. 218. — Section through Base of Tongue, showing Serous Glands. 
 
 brane and in the superficial muscular strata, being principally distin- 
 guished by the nature of their secretions. The gland-cells of the one set 
 are mucigenous, secreting mucin; these are the mucous glands. The 
 other set is productive of a serous fluid, 'thin, watery, and containing 
 albumin; these are the serous glands. 
 
 The mucous glands of the tongue are found along the lateral margins 
 and over the root of the organ, being most numerous in the latter situation. 
 
THE SALIVARY GLANDS 309 
 
 They are of the compound tubular variety, and in most particulars are 
 identical with the mucous glands of other parts of the oral cavity. The 
 ducts are lined with ciliated columnar epithelium, and the walls of the 
 duct consist of a homogeneous membrana propria and gland-cells. 
 The glands occupying the root of the tongue are frequently found with 
 their excretory ducts opening into the follicular crypts. The tubules 
 consist of a structureless membrana propria and numerous gland-cells, 
 the latter varying in appearance according to their function or functional 
 activity. The crypts of the follicles constitute reservoirs for the acinous 
 glands, and these receptacles frequently extend for some distance beneath 
 the surface, receiving at various points the main excretory ducts from 
 the mucous glands. These saccular-like reservoirs are lined by a well- 
 defined capsule surrounded by a fibrous sheath, internal to which is an 
 epithelial covering, a prolongation of the common epithelium of the 
 mouth. Between these two layers are a number of minute, closed lymph- 
 follicles placed in a single layer. The mucous glands on the lateral walls 
 of the tongue are, for the most part, situated near the middle or pos- 
 terior portion. The ducts from these glands usually open directly toward 
 the cheek, but in rare instances they pass obliquely downward and open 
 near the proper floor of the mouth. At the tip of the tongue, buried be- 
 neath the mucous membrane and some of the muscular fibers, may be 
 found a pair of -mucous glands (Nuhn's) which open by free orifices on the 
 under surface. At the root of the tongue, flat, lenticulated elevations of 
 the mucous membrane are present, beneath which is imbedded con- 
 globate, glandular substance. These show a central orifice leading to a 
 small pit lined with tessellated epithelium. 
 
 The serous glands of the tongue are compound tubular glands, and are 
 found in the region of the circumvallate papillae, closely associated with 
 the taste-buds. The excretory ducts, lined with a simple or stratified 
 columnar epithelium, the latter sometimes ciliated, open near the base of 
 the papilla, or between the papilla and its wall. The tubules are similar 
 to those in the mucous glands, consisting of a delicate, structureless mem- 
 brana propria and gland-cells. The gland-cells are composed of a frail, 
 transparent protoplasm, containing rounded nuclei. 
 
 The Salivary Glands. — The parotid, submaxillary, and sublingual 
 glands each consists of an excretory duct, branching frequently in a tree- 
 like manner into smaller ducts, lined throughout with a layer of epithelial 
 cells. From the smaller ducts terminal branches are given off, which in 
 turn are lined with epithelium. The other portions of the glands are 
 invested by columnar epithelium, and arranged like grapes about the 
 
310 HISTOLOGY 
 
 main excretory duct, and consequently belong to the group of racemose 
 glands. The terminal branches or alveoli attached to the smaller excre- 
 tory ducts are so numerous that they become much compressed from 
 pressure, and the grape-like appearance is more or less destroyed, and 
 but little space is left for interstitial tissue. Each gland is inclosed in a 
 fibrous connective-tissue capsule, and from this numerous septa of fibrous 
 trabecular pass to the interior and divide the glandular substance, first into 
 lobes, these being subdivided into lobules, the lobules by further sub- 
 division forming the alveoli. The glandular connective tissue is loose 
 in texture, containing many elastic fibers and lymphoid cells. Fine 
 bundles of fibrous tissue, together with branched connective-tissue cor- 
 puscles, constitute the connective-tissue matrix between the alveoli. 
 
 The Ducts. — Entering the interior of the gland, the chief duct divides 
 into a number of large branches, one of which passes to each lobe, each 
 of these giving off several branches which connect with the several lobules. 
 Upon close examination the central tube of each lobule is observed to 
 throw off several small tubes — the intralobular tubes. Following these 
 are the intermediate tubules, which continue into the terminal compart- 
 ments. The chief excretory duct consists of a double layer of cylindric 
 epithelium and fibro-elastic cartilage. Close beneath the epithelium is a 
 compact membrana propria. The intralobular tubes are each provided 
 with a distinct lumen. The walls are composed of a membrana propria 
 lined by a layer of columnar epithelium, the cells of which contain a 
 central round nucleus. 
 
 The Parotid Gland. — The distinguishing histologic feature in this 
 gland is found in its excretory duct (Stenson's duct), which is provided 
 with a membrana propria, especially broad and compact, placed imme- 
 diately beneath the epithelium. The duct is composed of a double 
 layer of cylindric epithelium and fibrous tissue, intermingled with elastic 
 fibers. The main duct divides and passes into the intralobular tubes, 
 beyond which are the intermediate tubules. The intralobular tubes are 
 lined by columnar cells, while the intermediate tubules are lined by 
 elongated, spindle-shaped cells. The salivary cells lining the acini are 
 different in character from those in the submaxillary and sublingual 
 glands. The parotid gland is a true salivary gland, and the serous 
 gland-cells composing its epithelial lining are disposed in a single layer. 
 The cells are columnar or pyramidal in form and composed of a dense 
 protoplasm, containing a spheric nucleus. 
 
 The Submaxillary Gland. — The excretory duct (Wharton's duct), 
 like the main duct of the parotid gland, is composed of a double layer 
 
NERVE-SUPPLY TO THE SALIVARY GLANDS 3II 
 
 of columnar epithelium, external to which is a layer of cellular connective 
 tissue, the whole being surrounded by a thin stratum of muscular fibers 
 placed longitudinally. The intralobular tubes are lined by a specialized, 
 elongated, cylindric epithelium, which, in the intermediate tubules, be- 
 comes clothed with cubic cells. The acini are lined either with serous 
 gland-cells similar to those lining the acini of the parotid gland or with 
 mucous gland-cells, the former being most constant in their presence. The 
 two kinds of acini are uninterruptedly connected. In most instances 
 there are but a few mucous acini present within the lobule, but occa- 
 sionally they are found in abundance. The submaxillary is a mixed 
 or mucosalivary gland. 
 
 1 The Sublingual Gland. — The excretory duct (Rivini's duct) is 
 similar in structure to the chief excretory duct of the submaxillary gland. 
 The intralobular tubes are lined with columnar epithelium. The inter- 
 mediate tubules are not positively known to exist, and it is quite probable 
 that the intralobular tubes pass directly into the terminal compartments. 
 The acini are composed of a membrana propria and gland-cells, both 
 mucous and serous. The former are much more numerous than in the 
 acini of the submaxillary. The membrana propria is composed of 
 stellate connective-tissue cells. This gland is also a mixed or muco- 
 salivary glands. 
 
 Blood-vessels and Lymphatics in the Salivary Glands. — The 
 lobules of the salivary glands are richly supplied with blood-vessels. The 
 many arterial branches break up into numerous capillaries, which, form- 
 ing a dense network, surround the acini, being supported by the inter- 
 alveolar connective tissue. The lymphatic vessels accompanying the 
 intralobular tubes are in communication with numerous lymph-spaces 
 which exist between the interalveolar connective tissue and the walls of 
 the acini. The substance of the gland is further supplied with blood 
 by numerous plexuses of lymphatics which are carried or supported by 
 the interlobular connective tissue. 
 
 Nerve-supply to the Salivary Glands. — The nerve-fibers distrib- 
 uted to the salivary glands are both of the medullated and non-medul- 
 lated variety, and other nerve-tissue in the form of ganglion-cells is 
 present. The medullated nerve-fibers are abundantly numerous, and 
 are distributed to all parts of the gland. In many respects the fibers are 
 peculiarly constructed. They are extremely delicate, made so by the 
 frail nature of this medullary sheath; they divide and give off so many 
 branches as to almost give them a feathery fineness. This peculiarity 
 is especially noticeable toward their extremities, where the fibers lie 
 
312 HISTOLOGY 
 
 between the alveoli and give off minute branches in all directions. The 
 nerve-fibers are placed in close relation to the tubes and tubules, which 
 they freely encircle; they perforate the membrana propria and break up 
 into finer subdivisions, from which they are distributed to the exterior 
 of the epithelial cells. In the alveoli two kinds of nerve terminations are 
 found. The primitive fibers branch between the alveoli and are distrib- 
 uted to the membrana propria, upon entering which numerous branches 
 are thrown off which pass to the epithelial cells beneath. The non- 
 medullated fibers, which are much less numerous, are composed of an 
 extremely delicate fasciculi of transparent fibers resembling axis-cylinders, 
 and invested by a sheath of connective-tissue cells containing nuclei. 
 The distribution of these fibers is similar to the medullated fibers, encircling 
 the tubes and penetrating the membrana propria, being similarly distrib- 
 uted to the alveoli. 
 
 Literature. 
 
 Stohr, "Text-book of Histology." 
 
 Strieker, "Human and Comparative Histology," vol. i, 1870. 
 
 Klein, "Elements of Histology," 1889. 
 
 Sebastian, "Recherches anatomique, physiologiques, pathologiques, les Glans 
 
 Labiales," 1842. 
 Ward, "On Salivary Glands," Tood's "Cyclopedia of Anatomy and Physiology.' 
 Klein and Vernon, Strieker's "Human and Comparative Anatomy," vol. i, chap. xvi. 
 Sudduth, "Embryology and Dental Histology," "American System of Dentistry," 
 
 vol. i, part hi. 
 Brubaker, "Transactions Odontological Society of Pennsylvania," i890-'95. 
 Todd and Bowman, "Physiological Anatomy," vol. i. 
 Szontagh, "Essays on Minute Anatomy of Hard Palate in Man," 1866. 
 
CHAPTER VII. 
 
 Tissues of the Teeth — Enamel; Dentin; Cementum; the Tooth- 
 Pulp. — The Alveolodental Membrane. 
 
 TISSUES OF THE TEETH. 
 
 Enamel. — The enamel, which forms a cap-like covering of varying 
 thickness over the entire crown of the tooth, is a vitreous, hyaline sub- 
 
 Fig. 219. — Section of Enamel from Human Tooth (Specimen by J. Howard Mummery.) 
 
 X350. {After Williams.) 
 
 stance, containing but little, if any, organic matter. The thickness of 
 the enamel cap appears to be strongly influenced by the function of the 
 
 313 
 
314 HISTOLOGY 
 
 different parts of the tooth-crown, being thickest over the cutting-edges 
 of the anterior teeth, while in the cuspidate teeth, the entire occlusal 
 surface is provided with the thickest enamel layer. It is about evenly 
 distributed over the lateral walls of the crown, but as the cervical line is 
 approached, its thickness is gradually diminished (Fig. 339). Chemically, 
 enamel is composed of the salts of lime, calcium phosphate predominat- 
 ing. Calcium carbonate, magnesium phosphate, and calcium fluorid 
 are present in smaller quantities. The proportionate quantity of lime- 
 salts in enamel is not fixed, a slight variation in density occurring in the 
 enamel of different individuals. These essential differences are regulated 
 by the proportionate quantity of calcium phosphate and carbonate — a 
 greater amount of the former being productive of additional hardness, 
 while an increase in the latter beyond the minimum amount decreases 
 this quality. As a general rule, the teeth of males contain a greater 
 amount of calcium phosphate than the teeth of females, as shown by the 
 following analysis by von Bibra: 
 
 Man. Woman. 
 
 Calcium phosphate and fluorid 89 
 
 Calcium carbonate 4 
 
 Magnesium phosphate 1 
 
 Other salts 
 
 Cartilage 3 
 
 Fat 
 
 Total organic, 3 
 
 Total inorganic 96 
 
 In general structure enamel is composed of numerous hexagonal 
 prisms, with a common direction at right angles to the long axis of the 
 tooth. These prisms are known as enamel prisms, enamel fibers, or 
 enamel rods. While the general direction of the fibers is, as previously 
 stated, nearly at right angles to the body of the tooth, they do not pursue a 
 perfectly straight course in passing from the dentin to the surface, but 
 are disposed in a tortouus or wave-like manner. The enamel prisms 
 may be said to sit on end against the surface of the dentin, minute depres- 
 sions in the latter receiving the extremities of the rods. The direction 
 of the enamel prisms, as compared to the body of the tooth-crown, varies 
 according to the part of the crown which they occupy. Taking the 
 entire crown of the tooth, they radiate in such a manner from the surface 
 of the dentin that at the cutting-edge or occlusal surface of the tooth they 
 are more or less vertical, while over the lateral surfaces they tend to the 
 horizontal direction. An examination of the prisms when isolated and 
 
 .82 
 
 81.63 
 
 ■37 
 
 8.88 
 
 ■34 
 
 3-55 
 
 .88 
 
 ■97 
 
 •39 
 
 5-97 
 
 . 20 
 
 a trace 
 
 •59 
 
 5-97 
 
 ■41 
 
 94 03 
 
ENAMEL 
 
 315 
 
 decalcified exhibits numerous evenly distributed varicosities, producing 
 a transversely striated appearance to the rods. Tomes has pointed out 
 that the enamel rods are variously disposed in that portion of the enamel 
 most closely associated with the dentin. On the cusps of the teeth they 
 are twisted and curved in various directions, while near the surface on the 
 incisors they are uniform and straight. In general, the enamel rods, 
 which begin on the surface of the dentin, are continuous through the 
 entire thickness of the enamel. In passing from the interior to the exte- 
 rior, the individual rod, to occupy a proportionate space in all parts, 
 
 Enamel 
 
 Dentin 
 
 Fig. 220. — Enamel and Dentin from Human Tooth, showing Gradual Reduction in the 
 Thickness of the Former as the Cervix is Approached. X20. 
 
 would have to increase in diameter; but this it does not do. In conse- 
 quence of this arrangement there exist numerous supplemental or per- 
 ipheral rods, which extend but a short distance from the surface, filling in 
 the interprismatic spaces formed by the longer rods. With the exception 
 of the faint transverse striations, the enamel prisms appear to be structure- 
 less. A variety of opinions have been expressed in regard to the cause 
 for the striated appearance of the enamel rods. It was claimed by Hertz 
 to be attributable to a temporary arrest of calcification, but more recent 
 investigation has shown the cause to be the presence of varicosities in the 
 individual fibers, precisely as the varicosities in the muscular fibers pro- 
 
316 
 
 HISTOLOGY 
 
 duce the striated appearance in that tissue. Bodecker asserts that fully- 
 developed normal enamel is non-striated, and Von Ebner practically 
 makes the same statement, claiming that they are due to the preparation 
 of the specimen, which usually being mounted in Canada balsam is in- 
 fluenced sufficiently from the slight acid reaction to produce the striated 
 appearance. These statements Williams emphatically denies, saying 
 that, while in some specimens the varicosities are apparent in some 
 parts, they are decided in others. If this be accepted, it would seem 
 to entirely overthrow the theory of Von Ebner in regard to the action of 
 the acid, which would be distributed to all parts alike. 
 
 Enamel 
 
 Dentin. 
 
 FlG. 221. — Comparison in the Appearance of the Enamel and Dentin under Low Power 
 
 of the Microscope. X40. 
 
 According to Williams, the varicosities of one enamel prism are 
 opposite those of the adjoining prisms, and by the coming together of 
 the varicosities the prisms become united by means of processes which 
 they send out. In like manner the varicosities upon the same rod are 
 connected by processes running parallel with the prism. According to 
 Von Ebner, enamel is traversed by numerous minute canals, and Heitz- 
 mann claims to have found organic fibers in its substance. Williams, 
 while admitting the enamel structure to be far more complex than past 
 research has shown, appears to have fully demonstrated that neither canals 
 nor organic fibers are present — in fact, he denies the presence of the least 
 
ENAMEL 
 
 317 
 
 trace of organic matter in this structure. The interprismatic matrix, 
 heretofore considered by most authorities to be an organic structure, 
 now appears, by the thorough methods employed by the last-named 
 gentleman, as a transparent, inorganic substance. By numerous experi- 
 ments he was enabled to secure a specimen in which the interprismatic 
 spaces of one layer were not backed up by the rods of another layer. 
 In some instances the specimen showed the rods well separated and the 
 interspace closed by a perfectly transparent substance, in the interior of 
 which might be seen connecting processes passing from one rod to another. 
 
 Enamel 
 Rods Fully 
 in Trans- 
 verse Sec- 
 tion 
 
 Ename 
 Rods No t 
 Fully n 
 Trans- 
 verse 
 Section 
 
 Enamel 
 Rods of 
 Irregular 
 Form 
 
 Fig. 222. — Human Enamel. Transverse Ground Sections. {After Cysi.) 
 
 That the enamel is practically of inorganic material, and therefore not 
 capable of transmitting or receiving sensations, may be demonstrated by 
 the simple experiment of immersing a thin section of this tissue in a weak 
 solution of chromic acid, the result of which will be a speedy separation 
 of the enamel prisms which have been liberated by the destruction of the 
 interprismatic substance. What does this signify ? Chromic acid is 
 one of the best preservatives of organic tissue known, and if the cement- 
 ing substances in enamel were organic or even partly so, the prisms by 
 this test would not be freed, but instead would become more firmly ce- 
 mented together. Therefore we infer from this that the interprismatic 
 
318 HISTOLOGY 
 
 substance is even less highly organic than the prisms themselves, these 
 not being acted upon until after the material which holds them together. 
 In addition to the striated appearance formed by the varicosities of the 
 individual prisms of fully developed enamel, other structures of a different 
 character, and upon a much larger scale, are present and known as the 
 " brown stria of Retzius" (Fig. 223). These markings, readily seen 
 with a low power, are of a brownish color, and run nearly parallel with 
 the surface of the dentin or enamel. Those striae nearest the surface of 
 the dentin are inclined to follow the contour of that structure, extending 
 
 ^fc^WFV^ 
 
 >*■>-% w 8 * 
 
 Fig. 223. — Thick Section of Enamel of Human Tooth, showing Brown Striae of Retzius. 
 
 X40. 
 
 in many instances the entire length of the crown. The lines nearest the 
 surface are the longest in the region of the cutting-edge, or occlusal surface 
 of the crown, becoming shorter as the neck of the tooth is approached, 
 being directed at an acute angle to the surface of the dentin at that point. 
 A number of theories are advanced to account for the presence of the 
 "brown striae of Retzius." Tomes suggests that, coinciding as they do 
 with the outer surface of what was at one time the primitive enamel cap, 
 they might be considered as in a measure outlining the stratifications of 
 the primary deposit. Another theory, but one seemingly without founda- 
 tion, is to the effect that the striae are produced by an arrest in the calci- 
 
ENAMEL 
 
 3 J 9 
 
 
 / 
 
 
 j&^llk. 
 
 Brown striae 
 of Retzius. 
 
 EviM s 
 
 
 
 ^5> '."^"§|aES5?w 
 
 '■ J| 9 
 
 Lines of f 
 Schreger. \ 
 
 TV" 
 
 ■-' ■ -.^'",'1 ■ '-^- ■ i *^CT*/'* 4 
 
 
 Enamel. 
 
 Dentin. 
 
 Fig. 224. — Enamel and Dentin, Human Tooth. {After Gysi.) 
 
 a .*- 
 
 wk 
 
 
 
 Oral Epithelium 
 Heaped up 
 over Band 
 
 Band 
 
 Connective Tissue 
 
 Fig. 225. — Vertical Section through Jaw of Human Embryo. Formation of Tooth-band, 
 
 about Sixtieth Day. X300. 
 
3 2 ° 
 
 HISTOLOGY 
 
 fying process; while a third theory attributes the cause to a variation in 
 the character of the nourishment taken by the mother during pregnancy. 
 The acceptance of this latter theory would seem to indicate a set diet 
 for all mothers and at stated intervals, the striae always being present 
 and somewhat regularly distributed throughout the tissue. 
 
 Still another set of lines or markings are to be observed in the sub- 
 stance of sections of enamel, these being known as the "lines oj Schreger." 
 Figure 224 shows these lines as they appear in the enamel by reflected 
 light, the same being quite invisible by transmitted light. The presence 
 
 Epitheliu 
 
 Band 
 
 Fig. 226. — Vertical Section, Tooth-band, Human Embryo. Tenth Week. X300. 
 
 of these lines is due to the various directions assumed by the contiguous 
 groups of enamel rods. Beginning at the surface of the dentin they are 
 well denned, but gradually become less marked as the exterior of the 
 enamel is approached. At the line of union between the enamel and den- 
 tin irregularly formed cavities are occasionally observed, into which the 
 dentinal tubules may extend, and in rare instances individual tubules may 
 pass beyond the boundary-line of the dentin and enter the enamel, but in 
 all probability both of these conditions are pathologic. Such a state of 
 affairs could hardly, be considered normal when we take into considera- 
 tion that the dentin and enamel calcify in opposite directions, and that the 
 outer wall of the former is completed before enamel calcification begins. 
 
ENAMEL 321 
 
 Development of Enamel. — Preparations for the development of the 
 enamel begin toward the close of the second fetal month, appearing first 
 as a multiplication of the primitive epithelial cells in the form of a con- 
 tinuous linear projection extending somewhat obliquely into the sub- 
 jacent connective tissue. From this crest, or tooth-band, the .germs for 
 the future enamel organs are given off. These primary dental bulbs, 
 as they are called, number one for each tooth to be generated, and coinci- 
 dently with their appearance an aggregation of closely associated connec- 
 tive-tissue cells make their appearance in the surrounding submucous 
 
 Dental Ridge 
 
 «*2Bfc 
 
 
 "^F^s^ 
 
 Band 
 
 Location of Future 
 Enamel Organ 
 
 ** • '■ 
 
 ' if' : "'Si. '"' '5V* ■ 
 
 
 Connective Tissue r k'V ti'i't+V.-r- 
 
 epithelium 
 
 Fig. 227. — Same as Figure 226. About Twelfth Week. X200. 
 
 tissue. This papilla-like specialization of the submucous tissue is the prim- 
 itive dentin germ, or dentin papilla. It will thus be observed that the 
 enamel is a product of the surface epithelium, ec to dermic, while the dentin 
 is generated from the connective tissues, mesodermic. Soon after the ap- 
 pearance of this club-shaped thickening, or tooth-bulb, by further differ- 
 entiation its form becomes bell-shaped, with the concavity directed toward 
 the surface. The dentin papilla gradually pushes into the concavity 
 of the forming enamel organ, and at a later period the odontoblastic cells 
 are generated about the periphery of the papilla, closely followed by a 
 surface calcification of the dentin. Soon a'fter the forming of the external 
 layers of dentin the ameloblasts or enamel-forming cells become active, 
 
322 
 
 HISTOLOGY 
 
 and a deposition of enamel prisms takes place upon the exterior of the 
 dentin cap. 
 
 Before taking up the subject of enamel calcification, brief reference 
 will be made to the further development of the enamel organ. As the 
 growth of this organ proceeds there is, as the result of a rapid proliferation 
 of the cellular structure, a marked tendency for the organ to become 
 separated from the tooth-band. The peripheral cells or external epithe- 
 lium are columnar or prismatic, and remain so, while those in the center, 
 primarily polygonal, soon become transformed into a radiating network 
 
 Epithelium of A*. 
 Upper JawiSTS 
 
 Epithelium a^ 
 9 
 
 &J%> *?*''? '-<":> ' . 
 
 ^S2 *^>'- t :> &>$*& 
 
 Surface of Jaw 
 
 *"S& 
 
 j&Neck of Cord 
 
 stellate Retic- 
 ulum of 
 Enamel Organ 
 
 J 
 
 4 
 | 
 
 ' Enamel Organ 
 
 )entin Papilla 
 
 Forming Bone 
 
 Fig. 228.- 
 
 -Vertical Section through Tooth-band, Cord, Dentin Papilla, and Enamel Organ, 
 about Fifteenth Week. X150. 
 
 or stellate reticulum. The appearance of a stellate reticulum is first 
 observed to take place in the cells occupying the central portion of the 
 enamel-organ, this cellular transformation progressing from the center 
 outward, but ceasing before reaching the columnar surface cells contigu- 
 ous to the dentin papilla. Between the enamel cells and the stellate 
 reticulum is a layer of unaltered cells — the stratum intermedium. In 
 the earlier stages of the development of the enamel organ the peripheral 
 cells are alike, being columnar or prismatic, but almost coincident with 
 the appearance of the dentin papilla, the cells most closely related to it 
 are observed to become elongated, and form the internal epithelium of the 
 organ. As the cells forming this internal epithelium become elongated, 
 
ENAMEL 
 
 323 
 
 their nuclei, instead of occupying the center of the protoplasmic body, 
 are carried to their extremities. It will thus be seen that the completed 
 enamel organ consists of four divisions or layers of cells. Beginning 
 with its convex surface is an external epithelium or outer tunic, succes- 
 sively followed, in passing toward the dentin papilla, by a stellate reticu- 
 lum', stratum intermedium, and an internal epithelium or inner tunic. 
 As the growth of the enamel organ proceeds, the tooth-band becomes 
 
 / 
 
 Fig. 229.— Section of a Developing Tooth of Lamb. 
 a, Ameloblasts; D, dentin; P, dental pulp. 
 
 X600. 
 
 smaller and smaller in size, until finally a complete rupture takes place. 
 This rupture, however, does not occur until the enamel organ has about 
 or fully completed its development, and, after remaining so long under 
 the influence of the oral epithelium, it must be considered, as before stated, 
 an epithelial structure. It is through the agency of the internal epithelial 
 cells of the enamel organ, the enamel cells or ameloblasts, that calcification 
 of the enamel takes place, and that subject will next be considered. 
 
3 2 4 
 
 HIS IOI.0GY 
 
 Amelijicalion (Fig. 229). — Two theories are advanced in regard to 
 the calcification of the enamel. In one it is claimed that the ameloblasts 
 or enamel cells became directly calcified or converted into enamel; in the 
 other the ameoblasts are simply considered as controlling agents, by 
 secreting or depositing the calcium salts which form the enamel prisms. 
 In the latter theory it is generally believed that the enamel is secreted or 
 shed out from the extremities of the ameloblasts, thus being productive 
 
 m>,. 
 
 Dentin 
 
 
 Enamel 
 Rods 
 
 / Enamel' 
 
 Amelo- 
 blasts 
 
 Section of 
 Papilla? 
 
 Fig. 230. — Section of Incisor of Rat. X200. {After Williams.) 
 
 of enamel fibers corresponding in size and position to the secreting cells. 
 By the direct calcification of the ameloblasts, it would be natural to 
 expect the process to begin on the exterior of the cell and gradually pass 
 into its interior, the central portion being the last to calcify. This 
 would result in an enamel prism corresponding in size and form to the 
 generating cell, and in a measure this similarity between the calcified 
 and uncalcified structure does exist and is one of the potent factors 
 
ENAMEL 325 
 
 in the recognition of this theory, but it is hardly sufficiently convincing 
 to warrant a general acceptance of the belief. 
 
 By examining figure 229 the importance of the secretory theory is 
 favored. Here we note that the early formed enamel at A records what 
 appears to be the definite action of the ameloblasts by prolongations of 
 partly calcified tissue extending from the cells, these markings corre- 
 sponding in number and location to the cells themselves. Between these 
 prolongations and the ameloblasts are many highly refractive granular 
 bodies which seem to be the actual lime deposit shed out from the free 
 extremities of the cells, from which they pass into the substance of the 
 then organic matrix beyond at A and form the enamel prisms. Williams 
 takes exceptions to this latter view, substantiating his opinion by stating 
 that while the ameloblasts of many animals are similar in shape and 
 arrangement, the enamel produced from these similarly arranged cells 
 varies greatly in structure. The same writer also states that, when such 
 a similarity of arrangement exists between the ameloblasts and the enamel 
 prisms, it occurs near the commencement of enamel calcification, and 
 that at a later period the relative position of the ameloblasts and the 
 prisms is always in longitudinal section. Dr. Williams calls attention 
 to the fact of the enamel prisms or rods not extending through the entire 
 distance between the enamel cells and the calcified dentin (Fig. 229). 
 That part of the structure lying between the ameloblasts and the extrem- 
 ities of the enamel rods is made up of a double set of fibers, some of which 
 are almost at right angles with long axis of the ameloblasts. In figure 
 231, D, the two sets of fibers previously mentioned are found to join 
 and become closely interwoven. 
 
 The ameloblasts are connected with the cells of the stratum inter- 
 medium, and more recent investigation goes to prove that this latter 
 structure is directly interested in furnishing to the ameloblasts the proper 
 material for the calcifying process. The stratum intermedium cannot, 
 however, take part in the primary enamel calcification, as this process 
 commences before the stratum intermedium is fully developed. This 
 being the case, it is generally supposed that the stellate reticulum furnishes 
 the material for the upbuilding of the first enamel prisms. 
 
 Lying between the free extremities of the ameloblasts and the enamel 
 in the course of formation is what has been generally considered as a 
 structureless basement membrane or membrana prcrformativa. The 
 existence, exact location and structure of this membrane has been and 
 still remains a matter of conjecture. The generally accepted theory 
 appears to be that given above, but Williams refers to it as a layer of 
 
326 
 
 HISTOLOGY 
 
 newly formed enamel, and does not consider it as a structureless mem- 
 brane. The structure is to be observed in figure 231, at either extremity 
 of the ameloblastic cells, this writer claiming that the so-called structure- 
 less membrane is present at both of these points. 
 
 As to the formation of the enamel rods, Dr. Andrews, in 1894, 
 referring to the presence of calcoglobulin in the enamel cells considered 
 
 Ameloblasts 
 
 Position of Capillary Loop 
 
 Fll 
 
 231. — Section of Incisor of Rat, showing Partial Decalcification of Enamel. 
 
 {After Williams.) 
 
 X600. 
 
 these refractive bodies as calcospherites, which, after being taken up by 
 the ameloblasts, were excreted by them, and after coalescing formed 
 globules of larger size, from which the rods were built up. Williams 
 partly agrees with this statement, but he is of the opinion that the calco- 
 spherites coalesce while in the ameloblasts, forming large, spheric bodies, 
 but that the deposit of this substance is in no way productive of building 
 
ENAMEL 
 
 3 2 7 
 
 the enamel rods. The theory of Tomes in regard to the forming of 
 enamel rods was that the walls of the ameloblasts themselves became 
 calcified, while the contents of the cells also became solidified, the first 
 forming the interprismatic substance, while the second became the 
 enamel rod. Whatever theory be accepted as to the formation of the 
 enamel rods or prisms, there appears to be no question in regard to the 
 
 Fig. 232. — Section of Developing Tooth of Embryo Lamb. X150. {After Williams.) 
 
 a, Forming Dentin; b, Forming Enamel; c, Stratum Intermedium; d, Inner Ameloblastic 
 
 Membrane; e, Outer Ameloblastic Membrane;/, Ameloblasts. 
 
 general process of enamel calcification. In the first place, an organic 
 matrix is formed, into which the first-formed layer of enamel is disposited. 
 Gradually the organic matter disappears, leaving behind the inorganic 
 elements closely resembling in appearance the organic matrix, which 
 it has by atomic change supplanted. The question of an organic inter- 
 
328 
 
 HISTOLOGY 
 
 prismatic cement-substance is also one upon which various writers dis- 
 agree. Klein partly believes that such a substance does exist, basing 
 his opinion upon the fact that the ameloblastic cells, in common with all 
 epithelial cells, are separated from one another by a homogeneous inter- 
 cellular substance, and that a certain proportion of this organic substance 
 must remain between the enamel prisms after calcification. Dr. Sudduth, 
 by a series of experiments made some years ago, appeared at that time 
 
 Fig. 233. 
 
 to have furnished conclusive proof that an organic, interprismatic cement- 
 substance does not exist between the enamel prisms of fully developed 
 enamel. By the use of a dilute solution of chromic acid, the action 
 of which is the preservation of organic substance, the prisms were liberated, 
 which would not have been the result had they been cemented by an 
 organic cement-substance. By substituting dilute muriatic acid, the 
 action of which is the destruction of organized tissue, the prisms were not 
 
ENAMEL 329 
 
 liberated, the acid acting evenly upon the whole mass of enamel, and 
 finally resulting in its complete destruction, not leaving the slightest trace 
 of an organic matrix behind. Dr. Williams claims that this transparent 
 cement-substance is formed by the distribution of a translucent liquid 
 substance about the previously formed pattern for the enamel rods. 
 This pattern, generated through the activity of the enamel cells, is com- 
 posed of a translucent material somewhat more solid than that substance 
 which surrounds it. These two substances calcify together, the latter 
 forming the enamel prisms, while the former creates the cement or inter- 
 prismatic substance. There is no better method by which to study the 
 character or mode of development of a growing or matured tissue than 
 by an artificial disassociation of its component parts. In enamel it 
 matters but little in what part of the tissue or at what period of its growth 
 the examination be made to learn of the action of the decalcifying agent; 
 it will never be found to take place in a manner corresponding to the 
 direction of the enamel fibers, but decalcification takes place in older 
 enamel more in the form of a general breaking up of the structure, as 
 shown at A in figure 233, while at B, which represents the newly formed 
 tissue, the action is one which appears to indicate a breaking up of the 
 interprismatic substance, favoring the theory of secretory amelification. 
 
 This distinction between the enamel cells and their product possesses 
 none of the characteristics to properly classify it as structureless. There 
 is but little doubt as to its character, being the primary product of the 
 ameloblasts, while the corresponding zone at the distal end of the ena- 
 mel-forming cells results from the functional activity of the stratum 
 intermedium. 
 
 Figure 242 shows a section near the point of the cusp of a developing 
 molar and exhibits a portion of the enamel organ at a time immediately 
 prior to the beginning of enamel calcification. A is the uncalcified dentin, 
 B the ameloblasts, now closely associated, and a very regular layer of 
 elongated cells, and behind these another layer of cells, which undoubt- 
 edly serve as feeders to the ameloblasts, the stratum intermedium. This 
 section is especially valuable in that it shows a number of capillaries 
 distributed through the body of the stellate reticulum and actually pene- 
 trating the stratum intermedium, as seen at B. 
 
 When viewed with a low power, these minute blood-vessels appear 
 to form a complete network, and in the district between the cusps pervade 
 the entire structure, from the stratum intermedium on one side to the 
 same cells on the other. The appearance of this animated vascular sup- 
 ply to the enamel is coincident with the process of calcification, for during 
 
330 
 
 HISTOLOGY 
 
 the early life of the tooth-germ it is non-vascular. The growth of enamel, 
 strata upon strata, from within outward is therefore by the direct cal- 
 cification of the enamel cells or ameloblasts, and while this is going on, and 
 as long as the crown of the tooth is incased in its epithelial cap, the enamel 
 organ, the growth of the tissue is stimulated through the blood-vessels 
 
 Fig. 234. 
 
 everywhere present in the stellate reticulum. The presence of this special- 
 ized blood supply to the central portion of the enamel organ was for a long 
 time doubted, but at present it can be readily observed (Fig. 233). As 
 soon, however, as the tooth passes through the surface tissue, carrying 
 with it the external epithelium of the enamel organ as the enamel cuticle, 
 the possibility of nourishment has been cut off, and after a little time it 
 
DENTIN 
 
 33* 
 
 becomes a petrified dental epithelium, no longer nourished and absolutely 
 non-vital. 
 
 Dentin. — This tissue, which constitutes the principal bulk of the 
 hard part of the tooth, forms a complete cap-like investment over the 
 pulp, from which it is generated. It is white or slightly yellowish-white 
 in color, somewhat elastic, and a trifle harder than bone, which it resem- 
 bles in many of its characteristics. In a perfectly developed tooth no part 
 of the dentin appears upon the surface, that part within the crown being 
 covered by the enamel, while that of the root is inclosed by the cementum. 
 
 Ename 
 
 Dentin 
 
 Fig. 235. — Section through Crown of Human Cuspid. X30. 
 
 While the thickness of the dentin varies somewhat over the different parts 
 of the tooth, there is a decided disposition to an equal distribution in 
 every direction. Dentin, unlike enamel, consists of an organic matrix — ■ 
 a reticular tissue of fine fibrils richly impregnated with the salts of cal- 
 cium, in this resembling the matrix of bone. Traversing the matrix 
 are long, fine canals or tubes (Fig. 243) — the dentinal tubules — which 
 pass from the margins of the pulp toward the surface. Immediately 
 surrounding the dentinal tubules the matrix is especially dense, forming 
 a lining or sheath to the tubes, known as the dentinal sheaths. Occupying 
 the lumen of the dentinal tubules are solid elastic fibers — the dentinal 
 fibers. Dentin, therefore, presents for examination, first, the matrix; 
 
332 HISTOLOGY 
 
 second, the dentinal tubules; third, the dentinal sheaths; and fourth, 
 the dentinal libers. 
 
 The Matrix. — As previously stated, the matrix is composed of organic 
 and inorganic substances, but the proportionate quantity of organic 
 and inorganic constituents is so variable that it is impossible to furnish 
 a definite chemic analysis. The relative quantity of organic and in- 
 organic matter is not only variable in the teeth of different individuals, 
 but is continually changing in the teeth of the same individual, the 
 former being present in larger quantities during youth and gradually 
 
 
 Fig. 236. — Section through Root of Human Incisor, showing many Dentin Tubules in 
 
 Transverse Section. X200. 
 
 diminishing as age advances. From an examination of perfectly dried 
 dentin, the following approximate analysis has been obtained: 
 
 Organic matter (tooth-cartilage) 27.61 
 
 Fat 0.40 
 
 Calcium phosphate and fiuorid 66.72 
 
 Calcium carbonate 3-3^> 
 
 Magnesium phosphate 1.08 
 
 Other salts o . 83 
 
 The organic basis of the matrix appears to be structureless and 
 transparent, and, although closely resembling the matrix of bone, is 
 not identical with it. While the matrix is usually structureless, there 
 
DENTIN 
 
 333 
 
 are instances in which the presence at one time of connective-tissue fibers 
 is indicated. 
 
 The Dentinal Tubules (Figs. 237, 238, 239). — Beginning by a free 
 opening about the walls of the pulp-cavity, the dentinal tubules permeate 
 the matrix in all directions. The tubules are generally disposed in a 
 direction perpendicular to the surface, so that in different parts of the 
 tooth they radiate in various directions. Beginning upon the surface 
 of the pulp-cavity, at which point they are of greatest diameter, they 
 pass more or less in a spiral manner toward the surface (Fig. 239), before 
 reaching which they become gradually reduced in size, as a result of the 
 
 Lacuna 
 
 Dentinal Tubules 
 
 Cementum 
 
 Dentin 
 
 Fig., 237. — Dentin and Cementum from Root of Human Molar. (After Gysi.) 
 
 numerous branches which they give off (Fig. 238). The branches given 
 off from the main tubes are quite variable in size, and anastomose with 
 one another or with the branches from other tubules. In the region of 
 the pulp the tubules are so closely associated that but little space is pro- 
 vided for the intertubular substance or matrix; but as the surface is 
 approached they become widely separated, and, in consequence, the 
 matrix substance is present in greater abundance. While the general 
 direction of the tubes is perpendicular, they do not pursue a direct course, 
 but are more or less curved as they pass from within outward. The 
 curvature of the tubuli may be divided into two classes — long curves 
 and short curves — usually referred to as the primary and secondary curva- 
 tures of the dentinal tubules. The primary curvatures are few in number 
 and are most prominent in the crown, while the secondary curvatures, prin- 
 
334 
 
 HISTOLOGY 
 
 cipally found in the roots, are smaller and more numerous. The branches 
 from the main tubes terminate in various ways, either by anastomosis, 
 by gradually fading out into hair-like terminals, or by ending in hooks 
 and loops. In rare instances they are said to enter the substance of 
 the enamel or cementum, but it is doubtful if they do so normally. The 
 branches from a main tube are usually two in number, the latter being 
 almost equal in diameter to the former, and from this first set of branches 
 a number of minute branches are given off almost at right angles. In 
 the crown this latter class of tubules are seldom observed, excepting 
 near the enamel margin, but in the root they are everywhere noticed. 
 
 Fig. 238. — Longitudinal Section through Root of Human Molar. Branching of the Dentinal 
 
 Tubules. X200. 
 
 Small varicosities are frequently present, but not in sufficient numbers 
 to produce a striated appearance on the surface of the dentin. 
 
 The Dentinal Sheaths. — While the dentinal tubules ramify through 
 the matrix in the form of well-defined channels, the walls of the channels 
 are not formed by the matrix, but by an indestructible substance the 
 exact character of which is not fully understood. The walls of the tubes, 
 or the dentinal sheaths, as they are termed, are believed by some histolo- 
 gists to be calcified, while others, though acknowledging their apparent 
 indestructibility, are doubtful as to the correctness of this theory. Neu- 
 mann being the first to accurately describe the walls of the tubules, they 
 have become known as "Neumann's sheaths." The existence of the 
 
DENTIN 
 
 335 
 
 dentinal sheaths may best be demonstrated by subjecting the tissue to 
 the action of strong acid for a sufficient time to destroy the intervening 
 matrix, which process usually requires several days. The fibrous mass 
 remaining will be found to contain a collection of tubes, which, however, by 
 careful examination, are found not to be the dentinal tubules themselves, 
 but the walls of these canals. Magitot and Sudduth deny the existence of a 
 wall to the dentin tubes. Tomes, while inclined to the belief that the tubes 
 are provided with definite walls, suggests that they may have been pro- 
 duced artificially during the preparation of the specimen, and that they 
 are only brought into existence by the action of the agent used for this 
 
 Fig. 239. 
 
 -Transverse Section through Root of Human Molar, showing the Curvature of 
 the Dentin Tubules about the Pulp-canal. X40. 
 
 purpose. In conclusion, the same writer adds that that part of the 
 matrix immediately surrounding the fibril differs in its chemic con- 
 stituents from the body of the matrix. 
 
 The Dentinal Fibers. — Occupying the lumen of each dentin tube is 
 a soft, elastic fiber, which is continuous with and has its origin from the 
 odontoblastic cells upon the periphery of the pulp. The existence of 
 these elongated processes of the odontoblasts having first been demon- 
 strated by Tomes, they are otherwise known as Tomes' fibers. By 
 means of these fibers, which not only fill the lumen of the larger tubes, 
 but the minute branches as well, the substance of the dentin is both 
 
33^ 
 
 HISTOLOGY 
 
 nourished and rendered slightly sensitive. There is still some doubt as 
 to the real nature of the fibrils, but, if they are processes from the odonto- 
 blasts, it would appear that the substance would be identical with that 
 of the cell-protoplasm. Bodecker claims that they are not round but 
 inclined to angularity, but Tomes infers that this form has been produced 
 by the action of some reagent. Klein advances the theory that the odon- 
 toblasts are active in the generation of the matrix for the dentin only, 
 and that the dentinal fibrils are not processes from them, but originate 
 from cells intervening between the odontoblasts and connecting with the 
 dentin tubes. It has never been fully demonstrated that true nerve- 
 
 X&Z 
 
 Fig. 240.— Longitudinal Section through Root of Human Tooth, showing Primary Curvature 
 of Dentin Tubules. X40. 
 
 fibers enter the dentin along with or in the substance of the dentinal 
 fibril, but, while 'the evidence is not at present forthcoming, there is but 
 little doubt that the sensitiveness of the dentin is produced by the presence 
 of organized tissue in the tubuli. Some contend that the contents of the 
 tubules are made of, first, a creative portion, that given off directly from 
 the odontoblasts; second, a circulatory portion, a minute vessel traversing 
 each tubule, entering either by the side of the cell-bodies or passing 
 through them, and that the nerve terminals are distributed in the same 
 manner. Others say that minute nerve-filaments from the pulp pass 
 directly through the odontoblasts and are continued in the center of the 
 tubule surrounded by a simple connective tissue, the cell process, and 
 
DENTIN 337 
 
 that in this way sensations are conveyed. It is now generally conceded 
 that dentin is a highly organized connective tissue; that it has a circula- 
 tory system and is endowed with sensation to a slight degree; that these 
 conditions are brought about not by actual entrance into the tubules of 
 separate vessels and nerve-filaments, but more in the way of the tubules 
 being occupied by a general connective-tissue substance resembling in 
 all essential features the pulp itself, being the semi-fluid interfibrillar 
 ground-substance of the pulp; that dendrites of sensory neurons every- 
 where present in the pulp, after losing their medullary sheaths divide 
 into fine varicose fibers and become closely associated with the peripheral 
 
 Cemen- .--re- 
 
 turn | . v. "* 
 
 
 Dentin 
 
 iljp 
 
 
 
 Pulp-canal 
 
 -r: * 
 
 *ye0^^S^/- 
 
 Fig. 241. — Transverse Section through the Root of a Human Incisor, showing the Dentin 
 Surrounded by the Cementum. X30. 
 
 cells, pass between these, and enter the cone-shaped openings of the 
 tubules and terminate soon after doing so. 
 
 While the microscope reveals in some instances what appear to be 
 prolongations from the dentinal fibers penetrating the enamel, or between 
 its prisms, such a condition is improbable if not impossible. If this 
 arrangement is present at all, it is so slight as to have no influence what- 
 ever over the enamel either as to nourishment or sensation. No conclu- 
 sions can be drawn with positive certainty from sections, since the slightest 
 deviation from parallelism in the surfaces may easily produce deceptive 
 appearances. It is just as common, and even more so, to find hair-like 
 lines interwoven and running parallel with the surface of the dentin 
 
33& 
 
 HISTOLOGY 
 
 immediately between this tissue and the enamel, as it is to see slight 
 fibers crossing beyond their boundary-line to penetrate the enamel. 
 The most likely place of all to find such a condition would be in the 
 beginning of calcification, and here it is never observed. The peripheral 
 pulp-cells, usually all classed as odontoblasts, are never found outside 
 their own territory, the dentinal papilla; but their location in the beginning 
 on the very "surface of the papilla, almost in direct contact with the inner 
 tunic of the enamel organ, would make it possible for their processes, 
 when appearing, to penetrate between the cells of the enamel organ if 
 they were grown out from the bodv of the cells from which they spring. 
 
 Cementum 
 
 Dentin 
 
 Granular 
 Layer 
 
 Fig. 242. — Tomes' Granular Layer. X40. 
 
 This, however, they do not do. They do not grow out from the cell- 
 body, so to speak, but the cell recedes, leaving them behind. By this 
 arrangement the terminals of the future fibers become definitely estab- 
 lished, all increase in length taking place in the opposite direction, toward 
 the pulp. While the active enamel-forming cells are present some little 
 time prior to the odontoblasts, calcification of the enamel does not take 
 place until after a definite cap of dentin has been formed, imprisoned in 
 which are the terminal branches of the fibers. Therefore the fact that 
 this cap of dentin is formed first, and this is not a question in dispute, 
 with the fibers or cell processes securely encapsulated within it, would 
 
DENTIN 339 
 
 seem to be sufficient evidence to qualify the statement that the dentinal 
 fibers do not penetrate the enamel. The examination of very many 
 sections of young growing teeth exhibits the fact that the early formed 
 dentin and enamel will separate bodily, leaving a positive clear line of 
 separation and a surface absolutely devoid of anything resembling the 
 prolongation of the fibers extending from the surface of the dentin. 
 
 Interglobular Spaces.- — In that part of the dentin which immediately 
 underlies the cementum numerous intercommunicating, irregularly 
 branched spaces are found. These are known as the interglobular spaces 
 (Fig. 242). On account of the granular appearance which this portion 
 of the dentin exhibits under low magnifying power, Tomes has desig- 
 
 Fig. 243. — So-called Interglobular Spaces in Dried Section of Dentin. Xioo. 
 
 nated it as the "granular layer." The granular layer is also found upon 
 that portion of the dentin which underlies the enamel, but in this region 
 it is far less marked. Many of the dentin tubes have their endings in 
 these spaces. While the interglobular spaces are most numerous near 
 the peripheral portion of the dentin, they are by no means confined to these 
 parts. They are present in all parts of the dentin, but not so closely 
 associated, and may be observed, when a dried section of dentin is exam- 
 ined, as spaces with irregular outlines and sharp-pointed processes ex- 
 tending in various directions (Fig. 243). The term "interglobular spaces" 
 becomes partly a misnomer when the so-called "spaces" are more care- 
 fully examined. In normal dentin the "spaces" are filled with a soft, 
 living plasma, having a structural arrangement similar to the general 
 
34© 
 
 HISTOLOGY 
 
 matrix of the dentin, and it is only in a dried specimen that a true space 
 is found by the shrinking or shriveling of the organic contents. The 
 interglobular spaces forming the granular layer, which are much more 
 numerous, but of smaller size, than those found in the body of the dentin, 
 are also filled with a soft living plasma, and they communicate, on one 
 hand, with the dentinal fibers, and, on the other, with the lacunae and 
 canaliculi of the cementum. According to Sudduth, the interglobular 
 spaces (so called) are occupied by masses of calcoglobulin which have 
 not become fully calcified. 
 
 N Dentinification. — The dentin bulb, or papilla from which the dentin 
 is forced, having already been described in Part I, the process of calcifi- 
 
 Pulp-cells 
 
 Odontoblasts v?i? . o*- 
 
 Odontoblasts 
 
 '. Calcified Dentin 
 
 Uncalcified Dentin 
 
 Fig. 244 — Pulp and Forming Dentin from an Incisor Tooth. (After Gysi.) 
 
 cation will at once be taken up. It will be recalled that calcification of 
 the dentin does not begin until the dentin papilla has developed to the 
 form and size of the dentin of the future tooth-crown. When this has 
 taken place, there is generated upon the surface of the papilla a modified 
 form of connective-tissue cells called odontoblasts (Fig. 244). These 
 cells, which are arranged in a single row upon the exterior of the papilla, 
 vary in form according to their activity. When most active, they are 
 broadest at the extremity directed toward the interior of the papilla. 
 Proceeding from a single odontoblast there may be one or more processes, 
 which are supposed to eventually occupy the tubes of the dentin, as the 
 dentinal fibers. These cells each contain an oblong nucleus, which occu- 
 pies the extremity of the cell most distant from the dentin, but during 
 
DENTIN" 
 
 341 
 
 the period of greatest activity becomes elongated or pointed in the direc- 
 tion of the process. The odontoblastic cells, while actively engaged in 
 the calcifying process, are closely asso:iated or crowded together, but 
 previous to this time there is more or less space between them, which is 
 filled with an indifferent tissue. The first layer of dentin being formed 
 upon the surface of the papilla, it will be observed that all additions to 
 its bulk take place from within (the reverse being true of enamel). 
 
 Fig. 245. — Section through Pulp and Forming Dentin. 
 
 As stated elsewhere, calcification of the dentin begins upon the 
 coronal extremities of the crowns, the cutting-edges of the incisors and 
 cuspids, and the summits of the cusps in the cuspidate teeth first receiving 
 their lime-salts. While the odontoblasts undoubtedly superintend the 
 calcifying process, the part taken by these cells appears to be somewhat 
 indefinitely determined. It is generally supposed that the li>ne-salts are 
 secreted under the superintendency of the odontoblasts. The secretion, 
 however, does not take place around the cells, and in that way completely 
 
34 2 HISTOLOGY 
 
 encapsule them, but around their fibrils. While this is taking place the 
 odontoblasts remain free upon the surface of the pulp, and the fibrils 
 assume their places as the organic dentinal fibrils. As the body of dentin 
 becomes thicker, the odontoblasts are forced to recede, and in so doing 
 the fibers lengthen. The dentinal tubules are, of course, formed in a 
 like manner, the walls of the tube being first calcified from the secretion 
 of lime-salts by the fibrils; and as the fibrils lengthen by the increasing 
 thickness of dentin and the receding of the odontoblasts, the tubes also 
 lengthen. 
 
 The general character of the pulp cells at a time immediately prior 
 to the appearance of the ameloblasts is vastly different from the same 
 cells at maturity or after calcification of the dentin has taken place. The 
 reason for this is obvious, considering that the connective-tissue mass 
 does not assume its principal function until the odontoblasts are generated 
 about its periphery. While at a later period the cells of the pulp are 
 oblong in shape, with slender tail-like processes given off from each end, 
 we find the same cells in the early embryo (sixteenth to twentieth week) 
 spheroidal in outline and distributed as they continue to be, at irregular 
 intervals about the semi-gelatinous matrix. (See Fig. 245.) 
 
 When the periphery of the pulp is reached, a definite layer of cells is 
 present, corresponding in every particular to those of the interior, and 
 it is from these spheric bodies that the dentin-forming cells are derived. 
 While the odontoblasts are usually characterized as spindle- or flask- 
 shaped cells, this can only apply to the cells of later life, as those active 
 at the beginning of calcification do not partake of either of these forms. 
 Figure 246 (twentieth to twenty-fourth week) shows the first formed 
 odontoblasts actively engaged in their function of dentinification. It will 
 be observed that the cells, instead of being individualized as they appear 
 at a later period, now present a racemose arrangement, such a cluster 
 appearing about the entrance to each dentinal tubuli, which at this period 
 are widely separated and apparently without anastomosing branches. 
 The nearer the summit of the crown is approached, the less apparent is 
 this grape-like association of the cells, showing conclusively that it is 
 a primary condition. 
 
 After a definite thickness of calcified dentin appears about the surface 
 of the tooth-pulp, the character of the odontoblastic cells becomes mate- 
 rially changed (twenty-fourth week), but even yet they do not answer 
 to the description accorded them. The elongated, spindle-shaped or 
 club-shaped odontoblasts are without question found in connection with 
 the tissue only after calcification has progressed to a considerable extent; 
 
DENTIN 
 
 343 
 
 and while it does not appear possible to detect the minute processes which 
 penetrate the calcifying structure before this stage of the phenomena has 
 been reached, they have nevertheless existed from the earliest inception 
 of this specialized layer of cells. 
 
 In this connection the query presents itself in regard to the manner 
 in which the intercommunication between the dentinal fibers is established, 
 and the probable cause for the so-called interglobular spaces about the 
 
 ,-*•«■ . : y j y * • 
 
 
 
 
 4k' . uim gL 
 
 _~rU T . 
 
 ^»w? ^avr ^ 
 
 — J)t»>*»»». 
 
 — • • • *** . ■!*£i£M^;lSSs& 
 
 
 ^F^HI r 
 
 
 *v3^^ r 
 
 
 ^'^BeSS®*^^ 
 
 
 Fig. 246. — Young Odontoblasts Attached to Forming Dentin. 
 
 periphery of the dentin. The former can probably be explained by an 
 examination of the peripheral cells of the pulp at a time immediately 
 prior to the beginning of calcification, when it will be found that these 
 primitive odontoblasts communicate with one another in a manner quite 
 similar to the canaliculi between the lacunae of the true bone, the connect- 
 ing processes being encapsuled within the substance of the calcifying 
 tissue. 
 
 Figure 247 is taken from a very thin section of a growing tooth at 
 
344 
 
 HISTOLOGY 
 
 a time in which we would most naturally look for the appearance of the 
 interglobular spaces, and many such imperfections, if they be so classified, 
 arc observed within the substance of the new r ly formed tissue. 
 
 Exceptions might be taken to the statement concerning the racemose 
 appearance of the early odontoblasts previously referred to, by claiming 
 the section to be one not directly through the long axis of the cells, or 
 
 Fig. 247. — Section through Crown of Growing Tooth of Lamb. 
 
 perhaps transversely through them. The examination of a number or 
 sections, one of which is shown in figure 247, shows the cells cut trans- 
 versely; the forming dentin at a appears with the tubuli squarely cut off, 
 showing the dentinal fibers confined, or rather appearing as though pro- 
 jecting from the lumen of the tubes. It will also be noted that the odonto- 
 blasts are irregular in outline, some of them being almost hexagonal, and 
 
DENTIN 345 
 
 as the calcified tissue is approached, they gradually become reduced in 
 size and much modified in contour. 
 
 After a dentin cap or matrix of considerable thickness has made its 
 appearance and the enamel cells are about to assume their functional 
 activity, the odontoblasts for the first time begin to resolve themselves 
 into ' the elongated flask-shaped cells, thus answering the description 
 usually accorded them, as illustrated in figure 244. In fact it would 
 
 Fig. 248. — Odontoblasts in Transverse Section. 
 
 appear that they assume this shape only when the actual lime deposit 
 begins. If we examine the line of union between the dentin and enamel 
 during the early growth of these tissues, it will be ascertained that, not- 
 withstanding the dissimilarity of the two structures at maturity, there 
 appears at this line of junction a matrix which may be defferentiated only 
 by the free extremities of the ameloblasts, as shown in figure 249. When 
 the enamel matrix begins to form, a faint line of demarcation between 
 the two may be observed, the difference in the appearance of the ground- 
 
346 
 
 HISTOLOGY 
 
 work of the two structures being one brought out by differential staining, 
 that of the enamel taking the darkest stain. 
 
 It will be observed, therefore, that after the dentin germ has assumed 
 the exact size of the dentin of the future tooth, certain cells appear upon 
 its periphery, and under their superintendence a definite layer of dentin 
 
 Fig. 249. — Section through Pulp, Dentin, and Forming Enamel. 
 A, Calcified dentin; B, Pulp; C, Ameloblasts. 
 
 soon results. This first formed layer of dentin is definite and unchange- 
 able in location, and it has within its substance the minute processes from 
 the dentin -forming cells which are destined to become and really are the 
 terminals of the dentinal tubules. All who have given the subject of 
 dentin calcification careful consideration are practically agreed as to the 
 part which the peripheral pulp-cells play in the process. This is to the 
 
DENTIN 347 
 
 effect that not about the body of the cells themselves, but around their 
 processes the lime salts are deposited. After a distinct layer of specialized 
 cells has become fully established upon the very periphery of the papilla, 
 the first change which takes place is a slight withdrawal of these cells 
 from this point, leaving behind slender hair-like processes which occupy a 
 portion of the space previously taken up by them, and about the ex- 
 tremities of the cells and their processes which are directed toward the 
 enamel organ calcified material is generated. Fone upon zone of calcified 
 dentin appears in this way, the body of the cell receding, leaving in its 
 wake its processes encapsuled within the calcified structure as the dentinal 
 fibers. 
 
 In connection with the primitive layer of dentin-forming cells, there 
 are usually described lateral processes passing from cell to cell, apparently 
 serving the purpose of communication between the cells. But these 
 have recently been shown to be simply a network of connective-tissue 
 fibers supporting the body of the cells. The theory of Andrews, brought 
 out some years ago in regard to the specialized layer of pear-shaped 
 cells — dentin corpuscles, as he termed them — may be accepted, and 
 these should be considered as having something to do with the process of 
 dentinification. The presence of these pear-shaped cells at the beginning 
 of calcification and during the continuance of this process can be easily 
 demonstrated, and if we accept them as being concerned in the process 
 of dentin formation, they might in a measure modify the function now 
 accorded the elongated club-shaped cells, the odontoblasts. It is ques- 
 tionable whether the odontoblasts alone are responsible for the growth 
 of dentin; they undoubtedly control the actual process of lime deposit, 
 but the additional cells, no doubt, contribute to the structural make-up 
 of the tissue. It may be that by modifying certain parts of the matrix, 
 the result in the general structure is the dentinal sheaths; this part of the 
 tissue being so markedly different from the bulk of the intercellular sub- 
 stance would lead us to believe that it was developed from specialized 
 cells. Further, it is said that while the dentinal tubules are filled with 
 a living substance, this substance is not solely the product of the processes 
 of the odontoblasts. That there is a special distribution of non-medul- 
 lated nerve terminals as well as a rich plexus of blood-vessels about the 
 periphery of the pulp is unquestionable, and this supply is just as plentiful, 
 or perhaps more so, at maturity as it is at the beginning of calcification 
 when the dentin cells are most active. From this we might be led to 
 believe that this special blood and nerve supply to the periphery of the 
 pulp is not solely for the upbuilding of the dentin and therefore distributed 
 
348 
 
 HISTOLOGY 
 
 to the peripheral cells, but also for the permanent welfare of the resultant 
 tissue, this being brought about by some circulatory system throughout 
 the tubules of the dentin. 
 
 Cementum (Fig. 250). — Investing the roots of the teeth is a sub- 
 stance which, both chemically and physically, is closely allied to bone. 
 This external covering is known as the cementum, and while generally 
 regarded as being confined to the roots of the teeth, by some it is considered 
 
 Lucun; 
 
 Fig. 250. — Cementum from Root of Molar. X200. 
 
 to extend to and completely invest the crowns during the early part of 
 their existence, in this latter location being known as the enamel cuticle, 
 or membrane of Nasmyth. 
 
 Generally speaking, the cementum begins by a thin margin at the 
 neck of the tooth or cervical line. It may commence at the free enamel 
 margin of the crown, or it may slightly overlap this structure. It is 
 thinnest at the neck of the tooth, and gradually increases in thickness as 
 the apex of the root is approached. In teeth with closely associated 
 
CEMENTUM 
 
 349 
 
 roots the cementum frequently extends from one root to the other, result- 
 ing in a firm, osseous union. Histologically considered, the structure of 
 cementum, like ordinary bone, consists of a gelatinous, basal substance, 
 combined with the salts of lime, and of numerous little hollow spaces — 
 lacuna. Branching in every direction from the lacunae are many minute 
 processes — canaliculi. 
 
 The Matrix. — The matrix is so nearly identical with that of bone 
 that it is with difficulty that they can be distinguished. By decalcification 
 it retains its form and structure, and by the intimate blending of organic 
 
 Fig. 251. — Longitudinal Section through Root of Human Molar. 
 Incremental Lines of Cementum. X30. 
 
 and inorganic substances it is provided with hardness, solidity, and 
 elasticity. Calcium salts and collagenous fibrils, united by a small 
 amount of cement-substance, in finer or coarser bundles, compose the 
 ground-substance, or matrix, of cementum. 
 
 Let us first take up the study of this tissue at different periods of its 
 existence, and in this manner learn of its character, its mode of develop- 
 ment, and the changes which take place as its growth proceeds. The 
 striated markings of the tissue have led to the belief that there are, during 
 the process of cementification, periods of activity and periods of rest or 
 little activity. An examination of the structure under low power (Fig 
 251) shows the incremental lines placed, with more or less regularity, one 
 
35° 
 
 HISTOLOGY 
 
 beyond the other, and when thus studied adds much to the strength of the 
 theory of interrupted development. 
 
 Figure 252 is prepared from a developing deciduous incisor three 
 months after birth. At this period the devloping organ is made up of 
 enamel and dentin alone, the process of cementification not yet being 
 under way. The establishment of the dentinal periphery, which surface 
 is unchangeable, provides a basis for the first layer of cementum generated 
 by the cementoblasts, which at this period are forming about the inner 
 wall of the tooth-follicle. In close proximity to the surface the inter- 
 
 Enamel 
 
 
 ^ 
 
 Surface of I 
 Dentin 
 
 
 
 
 • r" J 
 
 
 W 
 
 Fig. 252. — Section through Developing Incisor, Three Months after Birth. X30. 
 
 globular spaces are observed somewhat widely distributed, and propor- 
 tionately large in size, resulting in a surface poorly calcified and forming a 
 ready attachment for the cemental tissue. Figure 253 shows the process 
 of cementification under way, the section being prepared from a six- 
 month-old tooth. In an examination of the ground-substance of this 
 developing tissue there is an unbroken granular appearance, possessing 
 neither striations, fibers, nor cement-corpuscles. This appearance is 
 one which persists in the oldest or first-formed stratum, and is again 
 noticeable in the outermost or youngest stratum. While the oldest 
 stratum or strata retain this primary character, this cannot be said ^of 
 
CEMENTUM 
 
 351 
 
 those subsequently laid upon it, for they successively develop in their 
 matrix the partially calcified cells and fibers from the formative tissue. 
 
 Figure 254, taken from a one-year-old tooth, shows a further advance 
 in the process of cementification. Many of the transverse fibers of the 
 alveolodental membrane are observed penetrating the developing tissue, 
 and will, at a later period, by their partial calcification, become a part of 
 its substance. Already there has been established an intimate blending 
 of the cemental tissues with the dentinal tissues through the medium of the 
 granular layer, and by the further calcification of the latter this union 
 
 Fig. 253. — Developing Cementum, from Six-month-old Tooth. X200. 
 a, Developing Cementum; b, Granular Layer; c, Terminals of the Dentinal Tubuli. 
 
 gradually becomes more thorough. Figure 255 illustrates three distinct 
 zones of developing cementum; the older unbroken granular zone at A, 
 now beautifully cemented to the granular layer; a second or intermediate 
 zone, B, having encapsuled within its ground-substance many of its 
 formative cells; and an outer zone, C, but recently laid down, showing 
 numerous, longitudinal, wave-like striations, emblematic of the cemento- 
 blastic activity. In this outer zone the minute laminations disappear 
 as the tissue becomes more thoroughly calcified and the matrix gradually 
 partakes of the nature of the older tissue. 
 
35 2 
 
 HISTOLOGY 
 
 The position occupied by the cementum on the root has much to do 
 with its character. In the region of the cervix the cement-corpuscles 
 
 Fibers of 
 Cementum 
 
 Older Strata 
 
 ^J^Granular 
 ^F Layer 
 
 Fig. 254. — Section through One-year-old Tooth. X60. 
 
 
 
 
 
 
 V 
 
 4? 
 
 T 
 
 
 
 
 
 ■ 
 
 A 
 
 ;*N^>Cv 
 
 
 c<*-» 
 
 Fig. 255. — Developing Cementum, from Transverse Section of Bicuspid. X 100. 
 
 are few in number, and when present possess extremely short and irregular 
 processes. In the region of the apex the structure is much more complex 
 
CEMENTUM 
 
 353 
 
 in character, longitudinal striae, transverse fibers, cement-corpuscles, 
 and zones of apparently unbroken granular matrix all serving to this end. 
 
 To continue the study of this tissue let us examine in detail the 
 lamellae, the cement-corpuscles, and the cement-fibers. 
 
 The Lamella. — We are told that the lamellae are about the same in 
 number over all parts of the tooth-root, but that they are much thinner 
 at the neck than at the apex. In addition to this they are usually con- 
 sidered as running parallel, or nearly parallel, to the surface of the dentin. 
 While these statements might, and probably do, describe the -disposition 
 
 
 
 
 MS 
 
 
 
 Fig. 256. — Transverse Section from Root of Bicuspid, showing Variation in the 
 Disposition of the Lamellae. X40. 
 
 of the lamallae in young cementum, they do not apply with so much 
 certainty to the conditions after the adult period. The lamellae in the 
 region of the apex are not only of greater width, but are usually greater 
 in number than those occupying the cervix of the same root. 
 
 Figure 256 is prepared from the transverse section of an adult bicuspid 
 in the region of the apex, and shows how the disposition of the lamellae 
 may vary in thin, normal cementum. At A, which represents the granular 
 union of the cementum with the dentin, the incremental lines are ob- 
 served to follow the surface of the dentin. As the center of the area is 
 approached this regularity is much interfered with, some of the lamellae 
 being discontinued, others greatly thickened, while the field, taken in its 
 23 
 
354 
 
 HISTOLOGY 
 
 entirety, exhibits anything but regularity in the laying down of the 
 different strata. This same condition may be observed in longitudinal 
 section. While the lamellae are usually characteristic of the cemental 
 tissue in general, they are seldom found in interdentinal cementum, or 
 that growth which takes place between roots, resulting in their fusion 
 (Fig. 257). This, of course, refers to the tissue as formed between closely 
 associated roots of an individual tooth, and not to that union which 
 sometimes takes place between the roots of different teeth. The inter- 
 dentinal tissue previously referred to appears to have many characteristics 
 
 Interdentinal 
 Cementum f 
 
 Fig. 257. — Transverse Section through Fused Roots of Molar Tooth, showing 
 Interdentinal Cementum. X30. 
 
 common to itself; thus, the cement-corpuscles are peculiar in form, fibers 
 are few in number, and, as before stated, the lamellae are not decided. 
 Cement-corpuscles. — Many of the cementoblasts of the peridental 
 membrane, like the osteoblasts of the periosteum, become encapsuled 
 within the developing tissue, and persist as irregularly shaped spaces, 
 filled with a protoplasmic mass, and are known as cement-corpuscles. 
 These correspond to the lacuna, of bone, but, unlike these, are very variable 
 in size, in form, and in the number and direction of their processes. 
 Figure 250 shows a number of cemental lacunae and canaliculi. In 
 the majority of instances the body of the corpuscle will be found to be 
 oval or slightly oblong, with its long axis parallel to the surface; but it is 
 by no means uncommon to find them very irregular in outline, with the 
 
CEMENTUM 
 
 355 
 
 greatest diameter in the opposite direction. The processes are quite 
 variable in length and irregular in their course, and, while there is a 
 general disposition for them to extend toward the surface, they in many 
 instances radiate in various directions. All of these features are in con- 
 tradistinction to the lacunae and canaliculi of bone, which are placed 
 with much more regularity in the osseous matrix, the corpuscles being 
 oblong or cylindric in outline, with their processes about equally distrib- 
 uted in every direction, and uniting directly and positively with the 
 canaliculi of neighboring lacunae. As previously stated, the cement- 
 
 Fig. 258. — Cement-corpuscles of Outer or Younger Strata. X40. 
 
 corpuscles are very variable in outline, this difference in form appearing 
 to be much influenced by the part of the tooth examined. The younger 
 corpuscles (Fig. 258), or those associated with the outer strata, are usually 
 distinctly outlined and provided with delicate processes, the majority of 
 which are directed toward the surface. In the older strata the outlines 
 of the corpuscles are much more irregular, the processes short and ex- 
 tremely clumsy. 
 
 The proportionate distribution of the corpuscles to the various parts 
 of the tooth-root is as follows: The innermost or oldest zone and the 
 outermost or youngest zones contain but few; in the intervening strata 
 they are most abundant, especially in the region of the apex, becoming 
 less numerous in passing crownward. In interdentinal cementum the 
 
356 
 
 HISTOLOGY 
 
 corpuscles are somewhat regularly distributed throughout the ground- 
 substance adjacent to the granular layer, but near the center of this 
 confused mass of imperfectly calcified tissue they are seldom present. 
 When the interdentinal space is slight, peculiarly formed corpuscles are 
 often observed (Fig. 259), provided with a long, rod-like, central portion 
 or trunk, from which are given off numerous tree-like branches, the 
 terminals of which are frequently lost in the granular layer upon either 
 side. 
 
 Cement-fibers. — In a manner similar to that in which the cemento- 
 blasts become encapsuled within the developing cemental tissue forming 
 
 
 Fig. 259. — Cement-corpuscles Common to Interdentinal Cementum. X ioo. 
 
 the cement-corpuscles, many of the fibers of the peridental membrane 
 undergo a like transformation, and are found in the tissue as more or less 
 imperfectly calcified fibers transversely disposed. By many writers these 
 filamentary, thread-like structures have been compared to the delicate, 
 net-like processes which pass through the concentric lamellae of bone, 
 serving to hold them together and designated as Sharpens fibers; but, 
 according to Black, these are the principal fibers of the alveolodental 
 periosteum, and, as already stated, become a part of the cemental tissue 
 during its evolution. In figure 260 the fibers are shown under high 
 power; A represents the primary or older stratum of the tissue, and it is 
 from the outer margin of this zone that the fibers first make their appear- 
 ance, passing more or less directly in the direction of the surface until the 
 
CEMENTUM 357 
 
 next incremental line is reached, at which point they gradually disappear, 
 but recur in the succeeding lamellae. There is a marked disposition for 
 the fibers of each concentric lamella to keep within its borders, or, in 
 other words, to become individualized; but in many instances they pass 
 through from one lamella to another, and occasionally extend unbroken 
 through the entire thickness of the tissue. It occasionally happens that 
 the fibers are plentifully distributed to a region comprising three or four 
 lamella?, followed by a zone of similar proportions in which they are 
 entirely absent. The cement-fibers, considered as the partially calcified 
 residue of the principal fibers of the peridental membrane, would naturally 
 
 
 ft 
 
 
 
 1 
 
 
 < 
 
 . V 
 
 
 •>; 
 
 
 Fig. 260. — Transverse Section through Root of Molar, showing Cemental Fibers. X300. 
 
 assume a general direction relative to their manner of distribution before 
 this change had taken place, and in most instances they are thus disposed, 
 In figure 261, taken from the center of a long axis of a growing bicuspid, 
 the disposition of the fibers, which are alone observed in the second 
 lamella, is slightly crownward. The inclination for the fibers to be thus 
 disposed is most pronounced in young cementum, but after middle life, 
 or at a period when the tissue has greatly increased in thickness, the 
 course of the fibers, even in the same locality, is greatly at variance. 
 
 In figure 254, also from a young tooth, the fibers are shown springing 
 directly from the peridental membrane, with their free extremities 
 penetrating this tissue. This illustration is prepared from a transverse 
 section in the cervical region, and the inclination of the fibers is such as 
 
358 
 
 HISTOLOGY 
 
 to warrant the belief that they were some of those whose function it has 
 been to return the tooth to its normal position when slightly rotated upon 
 itself. Another class of fibers common to the cement-tissue are those 
 which appear to be grouped in bundles, springing more or less regularly, 
 at intervals, from the granular layer and penetrating the basement layer 
 of the cementum as though serving to tie this tissue to the periphery of 
 the dentin. In figure 262 a number of these bundles are shown at A, B, 
 and C. While the field is but a small proportion of the circumference 
 of the root, they are observed, under low power, to be distributed in a 
 
 Fig. 261. 
 
 like manner to all parts. These circumferential fibers, as they may be 
 called, are also observed in longitudinal section, being distributed with 
 considerable regularity throughout the whole extent of the root. They 
 are also present in the tissue at the earliest period at which its character 
 may be studied, the individual bundles at this time being proportionately 
 larger. These might be, and probably are, considered as prolongations 
 from the dentinal fibers, but it is doubtful if the true fibers of the dentin 
 are ever found penetrating the cementum. 
 
 Cementification. — We have seen in the study of the development of 
 the teeth, that the tooth-generating organs were confined in a closed sac 
 or follicle, and while the walls of this sac were not directly interested in 
 the calcification of the dentin or enamel, this cannot be said of the cemen- 
 tum. Attention has also been directed to the fact that at the time of 
 
THE DENTAL PULP 
 
 359 
 
 the eruption of the crown of the tooth a portion of the root only is cal- 
 cified. As the growth of the root continues, the follicular wall becomes 
 closely adherent to it. Upon the inner face of this vascular membrane 
 a layer of osteoblastic cells (cementoblasts) is generated, and as a result 
 of the calcification of these cells the cementum is formed. It will thus be 
 seen that the process of cementification is but a slightly modified form of 
 subperiosteal bone development. At the beginning of cementum cal- 
 cification the diameter of the dentin of the root is as great as it will ever 
 be, all additions to its bulk taking place from within. But while the 
 
 diameter of the dentin is thus fixed, the diameter of the root is increased 
 by the additional layers of cementum as they are deposited upon its 
 surface. As previously stated, 'a single layer of cementoblasts is first 
 formed in the membrane surrounding the root, these soon becoming 
 inclosed in a spherule of lime. By the time this has taken place another 
 layer makes its appearance, assuming all the characteristics of the first 
 formed layer. Other layers are formed in turn until the cementum 
 assumes its mature thickness. 
 
 The Dental Pulp (Fig. 263.). — The tooth-pulp, or formative organ 
 of the dentin , occupies the central or pulp-cavity, and in the fully developed 
 tooth assumes a general outline closely corresponding to the exterior 
 
3 6 ° 
 
 HISTOLOGY 
 
 of the organ.* Along with its primary function of generating the dentin, 
 it becomes the medium through which this structure receives its vascular 
 and nervous supply. 
 
 Histologically considered, the pulp may be described as a mucous- 
 like, protoplasmic matrix, containing delicate connective-tissue fibers 
 not formed into bundles and numerous nucleated cells, the latter being 
 especially numerous on the periphery of the pulp, or that portion which 
 comes in contact with the dentin. The cells are not closely enough 
 
 Enamel 
 
 Cementum 
 
 Dentin 
 
 Pulp 
 
 Blood-vessels 
 
 Fio. 263. — Longitudinal Section through Human Cuspid, showing Tooth-pulp. 
 (After Gysi.) X 10. 
 
 associated to form a complete tissue in themselves, but are found em- 
 bedded in a mucoid matrix, with always a definite space between them. 
 In general the cells are elongated or spindle-shaped, with a delicate, 
 hair-like process attached to either extremity. In the pulp-chamber the 
 cells vary somewhat in outline, in some instances being spheroid, in 
 others appearing as slender filaments, so that the cell proper can scarcely 
 be distinguished from its processes. A third class of cells may be met 
 
 * The pulp not only occupies the central cavity in the tooth-crown, but the canals of the 
 roots as well; therefore the form of the pulp corresponds to the outline of the pulp-cavity, 
 already described. 
 
THE DENTAL PULP 
 
 361 
 
 with, from which three or more filaments are given off. As stated, the 
 distribution of cells varies considerably in different parts of the pulp, 
 this being true not only as regards numbers, but also as to the relations 
 existing between the cells. In the coronal portion of the pulp the posi- 
 tion assumed by each individual cell appears to be without regard to 
 the position of neighboring cells, while in that portion of the pulp oc- 
 cupying the root L canals the cells are arranged parallel with the length 
 
 Fig. 264. — Transverse Section through Pulp. Blood-vessels and Nerves in Cross Section, 
 
 of the root. The cells are least in number in the interior of the pulp, 
 but gradually become more plentiful as the periphery is approached. (See 
 Cells of Dentin Papilla, page 408). 
 
 The Odontoblasts (Fig. 265). — The most active cells of the pulp are 
 those directl} on its periphery, in contact with the dentin, and known 
 as the odontoblasts. The odontoblastic layer, otherwise known as the 
 membrana eboris, is composed of a single row of cells, each of which 
 contains, near the extremity most distant from the dentin, a well-defined 
 nucleus. They are large, elongated cells, each furnished with three sets 
 
3 62 
 
 HISTOLOGY 
 
 of fibers or processes — the dentinal process, the pulpal process, and the 
 lateral process. The dentinal process or processes — there may be more 
 than one present — communicate with the deeper-lying cells of the pulp, 
 while by means of the lateral processes the cells are brought into com- 
 munication with neighboring cells. The processes given off in the 
 direction of the dentin, or the dentinal processes, may be one for each cell, 
 in which case they are of considerable size, and are inclined to taper as 
 
 Dentin 
 
 Blood- 
 vessels 
 
 Fig. 265. — Pulp and Dentin in Longitudinal Section. 
 
 they enter the substance of the dentin. Again, a single cell may give off 
 a number of smaller processes in this direction. The odontoblasts vary 
 much in form according to their functional activity. Before the period 
 of dentinification they are spheroid or pyriform, during the period of 
 calcification the dentin extremity becomes somewhat flattened and 
 square, while in advanced years they again return to their primitive, 
 rounded form. Covering the entire surface of the pulp like an epithelium, 
 the odontoblasts are especially closely associated at the end nearest the 
 dentin, forming an unbroken layer, while the pulpal extremities are 
 inclined to assert their individuality by disassociation. 
 
THE DENTAL PULP 
 
 363 
 
 Blood-vessels of the Pulp. — The pulp is richly supplied with blood- 
 vessels, forming networks extending principally in a direction parallel to 
 the long axis of the tooth, and finally terminate in a capillary plexus 
 closely associated with the odontoblastic layer. The veins of the pulp 
 are. ordinarily somewhat larger than the arteries, and form numerous 
 anastomoses. This organ appears to be destitute of lymphatics — at 
 
 Fig. 266. — Section through Pulp. Fig. 265 in Transverse Section. 
 
 least, none are known to occur in its substance. The blood-vessels of the 
 pulp are provided with a longitudinal layer of thinly distributed muscular 
 fiber, but otherwise the walls of the vessels are noted for their delicacy. 
 Nerves of the Pulp. — After entering the apical foramen either by one 
 large trunk or by two or more minute ones, the fibers pursue a parallel 
 course, breaking up but little or giving off but few fibers in that portion 
 of the pulp confined to the canal. When the expanded or coronal portion 
 of the pulp is reached, numerous subdivisions occur which are distributed 
 in every direction, and ending in a rich plexus beneath the odontoblastic 
 layer, or membrana eboris. In the body of the pulp the fibers are medul- 
 
3°4 
 
 HISTOLOGY 
 
 lated, but those occupying the periphery are non-medullated and sup- 
 posed to pass into the dentinal tubes. While this latter hypothesis 
 is in all probabiltiy correct, such a distribution of the germinal fibers has 
 never been definitely demonstrated. Two investigators (Ball and 
 Magitot) claim to have partially satisfied themselves in regard to the final 
 distribution of the non-medullated fibers. The former states that he has 
 traced these fibers into continuity with the larger medullated fibers in the 
 deeper pulp-tissue, and claims to have found them passing through the 
 
 Main Blood 
 vessels 
 
 Branching of Main 
 Nerve-trunk into 
 Single Fibers 
 
 Branching ofTMain 
 Blood-vessels into 
 Capillaries 
 
 Fig. 267 — Distribution of Blood-vessels and Nerves to the Pulp of Human Molar. 
 
 {After Gy si.) X20. 
 
 membrana eboris, beyond which point they assumed a direction parallel 
 to the dentinal tubules. This theory is controverted by Magitot, who 
 claims that the dentinal fibers are, in a measure, themselves prolongations 
 of the nerves, being so constituted through the medium of the branched 
 stellate cells which lie immediately beneath the membrana eboris, and 
 by which the nerves are made continuous. 
 
 Nasmyth's Membrane. — Nasmyth's membrane, otherwise known 
 as the enamel cuticle or persistent dentinal capsule, is an exceedingly thin 
 and peculiarly indestructible structure, entirely covering the enamel. 
 As to the presence of this membrane, which can be demonstrated only by 
 chemic detachment, there appears no doubt, but in regard to its origin 
 and definite structure much difference of opinion has been expressed. 
 
THE DENTAL PULP 
 
 365 
 
 By some writers (Tomes and Magitot) it is maintained that it is continuous 
 with, and similar in structure to, the cementum covering the root, being 
 an extension of the outermost layer in the region of the neck of the tooth; 
 and, in view of the fact that lacunae are found in its substance, this theory 
 would appear to be correct. On the other hand, it is considered to be a 
 product of the epithelium (Huxley and Koiliker) and in no manner con- 
 nected with the cementum. In the opinion of the author, it would be 
 difficult to understand how the theory advanced by Tomes could be 
 accepted. During the entire period of saccular development the crown 
 
 Alveolar Wall 
 
 ■ 
 - 
 
 Cementum 
 
 AlveolodentE 
 Membrane 
 
 Blood-vessels of 
 Alveolodental Mem- 
 brane in Trans- 
 verse Section 
 
 Dentin 
 
 Fig. 268. — Transverse Section through Root of Human Incisor and Surrounding 
 Alveolar Wall, with Alveolodental Membrane Intervening. X40. 
 
 of the tooth is in close relationship to the enamel organ, this structure 
 intervening between the forming enamel and the wall of the tooth-sac, 
 from which the cementum is developed. It would, therefore, appear 
 that this membrane is generated from the external epithelial layer of the 
 enamel organ by a change in the character and form of these cells. Sud- 
 duth attributes its formation to a metamorphosis of the ameloblastic 
 layer, the prismatic cells assuming a horizontal direction. The amelo- 
 blasts are observed to be prismatic in form up to the point at which the 
 enamel prisms are yet unfinished, but as the surface is approached they 
 are observed to shorten and widen, and near the gum-margin they as- 
 sume a longitudinal direction instead of being at right angles to the 
 
3 66 
 
 HISTOLOGY 
 
 body of the crown. Mrs. Emily Whitman devoted much time to the 
 study of the development of mammalian teeth, and appears to be of the 
 opinion that the cuticula dentis is the result of a change in the form and 
 character of the enamel cells, this metamorphosis taking place either 
 before or after calcification of the underlying tooth-tissues. Nasmyth's 
 membrane shows many characteristics which differ from those of the 
 body of enamel subjacent to it, serving as an indestructible, highly 
 polished surface-capping to the enamel prisms. The indestructible 
 nature of this membrane by reagents would appear to indicate that in 
 
 Fig. 269. — Section through Root of Tooth, Alveolodental Membrane, and Alveolus. 
 a, Alveolus; b, Blood-vessel; c, Alveolar Portion; d, Dental Portion; e, Cementum. 
 
 structure it is closely akin to the structure lining the dentinal tubules, 
 the lacunae, etc. 
 
 Alveolodental Membrane (Figs. 268 and 269). — As a general descrip- 
 tion of this membrane has already been given in Part I, it alone remains 
 to treat of its histologic character, which may best be accomplished by first 
 referring to the duties which it has to perform. These may be divided 
 into three classes — functional, physical, and sensory. The functional 
 office is accomplished through its cellular elements — the osteoblasts and 
 cementoblasts; the physical office is performed by the fibrous elements, 
 through which the tooth is fixed in its position; and the sensory office 
 
ALVEOLODENTAL MEMBRANE 367 
 
 through the abundance of nerves, which are richly distributed to all parts 
 of the membrane. We, therefore, find in this structure, besides con- 
 nective tissue, cells, fibers, nerves, and blood-vessels. The principal 
 cells, as already stated, are the osteoblasts and cementoblasts, but there 
 are aso present fibroblasts and osteoclasts. 
 
 The osteoblasts, which are instrumental in the upbuilding of a portion 
 of the alveolar walls, are found lying against the bone, between the 
 principal fibers. These cells do not appear to be evenly distributed, 
 being numerous and crowded together in some parts, while others will 
 
 Epithelium. 
 
 Alveolar 
 Wall 
 
 Fig. 2 70. — Transverse Section through Root of Tooth, Alveolodental Membrane, Thin 
 Wall of Alveolus, and Gingival Tissue. 
 
 appear to be almost destitute of them. They are most plentiful in the 
 young subject, and seldom present at all in old age. In youth the 
 alveolodental membrane is thickest, and, as the building of bone occurs 
 on the inner wall of the alveolus, it can only progress as the membrane 
 becomes reduced in thickness. The osteoblasts are polygonal cells, in- 
 clining to the oval form, and vary greatly in size, with their longest 
 diameter at right angles with the surface of the forming bone. During 
 the period of the development of the young alveolar wall they are inclined 
 to be crowded together, and are frequently much distorted from pressure 
 
368 HISTOLOGY 
 
 upon one another. As age advances this condition becomes less pro- 
 nounced, and the cells separate into groups. 
 
 The Cementoblasts. — Stationed upon the opposite side of the mem- 
 brane, or that in contact with the root of the tooth, are another class of 
 cells — the cementoblasts — or those cells which are concerned in the 
 formation of the cementum. Like the osteoblasts, these cells are found 
 lying between the principal fibers of the root-membrane. They differ 
 in form from the osteoblasts, notwithstanding that they have a similar 
 function. Instead of the polygonal form common to the osteoblasts, we 
 
 Dentin 
 
 Fig. 271. — Longitudinal Section through Root of Growing Tooth near the Cervix. 
 
 find these cells to be more or less flattened, with outlines somewhat 
 irregular. Extending from the body of the cell, which contains a well- 
 defined nucleus, are a number of irregular processes, which penetrate 
 the neighboring fibers or the interfibrous substance. Unlike the osteo- 
 blasts, the cementoblasts appear at all times to be evenly distributed 
 over the surface of the cementum, occupying all the space except that 
 taken up by the fibers as they leave the cementum. As to the develop- 
 ment of the osteoblasts and cementoblasts, they appear to be carried to 
 the fibrous meshes of the membrane by the blood as leukocytes or ameboid 
 cells, after which, by differentiation, they become fitted for the develop- 
 ment of bone or cementum, assuming their respective places against 
 the surface of one or the other of these structures. 
 
ALVEOLODENTAL MEMBRANE 369 
 
 Fibroblasts and Osteoclasts. — Fibroblasts and osteoclasts are also 
 present in the alveolodental membrane, the former for the purpose of the 
 increase or renewal of the fibrous tissue, the latter being functionally 
 concerned in the removal of a part of the alveolar waUs to accommodate 
 the ever- varying position of the teeth, or acting in a similar manner upon 
 the cementum of the root. The osteoclasts, or giant-cells, are generally 
 inclined to the round or oblong form, and usually contain a number of 
 nucleoli. They vary much in size, and are seldom branched or provided 
 with processes. In addition to the four classes of cells already mentioned 
 
 Cementum 
 
 Blood-vessel' 
 
 Alveolodental 
 Membrane 
 
 Gingivae, 
 
 Fig. 272. — Longitudinal Section through Alveolodental Membrane, Gingival Tissue, 
 and Root of Tooth. Cervical District. 
 
 as being present within the meshes of the fibrous tissue of the root-mem- 
 brane, there is another class, present, however, during youth only, which 
 appears to be in course of development, and, therefore, without apparent 
 function. 
 
 The Fibers oj the Alveolodental Membrane. — The principal fibers of 
 the alveolodental membrane are those which extend from the cementum 
 on one side to the alveolar wall on the other, and become firmly fixed 
 at either extremity by penetrating the calcified structures. The fibers 
 are all of the white, or inelastic, connective-tissue variety. It is by means 
 of the connective-tissue fibers that the actual attachment of the membrane 
 24 
 
37° 
 
 HISTOLOGY 
 
 both to the bone and to the cementum takes place, the fibers passing 
 directly into the hard tissues, which they traverse for some distance, 
 being here known as Sharpeys fibers. 
 
 The arrangement of the fibers is somewhat different over the various 
 parts of the root. In the region of the gingival margin they pass out 
 from the substance of the cementum, retaining their solid form or dividing 
 into fasciculi of finer fibers. In general the fibers lie parallel with one 
 
 Fibers 
 
 Fig. 273. — Section showing Fibers of Alveolodental Membrane, Attached to' and Passing 
 
 Out from the Cementum. 
 
 another, deviating only to give place to blood-vessels and nerves. There 
 is some variation in the distribution of the fibers about the different 
 gingival surfaces. Upon the labial and lingual surfaces they pass out 
 directly into the fibrous tissue of the gum, and soon become lost in this 
 tissue. On the mesial and distal surfaces the fibers passing the lower 
 margin of the alveolar wall join the fibers of the neighboring tooth. 
 This disposition for the fibers to bend toward the adjacent tooth is first 
 observed at the various angles of the gingival margin. All about the 
 
ALVEOLODENTAL MEMBRANE 
 
 371 
 
 free border of the gum the fibers from the alveolodental membrane 
 assist in forming this tissue, which is covered by a dense epithelial coating 
 of moderate thickness, surrounded or surmounted by the peridental 
 fibers. As the border of the alveolar wall is approached, the fibers are 
 observed to pass under the proper tissues of the gum, and unite with the 
 outer periosteal layer overlying the outer alveolar wall. The fibers 
 immediately within the alveolus are slightly inclined in an apical direction, 
 while those occupying the central portion of the membrane, or that 
 midway between the apex and the gingiva, pass nearly straight across 
 
 Alveolus 
 
 Cementum 
 
 Pulp 
 
 Dentin 
 
 Fig. 274. — Transverse Section through Growing Roots and Alveolus. 
 
 from the cementum to the bone. It is in this locality that the largest 
 and strongest fibers are found. As the apex of the root is approached 
 the inclination of the fibers is crownward from the cementum to the 
 alveolar wall. In this situation the single fibers are inclined to break up 
 the fasciculi. Immediately surrounding the apex of the root the fibers 
 are irregular during youth, but are disposed more regularly or fan-like 
 in older subjects. 
 
 While this account briefly furnishes a description of the distribution 
 of the fibers in various locations, and is in most instances correct, they 
 occasionally vary from this arrangement. While in most respects the 
 fibers of this membrane closely resemble the corresponding fibers of 
 
372 HISTOLOGY 
 
 attached periosteum, they possess some peculiarities. It might be 
 supposed that the fibers passing out from the cementum would in some 
 way differ from those springing from the alveolar wall, but, with the 
 exception of being somewhat less in size, they are otherwise of the same 
 character. 
 
 Interfibrous Elements. — Besides the various forms of cells, blood- 
 vessels, and nerves, there is present in the alveolodental membrane an 
 interfibrous tissue. This tissue is principally composed of the fibroblasts 
 belonging to the principal fibers, and other fibroblasts accompanied by 
 delicate fibers which appear to be independently distributed. This 
 interfibrous tissue, which is thus seen to be ordinary fibrous connective 
 tissue, appears to pervade the entire membrane wherever sufficient space 
 is found to permit, of its presence. In some parts of the membrane this 
 tissue appears to be more plentiful than the principal fibers themselves. 
 The interfibrous tissue also forms an investment for the blood-vessels 
 and nerves in addition to the tissues properly belonging to their walls. 
 
373 
 
 Fig. 275. — Evolution of the Face. (After Haeckel.) 
 
CHAPTER VIII. 
 
 Embryology of the Mouth and Teeth. 
 
 The nourishment required for the growth of the embryo and its 
 maintainance during the earliest part of its development, and in higher 
 animals during the whole of this period, is supplied either from the 
 mother by means of a placenta, or it is drawn from the supply of concen- 
 trated food-material, stored up for that purpose in the form of yolk in 
 the egg. The formation of the future digestive organs of the mature 
 organism begins, however, already at a very early stage of the development 
 of the embryo. The essential epithelial parts of the digestive canal 
 are derived from the entoderm, while the more auxiliary parts, such as 
 
 muscles, connective tissue, blood-vessels, etc., 
 which are not present in the lower animals, 
 but become more and more conspicuous as we 
 advance in the scale of animal organization, 
 are derived from the visceral layer of the 
 mesoderm. At first, the primitive gut presents 
 a shallow, wide groove, but it becomes grad- 
 ually deeper by a folding up of the sides; the 
 edges approximate more and more, and when 
 finally a union has taken place, there is a tube 
 formed — the so-called primitive digestive tract. 
 While the latter still has for a time an outlet 
 in the form of the vitelline or umbilical duct, 
 the tube is closed anteriorly, as well as pos- 
 teriorly. As development proceeds, there becomes noticeable a depression 
 of the ectoderm at the anterior end of the tube, opposite the ventral side of 
 the latter, and here ectoderm and entoderm lay closely attached to each 
 other, forming a thin membrane. This point of contact of the two layers 
 is known as the oral plate. Soon the plate ruptures, and thus an anterior 
 aperture of the gut, the oral sinus or primitive month, is formed. 
 
 The gradual transformation of the primitive aperture into the per- 
 manent mouth and face is brought about mainly by the development 
 and transformations of its boundaries, and are, in brief, the following: 
 About the end of the third week, a series of conspicuous elevations or 
 
 374 
 
 Fig. 276. — Diagram showing 
 the relation between Ectoderm 
 and Entoderm in the Mouth of 
 a Mammalian Embryo. 
 
 a. I. and p.l., Anterior and 
 posterior lobes of the hyphysis 
 m. L, medullary tube; ph., 
 pharynx; o.p., oral plate; x 
 and y, ectoderm which pro- 
 duces the lip and teeth of the 
 lower and the upper jaw, re- 
 spectively. [Stbhr.) 
 
EMBRYOLOGY OF THE MOUTH AND TEETH 
 
 375 
 
 processes are developed around the opening, from the front and the sides. 
 The one coming down from the head is called fronto-nasal process; those 
 coming from the sides are arranged in form of parallel bars and are 
 known as arches. There are five pairs of arches; in fishes they give rise 
 to the formation of the gills, and the individual pairs are separated from 
 
 gc 
 
 Fig 277. 
 
 Fig. 277. — Head of a Young Dogfish. (Stohr.) 
 
 Fig. 278. — Head of Human Embryo of 10 mm. It Illustrates the Phylogenetic Rela- 
 tion of the Visceral Arches. (Stohr.) 
 
 g.c, Gill cleft; m, mouth; n, nasal pit; c.s., cervical sinus; g.c 2 , second gill cleft; h, hyoid 
 arch; md, mandibular arch; sp., (spiracle) auditory groove. 
 
 one another by slits. In human embryos there is no communication 
 between the inside and outside of the arches, but there are distinct 
 furrows which serve as lines of demarcation between the individual 
 bars. On account of these formations having connection genetically 
 with air-absorbing viscera, the following names are given: Visceral or 
 branchial arches, inner visceral furrows or pouches, external visceral fur- 
 rows or clefts. From the accompanying illustrations it can be clearly 
 
 Fronto-nasal 
 
 Process 
 Lateral Nasal 
 Process 
 Mesial Nasal 
 Process 
 Maxillary 
 Process 
 Roof of the Oro- 
 pharynx 
 
 Fig. 279. Fig 280. 
 
 Figs. 279 and 280 Present the Development of Various Parts of the Palate. 
 
 Median Portion 
 of the Palate 
 
 Dental Ridge 
 
 Lateral Portion 
 of the Palate 
 
 {After His.) 
 
 seen that only the frontal process and the first visceral arch participate 
 in the formation of the permanent mouth and face; the other four arches 
 give rise to parts, the consideration of which is beyond the scope of this 
 book. In regard to the details of the formation of the individual parts, 
 tbe following statements may be made: 
 
376 
 
 HISTOLOGY 
 
 The Buccal Cavity. — The early appearance of the entrance to the 
 alimentary canal is found in the formation of an open cavity bounded by 
 the primitive maxillary processes above and mandibular arch below. 
 
 The cavity thus formed is the common buccal space, the upper 
 portion being the respiratory or nasal section, while below is the true 
 mouth. The cavity of the mouth, as such, does not exist until these two 
 are completely separated by the palatal plates forming the future roof 
 of the mouth. 
 
 Figure 281 shows a vertical transverse section through this common 
 
 Nasal Cavity 
 
 V Nasal Cartilage 
 
 Buccal [Cavity 
 
 Dental Ridge, 
 Upper Jaw 
 
 s. 
 
 Dental Ridge, 
 Lower Jaw 
 
 Tongue 
 
 f Dental . Ridge, 
 / Lower Jaw 
 
 Fig. 281. — Vertical Transverse Section through Head of Human Embryo, about the 
 
 Tenth Week. X30. 
 
 buccal cavity. At this early period the lateral walls and floor of the 
 mouth are manifest by certain cellular elements, but the roof of the 
 cavity, as already stated, is not complete until the palatal plates, now 
 separated by the tongue, grow inward and unite at the median line. 
 
 The Oral Cavity. The Roof of the Mouth. — When these two 
 processes which arise from the mesoblast unite at the median line, they 
 establish a permanent horizontal septum, dividing this part of the stomo- 
 deum into a respiratory or nasal section and an oral section, the mouth. 
 The cells entering into this part of the fetal head at this time (eighth to 
 tenth week) are of three varieties, being connective-tissue cells, cartilage 
 
THE FLOOR OF THE MOUTH 
 
 377 
 
 cells, and epithelial cells, the latter being distributed in a layer of varying 
 thickness over those parts destined to become a part of the lining mem- 
 brane of the mouth. 
 
 In figure 282 (twelfth week) the superior maxillary processes are 
 shown united and the permanent separation between the mouth and nasal 
 cavity established. This embryonal bridge is for the most part made up 
 of connective-tissue cells, about isolated bundles of which osteoblasts 
 arrange themselves, resulting in the production of two intermembranous 
 bony plates. 
 
 Nasal Cavity 
 
 Dental Ridge 
 
 Tongue 
 
 Meckel's Cartilage 
 
 r-\ ; - 
 
 
 Cartilagenous Septum 
 of Nose 
 
 ) Dental Ridge 
 
 Oral Cavity 
 
 Dental Ridge 
 
 Anlage of Lower Jaw 
 
 Fig. 282. — Vertical Transverse Section through Head of Human Embryo, about the 
 Twelfth Week, showing the Single Buccal Cavity Transformed into the Oral and Nasal 
 Cavities. X30. 
 
 By the fourteenth week a further advance in the generation of the 
 hard palate is noted, the septum now being largely composed of calcified 
 tissue. The disposition for these primitive bony plates to exist as separate 
 and distinct processes is exemplified at the median line by a definite 
 separation formed by the connective-tissue sheath from which they are 
 derived. Covering the surface of the hard palate there now appears a 
 thin layer of mucous membrane. 
 
 The Floor of the Mouth.— Having thus briefly noted the evolution 
 of the roof of the mouth, let us next consider the floor of the cavity, the 
 tongue and its attached muscles, together with considerable glandular 
 
378 
 
 HISTOLOGY 
 
 tissue making up the bulk of this district. Figure 283 is a vertical 
 transverse section through the floor of the mouth about the tenth week 
 in the human fetus, or at a period somewhat later than that shown in the 
 previous illustration. An examination of the parts in general at a time 
 prior to this is of little value, save the early preparation for the develop- 
 ment of the teeth, which will be referred to later. The tissues and or- 
 
 Fig. 283. — Section through Base of Tongue and Lower Jaw. X40. 
 
 gans here shown will be recognized as the tongue (.4), the glandular 
 tissues (B), the forming jaw (C), with developing tooth-germs at D D. 
 The tongue appears on the floor of the mouth between the thirtieth 
 and thirty-sixth days as a bud from the mesoblast covered by a layer of 
 cells of epiblastic origin. The muscle-fibers, be they intrinsic or ex- 
 trinsic, are all of the striated variety. In a very short time, and at a 
 comparatively early period, the tongue becomes an independent organ, 
 presenting most of the characteristics common to it after birth. Not a 
 small portion of the floor of the mouth is made up of another class of 
 
THE FLOOR OF THE MOUTH 
 
 379 
 
 tissue which, although eventually a distinct organism, is composed almost 
 entirely of epithelium. These cells, together with the connective-tissue 
 cells, and eventually blood-vessels, unite in the production of a true 
 salivary gland, the sublingual. Figure 284 shows the early character of 
 the. tissue, together with its relation to surrounding parts. The section 
 is one from the region of the premolars, and is bounded above by the 
 tongue, laterally by the borders of the jaw, and below by fibers of the 
 
 Fig. 284.— Section of Sublingual District. X 100. 
 
 mylohyoid and digastric muscles. Three distinct lobes or sections of 
 the gland are observed, the two largest being separated by a reticular 
 network of connective tissue. 
 
 The general character of these developing glands even at this early 
 period (about the twelfth week) appears to be very similar to the matured 
 organ, being composed of a number of small tubes emptying into a single 
 duct, constituting a gland of the compound tubular variety. 
 
 Let us next give some consideration to the embryology of the mouth 
 
380 HISTOLOGY 
 
 in its entirety; and to do this, it is necessary to make sections of the parts 
 in various directions. 
 
 The growth of the cavity is usually studied, and probably to the 
 best advantage, by vertical transverse sections, and attention will first be 
 called to a number of sections made in this way, beginning at the lips and 
 passing backward through the incisor region, and finally through the 
 districts occupied respectively by the cuspids and molars. The period 
 
 Fig. 285. — Embryonal Labial Mucous Membrane. 
 
 at which such an investigation is made has much to do with the character 
 of the tissue involved, but the time best suited to the purpose is included 
 between the fortieth and sixtieth days. At this time nearly all the tissues 
 making up the organs and parts which enter into the construction of the 
 cavity have advanced to such a degree of perfection that the investigation 
 may proceed with considerable satisfaction. 
 
 Figure 285 shows a cross-section through one of the primitive labial 
 folds about the period named. Little is to be observed in this district 
 
THE FLOOR OF THE MOUTH 
 
 38l 
 
 at this early period except the simple cells of three varieties which serve 
 to make up the parts, but attention is at once attracted to the abundant 
 thickness of the epithelium given to the lip. 
 
 If a section be made somewhat to the distal of that previously shown, 
 a marked change in the relationship existing between the various cell 
 layers is observed in a body of cells of another character, those which are 
 
 Fig. 286. — Section through Base of Jaw. 
 
 destined to become the cartilage of Meckel, and about which the younger 
 layer of cells of mucous membrane are observed outlining a new district. 
 If a section be made through this same location, say about the forty- 
 eighth day, a vast change in the appearance of the parts is noticed (Fig. 
 281). The buccal walls of the mouth have in a measure become com- 
 plete by a union of the upper and lower sections, the union at this time 
 being accomplished through the agency of the embryonal epithelium. 
 A cartilaginous nasal septum has made its appearance, and active prepa- 
 ration for the ossification of the maxillae is apparent. In the center of the 
 
3 82 
 
 HISTOLOGY 
 
 section is a distinct body of cells forming Meckel's cartilage, and early 
 preparations for the growth of the teeth may be seen at a by a dipping 
 down of the surface epithelium. 
 
 A transverse section through the same district about the sixtieth 
 day (Fig. 287) shows all the parts strongly differentiated. Ossification 
 has taken place to a considerable extent in the lower jaw, the two halves 
 being at this period, and for some months afterward, separate and distinct. 
 
 
 w 
 
 ■ ; 
 
 # 
 
 ;*¥■ ■ * 
 
 ■ 
 
 ■ _ 
 
 Fig. 287. — Section through the Wall of the Mouth of an Embryo, Sixtieth Day. 
 
 Many muscle bundles are observed beneath the jaw, and beyond these 
 the integument with its numerous blood-vessels and nerves, most of which 
 are seen in transverse section. A cross-section upon the same subject 
 about the sixtieth day in the region of the cuspids finds the tissues and 
 organs advanced to a certain degree of perfection. (See Fig. 283.) The 
 tooth-germs of the cuspid teeth have their crowns outlined by the cells 
 composing them; the tongue with its complex muscular arrangement 
 has become a specialized and independent organ, while beneath it we see 
 
THE FLOOR OF THE MOUTH 
 
 3*3 
 
 that product of the epiblast, the glandular structure, so plentifully supplied 
 to the floor of the mouth jn this locality. 
 
 Passing further back into the region of the molars, the appearance 
 of the parts does not differ to any marked degree from that in the cuspid 
 district, except in the general distribution of the muscular fibers of the 
 
 Fig. 288. — Longitudinal Section through Chin of Embryo Lamb. 
 
 tongue, and the appearance of the submaxillary gland, here appearing in 
 three distinct lobes or parts. 
 
 It has been previously stated that sections made in the direction of 
 those already considered are usually employed to study these parts, but 
 much is to be gained by supplementing these with sections made in other 
 directions. 
 
384 
 
 HISTOLOGY 
 
 Figure 288 shows a longitudinal section, or one made from mesial to 
 distal through the lower jaw at or near the median line, the parts included 
 within the field being the labial folds at A, the mandible at C, the tongue 
 at D, and a tooth-germ at E. An examination of the lips shows them to be 
 covered with a varying thickness of embryonal epithelial cells which are 
 continued backward over the future alveolar ridge and thence to the 
 hard palate above, or over the floor of the mouth and the surface of the 
 tongue below. 
 
 Meckel's Cartilage. — One of the earliest products of the mesoblast 
 is that which results in the production of Meckel's cartilage, which is 
 
 Fig. 289. — Diagram to Illustrate the Metamorphosis of the Visceral 
 Arches during Development. 
 1, Presents the first visceral arch, it gives rise to Meckel's cartilage, which 
 becomes transformed into the mandible. {After Wiedersheim.) 
 
 closely associated with the growth and early support of the lower jaw. 
 In the beginning, as already pointed out, the mandibular and hyoid arches 
 resemble one another, but soon after they become fully established they 
 take on different functions, and with this become dissimilar. The first 
 appearance of this cartilage as a distinct body of cells is found about 
 the middle of the second month, and when a transverse section of the 
 jaw is made for the purpose of studying its location and environments 
 (see Fig. 290), it is found near the base of the fetal head, considerably 
 below and to the outside of the base of the tongue. At mid-jaw it appears 
 as a circular body of cells separated from the surrounding parts by a 
 distinct layer of elongated cells. Even at this early period a portion of the 
 bony structure of the jaw is outlined by an aggregation of connective- 
 tissue cells, and the forming cartilage appears to subserve the purpose of 
 
Meckel's cartilage 
 
 385 
 
 controlling the outline of the future jaw. The bow-shape of the cartilage 
 is manifest as We pass toward the symphysis by the lateral halves approach- 
 ing each other (Fig. 291), but the circular character of the cartilage in 
 cross-section is still retained. 
 
 Figure 292 represents a section through the symphysis about the 
 eighteenth week, and shows the two halves of the cartilage closely asso- 
 
 Fig. 290. — Section through Base of Lower Jaw, showing Meckel's Cartilage. 
 
 dated, but not united, the separation being by a layer of connective- 
 tissue cells passing between the two. It will be noted also that the 
 cartilage, instead of being near the base of the jaw as in figure 291, now 
 appears near the floor of the mouth. 
 
 Figure 293 shows the relations existing between the two halves of 
 Meckel's cartilage and the growing mandible at the median line (a). 
 25 
 
3 86 
 
 HISTOLOGY 
 
 It also illustrates how little the development of the bone is dependent upon 
 the cartilage, the growth of the former being in this district far below 
 and apparently distinct from the latter. Here, as in the upper jaw, the 
 periosteal cells from either side are observed to unite at the symphysis and 
 pass as a somewhat thickened layer between the two bones, the only 
 difference in the final change which takes place between the two being 
 that in the upper jaw a suture results, while in the lower jaw a layer of 
 
 Fig. 291. — Section through Lower Jaw. M. C, Meckel's cartilage. 
 
 solid bone is formed. The character of this cartilaginous framework as 
 well as the cells which divide the two halves is shown in figure 294, the 
 cartilage cells being oblong or cylindrical, with a bountiful supply of 
 intercellular substance, while the connective-tissue cells are oblong or 
 spindle-shaped. 
 
 As soon as ossification in the jaw takes place to any extent, the car- 
 tilage begins to atrophy, that portion lying next to the jaw degenerating 
 
MECKEL S CARTILAGE 
 
 387 
 
 Fig. 292. — Meckel's Cartilage (M. C.) at the Symphysis. 
 
3 88 
 
 HISTOLOGY 
 
 first, so that by the tenth or twelfth week it has entirely disappeared, but 
 before this takes place we find it surrounded by the periosteum, and 
 finally completely inclosed within the bone. 
 
 Figures 294 and 295 show the character of the cartilage cells about the 
 
 Fig. 293. — Ossification of the Mandible at the Median Line. 
 
 time that they are beginning to atrophy. It will be observed that the cells 
 are inclined to a change in form, and that they are proportionately larger 
 with large nuclei and nucleoli. A represents the district nearest the jaw, 
 and the cells in this region have already lost their characteristic outline. 
 
Meckel's cartilage 
 
 389 
 
 * 
 
 -*' "*" «^- ■**/* ^\hWB 
 
 
 
 Fig. 294. — Section of Meckel's Cartilage at Median Line. 
 
39° 
 
 HISTOLOGY 
 
 Fig. 295. — Cartilage cells in the beginning of atrophy. 
 
CHAPTER IX. 
 
 Development of the Teeth. — The Dental Germs, Enamel Organ, 
 and Dentin Organ; the Dental Follicle; Calcification and 
 Eruption. 
 
 DEVELOPMENT OF THE TEETH. 
 
 Tooth Band 
 
 Stellate Retic- 
 ulum 
 
 Ameloblasts 
 
 Dentin Papilla 
 
 Fig. 296. — Developing Tooth-germs. X300. 
 
 In order that the student may obtain a general idea of the structural 
 changes which take place at a very early period, and which eventually result 
 in the formation of the teeth, the genesis of the subject will be briefly re- 
 ferred to. Preparation for the development of the teeth takes place as early 
 
 39i 
 
392 HISTOLOGY 
 
 as the middle of the second fetal month, prior to the formation of the bony 
 structures which finally surround and give support to the organs. At 
 this early period there will be found following the line of the future alveolar 
 ridge a slight heaping up of the surface epithelium {epithelial ridge), 
 while immediately beneath this proliferation of cells there appears a dip- 
 ping in of the deep or infant epithelial layer (early called the dental 
 groove), in the direction of the future alveolar walls. This epithelial 
 reflection is later known as the epithelial band or tooth-band. This so- 
 called tooth-band is not, as might be supposed, a special inflection for 
 each tooth-germ, but is continuous from one distal end of the future jaw 
 to the other. It must be remembered that at this time the outline of the 
 jaws has not been established, and the tooth-band, is principally instru- 
 mental in directing the location of the dental organs. The position and 
 form of this epithelial band may best be studied in vertical transverse 
 section. When first making its appearance it is somewhat broad and 
 shallow (dental groove), but as it passes more deeply into the embryonic 
 tissue it partakes of the shape of the letter V with its open end directed 
 toward the surface. As it grows in length the free extremity of the band 
 is inclined to the lingual, its external surface is slightly convex, and its 
 internal surface correspondingly concave. After the tooth-band has 
 assumed certain proportions, there appears on its inner or concave 
 surface a thin membranous plate, tooth lamina which is likewise a con- 
 tinuous structure, extending the full length of the epithelial band. 
 
 This lamina does not spring from the free margin of the tooth-band, 
 but is given off at a point about midway between this border and the base 
 of the band. The structure of this secondary band is so similar to that 
 of the primary one that it should be considered as an inflexion from it 
 rather than a new structure. We find them between the seventh and 
 eighth week, the maxillary regions giving place to two bow-shaped 
 bands (one for each jaw), each of which is preparing to throw out from 
 its secondary lamina ten little buds, tooth buds which soon develop into 
 the germs for the twenty deciduous teeth. When these buds make their 
 appearance they are simple, rounded bodies, placed somewhat closely 
 together, but they do not long retain this simple form. The first change 
 which takes place is one in which they appear to lengthen out into slender 
 cords, the extremities of which soon begin to extend laterally, and a pear- 
 shaped enlargement of the epithelial cells appears, which by invagination 
 later assumes a bell-shaped outline, which phenomenon is rapdly in- 
 creased by a proliferation of connective-tissue cells forcing into the 
 concavity. 
 
DEVELOPMENT OF THE TEETH 
 
 393 
 
 This bell-shaped proliferation of cells, given off directly from the 
 tooth-band, to which it continues for a time attached, together with the 
 specialized connnective-tissue cells crowding into its concavity, constitute 
 the tooth-germs, the former being the enamel organ, and the latter the 
 dentin organ. It will, therefore, be seen that the enamel is dependent upon 
 the oral epithelium for its development (ecdermic), while the dentin 
 springs from an entirely different source — the connective tissue of the jaw 
 
 Epithelium of 
 Tongue 
 
 , 
 
 Epithelium of Lower Jaw 
 
 '; I'/' ... Tooth Band 
 
 Infant Layer *&&' 
 of Cells K" ' 
 
 Forming Bone 
 
 • - - 
 
 
 V 
 
 Stellate Retic- 
 ulum 
 
 » Ameloblasts 
 Dentin Papilla 
 
 
 Fig. 297. — Developing Tooth-germs, Twelfth Week. X40. 
 
 (endermic). The enamel organ rapidly undergoes a cellular transfor- 
 mation: its concavity is increased, and the bell-shaped outline more 
 strongly defined. Accompanying this change in form it gradually recedes 
 from the surface, and its connection with the tooth-band becomes less 
 secure. The connective-tissue cells, which have been rapidly filling in 
 the concavity of the enamel organ, are also preparing to take upon them- 
 selves a special function, that of the formation of the dentin. Up to this 
 period (tenth week) the enamel and dentin germs are not definitely 
 separated from the surrounding cellular structure, but now a gradual 
 transformation takes place, whereby the tooth-germs become enveloped 
 in a sac-like covering — tooth-sac, this together with its contents forming 
 the tooth- follicle. 
 
394 HISTOLOGY 
 
 Enamel Organ (Figs. 296, 297, and 298). — This portion of the 
 tooth-germ, as previously stated, is derived from the concave or lingual 
 surface of the tooth-band, which in turn is derived from the surface 
 epithelium. From the free extremity of its slender cord-like attachment 
 it spreads out and forms a hood-like covering to the dentin germ. The 
 surface of the organ contiguous to the dentin germ, or dentin papilla, as 
 it is frequently called, is concave in the direction of the oral surface, 
 being thickest over the center of its concavity, thinning down as its per- 
 iphery is approached. Externally, the enamel organ is covered by an 
 epithelial layer, which is reflected upon its inner surface or that in contact 
 with the dentin papilla. These two layers are named according to their 
 location, the external and internal epithelium of the enamel organ. Placed 
 between these two layers, and constituting the bulk of the organ, are 
 numerous stellate bodies which penetrate a layer of rounded cells, the 
 stratum intermedium, and finally reach the internal epithelial layer 
 known as the enamel cells or ameloblasts. It is from this internal layer 
 of epithelial cells that the enamel is calcified, and they are, therefore, 
 the essential cells of the enamel organ. In the fully developed enamel 
 organ, there are to be found, therefore, four distinct layers of cells, the 
 external epithelium, stellate reticulum, stratum intermedium, and internal 
 epithelium, or ameloblasts. As its name implies, the function of the 
 enamel organ is principally that of enamel calcification, but in the opinion 
 of many writers its primary activity is that of molding the tooth-form 
 as represented by the dentin papilla, and it is not until this latter organ 
 has assumed the form and extent of the dentin of the future tooth-crown 
 that calcification begins. 
 
 The life of the enamel organ may properly be considered as begin- 
 ning when the bulbous extremity of the specialized cells given off from 
 the lingual face of the tooth-band become invaginated, and from this by 
 a rapid proliferation of its cells it passes on by successive stages assum- 
 ing the various forms common to it. This proliferation and differentia- 
 tion of cells continues up to the time of beginning of calcification, but 
 with the advent of this phenomenon certain parts of the organ begin to 
 degenerate. This degeneration may or may not be classed as an atrophy 
 of the cells interested, but the fact that a new tissue is generating, and 
 gradually occupying the space previously taken up by the formative cells, 
 calls forth a demand for the removal of the latter by the former. The 
 cells which first undergo this change are those of the internal epithelium 
 and stratum intermedium, the individuality of these two layers evidently 
 being kept up by migratory cells from the stellate reticulum. It is argued 
 
DEVELOPMENT OF THE TEETH 
 
 395 
 
 by some writers that the external epithelium begins to atrophy at this 
 period; by others this change is not recorded until the enamel cuticle has 
 been deposited to effectually seal the young tissue and protect it until well 
 desiccated. While there appears to be a decided disposition upon the 
 part of this outer layer of cells to change, they do not dissappear, and the 
 alteration is not one which affects the shape of the cells, for they remain 
 flattened or prismatic with their long axis parallel with the anlage 
 of the crown and most likely become the enamel cuticle. The stellate 
 
 FlG. 298. — Developing Tooth-germs, Enamel Organ, and Dentin Papilla. 
 
 cells making up the bulk of the organ are, in common with those which 
 inclose them, continually undergoing a degenerative change, at least this 
 is true of those cells closely associated with the stratum intermedium, for 
 in this location they rapidly proliferate, shed their many processes, 
 and gradually take on the characteristics common to this layer of which 
 they eventually become a part. A careful examination of no less than one 
 hundred enamel organs in all stages of development, and by section cut 
 transversely, obliquely, longitudinally, etc., fully justifies the statement 
 that the real life of the enamel organ begins as previously stated, and 
 continues until the structural arrangement of the enamel is completed. 
 
!9 6 
 
 HISTOLOGY 
 
 The question of form in the enamel organ — that is, its external epithe- 
 lium — is one which may be advantageously used in the consideration of 
 the life and function of its different cell layers. It has been said that the 
 apparently extravagant area taken up by the enamel organ subserves 
 the purpose of reserving space for the growing tooth-crown, but there are 
 many reasions why this theory cannot be accepted. In the first place, 
 the extent of the organ or the space existing between the dentin papilla 
 and the outer enamel epithelium does not in very many instances corre- 
 
 Fig. 299. — Developing Tooth-germs. Longitudinal Sections from Buccal to Lingual. 
 
 spond to the bulb of enamel when this tissue is completed at a given point. 
 In the developed tooth we find the enamel thickest over the cutting-edges 
 of the anterior teeth and about the summits of the cusps of the cuspidate 
 teeth, while these same parts are represented during the cellular stage of 
 development by the external layer of cells closely associated with the 
 surface of the papilla. 
 
 Again, the outline of the tooth is definitely represented by the cells 
 making up the dentin papilla (Figs. 298 and 299), but the surrounding 
 epithelial cells are characterized by an unbroken semicircular margin 
 
DEVELOPMENT OF THE TEETH 
 
 397 
 
 describing the extent and form of the enamel organ. Exception may be 
 taken to this hypothesis from the standpoint of generative changes, and 
 these in a great measure have much to do with the relative outlines as- 
 sumed by the two organs, but by studying very many sections representing 
 nearly every stage of the process, and all of them in a measure showing 
 the same characteristics, nothing but a definite opinion can result. 
 
 Of the many changes in general form which the enamel organ under- 
 goes, none are so pronounced and positive in character as those de- 
 
 Fig. 300. — Developing Tooth-germs. Longitudinal Section from Mesial to Distal. 
 
 scribed by the inner tunic, and first recorded when the bulbous end of 
 the specialized cell becomes invaginated by the mesodermic connective- 
 tissue cells forcing themselves into it. This is an alteration which is 
 gradual and continuous up to the time of beginning of calcification, and 
 while the cells forming the dentin papilla are generally accorded the power 
 of "pushing" or "forcing" their way into those derived from the epi- 
 blast, the latter has always been recognized as having a controlling influ- 
 ence over the former. In this connection a reasonable doubt presents 
 
398 
 
 HISTOLOGY 
 
 itself covering the theory so long accepted that the early function of the 
 enamel organ is one which in a measure superintends the contouring of 
 the tooth-crown as first represented in the dentin papilla. When the 
 character of the two embryonal tissues making up the two germs is com- 
 pared, we find the dentin germ possessing all the characteristics favor- 
 able to a rapid proliferation of its cells resulting in a highly vascular, 
 compact tissue. On the other hand, the bulk of the enamel organ is a 
 gelatinous-like mass, one that would readily succumb to the pressure 
 
 Fig. 301. — Developing Tooth-germs in Transverse Secton. 
 A, Stellate reticulum. B, Papilla. C, Cartilage cellsi. 
 
 exerted by the active connective-tissue cells within its borders. When 
 thus considered, the evidence is almost sufficiently convincing to reverse 
 the generally accepted theory, placing the general form of the enamel 
 organ as subservient to the dentin papilla. 
 
 Figures 298, 299, and 300 illustrate some of the variations common to 
 the general form of the enamel organ, and afford a good idea of the rela- 
 tionship existing between the enamel organ and the dentin papilla, in teeth 
 both of the simple and complex class. These were taken from sections- 
 
DEVELOPMENT OF THE TEETH 
 
 399 
 
 which represent a period just prior to the generation of the ameloblasts 
 and odontoblasts, at which time the external and internal epithelial 
 layers of the enamel organ most closely resemble one another in general 
 outline. It is from this aspect and from sections cut longitudinally that 
 most of the information given by the older writers has been derived. 
 Few attemps have been made to show this organ in sections transverse to 
 the long axis of the tooth. In figure 301 the germs of two teeth are 
 shown by a section made in this direction. One striking feature here 
 
 a 
 
 Fig. 302. 
 A, Inner tunic of enamel organ. B, Cells of dentin papilla. 
 
 illustrated is the relationship existing between the inner and the outer 
 tunic of the enamel organ, and attention is called to the apparent coales- 
 cence of these two layers at those points which represent the mesial and 
 distal surfaces of the developing crowns. This condition is apparently 
 brought about by the cartilage cells forcing the peripheral cells of the 
 enamel organ into direct contact with the inner tunic completely obliterat- 
 ing the stellate reticulum in these localities. As a result of this lateral 
 pressure the outer epithelial cells representing the labial and lingual 
 
4-00 
 
 HISTOLOGY 
 
 surfaces have become widely separated, but with no perceptible alter- 
 ation in the character of the cells composing the stellate reticulum. The 
 relationship existing between the tooth-germs and the surrounding parts 
 is one that will continue throughout the generation of the organs, and 
 makes questionable the theory that the stellate reticulum performs 
 the function of reinforcing or providing the ameloblasts with nutrient or 
 calcific materal. If these same germs were examined, as they usually 
 are, in longitudinal section (see Figs. 298, 299, and 300), the investigator 
 
 Fig. 303. 
 A, Cells of Dentin papilla. B. Elongated cells of inner tunic. 
 
 would at once arrive at the conclusion that there was an equal distribu- 
 tion of the stellate cells about all sides of the dentin papilla. In their 
 very early life they apparently establish an equal bulk about all sides of 
 the dentin germ, but with the preparation for the growth of the alveolar 
 walls they may assume the proportions shown in figure 301. At a period 
 corresponding to the complete envelopment of the dental germs by the 
 dental follicle the development of the buccal and lingual walls is well 
 under way, but as yet no provision has been made for the septa between 
 
DEVELOPMENT OF THE TEETH 4OI 
 
 the teeth, and it is undoubtedly to the approach of this latter phenomenon 
 that a definite lateral pressure is brought to bear upon the approaching 
 of the follicles. 
 
 Now let us pass to a consideration of some of the characteristics of 
 the various cell layers composing the enamel organ. These are desig- 
 nated according to their location, so far as three of the four layers are 
 concerned; in fact, the remaining cells, or those which receive their name 
 from their form, can scarcely be classified as a distinct layer, these stel- 
 late cells not being of uniform thickness in all parts of the organ. The 
 first layer of cells, or those making up the inner tunic, will be traced from 
 their primary spherical condition to their final generation into ameloblasts. 
 Figure 302 shows the character of these cells at a very early period, corre- 
 sponding to the sixteenth week in the human embryo. They are for the 
 most part spherical or slightly oblong multinucleated cells, and are more 
 or less closely associated. They partake very much of the nature of the 
 connective-tissue cells surrounding them, being differentiated from these 
 principally by a transparent zone not unlike the specialized matrix im- 
 mediately surrounding cartilage cells. About the first change recorded 
 in these ceils (see Fig. 303) is one in which they become markedly elon- 
 gated or cylindrical, but during this process of differentiation some of the 
 cells apparently recede, while others advance in the direction of the 
 papilla, lining up in a single layer to become the early enamel cells, the 
 cells which have been thus forced to the rear subsequently developing 
 into ameloblasts as the older cells atrophy. At this period the stratum 
 intermedium also asserts itself in the form of a distinct layer of rounded 
 cells, to be described later on. When first observed, these cylindrical 
 cells are devoid of processes, but are provided with rounded extremities, 
 with little or no variation between the end directed toward the papilla 
 and that looking in the opposite direction. This form is one which 
 persists in all of the cells included in this layer until a definite body of 
 cells is formed contiguous to the dentin papilla, these latter cells becoming 
 more markedly elongated and further differentiated by the addition of 
 processes, while the remaining cells, or those nearest the stratum inter- 
 medium, continue for a time unchanged. The next alteration in the 
 character of the inner tunic is one well illustrated in figure 303, in which 
 the body of the generating ameloblastic cells rapidly recede from the surf ace 
 of the papilla, while the elongating processes reach out to this latter point, 
 all of this occurring before the appearance of the odontoblasts. Soon 
 after this latter change in the cells of this layer the odontoblasts are 
 developed and form the periphery of the dentinal tissue. All of these 
 26 
 
402 
 
 HISTOLOGY 
 
 changes are of course first recorded about the free extremity of the tooth- 
 crown, becoming less noticeable as the union of the outer and inner tunics 
 is approached. 
 
 A study into the special characteristics of the fully developed amelo- 
 blasts shows that these active cells are the result of a gradual change in 
 the character of the columnar epithelia common to both the external and 
 internal epithelial layers in the primitive enamel organ. 
 
 A, Stellate reticulum. 
 
 Fig. 304. 
 
 B, Stratum intermedium. C, Ameloblasts. 
 E, Calcined dentin. F, Odontoblasts. 
 
 D, Forming enamel. 
 
 Next in importance to the internal epithelial layer are those closely 
 associated cells making up the stratum intermedium (Fig. 305). Primarily 
 oval or spheroidal in form, we find these cells gradually assuming a 
 columnar outline and occupying a position parallel to the long axis of the 
 crown. It may be said that the general character of these cells is inter- 
 mediate between those destined to become the proper enamel cells and 
 
DEVELOPMENT OF THE TEETH 
 
 403 
 
 those stellate cells making up the bulk of the organ. There appears to 
 be much confusion, at least considerable doubt, in regard to the office of 
 the cells of the stratum intermedium. It is most likely that these cells are 
 not only intermediate in character, but are also intermediate in func- 
 tion to those cells upon either side of them, recruiting the ameloblasts as 
 they fall, while in turn they themselves are supplied with nutriment from 
 the enamel pulp or stellate reticulum. No stronger proof that these 
 cells are secondary in importance to the ameloblasts need be mentioned 
 
 A, Generating ameloblasts. 
 
 Fig. 305. 
 B, Rounded cells of stratum intermedium. 
 
 C, Stellate reticulum. 
 
 than reference to the fact that they are always more generously supplied 
 to those parts about to undergo calcification. Nor is their increase in 
 numbers the only reason for believing that they are thus employed, for 
 at the same time those cells most closely associated with the developing 
 ameloblasts take upon themselves a decided change in outline. This 
 alteration may be brought about by the conditions which influence the 
 shapes of all cells, i.e., by the pressure of surrounding cells or by their 
 preparation for functional activity, or both. Some of the older writers 
 
404 HISTOLOGY 
 
 speak of the cells of this layer as being branched, and in this way closely 
 resembling those of the stellate reticulum. By strong amplification 
 it is somewhat difficult to distinguish between the two layers, but most 
 certainly if there are branched cells present they are confined to the 
 intermediate zone, and should properly be classed with those of the 
 stellate reticulum. 
 
 The cells of this layer do not long remain columnar with a general 
 direction at right angles to the forming ameloblasts, but they become 
 spheroidal and extremely closely associated in the deeper portion of 
 the layer; in fact, cells corresponding to these in general appearance 
 may be found in connection with the fully developed ameloblasts, being 
 observed to best advantage by the aid of a high power objective and 
 a full flood of light from a powerful sub-stage condenser. The cells 
 thus found appear to be distributed at regular intervals about the amelo- 
 blastic layer, and are so closely allied to the cells of the stratum inter- 
 medium that they may be considered as migratory cells from this layer. 
 In the earlier stages there appears to be no definite line of demarcation 
 between the cells of the inner tunic, and those composing the stratum 
 intermedium, but soon after the establishment of the ameloblasts the 
 two layers are strongly differentiated by the interposition of a highly 
 transparent membrane covering the outer extremities of the amelo- 
 blasts. After they are thus definitely separated from the enamel-forming 
 cells, a most radical change takes place in their character; they become 
 markedly elongated, and by anastomosing form a series of continuous 
 chain-like belts about the ameloblastic layer, the number and further char- 
 acter of which are dependent upon the extent to which the ameloblasts 
 have performed their function. If at any time there is a similarity between 
 the cells of the stratum intermedium and the stellate reticulum it is at 
 this period, because the former cells begin to lose their individuality, 
 although under low power they still appear as a distinct layer (Fig. 303). 
 
 There is probably no body of cells directly interested in the develop- 
 ment of the tooth tissues so widely discussed as those making up the so- 
 called stellate reticulum, and while the chief basis for argument has been 
 with reference to their function, the general character and form of the 
 cells have received but little consideration. Ever since the first description 
 of this portion of the enamel organ, the cells therein have been charac- 
 terized as "star-shaped," and while this stellate form is the most common, 
 it is by no means a universal condition. The form of the cells in common 
 with the other cells composing the organ appears to be much influenced by 
 the position which they occupy, and by the age of the organ, those cells in 
 
DEVELOPMENT OF THE TEETH 405 
 
 the region of the inner tunic partaking of the globular form characteristic 
 of this layer (see C, Fig. 304), while those closely associated with the outer 
 tunic are inclined to be columnar or somewhat elongated. While the 
 cells in these respective locations are more or less influenced by their 
 environments, they still retain to a certain extent the stellate feature 
 by their many processes. But it is in the center of this myxomatous 
 epithelial product that the most perfect stellate cells are located. We 
 find, therefore, where this part of the organ is of the greatest width, 
 that the true stellate cells are the most numerous, while at the summit of 
 the crown and at the base of the organ, at both of which points the outer 
 and inner epithelial layers are closely associated, the star-shaped cells 
 are almost wanting. In the study of this layer very much depends upon 
 the thinness of the section, only the thinnest possible sections affording 
 an opportunity for a correct conception. This is, of course, true of all 
 parts of the organ, but the peculiar character of the stellate reticulum 
 makes it especially necessary that great care be bestowed upon the prep- 
 aration of the section. In transverse section the cells present no charac- 
 teristic differences from those shown when the section is made longitudi- 
 nally. One very pronounced feature about the cells of the stellate retic- 
 ulum is the granular appearance of their protoplasm, resembling very 
 closely the flattened squamous cells from the epithelium of the mouth, 
 and it is no doubt this special feature which furnishes the ground for the 
 opinion of many writers that it is a peculiarly modified epithelium. One 
 peculiarity in connection with this tissue which is contrary to the generally 
 accepted character of epithelial cells is the abundance of intercellular 
 cement substance; but when the many minute spines or processes are 
 considered as a part of the individual cell, the proportionate quantity 
 of cellular and intercellular substance is somewhat decreased. The 
 connecting processes are quite similar to those described by Stohr as 
 connecting bridges of protoplasm, while the cells themselves may be 
 otherwise described as prickle-cells. The change in the form of the cells 
 of this layer is not due to the presence of neighboring cells, as in the case 
 with most epithelial ceils, but, being soft and extremely plastic, it is more 
 than likely that their form is strongly influenced by the tension of their 
 connecting filaments. One of the most marked alterations in the general 
 character of this part of the enamel organ is that which takes place 
 at a time corresponding to the beginning of amelification, and is no 
 doubt attributable to this phenomena. The cells which up to this period 
 have remained widely separated now become more closely associated, 
 not so much by a change of position as by what appears to be an increase 
 
406 HISTOLOGY 
 
 in the size of the cell body with a corresponding decrease in the length 
 of the anastomosing processes. 
 
 It is a fact admitted by most histologists that the peculiar star-like 
 nature of the cells of the stellate reticulum is one principally brought 
 about by postmortem changes, and that in reality they are polygonal cells 
 filling up a greater part of the tissue with but little intercellular sub- 
 stance. That some shrinkage and distortion does take place may be 
 proved by the examination of a section which has accidentally or other- 
 wise become for a moment dry during its preparation, in which case 
 little can be seen but the connecting processes, and even these are much 
 shrunken. All the cells contained within the organ are more or less 
 affected by this procedure, but none of them exhibit such a marked 
 change in the outline as those of the stellate reticulum. 
 
 The layer of , cells which is usually considered of least importance 
 is that which makes up the outer tunic. In the young enamel organ 
 the cells partake very much of the nature of these forming the inner 
 tunic, but the older the organ becomes, the more dissimilar are the two 
 layers. Primarily this layer is constructed of a single row of elongated 
 cells, placed with remarkable regularity, upon the inner side of which are 
 a number of similarly formed cells variously disposed, but with a common 
 direction at right angles to those previously referred to. Like the inter- 
 nal epithelial layer, the cells of the outer tunic partake more or less of 
 the nature of the stellate cells in passing from the single row of well- 
 defined cells toward the stellate reticulum. While in the beginning 
 the external epithelial layer is strongly differentiated from the sur- 
 rounding cells, this is of but short duration. The atrophy of this layer 
 begins with the appearance of the fully developed ameloblasts, by which 
 the regular arrangement of the cells is greatly disturbed by an apparent 
 breaking up of the entire layer. Many reputable writers claim that the 
 external epithelial layer is of little or no interest, save, as Tomes puts 
 it, "as a matter of controversy." This admission upon the part of 
 so eminent an authority practically opens up a new field for research, 
 especially so when we consider that various other writers (Waldeyer, 
 Kolliger, and Magitot) have expressed conflicting opinions in regard 
 to it. After carefully following the changes which occur in this layer 
 fom its earliest inception up to an advanced stage of calcification, it 
 would appear that while marked changes occur in the character of the 
 individual cells as well as in the general appearance of the layer, it is 
 nevertheless persistive, and in some way is essential to the process of 
 amelification even to a more marked degree than are the cells of the 
 
DEVELOPMENT OF THE TEETH 407 
 
 stellate reticulum. One important reason for this belief is based upon 
 the fact that in the cuspidate teeth there appears at a time corresponding 
 to the beginning of amelification, a decided disposition in the cells of 
 this layer to dip down and completely divide the stellate reticulum be- 
 tween the forming cusps. That this alteration is one instrumental or 
 essential to the calcifying process receives additional proof by referring 
 to figure 298, which shows the fully developed enamel organ with the 
 
 Oral Cavity- 
 
 Tongue n l§13iSlk-- ' ' 
 
 Fig. 306. — Section through the Floor of the Mouth of Human Embryo. 
 Twelfth Week. X30. 
 
 exception of the actual appearance of the ameloblasts, and the lack 
 of any attempt upon the part of the external epithelial layer to penetrate 
 between the cusps. 
 
 Dentin Organ (Fig. 297). — This part of the tooth-germ, formed from 
 the connective tissue of the primitive jaw, occupies the concavity of the 
 enamel organ, and at an early period begins to assume the form of the 
 future tooth-crown. Thus, primarily, the papillae for the incisors will 
 have their cutting-edges outlined by three small lobes, each of which 
 represents a separate point of calcification, while the papillae for the molars 
 will be molded according to the number of cusps of the future tooth, a 
 small tubercle making its appearance for each cusp. In its inception the 
 dentin papilla is composed of cellular tissue identical with that of the sur- 
 
408 HISTOLOGY 
 
 rounding parts. The growth of the papilla is in the direction of the sur- 
 face; at the same time the enamel organ forces itself more deeply into the 
 substance of the parts, not only overhanging the coronal extremity of the 
 papilla, but extending about and inclosing its lateral walls. Accompany- 
 ing the growth of the papilla is a rapid change in its structure, becoming 
 more vascular throughout, and its peripheral cells, differentiating, form 
 the essential dentin-forming cells — the odontoblasts. This layer of cells in 
 in close relation to the enamel cells of the enamel organ, the combined 
 activity of the two finally resulting in the calcification of the tooth-crown. 
 The dentin papilla, which eventually becomes the tooth-pulp, decreases 
 in size as calcification proceeds in the dentin, all additions to the calci- 
 fying surface taking place from within; while the enamel organ may be 
 said to increase in size, the calcific action in the enamel progressing from 
 within outward. 
 
 The Cells of the Dentin Papilla. — In the early life of the dentin 
 germ, the cells are all simple embryonal connective-tissue cells. After 
 differentiation takes place they are widely scattered and are of four 
 varieties; spindle-shaped, round, stellate, and the elongated or club- 
 shaped odontoblasts. None of these are constant in location except the 
 layer of odontoblasts, which, as has been said, are arranged in a single 
 row on the surface or the periphery of the organ, this zone being classed 
 by the older writers as the membrana eboris. Like the ameloblasts of 
 the enamel organ, the odontoblasts do not make their appearance until 
 the papilla has assumed certain proportions, this about corresponding 
 to the size and form of the dentin of the future tooth. Immediately 
 beneath the layer of odontoblasts appears a zone almost devoid of cells. 
 This is followed by a district in which the cells are quite numerous, and 
 finally when the central portion of the papilla is reached the cells are again 
 few in number and widely scattered. For the most part the cells in the 
 interior of the papilla are spindle-shaped or stellate, having rounded 
 nuclei about which there is a small amount of protopalsm which pene- 
 trates the intercellular substance by numerous hair-like processes. 
 
 The Odontoblasts. — These are club-shaped or flask-shaped cells, 
 each provided with a large nucleus which usually assumes the outline of 
 the enlarged end of the cell which is directed toward the interior of the 
 papilla. From the opposite end of the cell, or that directed toward the 
 enamel organ, and in close proximity to its concave surface, one or more 
 protoplasmic processes are given off. These persist and are finally encap- 
 suled within the calcified dentin, forming the dentinal fibers. These 
 cells are very closely associated, so much so, in fact, that their enlarged 
 
DEVELOPMENT OF THE TEETH 409 
 
 extremities are almost or quite in actual contact, more or less space ex- 
 isting between the constricted portion of the cells as they pass toward 
 the surface. 
 
 Germs for Permanent Molars. — Reference has been made to the 
 fact that the enamel organs for the deciduous teeth are given off from the 
 tooth-band at a point somewhat distant its free margin, so that the 
 tooth-band is continued beyond the primitive enamel germ, this free 
 margin of the band afterward generating the enamel organ for the succeda- 
 neous tooth. As the twelve permanent molars are not succedaneous 
 teeth, some other means must be provided for their development. 
 
 Opinions of various writers upon this subject are somewhat conflict- 
 ing. The theory is advanced by some that as the jaw increases in length 
 the tooth-band and lamina primarily provided for the deciduous teeth 
 are extended backward, first giving off a bud for the first permanent 
 molar; at a somewhat later period, and with the increase in the growth 
 of the jaw, an additional bud is generated for the second molar, the third 
 molar being provided for in a like manner. Another theory, and one gen- 
 erally accepted as correct, is that the cords for the permanent molars spring 
 individually and directly from the subepithelium. There may be found 
 an exception to this in the case of the first permanent molar, which some- 
 times appears to have its origin from the distal follicular wall of- the second 
 deciduous molar. Whatever theory be accepted in regard to the genesis 
 of these permanent organs, the process of development after the appear- 
 ance of the primary bulb or enamel germ is identical with that of the de- 
 ciduous teeth. 
 
 The Dental Follicle, or Tooth-sac. — During the early life of the 
 tooth-germ, both the enamel organ and the dentin papilla are differen- 
 tiated from the surrounding parts by dissimilarity of structure only, but 
 as development proceeds, a more definite separation appears between the 
 tooth-generating organs ond the general tissues of the primitive jaw, 
 this separating medium being known as the dental follicle. The term 
 ''follicle" is only one of a number applied to these parts, "dental saccu- 
 lus," " tooth-sac," and other appellations being employed with equal 
 significance. By some writers it is customary to apply the term " follicle" 
 up to the period of complete closure, the term "sac" or "sacculus" 
 being employed after that time. There appears, however, to be little 
 foundation for such a distinction, the terms being synonymous. Again, 
 the follicle is frequent referred to as meaning the sac and its contents, 
 but this usage is a misapplication of the term. There appears to be no 
 well-founded reason why the follicular wall or sac should not be referred 
 
4io 
 
 HISTOLOGY 
 
 to as such, regardless of the formative organs within. As to the develop- 
 ment of the tooth-follicle, it appears to be a generally accepted theory 
 that at a very early period there is developed from the base of the papilla, 
 cells which, differentiating, form the walls of the follicle. By this growth 
 of cells the periphery of the papilla is first surrounded, and this step is 
 soon followed by an extension of the cellular structure in the direction 
 of the surface epithelium, to the deep layer of which the cells become 
 firmly attached, and in so doing inclose the enamel organ, which hangs 
 like a hood over the extremities of the papilla. The tissue thus formed 
 
 Wall of Follicle 
 
 Calcified Dentin 
 
 Dentin Papilla, 
 or Tooth. -pulp 
 
 Wail of Follicle 
 
 Developing Bone 
 
 FIG. 307. 
 
 -Developing Tooth about the Fourth Fetal Month. Appearance of the 
 Tooth-follicle. 
 
 from the base of the dentin germ is continuous with and similar in its origin 
 to the pulp-substance. The primitive tooth-germ, during the formation of 
 the follicular wall, is found swinging in a membranous pocket, being 
 supported by the epithelial band, which, in turn, is attached to the oral 
 epithelium; but as the walls increase and completely inclose the germs, 
 which is accomplished about the fourth fetal month, the epithelial band 
 is broken and the second or saccular stage of tooth-development is reached. 
 The walls of the follicles are made up of two layers; the outer layer is 
 dense and firm, and finally becomes the dental periosteum; the inner 
 layer is thin, frail, and in the recent state somewhat transparent, and at 
 an advanced period assists in the formation of the cementum, the two 
 layers finally evolving into the alviolo-dental membrana. 
 
DEVELOPMENT OF THE TEETH 
 
 411 
 
 SECONDARY OR SACCULAR STAGE OF TOOTH 
 DEVELOPMENT. 
 
 Having thus briefly described the primary stage of tooth-develop- 
 ment, the careful study of which can only be pursued with the aid of the 
 microscope, we will now pass to the secondary or saccular stage. By 
 the introduction of a number of illustrations, prepared from original 
 dissections by the author, this phase of the subject will be readily 
 comprehended. 
 
 Before continuing the subject of tooth-development, it will be emi- 
 nently proper to briefly describe those parts directly concerned in the 
 process. At a very early period of fetal life we find preparations are being 
 
 Calcified Dentin 
 Enamel 
 
 Outer Enamel 
 Epithelium 
 
 Epithelium of 
 Jaw 
 
 Dental Ridge 
 
 Germ for Perma- 
 nent Incisor 
 
 Dental Papilla 
 
 Fig. 308. — Development of Deciduous Incisor, from Human Fetus. — 
 {After Gysi.) 
 
 made for the development of the maxillary bones. That these are about 
 the first bones to be called into functional activity accounts in a measure 
 for their very early development. The osteoblastic activity in the inter- 
 cellular substance destined to become the inferior maxilla begins about 
 the middle of the second month of fetal life, while at a somewhat later 
 period a similar action takes place in the region of the superior maxilla. A 
 detailed description of the body of these bones having been given on another 
 page, it will not be repeated here, but to that portion which gives lodg- 
 ment to the tooth-germs, and which in a measure is controlled by their 
 presence, some attention must be given, and for this purpose the mandi- 
 ble will principally be used. 
 
412 
 
 HISTOLOGY 
 
 Figure 309 represents the lingual face of the lower jaw after removal 
 from a three months' fetus. Attached to it is the remaining portion of 
 Meckel's cartilage, which by this time is much wasted. It will be re- 
 called that this cartilaginous band appears in the mandibular processes 
 before the beginning of the second fetal month, being formed in two 
 distinct halves, the free ends of which finally unite at the median line, 
 forming a continuous support or framework, about which ossification 
 takes place. At a corresponding period, and in a similar manner, two 
 like processes are thrown out for the superior maxillae, but, unlike Meckel's 
 cartilage, these do not unite at the median line, but stop short of this, 
 the space thus resulting being provided for by two additional processes, 
 
 That Part of Cartilage not yet Ossified. 
 
 Head of Malleus 
 Cartilage of Incus 
 
 Handle of Malleus 
 
 Ossified Mandible 
 Fig. 309. — Developing Mandible, Three-month Fetus. 
 
 which shoot down from the region of the forehead and provide for the 
 development of the intermaxillary bones. About the middle of the 
 second month a center of ossification appears in the neighborhood of the 
 future mental foramen, quickly followed by others at the symphysis and 
 at the angle of the mandible. These secondary centers soon unite with 
 the primary one, and by the end of the second fetal month the osseous con- 
 tour of the primitive jaw is established. While ossification takes place 
 in the membrane surrounding Meckel's cartilage, the cartilage itself 
 does not appear to be directly concerned in the process, and by the sixth 
 or seventh embryonic month the mandibular portion completely disap- 
 pears, while that portion near the tympanum is ossified into the malleus. 
 That portion of the bone which forms above Meckel's cartilage and the 
 inferior dental nerve is that which finally gives support to the tooth-germs. 
 This cartilage is not confined to the human species, but is the common 
 heritage of reptiles, rodents, birds, and fishes, in all of which it gives 
 support to the developing lower jaw. 
 
 Figure 310 represents the evolution of the mandible from the middle 
 
DEVELOPMENT OF THE TEETH 
 
 413 
 
 > 
 
 Fig. 310. — Evolution of the Mandi- 
 ble from the Third Fetal Month to 
 Birth, Two-thirds Actual Size. 
 
 of the third fetal month to the time of birth. It will be observed that 
 during this interval there is a gradual increase in the size of the bone, . 
 but little alteration in its contour. By a constant and gradual osseous 
 deposit about the distal extremity of 
 the bone its length is increased to accom- 
 modate the additional teeth as they 
 make their appearance. While the 
 external form of the bone shows but 
 slight variation during this period, the 
 internal structure, or that wherein the 
 tooth-germs lie, is undergoing a com- 
 plete transformation. 
 
 Figure 311 is illustrative of these 
 changes; beginning with a simple 
 groove, or gutter, into which the tooth- 
 follicles hang, the follicles exerting a 
 controlling influence over its form. 
 Next comes the appearance of septa between the anterior follicles, which 
 at this period are somewhat irregularly placed in the arch, followed in a 
 few weeks by a well-defined partition between the cuspids and molars, 
 until finally, at birth, each follicle is inclosed in its individual crypt, with 
 
 the single exception of the 
 second molar, in which the 
 distal septum, or that which is 
 to separate it from the perma- 
 nent first molar, has not yet 
 made its appearance. As the 
 tooth-follicles increase in size, 
 by the development of the 
 teeth within, they become 
 more perfectly inclosed in the 
 bony vaults, the sides of the 
 alveolar walls arching over 
 and almost completely in- 
 closing the developing teeth. 
 Figure 312 shows the lower jaw of a seven-months-old child embody- 
 ing the condition above referred to. No sooner have the crypts grown 
 to this extent, than the resorptive action produced by, or provided for, 
 the advancing crowns speedily results in their downfall, to be again 
 built up with the evolution of the permanent teeth. 
 
 Fig. 311. — Evolution of the Mandible from Third 
 Fetal Month to Third Month after Birth. 
 
414 
 
 HISTOLOGY 
 
 About the first visible sign of preparation for the development of the 
 teeth, other than that made apparent by dissection, may be observed as 
 early as the beginning of the third fetal month, when, upon opening the 
 cavity of the mouth and looking upon the palate, a well-defined infold- 
 
 Mucous Membrane and Periosteum Lifted.. Up 
 
 FlG. 312. — Inferior Maxilla of Seven-months-old Child. 
 
 ing of the epithelial eminence will be seen. In figure 313 this condition 
 is shown by a dissection through the oral cavity of a four months' fetus, 
 the outer fold being that of the cheeks and lips, while within are the hard 
 palate and primitive alveolar ridge. The mouth at this period has 
 
 Labiodental Space 
 
 Labial Fold 
 
 
 
 
 
 
 i!k— - — —--"""" 
 
 
 j^^lfin *•*!*' 'fcS 
 
 Mm 
 
 • f. 
 
 Bi 
 
 
 . 
 
 Primitive Dental 
 Groove, so called 
 
 Fig. 313. — Section Through the Mouth of Four-month Fetus. 
 
 passed the rudimentary state, the transverse plates which contribute to 
 the formation of the hard palate having approached each other until the 
 oral and nasal cavities, heretofore existing as a single buccal cavity, 
 have become separate and distinct. The infolding of the oral epithelium, 
 
DEVELOPMENT OF THE TEETH 
 
 415 
 
 Maxilla 
 
 Oral Mucous Membrane 
 Superior Fetal Maxilla, 
 
 as outlined on the summit of the primitive alveolar ridge — the primitive 
 dental furrow or groove — marks the position of the tooth-band, from 
 which are given off the incipient tooth-bulbs. For the purpose of further 
 investigation, a dissection of these parts was made and the maxillary 
 bones removed, after which they were divested of their fibrous covering, 
 including the periosteum. That portion which overlies the palatal 
 processes was readily lifted in one sheet, while that upon the facial surface 
 was separated at the median line and 
 stripped independently of the other 
 (Fig. 314). The removal of these 
 tissues is readily accomplished until 
 the margins of the partly formed 
 alveoli are reached. Here the peri- 
 osteum dips down into the various 
 crypts, and serves as a lining mem- 
 brane for them, and probably con- 
 tributes fibers to the outer layer of 
 the follicular walls. After advancing 
 
 thus far. the detached tissues may 
 
 Fig. 314. 
 be grasped, and by Careful manipula- showing manner of Dissection to Expose 
 
 tion the tooth-follicles containing the Tooth - sacs - 
 
 formative organs removed from their respective vaults and turned over 
 
 for examination. 
 
 Figure 315 shows the result of such a dissection. On the left, the 
 palatal plates and alveolar walls of the divested bones may be observed; 
 on the right, is the fibrous covering, which has been turned completely 
 over after removal from the bones, having firmly attached to it the 
 ten tooth-follicles for the deciduous teeth. It this dissection be made 
 without the precaution of lifting the periosteum, the follicles would not 
 cling to the oral membrane with sufficient tenacity to permit of their 
 ready removal. It may be of some interest to note that at this early 
 period the position of the follicles containing the germs for the lateral 
 incisors is that which the tooth is forced to occupy up to and frequently 
 beyond the eruptive period, being crowded within the tooth-line by the 
 central and cuspid follicles, in consequence of which the lateral crypts 
 are thrown well into the palatal plates. 
 
 Figure 316 represents the sacs broken down, exposing to view the 
 dentin papillae, or those structures destined to become the tooth-pulps. 
 The position which these occupy in the illustration is exactly the reverse 
 from that which they assume when in position in the follicle, being thus 
 
4i6 
 
 HISTOLOGY 
 
 reversed that a better idea of their shape may be obtained. Prior to the 
 twelfth or thirteenth week of fetal life this incipient bulb or papilla is 
 without definite form; but by the latter period each papilla begins to 
 assume the contour of the future tooth-crown, as faintly outlined in the 
 illustration, those of the incisors presenting the angular form of the 
 
 Tooth-follicles 
 
 Fig. 315. 
 
 Maxilla Oral Mucous Membrane 
 
 -Dissection upon Superior Maxilla, Fourth Fetal Month, Exposing 
 Tooth-follicles. 
 
 future cutting-edge, the cuspids that of the single cone, while the coronal 
 extremities of the molars are represented by outlines corresponding to 
 the future cusps and marginal ridges. Besides the dentin papilla, there 
 is contained within the follicular walls the organ which later on is pro- 
 ductive of the enamel, but up to this time has been actively engaged in 
 
 Dentin Papillae 
 
 Maxilla 
 
 Oral Mucous Membrane, Turned Completely Over 
 
 Fig. 316. 
 
 molding the tooth-form as outlined by the papilla. To return to the tooth- 
 follicle, the dissection in this instance being upon the lower jaw of a 
 four months' fetus. Figure 317 shows the mandible removed and the 
 dissection carried to the point at which the follicles may be lifted from 
 their bony encasements, this being accomplished by an incision along 
 
DEVELOPMENT OF THE TEETH 
 
 417 
 
 the base of the bone, followed by a stripping of the membrane first from 
 the facial and then from the lingual side of the bone. When these two 
 flaps reach the margin of the crypts, they are firmly grasped and the 
 follicles removed from their sockets, as illustrated in figure 318. 
 
 At the beginning of the saccular stage of development the form of 
 
 Oral Mucous Membrane and Periosteum Dissected from Mandible 
 
 Attachment of Follicle to Oral Membrane (Enlarged One-third). 
 Fig. 317. — Manner of Dissection to Expose Tooth-sacs. 
 
 the future tooth-crown is well outlined by the dentin papilla, which in 
 figure 319 is brought into view by a dissection of the walls of the follicles 
 shown in figure 3 18 without breaking the attachment existing between the 
 two. As in the case of the follicle, much confusion of terms in regard to 
 this structure exists, the "dentin bulb," the "pulp," "dentin germ," and 
 
 Tooth-follicles 
 
 Oral Membrane 
 
 
 Follicle or Perma- 
 nent First Molar 
 
 Fig. 318. — Tooth-follicles Removed from Mandible, Fourth Fetal Month. 
 
 the " papilla" being used ad libitum, without apparent regard for the struc- 
 tural changes which are continuously affecting the organ. It will, there- 
 fore, be proper to simplify this conglomeration of terms by a classification 
 appropriate to the various stages of development. The term "dentin 
 papilla" will best describe this part of the tooth-germ up to the time of 
 
 2 7 
 
4i8 
 
 HISTOLOGY 
 
 beginning of calcification, subsequent to which the term "pulp" should 
 be employed. As previously stated, the enamel organ, until this period, 
 has been principally devoting its energies to the molding of the tooth- 
 form, and it is not until this model, as represented in the papilla, is com- 
 plete that the process of calcification begins. About the fourth fetal 
 month preparations for the calcification of the deciduous teeth are begun 
 by the development of the odontoblastic cells for the dentin, which first 
 made their appearance on the periphery of the dentin papilla, the summits 
 of the various cusps in the molars and the future cutting-edges of the 
 incisors being first affected. This phenomenon is soon followed by the 
 
 Space Occupied by Oral Mucous 
 Enamel Organ Membrane 
 
 Follicular Wall 
 
 Dentin Papillae 
 Prior to Beginning 
 of Calcification 
 
 Fig. 319. — Tooth-follicles shown in Figure 314 Opened. 
 
 appearance of the ameloblastic cells for the enamel, which establish 
 themselves in the internal epithelial layer of the enamel organ. 
 
 Figure 320 is prepared from a dissection made in a manner similar 
 to that shown in figure 316, but at a period about a month later, being from 
 the superior maxilla of a five months' fetus. The dissection shows 
 the extent of calcification at this period, which process also defines the 
 position of the odontoblastic cells upon the extremity of the papillae. In 
 the incisors (one of which was lost in the preparation of the specimen) 
 the dentin may be seen capping the cutting-edges. The cuspids in this 
 subject have not yet begun to calcify, although it is not unusual to find 
 the cusp of this tooth receiving its lime-salts at this early period. In the 
 molars the summits of the various cusps, as well as a portion of the various 
 ridges descending therefrom, are undergoing the change produced by 
 the impregnation of the lime-salts. It is quite probable that these delicate 
 caps are at this time composed of dentin alone, the calcoglobulin which 
 
DEVELOPMENT OF THE TEETH 
 
 419 
 
 precedes the enamel calcification forming somewhat later. From this 
 time forward the pulp undergoes a gradual transformation as to size and 
 form, and there is likewise a change in its cellular construction on those 
 parts adjacent to the calcific action. While the outline of the pulp is 
 
 Calcified Caps Tooth- pulp 
 
 Maxilla; 
 
 Oral Mucous 
 
 Membrane, 
 
 Turned Over 
 
 FlG. 320. 
 
 gradually changing, its original form is permanently recorded upon the 
 periphery of the dentin cap, which, when once formed, is immutable, all 
 additions taking place from within. 
 
 Figure 321 illustrates the result of a dissection upon the lower jaw 
 of the same subject, disclosing practically the same conditions, with the 
 
 Calcified Dentin Oral Mucous Membrane 
 
 Space Occupied by 
 Enamel Organ 
 
 Dentin Papillas 
 
 Fig. 321.— Tooth-follicles Opened, Exposing Dentin Papilla; and Beginning of Calcification 
 
 Fifth Fetal Month. 
 
 exception of the sac containing the developing cuspid, which was found 
 with a slightly calcified cap of dentin. This slight variation between the 
 development of the upper and lower teeth is one that is present in nearly 
 every instance, the latter being somewhat in advance of the former. 
 
420 
 
 HISTOLOGY 
 
 In figure 322 the tooth-pulps, with their primitive cappings of dentin, 
 have been removed from the follicles, and a better opportunity of study- 
 ing the relations between the two parts is presented. By the conversion 
 of the coronal extremities of the dentin papilla into odontoblasts, and 
 their active calcification, some positive union between the two parts 
 might be expected. On the contrary, the dentine caps are readily 
 
 Fie 322. 
 
 removed, leaving the pulp beneath without the slightest rupture, so that 
 we find calcification is not a secretory or excretory metamorphosis, but 
 that the change takes place within the substance of the papilla itself, 
 whereby it is altered from an organic to an inorganic substance. 
 
 The next dissection was one upon the mandible of a six months' 
 fetus. Figure 210 shows the tooth-follicles removed from the partially 
 formed bony crypts in which they have been incased. At an early 
 
 Tooth-sacs of 
 Deciduous Teeth 
 
 Lingual Surface of Mandible 
 FiG. 323. 
 
 period of fetal life, and at a time prior to the completion of the tooth- 
 follicles, there is deposited beneath the tooth-germs a thin layer of bone, 
 which at once begins to assume the form of the partially developed fol- 
 licular walls. As the growth of the follicle proceeds, there is a correspond- 
 ing increase in the osseous deposit, the alveolar walls extending about 
 and accommodating themselves to the membranous sacs. Thus we find 
 
DEVELOPMENT OF THE TEETH 
 
 421 
 
 in this portion of the maxillary bones a feature peculiar to itself— that 
 of a continuous transformation from its earliest inception to the adult 
 period, first developing about the temporary tooth-sacs and completely 
 incasing them, which is speedily followed by complete resorption of the 
 walls, again followed by a rebuilding during the evolution of the perma- 
 nent teeth, and again swept away with the loss of these organs. Figure 324 
 
 Follicular Wall Calcified Caps Oral Mucous Membrane 
 
 Dentin Papilla 
 First Perma- 
 nent Molar 
 
 Tooth-pulps 
 
 FiG. 324 
 
 illustrates the opposite side of the same jaw, with its outer or facial plate 
 removed, together with the intervening septa. The follicles are opened, 
 and the extent of calcification at this period (six months) made apparent. 
 The pulps and calcified caps are approximately as found when dissected, 
 save a slight settling of all the parts. The incisors have calcified to about 
 one-third their full coronal length; the unicusped contour of the cuspid 
 has been established, as shown by the deposit of the lime-salts upon its 
 
 m m M **u* 
 
 Fio. 325. 
 
 summit, and the upbuilding of the mesial and distal cutting-edges. The 
 first molar has about completed its occlusal surface, and, while the cusps 
 of the second molar are nearing completion, there is a lack of union in the 
 central and distal fossae. Immediately posterior to the second deciduous 
 molar, the sac containing the formative organs for the first permanent 
 molar is shown opened, exposing to view the dental papilla, which at 
 
422 
 
 HISTOLOGY 
 
 this early period has assumed the form of the future tooth-crown, and 
 calcification is about to begin. Figure 325 illustrates the extent of calcifi- 
 cation in the deciduous teeth at the sixth fetal month. As the growth of 
 the teeth proceeds, it will be observed that the angularity which originally 
 accompanied the calcifying caps of the molars is gradually disappearing, 
 as is also the tritubercular form of the incisors and cuspids, this change 
 being brought about by the deposition of enamel to the parts. 
 
 Figures 326 and 327 show the calcified caps removed from the pulps, 
 
 9' 9 M 
 
 m*> 
 
 Fig. 326. 
 
 and so arranged that both their external and internal anatomy may 
 be studied. Prior to this time there has been but little alteration in 
 the form of the dentin pulp, only a gradual decrease in its size being 
 noted; but now we find it being divested of many of its angles, particularly 
 those which orginally served as a basal form for the coronal extremities 
 of the future tooth. With the disappearance of these the concavity within 
 the cap is slowly assuming the form of the future pulp-chamber. 
 
 That a better understanding of the saccular stage of tooth-develop- 
 
 Fig. 327. 
 
 ment might be had, a transverse section was made through the molar folli- 
 cles, as shown in figure 328. In this the attachment of the follicular walls 
 to the deep epithelial layer is visible, while within the walls is the enamel 
 organ, the calcified dentin, and the tooth-pulp. As previously stated, 
 the enamel organ is seen suspended above and forming a hood-like invest- 
 ment to the calcifying structure. This organ not only overhangs the occlu- 
 sal surface of the tooth-crown, but completely envelops the sides of the 
 calcified cap and dentin pulp. Previous to the beginning of calcification 
 
DEVELOPMENT OF THE TEETH 
 
 423 
 
 the enamel organ is in close proximity to the dentin papilla, the original 
 form of the latter being represented by the calcified dentin. 
 
 We have now arrived at that period of fetal existence when it is possi- 
 ble to study the macroscopic development of the permanent first molar. 
 Figure 329 represents a section of the lower jaw of a six months' fetus, 
 
 Enamel Organ Oral Mucous Membrane or Gum 
 
 Follicular Wall 
 
 Tooth-pulp Floor of Tooth-follicle 
 Fig. 328. — Dissection Showing Pulp, Calcined Cap, and Enamel Organ. 
 
 Oral Mucous Membrane 
 
 and displays not only the sacs of the deciduous teeth, but also that of 
 the permanent first molar. In most respects the evolution of this tooth 
 is similar to that of the temporary organs, having its origin from the 
 deep epithelial layer, either directly or by continuation of the tooth- 
 band backward. Preparations for its growth are begun as early as 
 the third fetal month, at which time the enamel organ is given off, and 
 thereafter the developmental process is 
 identical with that of the deciduous teeth. 
 There is one structure, however, 
 intimately connected with the develop- 
 ment of the permanent teeth not found 
 in connection with the deciduous organs 
 — the gubernaculum, or leading cord. 
 Figure 330 represents another section of 
 a six months' fetal mandible, with the 
 dentin papilla for the permanent first lg ' 329 ' 
 
 molar turned out from the follicle after being rolled from its bony in- 
 casement. Attached to the apex of the tooth-sac (which has been 
 turned back), and leading from it to the epithelium of the jaw, is the 
 gubernaculum. This fibrous structure was at one time thought to be 
 directly concerned in the development of the tooth. Although this is 
 denied at present, little is said in regard to its function, but it undoubtedly 
 
424 
 
 HISTOLOGY 
 
 serves the purpose of directing the tooth to that position which it should 
 occupy in the jaw, and where the least resistance to its eruption is 
 formed by the foramen which the cord has established. Each of the 
 permanent teeth is provided with a similar membranous cord, an illus- 
 tration and more complete description of which will follow later on. 
 We have now arrived at a period when the subject under consideration 
 
 Oral Mucous Membrane Gubernaculum 
 
 Dentin Papilla 
 Fig. 330. 
 
 naturally becomes of deeper interest. I refer to that time when the being 
 changes from a complex dependent condition to one of self-providing inde- 
 pendence. Previous to the time of birth the teeth appear to be but little 
 disturbed by certain morbid conditions which might be present in the par- 
 ent and from their earliest inception up to this period their development 
 proceeds with but little interruption and with much regularity. Figure 33 1 
 
 Hi 
 
 it* 
 
 Fig. 331. — Deciduous Teeth at Birth. (Reflected picture.) 
 
 illustrates the condition of the deciduous teeth at birth; the central incisors 
 are calcified externally to the cervical line, the lateral incisors to a point cor- 
 responding to the summit of the palatocervical ridge; the cuspids have ad- 
 vanced somewhat beyond the angles of the crown, while the molars have 
 their crowns calcified to about one-half their completed length. With all 
 this progress as represented by the external contour of the tooth-crowns, 
 
DEVELOPMENT OF THE TEETH 
 
 425 
 
 the internal form appears to be somewhat slow in assuming the shape of 
 the future pulp-chamber. From the beginning of the saccular stage of 
 development up to the time of birth there is but little increase in the 
 diameter of the tooth-sac, but there occurs a gradual increase in its length. 
 Figure 332 shows the mandible from a child one week old, with the 
 greater part of the external or facial surface of the bone removed, exposing 
 
 Oral Mucous Membrane 
 
 Fig. 332. 
 
 not only the sacs containing the developing deciduous teeth, but also 
 that of the permanent first molar. The relation of the sacs to the inferior 
 dental canal is apparent, as well as the firm attachment of the follicular 
 walls to the oral membrane. In figure 333 the tooth-sacs have been dis- 
 sected and the pulps removed from the calcified caps, presenting an 
 additional illustration of the amount of dentin deposit at this age. To 
 
 Calcified Caps 
 
 Dentin Papilla of 
 Permanent First 
 Molar 
 
 Pulps 
 
 Fig. 333. 
 
 further illustrate the size and form of the pulp as compared with the 
 calcified cap at birth, a transverse incision was made through the left supe- 
 rior maxilla, at a point corresponding to the base of the pulp, as shown 
 in figure 334. The calcified parts remain in position resting against the 
 remaining portion of the enamel organ, while the pulps are dislodged and 
 may be observed resting upon the incised surface. In this illustration 
 
426 
 
 HISTOLOGY 
 
 the dentin papilla for the first permanent molar is also seen, being sup- 
 ported by the walls of the follicle, which in turn are attached to the oral 
 
 Calcified?Caps, Resting against 
 Enamel Organ 
 
 Tooth-pulps'.Rolled Out 
 
 Section of Superior Maxilla 
 
 Pulp of Permanent First Molar 
 
 Fig. 334. 
 
 epithelium by the gubernaculum. It may also be noted that those parts 
 of the pulp corresponding to the cusps and marginal ridges show a decided 
 convergence of the surface toward the center. 
 
 
 J£ 
 
 )entin Papilla for 
 Permanent First 
 Molar 
 
 Papilla 
 Calcined Cap 
 
 St 
 
 —^r 
 
 Walls of Tooth- 
 sacs 
 
 3* 
 
 Maxilla 
 
 Oral Mucous 
 Membrane 
 
 Fig. 335. 
 
 Reference has been made to the formation of the follicular walls by a 
 differentiation of cells, which at an early period are given off from the 
 
DEVELOPMENT OF THE PERMANENT TEETH 
 
 427 
 
 base of the papillae and to the continuity of the two structures. Figure 
 335 was prepared for the purpose of showing these intimate relations. 
 The sides of the sacs were opened and turned back, the calcified tooth- 
 caps with pulps in position were grasped and given several revolutions, 
 thus twisting the remaining portion of the walls, the floor of which is 
 seen as a continuous structure given off from the pulp and connecting it 
 with the epithelium of the jaws. 
 • 
 PREPARATIONS FOR THE DEVELOPMENT OF THE 
 PERMANENT TEETH. 
 
 A little before the time of birth sufficient advance has been made in 
 the development of the permanent teeth to permit a study of their rela- 
 tions with the temporary organs. The early preparations for the growth 
 of these teeth was for a long time a subject of much controversy, some 
 writers advancing the theory that the buds for the permanent teeth were 
 produced or given off from the sacs of the temporary teeth; others contend- 
 
 Tooth-sac of Advancing 
 Deciduous Tooth 
 
 Gum 
 
 Gubemaculum Tooth-sac of Receding Permanent Tooth 
 Fig. 336. — Section through Superior Maxillae, Sixth Month after Birth. 
 
 ing that the cords were derived from the remnants of the primitive cords 
 immediately after their rupture. The theory now generally acceptee 
 is that the cord is given off from the primitive cord at a point in clods 
 proximity to its attachment to the deciduous enamel organ. This can 
 only apply to those teeth which are succedaneous, and, therefore, does 
 not include the permanent molars, as heretofore stated. Whatever 
 theory be accepted in regard to the genesis of the permanent teeth, there 
 can be no mistake in regard to that part of the process which we are 
 permitted to ocularly investigate. I shall, therefore, proceed to describe 
 
428 
 
 HISTOLOGY 
 
 the position and contents of the permanent tooth-sacs at birth. Figure 
 336 presents the result of a vertical dissection through the superior 
 maxillary bones, the inner surface of the right maxilla being exposed 
 to view. Many of the frail processes, particularly those entering into 
 the construction of the nasal cavities, were lost during the preparation 
 of the specimen, the whole purpose of the dissection being to' show the 
 sac of the permanent incisor and its relationship to its predecessor. 
 By this time calcification in the deciduous incisor has so far advanced 
 that the contour of the tooth-crown may be plainly outlined through 
 the walls of the sac. Resting against the lingual concavity of the crown 
 
 Foramina for Gubernacula 
 
 Fig. 337. 
 
 of the temporary incisor is the sac containing the formative organs 
 for its permanent successor. This permanent tooth-sac does not long 
 remain in such close proximity to the deciduous tooth-crown, for as 
 the latter advances toward the surface of the gum the former recedes 
 and is soon inclosed in a separate crypt, which, were it not for the guber- 
 nacular foramen, would completely inclose it. Figure 337 represents 
 a section of the left superior maxilla, introduced at this point for the 
 purpose of showing the position of the foramina for the gubernacula. 
 These may be observed immediately posterior to the incisor and cuspid 
 teeth. At birth these foramina do not exist as such, the partially formed 
 ed vaults containing the sacs for the permanent teeth appearing as an 
 extension of the temporary crypts in a palatal direction; but as the 
 
DEVELOPMENT OF THE PERMANENT TEETH 
 
 429 
 
 temporary teeth advance and the permanent teeth recede, the roof of the 
 crypt is completed, and the foramen established by the presence of the 
 gubernaculum. This extension of the temporary crypts is more clearly 
 demonstrated in figure 338, which represents one side of the lower jaw at 
 birth with the partially calcified deciduous teeth in position in the bone. 
 
 Tooth-sacs for Permanent Incisors 
 
 Gum 
 
 Tooth-sac for Perma- 
 nent First Molar 
 
 Deciduous Teeth in Bony Crypts 
 
 Fig. 338. 
 
 Suspended above this is the gum, which has been dissected from the 
 bone, having attached to its under surface the sacs for the permanent 
 teeth. Those for the incisors are particularly well defined and their 
 place of lodgment in the bone readily noted. In the case of the cuspid, 
 both the deciduous tooth and the permanent tooth-sacs are in position 
 in the crypt. The cords which support the incisors are somewhat length- 
 
 Oral Mucous Membrane 
 
 Follicular Wall, Turned 
 Wrong Side Out 
 
 Papilla for Permanent First 
 Molar 
 
 Dentin Papillae for Permanent Teeth, Upside Down 
 
 Fig. 339. 
 
 ened from the weight of the sacs, the gubernacula not assuming this 
 thread-like form until the permanent sacs have further receded. 
 
 Figure 339 shows the result of a dissection upon these permanent 
 tooth follicles. The dentin papillae of the various teeth are seen in a 
 reversed position, with the follicular walls attached to their bases. At 
 this period the papillae for the permanent incisors may be compared to the 
 
43° 
 
 HISTOLOGY 
 
 tail of a fish, being perfectly transparent over its free extremity, which 
 feature is gradually lost as its thickened base is approached. The fish- 
 tail appearance is further represented by the division of the free extremity 
 into three distnct parts, each of which provides a separate point of calcifi- 
 
 Tooth-sacs of Permanent Teeth 
 
 Tooth-sacs of 
 Deciduous Teeth 
 
 Lingual Surface of Mandible 
 
 Fig. 340. 
 
 Tooth-sacs of Deciduous Teeth 
 
 Tooth-sac of Permanent Tooth 
 
 Periosteum and Mucous Mem- 
 brane from Hard Palate 
 
 Gubernaculum 
 
 Tooth-sac for Permanent 
 First Molar 
 
 Fig. 341 -Same as Figure 340, Except on Upper Jaw. 
 
 cation. This cuspid papilla is missing, and the bicuspids have advanced 
 little beyond the form of the primitive bulb. The first molar is shown 
 with the full diameter of the crown represented by the pulpal mass, and 
 the tips of the cusps are already beginning to take on the calcific 
 action. 
 
DEVELOPMENT OF THE PERMANENT TEETH 
 
 43 1 
 
 A further illustration of the progress of the development of the per- 
 manent teeth and their relation to the deciduous organs may be seen 
 in figure 340, the dissection in this instance being upon the lower jaw of 
 a one-month-old child. The membrane has been lifted from the bone 
 with the tooth-sacs attached to it. Immediately posterior to the sacs 
 containing the crowns of the temporary incisors and cuspids are those 
 for their corresponding successors, the papillae of which have already 
 assumed the tubercular outline of the future cutting-edge. While the 
 permanent tooth-sacs are distinctively independent pouches, there ap- 
 pears, nevertheless, to be a well-established fibrous connection existing 
 
 Tooth-sacs of 
 Permanent Teeth 
 
 Tooth-sacs of 
 Deciduous Teeth 
 
 Periosteum of Hard Palate 
 Fig. 342. — Tooth-follicles for Deciduous and Permanent Teeth, Three Months 
 
 after Birth. 
 
 between the outer layer of the two follicular walls. This fibrous union 
 is gradually broken as the permanent sacs recede and become incased 
 in their own vaults. 
 
 While the permanent tooth-sacs are generally referred to as "reced- 
 ing," it is a question if this term is fully justified. While the follicles 
 do not remain in close relation with their predecessors, the change in the 
 relative position of the two is principally brought about by the advance 
 in the deciduous sacs, this forward movement being accompanied by a 
 marked growth of the bone in the direction of the future alveolar ridge, 
 thus leaving the permanent tooth-sacs well buried in the substance of 
 the jaw. 
 
 Figure 342 represents the result of a dissection upon the superior 
 maxillae of a three-months-old child. The mucous membrane coveting 
 the hard palate, together with the periosteum, has been dissected from 
 the bones and turned over for examination. The tooth-follicles for all 
 
432 . HISTOLOGY 
 
 the deciduous teeth, as well as those of the succedaneous permanent 
 organs, may be observed firmly attached to the fibrous tissue. The 
 permanent incisor sacs at this age are almost equal in size to those of 
 their predecessors, and the dentin papillae within possess a mesio-distal 
 diameter almost equal to those of the calcified temporary caps. The 
 sacs containing the germs for the permanent cuspids and bicuspids are 
 somewhat diminutive, but the enamel organs within are already molding 
 the contour of the future tooth-crowns upon the dentin papillae. 
 
 By the beginning of the second month after birth calcification in the 
 crowns of all the deciduous teeth is about complete, and preparation for 
 growth of the roots is under way. While at this period the tooth-crowns 
 may be said to be almost completely calcified, this does not apply to the 
 
 FiG. 343. 
 
 interior of the crowns, the deposit of dentin internally being a continuous 
 process, resulting in a gradual reduction in the capacity of the pulp- 
 cavity. It is also quite probable that the enamel organ is somewhat active 
 up to the eruptive period, and, if this be true, the enamel covering of the 
 crown is not complete until this time. Whatever be the condition in the 
 crowns, the time for the formation of the roots has arrived, and it is princi- 
 pally through the activity of the tooth-pulp that they are generated. 
 We have seen that the contour of the tooth-crown was first molded upon 
 the dentin papilla; so it is with the tooth-root: by a gradual elongation of 
 the sac, accommodations are afforded the tooth-pulp for a corresponding 
 growth. As the pulp lengthens out toward the future apex of the root, 
 it is molded to the root-form, and calcification takes place by the genera- 
 tion of odontoblastic cells upon the periphery of this organic root-form. 
 While the process of root-formation in the single-rooted tooth may be 
 readily comprehended, the bifurcation or trifurcation of the molar roots 
 presents a complication which calls for special reference. Figure 343 
 will assist in explaining this phenomenon. In the illustration three 
 deciduous molar crowns are shown, two of which are incased in their 
 
DEVELOPMENT OF THE PERMANENT TEETH 433 
 
 tooth-sacs, the third being stripped of its membrane. The view is directly 
 upon the base of the tooth-sacs, immediately beneath which is the base 
 of the pulp. Up to this period the odontoblastic cells have been gener- 
 ating about the occlusal surface and lateral walls of the crown only, 
 but now an accumulation of these cells is to be found upon the base of the 
 pulp, lining up in the position of the future root-walls. This structural 
 change is faintly outlined in the illustration. By this inward extension 
 of the odontoblastic cells from various points about the margins of the 
 pulp, and their union near the center of the mass, provision is made for 
 the calcification of the various roots, which process is considered separately 
 by an extension and molding of the pulp into two or more divisions. 
 
 Calcification of the Cementum. — While the dentin of the root is 
 derived from the tooth-pulp, the external covering of the root (the cemen- 
 tum) is generated from another source. In every respect cementum is 
 closely allied to bone, and we find its development provided for in a 
 similar manner. As stated in another part of th'X chapter, the tooth- 
 sac is made up of an outer and an inner layer, both of which are rich in 
 blood-vessels. These membranous walls continue to invest the roots 
 of the teeth during their upbuilding. The outer layer of the sac remains as 
 a permanent structure placed between the root and the alveolar walls, 
 forming the alveolodental membrane, while upon the surf ace of the inner 
 layer osteoblasts (cementoblasts) are developed, which are speedily 
 converted into bone or cementum. In this process the tooth-root may 
 be compared to one of the long bones of the body, and the development 
 of the cementum considered under the head of Subperiosteal Ossification. 
 The only variation to be observed between this and subperiosteal develop- 
 ment of bone is in the presence of a single Harversian canal (as the pulp- 
 cavity may be considered), and even this difference is sometimes over- 
 thrown by small canals running at right angles to the pulp. These small 
 canals are generally found near the apex of the root, at which point the 
 cementum is the thickest. Like the enamel cap of the tooth-crown, the 
 cementum is deposited upon the surface of the dentin of the root, thus 
 increasing its diameter. The calcification of cementum is more fully 
 considered later on. 
 
 Eruption of the Teeth. — Up to this time no reference has been 
 made to that process by which the teeth burst forth from their bony incase- 
 ments, and, penetrating the mucous membrane, made their appearance 
 in the mouth. Attention has been called to the growth of the bone 
 about the tooth-follicles — first forming beneath them as an open gutter, 
 next surrounding their lateral walls and inclosing each follicle in a sepa- 
 28 
 
434 HISTOLOGY 
 
 rate compartment, and finally each tooth becoming more completely envel- 
 oped by an arching-over of the mouth of the bony vault. This condition 
 in the maxillary bones is reached between the seventh and eighth month 
 after birth, and, almost simultaneously with the completed incasement 
 of the teeth by the bone, active absorption begins, that portion of the 
 bone which was last in forming being gradually removed. The cause 
 of the absorption of the bone may readily be attributed to the advance- 
 ment of the tooth, but the forces which are responsible for this latter 
 phenomenon do not appear to be clearly understood. In a general way, 
 the advancement of the crown may be said to result from the elonga- 
 tion of the root by the addition of dentin to its free extremity. But, 
 when it is taken into consideration that the cuspid teeth, both decid- 
 uous and permanent, have their roots fully or nearly calcified before 
 they begin to advance toward the surface, an exception to the generally 
 accepted theory is established. The eruptive process takes place first 
 in the anterior teeth,* and the bone overlying the labial surface is first 
 removed. 
 
 This loss of the bony structure is continued until fully one-half 
 of the labial surface is uncovered, and, as the crowns continue to advance 
 toward the surface, they assume a more prominent position in the arch, 
 and thus their cutting-edges become bared. The palatal or lingual face 
 of the crypt serves a double purpose, forming not only a covering to the 
 deciduous tooth, but also serving the permanent tooth-sac in the same 
 capacity. This part of the crypt remains unabsorbed, the tooth-crown 
 glides by its margin, and, after penetrating the mucous membrane, makes 
 its appearance in the mouth. Closely following the absorptive process 
 comes a rebuilding of the parts, until, finally, when the tooth is fully 
 erupted, it is firmly supported by the new bone filling in about the base of 
 the root. Accompanying the eruption of the anterior teeth and their 
 establishment in the arch is an increase in the depth of this portion of the 
 jaw, and, as the molars advance and assume their position, there is 
 a corresponding increase in the depth of the jaw in this locality. At the 
 beginning of the eruptive period the roots of the deciduous teeth are but 
 partially calcified, but as the crowns advance the calcific action at the 
 extremity of the roots is continued, and, in the majority of instances, by 
 the time the crowns are fully erupted the roots are completely formed. 
 During the period of eruption the transitory nature of the alveolar por- 
 tion of the jaw-bone is made manifest, accommodating itself to the 
 growth of the teeth as well as to their change of position. The free 
 
 *See Description of the Teeth in Detail. 
 
DEVELOPMENT OF THE PERMANENT TEETH 
 
 435 
 
 margins of the alveolar walls are taking on new structure, which ad- 
 vances with, and becomes adapted to, the base of the tooth-root. Coin- 
 cident with this the deeper portion of the alveolar process is formed 
 by a rapid filling-in about the root as the tooth travels onward to assume 
 its final position in the jaw. The eruption of the teeth is usually by 
 pairs, with a slight intermission between each class. The central in- 
 cisors first make their appearance, followed by the laterals, after which 
 the first molars are erupted. The cuspids usually follow the first mo'ars, 
 and, finally, the second molars take their place in the arch. While 
 this brief description of the eruption of the teeth refers to the deciduous 
 organs only, the process in the permanent teeth is almost identical 
 with this. Further reference to the eruption of the permanent teeth 
 will be made in connection with the degeneracy of the temporary set. 
 
 Fig. 344. — Hard Palate from a Nine-months-old Child, Actual Size. 
 
 To return to the subject of tooth-development, attention is called to 
 figure 344, prepared from a dissection upon a nine-months-old child, 
 the illustration representing the hard palate, or roof of the mouth, at this 
 period. The four incisor teeth have made their appearance, the labial 
 surfaces of the crowns being fully exposed, while those facing the palate 
 are but slightly uncovered. The approach of the remaining deciduous 
 teeth is plainly indicated by the fullness of the alveolar borders, and the 
 margins of the crypts are now being removed by absorption. 
 
 If the mucous membrane should be removed, the crowns of the 
 advancing teeth would be brought to view after the removal of the walls 
 of the tooth-sacs, while the approaching cuspids and second molars yet 
 remain partly covered by an arching over of the walls of the crypts; 
 
436 
 
 HISTOLOGY 
 
 the resorptive process has also begun in these parts. It would also be 
 observed that, while the walls of the crypts are molded to the outlines 
 of the tooth-crowns, there exists a well-defined interspace between the 
 two. During the growth of the tooth this interspace is filled by the walls 
 of the tooth-sacs, and even after the teeth have passed the saccular stage 
 of development, and assumed their positions in the mouth, there yet 
 remains between the roots and the alveolar walls a slight space which 
 is occupied by the alveolodental membrane. 
 
 The next dissection is one upon the superior maxillae of a two-year- 
 old child (Fig. 345), representing the roof of the mouth of this subject. 
 
 Fig. 345. — Hard Palate from a Two-year-old Child, Actual Size. 
 
 In this specimen it will be noticed that all of the deciduous teeth are 
 erupted with the exception of the second molars. The lingual surfaces 
 of the incisors are fully uncovered, while in the cuspids the labial surfaces 
 are much more exposed than the lingual. In figure 346 the mucous mem- 
 brane and sufficient of the bone have been removed to expose the tooth- 
 sacs of the developing permanent teeth. The dissection furnishes no 
 additional information over that obtained from figure 345, excepting 
 that the primitive follicles for the permanent second molars make their 
 appearance at this time. 
 
 At this early period the jaw has not lengthened sufficiently to permit 
 of these follicles occupying their future position; consequently they are 
 found generating immediately over the tooth-sacs of the first permanent 
 molars. As the first molars advance and the jaw lengthens backward, 
 these follicles will be carried to the distal by the extension of the mucous 
 
DEVELOPMENT OF THE PERMANENT TEETH 
 
 437 
 
 membrane, to which they are firmly adherent. If these follicles were to 
 be dissected at this time, the papillae would be without definite form, 
 showing that the early function of the enamel organ has not yet begun. 
 The tooth-sacs containing the permanent lateral incisors are found 
 immediately beneath the palatal plates, and frequently during their 
 earlier life they are not even protected by the bone, being in immediate 
 contact with the mucous membrane. On account of the imperfect 
 protection frequently afforded these sacs, the germs are sometimes 
 injured and the teeth fail to make their appearance. 
 
 Sac for Perma- 
 , N nent Second 
 1 -Molar 
 
 Fig. 346.— Roof of the Mouth of a Two-year-old Child. 
 
 In figure 347 the walls of the sacs shown in figure 346 have been opened, 
 and the relations existing between the first and second dentition at the end 
 of the second year become apparent. The crowns of the permanent in- 
 cisors are deeply set in the substance of the jaw, while the partially calci- 
 fied crowns of the permanent laterals are in close proximity to the palatal 
 surface. The partially formed crowns of the permanent cuspids are still 
 more deeply seated >n the substance of the jaw than those of the central 
 incisors, and are not visible in the illustration. In this connection it will 
 be well to again refer to the gubernaculum, and to its function — that of 
 directing the tooth to its proper position in the arch. By reference to 
 the illustration the crowns of the permanent teeth will be observed head- 
 ing in various directions, and, while in this instance there appears to be a 
 general tendency for them to advance and assume their proper positions 
 in the arch, in many cases they will be found directed at right angles to 
 the point at which they should emerge from the bone. The gubernacu- 
 
438 
 
 HISTOLOGY 
 
 lum, which appears to be nothing other than an elongation of the follicular 
 walls, not only directs the tooth by the tension of its fibers, but the fora- 
 men which its presence creates stimulates absorption over the tract 
 
 Fig. 347. — Development of the Teeth about the Second Year. 
 
 to be traveled by the tooth. Figure 348 was prepared for the purpose 
 of better showing the gubernacula, and the manner of connecting the 
 tooth-sacs, with the oral mucous membrane. This condition is well 
 
 Guber- 
 naculum 
 
 Tooth-sac for First Bicuspid 
 Fig. 348-Dissection of Lingual Face of Lower Jaw, Child Nine Months Old. 
 
 shown in the anterior teeth, the tooth-crowns having receded well toward 
 the body of the jaw. The follicle for the first bicuspid may be observed 
 attached to the lingual face of the deciduous molar sac, and the lead- 
 
DEVELOPMENT OF THE PERMANENT TEETH 
 
 439 
 
 ing cord is not yet an adjunct to the developmental process, but this 
 structure will make its appearance as the follicle recedes from the surface. 
 In figure 349 the deciduous molars have been removed from the jaw 
 and the relations existing between 
 these teeth and the developing 
 bicuspids is shown. The tooth- 
 follicles for the succedaneous teeth 
 are found immediately beneath the 
 gingival margin, and apparently 
 attached to the deep layer of the 
 mucous membrane. This relation- 
 ship between the permanent and FlG 349 ._ De ciduous Molars with 
 
 temporary Organs is present about Tooth-sacs for Bicuspids Attached to the 
 . . • 1 i 1 Gingival Tissue. 
 
 the eruptive period, but as the 
 
 deciduous tooth advances and the permanent tooth-sac recedes, the 
 two organs become more widely separated, and the permanent follicle 
 is connected to the surface only by the elongated follicular fibers which 
 form the gubernaculum. 
 
 Decalcification of the Deciduous Teeth.— By the close of the 
 second year the twenty deciduous teeth have taken their place in the 
 
 Deciduous Molars 
 
 Deciduous Incisors 
 
 Permanent 
 Incisors 
 
 Permanent 
 Cuspid 
 
 Fig. 350. — Same as Figure 235, with Tooth-sacs Opened Showing Developing 
 Teeth in the Jaw. 
 
 dental arch, their roots have become fully calcified, and the apical fora- 
 mina established; it is only for a short period, however, that they remain 
 thus perfect, the process of decalcification beginning about the fourth 
 year. This process begins at the apical extremities of the roots and 
 
44© 
 
 HISTOLOGY 
 
 gradually progresses in the direction of the crowns. Commencing 
 about the fourth year with the central incisor, decalcification takes place 
 in the teeth in the order of their eruption, the lateral incisor following the 
 central, the first molar following the lateral, etc. By reference to figure 
 351 an approximate idea of the progress of decalcification may be obtained, 
 and it will be observed that about three years elapse from the beginning 
 of this rather obscure process to its completion, and the final casting-off 
 
 Fig. 351. — Development of the Teeth about the Sixth Year. 
 
 or shedding of the tooth-crowns. In reference to the causation of this 
 dissolution of the deciduous teeth, but little appears to be known. It 
 has been said to result from the presence and pressure of the advancing 
 permanent teeth, but there is no question but that it occurs absolutely 
 independent of these organs, decalcification frequently taking place when 
 from some obscure reason, one or more of the permanent teeth are 
 absent. During the entire period of root decalcification, the pulp of the 
 
DEVELOPMENT OF THE PERMANENT TEETH 
 
 441 
 
 tooth, which is also involved in the destruction, retains its vitality, but 
 with the loss of vitality in the pulp absorption of the root ceases; so that 
 the gradual removal of the root-substance must be considered as a purely 
 physiologic action. 
 
 Figure 351 shows a dissection upon the jaws of a six-year-old child, 
 by a careful study of which, a fair knowledge of the extent of absorption 
 in the deciduous teeth at this period may be obtained. 
 
 Advance of the Permanent Teeth. — By referring to figure 351, the 
 relations existing between the deciduous and the permanent teeth at 
 
 Fig. 352. — The Completed Dentition. 
 
 about the sixth year may be noted. While the crowns of the deciduous 
 teeth remain in position, a part of the space formerly occupied by their 
 roots is taken up by the advancing crowns of the permanent teeth, the 
 latter being calcined but little beyond their cervical lines. Between the 
 seventh and eighth years the crowns of the deciduous incisors are cast 
 off, and gradually the crowns of the permanent incisors force their way 
 through the gum, the arch by this time having sufficiently increased in 
 size to accommodate the additional width possessed by them. Previous 
 to this time, or about the sixth year, by a backward extension of the jaws, 
 the permanent first molars have erupted, assuming a position in the arch 
 immediately posterior to the deciduous second molars. Between the 
 
442 HISTOLOGY 
 
 tenth and eleventh years the crowns of the deciduous molars are lost, and 
 the bicuspids advance to take their places. Uusually by the twelfth year 
 there has been sufficient increase in the length of the jaws to permit of 
 an additional tooth, and the permanent second molar gradually takes its 
 position immediately posterior to the first. Between the twelfth and 
 thirteenth year the deciduous cuspids are lost by decalcification of 
 their roots, and they are succeeded by the permanent cuspids. We 
 therefore find, by the fifteenth year, fourteen fully developed teeth occupy- 
 ing the dental arch of each jaw, the full number, thirty two, or sixteen 
 in each jaw, not being present until the eruption of the third molar, 
 which, like the other teeth of this class, is compelled to await accommoda- 
 tions by a further increase in the length of the maxillary bones. This 
 tooth usually takes its place between the eighteenth and twenty-first 
 years, and thus completes the dentition (Fig. 239). 
 
 FURTHER CONSIDERATION OF TOOTH 
 DEVELOPMENT. 
 
 One phase of the subject to which special attention will be given in 
 this chapter is that which denotes the period at which the various events 
 take place. This is a part of the study which is very difficult to deter- 
 mine, and it would appear, for this reason, if for no other, that investiga- 
 tors of recent years have been perfectly satisfied to accept the results 
 arrived at by their predecessors without any apparent effort to qualify the 
 deductions. It has for a long time been conceded that the primitive 
 changes which ultimately result in the formation of a tooth-germ are 
 first noted in a heaping up of the epithelial cells over the district repre- 
 senting the surface of the future jaw. While in many instances this is 
 true, there are reasons why it cannot be considered an essential feature. 
 In the first place, such a condition is not always present, as shown in fig- 
 ure 353, a transverse section through the primitive jaw of a human fetus 
 about the fortieth day. The tooth bands at A and B have penetrated 
 the submucous tissue for a considerable depth, but the surface epithelium 
 does not show a greater thickness at these points than it does over the 
 general surface of the cavity. 
 
 Figure 354 shows a section made in the same direction upon a human 
 embryo about the sixtieth day, and while the tooth band (-4) has pene- 
 trated the embryonal connective tissue to a greater depth, there is yet no 
 increase in the thickness of the epithelium, but rather a disposition for 
 the parts to become depressed. Another reason why the heaping up of 
 
TOOTH DEVELOPMENT 
 
 443 
 
 the superficial layer of cells forming the embryonal mucous membrane 
 should not be considered the first sign of the preparation for tooth develop- 
 ment lies in the fact that these cells are not directly interested in the 
 process, but that the inflection of cells which results in the formation of 
 the tooth band results from the deep or infant layer of cells known as 
 Malpighi's layer, as shown at B (Fig. 354). 
 
 Fig. 353. — Transverse Section through Primitive Jaws of Human Fetus. 
 
 There is no question but that the location from which the section is 
 taken has much to do with the character and thickness of the older layer 
 of epithelial cells ; and that they do at certain points constitute an epithe- 
 lium exceeding in thickness that of other parts of the cavity, but this 
 condition most frequently occurs after the enamel organ has assumed 
 definite proportions. 
 
444 
 
 HISTOLOGY 
 
 After the formation of the tooth band, which, it must be remembered, 
 encircles the entire jaw in the form of a well-defined body of oval epithelial 
 cells from the infant layer (shown at A in cross-section, Fig. 355), the 
 next step in the process is one which concerns the location for the individ- 
 ual buds for the enamel organs of the various teeth, and the approximate 
 time at which these appear. 
 
 In the human subject we find ten such spots appearing upon the 
 lamina given off from the lingual face of the tooth band. These do not 
 
 Fig. 354. — Transverse Section through Lower Jaw, Human Embryo, Sixtieth Day. 
 
 appear, however, at the free extremity of the band, but at some little 
 distance toward the surface from this point, as shown in figure 356. In 
 this section the tooth-germ is severed from the surface epithelium, but 
 this is not a true condition at this period, as it still retains its connection 
 with the surface by a narrow band of cells, the neck of the enamel organ. 
 Two distinct classes of cells are now (sixtieth day) concerned in the 
 process of tooth development, those at A being of epithelial origin and 
 
TOOTH DEVELOPMENT 
 
 445 
 
 forming the future enamel organ, while at B an aggregation of cells from 
 the mesoblast provides for the generation of the pulp and dentin. At C 
 the narrow band of cells which should continue to the surface is shown, 
 while the free extremity of the same body of cells at D will persist and 
 eventually become the germ for the succeeding tooth. In regard to the 
 time at which the buds for the various teeth appear, it might be expected 
 that the same variation which follows the development and eruption of 
 
 Fig. 355. — Section through Tooth Band of Human Embryo. 
 
 the teeth throughout would obtain, but such is not the case, the buds for 
 the deciduous incisors appearing about the sixtieth day, while the germs 
 for their permanent successors are but little later in forming. 
 
 By referring to figure 347, which shows a longitudinal section through 
 the lower jaw about the twelfth week, the deciduous incisor is seen well 
 outlined by its formative cells, while immediately to the lingual appears a 
 section of the germ for the permanent cuspid. Notwithstanding this, 
 there elapses a period of several years between the eruption of these teeth. 
 
446 
 
 HISTOLOGY 
 
 The same relative progress will be noted between the first and second 
 teeth, be the subject human or otherwise; requiring many years to com- 
 plete dentition, in the former, while in most of the lower animals the same 
 process occupies but a comparatively short time. 
 
 The next stage in the development of a tooth to which attention will 
 be called is that in which the entire tooth-crown is outlined by the dentin 
 papilla and surrounded by its epithelial cap, the enamel organ. Such 
 
 Fig. 356. — Tooth-germ, Embryo Lamb, Corresponding to Sixtieth Day (Human). X40. 
 
 an advance in the process is shown in figure 357, together with the sur- 
 rounding structure. When this stage is reached, the individual cells 
 of the tooth-germ are strongly differentiated, and the odontoblasts are 
 making their appearance about the summits of the cusps. 
 
 Up to this time the cells present are those which result in the forma- 
 tion of but two of the calcified tooth tissues, but now there is a marked 
 disposition upon the part of the periosteum of the jaw to pass down by 
 the side of the enamel organ (A), this being the first indication of the 
 
TOOTH DEVELOPMENT 
 
 447 
 
 formation of the tooth follicle, the alveolodental membrane, and cemen- 
 tum. At this period it will be observed that there appears in this, the 
 molar region, a "heaping up" not only of the surface epithelium, but 
 also of the connective tissue as well. 
 
 Figure 358 shows a section through the growing mandible taken 
 from a portion of the tissue not occupied by a tooth-germ. This is of 
 interest, first, as giving a view of the detached tooth band in cross-section, 
 
 Fig. 357. — Tooth-germ, Premolar, Embryo Lamb. X40. 
 
 at A; second, by showing the distribution of the periosteum to the interior 
 of the jaw to serve the double function of the future tooth-sac and alveolo- 
 dental membrane; and, third, the thickened epithelium with the underly- 
 ing tissue pushing into it. 
 
 Although the germ for the second tooth may be observed at a period 
 somewhat prior to this, a study of some of its characteristics is best made 
 at this time. The fact has already been referred to that this interesting 
 phenomenon occurs soon after or even simultaneously with that for the 
 
448 
 
 HISTOLOGY 
 
 first tooth, a portion of the primitive cord for the latter persisting as the 
 germ for the former. 
 
 In figure 359 the cells forming the primitive germ for the enamel 
 organ of one of the permanent teeth are shown highly magnified. It 
 will be observed that they are of tte simplest epithelial character, and 
 
 Fig. 358. — Section through Jaw of Embryo Lamb, in a District not Occupied by 
 a Tooth-germ. X40. 
 
 that they are derived directly from the enamel organ of the pre-existing 
 tooth, on the one hand, while, on the other, they communicate with the 
 surface by a narrow band of cells. In this way it is for a time dependent 
 upon both of these parts for continuance and growth, but after a time 
 it, too, like its predecessor, severs its connection with the surface, but 
 
TOOTH DEVELOPMENT 
 
 449 
 
 remains intact with the epithelial cells of the former enamel organ until 
 these cells begin to atrophy. 
 
 The cells which made up this primitive germ are of three varieties: 
 the inner layer, or those derived from the epithelium of the enamel organ 
 of the first tooth, being small and spheroidal; those of the outer layer, 
 
 Fig. 359. — Primitive Bud for Enamel Organ of Permanent Tooth, Human 
 Embryo. X300. 
 
 which spring from the surface epithelium, being proportionately larger 
 and cylindrical or oblong; while those which intervene are markedly 
 irregular in outline. In this respect — that is, in the character of the early 
 cell layers — the tooth-germs for the permanent teeth differ from those 
 of the deciduous. 
 
 The question of the origin of those teeth which have no predecessors 
 is one upon which there has always been more or less discussion, some 
 29 
 
45° 
 
 HISTOLOGY 
 
 writers contending that they are derived directly from the oral epithelium 
 by a special generation of cells for each tooth, while others are of the 
 opinion that as the jaw grows backward certain changes take place which 
 result in the establishment of an epithelial fold or lamina, in every partic- 
 
 j& ML Mk jm^ 
 
 Fig. 349. — Diagram of Various Forms of Hare-lip following the Lack of Union of Various 
 Processes Participating in the Formation of the Mouth. 
 
 ular corresponding to the tooth band of the deciduous teeth. With these 
 two conflicting opinions in mind, a number of sections were made through 
 the extreme distal end of the jaw. The result favored the latter theory, 
 for here the tooth band is seen similar in form and location to that ob- 
 served in the jaw in those locations from which succedaneous teeth result. 
 
CHAPTER X. 
 
 Anomalies of the Teeth. 
 
 It has always been conceded that the dental organs of man are sus- 
 ceptible of much variation in form and structural arrangement, and 
 that frequently this variation is so positive that the organ is pronounced 
 anomalous in character. Just where the line of distinction between the 
 normal and abnormal should be drawn is a subject worthy of some con- 
 sideration. Some authorities define the word anomaly as a marked 
 deviation from the normal, while, in the opinion of others, a much broader 
 meaning is accorded it; and we find all those conditions- which are in 
 themselves an irregularity from the typical structure or occurrence in- 
 
 Fig. 361. — Anomalous Teeth. 
 
 eluded in this category. Under the first definition a given structure or 
 organ is accorded a wide field for its normal existence, while under the 
 latter but slight deviation is necessary to classify it among the abnormal. 
 
 Upon first thought it would appear that the ability or inability of a 
 tissue or an organ to perform its special function should, in a measure, 
 decide the question of the nature of its being, and no doubt to a certain 
 extent this is true; but while the action of an organ or a part of the body 
 may, by observation, appear entirely satisfactory, it is only so at the 
 expense of other organs or tissues, and these in the course of time, by this 
 extra exertion, become hypertrophied or in other ways pathologic. 
 
 While this is especially applicable to those organs or tissues which 
 have a wide range of function, it may with a good deal of force be applied 
 
 45 1 
 
452 ANOMALIES 
 
 to the dental organs and their immediate environments. Anomalous 
 conditions in the teeth may originate in, or be confined to, one or more 
 of the tooth tissues, in any of which the structural disarrangement may 
 eventually result in the death or degeneracy of the part. Enamel mal- 
 formation is of such a character that it may be observed upon the surface 
 either in the form of a multiplication of cusps, or by an extra development 
 of the various ridges formed by pronounced folds of this tissue. But 
 probably the most disastrous anomaly of the enamel, and one frequently 
 responsible for the downfall of this tissue, is found in some defect of its 
 structural arrangement other than those just referred to. In some 
 instances the enamel rods of a given district, instead of being normally 
 distributed by assuming a direction principally at right angles to the long 
 axis of the tooth crown, are arranged without regard to the base or per- 
 iphery of the tissue, and we have as a result an anomaly of structure. 
 The question of normal and abnormal rod distribution now presents 
 itself, because in certain locations — i.e., the summits of the cusps — 
 an arrangement of the rods similar to that referred to is so common that 
 it may be considered a normal condition, while, if a like distribution was 
 found in other locations, the tissues should properly be considered 
 abnormal. 
 
 Malformed teeth, in respect to the number and forms of the cusps 
 present, are not alone confined to the enamel, but also to the dentin 
 which first records the tooth form on its periphery. 
 
 Anomalies in the general contour of the tooth crown are usually 
 confined to the incisors and third molars, both the dentin and the enamel 
 contributing to the deformity. Here the defect is usually so pronounced 
 that but little difficulty is experienced in properly classifying the organ. 
 One of the most frequent variations in form met with in these locations 
 is found in the peg-shaped or cone-shaped crown. If it were possible 
 it would be interesting to trace the development of such a malformation; 
 but with our present knowledge of this process in general, there is little 
 doubt as to its origin, the enamel organ failing to fulfill its early and pri- 
 mary function of moulding the tooth crown in the dentin papilla, the 
 responsibility for this resting in the special cells composing it, as well as 
 the so-called stellate reticulum, which, it is believed, exerts a controlling 
 influence over the form of the enamel cap. 
 
 While the organic defects of tooth crowns are numerous and varied, 
 those which are confined to the roots are most frequent, in many instances 
 interfering to some extent with the function of the organ. When a given 
 peculiarity is confined to this portion of the tooth, it is frequently difficult 
 
SUPERNUMERARY TEETH 453 
 
 to discriminate between the normal and the abnormal. Certain teeth 
 are recognized as normal when either a single root or two roots are pres- 
 ent, and the acceptance of this fact increases the difficulty of a proper 
 classification of its peculiarities. 
 
 In very rare instances do we find the roots of the cuspidate teeth 
 more or less crooked; yet, at the same time, many decidedly crooked 
 roots are considered within the natural law; while, on the other hand, 
 roots with but little more deflection are classed as anomalous. 
 
 Marked flexions of roots or crowns, cases of fusion or concrescence, 
 are usually so positive in character that an anomalous condition is at 
 once acknowledged. While tooth anomalies are usually referred to as 
 external, or as belonging to the hard tissues of the organ, they are not 
 infrequently found in the pulp or pulp cavity. This cavity, normally 
 following the external contour of the tooth, is subject to much variation 
 in outline and capacity, regardless of those changes which are incident to 
 the continuous process of dentinification. 
 
 Among these are a complete division of the pulp chamber; horn- 
 like processes penetrating the dentin in the direction of the occlusal sur- 
 face in locations where they would be least expected; an unusual number, 
 or a peculiar distribution of the canals, etc. 
 
 Lack of Dentition. — Cases in which there is a total absence of 
 teeth have been reported. Guilford reports the case of a man fifty years 
 of age who never had teeth, the jaws not differing in appearance or form 
 from those of a person whose teeth have been extracted. The mother 
 of the subject had the usual number of teeth, but the grandmother and 
 an uncle were both edentulous and hairless from birth. J. Tomes 
 mentions two cases having been reported to him, and Linderer mentions 
 one. 
 
 On the other hand, instances of a third dentition have been reported,, 
 but there is great possibility of such observation being erroneous. Teeth 
 belonging to the permanent set which may have remained unerupted 
 for years could readily be mistaken as predecessors of a third set, when 
 in after years they made their appearance. 
 
 Supernumerary Teeth. — All teeth appearing in the mouth in addition 
 to the normal number are designated as supernumerary teeth. These are 
 divided into two classes, those normal in size and form and those abnormal 
 in size and form. The first-named are most likely to be of the simple 
 class, incisors and cuspids, and they may occupy a regular position in 
 the arch or may be found inside the arch closely associated with teeth of 
 the same type. Supernumerary bicuspids and molars are sometimes. 
 
454 AMONALIES 
 
 present cither in regular position in the arch or inside of it. When the 
 jaws are long enough to accommodate them, an additional molar may 
 appear back of the third molar, making four molars instead of three, 
 and cases in which two extra molars were thus placed have occasionally 
 been met with. 
 
 Supernumerary teeth of the second class or those which are abnormal 
 in size and form are usually inclined to be cone-shaped and small in size, 
 this in respect to the root as well as the crown. Supernumerary teeth 
 of this charater are usually found in the incisor region, but it occasionally 
 happens that they are found in the molar district, but here, instead of 
 having a single cone for the crown, they are mostly made up of a number 
 of smaller cones resembling many small cusps on the occlusal surface. 
 The number of supernumerary teeth may vary from one to eight or ten. 
 In the latter instance they are usually scattered through the entire alveo- 
 lar border of the hard palate. As many as ten or twelve teeth thus 
 located have been reported. 
 
 Again, certain teeth are frequently missing from the arch. This 
 may be occasioned by delayed eruption, or it may be the result of im- 
 proper activity within the tooth-germ itself, so that the tooth may have 
 failed to develop. The teeth most frequently missing are the upper 
 lateral incisors. The probable reason for this lies in the fact that the 
 germs for these teeth are located very near the surface of the bone, and 
 in some instances they are not even protected by a thin layer of bone 
 over the follicle. Being thus situated, they are more or less exposed to 
 violence sufficient to destroy the germs and thus render development of 
 the teeth impossible. Lack of certain teeth in the mouth appears to be 
 to some extent an hereditary feature, the ondition being transmitted 
 from parent to child. Anomalies as to the size of individual teeth are 
 frequently noticed. When this is the case, it does not usually include 
 the entire set, but is confined to one or two, usually to the same teeth 
 on each side. Teeth that are above the usual size are generally found 
 in persons of large built, but if all the teeth are proportionate in size 
 they cannot be included within the abnormal class. The upper incisors 
 are most frequently affected in this way. Cases have been reported in 
 which the central incisors in the upper jaw have been fully twice the size 
 which they should normally have been, and all the remaining teeth in 
 the mouth perfect as to shape and size. Accompanying the abnormal 
 condition it is usual to find the teeth thus affected more or less abnormal 
 in outline, but, notwithstanding this, retaining their form sufficiently 
 well to permit their proper classification. 
 
SUPERNUMERARY TEETH 
 
 455 
 
 There are likewise teeth that are deficient in size. This does not 
 refer to teeth all relatively small, as frequently found in persons of small 
 frame and stature. This anomalous condition is often present in the 
 upper lateral incisors and in the third molars. Teeth that are deficient 
 in size generally possess their normal shape. 
 
 While the crowns of the teeth are susceptible to the above variations, 
 the roots appear to be anomalous much more frequently and to a more 
 marked extent than are the crowns. Most important among these may 
 be mentioned flexions of the roots. Curvatures in the roots may be 
 found either in single-rooted teeth or in multi-rooted teeth, and the point 
 of flexion may be located either at the center of the root or near its apex, 
 
 Fig. 362. — Anomalous Roots. 
 
 or both of these points may be affected. Flexions in the roots of the 
 teeth present a great variety in form. A single curve in one direction 
 may be present or a number of curvatures in different directions. In 
 multi-rooted teeth the roots may be so flexed that they will entwine about 
 each other, and the overlapping portions may be or may not be fused. 
 Probably the most important cause for the flexions of the roots of the 
 teeth is delayed eruption, this being particularly true if some positive 
 force prevents the progress of the organ. The roots of the teeth are 
 frequently anomalous in regard to number. These may have the same 
 general form and the same approximate length as the normal root or 
 roots would be, but they are proportionately smaller in diameter. Cases 
 are on record in which the incisor teeth have had two distinct roots, but 
 probably the most frequent location for the multiplicity of roots is found 
 in connection with the third molar. While this tooth when normally 
 developed as to crown is usually supported by three roots, it sometimes 
 possesses five or six smaller roots. 
 
45 6 
 
 ANOMALIES 
 
 It not infrequently happens that the roots of the teeth are less in 
 number than they should normally be. This is usually brought about 
 by the blending of the roots, which occurs during the developmental 
 stage. This may be of two distinct kinds. In the first place, it may 
 be the result of the conversion of two or more pulp canals into one, or the 
 individual roots may be united by a body of cementum being interposed 
 between them. In the former instance a single pulp canal is mostly 
 found within the blended roots, while in the latter the number of canals 
 is usually normal. With this blending there is generally a line of demar- 
 cation between individual roots as they should normally exist, in the 
 
 Fig. 363. — Fusion of Molars 
 
 way of longitudinal depressions extending through the entire root from 
 cervical line to apex. This anomaly, like that of the multiplicity of roots, 
 is principally confined to the third molars, and is more frequent in the 
 upper jaw than in the lower. 
 
 Fusion and Concrescence. — The union of two or more teeth is 
 known as fusion or concrescence, and may occur either during the develop- 
 ment of the organ or after this process has been completed. When union 
 takes place during development, it is characterized as fusion; when it takes 
 place after the completion of this process, it is known as concrescence. 
 There seems to be but little doubt that fusion of the teeth occurs through 
 some irregularity in the tooth-germ or germs, the beginning of the develop- 
 mental process taking place generally between two germs and continuing 
 together until the complete calcification of the organs. Teeth thus 
 united may have an internal anatomy corresponding in nearly every 
 respect to two separate teeth, and, on the other hand, they may possess 
 
FUSION AND CONCRESCENCE 
 
 457 
 
 but a single pulp chamber and canal. Fusian may take place in the 
 roots of the teeth alone, when it is called partial fusion; but when it is 
 confined to the roots and crowns alike, it is classified as complete fusion. 
 The teeth most likely to be affected in this way are the upper incisors 
 and the second and third molars. Some distinction must be made be- 
 tween those teeth which are united by a layer of cementum, and those of 
 
 Fig. 364. — Fusion and Concrescence. 
 
 Fig. t,6>. — Fusion and Concrescence. 
 
 true fusion, the latter existing only when there has been a union between 
 dentin and dentin. 
 
 Concrescence. — In concrescence, the roots of the teeth only can be 
 affected, as the union takes place after the complete development of the 
 organ. The roots of one tooth become united to the roots of another 
 through an additional growth of cementum, this growth being sufficiently 
 
45« 
 
 ANOMALIES 
 
 extensive to cause absorption of the alveolar septa by pressure. Follow- 
 ing this there is a resorption of the pericementum. The cemental tissues 
 of the teeth are then brought in contact, and coalescence gradually takes 
 place. Concrescence may take place not only between the roots of two 
 teeth, but between the various roots of an individual tooth, this resulting 
 in the same manner as above described by the destruction of the septa 
 within the tooth socket. When concrescence takes place, the process 
 is usually confined to the apices of the roots, although they may become 
 coalesced throughout their entire length. The teeth most commonly 
 affected by concrescence are the molars and bicuspids, from the fact 
 that the alveolar walls, particularly the septa about these teeth are espe- 
 cially thin, owing to the form and relative location of the roots. 
 
 Fig. 366. — Geminous Tooth. 
 
 Geminous Teeth (Fig. 366). — It occasionally happens that two 
 separate germs are confined within a single sacculus, and from this 
 results two teeth, either similar or dissimilar in size and form. One of 
 the pair may be normal as to form and size, while the other may be much 
 below the normal size, but more or less perfect in outline. Geminous 
 or twin teeth may be united or entirely separate. This condition is 
 most frequently found in the molar teeth, although cases in which the 
 bicuspids and incisors have been thus affected are recorded. Teeth 
 thus formed must not be confounded with those in which fusion is the 
 anomaly. In geminous teeth a single sac contains two tcoth-germs 
 from which results two similarly formed teeth; in fusion two follicles 
 coalesce, each of which contains its own germ. 
 
 Besides the foregoing, the roots of the teeth are subject to anomalies 
 in size in some instances being abnormally small, in others abnormally 
 large. In the former they may nearly always be characterized as anomal- 
 
GEMINOUS TEETH 
 
 459 
 
 ous; but this is not always true of the latter. When the roots are abnor- 
 mally large, it is somewhat difficult to discriminate between an anomalous 
 and a pathologic condition, the latter usually being the case when the 
 
 Fig. 367. 
 
 Fig. 368. 
 
 roots of individual teeth are affected. Roots, to be considered anomal- 
 ous in size should in a measure retain their normal form, cases of hyper- 
 trophy (hypercementosis) usually resulting in the destruction of the 
 normal contours (Figs. 367 and 368). 
 
INDEX. 
 
 Accessory palatal foramina, 18 
 Achromatic spindle, 241 
 Achrornatin substance, 238 
 Acidophiles, 273 
 Adipose tissue, 36 
 Adrenal, 282 
 Adventitia, 276 
 Alveolar process, 44 
 
 development of, 48 
 Alveoli, 45 
 Alveolodental membrane, 76, 297, 366 
 
 blood supply to, 299 
 
 cells of, 367 
 
 fibers of, 369 
 
 histology of, 366 
 
 interfibrous elements of, 372 
 
 nerve supply to, 299 
 Amelification, 324 
 Ameloblasts, 323, 325, 394 
 Amoeba, 242 
 
 Amoeboid movement, 268 
 Anatomy, macroscopic, 233 
 
 microscopic, 233 
 Angioblasts, 270 
 Angular artery, 13 
 Anomalies of the teeth, 451 
 Anterior palatal foramen, 18 
 
 nerve, 18 
 
 superior dental nerve, 100 
 Antrum of Highmore, 48 
 Apical foramina, description of, 190 
 Areolar tissue, 35 
 Artery, deep facial, 18, 32 
 
 inferior dental, 96 
 
 internal maxillary, 18, 107 
 
 lingual, 32 
 
 ranine, 32 
 
 structure, 276 
 
 sublingual, 32 
 Axon, 263 
 Azygos uvula;, 23 
 
 Basophiles, 273 
 Bicuspid, lower first, 172 
 second, 175 
 upper first, 125 
 second, 135 
 Blastoderm, 247 
 
 layers of, 247 
 Blood and lymph, 234, 326 
 cells, 267 
 corpuscles, 267 
 development, 267 
 physiology, 267 
 
 Blood plasm, 266 
 
 platelets, 273 
 structure, 267 
 course of, from heart to cheeks, 15 
 from heart to hard palate, 18 
 from heart to lips, 8 H 
 
 from heart to soft palate, 23 
 from heart to tongue, ^t, 
 supply to the teeth, 94 
 vessels, 274 
 Bone, 255 
 
 canaliculi of, 256 
 cells of, 256 
 
 Haversian canals of, 256 
 histologic examination of, 256 
 hyoid, 25, 62 
 inferior maxillary, 54 
 lacuna; of, 256 
 marrow of, 258 
 matrix of, 255 
 osteoblasts of, 258 
 palate, 50 
 periosteum of, 258 
 Sharpey's fibers of, 257 
 superior maxillary, 37 
 Bones, of the mouth, 37 
 
 superior maxillary, 37 
 Branchial arches, 374 
 Brown striae of Retzius, 318 
 Buccal cavity, 376 
 
 embryology of, 375 
 glands, 300, 307 
 orifice, 2 
 Buccinator muscle, 9 
 
 Calcification, beginning of, 419 
 
 of cementum, 358, 433, 453 
 
 of dentin, 340, 419 
 
 on enamel, 324 
 Canal, inferior dental, 55 
 
 infra-orbital, 38 
 
 posterior palatal, 41 
 Canaliculi, 256 
 Canals, Haversian, 256 
 
 pulp, 190 
 Canine eminence, 38 
 
 fossa, 38 
 Capillaries, 274 
 Capsular ligament, 67 
 Cartilage, 253 
 
 cells of, 254 
 
 elastic, 254 
 
 fibro-, 254 
 
 hyaline, 254 
 
 461 
 
462 
 
 INDEX 
 
 Cartilage, matrix of, 253 
 
 Meckel's, 60, 384, 412 
 
 permanent, 255 
 
 temporary, 255 
 
 varieties of, 253 
 Cell, amitosis, 243 
 
 conductivity, 240 
 
 definition, 236 
 
 division, 242 
 
 fat, 253 
 
 gland, 253 
 
 growth, 239 
 
 irritability, 239 
 
 karyokinesis, 242 
 
 membrane, 235, 238 
 
 metabolism, 237, 239 
 
 mitosis, 242 
 
 motion, 240 
 
 nucleus, 234, 237, 238 
 
 origin, 236 
 
 plasma, 253 
 
 pigment, 253 
 
 protoplasm, 234, 235, 237 
 
 spontaneous generation, 236 
 
 wall, 238 
 
 vital manifestations, 239 
 Cement corpuscles, 354 
 
 fibers, 356 
 Cementification, 358, 433 
 Cementoblasts, 368, 433 
 Cementum, 75, 348 
 
 calcification of, 282, 358, 433 
 
 canaliculi of, 349 
 
 fibers of, 356 
 
 histology of, 348 
 
 lacuna; of, 349 
 
 lamella?, 353 
 
 matrix of, 349 
 Central incisor, lower, 165 
 
 upper, 102 
 Centrosome, 237 
 Cervical line, 80 
 Cheeks, 8 
 
 blood supply to, 13 
 
 external covering of, 9 
 
 glands of, 9, 300, 307 
 
 integument of, 9 
 
 internal covering of, 9 
 
 mucous membrane of, 9, 285 
 
 muscles of, 9 
 
 muscular tissue of, 35 
 
 nerves of, 1 5 
 
 substance of, 9 
 Chromatin substance, 238 
 Chromosomes, 242 
 Circulatory organs, 274 
 Circumvallate papillae, 27 
 Close skein, 242 
 Coelom, 251 
 
 Concrescence of teeth, 440 
 Condyle, neck of, 59 
 Condyloid process, 59-66 
 
 forms of, 66 
 Connective tissue, cells of, 252 
 
 classification of, 252 
 
 Connective tissue, fibrous, 252 
 
 intercellular substance of, 252 
 origin, 250 
 
 Coronoid process, 59 
 
 Crowns, anomalous, 438 
 
 Cuspid, lower, 169 
 upper, 116 
 
 Cytoplasm, 237 
 
 Decalcification, 441 
 Deciduous lower central incisor, 226 
 cuspid, 227 
 first molar, 223, 228 
 lateral incisor, 220, 226 
 lateral incisor, measurement, 219 
 second molar, 225, 230 
 teeth, decalcification of, 219, 290 
 detail description of, 219 
 enamel organs for, 394 
 general description of, 216 
 occlusion of, 218 
 
 pulp chambers and canals of, 230 
 upper central incisor, 219 
 cuspid, 221 
 Deep facial artery, 18, 23 
 
 branches of, 23 
 Dendrites, 262 
 Dental arch, 80 
 
 arrangement of teeth in, 81 
 curve described by, 85 
 influence of temperament on, 86 
 follicle, 393, 409 
 formula, 78 
 Dental furrow, 415 
 
 Pulp, 359 
 
 sacculus, 409 
 Dentin, 75, 331 
 
 calcification of, 340 
 
 cells, 408, 409 
 
 chemical analyses of, 332 
 
 exposed by dissection, 417 
 
 fibers of, 331, 335 
 
 granular layer, 339 
 
 history of, 331 
 
 matrix of, 332 
 
 organ, 393, 407 
 
 papillae, 340, 408 
 
 tubules of, 331, 333 
 Dentinal fibers, 335 
 
 sheaths, 331, 334 
 
 tubules, 331, 333 
 walls of, 334 
 Dentinification, 340 
 Dentition, completion of, 441 
 
 lack of, 453 
 Depressor anguli oris, 12 
 
 labii inferioris, 7 
 
 labii superioris, 6 
 Deutoplasm, 238 
 Development of enamel, 321 
 
 of permanent teeth, 427 
 
 of teeth, 391, 442 
 Diaster stage, 242 
 Digastric fossa, 56 
 
INDEX 
 
 463 
 
 Duct, parotid, 303 
 sublingual, 304 
 submaxillar)-, 303 
 
 Ectoderm, 247 
 Elastic cartilage, 254 
 Embryology, general, 233, 240, 244 
 
 of mouth and teeth, 374 
 Enamel, 75, 313 
 
 ameloblasts of, 323 
 brown striae of, 318 
 calcification of, 324 
 cells, 324, 394 
 chemic composition of, 314 
 cuticle of, 364 
 development of, 321 
 histology of, 313 
 organ, 361, 393, 394 
 cells of, 394, 401 
 external epithelium of, 322, 394 
 form of, 394. 396 
 internal epithelium of, 322, 394 
 stellate reticulum of, 322, 394 
 stratum intermedium of, 322, 394 
 prisms of, 314, 325 
 rods, formation of, 314 
 Endocardium, 278 
 Endomysium, 261 
 Endoplasm, 238 
 Endothelium, 275 
 Entoderm, 247 
 Epicardium, 278 
 Epimysium, 261 
 Epithelial cells, 249 
 
 tissues, 234, 249 
 Eruption of the teeth, 433 
 Erythroblasts, 270 
 Erythrocytes, 270 
 number, 269 
 Exoplasm, 238 
 External maxillary artery, 13 
 oblique line, 55 
 pterygoid, 73 
 
 Facial angle, 91 
 
 artery, 15, 32 
 Facial artery, branches of, 13 
 
 nerve, 15 
 
 branches of, 15 
 Falciform papillae, 27 
 Fat cells, 253 
 Fauces, anterior pillars of, 20 
 
 isthmus of, 20 
 
 pillars of, 20 
 
 posterior pillars of, 20 
 Fibers, 237 
 
 dentinal, 335 
 
 Sharpey's, 370 
 Fibroblasts, 369 
 Fibro-cartilage, 254 
 Fibrous connective tissue, 252 
 Fifth nerve, 99 
 
 • division of, 99 
 Filiform papilla', 27 
 
 Floor of the mouth, boundaries of, 25 
 
 embryology of, 377 
 
 framework of, 25 
 Follicle, dental, 393, 409 
 Foramen, anterior palatal, 18 
 
 apical, description of, 190 
 
 caecum, 27 
 
 incisive, 44 
 
 infra-orbital, 38, 39 
 
 mental, 56 
 Foramina, accessory palatal, iS 
 
 posterior palatal, 18 
 Fossa, glenoid, 65 
 Frenae of the mouth, 295 
 Fungiform papillae, 27 
 Fusion of teeth, 456 
 
 and concrescence, 456 
 
 complete, 457 
 
 partial, 457 
 
 Ganglion, Gasserian, 98 
 
 Meckel's, 19 
 
 sphenopalatal, 19 
 Geminous teeth, 458 
 Genial tubercles, 56 
 Geniohyoglossus, 29 
 Germinal cells, 244 
 
 layers, 247 
 Germs for permanent molars 409 
 Gills, 374 
 Gingivae, 17 
 Gingival border, 17 
 
 margins, outlines of, 295 
 Gland cells, 253 
 
 follicles, 305 
 
 parotid, 301, 310 
 
 saccular, 305 
 
 sublingual, 304, 311 
 
 submaxillar}-, 303, 310 
 
 tubular, 305 
 Glands and ducts of the mouth, histology of, 
 
 3°5 
 buccal, 300, 307 
 
 excretory ducts of, 306 
 ductless, 282 
 labial, 293, 300 
 lingual, 301, 305 
 molar, 300, 306 
 of the cheeks, 307 
 of the hard palate, 307 
 of the mouth, 300, 305 
 of the soft palate, 301, 307 
 palatal, 301, 307 
 salivary, 301, 309 
 Glenoid fossa, 65 
 Gomphosis, 76 
 (Jranules, 272 
 
 acidophilic, 272 
 basophylic, 272 
 neu trophy lie, 272 
 Groove, infra-orbital, 39 
 Ground substance, 234 
 Gubernaculum, 423 
 foramina of, 429 
 
464 
 
 INDEX 
 
 Gums, 294 
 
 epithelium of, 288 
 fibrous tissue of, 288 
 general des< ription of, 204 
 mucous membrane of, 2S7, 295 
 
 Hard palate, 16 
 arch of, 17 
 
 blood supply to, 18 
 
 bones of, 17 
 
 covering of, 18 
 
 formation of, 18 
 
 glands of, 17, 307 
 
 mucous membrane of, 17, 289 
 
 nerves of, 19 
 Hare-lip, 449 
 Haversian canals, 254 
 Heart, 278 
 Hematoblasts, 270 
 Hemoglobin, 269 
 Heredity, 242 
 Highmore, antrum of, 48 
 Histogenesis, 233, 244, 247 
 Histology, 233 
 Hooke, Robert, 234 
 Hyaline cartilage, 313 
 Hyaloplasm, 238 
 Hyoglossus, 28 
 Hyoid bone, 62 
 
 development, 63 
 
 greater cornua, 62 
 
 lesser cornua, 62 
 
 muscles attached to, 63 
 Hypoblast, 247 
 Hypophysis, 282 
 
 Incisive foramen, 44 
 
 fossa, 39, 52 
 Incisor crest, 44 
 
 lower central, 165 
 
 lateral, 165 
 upper central, 102 
 lateral, no 
 Inferior coronary artery, 8, 13 
 vein, 13 
 dental artery, 96 
 
 incisive branch of, 98 
 mental branch of, 98 
 canal, 59 
 meatus, 43 
 nerve, 101 
 lingualis, 32 
 maxillary bone, 54 
 body of, 55 
 development of, 60 
 facial surface of, 55 
 internal surface of, 56 
 muscles attached to, 60 
 vertical portion of, 58 
 palatal vein, 18 
 turbinated crest, 40, 43 
 Infra-orbital canal, 39 
 foramen, 39 
 groove, 39 
 Intercellular substance, 251 
 
 Interglobular spaces, 339 
 
 Internal maxillary artery, 18, 94 
 
 infra-orbital branch of the, 94 
 superior maxillary branch of the, 94 
 oblique line, 56 
 pterygoid, 73 
 
 Interproximate spaces, 86 
 
 Intima, 276 
 
 Involuntary muscular tissue, 260 
 
 Isthmus of fauces, 20 
 
 Labial glands, 300, 306 
 Lacrimal canal, 41 
 
 tubercle, 41 
 Lacunae, 256 
 Lateral incisor, upper, 105 
 
 lower, 168 
 Layer, granular, 339 
 Malpighi's, 443 
 Leucocytes, 271 
 
 large mononuclear, 272 
 small mononuclear, 272 
 polymorphonuclear, 272 
 Dolynuclear, 272 
 transitional, 272 
 Levator akeque nasi, 4 
 anguli oris, n 
 labii inferioris, 6 
 
 superioris, 6 
 palati, 22 
 Lines of Schreger, 320 
 Lingual artery, 32 
 glands, 300, 342 
 vein, 33 
 Lips, 2 
 
 blood supply to, 7 
 external covering of, 3 
 frenae of, 3, 295 
 glands of, 3, 300, 306 
 integument of, 3 
 internal covering of, 3 
 mucous membrane of, 3, 284 
 muscles of, 3 
 muscular tissue of, 34 
 nerves of, 8 
 substance of, 3 
 Loose skein, 242 
 Lower bicuspid, first, 172 
 
 buccal surface of, 173 
 calcification of, 172 
 cusps of, 172 
 distal surface of, 174 
 lingual surface of, 174 
 measurements of, 172 
 mesial surface of, 174 
 neck of, 174 
 occlusal surface of, 172 
 pulp cavity of, 213 
 root of, 175 
 second, 175 
 
 buccal surface of, 176 
 calcification of, 175 
 distal surface of, 178 
 lingual surface of, 177 
 measurements of, 175 
 
INDEX 
 
 465 
 
 Lower bicuspid, second, mesial surface of , 177 
 neck of, 178 
 
 occlusal surface of, 175 
 root of, 178 
 bicuspids, general description of the, 172 
 
 pulp cavities of, 213 
 cuspid, 169 
 
 calcification of, 169 
 cusp of, 171 
 distal surface of, 170 
 labial surface of, 169 
 lingual surface of, 170 
 measurements of, 169 
 mesial surface of, 170 
 neck of, 171 
 root of, 171 
 cuspids, pulp cavities of, 212 
 incisor, central, 165 
 
 calcification of, 165 
 cervical margin of, 167 
 cutting-edge of, 167 
 distal surface of, 167 
 labial surface of, 165 
 lingual surface of, 165 
 measurements, 165 
 mesial surface of, 166 
 neck of, 168 
 root of, 168 
 incisors, pulp cavities of, 211 
 lateral incisor, general description of, 168 
 molar, first, 178 
 
 buccal surface of, 1S1 
 calcification of, 178 
 cusps of, 180 
 distal surface of, 183 
 lingual surface of, 182 
 measurements of, 178 
 mesial surface of, 182 
 neck of, 183 
 occlusal surface of, 178 
 roots of, 183 
 second, 184 
 
 buccal surface of, 185 
 calcification of, 184 
 distal surface of, 187 
 lingual surface of, 186 
 measurements of, 184 
 mesial surface of, 186 
 occlusal surface of, 184 
 roots of, 187 
 third, calcification of, 187 
 
 general description of the, 187 
 roots of, 188 
 types of the, 189 
 molars, pulp cavities of, 2 14 
 Lymph, 266 
 cells, 267 
 corpuscles, 267 
 development, 267 
 plasma, 267 
 vessels, 267 
 Lymphocytes, 271 
 Lymphoid cells, 281 
 organs, 281 
 tissue, 2 Si 
 
 3° 
 
 Malpighi's layer, 443 
 Mandible, 54 
 
 evolution of, 412 
 Masseter, n, 92 
 Mastication, muscles of, 71 
 
 active organs of, 1 
 Matrix, 251 
 Maxillary bones, development of, 411 
 
 sinus, 48 
 superior, 37 
 Meckel's cartilage, 62, 384, 412 
 
 ganglion, 19 
 Media, 276 
 
 Median raphe, 27, 297 
 Membrana eboris, 361, 408 
 
 praeformativa, 325 
 
 propria, 253 
 Mental foramen, 56 
 
 protuberance, 55 
 Mesoderm, 247 
 
 parietal, 251 
 
 visceral, 251 
 Mesothelium, 275 
 Metaplasm, 238 
 Microsomes, 231 
 Middle meatus, 43 
 
 superior dental nerve, 100 
 Migrator}' activity, 268 
 
 cells, 268 
 Moral glands, 307 
 
 lower, first, 178 
 second, 184 
 third, 187 
 
 upper, first, 138 
 second, 150 
 third, 157 
 Monaster stage, 242 
 Morula, 245 
 Motion, 240 
 
 amoeboid, 240 
 
 ciliary, 240 
 
 contractile, 240 
 Mouth, 1 
 
 angles of, 2 
 
 bones of, 37 
 
 boundaries of, 1 
 
 contents of, 1 
 
 development, 411 
 
 dissection of, 15 
 
 divisions of, 15 
 
 entrance to, 1 
 
 epithelium of, 283 
 
 floor of, 16, 24 
 
 frenac of, 295 
 
 general description of, 1 
 
 glands of, 297, 305 
 
 inferior portion of, 15 
 
 interior of, 15 
 
 lateral walls of, 8 
 
 mucous membrane of, 283, 296 
 
 muscular tissues of, 34 
 
 posterior boundary of, 20 
 
 primitive, 374 
 
 roof of, 15, 16 
 
 situation of, 1 
 
j.66 
 
 INDEX 
 
 Mouth, submucosa of, 284 
 superior portion of, 15, 16 
 tunica propria of, 284 
 vestibule of, 1 
 
 walls of, 8 
 Mucous membrane of the cheeks, histology 
 of, 286 
 of gums, histology of, 287 
 of lips, histology of, 284 
 of mouth, 283, 296 
 blood supply to, 284 
 histology of, 284 
 nerve supply to, 284 
 of tongue, histology of, 290 
 Muscle, external pterygoid, 73 
 internal pterygoid, 73 
 masseter, n, 71 
 temporal, 72 
 Muscles, angular series, 20 
 of mastication, 71 
 of soft palate, 20 
 of tongue, 28 
 Muscular tissue, 258 
 tissues of cheek, 35 
 
 endomysium of, 260 
 involuntary, 260 
 of lips, 34 _ 
 
 perimysium of, 260 
 sarcolemma of, 260 
 sarcoplasm of, 260 
 of mouth, histology of, 34 
 
 non-striated, 260 
 of soft palate, 35 
 
 striated, 260 
 of tongue, 36 
 voluntary, 260 
 Mylohyoid ridge, 56 
 Myocardium, 278 
 Myrtiform fossa, 39 
 
 Nasal crest, 44 
 
 spine, 44 
 Nasmyth's membrane, 348, 364 
 Nerve, anterior palatal, 19 
 
 cell, 261 
 
 corpuscles, 264 
 
 fiber, axis cylinder of, 264 
 neurilemma of, 264 
 
 Ibers, 264 
 
 medullated, 265 
 non-medullated, 265 
 
 inferior dental, 101 
 maxillary, 101 
 
 middle superior dental, 100 
 
 posterior superior dental, 100 
 
 process, 263 
 
 superior dental, 100 
 
 superior maxillary, 99 
 Nerves, 264 
 
 endoneurium of, 265 
 
 epineurium of, 265 
 
 funiculi of, 265 
 
 medullar}' sheath of, 265 
 
 perineurium of, 265 
 
 of tongue, 33 
 
 Nerves, system of, 262 
 
 Nervous tissue, 261 
 
 Neumann's sheath, 334 
 
 Neurilemma, 264 
 
 Neuroblasts, 262 
 
 Neurocyte, 261 
 
 Neuron, 261 
 
 Neutrophils, 273 
 
 Non-striated muscular tissues, 26c 
 
 Nuclear membrane, 238, 247, 307 
 
 reticulum, 238 
 
 stain, 236 
 Nucleolus, 239 
 Nucleoplasm, 237 
 Nucleus, 234, 237, 238 
 
 Occlusiox of the teeth, 88 
 
 of the deciduous teeth, 218 
 Odontoblastic cells, 408 
 Odontoblasts, 342, 361, 408 
 
 processes of, 374 
 Oral cavity, 374 
 
 embryology of, 374 
 
 plate, 374 
 
 sinus, 374 
 Orbicularis oris, 4 
 Organogenesis, 233 
 Organs and tissues, 243 
 Osteoblasts, 317, 367 
 Osteoclasts, 369 
 Overbite, 91 
 Ovum, 236, 244 
 
 fertilization, 244 
 
 maturation, 245 
 
 segmentation, 245 
 
 Palatal glands, 301, 307 
 raphe, 16, 297 
 ruga?, 16, 297 
 Palate, bone, articulation of, 53 
 
 attachment of muscles to, 53 
 blood supply to, 53 
 development of, 53 
 horizontal plate of, 51 
 vertical plate of, 51 
 bones, 50 
 hard, 15, 16 
 soft, 19 
 Palatoglossus, 20 
 Palatomaxillary suture, 18 
 Palatopharyngeus, 22 
 Papillae of the tongue, 27 
 Paraplasm, 238 
 Parotid duct, 303 
 
 _ gland, 301, 309, 345 
 Perichondrium, 3T3 
 Perimysium, 261 
 Periosteum, 258 
 
 Permanent incisors, papilla- for, 429 
 molars, germs for, 409 
 teeth, 76 
 
 advance of, 441 
 
 preparation for development of, 339 
 Phagocytes, 240, 271 
 Pigment cells, 253 
 
INDEX 
 
 467 
 
 Pillars of fauces, 20 
 Plasma cells, 253 
 Posterior foramina, 18 
 palatal canal, 41 
 superior dental nerve, 99 
 Primitive dental furrow. 4:5 
 Process, alveolar, 44 
 condyloid, 59 
 coronoid, 59 
 fronto-nasal, 374 
 Protoplasm, 234, 237 
 Protoplasmic stains, 236 
 Pulp, blood-vessels of, 363 
 canals, 190 
 
 cavities, description of, 190 
 dissections to show, 190 
 horns of, 191 
 of deciduous teeth, 230 
 of lower teeth, 211 
 of teeth, 190 
 of upper teeth. 192 
 cells of, 342 
 histology of, 342 
 nerves of, 363 
 odontoblasts of, 360 
 
 Quadratus menti, 7 
 
 Ranixe artery, 32 
 Raphe, 16, 27, 297 
 Retzius, brown striae of, 318 
 Ridge, mylohyoid, 56 
 Risorius muscle, 12 
 Roots, anomalous, 453 
 formation of, 433 
 Rouleaux, 269 
 Ruga;, 16, 297 
 
 Sacculus, dental, 247, 409 
 Salivary glands, 301, 309 
 
 blood-vessels of, 311 
 
 nerves of, 311 
 Sarcolemma, 260 
 Sarcoplasm, 260 
 Schleiden, 234 
 Schreger, lines of, 358 
 Schultze, Max, 236 
 Schwann, 235 
 
 sheath of, 325 
 
 white substance of, 325 
 Seventh nerve, 15 
 Sharpey's fibers, 37 
 Sheath, medullary, 262 
 
 of Schwann, 263 
 Simple saccular glands, 305 
 
 tubular glands, 305 
 Soft palate, 19 
 
 blood supply to, 23 
 
 glands of, 307 
 
 muscles of, 20 
 
 muscular tissue of, 35, 2I 
 
 nerves of, 23 
 
 substance of, 20 
 
 Spaces, interglobular, 339 
 
 interproximate, 86 
 Spermatozoon, 244 
 Sphenomaxillary ligament, 67 
 Sphenopalatal ganglion, 19 
 Spongioplasm, 238 
 Stains, 272 
 acid, 272 
 basic, 272 
 neutral, 272 
 Stellate reticulum, 322, 394 
 Stenson, foramen of, 44 
 Stratum intermedium, 322, 394 
 Striated muscular tissue, 319 
 Styloglossus, 31 
 Stylomaxillary ligament, 68 
 Sublingual artery, 32 
 duct, 304 
 fossa, 56 
 gland, 303 
 Submaxillar}- duct, 303 
 fossa, 56 
 gland, 303 
 Sub mucosa, 284 
 Superior coronary artery, 8, 13 
 vein, 13 
 lingualis, 32 
 
 maxillary bone, articulation of, 47 
 alveolar process of, 44 
 blood supply to, 47 
 development of, 47 
 facial surface of, 38 
 malar process of, 43 
 muscles attached to, 47 
 nasal process of, 42 
 orbital surface of, 37 
 palatal process of, 40, 43 
 proximal surface of, 40 
 sinus of, 48 
 tuberosity of, 42 
 zygomatic surface of, 42 
 bones, 37 
 meatus, 43 
 nerve, 96 
 palatal vein, 18 
 turbinated crest, 43 
 Supernumerary teeth, 453 
 Suture, palatomaxillary 7 , 18 
 Symphysis, 55 
 
 Teeth, 75 
 
 anterior, 79 
 
 apical extremities of, 79 
 articulation of, 88 
 attachment of, 75 
 blood supply to, 94-98 
 classification of, 75, 76 
 complex, 75 
 deciduous, 76, 216 
 
 description in detail of, 102 
 development of, 391, 442 
 dissections of, 190 
 division of, 75 
 eruption of, 289 
 names of, 78 
 
468 
 
 INDEX 
 
 Teeth, nerve supply to, 98 
 
 occlusion of, 88 
 
 permanent, 76 
 
 posterior, 79 
 
 pulp cavities of, 190 
 
 roots of, 78 
 
 simple, 75 
 
 surfaces of, 78 
 
 tissues of, 313 
 
 veins from, 98 
 Temporal muscle, 72 
 Temporomandibular articulation, 64 
 
 movements of, 69 
 Tensor palati, 23 
 Thyroid bodies, 282 
 Tissue, adipose, 253 
 
 areolar, 252 
 
 connective, 234, 250 
 
 elementary, 234, 243 
 
 epithelial, 234, 249 
 
 muscular, 234, 258 
 
 nervous, 234, 261 
 Tissues of the body, divisions of, 234 
 
 of the teeth, 313 
 Tongue, 25 
 
 attachment of, 26 
 
 base of, 26 
 
 blood-vessels of, 32 
 
 circumvallate papillae of, 291 
 
 dorsum of, 26 
 
 filiform papillae of, 290 
 
 frenum of, 27 
 
 function of, 26 
 
 fungiform papillae of, 291 
 
 glands of, 301, 308 
 
 median raphe of, 27 
 
 mucous membrane of, 290 
 
 muscles of, 28 
 
 muscular tissue of, 36 
 
 nerves of, 33 
 
 papillae of, 27 
 
 post-tip, 26 
 
 prebase of, 26 
 
 shape of, 26 
 
 size of, 26 
 
 substance of, 26 
 Tonsil, 20 
 Tonsillar recess, 20 
 Tooth band, 392 
 
 development, cellular stage of, 391, 442 
 saccular stage of, 411, 417 
 
 follicle, walls of, 410 
 
 fundamental parts of a, 75 
 general description of a, 72, 75 
 
 germs, 393 
 
 roots, preparations for development of, 
 
 433 
 sac, 409 
 
 sacs exposed by dissection, 415 
 sockets, 45 
 tissues of a, 75 
 Transverse facial'artery, 13 
 branches of, 13 
 vein, 13 
 Tunica propria, 328 
 
 Upper bicuspid, first, 125 
 angles of, 130 
 buccal surface of, 128 
 calcification of, 125 
 crown of, 126 
 cusps of, 127 
 distal surface of, 130 
 lingual surface of, 129 
 measurements of, 125 
 mesial surface of, 129 
 neck of, 130 
 occlusal surface, 126 
 pulp cavity of, 198 
 roots of, 131 
 types of, 132 
 
 second, calcification of, 135 
 general description of, 136 
 measurements of, 135 
 pulp cavities of, 201 
 cuspid, 116 
 
 calcification of, 116 
 
 crown, 117 
 
 cusp of, 121 
 
 cutting-edge of, 121 
 
 deciduous, 229 
 
 distal surface of, 120 
 
 labial surface of, 117 
 
 lingual surface of, 118 
 
 measurement of, 116 
 
 mesial surface of, 119 
 
 neck of, 122 
 
 pulp cavity of, 196 
 
 root of, 122 
 
 types of, 122 
 incisor, central, 102 
 
 calcification of, 102 
 cervical margin, 107 
 crown of, 103 
 cutting-edge of, 106 
 developmental grooves, 102 
 distal surface of, 106 
 labial surface of, 103 
 lingual surface of, 104 
 measurements of, 102 
 mesial surface of, 105 
 neck of, 107 
 occlusal surface, 107 
 pulp cavity of, 192 
 root of, 108 
 types of, 108 
 
 lateral, no 
 angles, 114 
 calcification of, no 
 crown of, 111 
 cutting-edge of, 1 13 
 deciduous, 220 
 distal surface of, 113 
 labial surface of, in 
 lingual surface of, 112 
 measurements of, no 
 mesial surface of, 113 
 
 neck of, 114 
 
 pulp cavity of, 195 
 root of, 114 
 types of, 115 
 
INDEX 
 
 469 
 
 Upper molar, first, 138 
 
 buccal surface of, 144 
 calcification of, 138 
 cusps of, 141 
 deciduous, 223 
 distal surface of, 146 
 fossae and grooves of, 143 
 lingual surface of, 143 
 marginal ridges of, 139 
 measurements of, 138 
 mesial surface of, 146 
 neck of, 147 
 occlusal surface of, 139 
 pulp cavity of, 205 
 roots of, 147 
 types of, 148 
 
 second, 150 
 angles of, 156 
 buccal surface of, 154 
 calcification of, 150 
 cusps of, 152 
 deciduous, 225 
 
 deciduous, 225 
 
 distal surface of, 155 
 fossae and grooves of, 154 
 lingual surface of, 154 
 marginal ridges of, 151 
 measurement of, 150 
 mesial surface of, 155 
 
 Upper molar, deciduous, neck of, 156 
 occlusal surface of, 151 
 pulp cavity of, 208 
 roots of, 156 
 third, 157 
 
 buccal surface of, 159 
 calcification of, 157 
 cusps of, 161 
 distal surface of, 158 
 fossae and grooves of, 162 
 lingual surface of, 159 
 marginal ridges, 161 
 measurements of, 157 
 mesial surface of, 158 
 occlusal surface of, 160 
 pulp cavity of, 208 
 types of, 163 
 
 Uvula, 20 
 
 Vein, inferior palatal, 18 
 
 lingual, S3 
 
 superior palatal, 18 
 Visceral arches, 374 
 
 furrows, 374 
 Voluntary muscular tissue, 261 
 
 Zygomaticus major, 12 
 minor, 7 
 
COLUMBIA UNIVERSITY LIBRARIES (hsl.stx) 
 
 RK 280 B79 1913 C.1 
 
 Anatomy and histology of the mouth and t 
 
 2002448327 
 
 RK?80 
 
 B79 
 1913 
 
 Broomell 
 
 Anatomv and histolosv of the 
 
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