, i '■■'. ]'■■■"' i j.i ! ,'. ; 34-7 H^Jf 'l tlWi«WL |l ,J l CORNELL UNIVERSITY. THE llosiucll ft. 3Plotuer Cibrarg THE GIFT OF ROSWELL P. FLOWER FOR THE USE OF THE N. Y. STATE VETERINARY COLLEQE. 1897 Cornell University Library QL 812.B47 Practcal anatomy of the rabbit an eleme -'VigjHB 3 1924 021 952 878 *r*t -k PM Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://archive.org/details/cu31924021952878 PRACTICAL ANATOMY OF THE BABBIT AN ELEMENTARY LABORATORY TEXTBOOK IN MAMMALIAN ANATOMY B. A. BENSLEY, Ph.D. Associate Professor of Zoology in the University of Toronto TORONTO: THE UNIVERSITY PRESS , I I ( r PHILADELPHIA: P. BLAKISTON'S SON & CO. 1910 EV. Nb.HCtSl Copyright, Canada, 1910, by The University Press B+7 V « ./ #/ /■> AV-> . r4 PREFACE. The object of the present book is to set forth the chief facts of mammalian structure in an elementary, practical form ; further, to use the anatomy of a typical mammal as a means of applying the more useful definitions of human anatomy and, so far as the limitations of a single type permit, also the broader conceptions of morphological zoology. On the practical side, its chief aim is to place before the student the materials necessary for a practical study of the type, rather than a descriptive account of its organization, though in some cases, notably in the treatment of the skeleton, it has been possible to follow a descrip- tive method without departing from the original plan. The inclusion of a section devoted to certain general aspects of the structure of the rabbit will, it is hoped, encourage the student to prose- cute his practical study with a more liberal point of view. As to the subject-matter of this section, its selection has been a matter of no little difficulty, and, doubtless, in many respects it might have been improved upon. One must feel, however, that the first question is not one of detail, but of general principle. Progress depends to a considerable extent on the ability to attack small problems with a large spirit. At the present time a vast amount of effort is being devoted to the planning of laboratory courses, and with increasing specialization it becomes more than ever the duty of the instructor to see that the student does not leave the laboratory, provided with a mass of detailed information, but with general conceptions as crude as when he entered it. As a laboratory type the rabbit has been made familiar to students through various zoological textbooks and especially through the "Zootomy" of Parker. The use of the animal, however, so far as one may judge, has not been as extensive as its general convenience would seem to warrant. It may, therefore, be of some value to direct atten- tion to this form by providing more facilities for its study. It is un- fortunate, in many respects, that no recent and adequate account of the anatomy of the rabbit is available, as is the case with other mammals used for laboratory study, the classic " Anatomie des Kaninchens" of Krause, published in 1884, being still the common source of information. Mammalian dissection is probably of most value to two classes of students, namely, medical or premedical students using it as an intro- duction to human anatomy, histology, or physiology, and zoological students using it as an introduction, or as part of the laboratory prac- tice of vertebrate zoology. For the latter class two aspects of the subject are especially worthy of consideration. One is the more or less detailed study devoted to a single type; the other, the study of a specialized type, the latter point being of more importance if the sub- iv Preface. ject is being used as an introductory one. In many respects the con- tinuous study of a single animal is a good corrective for the rough general kind of dissection as suggested by the zoological textbooks, and may be made to share the well-known merit of human anatomy as a laboratory discipline. Again, for the student who afterwards is con- cerned with vertebrate evolution, the study of a specialized type, such as a mammal, gives him, at the outset, something definite and concrete on which to base his conceptions of sequence. Primitive structure is of great value as a means of explanation, but the question, now as hereto- fore, is whether or not the study of primitive animals as a preliminary step represents the correct procedure from a laboratory standpoint. The more the student becomes interested in tracing sequence, the more he will be convinced of the necessity of stating his problem before he begins to solve it. The practical outlines on which the present book is based have been used for several years and in different forms in the laboratories of the University of Toronto. It would be difficult to make due acknowledg- ments to those colleagues and students who at one time or another have assisted in its preparation. Indeed, our chief obligation is to Professor Ramsay Wright, who, in establishing laboratory courses of this kind, has laid the foundation on which we have tried to build. B, A. BENSLEY. University of Toronto, January 3rd, 1910. CONTENTS Introduction Part I. A GENERAL CONSIDERATION OF THE STRUCTURE OF THE RABBIT. Divisions and Methods 3 Interpretation of Structure 4 Zoological Position. 6 General Anatomy 8 Epithelial Tissues 9 Connective Tissues 1'2 Muscular Tissues 19 Nervous Tissues i. . . 21 Terminology 23 The General Features and Ground Plan of the Organ Systems 25 The Skeletal System. 30 The Nervous System 34 The Digestive System 40 The Respiratory System .... 43 The Vascular System 43 The Urinogenital System. .• 46 The Serous Cavities 49 Regional Sections 52 Part II. OSTEOLOGY OF THE RABBIT. General Divisions of the Skeleton 68 The Vertebral Column 68 The Ribs 73 The Sternum . 74 The Skeleton of the Head 75 The Skull as a Whole 75 The Bones of the Skull 85 The Hyoid Apparatus N 97 The Skeleton of the Anterior Limb 98 The Skeleton of the Posterior Limb 103 vi Contents. Part III. DISSECTION OF THE RABBIT. 1. External Features 110 2. The Abdominal Wall . , 113 3. The Stomach and Spleen 116 4. The Liver 120 5. The Intestines 122 6. The Urinogenital System _ 127 7. The Abdominal Aorta, Inferior Vena Cava, and Sympathetic Trunks 133 8. The Anterior Limb 135 9. The Posterior Limb 144 10. The Head and Neck 157 11. The Thorax 172 12. The Vertebral and Occipital Musculature . 180 13. The Central Nervous System 184 Appendix. Directions for the Preservation of Material .. . 194 INTRODUCTION. AS a laboratory exercise, the anatomical study of an animal is largely a matter of applying a certain practical method of exposition, the student's attention being concentrated on those facts which may be made out by direct observation. For this reason, and also because continuity is a prime consideration, various important aspects of structure are of necessity left in the background. It is to be con- sidered, namely, that in studying the structure of any organism, the final object is not simply to determine in what its structure consists, that is, its anatomy in a restricted sense, but also to understand what the latter signifies when considered either as functional mechanism or, in general, as a product of the various factors underlying it. Every organism reflects in its structure the operation of a variety of influences, and consequently one cannot form an adequate conception of animal or- ganization without considering it from various points of view. In many respects, as indicated below, the interpretation of structure is not simply a matter of what is to be found in a given form, but also of what the latter represents in comparison with others. Assuming, as in the present case, that the student is principally occupied with the routine of a type dissection, the question of how far he may go afield in the consideration of accessory facts is one which must be determined by his own inclinations. His first need, one which the present book endeavours to fill, is to understand the sources of information. Part I, therefore, the subject matter of which has been selected especially with reference to the student who has had no previous experience in the biological sciences, is designed to indicate some of the possibilities in this connection, and also to serve in other ways as an adjunct to the practical account of the structure of the rabbit as outlined in Parts II and III. With the introduction obtained in this way by using the rabbit as an object lesson the student should be able to extend his informa- tion independently, using for this purpose special textbooks in the respective subjects. . \ PART I. A GENERAL CONSIDERATION OF ] THE STRUCTURE OF THE RABBIT. DIVISIONS AND METHODS. Biology, the science or study of living organisms, includes several related sciences, the chief of which are Anatomy, the study of organized structure ; Physiology, the study of function ; and Embryology, the study of development. Anatomy, or Comparative Anatomy, the latter referring to the comparative study of organisms, and Embryology are also considered as divisions, or as practical methods, of Morphology, the general science of the evolution of form. The term "Anatomy " was originally applied to the dissection or study of the human body, and is still considered as referring more especially to the latter. Even in the early stages of biological science, however, the use of the term was extended to organisms generally; and afterwards, chiefly as a result of the introduction of the microscope as a new method of examining- structure, it attained its present compre- hensiveness as a term applying to the study of structure generally. It has been found convenient, especially in human anatomy, to distinguish as Gross Anatomy, the study of that kind of structure which is displa} r ed by dissection, or is revealed by naked-eye appearances, and as Microscopic Anatomy, the study of finer structure through the ap- plication of the microscope ; or, again, to distinguish as Special or Des- criptive Anatomy, the study of the particular features of the organs of the body, and as General Anatomy, the study of its more fundamental composition. General Anatomy is practically equivalent to Histology, the latter considering the body from the point of view of the structure and arrangement of its cells and tissues. These distinctions are of interest in the present case chiefly as defining more exactly the practical method and the kind of structure to be considered. Thus, dissection is to be recognized as a method of displaying structure of a gross and special kind. It consists in the orderly exposure and displacement of organs with the object of observing their features and relations to surrounding parts. The plan is essentially one of analysis, since conceptions of structure are based on the recogni- tion of differences, the latter being estimated by various features, such as form, color, texture, or position. On the other hand, because of the class of structure with which it deals, dissection is also to be recognized as a preliminary method in comparison with various others involving the use of the microscope. Anatomy of the Rabbit. THE INTERPRETATION OF STRUCTURE. Gross structure is, in a sense, only the outward expression of the finer microscopic structure underlying it, the latter being the true basis of the body. This refers not so much to the individual features of organs as to the relation existing between their appearance as gross objects and their tissue composition. Since this relation is more fully discussed below under the head of general anatomy, it need only be mentioned here as an element in the interpretation of structure as viewed from the gross standpoint. All structure, however, may be con- sidered from two points of view — physiological, and morphological. The former is more easily understood. All parts of the body are constructed on a basis of use or function ; and although the various processes and activi- ties of the body are more properly considered under the head of physio- logy, a thorough conception of the anatomy of an organ is usually to be gained only by a consideration of its particular role in the general economy. The morphological aspect of structure concerns various features of form and arrangement which, although they have been developed on a basis of utility, cannot be explained directly on that basis, because the factors controlling them lie outside of the body of the individual, and are such as have operated only through a long series of gradually changing conditions in the evolution of its type. As applied to a particular animal, the morphological method consists in explaining its adult structure by reference either to its embryonic development, or to the equivalent conditions in lower existing, or perhaps fossil, forms. The recognized principle of embryology is that known as the Law of Re- capitulation. It is based on the general observation that the definitive structure of an organism is attained through a series of embryonic stages, in which it not only develops from a simple or ground type to a more complex condition, but also reflects in passing the features of lower, and presumably its own ancestral, forms. That of comparative anatomy depends on the comparison of higher, specialized animals with lower, or generalized ones, the latter being assumed, in one feature or another, to have remained in a backward or primitive state of specialization, and therefore to reflect in such features a low grade of structure of a kind possessed by the ancestors of existing higher forms. These relations . form a basis for the comparison of the embryonic development of or- ganisms with the evolution or history of the groups which they represent, the former being distinguished as ontogeny, the latter as phylogeny. The' interpretation of the adult structure of an organism is a matter of distinguishing its more general features from its more special ones, the former being in all cases those to which the ontogenetic and phylogenetic principles are especially applicable. All characters of animals have an evolutionary basis, the general Interpretation oe Structure. 5 nature of which is easily understood, although the process by which they have been developed is still a matter of uncertainty. In comparison with one another, animals present certain resemblances and differences — diagnostic features, which are used as a basis for classifying them into major and minor groups. In many cases characters of resemblance have been shown to be secondary, and are hence described as convergent. In some of these the resemblances are of a gross type, and the structures are described as analogous; in other cases they are exact or homoplastic. As a rule, however, characters of resemblance are broad marks of affinity, comparable to those seen on a small scale in human families, or in human races, and determined as in the latter cases by heredity. The chief basis of comparison of animals with one another is the general assumption that structures which are similar or identical are homogenous — of com- mon origin. On the other hand, their differences are chiefly marks of diver- gence in evolution. Although it is conceivable that many of the internal features of animals are the result of a general progressive development, more conspicuous in comparison with those of primitive types, the majority of their differences are such as have resulted from adaptive modifications of structure, by which they have become differ- ently adjusted to the particular conditions of their accepted habitats. Adaptation is one great factor in modifying animal form, producing first divergences, as between one type and its contemporaries ; although such features may afterwards become settled in particular groups, and thus appear for these as primitive, general, or group-characters. Adaptation, in other words, is not a matter of present conditions only, of fixed environment, or an environment of a general or special kind. The rabbit as a gnawing animal or rodent, for example, is also an air- breathing, walking vertebrate, and shares these larger and also more ancient features with many other vertebrates of otherwise different kinds. It is customary to include under the term specialization all those features in which an organism may be shown to be more highly modified in comparison with another type. If the latter is an ancestral type, or a lower form exhibiting ancestral features, its more* primitive features are said to be prototypal, because they indicate the form from which the higher modification has been derived. Such comparisons not only reveal the fact that different animals are specialized in different degrees, but also show that a given form may be greatly specialized in some respects and primitive in others. Moreover, it is to be considered that animals are at the present time, as they have been in the past, more or less changeable, or plastic types. Some of the most interesting features which they exhibit depend on the circumstance that the adjustment of structure which is rendered neces- sary by the opposing effects of heredity and specialization is not exact or immediate. Thus, it is not difficult to find in any specialized animal, in addition to those organs which are functional or in full development, others which are retrogressive in character and reduced in size. It is also to be assumed, although difficult of proof, except where the inter- mediate member of a known series is being considered, that there are also organs which are sub -functional or progressive. Anatomv of the Rabbit. ZOOLOGICAL POSITION. It will be evident from the foregoing statement that every specialized animal possesses in its organization a vast assemblage of features which, if referred to their proper categories, are found to represent many grades of morphological value. In so far as the adult structure of a particular form is concerned, it is possible to consider them anatomically without discrimination; but, on the other hand, if they are to be explained, it is necessary to proceed on a basis of function, embryonic development, or evolution. T he study of an animal as a type or representative of a group, however, concerns only in a general way the features common to its various members, since the majority of features present in any animal are of minor importance, and as such are significant chiefly as indicating the developments which may take place inside it. The question of what an animal is actually representative is a matter of comparison with other forms, in other words, of its zoological position. This is expressed through the medium of classification, the latter being arranged to indicate, so far as is possible, the relationships of organisms to one another. In this connection the following statement of the zoological position of the rabbit may be found useful ; and it may also be considered as illustrating, through the comparison of this animal with allied forms, some of the more general characters of animals as outlined above. The domestic rabbit is represented by several races, of which the common variously-colored forms, long-haired Angoras, Lop-Ear Rabbits, and Belgian " Hares" are more familiar. They are all descen- dents of the wild rabbit (Oryctolagus cuniculus, Lepus cuniculus) of Europe. The latter is thought to have belonged originally to the countries bordering the western portion of the Mediterranean, but its distribution has been greatly extended northward and to other continents through human agency. The family Leporidae contains a large number of closely related species formerly included in the single genus Lepus. They are variously known as hares and rabbits, but the latter designation is considered to apply more exactly to the European rabbit and its domesticated races, the others, with one or two exceptions, being more properly described as hares. The more familiar species include the North American Cotton- Tail (Sylvilagui floridanus, Lepus sylvaticus), and the Prairie Hare or Jack-Rabbit (Lepus campesiris) ; the European Common Rabbit (Oryctolagus cuniculus), and Hare (Lepus europceus) . The two European species differ in several well marked features, which form the basis of the accepted distinctions between hares and rabbits. The rabbit is dis- tinguished by its shorter ears and less elongated hind limbs ; also by its burrowing habits, and by the circumstance that the young are born in a blind and naked Condition. The hare is more nearly a running or cursorial type, and is distinguished by its longer ears — which, moreover, are tipped with black — longer hind limbs and prominent eyes. Unlike the rabbit it does not burrow, but inhabits only an open "form," and the young when born are clothed with hair and able to see. Zoological Position. 7 The various species constituting this family are distinguished by- several features, including the imperfectdevelopment of the clavicle, longer ears and limbs, and the presence of a distinct although greatly reduced tail, from the Picas or Tailless Hares (OchodontiilcB) of the mountain- ous districts of Central Asia and of North America (Rocky Mountains). The two families are allied, however, in the possession of a common feature, namely, the presence in the upper jaw of a second pair of incisor teeth. This feature distinguishes the sub-oider Duplicidentata from that of the Simplicidentata, the latter containing the majority of rodents and embracing all forms with a single pair of upper incisors. The mammalian order Rodentia, to which the family belongs, con- tains a very large assemblage of forms — the Squirrels, Marmots, Cavies, Beavers, Mice, and Porcupines being among the more familiar. This order is distinguished by the modification of the anterior incisors in both upper and lower jaws to form chisel-like cutting organs, the teeth having their enamel layer disposed chiefly if not wholly on their front surfaces, so that they remain in a permanently sharp condition. This modification is associated with an extensive obliteration of intermediate teeth, com- prising posterior incisors, canines, and anterior premolars; also with elaboration and often great complexity of the remaining premolar and molar teeth, the lower jaw, and, indeed, the parts of the skull generally. Characteristic of these animals is the extension, both for- ward and backward, of the jaw- musculature. The articulation of the lower jaw exhibits an elongated articular process fitting into a corresponding longitudinal fossa on the skull; the jaw being able to move forward and backward in addition to vertically and from side to side. The teeth are further arcuate in shape, and are provided with open roots, so that their growth is not limited, as it is in the majority of mammals. The rodents are in many particulars primitive types. For example, they tend to retain the five-toed (pentadactyl), plantigrade foot, characteristic of primitive mammalia and, indeed, of terrestrial vertebrates, and exhibit also unelaborated cerebral hemispheres in the brain. In other respects, however, as in the rodent characters above-mentioned and in the elaboration of the intestine, especially the caecum, they exhibit the characters of highly specialized herbivores. Like all higher or placental mammalia (Placentalia), the rabbit is viviparous, the young being retained through a period of gestation in the maternal uterus, to the wall of which they are attached by a vascular con- nection, the placenta. In this feature the placental mammalia differ from the marsupial mammalia (Marsupialia) of Australia and South America, the latter being viviparous, but, with one exception, without placenta; also from the egg-laying mammalia (Monotremata) of Aus- tralia, the latter being oviparous, like the majority of the lower, reptilian forms. These three sub-classes of mammals are united, however, by the common features of the class Mammalia. They are warm-blooded animals, provided with a complete double circulation, and with a hairy investment for the surface of the body. In all, the young are nourished for a time after birth through the secretion of modified cutaneous, milk- producing, or mammary glands. 8 Anatomy of the Rabbit. Many of the more general features of the rabbit are such as are not recognized by group designation, but yet are shared with other terrestrial vertebrates, including mammals, reptiles, birds, and, in part, amphibians. This refers to the development of the lungs and associated respiratory tracts, both the true respiratory tracts and the accessory respiratory passages traversing the skull ; further, the loss of the branchial or fish-type of respiration and the new disposition of the branchial structures; the development of a tri-segmented type of limb with a full complement of muscles, and originally a pentadactyl, plantigrade foot, for support of the body and for locomotion. The regional differentiation of the vertebral column, especially the mobility of the neck, the free occipital articulation, and the definition of the sacrum, the latter associa- ted with the elaboration of the pelvic girdle, are all features of general significance in the terrestrial vertebrates. Finally, the rabbit agrees with other members of the phylum Chordata in the possession of a ground-plan underlying the most general features of its organs, and the position, arrangement, and plan of development of its organ-systems. All Vertebrata or back-boned animals possess an axial skeleton formed by the segmented vertebral column. In a very com- prehensive way they possess as chordates a still more fundamental axial support, the notochord, the latter being an embryonic structure except in the lowest chordates. In a more restricted sense, as Craniota, they possess an organized head region with differentiated brain, special sense organs, and enclosing primary skull. They possess a series of branchial (branchiomeric) structures,"appearing either in the adult condition, as in fishes, or as part of the underlying plan in the embryonic condition; and they add to their general features in the arrangement of the organ-systems the further feature of transverse segmentation (metamerism) of a considerable portion of the body. GENERAL ANATOMY. Although in every respect a continuous structure, the body is differ- entiated into a large number of parts, or organs, the latter being more or less individual in form, composition, or function. Organs are arranged for the most part in systems, each of which is concerned with some general or fundamental function, to which several organs may contribute. In a more general way the body may be considered as an assemblage of tissues. The latter are layers or aggregations of similarly differen- tiated \ cells. They are of several different kinds, and are variously associated in the formation of organs. Being structures of an inter- mediate position they may be considered either as organ components or as products of specialized cells. As a body-unit a cell consists of a small mass of living protoplasm, containing a central body, the nucleus, and surrounded or enclosed on its free border by a cell -membrane. The nucleus is a highly organized body, having an important function in the reproduction of the cell and Epithelial Structures. also in its general activity or metabolism. It contains a characteristic formed material, chromatin, and frequently also a minute spherical body, the nucleolus. The chief features of a typical cell are illustrated in the accompanying figure (1) of the developing ovum, the latter being a single cell, noteworthy for its large size, and also one in,which the external form is not greatly modified, as it is in the majority of the cells of the body. Its enclosing membrane, the zona pellucida, by which in its natural position in the ovary it is separated from the surrounding follicular cells, is considered to belong in part to the latter. As fundamental living matter, protoplasm possesses certain properties on which the functions of the body ultimately depend. Considered collectively, these functions are not so well illustrated in the higher or multicellular organisms, in which particular functions are assigned to particular cells, as in the lower unicellular organisms, in which all func- tions are discharged by a single cell. In simple or protozoan animals the protoplasm is seen to be capable of ingesting food-materials, of discharging waste, of changing its form, and of reacting in one way or b.{ another to stimuli arising outside of the body. Moreover, the protozoan cell is capable of giving rise to new cells by division of its substance into two parts, which process originates in the nucleus, and is associated at some stage, usually at least, with union or conjugation of parent cells. All the cells of the body of a multi- cellular organism are products of a single cell, the fertilized egg, but the latter is a product of fusion of two primary elements, the spermatozoon of the male parent and the ovum of the female. The fertilized egg does not exhibit the functions of a one-celled body, but possesses the potential of these functions, and the latter appear, to a large extent individually, in the differentiation of its division products into special- ized tissue-elements. The primary tissues of the body are of four kinds — epithelial, con- nective, muscular, and nervous. To these — the fixed tissues — are to be added the fluid substances, blood and lymph, in which the cell elements, the red and white corpuscles, or in the latter case the white elements alone, are suspended in a fluid medium. Fig. 1. Developing ovum of the rabbit. From a section of the ovary: chr., chromatin; n.m., nuclear membrane; p.f., cells of the primary ovarian follicle ; z. p., zona pellucida; pr., protoplasm. 1. Epithelial Tissues. Epithelial tissues are distinguished chiefly as surface investments, such as those of the exterior of the body, the interior of the alimentary canal, the lungs, the respiratory and accessory respiratory tracts, and the 10 Anatomy of the Rabbit. ducts of the urinogenital organs. In all epithelia the cellular feature is a prominent one, and it is largely for this reason that as lining membranes they are not conspicuous in gross structure. They are noteworthy, however, for their products, the hairs and the various kinds of secreting organs or glands. > The epithelium of the skin (Fig. 2, ep.) is known as the epidermis or scarf-skin. Although composed of several layers of cells, it forms an exceedingly thin membrane, extending over the surface of the body and connecting at certain points with the epithelia of the internal surfaces. It is supported by a layer of connective tissue which forms the true skin or corium. The epithelium of the internal surfaces forms the chief portion of the mucous membranes, and in the greater portion of the alimen- tary tract the epithelial layer is asso- ciated with a thin layer of smooth muscle to form a mucous tunic (Fig. 15, t.ms.). The coating of hairs on the surface of the body, the presence of which is a notable mammalian feature, is a protective investment arising from the epidermis. A hair is produced by the modification of the central por- tion of an ingrowth of the epidermis, termed the hair follicle (Fig. 2, f.). The latter contains at its base a small elevation of the underlying vascular connective tissue, the hair papilla, through which the structure is nourished. On the general surface of the body the hair follicles are arranged in groups (Fig. 4), and on the lips certain large follicles are set apart for the production of the greatly enlarged sensory hairs or vibrissae. Connected with the hair follicles are thin strands of smooth muscle, the arrectores pilorum (Fig. 2, a. p.). They are placed in the broad angles formed by the inclined follicles with the corium, and their contrac- tion throws the hair into a more nearly erect position. Epithelial glands are ingrowths of the general layer, the cells of which become greatly modified as secreting structures. The lumen or cavity of the gland, in most cases greatly complicated through the division of the gland substance, is connected with the general surface by a duct which thus serves to carry away its secretion. In some cases the con- nection of a gland with the epithelial surface is embryonic, and in the -+ 1 Fig. 2. From a section of the upper lip of a four-day-old rabbit: a. p., erector muscle of the hair fa.p. 1 ', is that of the adjacent fol- licle); c.t., connective tissue of the corium; ep., epithelium; f., hair follicle; s., hair shaft; s.g., sebaceous gland. Epithelial Structures. 11 adult condition the gland is found separated from the epithelium from which it was originally formed. This condition is represented by the thyreoid and thymus glands of the neck and thorax respectively. In other cases the secreting element is a single cell, the latter thus repre- senting a unicellular gland lying directly in the general layer. The mucus-secreting goblet cells of the intestinal wall are structures of this nature. The majority of multicellular glands conform to one of two types, namely, the tubular gland, in which the secreting portions are of uniform calibre, and the acinous or alveolar gland, in which the secreting portions are sacculated (Fig. 3, A). Both types occur in simple, little branched and greatly branched conditions. Cutaneous glands of two types are commonly present in mammals in association with the hairs, namely, sudoriparous or sweat-glands, which are glands of the tubular type, and seba- ceous glands, which are of the acinous type. In the rabbit, glands are absent from the general surface, but are found in special situations, as, for example, in con- nection with the hair follicles of the lips, the internal surface of the ear, and the external genital organs. The inguinal glands comprise both tubular and acinous portions. The mammary glands of the female are greatly modified cutaneous glands of an acinous type. Apart from the mucus-secreting cells of the general epithelium, the glands of the alimentary canal comprise the important but less elaborated glands of the wall, such as the gastric glands of the wall of the stomach, and the greatly elaborated, out- standing glands which lie beyond the wall and are connected with the interior of the canal only through their ducts. The latter comprise the oral glands, the liver, and the pancreas. The oral glands include chiefly the submaxillary, parotid, sublingual, and infraorbital glands — conspicuous structures in the dissection of the surrounding por- tions of the head and neck. Their ducts communicate with the cavity of the mouth. There is a close association between the epithelia of the surface of the body and the nervous tissues, arising from the circumstance that they are derivatives of a common embryonic layer, the ectoderm. In the adult we may distinguish as sensory epithelia special aggregations of cells lying either in a deep or superficial position, and associated more or less closely with the central nervous system. They comprise the olfactory epithelium of the nasal cavity, some of the cells of which are true nerve cells, the Fig. 3. A, Diagram of a multi- cellular gland: al., alveolar type; d., duct; ep., general epithelium; t., tubular type. B , The pancreatic duct and associated portion of the duo- denum in the rabbit. 12 Anatomy of the Rabbit. gustatory epithelium of the tongue, and the auditory epithelium of the membranous labyrinth of the ear. The retina — the nervous portion of the eye — is a modified portion of the central nervous system. As linings of surfaces, the ordinary epithelia may be distinguished from certain special coverings of internal spaces, the endothelia and mesothelia. The latter consist microscopically of thin pavement-like cells. They differ from epithelia in origin, being formed, not in con- nection with originally free surfaces, but in relation to spaces of the mesoderm or intermediate layer of the body. Endothelia form the linings of bloodvessels and lymph canals, while mesothelia are the chief layers of the smooth, moist serous membranes which line the peritoneal, pleural, and pericardial cavities. 2. Connective Tissues. The connective tissues form the supporting elements of the body.. As ordinary connective tissues they serve to connect organs or parts of organs together, and as skeletal tissues they provide the rigid framework or skeleton from which all soft parts of the body are suspended. They are distinguished by the presence of two main components — the cell basis, and the intercellular substance or matrix. The cellular portion is formative, and is much more conspicuous in the embryonic than in the adult condition. All connective tissues are products of an em- bryonic tissue, the mesenchyme (Fig. 20, ms.), which consists of branched cells con- nected together by their outstanding pro- cesses. Through the activity of the cells there is formed an intercellular material consisting either of a homogeneous matrix, or more frequently a matrix containing formed elements of a supportive nature. -m.c.m. Fig. 4. From a section of the skin of the side of the body of an adult rabbit. X about 10; showing the grouping of the hair-follicles: co., corium; ep., epithelium; m.c.m., cutaneus maximus muscle; t.s., subcutaneous tissue. A. Ordinary Connective Tissues. In the adult condition the ordinary connective tissues, with few exceptions, consist of the cell basis with two kinds of fibrous elements, the white, and yellow fibres (Fig. 5). White fibres are elements of great strength. They are fine, unbranched fibres which do not occur independently, but are joined in a parallel fashion with one another, forming in this way fibre bundles of larger or smaller size. The yellow fibres are of greater diameter. They branch and anastomose, but are not asso- ciated to form bundles. They also differ from white fibres in being highly elastic. The tissue produced in this way is known as fibrous connective tissue. It occurs in several forms according to the relative Connective. Tissue Structures. 13 -a-/- Fig. 5. Areolar connective tissue (subcutaneous tissue) of the rabbit; from an embalmed specimen: c.c, connective tissue cell; w.f., bundle of white fibres; y.f., yellow elastic fibre. concentration of the two kinds of fibres or the admixture of other materials. The commonest kind of fibrous tissue in the adult is that described as areolar. It is characteristic of the sub- cutaneous tissue (Fig. 4) which connects the skin with the body ; but areolar tissue occurs also in various positions where it has a similar function of joining structures loosely together. Subcutaneous connective tissue is a white material, the peculiar appearance and properties of which are due to the fact that the two kinds of fibrous elements are arranged in a loose felt-like network (Fig. 5). When stretched it is found to yield up to a certain point, beyond which it is tough and resistant. It tends to regain its original shape when the tension is removed. Fibrous connective tissue may be greatly modified through the concentration of either one of the fibrous elements. Con- centration of white fibres is most common. This condition is illustrated in the thick connective tissue layer forming the true skin or corium (Fig. 4), but is more conspicuous in the glisten- ing white tendons (Fig. 6) by which muscles are attached to bone surfaces, in the ligaments uniting bones with one another, and in the thin, broad aponeuroses which serve for muscular attachment. The structures known anatomically as fasciae are special sheets of connective tissue covering chiefly individual muscles or muscle groups. Fat or adipose tissue is a soft form of con- nective tissue in which the cells are greatly enlarged and contain fatty material in the form of globules. It tends to occur in certain situations and in particular association with the bloodvessels, but otherwise is found in situations where areolar connective tissue might be expected to occur. Special fat masses, sometimes distinguishable by their darker coloration, occur at the side of the neck in the rabbit. In the foetus (cf . Plate VI) large masses of vascular connective tissue are found in this situation, and are probably similar in origin to the storing or hibernating glands of certain other mammals. Coloration or pigmentation of certain portions o£ ^e t 6 endon° ?f originTf of the body, especially of the skin and hairs, and the biceps muscle: m.f., . , , . ' K. , i 1 • ■ c ,, •-, muscle fibres ending on the of the retina, ciliary body, and iris of the eye, is due tendon; t., fibrous con- partly to the presence of special connective tissue Zn™ tUsue ° f the ten " 14 Anatomy of the Rabbit. Fig. 7. Hyaline cartilage: cell; 1., lacuna; m., matrix. .1. cartilage cells, chromatophores, and partly to the presence of pigment granules in epithelium. The absence of pigmentation in animals belonging to species normally colored — albinism — is indicated by the white coloration of the hair and by the pink coloration of the eyes, the latter being due to the circumstance that the bloodvessels of the vascular tunic are not concealed by pigment. B. Skeletal Tissues. The skeletal tissues are solid forms of connective tissue which, on account of their more permanent shape, are better adapted to form a "> support for the body. They are pr~ ----- — -^g^—--- .. ; ,~i — m. of two kinds — cartilage and bone. Simple or hyaline cartilage (Fig. 7) is a semi-solid and some- what resilient material of a bluish or pearly coloration. It consists of a homogeneous matrix in which the cells are imbedded. The cells are distributed singly, or more often in groups of two to four, each group being contained in a small oval space, the cartilage lacuna. The size of the spaces, and also their distance apart, is subject to great variation. The addition of fibrous elements, usually of white fibres, produces a modification known as fibrocartilage. White fibro- cartilage occurs in certain situations, as in the symphysis of the pelvis, or in connection with the interarticular menisci and at the capsular margins of the joints. In the adult skeleton cartilage is present only in small amount. It forms the articular surfaces of joints, the ventral portions or costal cartilages of the ribs, and a portion of the nasal septum ; it is also found uniting the basal bones of the skull. In the embryo, how- ever, it forms the entire skeleton, with the exception of a small por- tion which, as described below, is formed of membrane bone. In the course of development, except in the situations indicated, the carti- lage is replaced by bone. Bone is a compact, resistant, but yet somewhat elastic tissue, possess- ing much greater strength than cartilage, and therefore forming a more perfect skeletal support. As indicated below, its appearance as a tissue differs somewhat according to its mode of formation. The more typical structure (intramembranous bone) is illustrated in the accompanying figure (8) of a ground transverse section of the dried shaft of the femur. Fig. 8. Intramembranous bone; from a ground section of the rabbhVs femur: c.l., circumferential lamellae; h.c, Haversian canal; h.l., Haversian lamellae; i.l., inter- stitial lamellae; lac., lacuna. Skeletal, Structures. 15 The bone' materials are deposited in layers or lamellae, which are com- parable to highly modified white fibres of fibrous connective tissue. The lamellae enclose between them greatly branched spaces, the lacunae, in which in the natural condition the bone-cells are accommodated. On account of certain differences in development, the lamellae are disposed in different ways. Some are disposed concentrically around longitudinal spaces, the Haversian canals, forming in this way the so-called Haversian systems. The canals are occupied in the natural condition by bloodvessels. Others of them, the interstitial lamellae, are disposed in a somewhat parallel fashion between the Haversian systems ; while a third series, the circumferential lamellae, are disposed in a parallel fashion with respect to the periphery or the internal surface of the bone. In the natural condition the bone is enclosed except on its articular surfaces by a layer of con- nective tissue, the periosteum. During the period of growth this membrane contains large numbers of bone-forming cells, the osteoblasts, through the activity of which the deposition of the bone lamellae is accomplished. It is only in a few cases that the bones of the skeleton may be looked upon as solid structures — as a rule they consist of a fairly thin shell of hard or compact bone surrounding a central mass of spongy or cancellous bone. This arrangement is one of great mechanical strength, combined with lightness, and at the same time provision is made in the interior of the bone for bloodvessels and marrow spaces. Thus in a long bone (Fig. 9) the central portion or shaft consists of a cylinder of com- pact bone surrounding an extensive space, the marrow-cavity. The ends or extremities consist each of a thin shell of compact bone continuing that of the shaft and surrounding a mass of can- cellous tissue. In the short, flat, or irregular bones of the skeleton no continuous marrow cavity is formed. In point of origin bones are of two chief kinds — membrane or derm bones, and cartilage or replacing bones, but a third group is represented by the tendon or sesamoid bones which are developed in the tendons of muscles. The mem- brane bones are few in number. They comprise the roofing and facial bones of the skull, most of which are distinguished by their flattened, tabulate character. They are formed in connective tissue membranes, and although they sometimes contain cartilage they are not formed on a cartilage basis. Moreover, they are elements lying in a superficial position with respect to the skeleton proper. They owe this condition, and also their designation as derm bones, to the circum- stance that they represent surface plates which in lower vertebrates are associated with the skin. "\ Fig. 9. Divided femur of the rabbit: at., can- cellous tissue; e., distal epiphysis; e.l., epiphysial line; m.c., marrow-cavity; s.. shaft. 16 Anatomy of the Rabbit. Cartilage bones, on the other hand, are the characteristic elements of the skeleton. They are known as replacing bones because they are formed on a cartilage basis, the latter, as indicated above, representing the primary cartilage skeleton of the embryo. In the course of develop- ment, except in certain regions where the cartilage persists throughout life, the cartilage material is replaced by bone, which tends to surround and also invade it. The way in which bones are formed on a cartilage basis serves to explain many peculiarities of the adult skeleton. In the embryonic condition the cartilage rudiments are associated to form a complete but primitive skeleton. In many cases the replacement of these elements by bone is not direct, certain readjustments being necessary, both for pur- poses of growth and on account of the much more special functional requirements of the adult skeleton. In the embryonic condition the cartilage rudiments are enclosed by a connective tissue sheath, equivalent to the periosteum of a bone, but described as the perichondrium. The osteoblasts of this layer are concerned with the formation of bone material, both in the interior (endochondral bone) and on the surface (intramembranous bone). The forma- tion of endochondral bone is preceded by certain changes which take place in X rr .X-—/( A^ *■ mter i° r °f tne cartilage. In the latter, in certain areas, known as the '■ttu. centres of ossification, the matrix becomes partly dissolved, the cells en- larged and ultimately broken down. These changes are associated with a fig. to. Outline sketch of the proximal deposition of calcareous material, or P ^„cta^epn?h u y ^ ?<2°S£? htTof \t calcification, by which the portion of the femur. The accessory epiphyses are for cartilage undergoing transformation is the great (tr.ma.). lesser (tr.mi.). and ., , ,, , t , .-, • third (tr.t.) trochanters. temporarily strengthened. Into this , area the active cells of the perichondrium are carried through the agency of vascular ingrowths, the periosteal buds, and the result of their presence is the deposition of bone material in association with the remaining portions of the matrix. This condition is partly illustrated in the distal epiphysis of the humerus shown in Fig. 12, the figure being that of a vertical section of the elbow-joint of a four-day-old rabbit. In the long bones the formation of the first or main centres of ossifica- tion takes place in the shaft, and there are formed afterwards accessory or epiphysial centres for the extremities. A divided extremity, such as the proximal end of the femur (Fig. 10), may possess several such centres — a principal. one for the chief epiphysis or actual extremity of the bone and several subsidiary centres for its outstanding processes. In the shaft the formation of endochondral bone is of short duration. Through the activity of the osteoblasts lying directly in the perichondrium, or later the periosteum, a process of formation of intramembranous bones goes on during the whole period of growth, and the result of the peripheral Skeletal Structures. 17 deposition of bone lamellae is, as described above, that the transverse diameter of the bone is greatly increased. The enlargement of the marrow cavity, with which this is associated, is produced by the absorp- tion of bone from the interior. In young animals both the epiphysial centres and the masses of cartil- age in which they are formed are sharply marked off from the body of the bone (cf. Figs. 12-13). This is largely because the formation of the epiphysial centres tends to lag behind that of the main centres, and thus the cartilage extremities of the bones are evident long after the forma- tion of the shaft is under way. In the epiphysial centres the bone formation is endochondral. The bone masses which they form are distin- guished as epiphyses. During the period of growth they are connected with the body of the bone by plates of epiphysial cartilage, into which the surrounding perichondrium extends as an ossification ridge. In this region bone formation takes place, with the result that the whole structure is greatly increased in .length. After the period of growth, the duration of which differs in different bones, the epiphyses become firmly coossified with the body of the bone, although the lines of junction or epiphysial lines may still be visible. Thus in the distal extremities of the radius and ulna, in the proximal extremities of ? °' the fibula, or in the bodies of the lumbar vertebrae, the epiphysial lines appear even in old animals. In the foregoing figure jM$t '-t >u \ \ - c ^- (9) of the divided femur it will be seen that the position of the epiphysial lines is indicated by bands of compact ^ ffl[ f.m. ^ ||.....ex.D.. tissue. Finally, in thoroughly macerated bones of young animals, the epiphyses are usually found to be readily separable from the bones. In a comparison of the adult skeleton FlG u The occipital portion rf the With the more primitive embryonic skull in a three-day-old rabbit: b.o., skeleton, several differences in the arrange- cartfiagfrf 1 chondrocra°ni'um° CCiP c'o a ! ment of the elements are evident. Thus occipital condyle; f.m foramen magnum; ex.o., exoccipital; s.o., many bones, notwithstanding their pos- supraorbital. session of several centres of ossification, are to be looked upon as individual either in the cartilage or in the bone condition. In other cases, as in the basal portion of the skull, separate bone elements are produced in a mass of cartilage primarily continuous. These either remain distinct throughout life, or, as in the occipital region, (Fig. 11), become fused together to form compound or composite bones. In still other cases, as in the vertebrae, the apparently single elements of the adult condition are the products not only of originally distinct bones, but also of primarily separate cartilage masses. The bones of the skeleton are united or articulated with one another by connective tissue in the form of ligaments, by cartilage, or in some cases by both together, i.e., by fibrocartilage. Ligamentous union, distinguished as syndesmosis, is the most general type of articulation. 18 Anatomy oi? the; Rabbit. Cartilage union or synchondrosis occurs in certain situations, as in the basal region of the skull. Union by fibrocartilage or symphysis is characteristic of the articulation of the two sides of the pelvis (symphysis pubis). The articulations of bones are of two types — immovable articulations or synarthroses, and movable articulations, diarthroses, or joints. In the former, motion is either absent or at least greatly restricted. In the latter, it is definitely provided for through the presence of joint-structures. Thus in a joint (Fig. 12) the apposed surfaces of the bones are accurately modelled in relation to one another, and are moreover covered by a layer of cartilage, the latter forming a joint cushion. Between the two surfaces is a space, the cavity of the joint, containing a viscid material, the synovia, which serves for lubrication. The space is enclosed by a connective tissue capsule. The strength of the joint depends largely on the enclosing capsule, but it is usually greatly increased by the presence of accessory ligaments. In the more complex joints, such as that of the knee Ftg 12. Section of the elbow-joint of a four-day-old rabbit: c, capsule; e.b., endochondral bone in the distal epiphysis of the humerus; e.a., extensor muscles of the forearm; e.m., extensors of the hand; f.a., flexors of the forearm; f.m., flexors of the hand; h., humerus; ol., olecranon; r.. radius; s.c, synovial cavity; u., ulna. (Fig. 13), interarticular cartilages (men- isci) are enclosed between the bone surfaces, and the latter are connected Fig. 1.1. Section of the knee-joint .of a four-day-old rabbit: c., capsule; f., distal epiphysis of femur; l.p., patellar ligament (insertion of quadriceps femoris); p.v., popliteal vessels and nerves; t., proximal epiphysis of tibia; +., ligament of the lateral meniscus: x., posterior cruciate ligament; xx , anterior cruciate ligament. directly by short ligamentous cords. The various ligaments of a joint permit free motion of the bones, but only up to a certain point. Several differences are observable in joints according to the form of the apposed surface and the kind of motion provided for. Thus in the ball-and-socket joint or enarthrosis, exemplified by those of the shoulder and hip, a bone is able to move in various directions about its base of attachment, although actually, in the limbs, this motion is almost restricted to an anteroposterior direction. In the ginglymus or hinge- joint, as exemplified by the distal articulations of the limb, motion is Muscular Tissues and Muscles. 19 restricted to a single plane. The gliding joint or arthrodia is one in which a slight degree of motion is made possible by one surface slipping over the other ; it is exemplified in the accessory articulations of the vertebral arches. 3. Muscular TisspES. Muscular tissues are the active portions of the individual muscles of the skeleton and of the muscle coats of visceral organs. Their chief feature consists in the elongation of the cells to form fibres. These fibres may be considered to possess the contractile properties of proto- plasm, but with the contraction limited to one direction. Except in a few cases the fibres are arranged in a parallel fashion, so that the line of contraction of the muscle or muscle layer is the same as that of each of its fibres. The result of contraction in both is the shortening of the longitudinal axis and the increase of the transverse axis. Two chief types of muscle fibres occur in the body — the smooth or unstriated fibres, which are characteristic of the involuntary muscles or muscle coats of the visceral organs, or of the skin, and the striated fibres which compose the individual or voluntary muscles of the skeleton Smooth fibres (Fig. 14 B) are elongated, spindle-like cells, the substance of which is longitudinally striated, but possesses no transverse markings. The single nucleus of the cell occupies 'a central position. The. muscles which they form are dis- tinguished as involuntary because their operation is not under the con- trol of the will, their connections being with the sympathetic nervous system. The striated fibres (Fig. 14 A) are very much larger, cylindrical structures, the substance of which possesses characteristic transverse striations. Each fibre is enclosed by a loosely attached membrane, the sarcolemma, on the inner surface of which many nuclei occur.* The presence of these nuclei indicates that the fibre is not a single cell but a syncytium, i.e., an association of cells unseparated by cell boundaries. The muscles formed by such fibres are under the control of the will, their connections being directly with the central nervous system. They comprise not only the typical muscles of the skeleton, but also the special muscles connecting the skeleton with the skin. The muscular substance of the heart differs both from striated and smooth muscle in being composed of branched anastomosing fibres, I ! I I Fig. 14. A, Striated (skeletal) muscle of the rabbit. B , Unstriated mubcle from the muscular tunic of the intestine. *The position of the nuclei is- characteristic of the so-called white muscles. In the semitendinosus of the rabbit, which is a red muscle, the nuclei occur between the fibril bandies of the interior of the fibre. 20 Anatomy of the Rabbit. which apparently form a continuous network. Like striated muscle, it possesses characteristic transverse markings, but, like involuntary muscle, it is under the control of the sympathetic nervous system. As gross structures the voluntary muscles are functional units, each of which has a particular action according to the movement per- mitted by the parts of the skeleton to which it is attached. They present a longitudinal striation which is roughly referred to as the direc- tion of the fibres, and which is of great value in identification. The striation is due to the circumstance that the fibres are arranged in parallel groups or muscle bundles, each of which is surrounded and separated from the adjacent bundles by a connective tissue enclosure, the perimysium. A muscle is typically spindle-shaped, consisting of a middle fleshy portion, termed the belly of the muscle, and of tapering ends which provide for attachment. The attachment is effected by a strong band of fibrous connective tissue, the muscle tendon (Fig. 6). In contraction it is usually seen that one end, thus designated as the origin of the muscle, remains stationary, while the other, known as the insertion, assumes a position nearer the origin, carrying with it either the structure which is to be moved, or another portion determined by the point of leverage or the position of the joint. The action of the muscle is estimated in a direct line between origin and insertion, although the effect of the contraction, on account of the position of the joint, may be otherwise. From these more typical conditions, however, ■ many modifica- tions occur. Some muscles, such as those of the abdominal wall, are disposed in the form of flattened sheets, the ends of which are attached by broad, thin sheets of connective tissue, the apon- euroses. In unipennate muscles the fibres are attached obliquely to the side of the tendon, or in bipennate muscles to both sides, like the vane of a feather. In the so-called biceps, triceps and quadriceps muscles of the limbs, the origin is divided into two, three or four portions. Furthermore, the recognition of origin and insertion depends on usual but not invariable relations. The exact effect of muscle contraction also depends as a rule on the synchronous action of other muscles. A muscle like the diaphragm does not possess an insertion after the fashion of ordinary muscles; and in some cases, as in the intrinsic muscle of the tongue or the so-called orbicular or sphincter muscles, both origin and insertion may be absent. Involuntary muscle is distinguished by its white or greyish coloration and by its smooth or homogeneous appearance. It forms characteristic Fig. 15. From a section of the pyloric end (antrum pyloricum) of the stomach of the rabbit: m.m., muscularis mucosae; t.m.c. circular layer of the muscular tunic; t.m.l., longitudinal layer of the muscular tunic; t.ms., mucous tunic; t.s., tela submucosa; t.sr., serous tunic. Nervous Tissues: 21 • layers in connection with visceral organs or with the skin, and is thus . much less individual than the voluntary muscles in its relations to particular parts. It forms the muscular portion (muscularis mucosae) of the mucous tunic of the alimentary canal, and also a separate muscular tunic lying in the outer portion of its wall (Fig. 15). In the muscular tunic the fibres are arranged in both circular and longitudinal directions. Involuntary muscle also forms a small constituent of many organs, such as certain glands, in which contractility is not a chief function. It forms a large constituent of the wall of the urinogenital tubes, particularly the bladder and the uterus. In association with elastic connective tissue it is an important constituent of the walls of the bloodvessels. Although there is an underlying community of structure in the walls of the bloodvessels, the two chief types of vessels, arteries and veins, present conspicuous differences, both in functional behaviour, and in their appearance in the dead animal. The differences are largely the result of differences in the relative amounts of the above-mentioned constituents. The arteries are thick-walled, elastic tubes, which, under the force of blood from the heart, first become greatly Expanded, and then gradually con- tract, so that the blood is forced into the smaller capillary vessels. The veins on the other hand are thinner- walled, less elastic vessels, through which the blood is forced largely through the pressure from behind. In the dead animal the arteries appear white, flat or collapsed, and Pnir ,t T Ttnp -17-pin = nn t)n> ntfipr bnH Fig. 16. Nerve-cell from the anterior grey empty, ine veins on tne otner nana column o£ the spinal cord {ri F!g lg) . di _ appear large and dark On aCCOUnt Of dendrites; e.g., chromatophUe granules; their distension with blood. 4. Nervous Tissues. Nervous tissues form the basis of the central nervous system and of the outlying nerves and ganglia. They comprise two kinds of elements — nerve cells and nerve fibres. In the central nervous system these elements are imbedded in a mass of neutral tissue, the neuroglia. Nerve cells are characteristic of the central nervous system and of the spinal and sympathetic ganglia. They differ greatly in form, but typically each consists of a cell-body (Fig. 16) bearing two kinds of processes — a fibre-process, the neuraxis or neurite, and a series of branched protoplasmic processes, the dendrites. The cell-body is distinguished by the presence in its interior of granular masses, the chromatophile or tigroid bodies. The latter extend into the dendrites, but not into the neuraxis. The dendrites may be greatly elaborated, and may be present to a considerable number. The neuraxis is a nerve.fibre process. Since it continues as the central portion or axial cord of a nerve fibre, it may traverse a relatively enormous distance on its way to a peripheral organ. Ti r 22 Anatomy of the Rabbit. A nerve fibre consists of a central core, the axial cord, enclosed, except in the case of those of the olfactory nerve, by certain membranes. Two kinds of fibres are distinguished — medullated fibres, and non-medullated fibres. The former are characteristic of the peripheral nerves. In these (Fig. 17) the axial cord is surrounded by a comparatively thick membrane of fatty material, the medullary or myelin sheath. The latter is continuous except at certain points, the nodes of Ranvier, where the axial cord appears free except for an external investment of the whole fibre, the neurilemma. In the non-medullated nerve fibres the myelin sheath is lacking. This type of fibre is chiefly distributed in connection with the sympathetic system. A nerve is an association of nerve fibres, the latter being disposed in a parallel fashion and united together into bundles of larger or smaller size by connective tissue, which also forms a general peripheral investment, the epineurium. The dead-white coloration of a nerve is due to the fatty materials of the myelin sheaths, but nerves are commonly found imbedded in a fatty con- nective tissue which is associated with the epineurium and is also of white coloration. Nerve fibres, and also nerves, are distinguished functionally as afferent and efferent. They are organs of conduction, which carry impulses either from the peripheral parts of the body to the central nervous system, or in the opposite direction. Sensory nerves are afferent, while motor nerves SI 7>P- are. efferent. Nerves, however, usually contain both afferent and efferent fibres and are hence described as mixed. In the distribution of afferent and efferent fibres there is a marked difference between the external or somatic por- tions of the body and the internal or visceral portions. Consequently, both somatic and visceral kinds of afferent and efferent fibres are con- veniently distinguished. On account of certain diff- erences in coloration, the cellular and white fibrous* constituents of the central nervous system produce characteristic patterns where either one is Fir. 17. Portions of two meoullated nerve - fibres from the sciatic of the rabbit; a.c, axial cord; m.s., v myelin . sheath, stained black with osmic acid; n., neurilemma; n.r., node of Ranvier. Jim.Q Fig. 18. Section of the spinal cord of the rabbit: c , central canal ; f.m. a., anterior median fissure; s.m.p., posterior median sulcus; f.a., f.L, and f.p., anterior, lateral, and posterior funiculi of the cord; e.g. a. and c.ff.p., anterior and posterior grey columns (horns of grey matter); r.a., and r.p., anterior and posterior nerve roots; s.a., white substance. Terminology. 23 concentrated. Thus the cellular material is greyish, and is hence distinguished as the grey substance, while white fibrous material pro- duces when concentrated an opaque white appearance similar to that seen in the peripheral nerves, and is hence described as white substance. In the spinal cord (Fig. 18) the grey substance is disposed as a central core, the white substance as a peripheral investment. A similar relation is found in the basal portion of the brain, but the characteristic pattern in the cerebral hemispheres and in the cerebellum is one in which the grey substance forms a peripheral, investing, or cortical layer. TERMINOLOGY. In special or descriptive anatomy it is necessary to employ an extensive system of terminology in order that the various structures of the body may be individually designated, classified, and referred to their respective positions. The terms used for this purpose may be classified into four groups, as follows: (1) general terms — those included in the names of parts, but applicable in themselves to similar structures (arteries, nerves, etc.) in various parts of the body; (2) specific terms or names of parts ; (3) regional terms — those defining areas (topographic) ; and (4) terms of orientation. Although few in number, the terms of orientation may be regarded as the most generally useful terms of descriptive anatomy. This is because they are based on very general relations of the body, and are therefore of wide application. For this reason they are here selected for definition to the exclusion of others of a more restricting or individualizing kind. In all vertebrates we may recognize a longitudinal axis, corresponding, in general, to the line formed by the vertebral column. In the usual or prone position of the body this axis is horizontal. The uppermost surface is then described as dorsal, the lowermost surface as ventral, the sides of the body as lateral. Any position forwards, with respect to the long axis, is anterior in comparison with any position backwards, which is posterior. In relation to the long axis it is convenient to recognize a median vertical plane of section, which is one dividing the body into right and left halves; also transversal planes, which are planes situated at right angles to the median plane and to the long axis, and sagittal planes. The latter are vertical planes parallel to, and also including (as mid- sagittal), the median vertical plane. The median vertical plane is the plane of bilateral symmetry, each half of the body as thus defined being in a general way the reverse counterpart of the other. Structures situated in part in the median plane are unpaired, and are described as median, while structures situated wholly outside of the plane are paired, right and left, or dextral and sinistral. In relation to the median plane and to the sides of the body, structures are described as medial when nearer the former, and as lateral when nearer the sides of the body. The term intermediate is applied 24 Anatomy of the Rabbit. especially to a position between medial and lateral, but this restriction is perhaps not justifiable. In considering the extent of bilateral symmetry, it is necessary to bear in mind that, although a fundamental feature in vertebrates, it is not perfectly retained in the adult condition. Symmetry is destroyed by the migration of an unpaired structure from a median to a lateral position, as is seen, for example, in the case of the abdominal portion of the ali- mentary canal ; or, again, by the reduction or disappearance of structures belonging to one side of the body, as, for example, in the case of the mammalian aortic arch. Referring to centre and circumference, either in the body as a whole, or in particular parts, the terms deep and superficial, central and periph- eral, or internal and external may be applied. It may be observed, however, that the terms internal and external are sometimes used in the sense of medial and lateral, both in descriptive language and in the. names of parts. In comparison with the terms medial (medialis) and median (medi- anus) the term middle (medius) may be used to designate the position of a structure lying between two others, the latter being otherwise desig- nated, for example, as anterior and posterior, or one in the median plane. The limbs being more or less independent structures, it is proper to apply to them certain terms otherwise applicable to the main portion of the body. The chief terms not properly applied elsewhere are proximal, meaning nearer the centre or base of attachment, and distal, toward the extremity. In the middle segment of the fore limb the respective positions of the bones (radius and ulna) are indicated as radial and ulnar. The terms tibial and fibular are also applicable, although with less reason, to the, corresponding segment of the hind limb. The dorsal and ventral surfaces of the fore foot are described respectively as dorsal and volar, those of the hind foot as dorsal and plantar. In determining the identity of structures in a quadrupedal mammal, considerable difficulty may at first arise from the fact that descriptive terms, such as those just defined, are frequently included in the names of parts, the latter being, at the same time, terms applied in the first instance to the human body, in which the recognized relations are some- what different. In comparison with that of a quadrupedal vertebrate the human body occupies a vertical or erect position, and is to be con- sidered as having been rotated upward through ninety degrees on the posterior limbs. The latter accordingly occupy for the most part their original position, and the human arm indeed largely reassumes this position when allowed to hang freely at the side of the body. As in all cases, the face retains its forward direction. Thus the terms anterior and posterior as used in human anatomy mean dorsal and ventral, provided they refer to parts of the body, such as the entire trunk region, which have been affected by rotation. The terms superior and inferior as applied to man are similarly anterior and posterior as applied to a lower form. Since it is unwise to change the form of the official terms of human anatomy, it becomes necessary to interpret all such terms when used for a quadrupedal mammal according to the relations exhibited by General Plan of Organization. 25 man. On the other hand, in ordinary description little advantage is to be gained from adherence to this principle. The terms anterior and posterior apply with greater force to a lower vertebrate than to man, while the terms superior and inferior are only of interest in the latter. In this case the rule here followed is to use the terms anterior and pos- terior for descriptive purposes without reference to the human relation. The same applies to the terms of direction — upward, downward, forward, and backward. It may be pointed out, however, that it has become the practice with some to employ in place of anterior and posterior, such terms as cephalic and caudal, thereby eliminating one of the difficulties; or similarly to use the termination — ad, in connection with words sig- nifying position, for the purpose of indicating direction {e.g., dorsad, cephalad, laterad, for dorsalward, etc.). Reference may also be made here to the fact that the human structures to which identifying names are applied frequently fail in one way or another to correspond to structures in a lower form. Composite structures to which individualizing names are applied, for example, may be represented by independent parts. Also, structures which are similar in form or function may be convergent. Finally, although it is essen- tial to endeavor to apply all terms as accurately as possible, it will be remembered that a terminology primarily arranged for one type cannot be exactly applied to another without considerable qualification. THE GENERAL FEATURES AND GROUND PLAN OF THE ORGAN SYSTEMS. In order to arrive at a proper understanding of the special features of organs, or of the relations to one another of organ groups, it will be found useful in many cases to consider them in the light of their deriva- tion from a general or ground form. A brief statement of what may be accomplished in this way and of the methods involved has already been given above (p. 4) in discussing the general interpretation of structure and the zoological position of the type, so that in the following pages only the actual facts of organization will be considered. In this connection it will be remembered that the subject is a very broad one, and in the present case it will not be possible to do more than select in the various organ systems the more important features, the con- sideration of which will be of direct assistance in the practical study of the type. Before taking up the individual organ systems, however, it will be advisable to refer in the first place to the plan of classification, and secondly to explain the more fundamental features of vertebrate organization as something underlying the disposition of the systems themselves. It will also be convenient to summarize the chief features of the embryonic organ systems, since it is by reference to embryonic conditions that the general features of the body are most readily recog- nized. 26 Anatomy of" the Rabbit. f r " Classification of the Organ Systems. — The term organ system is employed in descriptive anatomy to designate a group of organs which cooperate in a general function* In many respects the systems repre- sent primitive functions, and it is therefore largely on account of the independent elaboration of these that the systems may be recognized also on a structural basis as groups of organs allied in origin and de- velopment. The exact number of systems recognized depends on certain arbitrary distinctions, the following being those usually distinguished : (1) The integumentary system, comprising the skin, and its ap- pendages, namely, the hairs and the general cutaneous, mammary, and inguinal glands. (2) The skeletal system, comprising the cartilage and bone elements of the skeleton, with their connections. (3) The muscular system, comprising all contractile structures of the body. Since, however, the involuntary muscles are arranged as muscle layers in connection with visceral organs, the muscular system is usually considered as including only the individual or voluntary muscles of the skeleton and skin. (4) The nervous system, comprising the central nervous system (the brain and spinal cord) and the peripheral nervous system, the latter consisting of the paired cranial and spinal nerves with their associated ganglia. A special portion of the peripheral nervous system is set apart as the sympathetic nervous system. The latter consists of a pair of ganglionated sympathetic trunks lying along the ventral surface of the vertebral column, and of two series of ganglia, prevertebral and peripheral, connecting the trunks with the visceral organs. (5) The digestive system, comprising the digestive tube and its out- standing glandular appendages — the oral glands, the liver, and the pancreas. (6) The respiratory system,, comprising the lungs, and respiratory passages, namely, the bronchi, the trachea and the larynx. With this system may also be included the accessory respiratory passages formed by the nasal fossae. (7) The vascular system, comprising the organs of circulation, namely, the heart, arteries, capillary vessels, and veins. The lymph - conducting canals are also portions of the circulatory system, but since they are largely independent of the bloodvessels, they are usually con- sidered as forming with their associated lymph glands a separate lymphatic system. (8) The urinogenital system, comprising the reproductive and excretory organs, together with their common ducts — the urethra of the male and the vestibulum of the female — and the associated bulbourethral gland. The reproductive organs comprise, in the male, the central organs or testes, and the deferent ducts, both of which are paired, the unpaired seminal vesicle, and the paired prostatic and paraprostatic glands. In the female, the reproductive organs comprise the paired ovaries, uterine tubes and uteri, together with the unpaired vagina. The excretory General Pi,an oi? Organization. 27 organs of both sexes comprise the paired kidneys and ureters and the unpaired urinary bladder. Certain organs of the body are not included in this classification: (1) The thymus and thyreoid glands are connected with the digestive tube in the embryonic condition, and for this reason are sometimes included with the digestive system, although in the adult they occur as in- dependent structures. (2) The suprarenal body is situated close to the kidney of either side, but is independent of the latter, both in the adult condition and in point of development. (3) The special (olfactory, optic and auditory) sense-organs of the head are highly elaborated structures, the relations of which are partly with the central nervous system. General Organization. — In the rabbit, as in all vertebrates, .the general plan of organization involves three chief features, as follows : (1) axial orientation — the arrangement of the chief organ-systems longitudinally about a more or less central, axial support; (2) meta- merism — the arrangement of a con- siderable portion of the body on a segmented or metameric plan, according to which structures are repeated in a serial fashion along the axis; (3) branchiomerism — the arrangement of a small anterior portion of the body on a serial but branchial plan, the latter depending not on the succession of true metameres but of visceral arches. The extent to which the general disposition of the organ-systems' is dependent on a fundamental plan will be evident from a comparison of the accompanying figure (20) of a trans- verse section of the rabbit-embryo, or of the schematic section of a general- ized vertebrate (Fig. 21), with the actual sections of the rabbit-foetus as given in the plates, more especially the abdominal section of Plate VIII and the thoracic section of Plate VII. It will be considered more fully below in connection with the general features of the organs. Metamerism (Fig. 19, me.) is characteristic of a dorsolateral portion of the body, identifiable in the embryo as that including the dorsal and intermediate portions of the middle layer or mesoderm (Fig. 20, d.m. and n.). In the adult it determines a number of features of serial arrangement, including the succession of the vertebrae and ribs, the divisions of the related dorsal musculature and its extensions to the ribs and abdominal wall, and indirectly the succession of the spinal nerves and their branches, of the parietal branches of the aorta, and the parietal roots of the inferior caval vein, as well as of the tributaries of the azygos vein. Fig. 19. Rabbit-embryo of 10i days (4.8 mm.): m., mandibular; h., hyoid; 1 and 2, first and second branchial arches; a.l., anterior limb-bud ; me., metameres; p.l., posterior limb-bud. (After Minot and Taylor, in Keibels Normentafeln, V.; Fig. 12.) 28 Anatomy of the Rabbit. Branchiomerism is an adult feature of lower aquatic vertebrates, such as fishes, where it appears as a succession of true, gill (or branchial) structures, which support gill filaments as functional respiratory organs. In higher terrestrial animals it appears as an embryonic feature (Fig. 19, m.h. 1, 2) and is to be considered both as a determinant of adult form and as a mark of aquatic ancestry. As in lower vertebrates, it underlies not only structures of branchial significance (branchial arches in the restricted sense) (Fig. 19, 1, 2), but also modified branchial structures, including the first or mandibular arch (m.), and the second or hyoid arch (n.). So great is the modification of these structures in passing from the embryonic eet. 8 %-"% Fig. 20. Transverse section of a rabbit-embryo of about 10-£ days, showing the arrangement of the organ-systems: ao., aorta; ch., not- ochonl; coe., coelomic cavity; d.m., dorsal mesoderm (myotomic and sclerotomic divisions); e., primitive alimentary canal (enteron) ; ect., ectoderm ; l.b., limb-bud ; ms., mesenchyme ; my,, external portion of a myotome; n., nephrotome of embryonic kidney; inter- mediate mass of mesoderm; sp. and so., splanchnic (visceral) and somatic (parietal) portions of the ventral mesoderm. to the adult condition that the recognition of the ground-plan is perhaps here of less general importance. It, however, determines the position and relations of certain skeletal structures, including the auditory ossicles, the hyoid, and in part the laryngeal cartilages — a point of some value in the classification of the parts of the head skeleton. It also determines the succession of certain soft structures, including the fifth, seventh, ninth and tenth cranial nerves; also the chief arterial vessels of the heart, which are more fully referred to below. General Plan of Organization. 29 Embryonic Plan of the Organ Systems. — In the individual organ- systems the main features of the general plan, as estimated on embryonic development, may be outlined as follows: 1. The formation of an axial skeletal support, consisting primarily of a strand of cellular tissue, the notochord, and secondarily of a seg- mented cartilaginous, afterwards bony, vertebral column. 2. The formation of (a) a primary cartilage skull (chondrocranium) as a support for the brain and capsules of the special sense organs (neurocranium or cerebral cranium) ; and (b) a series of cartilaginous visceral arches (splanchnocranium or visceral cranium). 3. The formation of the chief skeletal muscle in a dorsolateral position along the axis. 4. The formation of the central nervous system as a tube of nervous matter (neUral tube), lying on the dorsal side of the axial support, and differentiated into a general posterior portion, the spinal cord, and an anterior expanded portion, the brain. 5. The formation of the digestive tube as a median structure, lying directly beneath the axial support, and of special glandular appendages arising from the epithelium of its wall. 6. The formation of the lungs as paired outgrowths of the ventral wall of the digestive tube, afterwards connected with the outside of the body by accessory respiratory tracts traversing the head. 7. The formation of the circulatory system primarily on an aquatic plan. This involves the formation of (a) the heart in a ventral position to the digestive tube and immediately behind the gills; (b) a ventral aorta, passing forward to the gills, and dividing into a paired series of branchial aortic arches; (c) a dorsal aorta, combining the aortic arches, and passing backward along the. ventral surface of the axial support ; and (d) a series of paired veins returning the blood from various parts of the body to the heart. 9. The formation of the reproductive organs or gonads in association with the dorsal lining of the coelomic cavity, and their connection with the outside of the body by modified kidney ducts. 10. The formation of the kidneys, either as embryonic or permanent structures, from an intermediate mass of tissue, lying in general between the dorsal musculature and the lining of the coelomic cavity (cf . position of embryonic kidney in Fig. 20, n.). 11. The formation, in the ventral portion of the body, of an extensive space, the coelomic cavity or coelom, afterwards differentiated into pericardial, pleural and peritoneal portions. 30 Anatomy of the Rabbit. THE SKELETAL SYSTEM*. ' That portion of the skeleton which in the adult is designated as axial includes the vertebral column, together with the associated ribs and sternum, and the skeleton of the head. The axial relation belongs in the first instance to the bodies of the vertebrae and to a small portion of the base|of the skull. A line drawn through the centres of the vertebral bodies indicates the position of the primary axial support, the notochord. .ius. ao..... Fig. 21. Schematic representation of the chief organ-systems of a generalized vertebrate as seen in a transverse section of the abdominal region: Integument — int. Skeleton — v., vertebral body; a. v., vertebral arch; c. v., vertebral canal. Muscular system — s.m., skeletal muscle;' v.m., visceral muscle., v . . . . . Nervous system — m.s., spinal cord, with the central canal, and the dorsal (pos- terior) and ventral (anterior) roots of the spinal nerves; g.r.p., ganglion of the posterior root; r.c, ramus communicans to sympathetic trunk; r.m.a. and r.m.p.. anterior and posterior rami of a spinal nerve; t.s., sympathetic trunk. Digestive system — i., intestine. Vascular system — ao., aorta. Urinogenital system — k., kidney; go., gonad (ovary or testis). Serous cavity — c.p., general coelom, pleuroperitoneal, or peritoneal cavity; p. v. and p.p., visceral and parietal parts of the serous tunic— visceral and parietal peritoneum; mes., mesentery. The arches of the vertebrae are identified with the general functions of support of the related musculature and enclosure of the spinal cord. In lower vertebrates the line formed by the vertebral column and the base of the skull is for the most part straight. In a mammal the vertebral *The majority of the features mentioned below are illustrated in the plate figures of regional sections of the rabbit-foetus which follow, and for this reason the references are not indicated except in special cases. The Skeletai, System. 31 column presents marked dorsolumbar and cervical flexures. The axis of the skull proper, the basicranial axis, identifiable as a line passing through the centres of the basioccipital, basisphenoid and presphenoid bones, may differ by a considerable angle from that of the related cervical vertebrae. Further, the basicranial line, if continued forward in the rabbit, would pass through the dorsal wall of the skull immediately in front of the orbits (See Plate II, or Fig. 33 of the divided skull). It accordingly differs by a considerable angle from the basal line of the face, or basifacial axis. The skull consists primarily in the embryo of a cartilage trough, the extent of which is roughly definable as the area occupied by the occipital, anterior and posterior sphenoidal, and ethmoidal portions (cf. Plates III-V). As a cartilage skull it is designated as the chondro- cranium, and after its conversion into bone as the osteocranium. It is no more than an enclosure for the brain, except that it has associated with it the cartilage capsules of the nasal, visual, and auditory organs, and, in the case of the first and last of these, the capsules are incorporated with the skull proper. Thus, the primary skull is designated as the neuro- cranium or cerebral cranium, to distinguish it from a second portion of the head skeleton, the splanchnocranium or visceral cranium, which includes the series of visceral arches suspended from the ventral surface of the neurocranium. The addition to the primary head skeleton of a large number of membrane bones results in more or less confusion of the original divisions, since the membrane portions of the visceral cranium are, with the exception of the mandible, united by suture with those of the cerebral cranium, while the true cartilage or cartilage bone portions of the former, occurring as the auditory ossicles, the hyoid and larynx (in part) (Plate II), although highly modified, remain in a more or less independent relation. The skull, or cranium — using that term in a general sense — may be described as consisting of a cranial portion (the cranium proper) and of a facial portion, the latter including as visceral structures the upper jaw and the mandible, and as cerebral structures the parts of the turbinated bones and the associated secondary respiratory tract formed by the nasal fossae. In a mammal the investing membrane bones of the face are largely associated with a great extension of the nasal capsules, or that portion of the primary skull from which the turbinated cartilages are formed. Thus, as illustrated in the transverse section of Plate III, the nasoturbinals and maxilloturbinals are formed on lateral extensions of the primary septum (mesethmoid cartilage), but are supported more directly, and in the adult wholly, by the investing membrane bones (nasals, maxillae and premaxillae) . The elements of the head skeleton may be classified as follows : 1. The CEREBRAL CRANIUM (cranium cerebrale or neurocranium), including: (a) The primary cartilage skull (chondrocranium), enclosing the brain, and containing in its wall the olfactory and auditory capsules (embryonic) ; 32 Anatomy otf the Rabbit. (b) The secondary bone skull (osteocranium), replacing (a) and comprising the occipital, sphenoid, ethmoid, inferior turbinal, and periotic bones; (c) The associated derm elements, comprising the interparietal, parietal, frontal, nasal, vomerine, lacrimal, tympanic,* and squamosal bones. 2. The VISCERAL CRANIUM (cranium viscerale or splanchno- cranium), including; (a) The primary mandibular and hyoid f visceral arches (embry- onic) ; (b) The secondary elements, replacing (a) — the malleus, incus, and stapes of the auditory chain; the hyoid bone and its connections with the skull ; (c) The associated derm elements of the face and palate, com- prising the premaxillary, maxillary, zygomatic, mandibular, palatine and vestigial pterygoid bones. The anterior and posterior portions of the appendicular skeleton include each a proximal or girdle portion and a distal portion, the free extremity. In each the free extremity is divisible into proximal, middle and distal segments. The proximal joint of the limb, either of the shoulder or hip, is an enarthrosis, although it will be seen by examination of the rabbit that the glenoid cavity of the shoulder is not greatly elaborated as a concave surface. The distal joints of the limbs conform to the type of the ginglymus, and with minor exceptions the movements are restricted to one, namely, the sagittal, plane. In each limb they are described as movements of flexion and extension, since the general effect of movement is to bend or straighten particular parts on one another. Similar motions are observable in the limb as a whole, but are not spoken of as of flexion and extension, since the shoulder and hip joints are of a totally different type. In the limb as a whole, the chief motions are of pre- and post-traction, but it is convenient to recognize also movements of ad- duction (toward the body) and of abduction (away from the body). In both cases the axis of the upper arm, or of the thigh, is assumed to be parallel to the median vertical plane. In mammals, as in other terrestrial vertebrates, the anterior and posterior limbs are equivalent part for part. They are said to be serially homologous or homoplastic. There is, however, particularly in the mammalia, a pronounced difference between the anterior and posterior limbs in the respective positions of the different segments. Consequently, in studying the related musculature, it is advisable to consider not only the functional relations of muscles, but also the equival- *The identification of the tympanic as a derm element has been questioned. fThe thyreoid cartilage of the larynx and its connection with the hyoid (greater cornu) are modified branchial arches, but the structure as a whole is not included with the head skeleton. The Skeletal System. 33 ence of the bone surfaces. This may be estimated either from the rotations of the limbs in embryonic development, or by reference to an ideal or neutral condition as indicated in the accompanying diagram (Fig. 22). The neutral type (A) may be defined as one in which the proximal segment (humerus or femur) is situated at right angles to the median vertical plane, the middle segment directed downward parallel, to the median plane, and the distal segment again at right angles to it. In this condition the bones of the middle segment are parallel, with the radius, or the tibia, according to the par ticular limb, anterior in position. The first digit is also anterior. The entire anterior surface is indicated in the accompanying diagram by shaded lines. The angles b. and c. are "extension" angles; e. and f. "flexion" angles; a. is an "abduction" angle, while d. is an " adduction" angle. In both limbs of a mammal, the entire appendage is rotated downward to a position more or less underneath the body. In the anterior limb (B) the proximal segment is rotated backward, the middle and distal segments forward. The two divisions thus come to differ from one Fig. 22. Schematic representation of the respective positions of the segments in the mammalian limbs: A, neutral; B, anterior limb; C, posterior limb. Ex- planation in text: tr.p., transverse plane another to the extent of two right angles. Also, in the front limb, the in- teresting condition is observable that the radius and ulna are crossed on one another, the position of the former being chiefly anterior, although its proximal end is lateral, and its distal end, like the first digit, medial. In the rabbit, as in most mammals, the hand is thus fixed in a prone position, while in man the hand may be rotated to a supine position in which the radius is wholly lateral, or in which the two bones are parallel. In the posterior limb (C) all three segments are rotated forward. Con- sequently the bones of the middle segment retain their original parallel position with reference to one another. The extension angles of the knee and ankle are anterior, the flexion angles posterior. They differ from the corresponding angles of the anterior limb only at the middle joint, but here the difference amounts to two right angles. It will be observed also that the distal or ankle joint of the posterior limb retains a primitive condition, in which the foot is placed approximately at right angles to the leg; in other words, it is of a plantigrade type, one in which 34 Anatomy of the Rabbit. the sole of the foot rests on the ground. Thus the muscles described as flexors in the foot pass over the heel. They are functionally flexors of the toes, but extensors of the foot as a whole. Those described as extensors, lie for the most part on the anterior surface of the tibia, and are turned from their original course to the dorsal surface of the foot. They are functionally extensors of the toes, but flexors of the foot as a whole. THE NERVOUS SYSTEM. In the central nervous system, the more general features relate to the form of the whole structure as a neural tube, containing a central cavity, the neurocoele. This is differentiated into two portions, namely, a less modified portion, the spinal cord, or spinal medulla, containing as its cavity the central canal, and a greatly expanded portion, the brain, or encephalon, which is divided into a series of paired and unpaired seg- ments, and contains in its interior a corresponding series ofjdivisions of the original cavity, known as its ventricles. Although the internal structure of the brain is a matter of the dis- position of its nerve-centres and fibre-tracts/it is customary to estimate its primary divisions by reference to a general plan, the latter being in most respects one that is common to vertebrates generally and thus appears in a less elaborated condition in the embryo, or in the adults of lower forms. The main features of this plan are indicated in the ac- companying diagram (Fig. 23). The brain as first formed in the embryo appears as an anterior ex- panded portion of the neural tube, or rather three expansions arranged in a linear series. They are described as the primary cerebral vesicles; or, as primary divisions of the future brain, they are designated in anteroposterior order as the prosencephalon, mesencephalon, and rhombencephalon. The first of the primary divisions, the prosencephalon, or primary fore-brain, becomes divided during development into two portions, namely, an anterior portion, the end-brain or telencephalon, which is largely a paired structure, and a second portion, unpaired, the dience- phalon, or inter-brain. The larger paired portion of the telencephalon is the basis of the cerebral hemispheres. It contains, as divisions of the primary cavity, a pair of cavities, the lateral ventricles. The anterior portion of the telencephalon, moreover, becomes differentiated, so that a small terminal olfactory segment, the rhinencephalon, is more or less perfectly marked off from the rest. In the mammalian brain this part is chiefly identifiable as the paired olfactory bulb, the latter being the anterior portion of the olfactory lobe or olfactory brain, and containing in its interior an extension of the lateral ventricle. The unpaired portion of the prosencephalon is considered as belonging in part to the telencephalon and in part to the diencephalon. Its cavity, the third ventricle, is connected with the lateral ventricles through the interventricular foramen. Its anterior wall is formed by a transverse connection of the cerebral hemispheres, the lamina terminalis. In all The Nervous System. 35 vertebrates this portion of the brain is remarkable for the manner in which its wall is differentiated. The ventral portion extends downward as a slender funnel-like structure, the infundibulum, the tip of the latter h.c. '..v.m.b. b.o. 'T- h. c.m.\ r v.(j,. b'.c r. hyb Fifi. 23. Plan of the divisions of the vertebrate brain: A, embryonic; B, adult, projection from dorsal surface ; C, adult, sagittal section. Primary divisions — PR, prosencephalon; T, telencephalon; DI, diencephalon ; MS, mesencephalon; RH, rhombencephalon; MT, metencephalon ; MY, myelence- phalon; S, spinal cord. a. a, cerebral aqueduct; b.o., olfactory bulb; cb., cerebellum; cm., nuimmillary body; o.o., optic chiasma; c.p., pineal body; c.q., corpora quadrigemina; ep., epitha- lamus; f.i., interventricular foramen ; h., hypophysis; h.c, cerebral hemisphere; hyp., hypothalamus; inf., infundibulum; l.t., lamina terminalis; p., pons; p.c, chorioid plexus of third ventricle: p.cr., cerebral peduncle; tv, thalamus, also in- dicates position of massa intermedia; v.l., lateral ventricle; v.m.p., posterior medullary velum; v.q., fourth ventricle. being in contact with the pituitary body or hypophysis and its base connected with a small grey elevation, the tuber cinereum. Its cavity is the recessus infundibuli. Immediately in front of the infundibulum the 36 Anatomy of the Rabbit. ventral portions of the optic tracts join to form the optic chiasma, and immediately behind it the floor is thickened, forming. externally a pair of rounded protuberances, the mammillary bodies. In the brain of the rabbit this structure consists superficially of a larger median portion with faint lateral elevations appended to it. Collectively, these structures are considered to form a major division, the hypothalamus, the latter con- sisting of two portions, namely, an optic portion, comprising the in- fundibulum, tuber cinereum, and the optic chiasma, and a mammillary portion, including the mammillary bodies. The more dorsal portion of the diencephalon, containing the major part of the third ventricle, is known as the thalamencephalon. Its lateral walls are greatly thickened, while its roof is extremely thin, especially in its anterior part. Here the actual roof of the ventricle is formed only of a thin layer of tissue, the epithelial chorioid lamina, but the latter has associated with it a series of vascular ingrowths of the investing pia mater, the latter being described in this relation as the chorioid web (tela chorioidea). The two structures together form a chorioid plexus. This extends downward into the third ventricle, reaching out also into the lateral ventricles. The dorsal portion of the thalamencephalon bears posteriorly the pineal body, the latter together with certain related structures, the habenulae and habenular commissure, forming the epithalamus. The general portion of the thalamencephalon bordering the third ventricle, and broadly connected across the latter by the massa intermedia, is the thalamus. In the brain of the rabbit it will be seen that the thalamus is chiefly indicated externally by a rounded protuberance, the pulvinar. The latter is dorsal in position and is imperfectly marked off from a second protuberance, the lateral geniculate body, lying on its postero- lateral side. To the medial side of this is a third protuberance, the medial geniculate body. The medial and lateral geniculate bodies as thus defined constitute the metathalamus (Fig. 53). The second of the primary divisions, the mesencephalon, or mid-brain, is noteworthy in a mammal as lacking a ventricle. Its cavity is a narrow canal, the cerebral aqueduct, leading from the third, ventricle backward to the fourth ventricle, or cavity of the rhombencephalon. Externally, its roof is differentiated into four rounded elevations, the corpora quadrigemina, of which the anterior pair are much larger than the posterior ones. Its floor is chiefly formed by a pair of divergent cords, the cerebral peduncles. The parts of the mesencephalon and prosencephalon together con- stitute the large brain, or cerebrum. The third primary division, the rhombencephalon, or primary bind- brain, is a greatly elaborated portion from which arise the majority of the cranial nerves. The constricted area joining it with the mesencephalon is known as the isthmus rhombencephali. It includes the anterior medullary velum and brachia conjunctiva (Fig. 55) . The rhombencephalon itself is divisible into two portions, especially well defined in the mammalia, namely, the metencephalon, or hind-brain', and the myelence- phalon, or after-brain. The former includes the small brain, or cerebellum, The Nervous System. Cerebrum 37 pi 43 "I « u 43 a vH I s ^ .2 •sx, m -a i— i « m fa- ts H fa M 8-3 8 s > 3 n w fa- rt H fa O ■Si, •3 U in o H s ni •■>» at ^ 43 o h-» -a, O u< >, ■o. 43 o a o g 2 -2 § •* a a ■« -ss ^ ^ fc, a a -si o .5 •a • a a s "S-g u u S3 » v -' s s 'a? »— , OJ .° ^-> "3 en OJ '3 43 S3 o 'S S3 •, CO O a) u at O at CO o •V a 4= a o 43 Pi •§ a S3 CO -a • i-h 'u 6t (Poste (Pri 38 Anatomy of- the; Rabbit. and a ventral structure of a commissural nature, the pons. The my- elencephalon is a transitional portion connecting the brain with the spinal cord. The cavity of the rhombencephalon is the fourth ventricle. It is a peculiarly shaped space, the floor and lateral walls' of which are very greatly thickened, while the roof is for the most part thin. The roof appears at first sight to be formed largely by the cerebellum, but is in reality formed by two membranes underlying the latter. One of these, the anterior medullary velum, is connected forwards with the mesence- phalon, while the other, the posterior medullary velum, covers a triangular space over which the posterior margin of the cerebellum does not extend. The posterior medullary velum has the same structure as the chorioid plexus of the third ventricle, but is much more poorly developed. Apart from its principal divisions, which, as indicated above, are more or less common to all vertebrates, the brain depends for its external form on the elaboration of certain parts in comparison with others. In the mammalia the cerebral hemispheres and the cerebellum are the chief form- determinants, although the pons and the corpora quadrigemina may also be considered in this connection. It will be seen also that the form of the brain is more or less dependent on the existence at certain places of well-marked flexures (cf. Plate II). The first of these, the cephalic flexure, is in the region of the mesencephalon, the anterior portion of the brain being bent downward ; the second, or pontine flexure, is at the fourth •ventricle; while the third, or cervical flexure , is at the point where the myelencephalon passes over into the spinal cord. The peripheral nervous system embraces two groups of paired and, for the most part, metamerically arranged nerves, namely, the spinal nerves — those arising from the spinal cord and leaving the vertebral column through the intervertebral foramina; and the cranial or cerebral nerves — those arising from the brain and passing through the foramina of the skull. Of these the spinal nerves are less modified, both in structure and distribution. A spinal nerve (Fig. 21) is a product'of two roots, one of which, the radix posterior, is sensory and arises from the dorsal side of the cord, while the other, the radix anterior, is motor and arises from its ventral side. The posterior root bears a root-ganglion. The combined nerve is distributed in three chief parts, of which two, the anterior and posterior rami, are distributed as mixed nerves to the skeletal muscle and the skin, — the anterior ramus being in practically all cases the prominent nerve — while the third, the ramus communicans, is connected with the sym- pathetic trunk. The sympathetic trunk is formed on either side by a series of ganglia joined together by connecting cords. It is connected not only with the spinal nerves through the rami communicantes, but also with a series of prevertebral (coeliac, superior mesenteric, etc.) ganglia and plexuses from which nerves pass, chiefly in association with the bloodvessels, to the peripheral ganglia and plexuses of various organs. The anterior and posterior rami of the spinal nerves contain both sensory and motor components, and are typical somatic nerves. The ramus communicans is a visceral connection containing similar com- The Nervous System. 39 ponents, here better described as afferent and efferent. It connects the central nervous system either with the actual viscera, or with visceral structures in a general sense, the chief relations of the sympathetic system being with the smooth muscle of visceral organs, or with that of the bloodvessels in various parts of the body. The cranial nerves are comparable in some respects to the spinal nerves, but in various ways are highly modified structures. The third, fourth and sixth, respectively oculomotor, trochlear and abducent nerves, which are distributed to the muscles of the eye, are considered as belong- ing to the somatic motor division. The second, or optic nerve, and the nervous portion, or retina of the eye, are specialized parts of the central nervous system, arising embryonically as an outgrowth of the latter. The nerves of the remaining special sense-organs, including the first, or olfactory nerve and the eighth, or acoustic nerve, have been con- sidered as parts of an extensive system of nerves, which, in lower aquatic vertebrates, also contains representatives of the branchiomeric series, and is distributed widely to sense-organs lying in the skin. Of the remaining cranial nerves the fifth, seventh, ninth and tenth are branchiomeric. Although the connections of these nerves are not fully considered in the dissection as here outlined, their chief characteristic as branchiomeric structures may be indicated. The fifth, or trigeminal nerve, the nerve of the mandibular arch, arises in two parts, one of which, the portio major, is sensory, the other, the portio minor, motor. The portio minor unites with the third or mandibular division of the portio major. Thus, the terminal branches of all three divisions, ophthalmic, maxillary, mandibular, are distributed as somatic sensory nerves to the skin of the head, and, in addition, the mandibular nerve distributes visceral motor branches to certain muscles (masticatory group, mylohyoid and digastric) regarded as belonging to this, the first arch. A visceral sensory con- nection with the mouth is considered to be formed by the lingual branch of the mandibular nerve and by the palatine branches of the spheno- palatine ganglion. Both are, however, connected with the central nervous system through the seventh nerve, the former by the chorda tympani, and the latter by the great superficial petrosal. The seventh, or facial nerve is the nerve of the second, or hyoid arch. It is chiefly distributed as a visceral motor nerve to the cutaneous muscles of the head. The ninth, or glossopharyngeal nerve, belonging to the third arch, the tenth, or vagus, belonging to the fourth and succeeding arches in lower forms, and the eleventh, or spinal accessory nerves, the latter apparently related to the vagus as a motor portion, are distributed as visceral motor nerves to the pharyngeal and laryngeal musculature, and as visceral sensory nerves to various visceral organs, the ninth nerve supplying the gustatory organs of the tongue. The spinal accessory, moreover, has a characteristic distribution to the cleidomastoid, sterno- mastoid and trapezius muscles of the side of the neck and shoulder. The twelfth, or hypoglossal nerve has the relation of the ventral or motor portion of a spinal nerve, and is distributed as a motor nerve to the muscles of the tongue. 40 Anatomy of the Rabbit. THE DIGESTIVE SYSTEM. The digestive system comprises as its chief portions the digestive tube and the digestive glands. The digestive tube is divisible into several parts, which, with the exception of the caecum and its vermiform process, are arranged in a linear series. The digestive glands comprise the oral glands, the liver, and the pancreas. They are parts of an extensive series of epithelial glands, otherwise contained within the wall of the tube, and for this reason not appearing as gross structures. The parts of the digestive tube may be classified as follows : 1. Oral Cavity. 5. Small Intestine. Oral cavity proper. Duodenum. Vestibulum oris. Mesenterial intestine. Jejunum. 2. Pharynx. Ileum. Nasal portion. Oral portion. 6. Large Intestine. Laryngeal portion. Caecum. Vermiform process. 3. Oesophagus. Colon. Rectum. 4. Stomach. In its most general features the digestive system is significant as an epithelial tube, in which the food during its passage is subjected to the action of digestive juices provided by the epithelial glands, and is modified, by solution or otherwise, so that it is capable of being absorbed through the epithelial surface. In the form of the digestive tube as seen in a vertebrate, however, a number of gross mechanical features are evident, such as, for example, the increase in capacity, or in absorptive area, through the folding of the mucous membrane, or the expansion of the wall ; or again, the presence of a special muscular tunic, and its modifica- tion at certain places, as in the oesophagus, the pyloric limb of the stomach, and the first portion of the colon. Moreover, many features of the abdominal portion of the tube, and, indeed, certain of its recognized divisions, depend on its relation to an extensive serous sac — in a mammal the peritoneal cavity. In this connection it is to be considered that the digestive tube is primarily a median structure. It has this relation in the earlier stages of embryonic development (Fig. 20), and in many of the lower vertebrates it does not deviate to a great extent from a median position. In all higher vertebrates, however, the tube becomes greatly elongated in comparison with the cavity in which it lies, and thus becomes extensively displaced to one side or other of the median plane. This development, while advanced in all mammals, may be said to reach an extreme in the herbivorous mammalia ; and in many cases it is further increased by the independent elaboration of the blind intestine or caecum. In the rabbit the combined length of the small and large intestines is approximately eleven times that of the body. The Digestive System. 41 In considering the divisions of the digestive tube in the rabbit, the posterior, or post-cephalic portion, comprising the oesophagus and suc- ceeding parts, may be distinguished from the anterior, or cephalic portion, the latter comprising the oral cavity and pharynx. The former is a free portion embracing the digestive tube proper, while the latter is a fixed portion exhibiting a variety of general mammalian features connected with the organization of the head. The oesophagus is a slender but greatly expansible tube leading from the pharynx to the stomach. In its passage backward it traverses the neck and the thorax, and in both regions occupies a median position. In the thorax (Plate VII) it will be observed that it lies between the heart and the dorsal aorta, thus exhibiting the original relation of the digestive tube to the aortic portion of the vascular system. The succeeding portions of the digestive tube are those associated with the peritoneal cavity, and with the exception of the terminal portion, the rectum, are displaced from a median position. Consequently, the divisions which are recognized are based partly on the differential characters of the wall, and partly on the position of structures more especially in relation to the supporting peritoneum. Thus, the chief features of the stomach depend on the expansion of the organ and the rotation of its pyloric end forward and to the right. In the intestinal tract as a whole the chief although by no -means most conspicuous feature of position depends on the looping of the entire structure on itself, so that the terminal portion, chiefly the transverse colon, crosses the ventral surface of the duodenum and then turns backward on the dorsal surface of the mesenterial small intestine. The duodenum is sharply marked off from the mesenterial intestine as an extensive loop, containing the major part of the pancreas and its duct, and lying on the right side of the dorsal wall of the abdomen. The mesenterial intestine is a greatly convoluted portion, lying chiefly on the left side of the abdominal cavity, and loosely supported by the broad, frill-like mesentery. From the pylorus to the sacculus rotundus there is no abrupt change in the character of the wall, although the first portion of the mesenterial intestine, that designated as the jejunum, and the duodenum may be considered together as a more vascular portion with thicker walls in comparison with the second portion, the ileum, in which the wall is less vascular and more transparent. T s he main portion of the large intestine, the colon, although greatly specialized, may be considered to consist as in man of ascending, trans- verse, and descending parts, that is to say, the ascending colon lies on the right side of the body and passes in a general way from its point of origin on the caecum forward to a point where it becomes flexed to the left as the transverse colon ; the latter crosses the body and is flexed backward as the descending coion. In the rabbit, however, that portion definable as the ascending colon is greatly elongated, and is composed of five principal limbs, united by flexures. Two of these, in dissection from the ventral surface, are concealed by the base of the superior mesenteric artery, since they lie on its right side. The descending colon is also only nom- inally related to the left 'side of the body wall, its supporting peritoneum, the descending mesocolon, being closely connected with the mesoduodenum 42 Anatomy of this Rabbit. of the ascending limb of the duodenal loop. The course of the caecum as it lies in the body is comparable to two turns of a left-hand spiral, its blind termination, the vermiform process, being dorsal in position and directed for the most part backward. It may be observed at this point that in their vascular supply the more typical divisions, namely, the transverse and descending colons, have arterial branches, respectively, the middle and left colic arteries, comparable to those of man ; while on the other hand the right colic relation, on account of the great complexity of its parts, is represented by a large number of vessels,, branches of a common ileo- caecocolic trunk. The form of the anterior, or cephalic portion of the digestive tube (Plate II) depends on its fixed relation with respect to the enclosing parts of the head-skeleton. In the rabbit, as in mammals generally, the oral cavity is divisible into two portions, of which one is the oral cavity proper, while the other, the vestibulum oris, is a space enclosed between the alveolar process of the jaws and the teeth oh the one hand and the cheeks and lips on the other. As in other vertebrates, the tongue is a muscular structure projecting upward and forward into the oral cavity from its base of attachment on the hyoid apparatus, but its greater elaboration, as well as the differentiation of special processes, the circumvallate and foliate papillae, for the accommodation of the gustatory organs, are features of mammalian significance. The roof of the oral cavity is formed by an extensive palatal surface, comprising the hard palate, and the membranous, or soft palate. These structures also form the floor of the accessory respiratory tracts of the nose, the posterior aperture being thus carried backward to a point more directly above the aper- ture of the larynx. As accessory structures the teeth present two mammalian features; they are heterodont, or differentiated according to particular regions, and the adult series, excepting those designated as molars, are permanent teeth, replacing deciduous, or milk teeth of the young animal. The condition is thus described as diphyodont in comparison with that in lower vertebrates, where there is usually a multiple tooth change, new teeth being developed as required (polyphyodont type). Moreover, in the rabbit, as in all mammalia, the number is restricted, so that, considering the differentiation of the teeth, it is possible to express their relations by a dental formula. In the mammalia generally the teeth are differen- tiated into incisors, canines, premolars and molars, and in placental mammals the full dental formula is indicated as i. f, c. \, pm. -J, m. f. In the rabbit as in other rodents, however, the dentition is greatly modified by the elaboration of two pairs of incisors and the corresponding obliteration of intermediate teeth, the place of the latter being occupied by an extensive gap, or diastema in which no teeth occur. The dental formula of the rabbit is i. f, c. §, pm. f , m. f . It will also be observed in this animal that the absence of the intermediate teeth allows the lips to be approximated behind the incisors, and since the lips are in this region also provided with hairs on their internal surfaces, the oral cavity is separated almost completely from a small space enclosing the incisor teeth. This adaptation, however, is not so perfectly developed in the rabbit as in certain others of the rodent order. The Respiratory and Vascular Systems. 43 The chief features of the pharynx depend on its relation as a common or general portion of the digestive tube with the tubes of the respiratory- system. It is divisible into an oral portion, representing the direct connection of the oral cavity with the oesophagus, a dorsal or nasal portion, connected with the nasal fossae, and a ventral or laryngeal portion, containing the aperture of the larynx. THE RESPIRATORY SYSTEM. In all air-breathing vertebrates the lungs arise embryonically as ventral outgrowths of the digestive tube, and are secondarily con- nected with the outside of the body through special perforations of the anterior portion of the head and through the oral cavity. As indicated above, this connection in a mammal is represented by an extensive nasal cavity bearing on its lateral walls the olfactory sense- organs. It is distinguished as an accessory respiratory tract from the true respiratory tract formed by the trachea and its terminal divisions, the bronchi. The respiratory system as represented by the lungs and related tubes, is nominally ventral to the oesophagus, but this relation is chiefly true of the trachea. In the thorax (Plate VII) the bronchi are, in general, interposed between the oesophagus and the heart, the lungs being expanded laterally into the paired pleural cavities. THE VASCULAR SYSTEM. In the rabbit/ as in all vertebrates, the vascular system embraces a central, muscular organ of propulsion, the heart, and a series of branched tubes, the bloodvessels, the latter being of three different kinds : (a) thick- walled, elastic, distributing vessels — arteries; (b) microscopic terminal canals in the peripheral organs — capillaries ; and (c) thin- walled collecting vessels — veins. The chief mammalian feature in this system consists in the division of the heart into two portions (Plate VII), each consisting of a receiving chamber, or atrium, and a driving chamber, or ventricle, and the arrange- ment of their vascular connections in such a way that two complete, circulations are established. One of these is the long, or systemic circula- tion. It is concerned with the distribution of blood to the various parts of the body, with the exception of the lungs. It is established by the left ventricle, the aorta, the carotid and subclavian branches of its arch, and the parietal and visceral branches of its thoracic and abdominal portions. The blood is collected from the anterior portions of the body through paired internal and external jugular and subclavian veins, com- municating with the right atrium of the heart through paired superior cavals; from the posterior portions of the body through the unpaired and also asymmetrical inferior caval vein, the latter passing forward on the right of the median plane and entering the posterior end of the right atrium. The second, short, or pulmonary circulation, is concerned with 44 Anatomy of the Rabbit. the distribution of the blood to the lungs for purposes of aeration. It is established by the right ventricle, the pulmonary artery and its paired branches, and by the capillaries of the lungs. The blood is delivered to the left atrium through several pulmonary veins. A similar division of the circulatory organs occurs as a homoplastic modification in birds, which, it will be observed, are also warm-blooded vertebrates. Many of the peculiar features of the mammalian circulation which at first sight do not appear to be general, but are so in reality, depend on the circumstance that the complete partition of the organs is a final stage of a general progressive development, observable in air-breathing verte- brates, in which the lungs and their vascular connections become perfected for pulmonary respiration. On the other hand, the vascular system as it appears in the embryo,, more especially its aortic portion, is arranged according to the type of branchial respiration as found in fishes. In this condition the blood is sent forward from the heart through a ventral aorta. The latter is connected with a series of paired branchial aortic arches, traversing the rudimentary gill structures, and thus passing upward around the sides of the primitive pharynx. The dorsal aorta is formed by the junction of the branchial aortic arches, and passes backward as a main distributing vessel on the ventral side of the axial support. The heart itself is formed primarily on two-chambered plan, similar to that in fishes, where all the blood is received by single atrium and is delivered forward to the gills by a single ventricle. The definitive condition of the chief arterial vessels is arrived at by an extensive modification of the branchial plan. As indicated in the accompanying diagram (Fig. 24) the arch condition is retained by the aorta and by the pulmonary artery, and it is interesting to note also that the primary connections of these vessels, repre- sented in the embryo by an open canal, the ductus arteriosus (Botalli), is indicated in the adult condition by a short fibrous cord, the arterial ligament. The adult aortic arch represents only the left one of a pair, and since that of the right is only represented imperfectly by the innominate artery and the base of the subclavian of that side, a condition of asymmetry results, which is mainly expressed by the sinistral position of the arch with reference to the oesophagus (Plate VII). By comparison with the embryonic plan, it is seen that the primitive features of the heart and the arterial vessels include the ventral position of the heart itself, the equivalence of the two atria and of the two ventricles — these structures being partitioned internally but Fig. 24. Plan of the branchial aortic arches. The adulfmamroalian vessels are indicated in black (systemic) or shaded (pulmonary). 1-6, primary arches; ao., aorta; a. p., pulmonary artery; c.e., external carotid; c.i., in- ternal carotid; d.a., ductus arteriosus (Botalli); i., innominate artery; s.d., right subclavian; s.s.. left subclavian. (From Weber, after Boas.) The Vascular System. 45 imperfectly divided externally — the forward position of the first portion of the aorta, and the position of the aorta as a median vertebral trunk. The vascular system is noteworthy for several departures from the condition of symmetry, one of these having already been mentioned. In addition, it is seen that in a mammal, as in terrestrial vertebrates generally, the base of the pulmonary artery is rotated in a spiral fashion about the base of the aorta, so that from its beginning on the right ventricle it passes across the ventral surface of the base of the aorta, and divides on the dorsal side of the latter into its two main branches. Moreover, the separation of the ventricular portion of the heart into two chambers is associated with an enormous increase in the muscularity of the wall in the left ventricle, or, in other words, in that portion which is concerned with the larger, systemic circulation. The inferior caval vein (Plate VIII), a highly specialized vessel, is asymmetrical, since from its begin- ning in the pelvic cavity to its termination on the right atrium it lies wholly to the right of the median plane. The azygos vein of the thorax (Plate VII), a vessel uniting the majority of the paired intercostal veins, and interesting as a remnant of the primitive circulation, is also asymmetrical, since the trunk lies to the right of the bodies of the vertebrae, and is connected at its base with the right superior caval vein. In general, the blood which is distributed to the various parts of the body passes through but one set of capillary vessels, and is then returned through the systemic veins to the heart. In all vertebrates, however, a special portion of the systemic venous circulation is set: aside as the hepatic portal system, distinguished by the possession of a second series of capillary vessels ramifying in the liver. Thus, in the rabbit, the blood distributed to the stomach, spleen, and intestine through the coeliac, superior and inferior mesenteric arteries, is collected into a main intestinal vessel, the portal vein, and the latter, approaching the liver through the lesser omentum, divides in that organ into a series of portal capillaries. The portal capillaries, like the systemic capillaries proceed- ing from the hepatic artery, unite in the tributaries of the hepatic veins. In lower vertebrates, although not in the mammalia, a second system of venous capillaries occurs in connection with the kidneys, and is known as the renal portal system. The lymphatic system, both in its functional relation and in origin, is an appendage of the venous portion of the vascular system. The system is an important one, of which, unfortunately, little may be seen by ordinary dissection, the structures which appear in this way being the lymph glands, or lymph nodes, centres of cell formation, occurring in the course of the conducting vessels. These as superficial structures are found either singly, as in the head and neck, or more or less grouped, as in the axillary and inguinal spaces. As deep structures they are conspicuous in the intestinal mesenteries, and in the walls of the digestive tube, occurring in the latter chiefly as continuous masses of lymph follicles, as, for example, in the walls of the sacculus rotundus, the vermiform process, or the tonsil ; or, again, as aggregated lymph follicles (Peyer's patches) at various points in the intestinal wall. The conducting portion of the system comprises an extensive series 46 Anatomy of th£ Rabbit. of canals, beginning as lymphatic capillaries in peripheral organs, and ending as lymphatic trunks which empty into the great veins. The lymphatic trunks of the anterior portion of the body are designated from their association with the corresponding veins as jugular and subclavian. They enter the venous system on either side at the point of junction of the internal and external jugular veins or of the common jugular and subclavian (Fig. 52). The lymphatic vessels of the posterior portion of the body, including, the intestine, unite to form a common canal, the t horaci c duct. The latter lies for the most part between the aorta and the verte- bral column, and traverses the thorax in this position to enter the venous system at the same point as the jugular and subclavian trunks of the left side. The lymphatic capillaries are terminal, absorptive vessels, differing from blood capillaries both in the character of their walls and in their relations to other portions of the system, since they are not interposed as in the vascular system between vessels'of a larger order. The lym- phatic vessels connecting the capillaries with the lymphatic trunks form extensive plexuses, in connection with which the lymph nodes are dis- tributed. THE URINOGENITAL SYSTEM. The urinogenital system comprises two primary systems — reproductive and urinary — differing widely in their central organs, but associated to a certain extent by having common ducts. In the rabbit, as indicated in the accompanying diagram (Fig. 25), this association extends only to the presence in the two sexes of a urinogenital canal, or urinogenital sinus connecting both urinary and genital structures with the outside of the body. This canal is designated in the male as the urethra, but in the female as the vestibulum, since the structure known from the human relation as the female urethra is only a urinary canal leading from the bladder, and in man is not associated with the reproductive ducts. In general, however, the relations of the urinary and reproductive organs involves two chief features. First, in primitive vertebrates, the urinary and genital ducts open into the posterior end of the digestive tube, the latter forming in this relation a common canal, the cloaca. In terrestrial vertebrates, the urinary bladder is developed as a ventral outgrowth of the digestive tube, and, except in amphibians, both sets of ducts undergo a migration from their original position on to the wall of its canal, the latter being thus transformed into a urinogenital sinus. This development reaches its extreme in the higher mammalia, where the urinogenital sinus is completely separated from the digestive tube, and where the urinary ducts are also transferred from a posterior or hypo- cystic position on the wall of the urinogenital sinus to an anterior or epicystic position on the dorsal wall of the bladder. Secondly, there is a more fundamental association between the reproductive and excretory organs, depending on the circumstance that the former are primarily in the vertebrates organs connected with the The Urinogbnitai, System. 47 lining of the coelom and discharging their products into the cavity, while the kidneys are primarily tubular structures communicating also with this cavity, so that they become modified in part as reproductive ducts. The coelomic connections of the kidney tubules are with few exceptions only seen in the embryonic condition, since the definitive kidneys in all vertebrates are structures in which the tubules are connected with the vascular system, and have either lost, or, as in the specialized permanent kidneys of the mammalia, have not developed, the coelomic apertures. The embryonic development of a mammal includes the formation not only of a final kidney or metanephros, but also of two embryonic structures, one of which, the first kidney or pronephros, is embryonic in all vertebrates, while the second, designated as the mesonephros, or embryonic kidney, is one occurring in the adult condition of intermediate Fig. 25. Diagrams of the male (A) and female (B) urinogenital systems of the rabbit: b., urinary bladder; k., kidney; ur., ureter; r, rectum. (A) — c.p., crus penis; d.d., ductus deferens; ep., epididymis; g., gubernaculum; t., testis; v.s., seminal vesicle; u. v., male urethra. (B) — c.c, crus clitoridis; o., ovary; t.u., uterine tube; u.m., female urethra; ut., uterus; va., vagina; vs., veslibulum. forms. It is from the ducts of the primary kidneys that the reproductive ducts arise. Thus, in the female of most vertebrates the oviduct opens by an expanded funnel into the coelomic cavity, usually at some distance from the ovary; and, although in the rabbit the structure is said to arise as a secondary development, it exhibits here as in other mammals the feature of a coelomic opening, coupled with that of close association with 48 Anatomy of thb Rabbit. the ovary, so that the products of the latter, while nominally discharged into the coelom, are actually received directly into the uterine tube. The oviduct itself is phylogenetically the oldest of the urinogenital ducts, since it is identifiable in the embryo as the duct of the pronephros. In the rabbit, as in all mammals, the testis is connected with the urethra through the canal formed by the epididymis and ductus deferens, these structures comprising in a modified form a portion of the mesonephros and its duct. Since the excretory functions are fully provided for by the development of a permanent kidney and its duct, the ureter, the ducts of the embryonic kidney have in the adult no urinary connections; but in intermediate vertebrates, in which the mesonephros occurs as an adult kidney, the interesting condition is observable in the male that the duct of this structure serves both urinary and reproductive, functions. It will be seen in the rabbit that the female genital ducts, apart from the common urinogenital sinus, include an unpaired portion, the vagina, and a paired portion, comprising the uteri and uterine tubes. In all vertebrates up to and including the monotrematous mammalia the oviducts open separately either into the cloaca, or into the urino- genital sinus. In the higher mammalia, however, a process of fusion is observable, extending from the partial coalescence of the vaginae, as in marsupial mammals, to the complete coalescence of both vaginae and uteri, as in man. Thus, there are recognized in the mammalia the types of (a) unpaired uterus — uterus simplex; (b) semi-divided uterus — uterus bicornis ; and (c) completely divided uterus, such as that of the rabbit — uterus duplex. In the respective positions of the central organs, as exemplified by the rabbit, there are several features of general significance. Thus, the kidneys (Plate VIII) are paired structures lying on the dorsal wall of the abdominal cavity, where they are interposed between the peritoneum and the dorsal musculature. They are covered by peritoneum only on their ventral surfaces. As indicated above, the permanent kidneys of a- mammal are highly specialized structures, but they show in these rela- tions features common to lower forms and dependent in both cases on the primary position of the intermediate mass, from which in the em- bryo the kidneys are formed. The gonads are primarily associated with the dorsal lining of the coelomic cavity, although certain observations on the development of these structures in the lower vertebrates appear to indicate that their elements are assembled from other parts of the embryo. In some of the mammalia, as in all lower vertebrates, the testis occupies an abdominal position in the adult condition ; but usually it undergoes an extensive mi- gration, passing from the abdominal cavity into a special sac of peritoneum enclosed by the scrotum. This change — described as the descent of the testis — is effected through the agency of a muscular cord, .he guber- naculum. In many cases, as in man, the cavity enclosing Ihe testis is completely separated from the abdominal cavity, but in the rabbit a more primitive condition is retained in which the sac of the testis is widely open to the abdominal cavity, and the organ thus passes freely The Serous Cavities. 49 from one cavity to the other. It is also observable in this animal that the gubernaculum is represented in the adult as a short thick cord connecting the end of the testis with the wall of the enclosing sac (Fig. 26). In the female the round ligament of the uterus is a structure similar in a general way to the gubernaculum, and in the rabbit will be seen to be inserted in a depression of the body-wall resembling both in form and position a rudimentary vaginal process. The ligament is continued, however, beyond this point, ending in the wall of the urinogenital aper- ture. The presence of this structure is an indication that the ovary, as well as testis, is subject to change in position. In the adult condition, however, the ovary occupies approximately an original position on the dorsal wall of the abdominal cavity ; and it will be observed in the rabbit that the position of the structure is one also defined by the points of origin of the spermatic arteries and veins of both sexes, with the ex- ception, however, of the left spermatic vein in the male. THE SEROUS CAVITIES. The organs collectively described as visceral are those associated with the serous cavities. They belong to several systems, but present the common feature of being projected into the membranous linings of these cavities so that they are more completely invested' by them. The serous sacs are extensive body-spaces, derivatives of a primary body cavity or coelom. They are usually considered as containing the visceral organs, but the condition is more accurately described as one in which the visceral organs encroach, chiefly from a dorsal position, on the enclosing membranes. The latter are thus divided into two portions, one of which is distributed as a parietal or peripheral layer, forming the enclosure of the sac, while the other is disposed as a visceral layer on the surface of the visceral organs. The serous sacs are enclosed by thin, moist, serous membranes, consisting chiefly of mesothelium, which give to the visceral organs their characteristic appearance. In lower vertebrates, where the diaphragm is absent or imperfectly developed, the coelom is divided into two chief portions — the pericardial cavity, enclosing the heart, and the pleuroperitoneal cavity, lodging the remaining visceral organs, including in terrestrial vertebrates the lungs. In the mammalia the pleuroperitoneal cavity is completely divided into two portions by the diaphragm, the smaller pleural portion being again divided into right and left pleural cavities through the presence of certain structures filling the median portion of the thorax. There are thus recognizable in a mammal four large serous spaces, namely, the pericardial, peritoneal, and paired pleural cavities. The pericardial cavity, the smallest of these spaces, is situated between the paired pleural cavities. Its enclosing membrane, the peri- cardium, forms a capacious sac for the heart, and is reflected directly over the surface of the latter as a thin membrane, the epicardium. The pleural cavities are those lodging the lungs, the latter being pro- jected into them from a medial position. The lining membrane or pleura 50 Anatomy of the Rabbit. is divided into three chief portions — the pulmonary pleura, investing the greater part of each organ, the costal pleura, lining the internal surface of the thorax, and the diaphragmatic pleura, covering the anterior surface of the diaphragm. The latter is broadly connected with the pulmonary pleura through. the pulmonary ligament. The peritoneal cavity, the largest of the serous spaces, comprises in a mammal a general portion, the abdominal cavity, and its posterior ex- tension into the pelvis — in the male also into the sac of the testis. The general relations of the cavity to the abdominal organs is indicated dia- grammatically in Fig. 21. Its lining membrane, the peritoneum, is divisible into two portions, the parietal peritoneum, lining the, abdominal wall, and the visceral peritoneum, investing the visceral organs. Of the latter the kidneys encroach only to a minor extent on the serous lining, so that they are covered by peritoneum only on their ventral sur- faces. The digestive tube, on the other hand, is removed to such an extent from the abdominal wall that the peritoneum forms a complete serous coat, and is connected with the parietal peritoneum of the wall through a thin transparent mem- brane, the mesentery. The latter consists of two plates of peritoneum, enclosing between them a thin layer of connective tissue, the lamina mesenterii propria, for the trans- mission of nerves, bloodvessels and lymph canals. As indicated above, the relations of the abdominal portion of the diges- tive tube are greatly modified by its elongation and displacement from a median position. Thus, while in the embryo the common mesentery is recognizable as a continuous median vertical fold, in the adult it follows the convolutions of the digestive tube, and is therefore considered as divided into corresponding parts. In many cases the relations of these are greatly complicated by secondary adhesions. , mesentery, and descending mesocolon will be recognized as parts in which a more typical arrangement is retained. Moreover, in the anterior portion of the abdominal cavity the peritoneum is concerned not only with the investment of two large visceral structures, the stomach and the liver, but also with the formation of a lining for the posterior surface of the diaphragm. Thus the general condition is less simple than in the small and large intestines. The peritoneum, v.ty. Fig. 26. Diagram showing (the relation of the testis to its investments: a.i., inguinal ring; c.e., caput epididymidis; cr., cremaster muscle; d.d., ductus deferens; g., guber- naculum; mes., mesorchium; p.t.v. and v.t.v., parietal and visceral layers of the tunica vaginalis propria; p. v., cavity of the vaginal process; s., integument of the scrotum; s.v., spermatic vessels; t., testis. In the rabbit the mesoduodenum, The Serous Cavities. 51 passing from the dorsal wall, successively invests the spleen, the stomach, and the liver, and is reflected from the last-named structure to the diaphragm and the ventral body-wall through the coronary, triangular, and falciform ligaments. Its gastric portion is differentiated into the mesogastrium (phrenicosplenic and gastrolienal ligaments), the greater omentum, and the lesser omentum. Similarly, in the posterior part of the body the peritoneum passes from the rectum to the urinary bladder, enclosing also in the female the vagina. It is then reflected to the ventral body-wall as the middle umbilical fold. In the male, as indicated in the accompanying diagram (Fig. 26), the peritoneal relations of the testis are greatly modified by the migration of the organ from an abdominal to a scrotal position. The entire sac lodging the testis is an evaginated portion of the abdominal wall, and since in the rabbit the cavity is widely open throughout life to the abdominal cavity, the lining membrane — that designated as the parietal layer of the tunica vaginalis propria— is continuous with the parietal peritoneum of the abdomen, and thus represents a permanent vaginal process. Like other structures of the abdominal cavity, the testis itself is covered by peritoneum, the latter being designated as the visceral layer of the tunica vaginalis propria. This investment is connected with the parietal layer by the mesorchium, and in the rabbit it will be observed that the latter is chiefly attached forwards on the dorsal wall of the abdomen, i.e., in a position indicating the original situation of the testis itself. In the female the ovary is closely associated with the -dorsal wall of the abdomen, and its supporting peritoneum, the mesovarium, is in- significant. Its duct in passing backward, however, becomes greatly displaced from a dorsal position, and thus comes to be supported by a broad fold of peritoneum. The latter is considered to consist of two portions, one, the mesosalpinx, being the support of the uterine tube, the other, the mesometrium, that of the uterus. The entire fold, how- ever, forms a continuous structure, and is known in this relation as the broad ligament. REGIONAL SECTIONS. The following plate-figures (I-VIII) are from characteristic sections of a rabbit-foetus of 56mm., and may be used either in connection with the general features of topography as outlined above, or for the identi- fication of various minor structures appearing in the dissection. Certain points regarding the sections are perhaps worthy of notice. First, in the longitudinal section illustrated in Plates I and II it will be noticed that paired structures frequently appear; this being because of the fact that the section is not exactly median, at least in certain places. Secondly, in using sections of the foetus for gross anatomical features it is necessary to make allowance in some cases for the different propor- tions of organs, and consequent slight differences in position, in the foetal as compared with the adult condition. .Finally, many of the features appearing in the original sections are such as could not be repro- duced in the plates, although they are indicated in the accompanying skeleton figures, and may be referred to in this way. _ a %$ c . ^j^j S £ rf ps w.H.S H O o M^.yrtw c ehOhhopqs;> w m S W Ej ..S-C £ SS.S . . . . < ■S-i.Sli s s ? " s s (Nco^iratot^oooso p o m w H 2 w W a H o o H O w ►J o M h W > 53 DESIGNATIONS FOR PLATE II. 9. 10. 11. 12. 13. 14. 15. Transverse sinus of dura mater. Dura mater. Pallium of cerebral hemisphere. Lateral ventricle. Olfactory bulb. Olfactory tract. Divided olfactory nerve in the cribriform plate. Chorioid plexus of third ventricle. Anterior commissure. Thalamus. Optic chiasma. Tuber cinereum. Mammillary body. Superior colliculus. Inferior colliculus. Anterior medullary velum. 32. Maxilla. 33. Hard palate (palatine and maxilla). 34. Presphenoid. 35. Inter sphenoidal synchondrosis. 36. Basisphenoid; hypophysial fossa. 37. Sphenooccipital synchondrosis. 38. Basioccipital. 38a. Supraoccipital. 39. Nasal portion of pharynx. 40. Soft palate. 41. Oral portion of pharynx. 42. Epiglottis and epiglottic cartilage. 43. Thyreoid cartilage of larynx. 44. Laryngeal cavity. 45. 45a. Cricoid cartilage. 46. Oesophagus. 47. Cricothyreoideus muscle. 16. Cerebral peduncle; cephalic flexure. 48. 17. Isthmus rhombencephali. 49. IS. Fourth ventricle. 50. 19. Pons; pontine flexure. 51. 20. Cerebellum. 52. 21. Posterior medullary velum. 53. 22 Cervical flexure. 54. %i. Central canal of spinal cord. 55. 24. Hypophysis. 5li. 2. r >. Frontal bone. 57. 2fi. Nasal bone. 58. 27. Nasal fossa. 59. 28. Mesethmoid cartilage. 5fla 29. Cartilage of vomeronasal organ. HO. 30. Premaxilla. 01. 31. Nasopalatine duct and cartilage. Ii2. Thyreoid gland. Sternohyoideus muscle. Genioglossus muscle. Geniohyoideus muscle. Mylohyoideus muscle. Mandible. Occipital musculature. Semispinalis capitis. Rhomboideus minor. Superior portion of trapezius. Atlas. Epistropheus. . Odontoid process. Third cervical vertebra. Median vertebral vein. Body of hyoid bone. 54 II. A Median Vertical Section of the Head. DESIGNATIONS FOR PLATE III. 1. Nasal bone. 2. Levator alae nasi muscle. 3. Nasal septum. 4. Nasoturbinal cartilage. 5. Maxilloturbinal (concha inf trior) 0. Nasal fossa. 7. Nasolacrimal duct. 8. Vomeronasal organ and cartilage 0. Pre maxilla. 10. Small tipper incisor. 11. Large upper incisor. 12. Nasopalatine ducts. 13 Oral cavity. 14. Tongue. 15. Vibrissae. 16. Caninus muscle. 17. Terminals of superior maxillary nerve. 18. Buccal glands. 19. Buccinator muscle. 20. Terminals of inferior alveolar nerve. 21. Quadratus labii inferioris muscle. 22. Mandible. 23. Lower incisor. 24. Meckel's cartilage (primary mandibular arch), 25. Mentalis muscle. III. A Transverse Section of the Anterior Nasal Region, 67 DESIGNATIONS FOR PLATE IV. 1. Superior sagittal sinus of dura mater. 2. Lateral ventricle. 3. Cerebral hemisphere. 4. Pia mater. 1 5. Frontal bone. 6. Cartilage of orbital wing. 7. Mesethmoid cartilage. 8. Cupula posterior cartilage. i 9. Obliquus superior muscle. 10. Ophthalmic vessels and nerves. 11. Levator palpebrae superioris muscle. 12. Rectus medialis muscle. 25. Nasal tract; choana. 26. Palatine bone. 27. Oral cavity. 28. Palatine nerve. 29. Sphenopalatine ganglion. 30. Infraorbital vein. 31. Internal maxillary artery. 32. Maxillary nerve. 33. Maxilla. 34. Zygomatic bone. 35. Submaxillary duct. 36. Buccinator muscle. -^24 13. Retractor bulbi muscle. 14. Rectus inferior muscle. 15. Sclera. 16. Retina and chorioidea. 17. Vitreous body. 18. Lens. 19. Posterior chamber of eye. 20. Anterior chamber. 21. Cornea. 22. Ciliary body and iris. 23. Upper eyelid. 24. Lower eyelid. 37. Masseter muscle. 38. Parotid duct.' 39. Facial nerve. 40. External maxillary artery and vein (anterior facial vein). 41. Platysma muscle. 42. Inferior labial artery and vein. ' 43. Mandible. 44. Genioglossus muscle. 45. Digastricus muscle. 46. Quadratus labii inferioris muscle. 47. Geniohyoideus muscle. 58 IV. A Transverse Section of the Orbital Region. 59 DESIGNATIONS FOR PLATE V. 1. Parietal bone. 21. Longus capitis. 2 Transverse sinus of dura mater. 22. Rectus capitis anterior. 3'. Superior colliculus. 23. Oral portion of pharynx. 4. Cerebral aqueduct. 24. Thyreohyoideus muscle. r>. Isthmus rhombencephali. 25. Sternohyoideus muscle. 6. Pons. 2fi. Greater cornu of hyoid. 7. Trigeminal nerve. 27. Stylohyoideus major muscle. s. Basilar artery. 28. Lingual artery. 9. Facial nerve. 29. Hypoglossal nerve. 10. Cartilaginous auditory capsule. 30. Tendon of digastricus muscle 11. Cochlea. 31. External maxillary artery. 12. Basioccipilal bone. 13. Tensor tympani muscle. 14. Tympanic cavity. 15. Mailuus. 16. Tributarj.s of posterior facial vein. 17. Squamosal bone. 18. Cephalic portion of median vertebral vein. 19. Nasal portion of pharynx. 20. Origin of basioclavicularis and levator scapulae major muscles. 32. Stylohyoideus minor. 33. Styloglossus. 34. Internal maxillary artery. 35. Tympanic bone. 36. Mandible. 37. Submaxillary gland. 38. Anterior facial vein. 39. Internal carotid artery; GO V. A Transverse Section of the Auditory Region. ci DESIGNATIONS FOR PLATE VI. 1. Rhomboideus minor. 2. Superior portion of trapezius, 2a. Levator scapulae minor. 3. Splenius. 4. Semispinalis capitis. 5. Rectus capitis posterior superficialis. 6. ObHquus capitis major. 7. Arch of epistropheus. S. Ganglion of posterior root. 9. Longissimus cervicis. 19. Oesophagus. 20. Inferior thyreoid nerve. 21. Inferior thyreoid vein. 22. Trachea. 23. Thyreoid gland. 24. Cardiac branch of vagus (n. depressor). 25. Sympathetic trunk. 26. Vagus nerve. 27. Common carotid artery 28. Internal jugular vein. 10. Longissimus capitis. 11. Yertebral artery and vein. 12. Longus atlantis. 13. Vertebral body. 14. Transverse process (anterior root). 15. Median vertebral vein. 16. Longus colli. 17. Longus capitis. 18. Fat-body. 29. Sternohyoideus muscle. 30. Sternothyreoideus muscle. 31. Sternomastoideus muscle. 32. Descending ramus of hypoglossal nerve. 33. External jugular vein. 34. Basioclavicularis muscle. 35. Levator scapulae major muscle. 36. Cleidomastoideus. 37. Platysma. 62 VI. A Transverse Section of the Anterior Cervical Region. DESIGNATIONS FOR PLATE VII. 1. Semispinalis dorsi. 2. Longissimus dorsi. Iliocostalis. Spinal cord. Ganglion of posterior root and intercostal nerve. Tubercle of rib. t . Head of rib. 8. Sympathetic trunks. 9. Azygos vein. 10. Thoracic aorta. 11. Oesophagus. 3. 4. 5. 6. 7. 24. Costal pleura. 25. Bone ribs. 26. Costal cartilage. .27. Sternum. 28. Cutaneus maximus muscle. 29. Inferior portion of trapezius 30. Rhomboideus major. 31. Inferior angle of scapula. ' 32. Latissimus dorsi. 33. Serratus posterior. 34. Intercostales externi and intcrni 34a. Intercostalis internus. 12 12a. Right and left vagi. 35. 13. Lung. 36. 14. Bronchi. 37. 15. Branches of pulmonary artery. 38. 16. Pulmonary veins. 39. 17. Right atrium. 40. IS. Tricuspid valve. 19. Right ventricle. 41. 20. Left atrium. 42. 21. Left ventricle. 43. 22. Pericardial cavity. 44. 23. Pulmonary pleura. 45. 35. Thoracic portion of serratus anterior. a Obliquus externus abdominis. Trans versus thoracis. Pect oralis major. Rectus abdominis. Anconaeus longus (caput longum of triceps). * Extensor antibrachii parvus. Anconaeus medialis. Anconaeus lateralis. -. Distal extremity of humerus. 45. Proximal portion of radius. G4 VII. A Transverse Section of the Thorax. c,-, DESIGNATIONS FOR PLATE VIII. 1. Spinal cord. 2. Vertebral canal. 3. Vertebral body. s 4. Sacrospinalis muscle. 5. Quadratus lumborum. 6. Psoas major. 7. Psoas minor. 8. Sympathetic trunk. 9. Abdominal aorta. 18, 18a. Posterior and anterior lobules of left lobe of liver. 19,_19a. Right lobe o£_liver._ _ 20. Obliquus internus abdominis and trans- versus abdominis. 21. Obliquus externus abdominis. 22. Rectus abdominis. 22a. Cutaneus maximus. 10. Inferior caval vein. 11. Descending mesocolon. 12. Ureter. 13. Renal pelvis. 14. Renal papilla. 15. Left kidney. 16. Parietal peritoneum. 17. Visceral peritoneum. 23. Middle umbilical fold. 24. Urinary bladder (canal of foetal allantois). 25. Umbilical arteries.' s 26. Duodenum. 27. Pancreas and mesoduodenum. 28. Descending colon. 29. Parts of mesenterial small intestine. •''-0. Caecum. VIII. A Transverse Section of the Abdomen. C7 PART II. OSTEOLOGY OF THE RABBIT. For a practical study of the rabbit's skeleton, a thoroughly cleaned, but otherwise rough, unmounted skeleton will be found most convenient. The skull should be divided with a fine saw at a little to one side of the median plane, or a second skull may be provided for this purpose (cf. Fig. 33). The most useful specimens for reference are: (1) a well- mounted skeleton of the adult animal, showing the natural relations of the bones; and (2) a rough skeleton of a young animal of from one to five weeks, showing the primary composition of cartilage bones. For the special study of the skull (pp. 85-97) a disarticulated specimen may be employed, but the majority of the features may be made out in the intact or divided skulls. The general account of the skull as given below will be found to cover most of the osteological points noted in the dissection. DIVISIONS OF THE SKELETON. The skeleton is divisible into two main portions, namely, the axial skeleton and the appendicular skeleton. The former comprises the vertebral column, the ribs, the sternum, and the skeleton of the head; the latter, the supports of the anterior and posterior limbs, and the associated pectoral and pelvic girdles. THE VERTEBRAL COLUMN. The vertebral column (columna vertebralis) is formed of a linear series of segments, the vertebrae. In accordance with its function as a general support of the body, and also its relations with the nervous system and the spinal musculature, the vertebrae, with minor exceptions, are constructed on the same plan. Those of particular regions also present certain features in common, so that it is possible to classify them into cervical, thoracic, lumbar, sacral, and caudal groups. A typical vertebra — for the characters of which any one of the thoracic or lumbar series may be taken (Fig. 27, D-F) — consists of a basal portion, the vertebral body (corpus vertebrae), and of a dorsal, vertebral arch (arcus vertebrae). The two portions enclose a large aperture, the vertebral foramen (foramen vertebrale). The successive foramina form an almost complete tube, the vertebral canal (canalis vertebralis), for the accommodation of the spinal cord. The body of a vertebra is a cylindrical, or somewhat dorsoventrally compressed, mass of bone, which bears at either end an articular surface for attachment to the adjacent vertebra. The articular surfaces are borne on thin plate-like epiphyses, the epiphysial lines being evident even in older animals, especially in the lumbar region. The dorsal por- es The; Vertebrae Column. 69 tion of^the body bears on either side the pedicle, or root of the vertebral arch .(radix arcus vertebrae), the dorsal surface of the body forming in this-ltfay the floor of the vertebral foramen. The dorsal portion of the arch, borne on the pedicle, is distinguished as the lamina. The anterior and posterior margins of the pedicle are notched, each notch p.a.i Fig. 27. Representative vertebrae: A, atlas, anterior surface; B, epistropheus, lateral surface; C, fifth cervical vertebra, anterior surface; D, fourth dorsal, lateral surface; E, F, second lumbar vertebra, anterior and lateral surfaces. a.a., anterior arch of atlas; a.p., posterior arch of atlas; a. v., vertebral arch; c.v., vertebral body; d., dens epistrophei; f.a.a., anterior articular facet of epistropheus; f.a.s., superior articular pit of atlas; f.a.s. 1 ., superior articular facet of epistropheus; f.c.i., inferior costal demifacet for head of rib; f.c.s., superior costal demifacet; f.c.t., costal facet of transverse process;' f.d., fovea dentis; f.i., intervertebral foramen; f.tr., foramen transversarium ; f.v., foramen vertebrale; 1., lamina of vertebral arch; m.l., lateral mass of atlas; p.a., accessory process of lumbar vertebra; p.a.i. inferior articular process; p.a.s. superior articular process; p.m., mammillary process; p.s., spinous process; p.s. a., anterior spinous process; p.t., transverse process; p.tn., trian- gular process; r. radix of vertebral arch; r.a., r.p., anterior and posterior radices of transverse process of cervical vertebra; t.a., t.p., anterior and posterior tubercles of atlas. or incisure being converted, through its association with that of the adjacent vertebra, into a rounded aperture, the intervertebral foramen (foramen intervertebrale), for the passage outward of a spinal nerve. 70 Anatomy of the Rabbit. The arch of the vertebra is noteworthy for its projections or processes. On either side is a horizontal plate of bone, the transverse process (pro- cessus transversus), and, dorsally, a median projection, the spinous process (processus spinosus), all three serving for the attachment of the vertebrae to one another by ligaments, and for the attachment of the spinal musculature. Special articular surfaces, borne on low articular pro- cesses (processus articulares), are found on the anterior and posterior margins of the arch. The anterior, or superior articular surfaces are directed for the most part toward the dorsal surface, and are overlapped in the natural condition by the inferior articular surfaces, which are directed toward the ventral surface. A certain amount of movement is permitted by one surface slipping across .the other, the mechanism illustrating the arthrodia, or gliding-joint. The cervical vertebrae (vertebrae cervicales) are seven in number. The posterior five are similar, while the anterior two are specially modified in relation to the skull. The posterior vertebrae (Fig. 27, C) are dorso- ventrally compressed, their arches low, and the spinous process short. In the seventh vertebra, however, the spinous process begins to be elon- gated as in the succeeding thoracic vertebrae. In each vertebra the transverse process is perforated by a costo-transverse foramen (foramen transversarium), the latter serving for the passage of the vertebral artery forward to the head: Through the presence of this aperture, the base of the transverse process is divided into two parts, namely, a dorsal, or posterior root (radix posterior), and a ventral, or anterior root (radix anterior). The anterior root is a coalesced rib, and is comparable in its general relations to the normal ribs of the thoracic vertebrae. The first vertebra is the atlas (Fig. 27, A). It is peculiar in lacking the vertebral body, the latter being represented by the odontoid process of the epistropheus (cf . Plate II) ; also in possessing special articular surfaces, and in having its transverse process greatly flattened in the dorsoventral direction. It consists of a ventral half-ring, the anterior arch (arcus anterior), a dorsal half-ring, the posterior arch (arcus pos- terior), with paired lateral masses (massae laterales) uniting them. The lateral masses also form the bases of the transverse processes. The anterior arch bears on its ventral side a small backwardly-directed process, the anterior tubercle (tuberculum anterius). A similar posterior tubercle (tuberculum posterius) on the dorsal surface of the posterior arch is comparable to the spinous process of an ordinary vertebra. The anterior surface of the atlas bears on either side an extensive concave smooth surface, the superior articular pit (fovea articularis superior), for articulation with the convex occipital condyles of the skull. Its posterior surface bears on either side a somewhat triangular inferior articular facet (facies articularis inferior) for articula- tion with the epistropheus. These surfaces take the place of the arch- articulations of ordinary vertebrae. Through the compression of the transverse process, the costotransverse foramen is converted into a canal. The anterior aperture of this leads by a shallow groove, the sulcus arteriae vertebralis, into a second aperture perforating the posterior arch. The Vertebral Coeumn. 71 The space enclosed by the atlas is divided into a dorsal portion, corresponding to the vertebral foramen of other vertebrae, and a ventral portion which in the natural condition lodges the odontoid process of the epistropheus. The division is effected partly by a small tubercle on the inner side of each lateral mass, and partly by a transverse liga- ment which is stretched . between them and over the dorsal surface of the odontoid process. On the floor of the ventral portion, a rounded articular surface, the fovea dentis, marks the point of articulation of the anterior articular facet of the odontoid process with the inner surface of the anterior arch. The second vertebra is the epistropheus (Fig. 27, B). It resembles the succeeding cervical vertebrae more closely than does the atlas. It is noteworthy for its great size, for the lateral compression of its arch and spinous process, and for the possession of a stout forwardly-directed odontoid process, or tooth (dens epistrophei). It is articulated with the atlas through an anterior articular facet, borne on the ventral surface of the odontoid process, and by large paired superior articular facets borne on its base. The spinous process of this vertebra and the trans- verse processes of the atlas form together three main points of attach- ment for the occipital musculature. The thoracic vertebrae (vertebrae thoracales) are twelve in number. They are distinguished chiefly by the possession of articular pits for the attachment of ribs (Fig. 27, D). A rib is articulated at two points, namely, one on the body of the vertebra, the other on the transverse process. The former is marked by a small round depression, the costal pit (fovea costalis), or costal facet. In the last two vertebrae the facet is borne wholly on the vertebral body to which the rib belongs. In the remaining vertebrae a complete articulating surface is formed by two demifacets, one being on the vertebra to which the rib belongs, the other on the vertebra immediately in front. The articulation of a rib with a transverse process is marked by an oval facet, the costal pit of the transverse process (fovea costalis transversalis) . It is present only in the first ten of the thoracic vertebrae. In all vertebrae of the thoracic series the spinous processes are well- developed. They increase in length to the third, and then become gradually shorter, although their surfaces are, on the whole, slightly increased in extent. The anterior ten are directed backward, the elev- enth is almost vertical, while the twelfth is directed forward, like those of the succeeding lumbar vertebrae. The lumbar vertebrae (vertebrae lumbales) are seven in number. They are large vertebrae, conspicuous for their extensive surfaces and processes for muscular attachment (Fig. 27, E, F). The transverse processes continue the general line of the ribs of the thoracic region. They are directed forward, as well as outward, and the tip of each is formed by a thin triangular plate (processus triangularis), which represents a fused rib. At the posterior side of the base of each is a short, flattened projection, the accessory process (processus accessorius). The spinous process is especially well-developed, and is directed forward. The articular processes are rotated upward, so that their surfaces are directed n Anatomy of the Rabbit. more nearly toward, or away from, the median plane, instead of to the dorsal or ventral surface. The anterior articular surfaces are borne on the bases of stout, upwardly-directed mammillary processes (processus mammillares) . The latter are most characteristic of the lumbar verte- brae, but may be seen to arise in the posterior thoracic region as small elevations of the transverse processes. Each of the first three of the lumbar vertebrae bears a median ventral projection, the anterior spinous process (processus spinosus anterior), for the attachment of the lumbar portion of the diaphragm. The sacral vertebrae (vertebrae sacrales) are four in number. In contrast to the true vertebrae — those united by ligament and articular surfaces — of the remaining portions of the vertebral column, they are false vertebrae, united in the young by synchondroses, and in the adult coalesced to form a composite structure, the os sacrum (Fig. 28). The A B c.Y. P<«- f' m - Fig. 28 The os sacrum: A, ventral (pelvic) surface; B, dorsal surface; c.v., bodies of coalesced vertebrae; fa., auricular surface; f.s.a., anterior sacral foramina; f.s.m., median sacral foramina; f.s.p., posterior sacral foramina; p.a.s., superior articular process of first vertebra; p.m., mam- millary process of first vertebra; pr. promontory; p.s., spinous processes. axis of the sacrum forms an obtuse angle with that of the lumbar ver- tebrae, the angle being indicated by a ventral projection, the promon- tory (promontorium), formed by the last lumbar and first sacral ver- tebrae. The sacrum is the medium through which the vertebral column — in other words, the posterior portion of the trunk — is supported on the posterior limbs. Its anterior dorsal portion bears on either side a roughened area, the auricular surface (facies auricularis), for articulation with the pelvic girdle. This surface is borne for the most part on the transverse process of the first sacral vertebra. The Ribs. 73 The sacrum exhibits many features resulting from its formation through the fusion of originally distinct vertebrae. On the ventral, or pelvic surface (facies pelvina) , the lines of junction may be traced either between the bodies, or between the transverse processes. Four pairs of apertures on this surface, the anterior sacral foramina (foramina sacralia anteriora), lead into the intervertebral foramina, and give passage to the sacral spinal nerves. On the dorsal surface (facies dorsalis) a pair of posterior sacral foramina lie in the line of junction of the first and second vertebrae. The spinous processes are evident in all four vertebrae. The combined articular and mammillary processes are conspicuous only in the first two, but are represented in the succeeding two by low, roughened tubercles. In the middle line dorsally the vertebral arches are separated by con- spicuous apertures, the median sacral fora- mina. The caudal vertebrae (vertebrae caudales) are sixteen in number. They are segments of small size, increasing slightly to the third, and then gradually decreasing to the end of the column. The arches are complete in the first seven. The transverse processes are vestigial in all except the third. At the end of the column the segments are reduced to slender cylinders of bone representing the vertebral bodies. THE RIBS. The ribs- (costae) are twelve in number on either side. Each is composed of a dorsal portion, the costal bone (os costale), or bone-rib, and a ventral portion, the costal cartilage (cartilago costalis) (Fig. 29). From their, attachment on the vertebral column the bone-ribs are directed outward, downward, and backward. The costal cartilages are directed for the most part inward, down- ward, and forward. The first costal cartilage forms a pronounced angle with the corres- ponding bone-rib. In the succeeding ribs the angle is gradually replaced by a broad curve. Ribs are classified as true ribs (costae verae), and false ribs (costae spuriae). The former — comprising the anterior seven — are those directly attached to the sternum. The latter — comprising the posterior five — are either indirectly attached, or unattached. The unattached ribs are designated as floating ribs. Generally speaking, the bone-ribs are cylindrical; but the anterior Fig. 29. The sternum and first rib, ventral view: 1-7, the true ribs; 8, first false rib; c.c., head of rib; cl.c., neck of rib; cr., costal cartilage; cr.c, body of rib; c.s., body of sternum; m.s., manubrium sterni ; o.c, bone-rib; p.x., xiphoid process; i.e., costal tubercle. 74 Anatomy oj? the Rabbit. five or six are more or less flattened, with their main surfaces respectively medial and, lateral. The compression is most marked in old animals. The first rib is extremely short. The succeeding ribs increase in length to the sixth, and then decrease to the twelfth. The arch formed by each rib has its greatest convexity, or angle, at, some point toward the dorsal surface. Passing backward, the point of greatest convexity changes from a medial to a lateral position. This, together with the elongation of the more posterior ribs, results in an enormous increase in the posterior extent of the thoracic cavity. The vertebral end, or head of the rib (capitulum costae), is articulated with the body of the vertebra to which it belongs, and also, in the case of the first ten, with the vertebra immediately in front. The articulation with a transverse process is marked by a small smooth elevation, the costal tubercle (tuberculum costae). It is present only in the first nine ribs. Except in the first rib, and in the last four, the tubercle bears a sharp, dorsally-directed process for muscular attachment. The slender portion of the rib intervening between the head and tubercle is the neck (collum costae), the remaining larger portion being distinguished as the body of the rib (corpus costae). The bony thorax is formed by the ribs and the sternum with the assistance of the thoracic vertebrae. It encloses a large space, the thoracic cavity (cavum thoracis). The latter is conical in shape, with the apex directed forward. The dorsoventral diameter of the cavity is considerably greater than the transverse diameter. Apart from the intercostal spaces, the cavity is open at two points : anteriorly, the first thoracic vertebra, the first rib, and the manubrium sterni together enclose a small opening, the superior thoracic aperture (apertura thoracis superior) ; posteriorly, the seventh and succeeding ribs, together with the posterior thoracic vertebrae and the xiphoid process of the sternum, enclose a much larger opening, the inferior thoracic aperture (apertura thoracis inferior). In the natural condition it is largely closed by the diaphragm. The curved boundary formed by the ribs in this region is the costal arch (arcus costarum), the angle formed at the point of attachment of the xiphoid process being the infrasternal angle (angulus infrasternalis) . Through their articulations with the vertebral column, and the nature of the costal cartilages, the ribs are capable of being moved, or rotated, forward. The movement results in an increase of the extent of the thoracic cavity, and is of importance in respiration. THE STERNUM. The sternum consists of a linear series of six segments, the sternebrae. The first segment is the manubrium sterni. It is about twice the length of the middle segments. It is somewhat triangular in section, two of its surfaces being ventrolateral, the third dorsal and directed toward the thoracic cavity. To its anterior tip is attached the sternoclavicular ligament, by which the greatly reduced clavicle is united with the sternum. The Skeleton op the Head. 75 The four middle segments aire similar in appearance, and form the body (corpus sterni). The sixth segment, described as the xiphoid process (processus xiphoideus), is an elongated strip of bone, to the posterior end of which is attached a broad, thin plate of cartilage. The first costal articulation is situated at about the middle of the manubrium, the remaining six at the points of junction of the segments. Five of them occur singly, while the sixth and seventh costal cartilages are attached together at the point of junction of the last segment of the corpus sterni with the xiphoid process. THE SKELETON OF THE HEAD. The head-skeleton comprises: (1) the series of elements constituting the skull; and (2) the hyoid bone, with its connections. The skull, or cranium — using that term in a general sense — includes the cranium proper, that portion enclosing the brain and containing in its wall the auditory capsules, and the bones of the face (ossa faciei), the latter in- cluding the series of elements related for the most part to the jaws and palate. The primary relations of the constituents of the head-skeleton have already been indicated above (p. 31). A. THE SKULL AS A WHOLE. The skull is a composite structure, consisting of a large number of elements, which, with the exception of the mandible, are united by synarthroses, so that they produce the effect of a continuous mass. The mandible is a more or less independent structure, articulated with the main hody of the skull by a typical joint. The skull is roughly divisible into two portions, namely, an anterior, facial portion, and a posterior, cranial portion. The cranial portion has a somewhat conical shape, its apex being directed forward. It is sepa- rated from the facial portion by a depression on either side of the skull, the orbital cavity (orbita), which serves for the accommodation of the optic bulb. Unlike the remaining special sense-organs, the eye is not included within the skull-wall. The two portions are united both medially and laterally, the lateral connection being established by the zygomatic arch (arcus zygoma ticus), which bridges the lateral portion of the orbit. The facial portion has also a somewhat conical shape, its apex being formed by the anterior extremity of the upper jaw and the incisor teeth. Its base is formed in part by the connection with the cranial portion, as already described, and also by the anterior walls of the orbits. The cranial portion exhibits an extensive posterior, nuchal surface (planum nuchale), situated in general at right angles to the cervical portion of the vertebral column and also to the dorsal, lateral, and ventral walls of the skull. This surface includes the external surface of the occipital bone, with the exception, chiefly, of the basilar portion of the latter." Its dorsal portion forms an area of attachment for the spinal and special occipital musculature. Its ventral portion is perforated by a 76 Anatomy of the Rabbit. largeTaperture, the foramen magnum occipitale, for the passage of the central nervous system from the cranial cavity into the vertebral canal. On either side of this is a smooth, ridge-like projection, the occipital condyle (condylus occipitalis), for articulation with the superior articular pits of the atlas. At a little distance lateral to the occipital condyle, the nuchal surface is continued downward through the medium of a some- what triangular, pointed jugular process (processus jugularis). This structure is separated from the occipital condyle by a pronounced notch, the posterior boundary of a deep narrow excavation, the jugular fossa (fossa jugularis), which lies between the condyle and the tympanic bulla. The jugular process serves for the attachment of muscles belonging to the tongue, hyoid, and mandible, namely, the styloglossus, stylohyoidei major and minor, and the digastricus, the suspensory ligament of the lesser cornu of the hyoid also being included in the ligament of the stylohyoideus minor. Toward the dorsal margin of the nuchal surface, the dorsal surface of the skull is projected backward as a shield-shaped -j>.o.e. Fio. 30. Lateral surface of the skull: AS, alisphenoid (ala magna); BO, basioccipital (basilar portion of occipital); BS, basisphenoid (body oi posterior sphenoid); F, frontal; I, interparietal; L, lacrimal; M, maxilla; MS, mastoid portion of petrosal (petromastoid) ; N, nasal; OS, orbitosphenoid (alaparva); P, parietal; PL, palatine; FM, premaxilla; SO, supraoccipital (squamous portion of occipital); SQ, squamosal; T, tympanic; ZY, zygomatic. a.p., piriform aperture of nose; d.i., incisor teeth; d.m., molars; d.pm , premolars; f.i., infraorbital foramen; f.mx., maxillary fossa; f.o., optic foramen; f.s., stylomastoid foramen; f.t., temporal fossa; 1.1., lateral lamina of pterygoid process; l.m., medial lamina; m.a.e., osseous portion of external acoustic meatus; p. a., alveolar process of maxilla; p.e., ethmoidal portion of orbitosphenoid; p.f., frontal process of premaxilla; p.j., jugular process of occipital; p.m., mastoid process of mastoid; p.mx., maxillary process of frontal; p.o., orbital process of maxilla; p.o.e., externa] occipital protuber- ance; p.s., squamosal process of parietal; p.s.a., and p.s.p., anterior and posterior supraorbital processes of frontal; p. z., zygomatic process of squamosal; p.z.m., zygo- matic process of maxilla; s, sphenoorbital process of maxilla; s.m., spina masseterica; sq., squamosal process of squamosal. promontory. The lateral margin of this projection is the superior nuchal line (linea nuchae superior). It forms a curved ridge, the position of which indicates the dorsal limit of the occipital musculature. The posterior, somewhat tri-radiate tip of the projection is the external occipital protuberance (protuberantia occipitalis externa), an important The Skuu, as a Whole. 77 median point of attachment for the occipital muscles and the ligamentum nuchae. The ventral wall of the cranial portion is the basal portion (basi- cranium) of the entire skull. As indicated above (p. 31), its axial line, the basicranial axis, continues, in general, that of the bodies of the vertebrae, and its posterior portion is equivalent, morphologically, to vertebral segments. It is formed by a linear series of three bones, namely, the basilar portion of the occipital, the body of the posterior sphenoid, and that of the anterior sphenoid (respectively basioccipital, basisphenoid and presphenoid bones). Its extremely narrow, anterior portion forms the roof of a deep groove which encloses the nasal portion of the pharynx. As viewed from the ventral surface, it is seen to dis- appear in the facial complex at some distance dorsal to the posterior margin of the bony palatine bridge. Laterally, it is separated from the orbit on either side by a vertical plate formed by the palatine bone, and also by two downward projections of the posterior sphenoid, the medial and lateral laminae of the pterygoid process (processus pterygoideus). These structures enclose between them the pterygoid fossa (fossa pterygoidea), the walls of which serve for the attachment of the external and internal pterygoid muscles of the mandible. The lateral wall of the cranial portion fo^rms anteriorly a large part of the boundary of the orbit. The cranial wall of the orbit is partly formed by two upward projections of the basicranium, namely, the lesser wing of the anterior sphenoid, or orbitosphenoid, and the greater wing of the posterior sphenoid, or alisphenoid. The remaining portion is formed by membrane elements, including the frontal bone of the roof of the skull and the squamosal bone, the latter distinguishable as the support of the posterior root of the zygomatic arch. Immediately behind 'the orbit, the root of the zygomatic arch projects outward and then downward. It is formed by a zygomatic process (processus zygomaticus) of the squamosal bone, and the tip of this forms a vertical plate, which is united by a horizontal suture with the zygomatic bone. On the ventral side, of the process, close to the cranial wall, is the mandibular fossa (fossa mandibularis), for articulation with the mandible. On its dorsal side, but more especially on the adjacent portion of the cranial wall, there is a shallow, horizontal groove, lodging in the natural condition the temporalis muscle of the mandible, and therefore representing a greatly reduced temporal fossa (fossa temporalis) . In the natural condition the anterior portion of the groove is converted into a foramen through the presence of a stout ligament extending from the posterior supraorbital process to the base of the zygomatic arch. The dorsal boundary of the fossa is formed by a pronounced ridge, the temporal line (linea temporalis) , the latter forming also the lateral margin of the roof of the skull in this region. Behind the posterior root of. the zygomatic arch, the external surface of the lateral wall is largely occupied by the swollen tympanic bulla (bulla tympani) , formed by the tympanic bone. It contains the capac- ious tympanic cavity (cavum tympanicum) and certain skeletal structures of the middle ear, namely, the auditory ossicles (ossicula auditus), the 78 Anatomy of th£ Rabbit. Z.Y' relations of which are more fully dealt with below (p. 90). The dorsal portion of the tympanic bulla is continuous with a short bony tube which opens at a short distance dorsad by a large oval aperture. This tube is part of a more extensive canal, the external acoustic meatus (meatus acusticus externus) which, in the natural condition, leads down- ward through the base of the external ear to the tympanic membrane. The tympanic bulla does not form the lateral wall of the skull in this region, and is not exposed to the cranial cavity. It is applied closely to' the external surface of the periotic or petromastoid bone, which forms the lateral boundary of the cranial cavity, and contains the structures of the internal ear. The external or mastoid portion of this bone appears in the space enclosed between the tympanic bulla and the jugular process of the occi- pital bone, where it is readily distinguishable by its pitted appearance. Its ventral portion bears a slender projection, lying parallel to the jugular process, the mastoid process (processus mastoideus). A series of foramina, lying partly within the orbit, and extending thence posteriorly along the boundary between the lateral, and ventral walls to. the occiput, put the cranial cavity in communication with the outside, and serve for the passage of nerves and vessels. The first and largest of these, the optic foramen (foramen opticum), occupies the middle portion of the orbit, and trans- mits, in the natural condition, the optic nerve. Following this is a vertical slit-like aperture — not to be confused with the perforations of the external lamina of the pterygoid process — the superior orbital fissure (fissura orbitalis superior). It represents both the superior orbital fissure of. the normal mammalian skull and the foramen rotundum, and provides for the passage outward of the third, fourth and sixth cranial nerves, together with the first and second divisions of the fifth. The lateral lamina of the pterygoid Fig. 31. Dorsal surface of the skull: F, frontal; I, interparietal; L, lacrimal; M, maxilla; MS, mastoid porl.ion of petrosal (petromastoid); N ( ■ nasal; P, parietal; SO, supraoccipital (squamous portion of occipital); SQ, squamosal; ZY, zygomatic. f.mx., maxillary fossa; f.t., temporal fossa; l.n.s., superior nuchal line; l.t., temporal line; p.f., frontal process of premaxilla; p.mx., maxillary process of frontal; p.o.e., external occipital protuberance; p.s.a. and p.s.p., anterior and posterior supraorbital processes of frontal; p.sc, subcutaneous process of lacrimal; p.z., zygomatic prooess of squamosal; p.z.m., zygomatic process of maxilla; s.f., frontal spine. s.m., spina masseterica. The Skuia as a Whoia 79 process presents three foramina, of which the largest, anterior, and medial one, the anterior sphenoidal foramen, serves for the transmission of the internal maxillary artery, while the remaining two, the middle and posterior sphenoidal foramina, transmit muscular branches (masseterico- temporal and pterygobuccinnator) of the mandibular nerve. On the medial side of the base of the medial lamina of the pterygoid process there is a shallow longitudinal groove, representing the pterygoid canal (canalis pterygoideus) of the human skull. Immediately in front of the tympanic bulla, on the ventral surface of the skull, an irregular aperture, the foramen lacerum, leads directly into the cranial cavity. It is incompletely divided into two parts by a slender bony splint. It contains, in addition to the foramen lacerum, the foramen ovale of the typical mammalian skull, and serves to transmit the mandibular portion of the fifth nerve and the internal carotid artery. Looking into the aper- ture from the front, it is seen to communicate not only with the cranial cavity, but also with two apertures in the anterior portion of the auditory complex. One of these — that toward the middle line — is the internal carotid foramen (foramen caroticum internum). It is the an- terior end of a canal transmitting the internal carotid artery; the pos- terior end of this canal, the point at which the internal carotid artery enters the tympanic bone, or the external carotid foramen (foramen caroticum externum), being visible as a rounded aperture lying on the posteromedial siirface of the tympanic bulla. The second, lateral aperture communicating with the foramen lacerum is that of the auditory (Eustachian) tube (tuba auditiva). It leads into the tympanic cavity, and in the natural condition the tube places this cavity in com- munication with the nasal portion of the pharynx. Associated with the mastoid process is a small aperture, the stylomastoid foramen (foramen stylomastoideum), the external opening of the facial canal, which serves for the passage of the facial nerve. A slit-like aperture, the jugular foramen (foramen jugulare),, lies in the • jugular fossa, between the posterior ventral margin of the tympanic bulla and the occipital condyle. It transmits the first portion of the internal jugular vein from the trans- verse sinus of the dura mater, and also the ninth, tenth, and eleventh cranial nerves. Finally, immediately in front of the dorsal portion of the condyle, the occipital segment is perforated by several small apertures together representing, the hypoglossal canal (canalis hypoglossi), and serving for the transmission of the hypoglossal nerve. The roof of the cranial portion is largely formed by two pairs of thin membrane elements, the frontal and parietal bones. The former occupy a general position between the orbits, while the latter are interposed between the fr.ontal bones and the occipital segment. A small portion of the roof is formed posteriorly, however, by a ' small lozenge-shaped element, the interparietal bone, and by the shield-shaped projection described above, which is part of the occipital bone. The space enclosed by the cranial portion of the skull is the cranial cavity (cavum cranii). Its form depends on the external configura- tion of the brain. It is divisible into three portions, known as the cranial fossae. The anterior cranial fossa (fossa cranii anterior) is a 80 Anatomy oj? the Rabbit. small division lodging in the natural condition the olfactory bulbs of the brain. The middle cranial fossa, the largest division of the cavity, lodges the enlarged cerebral hemispheres. The posterior cranial fossa is a small division extending backward to the foramen magnum, and lodging in the natural condition the cerebellum and related posterior portions of the brain. It is partly set off from the middle cranial fossa by a fold of the dura mater, the tentorium cerebelli, which projects inward from the dorsal and lateral walls of the skull. This fold is usually found adhering to the internal surface of the skull, unless the latter has been very thoroughly cleared, and in all cases its posi- tion is indicated by a low ridge of bone. The marked difference in diameter between the middle and posterior cra- nial fossae is accounted for by the great thickness of the auditory portion of the skull. The anterior surface of the periotic bone will be observed to form an extensive posterior wall for the middle cranial fossa. The floor of the middle and posterior cranial fossae is not smooth, like the external base of the skull, but presents in its anterior portion a pro- minent elevation, the sella turcica, which is borne on the body of the posterior sphe- noid. It contains a large central depression, the hypo- physial fossa (fossa hypophy- seos), which in the natural condition lodges the hypo- physis or pituitary body. The aperture of the fossa is partly enclosed laterally by a pair of pointed posterior clinoid processes (processus clinoidei posteriores), the tips of which are directed forward; and a corresponding pair of anterior clinoid processes lie at the anterior end of the fossa, with the tips directed backward, the fossa, described as the Fig. 32. Ventral surface of the skull: AS, alisphenoid (ala magna); B, basioccipital (basilar portion of oc- cipital); BS, basisphenoid (body of posterior sphenoid); EXO, exoccipital; M, maxilla; PL, palatine; PMX, premaxilla; PR, presphenoid (body of anterior sphe- noid); SO, supraoccipital (squamous portion of occi- pital); SQ, squamosal; T, tympanic; ZY, zygomatic. ch, choana; c.hy., hypoglossal canal; c.o., occipital condyle; f.c.e., external carotid foramen; f.in., incisive foramen; f.j, jugular foramen; f.l., foramen lacerum; f.m., mandibular fossa; f.m.o., foramen magnum; f.p ; m., greater palatine foramen; f.s.a., anterior sphe- noidal _ foramen ; m.a.e., osseous portion of external acoustic meatus; p.j., jugular process; p.o.e., external occipital protuberance; p.pl., palatine process of maxilla; p.pm., palatine process of premaxilla; p.pt., medial and lateral laminae of pterygoid process of posterior sphenoid; s.m., spina masseterica. The posterior, and also dorsal wall of dorsum sellae, leads by an abrupt curve The Skull as a Whole. 81 backward on to the floor of the posterior cranial fossa, the sloping portion of the floor, or clivus, supporting in the natural condition the pons and medulla oblongata. Toward the anterior end of the middle cranial fossa, the lateral walls of the skull are greatly compressed, so that the anterior portion of the basicranium, especially the body of the anterior sphenoid, is largely excluded from the cranial cavity. The usually paired optic foramina are here confluent, there being a single aperture for the transmission of the optic nerves. The posterior ventral boundary of this aperture contains a broad groove, the sulcus chiasmatis, which lodges in the natural condition the optic chiasma. In the anterior cranial fossa the floor is largely formed by a perforated area, borne on the cribriform plate (lamina cribrosa) of the ethmoid bone, and serving for the transmission of the divided olfactory nerves. Its median portion projects slightly into the cranial fossa as a ( low ridge, the crista galli, which is interposed between the tips of the olfactory bulbs. In the ventrolateral portion of the cranial cavity may be found the internal openings of the foramina described above, namely, the superior orbital fissure, the foramen lacerum, the jugular foramen, and the hypo- glossal canal. The superior orbital fissure is almost ventral in position to the foramen opticum, and is connected backward with the foramen lacerum by a broad groove, the sulcus sphenoidalis, which lodges in the natural condition the roots of the fifth nerve. This groove continues to the medial surface of the periotic bone, where it is bridged over by the tentorium cerebelli. On the lateral wall of the posterior cranial fossa, and enclosed by the compact, white, petrous portion of the periotic bone, is a series of three apertures leading into its substance. One of these, much larger than the remaining two, is the subarcuate or floccular fossa (fossa subareuata s. floccularis). It lodges in the natural condition the flocculus, a small stalked appendage of the cerebellum. Ventral to this fossa, and also somewhat in front of it, a thin ledge of bone extends over an oval open- ing, the internal aperture of the facial canal (canalis facialis), which serves for the transmission of the seventh cranial (facial) nerve. Imme- diately behind and below this aperture is the opening of the internal acoustic meatus (meatus acusticus internus) for the transmission of the eighth cranial (acoustic) nerve. The two apertures tend to be enclosed by a shallow bony ridge, largely formed by the projecting ledge described above, and resembling superficially the complete common tube repre- sented by the internal acoustic meatus of the human skull. The facial portion of the skull is largely formed by the investing bones of the upper jaw, palate, and mandible, but it encloses also the entire olfactory region of the primary skull, including the nasal fossae and associated turbinal bones. The upper jaw — the maxilla of the human skull— is formed of two primary, and, in the rabbit, separate, elements, the maxilla and premaxilla. They together form the greater portion of the facial region — in the adult condition also a large portion of the lateral walls of the nasal fossae — and bear in a ventrolateral position low alveolar- processes (processus alveolares) for the sockets or alveoli of the incisor and cheek-teeth. The maxilla bears the anterior 82 Anatomy of thb Rabbit. root of the zygomatic arch, the latter being formed partly by a short zygomatic process arising from its lateral surface, by the zygomatic bone, which is fused with it, and by the corresponding zygomatic process of the squamosal bone, constituting the posterior root. The anterior root of the zygomatic arch is perforated by a deep narrow infraorbital canal (canalis infraorbitalis), which opens on the facial surface by a vertical slit-like aperture, the infraorbital foramen. It serves for the trans- mission of the infraorbital vessels and nerves from the orbit to the face. The ventral portion of the maxilla is associated with the palatine bone to form the hard palate (palatum durum) . This structure is repre- sented chiefly by a bony palatine bridge connecting the two sides of the skull between the more anterior cheek-teeth. It forms a portion of the roof of the oral cavity and a portion of the floor of the nasal cavity Immediately in front of it, the palatal surface is perforated by a pair- of large incisive foramina' (foramina incisiva), which are broadly open to the nasal fossae. A considerable portion of the anterior and dorsal wall of the orbit is formed by the facial complex. Dorsally, the roofing element of this region, the frontal bone, bears a curved lateral projection, the supra- orbital process (processus supraorbitalis), which overhangs the orbit. Its narrower base expands into anterior and posterior tips, which lie parallel to the adjacent portion of the skull, and enclose with the latter corresponding anterior and posterior supraorbital incisures. The latter are converted by ligament into foramina. The anterior wall of the orbit is formed in part by a loosely articulated element, the lacrimal bone, the lateral margin of which projects from the orbital rim as a blunt sub- cutaneous process (processus subcutaneus). On the ventral side of its base is the orbital opening of the nasolacrimal canal (canalis nasola- crimalis), the bony enclosure of the nasolacrimal duct, which in the natural condition leads from the corneal surface of the eye to the anterior portion of the nasal fossa. A smaller projection forming the ventral boundary of the nasolacrimal aperture is the hamulus lacri- malis. Finally, in the ventral anterior angle of the orbit, the bases of the three posterior cheek-teeth encroach to a considerable extent on the orbital space. They are separated from the orbital wall by a deep infraorbital groove (sulcus infraorbitalis), which leads forward into the canal of the same name. They partly conceal two important apertures of this region, the orbital opening of the pterygopalatine canal (canalis pterygopalatine), leading to the palatal surface, and the sphenopalatine foramen (foramen sphenopalatinum), leading to the nasal fossa. The pterygopalatine canal opens ventrally in the palato-maxillary suture of the hard palate by a rounded aperture, the greater palatine foramen (foramen palatinum majus). The nasal cavity (cavum nasi) is enclosed by the maxilla and pre- maxilla, with the assistance of paired roofing elements, the nasal bones. Apart from the incisive foramina, which are closed in the natural con- dition, the cavity is open at two points. Posteriorly it communicates with the ventral surface of the skull by the choanae, which, in the rabbit, are incompletely divided. Anteriorly it opens to the outside by the- The Skull as a Whole. 83 piriform aperture (apertura piriformis). The cavity is divided into right and left portions, the nasal fossae. In the divided skull it is seen that the division is effected chiefly through a median vertical, cartilaginous plate, the nasal septum (septum nasi), or cartilaginous portion of the mesethmoid. This is continuous posteriorly with a small crescentic vertical plate of bone, the perpendicular plate (lamina perpendicularis) of the ethmoid bone — the bony portion of the mesethmoid — and the latter is also the terminal element of the series of median bones constituting the basicranium. Posteriorly, the ventral portion of the cartilaginous nasal septum is supported by a vertical bony plate, the vomer, the dorsal margin of which is grooved to receive it. Anteriorly, the nasal septum bears on its ventral margin the paired enclosures of the vomero- nasal organ, which are also supported by the grooved surface formed in the middle line by the adjacent dorsal surfaces of the palatine processes of the premaxilla. The relations of these structures, as well as of the cartilage supporting the nasopalatine duct, are best seen in very young animals (cf. Plate III). The delicate, folded, or scroll-like turbinated bones, characteristic of the nasal cavity, are borne on its posterior and lateral walls. Occupying Fig. 33. The skull in vertical section: BO, basioccipital (basilar portion of occipital); BS, basisphenoid (body of posterior sphenoid); ET, ethmoturbinal ; F, frontal; I, inter- parietal; M, maxilla; MT, maxilloturbinal ; N, nasal; NT. nasoturbinal; P, parietal: PL, palatine; PMX, premaxilla; PR, presphenoid (body of anterior sphenoid) ; PT, petrous portion of petromastoid; SO, supraoccipital (squamous portion of occipital) ; T, tympanic; V, vomer. a.p., piriform aperture of nose; c.f., internal aperture of facial canal; c.o., occipital condyle; f.c.a., f.c.m. and f.c.p., anterior, middle, and posterior cranial fossae; f.f., floccular fossa; f.h., hypophysial fossa; f. in., incisive foramen; f.s., sphenopalatine foramen; 1., per- pendicular plate of the ethmoid; m.a.i., internal acoustic meatus; o., optic foramen; p. a., alveolar process of maxilla; p.d., hard palate; p.o.e., external occipital protuberance; p.pt., pterygoid process of posterior sphenoid; s.n., nasal septum; t.c, tentorium cerebelli. the anterior portion of the lateral wall of the nasal fossa is a finely-ridged mass of bone, the concha inferior, or maxilloturbinal. It is easily dis- tinguishable from a more dorsal and posterior series of broader folds, which together constitute the ethmoturbinal. In the rabbit, as in mammals generally, the latter is divisible into a more dorsal elongated portion attached to the nasal bone, the nasoturbinal, and a more ventral portion, also posterior portion, the ethmoturbinal proper, composed of several 84 Anatomy of the Rabbit. shorter folds decreasing in length from above downward. In the natural condition the turbinated bones bear a considerable portion of the nasal epithelium, the surface of which is greatly increased by the folding of the underlying bone. That covering the ethmoturbinal contains the olfactory sense organs, while that covering the maxilloturbinal is non- sensory and possesses the mechanical function of freeing the air of the respiratory tracts from foreign materials, as well as of warming it slightly in its passage. On this account the respective structures are conveniently distinguished as sensory (olfactory) turbinals and respiratory turbinals. The mandible (mandibula) is composed of two portions, united anteriorly by the symphysis mandibulae. Each half comprises a hori- zontal portion, forming in conjunction with that of the opposite side the body of the mandible (corpus mandibulae), and a posterior, vertical portion, the ramus mandibulae, the latter serving for the insertion of the muscles of mastication and for artticulation with the skull. The Fig. 34. Lateral surface of the left ramus of the mandible: a.m., , angle; cm., body of mandible; cp.m., articular portion (head) of mandible; d.i., d.m., and d.pm., incisor, molar and premolar teeth; f.m., mental foramen; i.m.a. and i.m.p., anterior and posterior mandibular incisures; p.c, coronoid process; p.cd., condyloid process; t.m. and t.pt., masseteric and pterygoid tuberosities. body of the mandible bears on its dorsal margin the alveoli of the lower teeth. The mandibular ramus forms a broad plate, the lateral surface of which is occupied in the natural condition by the masseter muscle, while the medial surface forms an area of insertion for the external and internal pterygoids. The surface of the ramus is greatly increased in its posteroventral portion through the expansion of the bone to form the angle (angulus mandibulae), or angular apophysis. The elon- gated articular surface is borne at the end of a vertical, or slightly oblique condyloid process (processus condyloideus) . The nerve and vessels of the mandible enter at the mandibular foramen (foramen mandibulare), the latter being situated on the medial surface of the bone immediately behind the last cheek-tooth. The; Bones of the Skull. 85 B. THE BONES OF THE SKULL. 1. THE OCCIPITAL BONE. The occipital bone (os occipitale) is the first of the basicranial seg- ments as numbered from the occipital articulation forward. It forms the posterior boundary of the skull, and establishes the connection of the latter with the vertebral column. Its external surface is identifiable for the most part with the nuchal surface, but a portion of it falls in the horizontal plane of the basis cranii. The internal surface is partly exposed to the cranial cavity, and forms in this relation the posterior, dorsal, and ventral boundaries of the posterior cranial fossa. The re- maining portion is excluded from the cranial cavity, being applied instead to the broad posterior surfaces of the petrotympanic bones. The occipital bone is divisible into four portions, namely, the basilar portion (pars basilaris), or basioccipital, the paired lateral portions (partes laterales), or exoccipitals, and the squamous portion (squama occipitalis), or supraoccipital. ' All four portions take part in the forma- tion of the foramen magnum. In the young animal (Fig. 11) they are represented by separate elements, formed in a continuous mass of cartilage, and united for a time by synchondroses, but in the course of growth they become fused to form a single occipital bone. The basioccipital is that portion lying below and in front of the foramen magnum. Its main surfaces are respectively dorsal and ven- tral. Its anterior margin is united with the posterior margin of the basisphenoid by a thin, transverse cartilage union, the sphenooccipital synchondrosis (synchondrosis sphenooccipitalis). Posteriorly its dorsal and ventral surfaces come together in a thin concave edge which forms the ventral boundary of the foramen magnum. Laterally it is bounded by the petrotympanic bone and by the lateral portion of the occipital. The dorsal surface bears a median groove, deeper in its middle portion, where the lateral margins of the bone are raised to form a pair of rounded bosses for articulation with the petrotympanic. The groove represents the sloping porjtion or clivus of the occipital, and lodges in the natural condition, as described above, the ventral portion of the medulla oblongata. The ventral surface presents a similar groove, in the pos- terior portion of which there is a small ridge-like elevation, the pharyngeal tubercle (tuberculum pharyngeum). The exoccipital is directed dorsad from the basioccipital in such a way that it falls in the plane of the nuchal surface. It is applied to the posterior surface of the petrotympanic bone, and also extends down- ward beyond the latter as the jugular process. The occipital condyle is borne on the exoccipital, with the exception, however, of its ventral tip, which belongs to the basioccipital. The portion of the occipital bone con- necting the basioccipital and exoccipital contains the jugular fossa and the apertures representing the hypoglossal canal. Its anterior margin • bears a jugular incisure (incisura jugularis), forming the occipital 86 Anatomy of the Rabbit. boundary of the jugular foramen, the remaining portion of the latter being formed by the petrotympanic. The supraoccipital is the dorsal portion of the bone. Its dorsal margin is bent sharply forward, so that it tends to fall, like the basi- occipital, in a horizontal plane. Its external surface bears the superior nuchal lines and the external occipital protuberance. A pair of lateral wing-like expansions rest upon, and partly overlap, the dorsal margins of the petrotympanic bones. The anterior boundary is formed by the interparietal, parietal, and squamosal bones, but in young skulls the squamosal connection is represented by a vacuity. The internal sur- face bears a longitudinal groove, lodging in the natural condition the median vermis of the cerebellum. It is crossed at its anterior end by a shallow transverse groove (sulcus transversus), which marks the position of the transverse sinus of the dura mater. 2. THE POSTERIOR SPHENOID. The sphenoid bone, as identified from the human condition, is a com- plex of elements belonging to two segments, namely, the posterior sphenoid (os sphenoidale posterius) and the anterior sphenoid (os sphenoidale anterius). In the rabbit, as in mammals generally, these segments are separate throughout life. The posterior sphenoid comprises: (1) a median portion, the body, or basisphenoid ; (2) paired dorsolateral expansions, the greater wings (alae magnae), or alisphenoids ; and (3) paired ventral projections, the pterygoid processes. The basisphenoid continues the basis cranii forward from the basi- occipital to the body of the anterior sphenoid. It is united with the latter by the intersphenoidal synchondrosis. Its surfaces correspond for the most part to those of the basioccipital. The ventral surface forms the chief part of the bony roof of the nasopharynx. It is perforated in its middle by a round aperture, the foramen cavernosum, which leads into the interior of the bone. The dorsal surface is occupied, as described above, by the hypophysial fossa and related structures, namely, the dorsum sellae and the posterior clinoid processes. On the lateral surface of the base of the posterior clinoid process a faint groove, the sulcus caroticus, marks the course of the internal carotid artery. The interior of the bone contains a cavity of considerable size, the sphenoidal sinus (sinus sphenoidalis), which communicates both with the foramen cavernosum and the hypophysial fossa. The alisphenoid extends at first laterad, but soon changes its direction so that its axis becomes dorso ventral. At the same time the bone is rotated in such a way that its surfaces tend to fall in a transverse plane. It is bounded anteriorly by the orbitosphenoid, dorsally by the squamosal, and posteriorly by the petrotympanic. The anterior margin of its root encloses with the basisphenoid, and to a certain extent with the orbito- sphenoid, the superior orbital fissure. The foramen lacerum is formed ' by the posterior margin of its root in association with the petrotympanic. The external surface of the alisphenoid is convex, both toward the The Bones of the Skull. 87 orbit and toward the ventral surface of the skull. In the posterior por- tion of the orbit this surface bears a jagged elevation, the crista alae magnae. The internal surface forms a portion of the floor and antero- ventral wall of the middle cranial fossa. At its base a broad groove, the sulcus sphenoidalis, indicates the position of the root of the fifth nerve and the related semilunar (Gasserian) ganglion. The pterygoid process comprises the two plates described above as the medial and lateral laminae. The former is vertical, and its medial surface is directed toward the nasopharynx. The latter is almost hori- zontal. The medial lamina ends ventrally in a hooked projection, the hamular process (hamulus pterygoideus). In the young animal this por- tion is formed of an elevation of cartilage tipped by a separate mem- brane element, the pterygoid bone. The pterygoid fossa is formed in part by the medial and lateral laminae and in part by the divided pos- terior end of the palatine bone. The posterior base of the lateral lamina is extensively excavated, like the adjacent portions of the alisphenoid, It bears a shallow groove, representing a pterygoid canal (canalis pterygoideus), and is perforated by the three apertures described above as the anterior, middle, and posterior sphenoidal foramina. 3. THE ANTERIOR SPHENOID. The anterior sphenoid (os sphenoidale anterius) consists of two por- tions, namely, a median portion, the body, or presphenoid, and a pair of lateral expansions, the lesser wings (alae parvae), or orbitosphenoids. The presphenoid is a constricted bony splint which continues the basis cranii forward from the basisphenoid. It is joined anteriorly with the perpendicular plate of the ethmoid and with the cartilaginous nasal septum. In the divided skull, or better in one from which the roof has been removed, the actual dorsal surface of the bone is seen to be exposed to the cranial cavity only in its posterior portion, where it is occupied by the sulcus chiasmatis and the optic foramina. That part of the floor immediately in front of the optic foramina is formed by the coalesced roots of the orbitosphenoids, the dorsal surface of the pre- sphenoid being thus excluded. The orbitosphenoid forms a long, low plate, lying in the ventral por- tion of the orbit, and divided by a shallow notch at the level of the optic foramen into a posterior portion, the orbitosphenoid proper, and an anterior portion, the ethmoidal process (processus ethmoidalis). The orbitosphenoid proper lies behind the optic foramen. It is in contact dorsally with the orbital portion of the frontal, and ventrally with the alisphenoid ; it assists the latter in the formation of the superior orbital fissure. Its posterior tip is in contact with the squamosal. Its internal surface forms a considerable portion of the anteroventral wall of the middle cranial fossa. The ethmoidal process extends forward from the optic foramen. Its dorsal margin is articulated with the orbital portion of the frontal, and its ventral margin with the orbital portion of the palatine. Anteriorly it projects toward the lacrimal bone,- thus occupying, in part, a space 88 Anatomy of the Rabbit. which, in the typical mammalian skull, is filled by the lamina papyracea of the ethmoid. Its internal surface is associated with the ethmoid bone and with the nasal cavity. It falls for the most part below the level of the cranial cavity. 4. THE SQUAMOSAL BONE. The temporal bone, or temporal complex, as recognized from the human condition, is an association of three elements — squamosal, tympanic, and periotic — which in the human skull are coalesced to form a single bone. It is usually described as consisting of four portions, of which the squamosal and tympanic portions are two, while the periotic bone is considered to consist of two others, one of which, the petrous portion, is a solid white portion lodging the internal ear, while the second, or mastoid portion, is a mass of less compact character appear- ing externally in the wall of the skull. In the rabbit the original elements are not coalesced, but the periotic and tympanic bones are so closely associated that it is proper to describe them as forming a petrotympanic bone. ' The squamosal bone (os squamosum) is a rectangular plate, forming part of the lateral wall of the cranium, and bearing the posterior root of the zygomatic arch. It is articulated anteriorly with the orbitosphenoid and with the orbital portion of the frontal, dorsally with the frontal and parietal posteriorly with the supraoccipital and petrotympanic, and ventrally with the alisphenoid. Its posterior margin bears a prominent, slightly decurved squamosal process (processus squamosus). It lies on the lateral surface of the petrotympanic immediately above the opening of the bony external acoustic meatus. The posterior root of the zygo- matic arch is formed by a lateral and afterwards ventral projection, the zygomatic process of the squamosal. Its base bears ventrally the mandibular fossa, and dorsally, in association with the body of the squamosal, the temporal fossa. The internal surface of the squamosal forms a considerable portion of the wall of the cranial cavity, the middle cranial cavity being, in fact, broadest in this region. 5. THE PETROTYMPANIC BONE. ■ The petrotympanic bone (os petrotympanicum) is a somewhat oblong bone lying in the lateral wall of the cranium between the posterior sphenoid and occipital bones. It is chiefly indicated externally by the tympanic bulla and the bony external acoustic meatus. It is articulated anteriorly with the alisphenoid and squamosal, dorsally with the supra- occipital, and posteriorly with the exoccipital. Except for the presence of the squamosal process of the squamosal bone, the lateral and ventral surfaces are exposed to the outside of the skull. The internal surface is exposed to the posterior cranial fossa, with the exception, however, of a small ventral portion which is articulated with the basioccipital bone. Only a small portion of the anterior surface is in contact with the squamosal bone, the larger part being applied to the tentorium cerebelli and forming with the latter a posterior wall for the middle The Bones oe the Skull. 89 cranial fossa. The dorsal portion of the bone corresponds in thickness with the wins* of the supraoccipital with which it is articulated. The posterior surface is applied to the anterior surface of the exoccipital, and is thus excluded both from the cranial cavity and from the external surface of the skull. Viewing the skull from behind, however, it is seen that a small dorsal portion protrudes in a triangular space formed by the dorsolateral margin of the exoccipital and the ventrolateral margin of the supraoccipital wing. This portion is distinguishable by its pitted character. It forms the mastoid portion (pars mastoidea) as distin- guished from the solid white petrous portion (pars petrosa), which is exposed to the cranial cavity, and which contains the structures of the internal ear. The mastoid portion lies for the most part above the tympanic cavity, but it is also continued ventrad between the external acoustic meatus and the exoccipital as the mastoid process. The stylo- mastoid foramen lies between the latter and the external acoustic meatus. The petrous portion, as viewed from its medial surface, is roughly oblong; it is placed obliquely with reference to the basioccipital and basisphenoid. The fioccular fossa occupies its posterodorsal portion, and extends into the substance of the bone, forming a much larger depression than is indicated by the diameter of its rim. The related dorsal margin of the bone is occupied by a groove which leads into a canal at its posterior margin. It indicates the position of the lateral portion of the transverse sinus of the dura mater. The ventral, thicker portion of the bone, enclosing the apertures of the internal acoustic meatus and the facial canal, is also that lodging the vestibulum and cochlea of the internal ear. A small aperture at its anteroventral angle, only visible when the petrotympanic is freed from its connections, repre- sents the hiatus canalis facialis of the human skull. It transmits the great superficial petrosal nerve, a branch of the facial nerve passing to the sphenopalatine ganglion. The tympanic surface of the petrous portion is described below in connection with the structures of the tympanic cavity. The tympanic portion forms the spherical, expanded, shell-like, tym- panic bulla, which contains in its interior the tympanic cavity, and is continuous dorsally with the bony enclosure of the external acoustic meatus. The boundary between the two is indicated externally by a shallow oblique groove, the position of which indicates roughly that of the tympanic membrane within. The medioventral margin of the bone is articulated with the basioccipital, but the swollen portion is separated from the latter by a broad groove terminating posteriorly in the jugular fossa and the jugular foramen. Immediately in front of the jugular fossa, the rounded aperture of the external carotid foramen, transmitting in the natural condition the internal carotid artery, leads into the carotid canal of the interior of tympanic portion. At the anterior end of the groove, communicating with the foramen lacerum, is the anterior opening of the carotid canal, the internal carotid foramen, and on its lateral side the much larger aperture of the auditory (Eustachian) tube. The rela- tions of these apertures are seen to best advantage when the petro- 90 Anatomy op the Rabbit. tympanic is disarticulated from the associated posterior sphenoid bone. The auditory tube is then seen to lead directly into the tympanic cavity. A fine bristle may be passed through the carotid canal from one foramen to the other. 6. THE STRUCTURES OF THE TYMPANIC CAVITY. The relations of the tympanic cavity and associated structures may be studied with advantage in a skull from which the lateral wall of the tympanic bulla and external acoustic meatus has been removed, the sur- face displayed being as indicated in Fig. 35. The tympanum or middle ear is enclosed by the tympanic and petromastoid portions of the temporal complex. The attached margin of the tympanic bulla encloses a roughly triangular area, into the ventral part of which the petrous portion of the petromastoid projects as a smooth, white, convex ridge, the prom- ontory (promontorium) . Above and behind the promontory the tympanic cavity is extended toward the mastoid portion of the bone Fig. 35. Petrotympanic portion of the auditory complex of the left side x3. The lateral portions of the tympanic bulla and external acoustic meatus have been removed, exposing the structures of the tympanic cavity. MS, mastoid portion ; P, petrous portion; T, tympanic portion (bulla tympani). cm., mastoid cells; c.t., tympanic cavity; f.c, cochlear fenestra; in., incus; -- .. m.a.e., external acoustic meatus; m.m., manubrium of the malleus; m.so., ''■ supraoccipital margin of petromastoid; p.m., mastoid process; St., stapes; t.a., * . aperture of auditory tube. as the tympanic or mastoid antrum (antrum tympanicum), and the interior of the mastoid portion is partly occupied by small extensions of the tympanic antrum, termed the mastoid cells (cellulae mastoideae) . At the anteroventral angle of the area already described, a deep notch indicates the point of entrance of the auditory tube. The exposed surface of the petromastoid presents two apertures, one of which, situated posteroventrally, is open in the dried skull, and is the cochlear fenestra (fenestra cochleae). In the natural condition it is closed by a thin membrane which separates the tympanic cavity from the perilym- phatic space containing the membranous labyrinth. The second aperture, the vestibular fenestra (fenestra vestibuli), lies above and in front of that just described. It is closed by the base of the stapes. The auditory ossicles (ossicula auditus) comprise three elements, namely, the malleus, incus, and stapes, which bridge the space inter- The Bqnes of the Skull. 91 vening between the tympanic membrane and the opening to the internal ear as represented by the vestibular fenestra. They occupy the dorsal angle of the triangular area already described, and lie immediately above the promontory. The malleus is the lateral element. The main por- tion, termed the head, is concealed by the projecting edge of the external acoustic meatus. It bears a stout vertical process, the manubrium mallei, which in the natural condition lies in contact with the tympanic membrane. The incus is the intermediate element; it is directly articulated with the malleus, and bears a downwardly-directed long limb (crus longum), for articulation with the minute head of the stapes. The latter element is a small, stirrup-shaped bone, occupying an almost transverse position, and articulated at its base with the margin of the vestibular fenestra. 7. THE INTERPARIETAL BONE. The interparietal (os interparietale) is a small, lozenge-shaped element, surrounded by the two parietal bones and the supraoccipital. It is the first of the membrane roofing elements of the cranium proceeding for- ward from the supraoccipital, and in the rabbit's skull is not fused with the occipital segment. 8. THE PARIETAL BONE: The parietal bone (os parietale) is a characteristic roofing bone covering a large portion of the middle cranial fossa. It is somewhat rectangular in shape, and' is connected by serrate sutures with the sur- rounding elements and with its fellow of the opposite side, the sutures producing a characteristic pattern on the external surface of the skull. The sutures are medial, anterior, lateral, and posterior in position, and are designated respectively as sagittal, coronal, squamosal, and lamb- doidal. The posterolateral angle of the bone is produced ventrally into a long, curved squamous process (processus squamosus), which lies in the angle formed by the tentorium cerebelli and the lateral wall of the middle cranial fossa. It is not exposed to the external surface of the skull. 9. THE FRONTAL BONE. The frontal bone (os frontale) is a paired element, lying directly in front of the parietal, and forming with its fellow of the opposite side the anterior portion of the roof of the cranial cavity and also a considerable portion of its lateral, orbital wall. Unlike the condition in the human skull, the two bones are separate throughout life; so that there is a per- manent frontal suture. Each consists of a frontal portion (pars frontalis), the external or dorsal surface of which continues that of the parietal, and of an orbital portion (pars orbitalis), enclosing the dorsal portion of the orbit. The two parts are connected at the supraorbital border, with which is also connected the base of the divided supraorbital process. The anterior end of the frontal portion is deeply notched where it comes in contact with the nasal and premaxillary bones. Two processes are 92 Anatomy of the Rabbit. thus formed, one medial, the other lateral to the nasal. The medial process is associated with that of the opposite side to form a triangular frontal spine, while the lateral or maxillary process (processus maxillaris) projects forward between the nasal and premaxillary bones, on the one hand, and the subcutaneous process of the lacrimal, the orbital process of the maxilla, and the body of the latter, on the other. The orbital portion of the frontal forms a considerable portion of the orbital wall. Its anterior margin is in contact with the lacrimal bone, its ventral margin with the, slender sphenoorbital process of the maxilla, the ethmoid process of the orbitosphenoid, and the orbitosphenoid proper. Its internal surface is divided by a vertical ridge into anterior and posterior portions, in relation respectively to the anterior and middle cranial fossae. The anterior cranial fossa is enclosed by the frontal bones, with the exception, however, of a small portion of the floor which is formed by the cribriform plate of the ethmoid. 10. THE ETHMOID BONE. The ethmoid bone (os ethmoidale), the chief representative of the embryonic cartilaginous nasal capsule, is a delicate, greatly sculptured structure, almost completely enclosed by the membrane bones of the face. Its features may be studied either in the divided skull, or in one from which the roof of the nasal and cranial cavities has been re- moved. It consists of three main portions, namely, the cribriform plate, the perpendicular plate, and the paired lateral masses, or ethmoidal labyrinths. The cribriform plate (lamina cribrosa) is exposed to the anterior cranial fossa. It is somewhat heart-shaped, with its apex in contact with the ethmoidal processes of the orbitosphenoids. Its lateral por- tions are perforated by numerous foramina, giving passage in the natural condition to the branches of the olfactory nerves. Its median portion forms a low vertical ridge, the crista galli, continuous in front with the perpendicular plate. The perpendicular plate (lamina perpendicularis) is the bony, pos- terior portion of the nasal septum, and as such is exposed to the nasal cavity. It is united with the cartilaginous nasal septum and also with the presphenoid. It forms the terminal member of the chain of bones lying in the basicranial axis. The ethmoidal labyrinth (labyrinthus ethmoidals) occupies for the most part the posterior portion of the nasal fossa, but the nasoturbinal extends forward to its anterior end, and is attached for the greater part of its length to the internal surface of the nasal bone. It is broadest in its middle portion, where it projects into the space left between the ethmoturbinal proper and the maxilloturbinal, and contains at this point a pouch-like cavity, termed the marsupium nasale. The whole struc- ture is comparable to one of the folds of the ethmoturbinal proper; but it is frequently seen to be divided into anterior and posterior parts by a thin vertical line of cartilage, the anterior division being probably allied to the maxilloturbinal. Its middle, ventral, portion bears a Thu Bonbs op the; Skull'.. 93 stout, backwardly-directed uncinate process (processus uncinatus), which is applied to the medial, surf ace of the maxilla. The ethmoturbinal proper consists, as described above, of several shorter scrolls, decreasing in length from above downward. Like the posterior part of the nasoturbinal, they are attached directly to the cribriform plate, the perforations of which may be seen in the divided skull opening into the ethmoidal cells (cellulae ethmoidales) or spaces contained by them. They are roughly comparable to the superior and middle turbinated bones of the human skull, but in the rabbit, as in most mammals, the ethmoturbinal surfaces are relatively much more extensive than in man. In the typical mammalian skull the ethmoid bone is exposed to the orbit, where it forms a thin plate of bone, the lamina papyracea. In the rabbit, however, the space usually occupied by the lamina papyracea is partly filled by the lacrimal bone, the ethmoidal process of the orbito- sphenoid and the sphenoorbital process of the maxilla. 11. THE INFERIOR TURBINATED BONE. The inferior turbinated bone (concha nasalis inferior), or maxillo- turbinal, is a finely ridged structure, situated anteriorly in the nasal fossa, and supported by the maxilla and premaxilla. It represents the simi- larly-named structure of the human skull, the lowermost of three scroll- like bones, of which the remaining two, the superior and middle turbinated bones, belong to the ethmoturbinal. In the natural con- dition it is covered by a non-olfactory epithelium, and is thus distinguish- able in function as well as in position from the latter. 12, THE MAXILLA. The maxilla, the largest element of the facial region, is associated with its fellow of the opposite side to form the main portion of the upper jaw. It consists of a central portion, the body (corpus maxillae), and of five processes, namely, alveolar, palatine, orbital, zygomatic, and spheno- orbital. In the adult condition the zygomatic bone is fused with the maxilla, so that the extent of the zygomatic process appears to be greatly increased. The body of the maxilla is greatly fenestrated on its external sur- face, the perforated area extending backward to the anterior rim of the orbit, and thus including the maxillary fossa and the infraorbital foramen. The dorsal boundary of the bone is formed by the frontal process of the premaxilla and by the maxillary process of the frontal. Anteriorly, it is united with the premaxilla, the ventral part of the suture appearing in the diastema separating the incisors from the cheek-teeth. The ventral portion of the bone forms part of the lateral boundary of the incisive foramen. Behind the palatine bridge it is applied to the lateral surface of the palatine bone, and is projected into the orbit as a broad ridge enclosing the alveoli of the four posterior cheek-teeth. In the divided skull the medial surface of the body of the, maxilla is found to be concealed by the ethmoturbinal. It contains a deep longitu- "1 '1 94 Anatomy of the Rabbit. dinal excavation, the maxillary sinus (sinus maxillaris), widely open to the nasal fossa, but only seen to advantage when the ethmoturbinal is removed. The lateral wall of the sinus corresponds in position with the fenestrated area of the external surface. It bears the chief part of the nasolacrimal canal. The alveolar process (processus alveolaris) is that portion of the maxilla lodging the sockets of the cheek-teeth. In the rabbit it is separated by the diastema, in which no teeth occur, from a corresponding but imperfectly differentiated process of the premaxilla. The palatine process (processus palatinus) extends toward the median plane. It forms with its fellow of the opposite side about two-thirds of the palatine bridge. The orbital process (processus orbitalis) is directed obliquely toward the dorsal surface of the skull. In conjunction with the lacrimal bone and the maxillary process of the frontal, it forms the anterior orbital rim. It is continuous with the fenestrated portion of the body, and its appearance as a process is largely due to its solid character as com- pared with the perforated surface lying in front of it. . The zygomatic process (processus zygomaticus) forms the anterior root of the zygomatic arch, and in the adult condition has fused with it the anterior end of the zygomatic bone. Its ventral angle bears a prominent masseteric spine for the attachment of the ligament of the masseter muscle. The sphenoorbital process (processus sphenoorbitalis) lies on the medial wall of the orbit, in a position opposite to the middle portion of the ridge lodging the posterior cheek-teeth. It forms a stout buttress, the tip of which is applied to the anteroventral angle of the frontal bone. In this position it is visible from the orbit, lying between the lacrimal bone and the ethmoidal process of the orbitosphenoid. 13. THE PREMAXILLA. The premaxilla, or incisive bone (os incisivum), forms the anterior portion of the upper jaw. It comprises a central portion, the body — including with the latter the scarcely differentiated alveolar portion con- taining the large and small incisors — a frontal process, and .a palatine process. The body forms a portion of the palatal surface of the skull and of the lateral boundary of the incisive foramen. Its dorsal surface forms part of the boundary of the piriform aperture, the remaining por- tion of this being formed by the nasal bone. The palatine process extends backward on the medial side of the bone, closely applied on the palatal surface to its fellow of the opposite side, and forms in this way a medial boundary for the incisive foramen. Its dorsal surface, in conjunction with that of the corresponding process of the other side, bears a broad palatine groove (sulcus palatinus), lodging a portion of the cartilage of the vomeronasal organ and nasopalatine duct. The frontal process (pro- cessus frontalis) is a thin bony splint, extending backward between the nasal and maxillary bones, and terminating between the former and the maxillary process of the frontal. The Bones of- the Skull. 95 14. THE ZYGOMATIC BONE. The zygomatic bone (os zygomaticum) is a separate element only in very young animals. In the adult it is fused anteriorly with the zygo- matic process of the maxilla, the position of the original suture being roughly identifiable as the point where the free horizontal portion of the zygomatic arch arises from the transverse zygomatic process. It forms an almost sagittal plate of bone bridging the orbit and serving for the attachment of the masseter muscle of the mandible. Its dorsal margin forms posteriorly a smooth, horizontal articulation with the zygomatic process of the squamosal, the end of the bone projecting con- siderably behind the articulation. 15. THE NASAL BONE. The nasal bone (os nasale) is a thin, elongated bone forming the roof of the nasal fossa, and, in conjunction with its fellow of the opposite side, the dorsal boundary of the piriform aperture. It is loosely articulated with the maxilla and with the bone of the opposite side by smooth (harmonic) sutures. The medial margin is supported by the dorsal edge of the nasal septum. The internal surface bears the nasoturbinal scroll. 16. THE VOMER. The vomer is the median, -somewhat sickle-shaped, vertical plate of bone separating the ventral portions of the nasal fossae. It is visible from the palatal surface through the incisive foramina, but its extent is best shown in the divided skull. It forms a support for the ventral border of the nasal septum, and its posterior portion bears a shelf-like projection, the ala vomeris, which assists in the support of the ethmo- turbinal. 17. THE LACRIMAL BONE. The lacrimal bone (os lacrimale) is a small element lying in the anterior wall of the orbit. It is loosely articulated with the surrounding bones, and in the dried skull is frequently missing unless care has been taken to keep it in place. It consists of a basal portion, somewhat rectangular on its orbital surface, and of two processes, namely, the subcutaneous process and the hamulus lacrimalis. The subcutaneous process is the prominent hook-like projecting laterad beyond the orbital rim. The hamulus lacrimalis is a small process, directed toward the nasal cavity. It bears a groove which, in association with a corresponding groove of the maxillary bone, forms the first portion of the nasolacrimal canal. 18. THE PALATINE BONE. The palatine bone (os palatinum) forms the posterior portion of the palatine bridge and the major portion of, the lateral wall of the naso- pharynx. It consists of two portions — horizontal, and perpendicu- lar. The horizontal portion (pars horizontalis) is that lying in the plane 96 Anatomy of the Rabbit. of the palatal surface. It is articulated in front with the palatine process of the maxilla, the suture between the two bones enclosing the greater palatine foramen, the ventral termination of the pterygopalatine canal. The perpendicular portion (pars perpendicularis) is the vertical plate extending backward from the palatine bridge. Its medial surface is divided by a low ridge into a dorsal portion, in particular relation to the nasopharynx, and a ventral portion, in relation to the oral cavity, the ridge indicating the position of the soft palate. Its lateral surface is partly applied to the maxilla and partly exposed to the orbit. Its dorsal margin is articulated with the presphenoid and with the ethmoidal process of orbitosphenoid, but a small posterior portion is free, so that the anterior portion of the basisphenoid is visible from the orbit. The free ventral margin forms posteriorly a thick projecting angle, the pyramidal process (processus pyramidalis), the base of which is cleft where it articulates with the medial and lateral laminae of the pterygoid process. Between the pyramidal process and the alveolus of the last cheek-tooth there is a conspicuous palatine notch (incisura palatina), connecting the orbit with the palatal surface. In the entire skull only the posterior portion of the lateral surface is visible from the orbit, the anterior portion being concealed by the projecting bases of the posterior cheek-teeth. The ridge of bone on which the alveoli of these teeth are borne is separated from the palatine bone by the infraorbital groove. The medial wall of the latter, formed by the palatine bone, contains the orbital opening of the pterygopalatine canal and the sphenopalatine foramen. 19. THE MANDIBLE. The mandible (mandibula), or lower jaw, comprises the two dentary bones (ossa dentalia), which, in the rabbit, as in mammals generally, are united by a fibrous or fibrocartilaginous connection (symphysis mandibulae) ; not coalesced, as in the human skull, to form a continuous structure. As indicated above, each of the dentary bones comprises: (1) a horizontal, tooth-bearing portion which, in conjunction with that of the opposite side, forms the body of the mandible (corpus mandibulae) ; _ and (2) a posterior, vertical plate, the mandibular ramus (ramus mandi- bulae), for muscle attachment and articulation. The horizontal portion is deep posteriorly, where it lodges the alveoli of the cheek-teeth. Anteriorly, in the diastema separating the latter from the incisors, its dorsal surface is rounded and depressed, the space thus formed corres- ponding to a similar space in the upper jaw and serving chiefly for the accommodation of the lips, which in this region encroach medially on the oral cavity. The medial surface of the horizontal portion forms an acute angle with that of the bone of the opposite side, except anteriorly, where it bears a roughened area for articulation with the latter. Run- ning backward from the symphysis there is a broad horizontal ridge, representing the mylohyoid line (linea mylohyoidea), the line of attach- ment of the mylohyoid muscle. The mandibular foramen, through which, in the natural condition, the inferior alveolar nerve and artery gain access to the interior of the bone, lies on this surface at the junction The Hyoid. 97 of the horizontal portion with the ramus. The corresponding mental foramen (foramen mentale), through which branches of these structures leave the mandible, is situated on the lateral surface in front of the first premolar. The mandibular foramen is closely connected with a second aperture lying at the ventral end of the sulcus ascendens, directly behind the last molar, and serving for the transmission of a vein connecting the inferior alveolar and inferior orbital veins. The mandibular ramus forms in general an obtuse angle with the horizontal portion. As in other herbivores, the ventral part, distin- guished as the angle, is greatly increased in size at the expense of the condyloid process and to a still greater extent of the coronoid process, the latter being 'vestigial. In addition to a low pterygoid tuberosity (tuberositas pterygoidea), situated at the posterior projecting point of the angle, the posterior and ventral margins of the angle are excavated on the medial side of the bone, so that they form the boundary of a pro- nounced, though shallow, pterygoid fossa for the insertion of the ptery- goideus internus muscle. A somewhat similar, but less developed, masseteric fossa occupies the corresponding lateral surface of the angle, its raised ventral margin terminating posteriorly in the masseteric tuberosity (tuberositas masseterica) . The articular portion, or head of the mandible is greatly elongated in the anteroposterior direction in accordance with the anteroposterior action of the lower jaw, this feature being one which is of general occurrence in the rodent order, and more fully expressed in the great extension forward and backward of the attachment areas of the muscles of mastication. The connection of the articulating portion with the condyloid process, the so-called neck of the mandible (collum mandibulae), is a thin plate of bone, the anterior and posterior margins of which are barely notched by the anterior and posterior mandibular incisures. Connecting the anterior incisure with the rim of the alveolus of the last cheek-tooth there is a deep groove, the sulcus ascendens, the lateral margin of which is formed by the re- duced coronoid process (processus coronoideus). Its low medial margin is formed by a bony stay which extends to the medial surface of the horizontal portion opposite the last cheek-tooth, and is continued for- ward into the mylohyoid line. The sulcus ascendens lodges in the natural condition the insertion portion of the greatly reduced temporalis muscle. C THE HYOID APPARATUS. The hyoid bone (os hyoideum) (Fig. 36) is a stout, somewhat wedge- shaped bone lying in front of the larynx and between the angles of the mandible. Its ventral portion is connected with the thyreoid cartilage of the larynx by the median hyothyreoid ligament. With its lateral portion are articulated two independent elements, termed the lesser and greater cornua. The lesser cornu (cornu minus) is a small, partly car- tilaginous structure, attached to the anterodorsal angle of the hyoid, and connected through the stylohyoideus minor muscle with the jugular process of the skull. The greater cornu (cornu majus) is a larger element extending obliquely dorsad, and similarly suspended from the jugular 98 Anatomy of the Rabbit. process by the stylohyoideus major muscle. The connection of the lesser cornu with the styloid process through the stylohyoideus minor replaces the stylohyoid ligament of the human skull and the chain of elements commonly occurring in mammals and other vertebrates in this region. The muscle tendon contains near the jugular process a small ossification representing a detached styloid process. This connection, together with the hyoid bone itself, indicates the relation of the em- bryonic hyoid arch, from which the skeletal structures in question are derived. The greater cornu belongs to the succeeding visceral arch, and is connected with the superior cornu of the thyreoid cartilage of the larynx by the lateral hyothyreoid ligament. .i£.- THE SKELETON OF THE ANTERIOR LIMB. The skeleton of the anterior limb is divisible into two portions, namely, a proximal portion, comprising the scapula and the clavicle, - and a distal portion, comprising the supports of the free extremity. The scapulae and clavi- cles of the two sides together form the pectoral girdle. The pectoral girdle is lightly constructed, and, apart from its muscular connections, which constitute its main support, is directly attached to the axial skeleton only through the sternoclavicular ligament. The skeleton of the free extremity is divisible into proximal, middle, and distal segments. The proximal segment contains a single bone, the humerus; , the middle segment two elements, the radius and ulna ; while the distal segment comprises, in addition to the accessory sesamoid bones, twenty-eight elements of the regular series, of which nine form the carpus, five the metacarpus, and fourteen the phalanges of the digits. THE SCAPULA. met Fig. 36. Lateral hyoid and larynx: cartilage; c.c, cricoid cartilage; c.i., inferior cornu of thyreoid cartilage; cm., lesser cornu of hyoid; c.mj., greater, cornu of hyoid; c.s , superior cornu of thyreoid cartilage; c.t., left plate of the thyreoid cartilage; e. t epiglottic cartilage; f.t.s., thyreoid foramen; l.h., lateral hyothyreoid ligament; l.h.m., median hyothyreoid ligament; m.ct., cricothyreoideus muscle; o.h., hyoid bone; s.m., stylohyoideus minor muscle; s.mj., stylohyoideus major muscle; tr. t cartilaginous tracheal rings. The scapula (Fig. 37) is a somewhat triangular plate of bone lying in the natural position on the lateral surface of the anterior part of the thorax, with its apex directed downward and forward. In the rabbit, as in quadrupedal mammals generally, the main surfaces are respectively medial and lateral, and differ in this respect from the human condition, in which, from the transverse widening of the thorax, the corresponding surfaces are more nearly ventral and dorsal. Of its The Anterior Limb Skeleton. 99 three borders, one, the superior border (margo superior), is directed toward the occiput; another, the vertebral border (margo vertebralis), toward the vertebral column; and the third or axillary border (margo axillaris), toward the armpit. The corresponding angles are medial, inferior, and lateral. The lateral surface bears a stout bony plate, the scapular spine (spina scapulae), which arises from the body of the bone through about two-thirds of its extent, and ends ventrally in a free pro- jection, the acromion. The posterior margin of the acromion bears a backwardly-directed process, the metacromion (processus hamatus). Through the presence of the scapular spine, the lateral surface of the bone is divided into two areas for muscular attachment. One of these, the supraspinous fossa (fossa supraspinata), lies in front of the spine, the other, the infraspinous fossa (fossa infraspinata), behind it. The infraspinous fossa is the more extensive one. The medial surface, on the other hand, presents a single large shallow depression, the sub- scapular fossa (fossa subscapularis), which is triangular in shape and — m-v- a- l. Fig 37 Lateral surface of the left scapula: a., acromion; a.i., a.L, and a m„ inferior, lateral, and medial angles; e.g., glenoid cavity; c.s., neck of the scapula; f.s. and f.i., supraspinous and infraspinous fossae: m., metacromion; m.a., m.s„ and m.v., axillary, superior, and vertebral borders; p.c, coracoid process; s.s., scapular spine. occupies practically the entire surface. The apex or lateral angle of the scapula, sometimes termed the head of the bone, is expanded to a considerable extent in comparison with the slender portion — the so- called neck of the scapula (collum scapulae)— connecting it with the body of the bone. It bears a concave depression, the glenoid cavity (cavitas glenoidalis), for articulation with the humerus. The articu- lating surface is borne chiefly on that part of the bone corresponding to the axillary border, but it also extends in an anterior direction to the base of an overhanging projection, the coracoid process (processus coracoideus). The free portion of the latter forms a blunt, hook-like projection lying toward the medial surface of the bone, 100 Anatomy of thf, Rabbit, THE CLAVICLE. The clavicle (clavicula) is imperfectly developed in the rabbit, con- sisting of a slender, arcuate rod of bone, tipped by cartilage, which lies in the interspace between the manubrium sterni and the head of the humerus. It occupies only a portion of this interspace, being attached medially by the sternoclavicular ligament and laterally by the cleido- humeral ligament. THE HUMERUS. The humerus (Fig. 38) is typical of the long bones of the proximal and middle segments of the fore and hind limbs in consisting of a central portion, the body or shaft of the bone, and of proximal and distal extremities for muscle attachment and articulation. The proxi- mal extremity bears on its medial side a smooth, convex projection, the head of the humerus (caput humeri), for articulation with the scapula. The articulation is nominally a ball-and-socket joint, or enar- throsis, but the articulating surfaces are somewhat restricted, and the muscular arrangements of the limb are such that the range of lateral motion (abduction and adduction) is small. Immediately in front of the head of the bone there is a small elevation, the lesser tubercle (tuberculum minus). It is separated by a longitudinal furrow of the anterior surface, the inter- tubercular groove (sulcus intertubercularis), from a much larger lateral elevation, the greater tubercle (tuberculum majus). Ex- tending distad from the latter is a tri- angular area, the humeral spine (spina humeri), the tip of which reaches almost to the middle of the bone and forms a pronounced angle on its anterior surface. The distal extremity of the humerus bears a grooved articular surface, the trochlea humeri, for articulation with the radius and ulna. On its lateral side is a smaller surface, the capitulum humeri, for articulation with the radius alone. Im- mediately above the trochlea the medial and lateral portions of the bone are thickened to form two areas for muscular attachment. One of these, the lateral epicondyle (epicondylus lateralis), is a general point of origin for the extensor muscles of the dorsal surface of the hand, while the other, the medial epicondyle (epicondylus medialis), is Fig. 38. Anterior surface of the left humerus: c, capitulum; c.h., head of humerus; e.l. and e.m., lateral and medial epicondyles; f.r., radial fossa; s.h., spina humeri; s.i., intertuber- cular groove; t.h., trochlea humeri; t.mi. and t.mj., lesser and greater tubercles. The Anterior Ljmb Skeleton. a similar point of origin for the flexor muscles of the ventral or volar surface. Between the epicondyles the extremity of the bone is greatly excavated, so that the projecting portions of the radius in front and of the ulna behind are received into depressions of the surface when the forearm is greatly flexed or extended. On the anterior side is the radial fossa (fossa radialis) ; on the posterior side the olecranon fossa (fossa olecrani), so- called because it accommodates the olecranon process of the ulna. THE RADIUS AND ULNA. The radius (Fig. 39) is the shorter of the two bones of the forearm, since its proximal extremity does not extend backward beyond the front of the elbow joint. It is antero- dorsal in its general position, but is crossed on the ulna in such a way that its proximal extremity tends to be lateral, while its distal extremity is medial. The proximal extremity, termed the head of the radius (capitulum radii) is immovably articulated with the ulna. It bears an extensive articular surface, meet- ing both the trochlea and capitulum of the humerus, and thus forming a considerable portion of the elbow-joint. The body of the bone is solidly united with the ulna by the interosseous ligament of the forearm. The, distal extremity is largely formed by an epiphysis, which is well-marked even in older animals. It bears a grooved, carpal articular surface (facies articularis carpea), for articula- tion with the navicular and lunate bones. The ulna (Fig. 39) is a somewhat S-shaped bone, the shaft of which is vertically flattened, so that it possesses two main surfaces, respec- tively anterodorsal and posteroventral. The former, in conjunction with the related sur- face of the radius, continues the area of origin of the extensor muscles of the hand from the lateral epicondyle of the humerus distad on to the forearm, while the latter has a similar function with respect to the flexor, muscles. The proximal portion of the bone is laterally compressed. It bears a crescentic depression, the semilunar notch (incisura semilunaris), 101 Fig. 39. Skeleton of the fore- arm and hand from the dorsal surface: R, radius; U, ulna; C, carpus; M, metacarpus; P, phalanges ; I-V, metacarpal bones; c, central bone; cp. f capitate; c.r., head of radius; i. a. a, carpal articular surface of radius ; h., hamate bone ; i.s., semilunar notch of the ulna; 1., lunate bone; mi., lesser mul- tangular; mj., greater mul- tangular; n., navicular; ol., olecranon; p.s., styloid process 'of the ulna; tr., triquetral bone; u., ungual phalanges. 102 Anatomy of the; Rabbit. the articulating surface of which continues that of the medial portion of the head of the radius, and is received into the trochlea humeri. Behind the elbow-joint the bone forms the large projecting portion of the elbow, the olecranon, which is a strong process for the insertion of the extensor muscles (anconaei) acting on the forearm. The distal extremity of the bone is formed by an epiphysis, similar to but much longer than that of the radius. It is immovably articulated with the radius, and its tip is formed by a blunt styloid process (processus styloideus), which is articulated with the triquetral bone of the carpus. The elbow-joint is formed by the trochlea and capitulum of the humerus in conjunction with the semilunar notch of the ulna and the corresponding articular surface of the head of the radius. It is a hinge- joint, or ginglymus, permitting motion in one plane, i.e., extension and flexion of the forearm. The trochlear surface of the humerus, however, has a slight spiral trend, the anterior portion being medial in comparison with the posterior portion. Through the immovable articulation provided by the respective proximal and distal ends of the bones, and also through the interosseous ligament, the radius and ulna are unable to change their positions with respect to one another; in other words, the radius is unable to rotate on an axis formed by the ulna, the fore foot being fixed in a position comparable to that of pronation in the human hand (cf. p. 33). THE CARPUS. The carpus (Fig. 39) comprises nine small elements, the wrist or carpal bones (ossa carpi), which are interposed between the forearm and the digits. They are arranged in two main rows, namely, a proximal row, the elements of which are articulated with the radius and ulna ; and a distal row, the elements of which are articulated with the five bones of the metacarpus. Enumerated from the medial side of the wrist laterad, the proximal row contains four elements, namely, the navicular, lunate, triquetral and pisiform bones. The navicular and lunate are articulated with the distal extremity of the radius; the triquetral with the styloid process of the ulna. The pisiform bone lies on the ventral surface of the extremity of the ulna, and is therefore not exposed to the dorsal surface of the wrist. The distal row contains five elements, namely, the greater multangular, lesser multangular, central, capitate, and hamate bones. The first, second, and fourth are in association respectively with the first, second, and third metacarpals. The central bone lies to the lateral side of the articulation at the base of the second metacarpal. As its name implies, it is originally an element interposed between the proximal and distal rows. The hamate is a comparatively large element associated with the fourth and fifth metacarpals, but extending also to the articulation of the third, where it tends to replace the greatly reduced capitate. The Posterior Limb Skeleton. 103 THE METACARPUS AND PHALANGES. ■ The metacarpus (Fig. 39) comprises five stout elements, the meta- carpal, bones (ossa metacarpalia), which form the basal supports of the digits. Each consists, in addition to a main portion or body, of a flattened proximal end, or base, and a rounded distal extremity, or head. The four lateral bones are normally developed, while the first, which belongs to a reduced digit, is of very small size. The phalanges or bones of the digits are distributed according to the formula 2, 3, 3, 3, 3. They are similar- in form to the metacarpals, with the exception, however, of the terminal, ungual phalanges, which are laterally compressed, pointed, and cleft at their tips for the attachment of the claws. SESAMOID BONES. Accessory elements, sesamoid bones (ossa sesamoidea), developed in connection with the ligaments of muscles, are found on the volar surface of the foot in association with certain of the joints. They occur in trans- verse pairs at the metacarpophalangeal articulations and in linear pairs at the articulations of the second with the third phalanges. The pisi- form bone of the carpus is also a sesamoid, being formed in the insertion tendon of the flexor carpi ulnaris muscle. THE SKELETON OF THE POSTERIOR LIMB. In the posterior limb the proximal or girdle portion comprises the paired coxal bones, which are united ventrally at the pelvic symphysis, thus forming the pelvic girdle. The distal portion — comprising, as in the anterior limb, the supports of the free extremity — is divisible into proximal, middle and distal segments. The proximal segment contains a single element, the femur; the middle segment two elements, the tibia and fibula, which, however, are extensively coalesced; and the distal segment twenty-three elements, of which six form the tarsus, five the metatarsus, and twelve the phalanges. THE COXAL BONE. The coxal bone (os coxae) (Fig. 40) is a somewhat triradiate struc- ture, the posterior limbs of which are united, so that they enclose a large aperture, the obturator foramen (foramen obturatum). It is firmly articulated with the sacrum, and is united with its fellow of the opposite side by a thin strip of cartilage containing a small amount of fibrous material. The latter connection is the pubic symphysis (symphysis pubis), better termed in the rabbit the pelvic symphysis, since it is somewhat more extensive than the corresponding articulation of the human pelvis. In the young animal each half of the pelvis consists of three elements, namely, the ilium, ischium, and pubis. They form the three rays of the coxal bone, and are united with one another in the region of the acetabulum, which is the basin-like depression for the articulation of the pelvis with the femur. Only two of the original elements, however, actually take part in the formation of the acetabulum, the pubis being 104 Anatomy of the Rabbit. excluded through the development in the acetabular depression of .a small triangular element, the os acetabuli. Although completely coalesced in the adult condition, and showing but few traces of their original separation, the three chief elements are nevertheless described as if distinct. The ilium (os ilium) is the anterior, also somewhat dorsal, portion of the bone ; that part extending forward from the acetabulum. It comprises a basal portion, the body (corpus oss. ilium), which includes the anterior portion of the acetabulum and the cylindrical part of the bone in front of it, and an ex- panded portion, the iliac wing (ala oss. ilium), for muscle attachment and articulation with the sacrum. The body is somewhat triangular in section, its surface being divided into three areas, which are respectively medial, or sacral, ventrolateral, or iliac, and dorso-lateral, or gluteal. The corresponding borders are respectively ventral, or pubic, lateral, or acetabular, and dorsal, or ischial. The acetabular border terminates a short distance in front of the acetabulum in an abruptly truncated projection, the inferior anterior spine (spina anterior inferior) . The ischial border forms the anterior half of a long depression of the dorsal surface of the coxal bone, the greater sciatic notch (incisura ischiadica major). The pubic border presents on its medial side a faint, ridge-like eleva- tion, the iliopectineal line (linea iliopect- inea), which connects the sharp anterior border of the pubis with the articular surface for the sacrum. The wing of the ilium forms a shovel- like expansion, the natural position of which is almost sagittal. Its lateral surface provides a fairly extensive area for the origin of the gluteal muscles. Its medial surface is a muscle surface only in its anterior portion, the posterior portion being occupied by the roughened auricular surface (facies auricularis), for connection with the sacrum. The dorsal margin is thin and straight. Posteriorly, where it is associated with the greater sciatic notch, there is a small projection, the inferior posterior spine (spina posterior inferior) Anteriorly it passes by a broad angle into the anterodorsal margin of Fig. 40. coxal bone: pubis; a., _ Lateral surface of the left IL, ilium; IS, ischium; P, acetabulum; a.i., iliac wing; body of ilium; c.is., body of ischium; c p., body of pubis; cr., iliac crest; e.i., iliopectineal eminence; f.a., acetabular fossa; f.o., obturator foramen; i.a., acetabular notch; i.mi., lesser sciatic notch; i.rni., greater sciatic notch; l.i., iliopectineal line; p.l., lateral process of ischial tuberosity; r.i.i., inferior ramus of ischium; r.i.p., inferior ramus of pubis; r.s.i., superior ramus of ischium; r.s.p., superior ramus of pubis; s.a.i., inferior anterior spine of the ilium; s.a.s'., superior anterior spine; s.i., ischial spine; s.p., symphysis pubis; s.p.i., inferior posterior spine; t.i., ischial tuberosity; t.p., pubic tubercle. The Posterior Limb Skeleton. 105 the bone,, the latter forming the projecting end of the wing, which is dis- tinguished as the iliac crest (crista iliaca). This portion is considerably thicker than the related dorsal and ventral margins, and also bears on its medial side a somewhat hook-shaped process. Its anteroventral angle is the superior anterior spine (spina anterior superior). The ven- tral margin is slightly longer than the dorsal margin, and is also concave. It is associated with the pubic border of the body of the ilium, and is not connected with the inferior anterior spine. The ischium (os ischii) extends backward from the acetabulum, its axis continuing that of the ilium. It consists of a basal portion, or body (corpus oss. ischii) , a superior ramus, and an inferior ramus. The body of the ischium is for the most part cylindrical. It forms the posterior part of the acetabulum, and presents in connection with the latter a deep acetabular notch (incisura acetabuli), which tends to interrupt the articular surface. The acetabular notch leads forward into a depression of the centre of the articular basin, the acetabular fossa (fossa acetabuli). In the natural condition the combined depressions serve for the attach- ment of the round ligament of the head of the femur. The dorsal margin of the bone, belonging in part to the body and in part to the superior ramus, bears a short hook-like projection, the ischial spine, (spina ischiadica). The spine divides this margin into two parts, one of which forms the posterior half of the greater sciatic notch, already described, while the other forms a similar, and, in the rabbit, scarcely less extensive, posterior depression, the lesser sciatic notch (incisura ischiadica minor). The superior ramus of the ischium is the continuation backward of the body of the bone. It is a somewhat flattened plate of. bone, the thicker dorsal portion of which terminates in two blunt projections. One of these, the ischial tuberosity (tuber ischiadicum), forms the pos- terior end of the bone, while the other extends in a lateral direction, and is described as the lateral process (processus lateralis). The inferior ramus is that part of the ischium which extends from the superior ramus downward and forward between the obturator foramen and the symphysis to meet the corresponding ramus of the pubis. The pubis (os pubis) consists of a basal portion, or body lying imme- diately below the acetabulum, a superior ramus extending from the body to the symphysis, and an inferior ramus extending backward along the symphysis to its junction with the ischium. The anterior margin of the bone, described as the pecten oss. pubis, is thin and sharp. Near the symphysis it bears a minute elevation, the pubic tubercle (tuberculum pubicum), and laterally a more extensive elevation, the iliopectineal eminence (eminentia iliopectinea) . The latter is more conspicuous in older specimens, where it is easily recognizable by its jagged outline. Its lateral margin is continuous with the iliopectineal line. THE FEMUR. The femur (Fig. 41) is a somewhat S-shaped bone, the body being very slightly arcuate, while of the two extremities, the distal one is bent downward, forming the articulation of the knee, the proximal one, 106. Anatomy, of the Rabbit. with its various processes, slightly upward in association with the pelvis. In considering the general form, it will be remembered that in the natural sitting posture of the rabbit, the position of the femur is approxi- mately horizontal, the convex surface of the shaft, which is equivalent to the anterior surface in man, being uppermost. The proximal extremity of the femur bears an extensive rounded portion or head (caput femoris), for articulation with the pelvic girdle. This portion is separated from the main part of the extremity by a con- stricted area or neck (collum femoris), so that, unlike the case of the anterior limb, the points of muscle attachment fall a considerable distance from the point of articulation. The actual extremity of the bone is formed by a large process for muscular attachment, the great trochanter (trochanter major)! It is divided into two portions, one of which, the first trochanter (trochanter primus), forms the large termi- nal, hook-like projection, while the other, the third trochanter (trochanter tertius), is the smaller lateral crest. On the medial side of the bone, immediately distal to the head, there is a triangular elevation, the lesser, or second trochanter (trochanter minor s. secundus) . Posteriorly, these pro- jections form a smooth surface for muscle attachment, except, however, at the base of the trochanter major, where the surface of the bone presents a deep, though narrow, depression, the trochanteric fossa (fossa trochanterica). The distal extremity bears an extensive surface for articulation with the tibia. It is divided into two portions, known as the medial and lateral condyles, through the presence of a deep excavation, the inter- condyloid fossa (fossa intercondyloidea). Immediately above the condyles, on the anterior surface of the bone, the inter- condyloid fossa is replaced by a broad groove, the patellar ^surface (facies patel- laris), which, in the natural condition, accommodates the convex internal surface of the ' patella. The medial and lateral portions of the bone, intervening between the distal portion of the patellar surface and the tips of the condyles, provide slightly elevated, roughened surfaces, the medial and lateral epicondyles, for muscular attachment. cm- cm- Fig. 41. Anterior surface of the left femur: c.l. and cm., late ral and medial condyles; cl.f., neck of femur; cp.f., articular portion (head); e.l. and e.m., lateral and medial epicon- dyles; f.p., patellar surface; t.mi., trochanter minor; t.mj., trochanter major, including t.p. and t.t., the first and third trochanters. The Posterior Limb Skeleton. 107 C-m. THE TIBIA AND FIBULA. The tibia (Fig. 42) is the larger of the two bones of the leg, lying on the medial side of the fibula, and fused with the latter for more than one- half of its length. Its proximal extremity is triangular in section, the main surfaces being respectively anterolateral, anteromedial, and pos- terior. The anterior border is formed by a stout ridge-like elevation, the tuberosity of the tibia (tuberositas tibiae), which in the natural con- dition serves for the insertion of the quadriceps femoris, the extensor tendon being carried over the knee by the patella and the patellar ligament. The arti- cular portion is slightly differentiated into medial and lateral condyles corresponding to those of the distal end of the femur. On the articular surface the concave areas for the reception of the condyles of the femur are separated from one another by a small intervening, partly divided hillock, the inter- condyloid eminence (eminentia intercondy- loidea), and also posteriorly by a depression of the articular border, the posterior inter - condyloid fossa. A corresponding anterior intercondyloid fossa lies in front of the inter - condyloid eminence, but is poorly differ- entiated. The fibula (Fig. 42) is the smaller, lateral bone of the leg, and in the rabbit is so extensively fused with the tibia that scarcely more than a third of it is distinguishable. The free portion forms a flattened bony splint, the medial margin of which is firmly united with the tibia by the interosseous ligament of the leg. Its proximal extremity is connected with the lateral condyle of the tibia by an elongated epiphysis, the latter, like those of the distal ends of the radius and ulna, being distinguishable even in older animals. The combined distal extremities of the tibia and fibula bear a roughly rectangular articular surface for the tarsus. The tibial portion of this surface presents two grooves, separated by a ridge, for articulation with the trochlea tali. On its medial side is a small projection, the medial malleolus (malleolus medialis). The fibular portion presents a trans- verse groove for the convex articular surface diately above it, on the lateral side of the bone, is a prominent projec- tion, the lateral malleolus (malleolus lateralis). It forms the anterior boundary of a groove which in the natural condition lodges the inser- tion tendons of the peroneal muscles. mm Fig. 42. Anterior surface of the left tibia (T) and fibula / (P) : c.l. and cm., lateral and medial condyles; f.a.s., proximal articular surface for the femur; m.l. and m.m., lateral and medial malleoli; t.t., tuberosity of tibia. of the calcaneus. Imme- 108 Anatomy of the Rabbit. THE TARSUS. The tarsus (Fig. 43) comprises six elements, the tarsal, or ankle-bones (ossa tarsi), which, like the corresponding bones of the carpus, are arranged in proximal and distal rows. An exception is to he made, how- ever, for one element, the navicular, which occupies an intermediate position. The proximal row contains two elements, the talus and cal- caneus. The talus is medial and also slightly dorsal in position. Its proximal end, described as the body (corpus tali), bears an extensive pulley-like surface, the trochlea tali, for articulation with the tibia, these two surfaces together forming the chief portion of the ankle-joint. Its distal end, termed the head of the talus (caput tali), pro- vides a convex articular surface for the navi- cular bone, and is separated from the larger trochlear portion by a slightly constricted intermediate portion or neck (collum tali). Its ventrolateral border is extensively arti- culated with the calcaneus. The latter is a cylindrical element, fully twice as long as the talus, since it is extended backward behind the ankle-joint, as the tuber calcanei, or bone of the heel. Its dorsal surface bears a prom- inent elevation for articulation with the fibular side of the tibiofibula. Its medial surface bears a flat, shelf-like process, the sustentaculum tali, which forms a ventral support for the talus. The distal extremity of the bone articulates with the cuboid and also with the navicular. The intermediate element, the navicular bone, is a somewhat cubical bone, lying on the medial side of the tarsus between the talus, on the one hand, and the proximal end of the second metatarsal bone and the second and third cuneiform bones, on the other. Its position is more nearly that of a central element than is the case with the bone called by this name in the rabbit's carpus. In this connection it will be remembered that the carpus and tarsus, like other parts of the limbs, are primarily constructed on the same plan. The distal row of the tarsus contains three elements, namely, the second and third cuneiform bones and the cuboid bone. The two former and especially the first, are smaller bones, articulated respectively with the second (first developed) and third metatarsals. In the rabbit the first cuneiform bone — the first element of the distal row in the usual con- FlG. 43 . The bones of the left foot, viewed from the dorsal surface: T, tarsus; M, metatarsus; P. phalanges. II-V, the four me- tatarsal bones: cb.. cuboid; cl., calcaneus; c.s., second cuneiform; c.t v third cuneiform: f.a., articular surface for fibular side of the tibofibula: n navicular; t, talus; t.c, tuber calcanei; t.t., trochlea tali. The Posterior Limb Skeleton. , 109 dition — is fused with the proximal end of the second metatarsal. The cuboid is a larger element articulating, like the hamate bone of the carpus, with two distal elements, the fourth and fifth metatarsals. Its ventral surface bears a transverse elevation, the tuberosity of the cuboid (tuberositas oss. cuboidei), in front of which is a groove for the accom- modation of the peculiar insertion tendon of the peronaeus primus muscle. THE METATARSUS AND PHALANGES. The metatarsus (Fig. 43) comprises five elements, of which four are fully developed and greatly exceed in size the corresponding bones of metacarpus, while one, the first metatarsal, is vestigial. The vestigial element lies on the plantar surface of the foot, for the most part ventral to the navicular and at the base of the second metatarsal. In each developed metatarsal there may be distinguished a main portion or body, a proximal extremity or base, and a distal extremity or head, the last- named portion articulating with the proximal phalanx of the digit. The base of the fifth metatarsal bears a tuberosity for the insertion of the peronaeus secundus muscle. The phalanges are distributed accordingto the formula 0, 3, 3, 3, 3 the terminal, ungual phalanges being modified like those of the anterior limb. SESAMOID BONES. The sesamoid bones of the posterior limb occur at the knee-joint and on the plantar surface of the foot. On the anterior surface of the knee is the knee-pan or patella, through which, as indicated above, the tendon of the quadriceps femoris muscle is carried over the knee and continued as the patellar ligament to the tuberosity of the tibia. On the posterior surface there are three sesamoid bones, of which one lies in association with the medial condyle of the femur, while the remaining two are asso- ciated respectively with the lateral condyle of the femur and that of the tibia. The sesamoids of the foot are situated at the metatarsophalangeal joints and at those connecting the second and third phalanges. PART III. DISSECTION OF THE RABBIT. The plan of dissection as outlined in the following pages presupposes in the first place that the entire dissection is to be' made on a single specimen, and secondly that the latter has been prepared for gross dissection according to the method given in the appendix. These points may be 1 mentioned as explaining many details of procedure and also to a certain extent the selection in preference to others of those structures which are more readily made out by the method employed. Because of the convenience of dissecting in circumscribed regions, the plan has been divided, although of necessity very unequally, into several parts. The order of these is such that the visceral dissection is introduced at an early stage. The somewhat more logical plan of completing first the dissection of the anterior and posterior limbs may be followed, but on account of the fact that it involves a lengthy muscular dissection to begin with, it is perhaps not to be recommended. The account itself aims at a statement, of the various structures as met with in order of dissection and the features by which they may be identified, rather than at a full description. The student should make his own observations arid prove them by personal drawings and de- scriptions of selected parts. In this connection he will do well to bear in mind that while dissection is nominally a means of obtaining anatom- ical information, its chief value as a laboratory exercise consists in the training to be acquired from critical observation and analysis. It is therefore of quite as much practical importance that he should make his observations extensive and accurate as that he should employ only good instruments, or maintain the proper sequence in dissection. I. EXTERNAL FEATURES. The external structures, subdivisions of the body, and superficial skeletal points may be made out as follows: 1. The division of the body into the head (caput), neck (collum), trunk (truncus), tail (cauda), and the anterior and posterior limbs or extremities (extremitates). i 2. In the head: (a) The division into a posterior, cranial portion (cranium), and an anterior, facial portion (facies). (b) The mouth (os), bounded by the cleft upper lip (labium superius) and the undivided lower lip (labium inferius). The large sensory hairs or vibrissae. External Features. Ill (c) The nose (nasus), and its external apertures (nares an- teriores) . (d) The eye (oculus), and its coverings, the eyelids, including the upper eyelid (palpebra superior), the lower eyelid (palpebra inferior), and the third eyelid (palpebra tertia). The third eyelid occupies the anterior angle of the eye, and is compar- able to the conjunctival fold of the human eye.' (e) The external ear (auricula), and its canal, the external acoustic meatus (meatus acusticus externUs), leading to the tympanic membrane. (f) Points on the head skeleton, to be identified by feeling through the skin; zygomatic arch, supraorbital process, external occipital protuberance, angle of the mandible, symphysis of the mandible, and the hyoid bone. 3. In the trunk: (a) The division into thorax, abdomen, and back, or dorsum. (b) The inclusion with the trunk of the proximal portions of the limbs. The angle formed by the anterior limb with the trunk represents the axillary fossa (fossa axillaris) . The depression is much less evident than in man on account of the different positions of its enclosing folds formed by the pectorales and latissimus dorsi muscles. A corresponding inguinal furrow separates the posterior limb from the abdomen and pelvis. (c) The anal aperture (anus), and on either side of it the inguinal spaces, hairless depressions, on which the ducts of the inguinal glands open. (d) In the male: the urinogenital aperture at the extremity of the penis ; the latter enclosed by a fold of integument, the prepuce (praeputium) ; the scrotal sacs (scrotum), lateral sacs of the integument lodging the testes. (e) In the female: the urinogenital aperture, enclosed by folds of the integument, forming the vulva. The clitoris, the homologue of the penis, is contained in its ventral wall: The mammary nipples (papillae mammarum), eight (to ten) in number on the ventral surface of the breast and abdomen. (f) The following skeletal points: on the axial skeleton, the manubrium sterni, xiphoid process, costal arch, spinous processes of thoracic and lumbar vertebrae ; on the pectoral girdle, the acromion, clavicle, and respective borders and angles of the 'scapula; on the pelvic girdle, the iliac crest, pubic symphysis, and ischial tuberosity. 4. In the anterior limb : (a) The division of the free portion into three segments, the arm (brachium), forearm (antibrachium), and hand (manus). (b) The position of the elbow (cubitus) in comparison with the knee. 112 Anatomy of the Rabbit. (c) The five digits, designated from the medial side as: first (d. primus), or pollex, second (d. secundus), or index, third or middle (d. tertius s. medius), fourth (d. quartus), and fifth (d. quintus s. minimus). 5. In the posterior limb : (a) The division into three segments, the -thigh (femur), leg (crus), and foot (pes). (b) The knee (genu), and the popliteal fossa of its posterior surface, the latter not well defined. The projection of the heel (calx), and the angle formed by the foot with the leg. (c) The four digits (dd. secundus-quintus) . The vestigial first digit, or hallux, does not appear above the integument. The Abdominal Wall. 113 II. THE ABDOMINAL WALL. 1. Place the animal in the supine position. Make a median in- cision of the skin of the ventral surface extending from the pubic symphysis to the tip of the mandible, being careful not to cut through more than the skin itself. Make three transverse incisions on the left side, the first on the medial surface of the arm and extending to the elbow, the second midway between the anterior and posterior limbs, the third on the medial surface of the thigh and extending to the knee. Work the flaps loose from the surface, using the handle of the scalpel, until the side of the trunk is well exposed. On the right side of the body it is sufficient to clear the middle line. Identify the structures as follows : On the skin : (a) The thick compact connective tissue forming the cprium. (b) The imbedded hair-follicles. (c) The loose subcutaneous tissue (tela subcutanea) by which the skin is attached. (d) In the female: the mammary glands (mammae), forming a layer on the inner surface, and more or less closely aggregated about the mammary nipples. On the exposed surface: (e) The linea alba, a white aponeurotic line extending from the pubic symphysis to the xiphoid process of the sternum. (f) The cutaneus maximus muscle, a thin sheet of muscle fibres covering the entire lateral surface of the thorax and abdomen. Origin : The linea alba, the ventral surface of the sternum in its posterior portion, and the humeral spine. The last- named portion appears on the medial surface of the humerus. Insertion : The skin of the dorsolateral surface of the trunk. The fibres are directed upward and backward. The muscle is continuous across the back with its fellow of the opposite side, and is extended backward to the dorsum of the tail. It is used in shaking the skin. The artery passing forward for a short distance in the inguinal region, and lying in the subcutaneous tissue, is the superficial epigastric, a branch of the femoral (p. 149). The abdominal vein, a conspicuous vessel 'in the female, traverses the lateral portion of the abdominal wall from the inguinal furrow to the axillary fossa, lying on the external surface of the cutaneus maximus. It is a tributary of the inferior epigastric (p. 134), and anastomoses forwards with the external mammary vein of ' the axilla. The corresponding arteries are the external mammary branch of the long . thoracic (p. 137), and the abdominal branch of the inferior epigastric, the latter arising directly from an external spermatic trunk. The inguinal lymph nodes (lymphoglandulae inguinales) are small, oval, brownish bodies lying in the inguinal furrow. 114 Anatomy of the Rabbit. 2. Remove the cutaneus maximus from the surface. Identify the following points of attachment of the abdominal muscles proper: (a) The linea alba. (b) The linea semilunaris, a slightly curved line situated laterally a short distance from the linea alba. (c) The ribs and the costal arch. (d) The lumbodorsal fascia (fascia lumbodorsalis), a broad, white sheet of connective tissue extending over the posterior thoracic and lumbar regions. (e) The inguinal ligament (ligamentum inguinale), a stout white cord, stretched between the symphysis pubis and the iliac crest. 3. Identify on the surface the external oblique muscle (m. obliquus externus abdominis). Origin: The posterior ten ribs by separate slips, the xiphoid process, and the lumbodorsal fascia. Insertion: The linea alba and the inguinal ligament. The fibres are directed from an anterior dorsal position downward and backward, the more dorsal ones almost directly backward. Some of the anterior slips of origin interdigitate with those of the thoracic portion of the serratus anterior muscle. Some are concealed by the pectoral muscles. The muscle crossing the breast from the sternum to the arm is the pectoralis major. That passing forward from the lumbodorsal fascia to the medial surface of the humerus is the latissimus dorsi. The margins of these muscles may be raised where they conceal the external oblique. 4. Taking a line between the iliac crest and the xiphoid process, divide the muscle, and then separate it fully from the next, which may be distinguished by the cross direction of its fibres. Note the separate slips of origin and the difference in appearance between the fleshy and aponeurotic portions of the muscle; then remove it from the surface. Examine the following muscles, proceeding in a similar manner: (a) The internal oblique muscle (m. obliquus internus abdominis). Origin : The inguinal ligament, a second sheet of the lumbo- dorsal fascia, and the posterior four ribs. Insertion: The linea alba. The fibres pass downward, and forward. The ventral aponeurosis is much broader than that of the external oblique. It contains the rectus abdominis. (b) The transverse muscle (m. transversus abdominis). Origin: Seven posterior ribs, a third sheet of the lumbodorsal fascia, and the inguinal ligament. Insertion : The linea alba. The fibres are directed downward and slightly backward. (c) The rectus abdominis muscle. Origin : Lateral border of the sternum, including the xiphoid process; also the ventral surfaces of the second to seventh costal cartilages. Insert- ion : At the anterior end of the pubic symphysis. It is a thin, strap-like muscle, enclosed by the aponeurosis of the. internal The Abdominal Wall. 115 oblique, and separated from its fellow of the opposite side by the linea alba. The artery passing forward, for the most part in this muscle, is the inferior epigastric, a branch of the external iliac (p. 134). It anastomoses with the internal mammary (p. 174). It gives off the external spermatic artery, a small vessel which perforates the abdominal' wall and extends backward, supplying the sac of the testis in the male and ending in the female in the wall of the vulva. An abdominal branch of this vessel passes forward on the abdominal wall as described above. 5. Divide the remaining portion of the wall on the left side, and its whole thickness on the right, by a transverse incision, so that the viscera are fully exposed. Note on the internal surface of the wall the smooth serous investment here forming the parietal peritoneum (peritonaeum parietale). 116 Anatomy of the Rabbit. III. THE STOMACH AND SPLEEN. The cavity disclosed by the division of the abdominal wall is the peritoneal cavity (cavum peritonaei), the largest of the four great serous sacs representing the primary body-cavity, or coelom (p. 49). The major portion of the cavity is abdominal, but it extends into the pelvis, and, in the male, also into the scrotal sacs. Its lining membrane is that appearing on the body-wall as the parietal peritoneum, noted above, and on the visceral structures as the visceral peritoneum (peritonaeum viscerale). The visceral structures here include the major portions of the digestive and urinogenital systems. The general relations of the visceral peritonaeum should first be examined by raising a portion of the small intestine from the left side of the visceral mass. Note its enclosure by a complete serous coat, similar in appearance to the membrane covering the body wall, and the ex- tension of the latter into a mesentery for the support of the structure from the dorsal body-wall. Note the parallel arrangement of the arteries and veins, and also their frequent anastomoses. Lymphatic vessels (lacteal vessels) accompany the bloodvessels in the mesentery, but being transparent are not readily recognizable. Lymph nodes also occur, but in this portion of the mesentery they are aggregated at its dorsal attachment, or root (radix mesenterii). 1. Displace the posteroventral portion of the liver forward, exposing in this way the ventral surface of the stomach. Without injuring the enclosing peritoneum, examine the contour of the organ and its divisions, as follows : (a) The greater curvature (curvatura ventriculi major), its convex posterior surface. (b) The lesser curvature (curvatura ventriculi minor), the contracted, concave anterior surface. (c) The main portion, or body of the stomach (corpus ventriculi) . It lies for the most part to the left of the median plane. (d) The cardia, or area of junction with the oesophagus. (e) The fundus, a small sac-like portion lying to the left of the cardia. (f) The pyloric limb (pars pylorica), lying to the right of the body of the organ. (g) The pylorus, the point of communication of the stomach with the intestine (duodenum). It is marked by an annular constriction, preceding which is a greatly thickened muscular portion of the pyloric limb, known as the pyloric antrum (antrum pyloricum). 2. Raise the posterior portion of the stomach and turn it forward. Note on the dorsal surface of the greater curvature a dark red, elongated The Stomach and Spleen. 117 body, the spleen (lien). Trace the course of the peritoneum from the dorsal abdominal wall to the liver, as follows: (a) A broad fold of peritoneum, the mesogastrium, connects the dorsal abdominal wall and the diaphragm with the left side and dorsal surface of the greater curvature. Its posterior portion is divided into two parts by the spleen. The dorsal part, the phrenicosplenic ligament (lig. phrenicolienale) connects the spleen with the dorsal body-wall! The ventral part, the gastrosplenic ligament (lig. gastrolienale) connects the spleen with the greater curvature. (b) The peritoneum is projected backward from the greater curvature as a free fold, the greater omentum (omentum majus), which covers the intestines to a certain extent. It usually contains fat. It is composed of four leaves, of which two come from the surface of the stomach and two others pass forward in a dorsal position from the free posterior border to unite with the transverse mesocolon. (c) The lesser omentum (omentum minus) passes from the lesser curvature and the duodenum to the posterior surface of the liver. Its thickened margin on the right side forms the hepatoduodenal ligament (lig. hepatoduodenale) which carries three important vessels of this relation, namely, the common bile duct, the hepatic artery and the portal vein. Its left portion forms a thin membrane, the hepatogastric omentum, connecting the caudate lobe with the lesser curvature. 3. Working on the left side between the dorsal surface of the stomach and the body-wall, remove sufficient of the peritoneum to expose the first portion of the abdominal aorta as it emerges from the diaphragm. Passing in the direction of the stomach is a median ventral branch, the coeliac artery, the distribution of which may be traced. The following structures, however, should first be identified. (a) The superior mesenteric artery (a. mesenterica superior), a second, also much larger, median branch of the aorta, given off a little distance behind the coeliac artery and passing in the direction of the intestine. (b) The suprarenal gland (gl. suprarenalis) of the left side, a yellowish disc-shaped body lying some distance from the anteromedial margin of the left kidney. (c) The inferior caval vein (v. cava inferior), a large thin- walled vessel lying to the right of the aorta. It is not conspicuous if empty. (d) The coeliac ganglion (g. coeliacum) of the sympathetic nervous system lies in front of the superior mesenteric artery. (e) The superior mesenteric ganglion (g. mesentericum superius) of the sympathetic system lies immediately behind the su- perior mesenteric artery, and also on its lateral walls. JO 118 Anatomy of the Rabbit. (f) The nerves proceeding from the coeliac and superior me-, senteric ganglia accompany the corresponding arteries, forming the coeliac and superior mesenteric plexuses. (g) The (greater) splanchnic nerve (n. splanchnicus major) of the left side passes backward from its origin in the thorax (see p. 178), around the reduced left crus of the diaphragm and crossing the aorta obliquely enters the superior mesen- teric ganglion. (h) A portion of the pancreas (cf . p. 122) is seen in the peritoneum after the branches of the splenic artery have been severed (4, a). Trace the plan of branching of the coeiiac artery, beginning at the point of origin, and exposing the vessels in order. The coeliac artery (a. coeliaca) is a short trunk, its first main branch, the splenic artery, being given off near its origin from the aorta. The remaining portion of the vessel passes to the right in the direction of the lesser curvature, and divides into two parts, the left' gastric and hepatic arteries. Small vessels the inferior phrenic arteries (aa. phrenicae inferiores) are given off from the anterior wall of the coeliac and are distributed to the diaphragm. The distribution of the main branches is as follows: (a) The splenic artery (a. lienalis) passes in the direction of the spleen, giving off small branches (rr. pancreatici) to the pan- creas _and one or more large vessels, the short gastric arteries (aa. gastricae breves), to the left portion of the greater curvature. Passing along the concave surface, or hilus, of the spleen it gives off several splenic branches (rr. lienales) to that organ, and also several more branches, comparable to the short gastric arteries, to the greater curvature. Toward the end of the spleen the artery passes into the greater omentum, and at this point there is given off a large vessel, the left gastroepiploic artery (a. gastroepiploica sinistra), which passes to the right on the greater curvature and anastomoses with the right gastroepiploic artery. The gastrosplenic ligament, together with its vessels, may be divided, the spleen being allowed to fall backward toward the intestine. (b) The left gastric artery (a. gastrica sinistra) forms a short trunk, or more commonly a group of vessels, the branches of which pass in a somewhat radiate manner toward the lesser curva- ture of the stomach, reaching in this way both dorsal and ventral surfaces. Two larger vessels appear on the ventral surface respectively to the right and left of the cardia. That on the left distributes small branches (rr. oesophagei) to the cesphagus, while that on the right bears a small pyloric branch which anastomoses across the lesser curvature with the right gastric artery. The chief nerves of the coeliac plexus accompany the branches of the artery to the stomach where they connect with the terminal ramifications of the vagus (p. 119). The Stomach and Spleen. 119 ■ (c) The hepatic artery (a. hepatica), the continuation of the coeliac, passes forward and to the right, giving off' small branches to the pancreas. Its first main branch is the gastroduodenal artery (a. gastroduodenalis) . The latter is distributed chiefly to the first portion of the intestine.as the superior pancreaticoduodenal artery (a. pancreaticoduoden- alis superior), but a small recurrent branch, the right gas-, troepiploic artery (a. gastroepiploica dextra), traverses the greater omentum to the greater curvature where it ana- astomoses with the left gastroepiploic artery. After giving off the gastroduodenal artery, the hepatic enters the lesser omentum on its way to the liver. A small branch, the right gastric artery (a. gastrica dextra) passes to the pylorus and anastomoses across the lesser curvature with a branch of the left gastric artery. The veins of the stomach and spleen are tributaries of the portal vein. Accompanying the branches of the splenic artery are the tributaries of the splenic vein (v. lienalis), including the left gastroepiploic vein. Accompanying the branches of the left gastric artery are the tributaries of the coronary vein (v. coronaria ventriculi). The splenic and coronary veins enter the left wall of the portal vein through a short common trunk. On the right side of the stomach the superior pancreaticoduo- denal vein is united with the right gastroepiploic vein to form a short trunk, the gastroduodenal vein (v. gastroduodenalis), which enters the right wall of the portal vein. The left gastroepiploic vein receives tributaries from the dorsal surface of the pyloric antrum. The abdominal portion of the tenth cranial, or vagus nerve (n. vagus) may.be traced from the oesophagus to the surface of the stomach. The left cord appears on the left wall of the oesophagus; crossing the ventral surface of the latter obliquely to the right, it ramifies on the ventral portion of the lesser curvature. The right cord passes backward in a similar manner on the dorsal surface of the oesophagus. 4. Cut across the stomach at the pyloric antrum. Divide the oesophagus, and remove the stomach from the body. Open the organ by means of an incision extending around the greater curvature to the oesophagus. On the cut end of the pyloric antrum the mucous and muscular tunics (cf. Fig. 15) may be distinguished and separated from one another by dividing the loose tissue of the tela submucosa. On the surface of the mucous tunic may be seen the gastric areas (areae gastricae), formed by the longitudinal folds and imperfect transverse ridges which tend to connect them. They are well marked only in the contracted condition of the stomach. The mucous tunic of the stomach is sharply differen- tiated from that of the oesophagus. 120 Anatomy of the Rabbit. IV. THE LIVER. The liver (hepar) is noteworthy, first, as being the largest of the glandular structures of the body, and, secondly, as containing, in ad- dition to the primary circulation formed by the hepatic artery and veins, the ramifications of the portal system. It is an appendage of the digestive tube, its connection with the latter being through the common bile duct. 1. Examine the general contour and plan of division as follows: (a) The convex anterior surface, applied to the diaphragm; the concave posterior surface fitting the convexity of the stomach; the thickened dorsal portion of the organ, and its thin posteroventral margin. (b) The division of the organ into right and left lobes, and of each lobe into anterior and posterior lobules. (c) The gall bladder (vesica fellea), situated on the posterior surface of the right anterior lobule. (d) The quadrate lobe (lobus quadra tus), an imperfectly de- veloped lobe lying to the medial side of the gall bladder. (e) The caudate lobe (lobus caudatus), a small independent lobe projecting backward from the base of the left posterior lobule, and accommodated in the natural condition in the space enclosed by the lesser curvature of the stomach. 2. Trace the peritoneal connections as follows: (a) The lesser omentum, represented by the hepatoduodenal ligament. (b) The falciform ligament (lig. falciforme hepatis), a broad median sheet connecting the anterior surface of the liver with the diaphragm and extending backward to the ventral abdominal wall. The position of this ligament indicates the line of division of the liver into right and left lobes. The free arcuate border of the ligament contains a thin cord, the round ligament (lig. teres hepatis), which marks the position of the umbilical vein in the foetus. (c) The coronary ligament (lig. coronarium hepatis), a short circular fold, continuous with the falciform, and connecting the anterior surface of the liver with the middle of the diaphragm. (d) The left triangular ligament (lig. triangulare sinistrum), a lateral continuation of the coronary connecting the left lobe with the diaphragm. 3. Trace the branches of the common bile duct, the hepatic artery and the portal vein. These structures traverse the lesser omentum side by side and their branches are similarly arranged. The Liver. 121 (a) The common bile duct (d. choledochus) is formed on the posterior surface of the liver by the union of a left hepatic duct (d. hepaticus) with a similar duct from the right anterior lobule. The latter receives the cystic duct (d. cysticus) from the gall bladder. Special ducts from the right posterior lobule and from the caudate lobe enter the common bile duct through a short common trunk. The common bile duct passes backward on the right side of the portal vein, and enters the digestive tube on the dorsal surface of the first (superior) portion of the duodenum immediately beyond the pylorus. (b) The hepatic artery (a. hepatica) approaches the liver by passing forward on the right side of the portal vein ventral to the bile duct. It distributes branches to the right posterior lobule and the caudate lobe, and at the base of the liver proper divides into right and left rami. The right ramus sends a branch, the cystic artery (a. cystica), to the gall bladder. (c) The portal vein (v. portae), a vessel of large calibre, but usually found in a collapsed condition, enters the lesser omentum from the dorsal surface of the pyloric antrum. It distributes branches to the right posterior lobule and the caudate lobe; then passing directly forward to the base of the left lobe is distributed to the latter, a right branch being given off to the right anterior lobule. 4. Divide the lesser omentum with the structures described above. Divide the coronary and triangular ligaments, being careful not to injure the central tendon of the diaphragm, which resembles the coronary ligament. Remove the liver and examine its dorsal surface for the following : (a) The inferior vena cava ; it is accommodated in a depression of the thickened dorsal portion of the organ. The vessel should be opened lengthwise. (b) The hepatic veins (vv. hepaticae) open almost directly ffom the substance of the liver into the inferior cava. They are typically four in number, there being separate vessels for the anterior and posterior parts of the right lobe and for the caudate lobe, in addition to a large vessel for the left lobe. (c) The renal impression (impressio renalis), an extensive excavation of the right posterior lobule for the accommoda- tion of the right kidney. 122 Anatomy of The Rabbit. V. THE INTESTINES. The posterior portion of the digestive tube, or that portion extending from the pyloric aperture of the stomach to the anal aperture, is divisible into two main parts, not wholly distinguishable in calibre, namely, the small intestine (intestinum tenue), and the large intestine (intestinum crassum). Both are greatly elongated and convoluted. In examining them care must be taken to avoid injury to the bloodvessels and me- senteries, especially the dorsal attachments of the mesenteries, in which the chief plexuses and related ganglia of the sympathetic system will afterwards be traced. For the general relations of the intestines and mesenteries see pp. 41, 50. 1. Beginning at the pylorus, trace the course of the small intestine, as follows: Its first portion, the duodenum, forms a U-shaped loop lying on the dorsal wall of the abdominal cavity to the right of the vertebral column. The end of this loop, when traced from the right side, disappears in the peritoneum and may then be picked up in a forward position on the left side of the mass. This point marks the beginning of the second portion, the mesenterial small intestine (intestinum tenue mesenteriale), which may be traced to its termination on the greatly enlarged caecum. The connection with the caecum is through a rounded semi-expanded sac, the sacculus rotundus. The terminal portion is somewhat more difficult to follow on account of the adhesions of its peritoneum with that, of the large intestine. 2. Examine the divisions of the duodenal loop and related structures, as follows : (a) The superior, . descending, transverse (horizontal), and ns- cending portions of the duodenal loop. (b) The. common bile duct, opening on the dorsal wall of the superior portion. (c) The mesoduodenum, a fold of peritoneum joining the various parts of the loop. (d) The pancreas (Fig. ,3 B, p. 11). Its principal portion is here seen as a diffuse brownish mass lying in the mesoduodenum. Its duct (d. pancreatis) opens into the posterior portion of the ascending limb. Ce) The superior pancreaticoduodenal artery, a branch of the *>' gastroduodenal (see p. 119), passes backward on the first portion of the descending limb, (f) The inferior pancreaticoduodenal artery (a. pancreaticoduo- denalis inferior), a branch of the superior mesenteric (p. 125), enters the mesoduodenum from the left side and supplies the major portion of the loop. An anterior branch an- astomoses with (e). The Intestines. 123 3: In the mesenterial small intestine the following features may be identified : (a) The lighter coloration, due to the thicker wall and greater vascularity, of the first or duodenal portion, thus distin- guished as the jejunum (intestinum jejunum). (b) The darker coloration, due to the thinner walls, which allow the contents to show through, and diminished vascularity of the terminal or caecal portion, thus distinguished as the ileum (intestinum ileum) . The two portions are not distinctly separable. Also the circular folds (plicae circulares), or valvulae conniventes, of the mucous tunic, which in many mammals contribute to the thickness of the wall in the duodenum and jejunum, are in the rabbit not definitely expressed. (c) The mesentery, the peritoneal support of the mesenterial small intestine, is distinguished in its major portion by its broad frill-like character, which allows great freedom of movement to this portion of the digestive tube. Its terminal portion, however, beginning at a point where the intestine turns sharply forward on its way to the caecum, is adherent to the mesocolon. (d) The mesenteric lymph glands (lymphoglandulae mesentericae) are aggregated at the dorsal root of the mesentery, where they form a compact mass surrounding the base of the superior mesenteric artery. (e) The wall of the sacculus rotundus shows externally a hexagonal pattern on account of the presence in it of a large number of lymph follicles. Similar structures, forming oval areas about 3 mm. in diameter and 5 mm. in length, may be found along the intestinal wall (aggregated lymph nodules of Peyer). (f) The finger-like processes, or villi, of the mucous tunic of the small intestine may be seen by making an incision of the wall and examining its internal surface. A small portion of the wall may be excised and examined under water. 4. Trace the course of the large intestine, beginning at the sacculus rotundus, as follows: Its first portion, the blind intestine or caecum (intestinum caecum), distinguished by its great size, is connected with the large intestine proper only in the region of the sacculus rotundus. Its course is comparable to two turns of a spiral. Its closed end, formed by the vermiform process (processus vermif ormis) , lies in a dorsal position, and is directed backward. The second portion, the colon, comprising the major portion of the large intestine proper, leaves the caecum in the region of the sacculus rotundus, in which position it is distinguished by its greatly sacculated walls. The third portion, the straight intestine, or rectum, (intestinum rectum) is a small terminal division situated in the middle line and enclosed for the most part by the pelvis. It is scarcely distinguishable from the related portion of the colon. 124 Anatomy of the Rabbit. 5. In the caecum the following features may be distinguished: (a) The wall, which is otherwise smooth, is divided by a spirally arranged constriction, the latter denoting the position, internally, of a fold of the mucous tunic, the spiral valve. (b) The vermiform process is a narrow, light-colored tube of about five inches in length, the wall patterned externally by lymph follicles, in the same way as that of the sacculus rotundus, and greatly thickened in comparison with that of the caecum proper. Fig. 44. Plan of the connections of the large and small intestines: c, ascending colon; cae., caecum; h., haustra; i., ileum; s.r., sacculus rotundus; t.c, band of the colon; v.s., spiral valve. 6. The colon is divisible into ascending, transverse, and descending portions, the relations of which may be traced as follows: (a) The ascending colon (colon ascendens) passes from its origin on the caecum to a point forwards on the right side of the dorsal body-wall. This portion is greatly elongated in the rabbit, and instead of passing directly forward follows a tortuous course. It is composed of five principal limbs, united by flexures. Three of the limbs are directed for the most part forward, the remaining two backward. The first limb of the colon bears three rows of small saccu- lations, the haustra, separated by three longitudinal muscle stripes, distinguished as the bands of the colon (taeniae coli). Two of these bands are free, while the third is enclosed by the supporting peritoneum, the mesocolon. The two free bands run together toward the anterior end of the first limb. (b) The transverse colon (colon transversum)' is a short segment, beginning forwards on the right and crossing the middle line transversely to the left, where it bends sharply backward, and is replaced by the descending colon. The Intestines. 125 (c) The descending colon (colon descendens) passes backward to a point in front of the pelvis, where it is replaced by the rectum. The descending mesocolon, which connects this portion with the dorsal body-wall, should be noted on account of its relation to the inferior mesenteric artery and sympathetic plexuses. It is connected for a considerable distance with the mesentery of the ascending limb of the duodenum. 7. Displace the caecum, turning it over to the right. Lay out the mesenterial small intestine, so that the mesentery and its bloodvessels are exposed. Remove the lymph glands from about the base of the mesenteric artery, and trace its branches as follows: (a) The middle colic artery (a. colica media), a small vessel arising from the left wall and passing to the transverse colon. (b) The inferior pancreaticoduodenal artery (p. 122) arises at the same level, but from the right wall. (c) The ileocaecocolic artery (a. ileocaecocolica), a large branch, equalling in size the superior mesenteric trunk, is distributed to the terminal portion of the ileum, the caecum (including the vermiform process), and the ascending colon. Its branches are arranged in two series, a proximal group being given off near the point of origin of the main vessel, and a distal group, including the terminal portion of the vessel, at about two inches from the point of origin. The proximal branches include : (1) Small branches to the third, fourth, and fifth limbs of the ascending colon. (2) The appendicular artery (a. appendicularis) to the vermiform process. A branch of this vessel, given off near the point of origin, passes to the ileum, anastomosing with a branch of the superior mesen- teric trunk. (3) An anterior ileocaecal artery to the terminal fourth (third limb) of the caecum proper and related por- tion of the ileum. (4) An anterior right colic artery to the flexure uniting the first and second limbs of the ascending colon. (5) A posterior right colic artery to the second limb of the ascending colon. This vessel anastomoses with (4) and with the special branch to the third limb (1). The distal branches include : (6) A posterior ileocaecal artery to the middle portion of the third limb of the caecum and the adjacent portion of the ileum ; anastomosing with (3) . (7) A caecal artery to the posterior portion of the third limb of the caecum. 126 Anatomy of the Rabbit. (8) Terminal branches to the parts, about the sacculus rotundus; anastomosing with (4). (d) The intestinal arteries (aa. intestinales), about twenty in number, are given off from the superior mesenteric artery, and are distributed to the free portion of the mesenterial small intestine. The successive vessels are connected by anastomoses. The end of the superior mesenteric artery has the relation of one of the intestinal arteries. It anastomoses forwards with a single branch given off from the side of the vessel opposite to the intestinal arteries, and the latter in turn anastomoses forwards with a branch of the appendicular artery. 8. Locate in the descending mesocolon the inferior mesenteric artery (a. mesenterica inferior), a small median vessel arising from the abdominal aorta. It has two main branches — the left colic artery (a. colica sinistra) to the anterior portion of the descending colon (anastomosing with the middle colic), and the superior haemorrhoidal artery (a. haemorrhoidalis superior) to the posterior portion of the colon and the rectum. 9. The superior mesenteric vein (v. mesenterica superior), the chief tributary of the portal, collects the blood distributed by the superior mesenteric artery, its tributaries being similar in arrangement to the branches of the artery. The inferior mesenteric vein (v. mesenterica inferior) collects blood from the descending colon and rectum; it may be traced forward in the descending mesocolon. 10. Sympathetic plexuses. In the descending mesocolon will be found the inferior mesenteric ganglion (g. mesentericum inferius), a narrow curved body situated in front of the inferior mesenteric artery. Surrounding the abdominal aorta and appearing in the mesocolon is the abdominal aortic plexus (plexus aorticus abdominalis). It is connected anteriorly with the coeliac and superior mesenteric plexuses (p. 118) accompanying the corresponding vessels, and with the renal plexuses accompanying the renal vessels to the kidneys; posteriorly with the inferior mesenteric and spermatic plexuses about the inferior mesenteric and internal spermatic arteries, and with the hypogastric plexus about the pelvic vessels. 11. By dividing the rectum close in front of the pelvis, and severing the peritoneal attachments, the intestines may be separated and laid out in an extended condition. The relations to one another of the ileum, caecum, and colon are studied to much better advantage than in the natural position. The Urinqgenitai, System-. 12^ VI. THE URINOGENITAL SYSTEM. A. The Urinary Organs. The central organs of excretion, the kidneys (renes), occupy an anterior position on the dorsal wall of the abdomen. The right kidney is placed a little farther forward than the left, and is largely covered by the right posterior lobule of the liver. In addition to a fibrous coat immediately surrounding the kidney substance, each organ is imbedded in a mass of fatty material, the adipose capsule (capsula adiposa), and is also held in position by the peritoneum, which is stretched across its ventral surface. 1. By removing the peritoneum and adipose capsule from the left kidney, the external features and vascular connections may be made out as follows : (a) The general convexity of contour. (b) The renal hilus (hilus renalis), a concavity of the medial sur- face of the organ. (c) The ureter, or duct of the kidney, a white tube passing back- ward from the hilus. (d) The renal artery (a. renalis), arising from the abdominal aorta and entering the kidney at the hilus. A branch of this vessel, the twelfth intercostal artery (a. intercostalis xn), passes to the body-wall in front of the kidney, giving off a small suprarenal artery to the suprarenal gland. (e) The renal vein (v. renalis), leaving the kidney at the hilus, and joining the inferior cava. 2. Divide the kidney, beginning the incision at the hilus and re- moving the ventral half. Examine the cut surface of the dorsal half for the following: (a) The renal pelvis (pelvis renalis), formed by the expanded funnel-like end of the ureter, which is fitted into the renal hilus. The enclosed space is largely occupied by a conical projection of the kidney substance, the renal papilla (papilla renalis) . (b) The cortical substance (substantia corticalis) ; distinguish- able as a narrow peripheral zone of the kidney substance. (c) The medullary substance (substantia medullaris), forming the central and medial portion of the kidney, including the renal papilla. It is distinguished by its radial striations. (d) The fibrous coat (tunica fibrosa) of the kidney may be stripped from the surface. In the rabbit the kidney is not lobulated. Hence there is a, single renal papilla, and the division of the kidney substance into renal pyramids _ is imperfectly expressed. The medullary substance, however, possesses u slightly divided margin. 128 Anatomy of the Rabbit. The distinction of medullary and cortical portions depends on differ- ences in the arrangement of the minute tubules which compose the kidney. The medullary substance is largely composed of straight collecting tubules converging on the renal papilla, while the cortical substance is occupied by convoluted portions of the tubules and their vascular connections, the glomeruli. In the natural condition the two parts are also distinguishable in coloration, the cortical substance being darker. In embalmed animals, however, the color features are usually reversed. 3. The urinary bladder (vesica urinaria) lies in the ventral posterior portion of the abdominal cavity. It is a muscular sac, capable of a considerable amount of distension, but usually found in preserved animals in a greatly contracted condition. Its rounded anterior end, the vertex, projects forward into the abdominal cavity, while its posterior portion or fundus, narrows to a canal, the urethra, which receives on its dorsal wall the apertures of the genital ducts and those of the related glands. The connections may be made out as follows : (a) The peritoneum is reflected from the ventral surface of the rectum in the male and from the uterus in the female, to the bladder, and after investing the latter passes to the ventral abdominal wall. The dorsal peritoneum forms in the male a double rectovesical fold (plica rectovesicalis), and in the female a similar vesicouterine fold, a recess of considerable extent (rectovesical or vesicouterine pouch) being left between the adjacent structures. The ventral peritoneum forms a broad median vertical sheet, the middle umbilical fold (plica umbilicalis media). The free edge of this fold, extending from the vertex of the bladder to the umbilicus, contains a slender cord, the middle umbilical ligament (lig. umbilicale medium). The latter marks the position of the peripheral portions of the umbilical arteries in the foetus. (b) The umbilical artery (a. umbilicalis), a branch of the hypo- gastric, passes along the side of the bladder to the vertex. From the base of the artery branches are given off to the ureter (a. ureterica) and related portions of the genital ducts. B. The Male Genital Organs. 1. Continue the median ventral incision of the skin backward along the symphysis to the penis. Reflect the skin on both sides to clear the attachments of the penis to the ischium, and on the left to a point beyond the scrotum. Note the cremaster muscle (m. cremaster), a thin layer of muscle fibres forming the outer layer of the sac of the testis-. It is continuous with the internal oblique muscle of the abdominal wall, and also contains fibres from the transverse muscle. ■ Make a longi- tudinal incision through this muscle, cutting forward into the abdominal cavity. On spreading apart the two, flaps the following features may be made out: (a) The parietal layer (lamina parietalis) of the tunica vaginalis propria, a layer of peritoneum, continuous with that of the The Urinogenitae System. 129 abdominal wall, forms the internal lining of the sac of the testis (cf. p. 51). The sac is widely open to the abdominal cavity, so that the testis passes freely from one cavity to the other. (b) The male reproductive gland, the testis, with its associated vessels and duct, occupy the cavity of the sac, the testis being suspended from its dorsal wall. (c) The gubernaculum, a short thick cord containing smooth muscle fibres, joins the posterior end of the testis with the end of the sac. (d) The visceral layer (lamina visceralis) of the tunica vaginalis propria forms the peritoneal coat of the testis and is con- tinuous with the mesorchium, a broad vertical fold of peri- toneum connecting the testis forwards with the dorsal body- wall. (e) The first portion of the duct of the testis, the epididymis, forms a thickened mass at the anterior end of the testis, and extends backward as a thinner cord along its side. The thickened anterior portion is the caput epididymidis and the contracted posterior portion the cauda epididymidis. The second portion, the ductus deferens, leaves the testis at its posterior end, where it is firmly attached to the guberna- culum. The connection with the epididymis may be shown by carefully separating the duct from the gubernaculum and the side of the testis. (f) The internal spermatic artery (a. spermatica interna) arises from the abdominal aorta, that of the left side immediately behind the origin of the inferior mesenteric. It enters the anterior end of the testis. (g) The spermatic vein (v. spermatica) is formed by a network of vessels, the plexus pampiniformis, which surrounds the internal spermatic artery in its posterior portion. It opens forwards into the inferior caval at the angle formed by the latter with the renal artery. (That of the right side enters the inferior caval at about the level of the spermatic arteries.) Owing to the open communication of the testis sac with the abdominal the association of the ductus deferens with the spermatic vessels to form a spermatic cord (funiculus spermaticus) is very imperfectly expressed. 2. The connections of the deferent ducts with the common urino- genital tube and related parts may be displayed by dividing the sym- physis and dissecting in the interior of the pelvis. The structure and attachments of the penis should first be examined. Apart from its terminal portion, the glans penis, and the urethra, the body of the penis is chiefly formed of a pair of hollow fibrous structures, the cavernous bodies (corpora cavernosa penis). Each of these is firmly attached to the posterior margin of the ischium by a fibrous cord, the cms penis. The latter is largely concealed by a short thick ischiocaver- 130 Anatomy of the Rabbit. nosus muscle, the origin of which occupies a similar position on the ischium. The penis is also attached to the symphysis hy a short but stout suspensory ligament (lig. suspensorium) and by a small pubo- cavernosus muscle lying between the latter and the ischiocavernosus. The attachments of the penis should be severed and the symphysis divided. By pressing apart the two sides. of the pelvis the urinogenital tubes, with the penis and the pelvic portion of the rectum, may be dissected out and removed from the body. The middle haemorrhoidal artery (a. haemorrhoidalis media), a branch of the hypogastric, passes to the side of the rectum and to the seminal vesicle. The internal u.u. — pudendal artery (a. pudenda inter- na), accompanied by the corres- ponding nerve and vein, passes to the side of the penis, giving off the inferior haemorrhoidal artery to the terminal portion of the rectum and to the associated rectal or anal gland. The rectum is connected with the root of the tail by the rectocaudalis muscle, a somewhat spindle - shaped aggregation of smooth muscle fibres, arising from the body of the second caudal ver- tebra, and inserted a short distance forwards on the dorsal surface of the rectum. The sphincter ani externus and sphincter ani internus are two closely related muscles enclosing the rectum and urethra, the former i arising from the dorsum of the tail. The following parts of the urinogenital system may be made out : (a) The connection of the bladder with the outside of the body through the urethra. It com- prises a short pros- tatic portion in rela- tion to the genital ducts, a much longer membranous portion traversing the pelvis, and a terminal cavernous portion in the penis. (b) The seminal vesicle (vesicula seminalis) lies on the dorsal surface of the base of the bladder. It is a flattened median pouch, the anterior, slightly divided tip of which is directed forward. Fig. 45. The male urinogenital ducts and related structures, viewed from the lateral surface. After Rauther: a., anal aperture; bu., bulbourethral gland; c.c, corpus cavernosum; d.d., ductus de- ferens; g.a., anal (rectal) gland; g,i., inguinal gland; g.p., glans penis; par., paraprostatic glands; pr., prostate; r., rectum; ur., ureter; u.v., urethra (membranous portion); v.s., seminal vesicle; v.u., urinary bladder. The Urinogenitai, System. 131 (c) The deferent ducts lie between the seminal vesicle and the dorsal wall of the bladder. They terminate in the ventral wall of the seminal vesicle. (d) The prostate (prostata), a white or yellowish mass of glandular tissue, lies in the posterior portion of the dorsal wall of the seminal vesicle. Its ducts, three or four in number on either side, open into the urethra. Accessory paraprostatic glands (gl. paraprostaticae), minute finger-like projections of the urethal wall, lie on either side of the base of the seminal vesicle. (e) The bulbourethral gland (gl. bulbourethralis) ; situated on the dorsal wall of the urethra behind the prostate. 3. The internal surface of the dorsal wall of the urethra may be exposed by a longitudinal incision extending into the bladder. The crescentic aperture of the seminal vesicle lies immediately in front of an oval elevation, the colliculus seminalis, on either side of which some of the minute apertures of the prostate may be made out. C. The Female Genital Organs. 1. The organs may be traced from the abdominal cavity backward, as follows : (a) The ovary (ovarium) is a small — in young animals minute — elongated structure of greyish or yellowish coloration lying on the dorsal body-wall some distance behind the kidney. It is readily distinguished by the circular translucent dots representing the larger vesicular ovarian follicles. In some cases the darker radiate impressions (corpora lutea) left by extruded eggs are discernible. (b) The mesovarium, a short fold of peritoneum suspending the ovary from the body- wall. (c) The internal spermatic artery (a. spermatica interna) arises from the abdominal aorta, immediately behind the origin of the inferior mesenteric artery, and crosses the body-wall transversely to the ovary. (d) The spermatic vein (v. spermatica) leaves the medial side of the ovary, and crossing the body-wall enters the inferior caval vein. (e) The uterine tube (tuba uterina), the first portion of the ovi- duct, distinguishable by its narrow calibre, opens into the abdominal cavity through a broad funnel-like expansion, the ostium abdominale tubae uterinae. The margin bears a large number of short folds and processes, the fimbriae tubae, which tend to enclose the margin of the ovary. (f) The mesosalpinx is the peritoneum supporting the uterine tube. It is continuous with the mesovarium. (g) The uterus, the second portion of the oviduct ; distinguished 132 Anatomy of the Rabbit. by its greater diameter and muscular walls. The size of this portion is enormously increased in animals which contain or have borne young. (h) The mesometrium is the supporting peritoneum of the uterus, and is a continuation of the mesosalpinx. The mesometrium, mesosalpinx and mesovarium together constitute the broad ligament (lig. latum uteri). (i) The round ligament of the uterus (lig. teres uteri) crosses the broad ligament, and may be traced from the anterior end of the uterus to the body-wall below the posterior portion of the inguinal ligament where it is inserted (cf. p. 49). (j) The vagina is a flattened median tube with muscular walls; it receives anteriorly the apertures of the right and left uteri. 2. Preparatory to dissecting the urinogenital structures of the pelvis, the median incision of skin of the ventral surface should be continued backward to the base of the clitoris, the structure and attachments of the latter being then examined as follows : (a) The cavernous bodies (corpora cavernosa clitoridis) form the body of the structure, as in the male, and are attached to the posterior border of the ischium through short fibrous cords, the crura clitoridis. (b) The suspensory ligament (lig. suspensorium clitoridis) is a short median cord joining the base of the clitoris with the posterior end of the symphysis. (c) The pubocavernosus and ischiocavernosus muscles pass from the posterior border of the ischium to the base of the clitoris on either side, the former being medial in position. The attachments of the clitoris should be severed and the symphysis divided. By pressing apart the two sides of the pelvis the urinogenital tube may be dissected out and removed, together with the terminal portion of the rectum. The related structures, the middle haemorrhoidal artery, etc., appearing in this dissection are as in the male (see note p. 130). In the urinogenital ducts examine the extent of the vagina back- wards and its connection with the canal of the bladder to form the common vestibulum. The latter is comparable to the male urethra (cf. p. 47, Fig. 25). The bulbourethral gland (gl. bulbourethralis) (cf. p. 131) lies on the dorsal wall. By slitting open the vestibulum and extending the incision into the bladder and also forward into the left uterus the apertures of these structures may be examined from the interior. There is an external uterine aperture (orificium externum uteri) for each division of the uterus. The Abdominal, Aorta, etc. 133 VII. THE ABDOMINAL AORTA, INFERIOR CAVAL VEIN, AND SYMPATHETIC TRUNKS. The dissection and removal of the intestines and urinogenital organs clears the dorsal body-wall for an examination of the abdominal portion of the aorta, the inferior caval vein, and the sympathetic trunks. If the inferior cava does not contain blood its tributaries should be cleared first, in order to keep them from being damaged; otherwise the branches of the aorta should first be traced. The anterior portion of the inferior cava has been removed with the liver. 1. The abdominal portion of the aorta, described as the abdominal aorta (aorta abdominalis), extends from the hiatus aorticus of the diaphragm to the seventh lumbar vertebra, where it is replaced by the paired common iliac arteries. It passes backward in a median position along the ventral surfaces of the bodies of the vertebrae. Its natural continuation backwards on the sacrum and the caudal vertebrae is represented by the greatly reduced median sacral artery. The branches of the vessel are distributed in two series: (1) visceral branches (rami viscerales) to the parts of the digestive tube and the urinogenital organs; and (2) parietal branches (rami parie tales) to the body- wall. The visceral branches comprise the paired renal and spermatic arteries, and the unpaired coeliac, superior mesenteric and inferior mesenteric arteries, which have already been traced. The parietal branches comprise : (a) The superior phrenic arteries (aa. phrenicae superiores), small vessels arising in the hiatus aorticus and passing to the diaphragm. (b) The twelfth intercostal artery (a. intercostalis xu), arising on either side from the renal artery, and passing laterad to the body- wall. (c) The lumbar arteries (aa. lumbales), seven pairs of vessels distributed metamerically to the lumbar portion of the body-wall. Six pairs arise from the dorsal wall of the aorta, the seventh from the median sacral artery (d) , each through a common trunk. (d) The median sacral artery (a. sacralis media) arises from the dorsal wall of the aorta near its posterior end, and passes backward on the ventral surface of the sacrum. Its first portion is concealed from the ventral surface by the common hypogastric vein. 2. The common iliac artery (a. iliaca communis) is a short trunk, the branches of which pass to the posterior limb, the wall of the pelvis, and the pelvic viscera. It gives off the iliolumbar artery (a. iliolumbalis), which passes laterad to the body-wall, and then divides into two branches the connections of which may be traced as follows : li 134 Anatomy of the Rabbit. (a) The external iliac artery (a. iliaca externa) is the larger lateral branch, directed toward the inguinal ligament, over which it passes to the medial surface of the limb as the femoral artery. It gives off the inferior epigastric artery (a. epigastrica inferior), which passes forward on the medial portion of the abdominal wall (see note p. 115). (b) The hypogastric artery (a. hypogastrica) is the smaller medial branch, directed backward on the dorsal wall of the pelvis. At its junction with the external iliac the vessel gives off the umbilical artery (a. umbilicalis) to the bladder, or in the female first to the vagina and uterus (a. uterina). The obturator artery (a. obturatoria) passes laterad to the pelvic wall, and the middle haemorrhoidal (see note p. 130) to the side of the rectum. The main vessel leaves the pelvic cavity, passing to the lateral side of the abductor caudae anterior, and reappears posteriorly as the internal pudendal (note p. 130). The intermediate branches pass to the posterior limb and the side of the tail (p. 149). 3. The inferior caval vein (v. cava inferior) is formed on the dorsal surface of the posterior end of the aorta by the union of the paired external iliac veins with the common hypogastric. From this position it passes to the right side of the aorta (rarely to the left) almost to its ventral surface, and then runs forward on the right side to the diaphragm. Its visceral roots or tributaries (radices viscerales) comprise the paired renal and spermatic veins, and the hepatic veins from the liver (p. 121). Its parietal tributaries (radices parietales) include the inferior phrenic veins (vv. phrenicae inferiores), which enter the inferior cava from either side of the diaphragm, the lumbar veins (vv. lumbales), a series of vessels corresponding to the lumbar arteries, and the paired iliolumbar vein (v. iliolumbalis). 4. The external iliac vein (v. iliaca externa), the continuation of the femoral vein of the thigh, approaches the inferior cava from the dorsal side of the inguinal ligament. It receives the inferior epigastric vein from the abdominal wall and the vesical vein from the bladder, the latter including in the female also the veins of the uterus. 5. The sympathetic trunk (truncus s_ympathicus) . Its lumbar, sacral, and caudal portions may be traced on either side by working between the abdominal aorta (or its continuation, the median sacral artery) and the body-wall. Except on the ventral surface of the sacrum, the ganglia of opposite sides lie close together. The lumbar portion of each trunk comprises seven ganglia with their connections. The ganglia lie on the lateral surfaces of the lumbar arteries near the points where the latter disappear dorsally in the body-wall. The rami communicantes may be found passing from the ganglia toward the spinal nerve-roots. The sacral portion comprises four ganglia of which the first two are much larger than the others. The caudal portion comprises two minute ganglia and an unpaired terminal ganglion uniting the two trunks. The; Anterior Limb. 135 VIII. THE ANTERIOR LIMB. For this dissection the skin must first be reflected from the lateral surface of the limb and the side of the neck to the dorsal median line. Covering the side and ventral surface of the neck is a broad thin sheet of muscle, the platysma, replacing the cutaneus maximus of the trunk. It forms a continuous layer over the dorsal surface of the neck, at which place it is also continuous with the cutaneus maximus. Passing forward from the manubrium sterni is a narrow band of fibres, in- separable from the platysma but lying beneath it, the depressor conchae (parotideoauricularis) posterior, which is inserted into the external base of the ear. The entire sheet of muscle should be raised from the surface, separated posteriorly from its attachment, and turned forward on the head. The dissection is mainly muscular, but the arteries and nerves should be kept intact for later examination. For the general muscular relations of anterior and posterior limbs see p. 33. *1. Muscles arising from the axial skeleton and inserted on the scapula and clavicle. (a) The cleidomastoideus. Origin : Mastoid portion of the skull. Insertion : Middle portion of the clavicle. The muscle lying on its medial side and arising from the manubrium sterni is the sternomastoideus, one of the muscles of the head. (b) The basioclavicularis (basiohumeralis). Origin: Basiocci- pital bone. Insertion: Lateral third of the clavicle and the cleidohumeral ligament. (c) The levator scapulae major. Origin: Sphenooccipital syn- chondrosis. Insertion: Metacromion. The superficial cervical artery (p. 173) passes obliquely forward and outward under cover of the'se muscles, ramifying beneath the superior portion of the trapezius in the fat-mass of the side of the neck. Its ascending cervical branch passes forward on the lateral surface of the external jugular vein. (d) The trapezius. Origin in two portions : Superior (cervical) portion: External occipital protuberance and the ligamentum nuchae. Insertion: Metacromion and supraspinous fascia. Inferior (thoracic) portion. Origin: Spinous processes of the thoracic vertebrae and the lumbodorsal fascia. Insertion : Dorsal two- thirds of the scapular spine. The muscle forms a broad triangular sheet on the lateral surface of the shoulder. The levator scapulae major, basioclavicularis, and trapezius should be divided. On the ventral surface of the origin of the superior portion of the trapezius and levator scapulae- major may be found the ventral rami of the third and fourth cervical spinal nerves. The great auricular nerve (n. auricularis magnus) passes from the third to the ear. *The structures of Group 2 may be dissected first if preferred, the serratus anterior muscle being exposed from the lateral surface and divided together with the latissimus dorsi. 136' Anatomy of The Rabbit. (e) The rhomboideus minor. Origin : Ligamentum nuchae. Insertion : Anterior two-thirds of the vertebral border of the scapula. (f) The rhomboideus major. Origin: Spinous processes of the first seven thoracic vertebrae. Insertion: Posterior third of vertebral border. By dividing the rhomboidei the scapula may be displaced laterad. • The operation is facilitated by dividing the latissimus dorsi, the re- lations of which should, however, first be noted (2, a.). (g) The levator scapulae minor. Origin: Mastoid and supra- occipital portions of the skull. Insertion : Medial surface of the inferior angle of the scapula. (h) The serratus anterior. Origin in two portions : Cervical portion: Transverse processes of the posterior five cervical vertebrae and the anterior two ribs. Thoracic portion: Third to ninth ribs by separate slips alter- nating with those of the external oblique. Common insertion : Medial surface of the vertebral border of the scapula. The transverse artery of the neck (a. transversa colli) lies on the medial side of the cervical portion. 2. Muscles arising from the axial skeleton and the pectoral girdle and inserted on the humerus, for the most part at its proximal extremity. (a) The latissimus dorsi. Origin: Lumbodorsal fascia and four posterior ribs. Insertion: Humeral spine. A long flat triangular muscle, covering a considerable portion of the lateral surface of the thorax ; having its dorsal angle covered by the thoracic portion of the trapezius. Its insertion end passes to the medial side of the humerus. (b) The pectoralis primus (p. tenuis). Origin: Manubrium sterni. Insertion: Humeral spine. A branch of the thoracoacromial artery appears between this muscle and the deltoidens. (c) The pectoralis secundus (p. major). Origin: Entire lateral portion of the sternum. Insertion : Anteromedial surface of the humerus, beginning below the head, and extending to the boundary between the middle and distal thirds. The anterior fibres are covered by those of (b). Some of the posterior fibres are inserted highest on the humerus. The two muscles should be separated and cut across. (d) The pectoralis tertius (p. minor). Origin consisting of two portions.: First portion : Middle line of the sternum between the attach- ments of the second to fourth ribs. Second portion : Manubrium sterni, extending to the level of the first rib. Insertion : The superficial fibres of the first portion are attached to the clavicle. The remaining fibres, combined with those The; Anterior Limb. 137 of the second portion and those of the pectoscapularis, pass to the dorsal side of the clavicle and over the shoulder to be inserted on the scapular spine. (e) The pectoralis quartus. Origin: The sternum, from the attachment of the fourth to seventh costal cartilages, in- sertion : Anterior surface of the head of the humerus, passing thence to its medial side. The muscle overlaps the more posterior fibres of the first portion of (d). (f) The pectoscapularis. Origin: The sternum at the point of attachment of the first costal cartilage. Insertion as indi- cated above. A slender muscle lying behind and lateral to the second portion of (d). In order to make out its extent the first portion of (d) should be divided. Bloodvessels and Nerves of the Axillary Fossa. By dividing the remaining pectorals and the clavicle, the blood- vessels and nerves of the axillary fossa will be exposed. *The axillary artery (a. axillaris), the continuation of the subclavian, crosses from the first rib to the medial surface of the humerus, where it is replaced by the brachial artery. Its branches include : (a) The thoracoacromial artery (a. thoracoacromialis) ; it arises from the anterior wall and passes laterad to the shoulder, reach- ing the latter at a point directly above and in front of the head of the humerus. A superficial branch of this artery passes between the pectoralis primus and the deltoideus, giving twigs on either side. (b) The lateral (long) thoracic artery (a. thoracalis lateralis) is a larger branch given off from the posterior wall. Its branches include short anterior vessels to the pectoral muscles and long (external mammary) branches, especially developed in the female, to the side of the thorax. (c) The subscapular artery (a. subscapularis) is a large branch which passes around the axillary border of the scapula, per- forating the teres major muscle, and is distributed to the muscles of this region, including the cutaneus maximus. A branch of this vessel, the thoracodorsal artery (a. thoraco- dorsalis), passes backward to the latissimus dorsi. The superficial cervical artery (p. 173) crosses the ventral surface of the brachial plexus, passing obliquely laterad to the side of the neck. (d) The deep artery of the arm (a. profunda brachii) is given off at about the point of origin of the subscapular. It passes to the posterior surface of the humerus giving branches to the shoulder, and is continued distad on the lateral surface of the anconaeus medialis and afterwards of the lateral head of the brachialis as the radial collateral artery. This artery accompanies the branches of the radial nerve. 138 Anatomy of the Rabbit. **The axillary vein (v. axillaris) begins at the medial side of the humerus and crosses the axillary fossa to the first rib whence it is con- tinued as the subclavian. It receives the lateral thoracic and sub- scapular veins, which accompany the corresponding arteries, and also the cephalic vein (p. 143), which reaches the medial side of the shoulder from the anterior surface of the arm by passing between the teres major and subscapular muscles near the neck of the scapula. ***The brachial plexus (plexus brachialis) is the network of nerves formed from the ansae of the ventral rami of the posterior five cervical and first thoracic spinal nerves. The cervical ansae also take part in the formation of the more general cervical plexus embracing all nerves of the cervical series. The strands of the brachial plexus cross the axillary fossa and at the medial surface of the humerus are largely re- placed by the three chief trunks of the free extremity, the radial, median, and ulnar nerves (p. 143). The suprascapular nerve (n. suprascapularis) is a large nerve formed from the fifth cervical ansa; it enters the muscle in the region of the neck of the scapula and passing around the superior border of the bone is distributed to the supraspinatus and infraspinatus muscles. The subscapular nerves (nn. subscapulares), arising from the sixth and seventh ansae, are distributed to the subsca- pular, teres major, and latissimus dorsi muscles. The axillary lymph glands (lymphoglandulae axillares) are pink or brownish bodies lying in the fat of the axillary fossa. 4. Muscles arising from the pectoral girdle and inserted on the humerus. These muscles act on the humerus through the shoulder- joint, and except for the unimportant difference in origin are similar to those of Group 2. The course of the cephalic vein (p. 143) should be traced before separating the muscles of the front of the forearm. Note the supraspinous and infraspinous fasciae covering the corres- ponding portions of the lateral surface of the shoulder. (a) The deltoideus. Origin: Lateral portion of the clavicle and the cleidohumeral ligament. Insertion: Anterior surface of the humerus in its distal third. The muscle is a continuation of the basioclavicularis. (b) The abductor brachii superior. Origin : The acromion. In- sertion : Distal portion of the humeral spine. (c) The abductor brachii inferior. Origin: Infraspinous fascia. The end of the muscle forms a curved line over the dorsal portion of the infraspinatus, leaving only a small triangular portion of the latter exposed. Insertion: The distal portion of the muscle passes beneath the metacromion, which also serves as a point of attachment, and is replaced on the lateral surface of the humerus, beneath the abductor brachii superior, by the long thin tendon, through which it is inserted. The abductores are also considered as sesond and third portions of the deltoideus. The Anterior Limb. 139 The abductor brachii inferior should be separated from the in- fraspinatus and divided, the distal end being reflected together with the metacromion. (d) The infraspinatus. Origin: Posterior portion of the lateral surface of the scapula, including the spine. Insertion: Greater tubercle of the humerus. The muscle fills the infraspinous fossa. (e) The supraspinatus. Origin: Anterior portion of the lateral surface of the scapula (supraspinous fossa), supraspinous fascia, and, to a certain extent, the subscapular fascia. Insertion: Greater tubercle of the humerus. (f) The subscapularis. Origin: Entire medial surface of the scapula. Insertion: Lesser tubercle of the humerus. (g) The teres major. Origin: Dorsal portion of the &n«™- axillary border of the scapula. Insertion : In common with the latis- simus dorsi on the ante- rior surface of the humerus, (h) The teres minor. Origin : Ventral portion of the axillary border of the L a tml scapula. Insertion: Greater tubercle, (i) The coracobrachialis. Origin : Coracoid process. Insertion: Distal portion of the upper third of the humerus on its medial surface. 5. Muscles arising from the scapula and humerus and inserted on the proximal ends of the radius and ulna (extensors and flexors of the forearm), (Fig. 46). A. Extensor (anconaeus) group. The muscles arise for the most part behind the axis of the humerus, and are inserted on the olecranon. (a) The extensor antibrachii parvus (anconaeus quar- tus). Origin: Fascia of the medial surface of the humerus, Insertion: Medial surface of the olecranon. The muscle should be divided, or detached from its origin, and re- flected' , Posterior Fig. 46. Transverse section through the distal portion of the arm ; semidiagrammatic ; a.b.. brachial artery ; a.c.r., radial collateral artery; b., biceps: br.l. and br.m., lateral and medial heads of the brachialis; d., deltoideus (insertion) ; e.a.p., extensor antibrachii parvus; f., brachial fascia; h., humerus; n.m., median nerve; n.r., radial nerve ; n.u., ulnar nerve; tr.l-tr.3, long, lateral, and medial heads of the triceps; v.b.. brachial vein; v.c. cephalic vein. 140 Anatomy of the Rabbit. (b) The anconaeus minimus (epitrochleoanconaeus) . Origin: Medial epicondyle of the humerus. Insertion: Medial surface of the olecranon. (c) The triceps brachii. Origin in three portions: Caput longum (anconaeus longus) '. Ventral portion of the axillary border of the scapula. Caput laterale (anconaeus lateralis) : Greater tubercle and related portion of the lateral surface of the humerus. Caput mediale (anconaeus medialis) : Posterior surface of the humerus. The three portions are practically separate muscles. Insertion on the olecranon. B. Flexor group. The muscles arise in front of the axis of the humerus and are inserted on the radius and ulna in front of the elbow-joint. (a) The biceps brachii. Origin: Anterior border of glenoid cavity. Insertion : Ventromedial surface of the ulna and medial surface of the radius. The muscle possesses only one head in the rabbit. (b) The brachialis. Origin: Anterior and lateral surfaces of the humerus, divided unequally into a larger lateral and a smaller medial portion by the insertion tendons of the deltoideus and abductores muscles. Insertion : In common with the biceps. 6. Muscles arising from the distal end of the humerus and the radius and ulna and inserted on the hand (extensors and flexors of the hand and the individual digits). The long insertion tendons pass through perforations of the dorsal carpal and transverse (ventral) carpal liga- ments (Fig. 47). A. Extensor group. The muscles have a general area of origin from the lateral epicondyle of the humerus and the anterodorsal, or antero- lateral surface of the radius and ulna. Insertion dorsal. (a) The extensor carpi radialis longus. Origin: Lateral epi- condyle. Insertion: Base of the second metacarpal. (b) The extensor carpi radialis brevis. Origin: Lateral epi- condyle. Insertion: Base of the third metacarpal. The muscle is partly fused with the foregoing one, and the tendons are closely associated on the wrist. (c) The abductor pollicis. Origin : Anterolateral surface of the radius and ulna. Insertion: Base of the first metacarpal. The muscle is partly concealed by (e). Its tendon forms a conspicuous cross with those of (a) and (b). (d) The extensor pollicis et indicis. Origin: Anterolateral surface of the radius and ulna. Insertion: Ungual phalanx of the pollex and the head of the second metacarpal. Its tendon is the first of five in the centre of the carpus. (e) The extensor digitorum communis. Origin: Lateral epi- The Anterior Limb., 141 condyle and proximal end of the ulna. Insertion : By four tendons on all phalanges of the four lateral digits. (f) The extensor digiti quarti proprius. Origin: Lateral epi- condyle. Insertion: Ungual phalanx of the fourth digit. (g) The extensor digiti quinti proprius. Origin: Lateral epicondyle and lateral surface of the ulna. Insertion: Head of the fifth metacarpal and base of the first phalanx of this digit. (h) The extensor carpi ulnaris. Origin : Lateral epicondyle and proximal portion of the lateral surface of the ulna. Insertion : Base of the fifth metacarpal. Dorsal Lateral a.p. Medial. "V AJA " Volar. Fig. 47. Transverse section of the distal end of the forearm. ' Showing the relative positions of the muscle- tendons: a.p., abductor pollicis; a.r., radial artery; a.u., ulnar artery; e.c.u., extensor carpi ulnaris; e.d.c, ' extensor digitorum communis; e.m.p., extensor digiti quinti proprius ; e.p.i., extensor pollicis et indicis ; e.q.p., extensor digiti quarti proprius; e.r.b., extensor carpi radialis brevis ; e.r.l., extensor carpi radialis longus ; f.c.r., flexor carpi radialis; leu., flexor carpi ulnaris; f.d.p., flexor digitorum profundus; f.d.s., flexor digitorum sublimis; l.c.d., dorsal carpal ligament ; Let., transverse carpal ligament; n.m., median nerve; n.u., ulnar nerve; p., palmaris; r., radius; u., ulna; v.c, cephalic vein; v.r., radial vein; v.u., ulnar vein. B. Flexor group. The muscles have a general area of origin from the medial epicondyle of the humerus and the posteroventral or postero- medial surface of the radius and ulna. Insertion volar. (a) The pronator teres. Origin: Medial epicondyle. Insertion: Ventral surface of the radius. (b) The flexor carpi radialis. Origin: Medial epicondyle. In- sertion: Base of the gecond metacarpal. (c) The flexor digitorum sublimis. Origin : In common with the ulnar portion of the profundus from the medial epicondyle; proximal portion of the ulna. Insertion : Bases of the second phalanges of the four lateral digits. 142 Anatomy of the Rabbit. (d) The palmaris. Origin: Medial epicondyle. Insertion: Super- ficially on the volar fascia. This extremely slender muscle lies between the superficial portion of the profundus and the. flexor carpi ulnaris. (e) The flexor digitorum profundus. Origin in four portions : Superficial portion: Medial epicondyle. Radial portion: Ventral surface of the radius. Middle portion: Ventral surface of the ulna. Ulnar portion: Medial epicondyle in common with (c) Insertion: By five tendons on the bases 6i the ungual phalanges. (f) The flexor carpi ulnaris. Origin: Medial epicondyle and medial surface of the olecranon, forming two short but separate heads. Insertion : Pisiform bone. 7. Muscles arising from the bones of the hand and inserted on the individual digits:. (a) The flexor digiti quinti. Origin: Pisiform bone and tendon sheath of the flexor digitorum profundus. Insertion: Sesamoid bones of the metacarpophalangeal joint of the fifth digit, extending to the ungual phalanx. (b) The lumbricales. Origin: From the point of division of the tendon of the flexor digitorum profundus. Insertion: First phalanges of the second to fifth digits. (c) The interossei. Origin : In pairs from the bases of the second to fifth metacarpals and related portions of the carpal bones. Insertion : Sesamoid bones of the metacarpophalangeal joints. Bloodvessels and Nerves of the Arm and Forearm. *The brachial artery (a. brachialis), the continuation of the axillary, passes distad on the medial surface of the arm between the biceps and the anconaeus medialis. Crossing to the anterior surface of its distal extremity, it passes beneath the head of the pronator teres to the medial surface of the radius, dividing at this point — a short distance in front of the elbow — into the radial and ulnar arteries. Its chief branches on the arm are the ulnar collateral arteries (superior, middle, and inferior) to the muscles and the elbow joint. The radial artery (a. radialis) passes distad on the ventromedial border of the radius, in company with the median nerve, lying at first between the flexor carpi radialis and the radial portion of the flexor digitorum profundus. Toward the distal end of the radius it crosses the ventral surface of the tendon of the flexor carpi radialis, and appears in a superficial position on the medial border of the carpus. It reaches the volar surface of the hand after passing obliquely across the tendon of the flexor digitorum sublimis. The ulnar artery (a. ulnaris) crosses the ulna obliquely from its origin, reaching in this way the lateral border of the flexor carpi ulnaris, The Anterior Limb. 143 along which it passes to the end of the forearm and to the ulnar side of the pisiform bone. It passes to the ventral surface of the fifth digit, and then turns across the hand, forming the volar arch. **The single brachial vein (v. brachialis) accompanies the brachial artery and lies behind it. It is formed in front of the elbow by the union of two vessels, the radial and ulnar veins, which accompany the corres- sponding arteries, and join one another at the point of separation of the latter. The radial vein anastomoses with the radial portion of the cephalic at a point distal to the middle of the forearm. The cephalic vein (v. cephalica) is a large superficial vessel appearing on the dorsal surface of the forearm. From the radial side of the latter it receives a large tributary which anastomoses with the radial vein. It is accompanied by branches of the superficial ramus of the radial nerve. It passes to the front of the arm across the angle of the elbow, lying at first on the anterior margin of the lateral head of the brachialis, and afterwards on the lateral surface of the arm between the abductor brachii superior and the anconaeus lateralis. It disappears from this surface in the triangular space enclosed by these muscles and the inser- tion of the levator scapulae major, receiving at this point a large tributary from the shoulder. It appears on the medial surface of the shoulder at the distal end of the axillary border of the scapula between the teres major and subscapularis, entering the axillary vein at about the same place as the subscapular vein, or in common with the latter. ***The radial nerve (n. radialis) passes behind the brachial artery to the posterior surface of the humerus. It perforates the anconaeus medialis, appearing afterwards on the lateral side of the brachialis in company with the collateral radial artery. A superficial ramus, given off on the distal portion of the arm, accompanies the cephalic vein: it passes along the surface of the extensor carpi radialis, dividing into branches for the dorsum of the hand. The remaining portion is chiefly distributed as the ramus profundus to the extensor muscles of the fore- arm. The median nerve (n. medianus) passes distad along the medial sur- face of the arm, lying at first in front of the brachial artery and then on its medial side. It accompanies the brachial artery, passing beneath the head of the pronator teres, and then traverses the forearm, in com- pany with the radial artery, to the volar surface' of the hand. The ulnar nerve (n. ulnaris) lies behind the brachial artery. Toward the distal extremity of the humerus it accompanies the inferior ulnar collateral artery. It passes from the medial surface of the elbow, be- tween the, anconaeus minimus and the base of the olecranon, to the dorsal surface of the olecranon head of the flexor carpi ulnaris, and then crosses the ulna obliquely, in company with the ulnar artery, to the lateral border of the muscle and along it to the insertion tendon. At the wrist it crosses the dorsal surface of the tendon, and passing between the tendon of the sublimis and the pisiform bone, reaches the volar surface of the hand. 144 Anatomy of the Rabbit. IX. THE POSTERIOR LIMB. Dissection on the side opposite to that of injection. The dissection is largely a muscular one; to be conducted in the same way as in the anterior limb. The corresponding muscle groups should be compared with respect to the difference in orientation of the equivalent segments. 1. Muscles arising from the ventral surface of the posterior thoracic and lumbar vertebrae and inserted on the pelvic girdle, or on the lesser trochanter of the femur. These muscles are chiefly distinguished by their vertebral position, on account of which and the fixed condition of the pelvic girdle they combine the characters of vertebral and appen- dicular muscles. (a) The psoas minor. Origin: Bodies of the four posterior lumbar vertebrae. Insertion: Pecten of the pubis. The flat, pointed tendon forms a square cross with a dorsal (sacral) continuation of the inguinal ligament, which is stretched transversely from the middle of the inguinal liga- ment to the centre of the body of the first sacral vertebra, and on which it is also inserted. It is necessary to divide the inguinal, ligdment and reflect its sacral continuation, together with the tendon of the psoas minor. (b) The psoas major. Origin: Internal surfaces of the bases of the last three ribs and bodies of the corresponding thoracic vertebrae; also the lumbar vertebrae. Insertion: Lesser trochanter. (c) The iliacus. Origin: Bodies of the last lumbar and first sacral vertebrae, extending to the sacroiliac union. Inser- tion : With the psoas major on the lesser trochanter. The two muscles together form the iliopsoas. The lumbar portion of the lumbosacral plexus, beginning with the fourth lumbar nerve, lies on the ventral surface of the psoas major and between the latter and the iliacus. The fifth lumbar is the chief root of the femoral nerve, (p. 149), the trunk of which may be traced from a position between the two muscles distad over the dorsal surface of the inguinal ligament to the medial surface of the thigh. The remaining four nerves crossing the dorsal body-wall obliquely are the twelfth thoracic and first three lumbar. The psoas major should be freed at its lateral margin and turned toward the middle line, the fourth lumbar nerve being divided. (d) The quadratus lumborum. Origin: Bodies of the posterior five thoracic vertebrae and the bases of the corresponding five ribs; bodies and transverse processes of the lumbar vertebrae. Insertion: Triangular processes of six lumbar vertebrae and the posterior ventral angle of the iliac wing, together with the adjacent portion of its medial surface. 2. Muscles arising from the pelvic girdle and sacrum and inserted on the femur, for the most part at its proximal extremity. The muscles of this group enclose the proximal portion of the femur The Posterior Limb. 145 on its lateral, posterior, and medial sides. They are partly covered by the flexors of Group 3, namely, the biceps, sartorius, and gracilis, which must be divided. To begin the dissection, see directions for removing the biceps (p. 148). The position of the sciatic vein (p. 149) should be noted. Dissection on the lateral surface posteriorly. (a) The glutaeus maximus. Origin in two fleshy portions, joined by an aponeurosis : First portion : Fascia covering the sacrum in its entire length. This portion is triangular in shape, and is covered posteriorly by the first head of the biceps. Second portion : Anteroventral border of the iliac wing, fused with the tensor fasciae latae and the first head of the rectus femoris; also from the dorsal border through the aponeurosis mentioned above. Insertion: Third trochanter. The axis of the first portion is transverse, that of the second horizontal. Both portions of the muscle should be divided. (b) The glutaeus medius. Ventral border of .the wing of the ilium and the iliac crest. Insertion: Greater trochanter. Some of the fibres pass around the medial surface of the tip of the greater trochanter and are inserted in the lateral wall of the trochanteric fossa. The muscle should be divided. (c) The glutaeus minimus. Origin : Entire lateral surface of the body and wing of the ilium. Insertion : Greater trochanter. Remove the entire muscle. The piriformis is in contact with its dorsal margin posteriorly. (d) The tensor fasciae latae. Origin: Anterior portion of the ventral border of the wing of the ilium. Insertion: Broad fascia of the lateral surface of the thigh. The muscle is fused with the first head of the rectus femoris in front, and with the second portion of the glutaeus maximus behind. (e) The piriformis. Origin: Lateral portions of the second and third sacral vertebrae. Insertion : Tip of the great trochanter. The muscle should be divided, care being taken to avoid injury to the nerves and bloodvessels beneath it. (f) The gemellus superior. Origin: Tendinous from the ischial spine and fleshy from the body of the ischium immediately in front of it. Insertion: Lateral wall of the trochanteric fossa. The muscle extending from the ischial spine to the sacrum is the abductor caudae anterior (p. 183). (g) The obturator internus. Origin: Internal extent of the obturator foramen. Insertion: Trochanteric fossa. The muscle passes over the ischium in the lesser sciatic notch, only its insertion portion appearing from the lateral 146 Anatomy of the Rabbit. surface. To see its origin reflect the tendon through the lesser sciatic notch and examine the muscle from the internal surface of the pelvis. (h) The gemellus inferior. Origin: Posterior portion of the superior ramus of the ischium and the ischial tuberosity. Insertion : Trochanteric fossa. (i) The quadratus femoris. Origin: Ventral surface of the ischial tuberosity and the superior ramus of the ischium immediately in front of it. Insertion: With (j) in the trochanteric fossa. (j) The obturator externus: Origin: External extent of the obtur- ator foramen. Insertion : Trochanteric fossa. The muscle is largely concealed from this surface, but may be fully dis- played by the division of the pectineus and adductores brevis and longus. Dissection on the medial surface posteriorly, after division of the'gracilis. (k) The pectineus. Origin: Pecten of the pubis. Insertion: Immediately below the lesser trochanter. (1) The adductor brevis. Origin: Anterior portion of the symphysis pubis. Insertion : Below the pectineus. (m) The adductor longus. Origin : Posterior portion of the symphysis and inferior ramus of the ischium. Insertion: Posterior surface of the shaft of the femur to its distal third. (n) The adductor magnus. Origin: Ventral surface of the ischial tuberosity. Insertion: Medial surface of the distal end of the femur, extending to the medial condyle of the tibia. 3. Muscles arising from the pelvic girdle and' the femur and inserted on the proximal portions of the tibia and fibula' (extensors and flexors of the leg). (Fig. 48.) ^ A. Extensor group (quadriceps femoris). The muscles lie for the most part in front of the axis of the femur. They have a common insertion on the tibial tuberosity through the patella and the patellar ligament. (a) The rectus femoris. Origin in two portions : First portion: Superior anterior spine, fused with the tensor fasciae latae, and ventral border of the iliac wing. Second portion : By a stout round tendon from the inferior anterior spine, immediately in front of the acetabulum. This part is cylindrical in shape and is practically a separate muscle. The two portions of the muscle should be divided. (b) The vastus lateralis. Origin: Anterior surface of the great trochanter and the lateral intermuscular ligament. The muscle should be divided. Ti-i£ Posterior L,imb. 147 Lateral (c) The vastus intermedius. Origin in two portions: First portion: Great trochanter, below the origin of the vastus lateralis. Second portion : Anterior surface of the femur. (d) The vastus medialis. Origin: Medially, at the base of the collum femoris and adjacent portion of the shaft. Common insertion (a-d) : Tibial tuberosity. B. Flexor group (hamstring ^^ muscles). With the exception of the sartorius, the muscles lie behind the axis of the femur, and are in- serted on the medial and lateral surfaces of the knee-joint and the corresponding proximal portions of the leg. They form the bound- aries of the popliteal fossa. (a) The sartorius. Origin: Posterior portion of the inguinal ligament, espe- cially its sacral exten- sion. Insertion : Medial condyle of the ttfSi'a. This is an extrernely thin and narrow band of fibres, lying on the more anterior portion of the medial surface of the thigh. The muscle is fused distally with the gracilis. It has the position of a rotator muscle, and is only a flexor through its con- nection with the latter. (b) The gracilis. Origin : The entire extent of the pubic symphysis. Insertion: Through a broad tendinous expan- sion ending in the fascia of the proximal portion of the medial surface of Posterior Fig. 48. Transverse section through the middle of the thigh: a.l., adductor longus; a.m., ad- ductor magnus; b.f.l and b.f. 2, first and second heads of the biceps femoris; f., femur; gr., gracilis; n.p., peroneal nerve; n.s.m., greater saphenous nerve; n.t., tibial nerve; r.f. 1 and r.fT 2, first and second heads of the rectus femoris ; s., sartorius; sm., semimembranosus; St., semi- tendinosus; t.f.c, tensor fasciae cruris; t.f.l., tensor fasciae latae; v.i. 1 and v.i. 2, first and second heads of the vastus intermedius; v. is., sciatic vein; v.i., vastus lateralis; v.m., vastus medialis; v.s.m., great saphenous vein. the thigh. The muscle forms a broad, comparatively thin sheet, covering the pos- terior portion of the medial surface of the thigh. Its inser- tion tendon is perforated by the great saphenous artery and vein and the greater saphenous nerve. The sartorius and gracilis should be raised from the surface and divided. 148 Anatomy of the Rabbit. (c) The biceps femoris: Origin in two portions: First portion (caput breve) : Spinous processes of three posterior sacral and three anterior caudal vertebrae. This portion is triangular in shape, the distal end, or apex of the triangle, passing into a thin flat tendon which is inserted on . the lateral margin of the patella. Second portion (caput longum) : Dorsal surface of the ischial tuberosity, fused with the adductor magnus, and the lateral process, fused with the second, or deep portion of the semimembranosus. This portion is also triangular, the base being distal and providing a broad insertion on the. fascia of the proximal third of the lateral surface of the leg. The first, or superficial head of the semimembranosus covers this portion at its origin. To free the biceps and divide it : Incision along the aponeurotic line joining the tip of the great trochanter with the sacrum; also distad along the intermuscular septum of the lateral surface of the thigh from the great trochanter to the knee. The sciatic vein lies on the lateral surface towards the ischial tuberosity. An incision along it will free the superficial head pf the semimembranosus from the biceps. Raise the biceps slightly by working the handle of the scalpel under its distal portion. Divide the muscle from its posterior margin, being careful not to injure the slender tensor fasciae cruris muscle and the branches of the sciatic nerve which lie beneath it. The first head of the biceps may be separated and reflected in order to expose the short muscles of the thigh. (d) The tensor fasciae cruris. Origin : By a long, slender tendon, from the transverse process of the fourth sacral vertebra. Insertion: Lateral fascia of the leg. This slender muscular slip underlies the biceps femoris. (e) The semimembranosus. Origin in two portions: First (superficial) portion : Fascia covering the first head of the biceps. ' Second (deep) portion : Lateral process of the ischial tuberosity. Insertion: In common with the gracilis on the fascia of the proximal portion of the medial surface of the leg. This fascia is contracted into two ligaments, one of which carries • the insertion of the muscle to the distal end of. the tibial tuberosity, the other to the distal end of the leg, where it joins the tendon of the heel (tendo calcaneus). (f) The semitendinosus. Origin: Ischial tuberosity. Insertion: Medial condyle of the tibia. The muscle is completely en- closed by the adductor magnus, which must be split to expose it. The. Posterior L,imb. 149 Bloodvessels and Nerves of the Thigh. *The femoral artery (a. femoralis) traverses the medial surface of the thigh, beginning at the dorsal side of the inguinal ligament, where it continues the external iliac artery. Immediately distal to the inguinal ligament it gives off the deep artery of the thigh (a. profunda femoris). The latter passes to the dorsal side of the pectineus muscle, and is dis- tributed to the posterior proximal portion of the limb. A second branch the lateral circumflex artery (a. circumflexa femoris lateralis), passes forward from the anterior wall between the insertion of the psoas major and the origin of the vastus medialis. Its first branches are distributed medially to the vastus medialis and rectus femoris. The trunk passes to the lateral side of the head of the second portion of the rectus femoris, and is distributed chiefly to the first head of the rectus fenwris and to the tensor fasciae latae. A third branch of the femoral, the superficial epigastric artery (a. epigastrica superficialis), given off medially, and passing to the abdominal wall, has been divided (p. 113). At the beginning of the distal third of the thigh, a small branch, the a. genu suprema, passes over the medial condyle of the femur to the knee-joint, and at about the point of origin of this vessel a large branch, the great saphenous artery (a. saphena magna), arises from the posterior wall. It passes across the medial surface of the distal end of the adductor longus, and through the tendon of the gracilis, to the medial surface of the leg. The femoral artery passes between the adductores longus and magnus, continuing as the popliteal artery (a. poplitea). The hypogastric artery (p. 134) appears in the greater sciatic notch, continuing thence as the sciatic artery (a. ischiadica) . The vessel passes backward to the dorsal surface of the superior ramus of the ischium, where it divides into lateral caudal and internal pudendal branches. Its smaller branches are distributed to the glutaei and biceps femoris muscles. **The femoral vein (v. femoralis) traverses the medial surface of the thigh in company with the femoral artery. It begins at the proximal end of the lower third of the thigh as a continuation of the popliteal vein (v. poplitea), which accompanies the corresponding artery. Its tribu- taries comprise the great saphenous, superficial epigastric, lateral circum- flex, and profunda femoris veins. The sciatic vein (v. ischiadica) traverses the lateral surface of the thigh near its posterior margin, lying at first between the biceps and semimembranosus and afterwards on the posterior margin of the former. At the dorsal border of the ischium, in front of the ischial tuberosity, it receives the lateral caudal and internal pudendal veins. ***The femoral nerve (n. femoralis) arises from the lumbosacral plexus, chiefly from the fifth lumbar. Its position between the psoas major and iliacus muscles has already been noted (p. 144).. Immediately beyond the inguinal ligament it divides into two portions, one of which is distributed to the muscles of the anterior side of the thigh, while the other, the greater saphenous nerve (n. saphenus major), passes to the 12 150 Anatomy of the Rabbit. medial surface of the thigh and leg in company first with the femoral artery and afterwards with the great saphenous artery. The sciatic nerve (n. ischiadicus) appears laterally in the greater sciatic notch. It passes backward beneath the piriformis muscle, and then turns distad to the thigh, where it lies on the lateral surfaces of the adductores magnus and longus. It distributes branches to the posterior musculature of the thigh. In the proximal portion of the thigh it divides into two chief branches, which are closely connected as far as the knee. The anterior branch is the peroneal nerve (n. peronaeus), the posterior branch the tibial nerve (n. tibialis). The lesser. saphenous nerve (p. 154) is a small branch given off from the tibial above the knee- joint. For the origin of this and related nerves see p. 155. The superior gluteal nerve (n. glutaeus superior) appears in the greater sciatic notch, leaving the sciatic close to the inferior posterior spine of the ilium. It passes between the glutaeus minimus and the lateral surface of the ilium, ending in the tensor fasciae latae. Its branches are distributed to the glutaei medius and minimus and the piriformis muscles. The inferior gluteal nerve (n. glutaeus inferior) perforates the pos- terior portion of the piriformis, and is distributed to the glutaeus maxi- mus. The posterior cutaneous nerve (n. cutaneus femoris posterior) accom- panies the hypogastric artery backward to the ischial tuberosity, where it turns to the posterior margin of the thigh and the medial surface of the sciatic vein, ending in branches to the skin. The pudendal nerve (n. pudendus) accompanies the sciatic artery and afterwards the internal pudendal to the penis or clitoris. In preparation for the muscular dissection of the leg, the insertion tendons of the biceps femoris, tensor fasciae cruris, gracilis, and semi- membranosus muscles should be removed from about the knee-joint. The adductor magnus may be detached from the medial condyle of the femur, but the popliteal vessels must be kept intact. The superficial bloodvessels of the leg should be noted, since it is necessary to clear them away in separating the muscles. They include, medially, the great saphenous artery and vein, and laterally the sciatic vein, together with 'its continuation, the anterior tibial vein, and the accessory small saphe- nous vein (p. 153). 4. Muscles arising from the medial and lateral condyles of the femur and from the proximal portions of the tibia and fibula, including the tibial condyles; inserted on the foot. The group includes the typical extensors and flexors of the foot, together with the peronaei muscles, which individually are extensors and flexors, but collectively have the relation of lateral tractors (Fig. 49). A. Extensor group. Muscles occupying an anterior position on the leg and inserted on the dorsum of the foot. (a) The extensor hallucis longus. Origin : Middle portion of the anteromedial surface of the tibia and from the medial con- The Posterior Limb. 151 dyle behind the tibial collateral ligament. Insertion : The tendon passes around the medial malleolus of the tibia and beneath the base of the second metatarsal, uniting with the first tendon of the extensor digitorum longus. The posterior tibial artery, the continuation of the great saphenous, and the tibial nerve accompany the tendon in the malleolar groove. (b) The tibialis anterior. Origin: Lateral condyle of the tibia and corresponding surface of the tibial tuberosity. Insertion : Base of the second metatarsal. The tendon passes beneath the obliquely placed crural ligament of the lower portion of the leg. The muscle should be divided and its head reflected. The anterior tibial artery and vein, and the peroneal nerve appear on the anterior surface of the tibia at the lateral side of the crural ligament. (c) The extensor digitorum Mlerhr longus. Origin : By a flattened tendon from the lateral margin of the facies patellaris of the femur. This ten- don passes through the capsule of the knee - joint, and the fleshy portion of the muscle lies on the anterolateral surface of the tibia. Inser- tion: The distal ten- don passes beneath the crural ligament, then beneath the cru- ciate ligament of the dorsum of the foot, dividing into four por bf- Fig. 49. Transverse section of the proximal portion of the leg: a.p., peroneal artery; a.s.m., tions for insertion On great saphenous artery; 'a s.p. small saphenous artery; a.t.a., anterior tibial artery, b.f., biceps femoris (insertion); e.d.l., extensor digitorum longus; e.h.l., extensor hallucis longus; f., fibula; f.d.l., flexor digitorum longus; g.l., and g.m., lateral • and medial heads of the gastrocnemius; gr., gracilis (insertion tendon); n.s., greater saphenous nerve; n.s.m., lesser saphenous nerve; n.t., tibial nerve; pi., plantaris; s., soleus; t., tibia; t.a., tibialis anterior; t.f.c, tensor fasciae cruris (insertion); v.is., sciatic vein; v.s.m., great saphenous vein; v.s.p., small saphenous vein; 1-4, the peronaei (pri- mus-quartus). all the phalanges of the digits. The muscle may be dis- placed by dividing the crural ligament. The peroneal and ante- rior tibial arteries lie behind this muscle, the former in a medial posi- tion, in contact with the tibia, the latter on the peronaei muscles in company with the peroneal nerve. B. Peronaeus group. These muscles arise from the lateral surface of the leg, and are inserted on all surfaces at the lateral side of the foot. The insertion tendons reach the foot from beneath the lateral malleolus. 152 Anatomy of the Rabbit. (a) The peronaeus longus (p. primus). Origin: Lateral condyle of the tibia and head of the fibula. Insertion: End of the reduced first metatarsal. The tendon crosses the plantar surface of the foot, passing around the distal end of the cuboid bone. The muscle should be divided. (b) The peronaeus brevis (p. secundus). Origin: Lateral condyle of the tibia and corresponding surface of the shaft; also the crural interosseous ligament joining the tibia and fibula. Insertion : Tuberosity of the base of the fifth meta- tarsal. (c) The peronaeus tertius. Origin: The head of the fibula and the crural interosseous ligament, fused with the extensor digitorum longus. Insertion: Head of the fifth metatarsal, and distally, united with the tendon of the extensor digitorum longus, on the phalanges of this digit. (d) The peronaeus quartus. Origin: The fibula and the inter- osseous ligament, fused with the peronaeus brevis and with the extensor digitorum longus. Insertion: Head of the fourth metatarsal. C. Flexor group. The muscles arise from the medial and lateral condyles of tibia and femur (the flexor digitorum sublimis from the posterior surface of the tibia). They lie behind the axis of the tibia, and are inserted both on the heel and on the plantar surface of the foot. (a) The triceps surae comprises : (1) The gastrocnemius. Origin*: in two portions: Caput laterale: Lateral condyles of tibia and femur and related femoral sesamoid. Caput mediale: Medial condyle of the femur and its sesamoid. (2) The soleus: Origin: Head of the fibula. Insertion : Through the Achilles' tendon (tendo calcaneus) . The latter passes over the posterior end of the tuber calcanei, and is attached to its ventral surface. The tendon is covered by that of the plantaris muscle. The small saphenous artery and vein lie at the posterior margin of the lateral head of the gastrocnemius in company with the lesser saphenous nerve. (b) The plantaris. Origin: Lateral condyle of the femur and associated sesamoid. Insertion: The tendon passes over the heel to the plantar surface of the foot, and divides in four parts for insertion on the second phalanges of the four developed digits. The two muscles should be divided, c) The popliteus. Origin: Lateral condyle of the femur: The tendon passes through the capsule of the knee-joint. The muscle contains the tibial sesamoid. It crosses the pos- terior surface of the tibia obliquely, and is inserted on the proximal portion of its posteromedial angle. The Posterior Limb. 153' (d) The flexor digitorum longus. Origin : Lateral condyle of the tibia and head of the fibula, extending to the posterior surface of the interosseous ligament and associated portions of the tibia and fibula. Insertion: The tendon passes beneath the sustentaculum tali, reaching the plantar surface of the foot, where it is partly covered by the plantaris tendon. It divides into four parts for insertion on the ungual phalanges of the four developed digits. The tibial nerve lies on the medial surface of the head of the plantaris and afterwards on the medial surfaces of the popliteus and flexor digitorum longus. 5. Muscles arising from the foot and inserted on the individual digits. (a) The lumbricales. Origin: Tendon of the flexor digitorum longus. Insertion: Medial surfaces of the first phalanges of the three lateral digits. (b) The interossei (metatarsi). Origin: From the dorsal portion of the tendon-sheath of the flexor digitorum longus. In- sertion : Heads of the four metatarsals. Vessels and Nerves of the Leg and Foot. *The great saphenous artery passes distad on the medial surface of the leg, and is continued as the posterior tibial artery (a. tibialis posterior) around the medial malleolus to the plantar surface of the foot. Above the ankle-joint it gives off the malleolar artery (a. malleolaris) to the posterior surface of the distal end of the tibiofibula. The popliteal artery, the continuation of the femoral, passes between the medial head of the gastrocnemius on the one hand and the lateral head and the plantaris on the other, reaching the anterior surface of the popliteus, and afterwards the anterior surfaces of the tibia and fibula by passing between their proximal ends. It divides into two branches, the anterior tibial and peroneal arteries. It distributes branches to the muscles about the knee-joint, including a branch to the distal portion of the vastus lateralis, which is given off at about the point of origin of the small saphenous artery. The small saphenous artery (a. saphena parva) appears on the proximal portion, of the posterolateral margin of the leg, running along the border of the lateral head of the gastrocnemius in company with the corresponding vein and the lesser saphenous nerve. The anterior tibial artery (a. tibialis anterior) lies on the medial margin of the peronaeus longus, and passes to the fibular side of the crural ligament in company with the anterior tibial vein and the peroneal nerve to the dorsum of the foot. The peroneal artery occupies a more medial position, traversing the leg close to the tibia and the interosseous ligament. It reaches the dorsum of the foot after passing beneath the crural ligament. **The great saphenous vein (v. saphena magna), a large tributary of the femoral, accompanies the corresponding artery, and the greater 154 Anatomy of the Rabbit. saphenous nerve, and is continued as the posterior tibial vein (v. tibialis posterior), to the plantar surface of the foot. The popliteal vein, the root of the femoral, accompanies the corre- sponding artery in the popliteal fossa. It receives the small saphenous vein (v. saphena parva) from the posterior margin of the lateral head of the gastrocnemius. The sciatic vein is continued on the lateral surface of the leg as the anterior tibial vein (v. tibialis anterior) . It receives the accessory small saphenous vein (v. saphena parva accessoria) from the posterior surface, and is continued to the dorsum of the foot passing to the fibular side of the crural ligament. ***The greater saphenous nerve, the posterior branch of the femoral nerve, accompanies first the femoral artery and afterwards the great saphenous artery, passing distad to the medial surface of the leg. The tibial nerve, the posterior division of the sciatic, passes between the medial and lateral heads of the gastrocnemius to the medial surface of the head of the plantaris. It traverses the leg, lying on the medial surface first of the popliteus and afterwards of the flexor digitorum longus, and passing beneath the medial malleolus reaches the plantar -surface of the foot. In the proximal portion of the leg it distributes muscular branches to the flexor group. The lesser saphenous nerve (n. saphenus minor) accompanies the small saphenous artery and vein on the posterior margin of the lateral head of the gastrocnemius. The peroneal nerve, the anterior division of the sciatic, passes distad, lying at first between the insertion of the biceps and the lateral head of the gastrocnemius, and thus appearing on the surface after the removal of the former. It perforates the anterior portion of the lateral head of the gastrocnemius and afterwards the fused heads of the peronaeus tertius and flexor digitorum longus, traversing the leg at first behind the peronaeus longus and then around its medial margin to the front of its tendon. It passes to the fibular side of the crural ligament and thence to the dorsum of the foot. The nerve distributes branches to the tibialis anterior and extensor digitorum longus. The structure of the lumbosacral plexus may be examined by break- ing away the ventral portion of the pelvis, or by dividing the sacroiliac articulation in such a way that the two sides of the pelvis may be pressed apart, the ventral or pelvic face of the sacrum being thus exposed. The posterior portions of the psoas and iliacus muscles may be picked away with the forceps, and the abductor caudae anterior muscle (p. 183) may be detached from its origin on the ischial spine. The lumbosacral plexus (plexus lumbosacralis) is formed by the ventral roots of the four posterior lumbar and four sacral spinal nerves. It is divisible into a lumbar plexus (plexus lumbalis), from which arises the femoral nerve, and a sacral plexus (plexus sacralis), from which arises the sciatic nerve. The femoral nerve is formed from the fifth, sixth, and seventh lumbar, especially from the loop connecting the fifth and sixth (ansa lumbalis n). From the same loop arises a smaller, obturator nerve (n. obturatorius), The; Posterior Limb. 155 which accompanies the obturator artery, and is distributed to the obturatores, adductores, and gracilis muscles. The sciatic nerve, together with the superior and inferior gluteal nerves, arise from the loop connecting the last lumbar and first°sacral nerves (ansa lumbalis in). The internal pudendal nerve is formed from the loop connecting the second and third sacral nerves (ansa sacralis n), but chiefly from the second. The Articulations of the Posterior Limb. The more perfect development and larger size of the joints of the posterior limb make them much more favorable for examination than the corresponding parts of the anterior limb. The muscular attachments should be removed from about the articular capsules and the structures examined as follows: A. The hip-joint (articulatio coxae) is an enarthrosis, formed by the head of the femur with the parts of the ischium, ilium, and the os aceta- buli enclosing the acetabulum, together with the articular capsule (capsula articularis) and accessory ligaments. The articular capsule extends from the acetabular margin to the proximal end of the neck of the femur. It is strongest on its dorsal side. The external supports of the joint comprise the iliofemoral, ischiocapsular, and pubocapsular ligaments. By dividing the capsule, the contents of the joint and the smooth articular surfaces may be examined; also the attachment of the head of the femur to the wall of the acetabular fossa through the round liga- ment (lig. teres femoris). The glenoid lip (labrum glenoidale) is the ring of fibrocartilage surrounding the margin of the acetabulum and connecting with the articular capsule. B. The knee-joint (articulatio genu) is a hinge-joint, or ginglymus with a slight spiral trend. It is formed by the articular surfaces of the medial and lateral condyles of the femur and tibia, with the associated articular capsule, ligaments, and inarticular fibrocartilages (see section, Fig- 13)- The tibial collateral ligament (lig. collaterale tibiale) is a stout band of connective tissue stretching from the medial condyle of the femur to the posteromedial angle of the medial condyle of the tibia. The fibular collateral ligament is a similar structure connecting the lateral condyle of the femur with the lateral surface of the tibia imme- diately in front of the head of the fibula. The sesamoid bones of the popliteal region have articular surfaces taking part in the formation of the joint. That on the medial condyle of the femur is contained in the medial head of the gastrocnemius, that on the lateral condyle of this bone in the lateral head of the gastroc- nemius and the plantaris, and that on the lateral tibial condyle in the popliteus. The common tendon of the quadriceps femoris, the patella, and the 156 Anatomy of the Rabbit. patellar ligament are associated with the capsule, forming the anterior wall of the joint. Between the apposed surfaces of the condyles, in the interior of the joint, there are two short, cruciate ligaments and two thin plates of fibrocartilage, the medial and lateral menisci. The anterior cruciate ligament (lig. cruciatum anterius) passes from the lateral wall of the intercondyloid fossa of the femur to the anterior end of the intercon- dyloid eminence of the tibia. The posterior cruciate ligament passes from the medial wall of the intercondyloid fossa of the femur to the posterior intercondyloid fossa of the tibia. The medial meniscus (meniscus medialis) lies on the articular surface of the medial condyle of the tibia, and is connected by ligament with the anterior and posterior intercondyloid fossae of the bone. The larger, lateral meniscus lies on the lateral condyle of the tibia, and is attached by ligament anteriorly to the medial portion of the articular surface, and posteriorly to the medial wall of the intercondyloid fossa of the femur. C. The ankle-joint (articulatio talocruralis) is a ginglymus with a considerable amount of spiral torsion. The articulating surfaces are chiefly formed by the tibia and talus, but also by the fibular side of the tibiofibula and the calcaneus. On the tibial side the calcaneotibial liga- ment (lig. calcaneotibiale) connects the medial malleolus with the sus- tentaculum tali. On the fibular side the calcaneofibular ligament (lig. calcaneofibulare) connects the posterior portion of the groove for the peronaei muscles forwards with the lateral surface of the calcaneus, and a second ligament extends from the anterior margin of the groove back- ward to the lateral surface of the calcaneus. The tibionavicular liga- ment (lig. tibionaviculare) connects the anterior surface of the distal end of the tibia with the dorsal surface of the navicular bone. Thb Hbad and Neck. 157 X. THE HEAD AND NECK. This dissection includes the various structures of the region, with the exception of the cervical and occipital musculature and the central nervous system, which are treated in the succeeding parts, and the special musculature of the ear, which has been omitted. To begin the dissection, the skin, which has already been divided to the mandibular symphysis and partly reflected, should be separated from the underlying platysma along the side of the head, and reflected until the surface is clear to a point near the dorsal median line of the skull. The insertion of the platysma on the cheek, and that of its special portion, the depressor conchae posterior, on the base of the ear, should be noted. A second band of muscle, similar in its relation to the platysma, arises from the lateral border of the mandible, immediately in front of the masseter muscle, and is inserted into the base of the ear. This is the depressor conchae (parotideoauricularis) anterior. In removing the skin of the upper and lower eyelids, two muscles, the orbicularis oculi and the depressor palpebrae inferioris, may be observed. The former is a somewhat circular band of fibres enclosing both eyelids, the fibres lying directly on the skin, and being concen- trated at the anterior and posterior angles. The latter is a very slender muscle arising from the zygomatic arch and inserted into the skin of the lower eyelid. The corresponding levator palpebrae superioris arises from the orbital wall, and is here concealed by the projecting supraorbital process. 1. On the lateral surface of the head the following structures may be made out without further dissection : (a) The masseter muscle. Origin: The zygomatic arch; tendinous from its anterior angle. Insertion: Lateral sur- face of the angle of the mandible, (b) The parotid gland (gl. parotis), a diffuse, white or brownish gland lying immediately behind the angle of the mandible. Its duct (d. parotideus) crosses the masseter and perforating the mucous membrane of the cheek opens into the oral cavity. (c) The chief part of the seventh cranial or facial nerve (n. facialis) appears in the anterior portion of the parotid gland, its branches crossing the masseter. They are distributed as motor nerves to the cutaneous muscles of the face, including the platysma. tube is clamped. By drawing on one cord of the ligature the knot loosened sufficiently to withdraw the cannula, and by keeping a jer pressed on the end of the vessel, the knot may then be drawn (it without loss of injection. It sometimes happens, despite ordinary precautions, that the cannula :omes clogged either with settled starch or with coagulated blood, this case it may be easily removed, cleaned, and replaced. The same inula should always be used. Material prepared according to the directions given above will keep efinitely, provided, however, that precautions are taken to avoid con- lination from the surface. These are especially necessary in view of thick coating of hairs. It is a good plan, therefore, to sponge the tnal with a preserving fluid which will penetrate the coat imme- tely, or if many specimens are being prepared, to immerse the whole tnal for a moment. A suitable fluid for this purpose is formalin- >hol, made by adding 2% of formalin to a mixture of equal parts of inary spirit and water. The alcohol ensures immediate penetration assists the formalin in preservation. The fluid should be squeezed of the coat as much as possible. The presence of a considerable ntity is not harmful, unless, after the dissection has begun, the fluid ild gain access to the tissues and destroy the effect of the glycerin he embalming fluid. 198 Anatomy of the Rabbit. For the storage of material either before or during dissection no pre- caution is necessary except that of protecting the body from undue exposure to evaporation. A convenient plan for handling the material, however — one that is in use at the present time in the laboratories of the University of Toronto — is that of providing for each specimen a zinc- lined copper box, with sliding top, and of dimensions ample for the largest specimens — namely, 6x6x20 inches. This type of storage box was designed several years ago by Professor Ramsay Wright for the purpose, but is one which has proved useful for storing purposes in many other ways. ■a' INDEX Abdomen, 111 Abdominal aorta, 133 cavity r 50, 116 wall, muscles of, 113 Abduction in limbs, 32 Accessory respiratory- tracts, 43 See" Nose. Acinous glands, 11 Acoustic meatus, external, 77, 88, 89, 111 internal, 81, 89 Adaptation, 5 Adduction in limbs, 32 Adipose tissue, 13 Afferent nerves, 22 Albinism, 14 Alveolar glands, 11 Anal aperture, 111 glands, 11, 130 Analogy, 5 Anastomoses of intestinal vessels, 116 Anatomy defined, 3 Ankle, bones of, 108 joint, 156 Aorta, see Arteries. Aortic arches, branchial, 44 Aperture, anal, 111 ot auditory tube, 165 of larynx, 165 of mouth, 110 of nose, external, 111 internal, 82, 165 piriform, 83, 94 of thorax, inferior, superior, 74 urinogenital, 111 of uterine tube, internal, 131 of uterus, external, 132 Aponeuroses, 13 Appendicular skeleton, 32, 98 Arachnoidea, 184