ses gaa sa aoe ea Cortes vs gays rs an uf i z vais oh - ie 1 i Js 2 ate 2 iat a; ‘> Sar ne akin ae i Ros oa zs ip hat ean eri ; ie ac Lee i 33 apes SRNR sath an : ra z ee ees = re Ss ss ee ars ee ~ es Rte ee Nh ~~ i Bh oh : “i o 4 3 2 A a v 3 ~ s 6 Oo a : He A A TEXT BOOK PHYSIOLOGY A TEXT BOOK OF PHYSIOLOGY BY M. FOSTER, M.A., M.D, LLD., F.RBS., PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE, AND FELLOW OF TRINITY COLLEGE, CAMBRIDGE. ASSISTED BY C. S SHERRINGTON, M.A. M.D., FBS, PROFESSOR OF PHYSIOLOGY IN UNIVERSITY COLLEGE, LIVERPOOL. WITH ILLUSTRATIONS. SEVENTH EDITION, LARGELY REVISED. PART III. The Central Nervous System. Dondon: MACMILLAN AND CO., Limitep. NEW YORK: THE MACMILLAN COMPANY. 1897 [The Right of Translation is reserved.] Ft PIs 1/897 Aton bye - anweinge > PRINTED BY J. AND C. F. CLAY, AT THE UNIVERSITY PRESS, Mo 4 First Edition 1876. Second Edition 1877. Third Edition 1879. Fourth Edition 1883, Reprinted 1884, 1886. Fifth Edition 1890. Siath Edition 1892, Seventh Edition 1897. GINCE the time when I was engaged in preparing the last edition of this Part of my Text Book, the progress of our knowledge in the branch of Physiology dealt with in it has been very great, has indeed been remarkable. It has compelled me wholly to rewrite numerous paragraphs; I have thought it wiser to use a free hand in this respect rather than to attempt merely to correct or add to what I had written before. My friend, Prof. Sherrington, has given me throughout such large help, that I have thought it due to him that his name should appear with mine on the title-page; but I have also received great assistance from other friends, more especially Dr Langley and Dr H. Anderson. M. FOSTER. CAMBRIDGE, August, 1897. CONTENTS OF PART III. BOOK III. THE CENTRAL NERVOUS SYSTEM AND ITS INSTRUMENTS. § 558. § 559. $ 560. § 561. § 562. § 563. § 564. § 565. § 566. § 567. CHAPTER I. THE SPINAL Corp. SECTION I. On Some FEATURES OF THE SPINAL NERVES. PAGE The spinal nerves. i ‘ Z ‘ 3 é . 915 On efferent and afferent ‘anptlees ‘ , : : 916 Efferent fibres run in the anterior root and afferent fibres in the posterior root ‘ ‘ 918 The ‘ trophic” influence of the saaelion of fhe postarion oat: fie degeneration of nerve fibres. : . . . : . . 919 SECTION II. THE STRUCTURE OF THE SPINAL CoRD. The general features of the cord; grey and white matter ? ‘ 922 The structure of the white matter; neuroglia . 5 : 925 The structure of the grey matter; the features of a nerve sal 4 927 The central canal, the substantia gelatinosa centralis, and the substantia gelatinosa of Rolando . . 5 x A ‘ 5 932 The grouping of the nerve cells. The cells of the ventral and dorsal horn, the lateral group, Clarke’s column, and the lateral horn. The reticular formation . 5 935 The tracts of white matter. Median nosibniod dain, caterial posterior column. The evidence of the differentiation of the white matter into tracts. Ascending and descending degeneration. Descending tracts: crossed and direct pyramidal tracts, antero- lateral descending tract. Ascending tracts: cerebellar tract, antero-lateral ascending tract, median posterior tract. Fibres of short course . : 3 i 5 ‘ ; , ‘ ; . 939 viii § 568. § 569. § 570. § 571. § 572. § 573. § 574. § 575. § 576. § 577. § 578. § 579. § 580. § 581. § 58%. § 583. § 584. § 585. § 586. § 587. § 588. § 589. § 590. g 591. § 592. § 593. § 594, CONTENTS. The meaning of the terms “ascending” and ‘‘descending” de- generation, and the inferences to be drawn from them . The connections of the nerve roots; of the ventral root; of tis dorsal root, median, lateral and intermediate bundles . The special features of the several regions of the spinal cord. The conus medullaris, the lumbar and cervical swellings. Variations in the sectional area of the white matter 4 é ‘ . Variations in the sectional area of the grey matter . The relative size, form and features of transverse sections of fie cord at different levels Variations in the disposition of nerve vada anit: groups an nerve seals at different levels . aft . Variations in the several daltaniate of white ee at different ievdls, The course of the crossed and of the direct pyramidal tract along the length of the cord . éj ; x : The course of the cerebellar tract along the janet of ihe ed The course of the median posterior tract along the length of the cord - ‘ . The course of the aintae: jntdral wecentting oe ding the ianeits of the cord 2 ‘ 3 The nature of fhe grey aaisttes of the wavs the nerve esi, ita relations and its probable mode of action . ‘ ¥ - . Spinal mechanisms as instruments of the brain Intrinsic spinal mechanisms . : 2 . SECTION IIL Tur REFLEX ACTIONS OF THE SPINAL CoRD. The difficulties attending the experimental investigation of the cen- tral nervous system; ‘shock’ and other effects of an operation The differences, as regards reflex movements, between different kinds of animals . ‘ ‘ ‘ The features of a reflex act dependent on the dsitcted of the afferent impulses . j ‘ The complex nature of the eeaeal propasned ina vedio meremninit = The characters of a reflex movement dependent on the strength of the stimulus . 3 . The characters of a retier. morenient dependent on the pait of tha body to which the stimulus is applied The complexity of many reflex movements ; their celutivai to ata. ligence Reflex movements soeraiaie ‘by siterent drapullees iter ‘ie the exciting impulses; relations to consciousness The characters of a reflex movement determined by the inivinate condition of the cord ‘ The reflex movements carried out by the sinal nord é inman . Reflex actions resulting in changes other than movements The inhibition of reflex actions The time required for reflex actions . PAGE 948 948 950 952 954 955 958 960 962 964 967 968 970 971 973 975 976 977 977 979 980 981 983 984 986 987 990 CONTENTS. ix SECTION IV. THE Automatic AcTIONS OF THE SpInaL Corb. PAGE § 595. Automatic actions of the spinal cord in the frog and in the dog 3 992 § 596. Automatic activity dependent on afferent impulses . ‘ , 3 993 § 597. Tone of skeletal muscles . 5 | ‘i 7 . : ‘ ; 994 § 598. Tendon phenomena, knee-jerk . Z ‘4 ‘ ‘ ‘ ‘ : 998 § 599. Rigidity of muscles through spinal action : : : : - 1000 CHAPTER IT. Tue Brain. SECTION I. On Some GENERAL FEATURES OF THE STRUCTURE OF THE BRAIN. § 600. The embryonic brain ; the three primary cerebral vesicles ‘ - 1001 § 601. The transformation of these into the bulb and fourth ventricle, the cerebellum and pons vavrolii, the crura cerebri, corpora a gemina, and third ventricle . ‘ . 1002 § 602. The vesicles of the cerebral hpnilephens: fied growl aa trans- formation into the cerebrum; the cerebral hemispheres, corpus striatum, corpus callosum, fornix, and choroid plexus . ‘ . 1002 § 603. The parts of the adult brain corresponding to the main divisions of the embryonic brain , ‘ : : a ‘ ‘ . 1007 § 604. The cranial nerves . ‘ , ‘ 7 : i rf - 1008 SECTION II. THE Bute. § 605. The main changes by which the cervical spinal cord becomes trans- - formed into the bulb; the pyramids and their decussation, the olivary bodies, the fasciculus cuneatus and fasciculus gracilis - 1009 § 606. The superior or sensory decussation . i . 1014 § 607. The opening up of the central canal of the Sal ead into the _ fourth ventricle of the bulb; the calamus scriptorius . A - 1015 § 608. The changes in i*p grey matter; the reticular formation and the areuate fibres. “*. 3 ‘ - 1016 § 609. The olivary nucleus, or fhufanloy “olive fied inner ni outer accessory olivary nuclei, the antero-lateral nucleus. . . 1017 § 610. The gracile and cuneate nuclei; the changes in the ‘gelatinous substance of Rolando . , . . r ‘ : . . 1018 § 611. The fibres of the bulb ; Z . 1020 § 612. The relations of the gracile and winents ‘witha! to the es ohigary : layer, to the fillet, and to the restiform body : é “ . 1021 § 613. § 614. § 615. § 616. § 617. § 618. § 619. § 620. § 621. § 622. § 623. § 624. § 625. § 626. § 627. 8 628. § 629. § 630. § 631. § 632. CONTENTS. SECTION III. Tur Dispostrion AND CONNECTIONS OF THE GREY AND WHITE MatTER OF THE BRAIN. The chief collections of grey matter . 1. The central grey matter and the nuclei of the cranial nerves. The central grey matter The nuclei of the cranial nerves, ‘hei dancgeaniiieal distribution The nucleus of the twelfth or hypoglossal nerve The nuclei of the eleventh or spinal accessory, tenth or oan, iota ninth or glossopharyngeal nerves . Z . : The nuclei of the eighth or auditory nerve The nucleus of the seventh or facial nerve The nucleus of the sixth or abducens nerve The nuclei of the fifth or trigeminal nerve i The nucleus of the fourth or trochlear nerve . ‘ : : The nucleus of the third or oculomotor nerve . On the nature and relations of the several nuclei of ihe centr nerves . . 2 ‘i A ; 5 5 : ‘ , : 2. The superficial grey matter. The cortex of the cerebrum and the superficial grey matter of the cerebellum 3. The intermediate grey matter of the Crural system. The corpus striatum and optic thalamus, their positions and relations. The crus composed of pes and tegmentum. The internal capsule and corona radiata 7 The nucleus lenticularis; or pallidus and putamiek, "The nucleus caudatus . : The optic thalamus, its nuclei ; thie valeaniae ; The substantia nigra, the red nucleus, and the corpus suhihalaenicien, The grey matter of the pons, the upper olive ‘ 4. Other collections of grey matter. The corpora quadrigemina. The corpora geniculata. The corpus dentatum of the cerebellum . - ‘ : F A The arrangement of the Fibres of the Brain, The pedal and tegmental systems . é - 3 Longitudinal Fibres of the Pedal System. The pyramidal tract. Anterior or frontal cortical fibres. Posterior or temporo-occipital fibres. Fibres from the nucleus caudatus to the crus . . PAGE 1024 1025 1025 1026 1027 1029 1034 1034 1035 1037 1040 1040 1043 1043 1047 1051 1054 1056 1057 1061 § 633, § 634, § 635, § 636. § 687. § 638. § 639. § 640. § 641. § 642. § 643. § 644. § 645. § 646. g 647. § 648. § 649. § 650. § 651. § 652. CONTENTS. Longitudinal Fibres of the Tegmental System. Cortical fibres, the optic radiations . 7 The superior peduncles of the cerebellum. The fillet. The lonuttn- dinal posterior bundle. Tracts from the corpora quadrigemina . Transverse or Commissural Fibres. : The corpus callosum. The anterior white commissure, The fornix. The middle peduncles of the cerebellum 7 . A summary of some of the chief relations of the seit vais of the brain to each other and to the spinal cord SECTION IV. On THE PHENOMENA EXHIBITED BY AN ANIMAL DEPRIVED OF ITS CEREBRAL HEMISPHERES. The absence of distinct signs of volition and intelligence The characters of the movements of a brainless frog The phenomena exhibited by birds after removal of their cerebral hemispheres The effects of removing tha eeeura hatiiapheros | in ‘emondie The effects of removing the cerebral hemispheres in the dog SECTION V. THE MACHINERY OF CooRDINATED MoVEMENTS. The effects of injury to the semicircular canals. Our appreciation of the position of our body, the sense of equilibrium Afferent impulses and sensations as factors of the coordination of movements ‘ The phenomena and eaieation of erties Forced movements The parts of the middle ints sanbathied in tlie ‘eordinatien of movements . ‘ : 7 ‘ . ; SECTION VI. On Some HistoLogicaL FEATURES OF THE BRAIN. The structure of the central grey matter and some other collections of grey matter The histology of the snipartintal grey ahs of ‘the eerhaliann : itis structure of the nuclear layer and molecular layer; the eonetitnent cells, their relations to each other and to the fibres of the white ‘matter . : ‘ : ‘ . ‘ 7 : te The cerebral cortex. The general features of the cortex in the frog and in the mammal; the pyramidal and other cells The layers of the cortex . ‘ 3 The histological features of the parletal, eedtnital aia frond regions The probable significance of the structure of the cortex . xi PAGE 1064 1065 1067 1070 1072 1073 1076 1078 1080 1082 1086 1088 1091 1093 1096 1097 1103 1111 1113 1114 xii CONTENTS. SECTION VII. On VoLuNTARY MOVEMENTS. § 653. The real distinction between voluntary and involuntary movements . § 654. The cortical areas of the dog; the characters of the movements resulting from cortical stimulation § 655. The cortical areas in the monkey , § 656. The cortical areas in the anthropoid ape . § 657. The movements of cortical origin carried out iy means of the pyramidal tract ; the nature of the movements so carried out § 658. The results of the removal of a cortical area in dog and in the monkey . 7 § 659. The cortical areas in man; ihe: area for speech, § 660. The nature of the action oe a cortical area in carrying out a yoltintay movement; the relation of the Rolandic region of the cortex to sensations § 661. The characters of re ioyements saotted = stimulation of the occipital and temporal regions of the cortex . § 662. Distinction between ‘ skilled’ voluntary movements and phage § 663. The passage of volitional impulses ave the spinal cord in animals . § 664. Their passage in man ji F 4 § 665. A summary of the chief facts eoncarntig the aaerying out of voluntary movements . 6 ‘ ‘ ‘ * . : . SECTION VIIL On THE DEVELOPMENT WITHIN THE CENTRAL NERVOUS SYSTEM VISUAL AND OF SOME OTHER SENSATIONS. § 666. Visual impulses and sensations; visual fields, and binocular vision . § 667. The decussation of the optic nerves in the optic chiasma § 668. The course of the optic tract § 669. The endings of the optic tract in the iaiesl soraie seailoallatia ie pulvinar and the anterior corpus quadrigeminum; the results of degeneration and atrophy experiments . ‘ é é § 670. The connection of the three above bodies with the sarebrad dotiee: ; the meaning of the terms, blindness total and complete or partial, hemianopsia, amblyopia. The difficulties of interpretation attend- ing experiments on the vision of animals. . j § 671. The nature of the movements of the eyes caused by sGiniletig of the occipital cortex 3 % § 672. The effects on vision of removing pails of fis eustpital ee in monkeys and in dogs; the teachings of clinical histories , A § 673. The probable progressive development of visual sensations; lower and higher visual centres § 674. Sensations of smell. The structure of fhe ditactory bulb sia ives the connections of the tract with other parts of the cerebrum PAGE ~ 1119 1120 1123 1128 1129 1134 1138 1139 1145 1149 1151 1153 1154 1159 1162 1163 1164 1166 1171 1172 1175 1177 § 675. § 676. § 677. On § 678. § 679. § 680. § 681. § 682. § 683. § 684. § 685. § 686. § 687. § 688. § 689. § 690. g 691. g 692, CONTENTS, The cortical area for smell 7 Sensations of taste . ’ Sensations of hearing SECTION IX. xill PAGE 1180 1182 1182 THE DEVELOPMENT OF CUTANEOUS AND SOME OTHER SENSATIONS. Sensations of touch, heat, cold and pain . ‘ ‘5 . . ‘ The connections within the central nervous system of the sensory fibres of spinal and cranial nerves . 7 . The probable or possible paths for sensory impulses sagaestad by the above anatomical arrangements . . The conclusions concerning the development of lencian bigots which may be drawn from the results of experimental investigation and clinical observation The individual development of ie sevens sctineous sap ehinte: general considerations . - i ‘ The teachings of clinical dhietvations on this point A comparison of visual and cutaneous sensations The passage of sensory impulses along the spinal cord General conclusions touching the development of cutaneous sen- sations . i _ SECTION X. On Some oTHER ASPECTS OF THE FUNCTIONS OF THE BRAIN. Considerations touching the cerebellum . ; 7 A Considerations touching other structures of the middle brain , The splanchnic functions of the brain . . : General considerations on the processes taking ‘siueed in the potions SECTION XI. ON THE TIME TAKEN UP BY CEREBRAL OPERATIONS. The reaction period or reaction time . Elementary analysis of psychical processes, the fitne inten up by each. The time required for discrimination, for the development of perception, and of the will; the circumstances influencing them . 1189 1190 1193 1196 1198 1200 1201 1202 1204 1206 1212 1214 1216 1222 1224 xiv CONTENTS. SECTION XII. Tue LympHatic ARRANGEMENTS OF THE BRAIN AND SPINAL CORD. PAGE § 693. The membranes of the brain and spinal cord . . a F . 1228 § 694. The sources of the cerebro-spinal fluid . « ‘i ‘ 5 . 1229 § 695. The characters of the cerebro-spinal fluid - : 1231 § 696. The renewal of the cerebro-spinal fluid. The purposes spied a ihe fluid . , F . ‘ , : : i ‘ 5 . 1232 SECTION XIII. Tue VascuLarR ARRANGEMENTS OF THE BRAIN AND SPINAL CorD. § 697. The distribution and characters of the arteries of the brain. » 12385 § 698. The venous arrangements of the brain; the venous sinuses. 1237 § 699. The supply of blood to the brain relatively small. The methods of investigating the circulation of the brain : 1238 § 700. The supply of blood to the brain modified by the sesptradiani and by changes in the general arterial pressure. The want of clear proof of special vasomotor nerves for the cerebral arteries. 1241 § 701. The flow of blood through the brain nevertheless infinenced by changes taking place in the brainitself . . ; . a « 1248 InpEx x ‘ ‘ : , zs 5 7 4 ‘ a . 1245 LIST OF FIGURES IN PART III. FIG. 96. A transverse dorset section of the spinal cord (man) at the level of the sixth thoracic nerve 97. Diagram of a nerve cell with axon and dewariten 98. Diagram to illustrate the nature of the substance of Bolands 99. Transverse dorsoventral section of the spinal cord (man) at the level of the sixth cervical nerve . 100. Transverse dorsoventral section of the sna, ond (maj a ce level of the third lumbar nerve . : 101. Diagram to illustrate the general seaipeirent of the | ifaats of white matter in the spinal cord i . 102. Diagram shewing the tracts in the human spina solids spatiet as revealed by the date of myelination of the nerve fibres 103 4, 8, c. Diagrams to illustrate the degenerations ascending and geass ing after a cross section of spinal cord 104. Diagram shewing the united sectional areas of the ° sina nerves proceeding from below upwards 105. Diagram shewing the variations in the sertional area, of ihe: sey matter of the spinal cord, along its length 106. Diagram shewing the relative sectional areas of the iil nerves as they join the spinal cord ‘ 107. Diagram illustrating some of the features of ths. spinal cord at differ. ent levels : 108. Diagram shewing the variations: in the sectional area of ihe inteval columns of the spinal cord, along its length , 109. Diagram shewing the variations in the sectional area of the antenen columns of the spinal cord, along its length . ; 110. Diagram shewing the variations in the sectional area of the pontarie: columns of the spinal cord, along its length 111. Diagram illustrating the nervous mechanism of the knee-jerk 112. Outlines of parts of the brain; A dorsal, B lateral, C ventral aspect . 113. Transverse dorsoventral sections of the bulb at different levels . 114. Transverse dorsoventral section through the bulb just behind the pons 115. Transverse dorsal seation through the "bath (man) at he wae patt of the fourth ventricle . 116. Transverse dorsoventral section ihvough the pons ai the exit of ie fifth nerve PAGE 923 929 933 934 937 942 943 946 952 952 953 956 958 958 959 999 1010 1012 1020 1030 1033 xvi FIG. 117. 118. 119. 120. 121. 122, 123. 124, 125. 126. 127. 128. 128 129. 130. 131. 132. 133. 134, 135. 136. 137. 138. 139. 140, 141. 142, A. LIST OF FIGURES IN PART III. Transverse dorsoventral section through the fore part of the pons Transverse dorsoventral section i the crus and anterior corpora quadrigemina . . Diagram to illustrate the postion of the sisal of fie semuial nerves . Diagrammatic outline of a transverse dorsoventral section through the right hemisphere at a level ish posterior to the knee of the internal capsule. ‘ Diagrammatic outline of a wansvel se Aaneavantedl nation fateh the right hemisphere at a level anterior to fig. 120 Diagrammatic outline of a transverse dorsoventral section denial the right hemisphere through the frontal lobe Diagrammatic outline of a sagittal section taken through ie right hemisphere seen from the mesial surface A View of right half of brain, as disclosed by a jongitedinsl sietionl in the median line through the longitudinal fissure Outline of horizontal section of brain, to shew the internal capente Outline of a sagittal section through the hemisphere . Outline of a transverse dorsoventral section of the right half of tha brain : , . : Diagram of a leaflet of the earetidlizn Ghent in the hemor lee . The same taken in the longitudinal plane Diagram of the nerve cells of the cerebral hemisphere of the frog Diagram of a pyramidal cell Scheme of the principal nerve cells in the sevebed onter of. a stiararial The areas of the cerebral convolutions of the dog é Outline of brain of monkey to shew the principal sulci and gyri Left hemisphere of the brain of monkey viewed from the left side and from above Mesial aspect of the left half of the eas of agaeape 3 Diagram to illustrate the relative size of the pyramidal tract in man, monkey and dog Diagram of the convolutions nd: Aisenree on nthe ‘Tatital sata of the right cerebral hemisphere of man The same on the mesial surface . The right lateral aspect of the cerebrum re man in autliie . iltaatrate the cortical areas). : Mesial surface of the right pevebial hemisphere of man in pitta i illustrate the cortical areas x Diagram to illustrate the nervous eupaine of vision in man Diagram to illustrate the paths of auditory impulses . PAGE 1035 1036 1038 1046 1048 1049 1050 1052 1058 . 1059 1060 1098 1099 1104 1107 1110 1121 1124 1125 1127 1134 1140 1140 1141 1141 1160 1183 BOOK III. THE CENTRAL NERVOUS SYSTEM AND ITS INSTRUMENTS. F. Ill. 5Y CHAPTER I. THE SPINAL CORD. SEC. 1. ON SOME FEATURES OF THE SPINAL NERVES. § 558. WE have called the muscular and nervous tissues the master tissues of the body; but a special part of the nervous system, that which we know as the central nervous system, the brain and spinal cord, is supreme among the nervous tissues and is master of the skeletal muscles as well as of the rest of the body. We have already (Book 1. Chap. IIL.) touched on some of the general features of the nervous system, and have now to study in detail the working of the brain and spinal cord. We have to inquire what we know concerning the laws which regulate the discharge of efferent impulses from the brain or from the cord, and to learn how that discharge is determined on the one hand by intrinsic changes originating, apparently, in the substance of the brain or of the cord, and on the other hand by the nature and amount of the afferent impulses which reach them along afferent nerves. As we shall see the study of the spinal cord cannot be wholly separated from that of the brain, the two being very closely related. Nevertheless it will be of advantage to deal with the spinal cord by itself so far as we can. The spinal bulb (medulla oblongata) we shall consider as part of the brain. But before we speak of the spinal cord itself, it will be desirable to say a few words con- cerning the spinal nerves, that is to say the nerves which issue from the spinal cord. We have already seen (§ 96) that each of the spinal nerves arises by two roots, an anterior root attached to the ventral or anterior surface, and a posterior root attached to the dorsal or posterior surface of the cord. We have further seen that the latter bears a ganglion, a ‘ ganglion of the posterior root’ or ‘spinal 59—2 916 SPINAL NERVES. [Boox 111. ganglion,’ and we have (§ 97) studied the structure of this ganglion... ; : We stated at the same time that while the trunk of a spinal nerve contained both efferent and afferent fibres, the efferent fibres were gathered up into the anterior root and the afferent fibres into the posterior root; but we gave no proof of this state- ment. § 559. Before we proceed to do so, it will be as well to say a few words on the terms ‘efferent’ and ‘afferent.’ By efferent nerve fibres we mean nerve fibres which in the body usually carry impulses from the central nervous system to peripheral organs. Most efferent nerve fibres carry impulses to muscles, striated or plain, and the impulses passing along them give rise to movements; hence they are frequently spoken of as ‘motor’ fibres. But all efferent fibres do not end in or carry impulses to muscular fibres ; we have seen for instance that some efferent fibres are secretory. Moreover all the nerve fibres going to muscular fibres do not serve to produce movement ; some of them, as in the case of certain vagus fibres going to the heart, are inhibitory and may serve to stop movement. By ‘afferent’ nerve fibres we mean nerve fibres which in the body usually carry impulses from peripheral organs to the central nervous system. A very common effect of the arrival at the central nervous system of impulses passing centripetally along afferent fibres is that change in consciousness which we call a ‘sensation’; hence afferent fibres are often called ‘sensory’ fibres, and the nervous impulses passing centripetally along them, sensory im- pulses. But as we have already in part seen, and as we shall shortly see in greater detail, the central nervous system may be affected by centripetal impulses, and that in several ways, quite apart from the development of any such change of consciousness as may be fairly called a sensation. We shall see reason for thinking that centripetal impulses reaching the spinal cord, and indeed other parts of the central nervous system, may modify reflex or automatic or other activity without necessarily giving rise to a “sensation.” Hence it is advisable to reserve the terms ‘efferent’ and ‘afferent’ as more general modes of expression than ‘motor’ or ‘sensory.’ We have seen in treating of muscle and nerve, that the changes produced in the muscle serve as our best guide for determining the changes taking place in a motor nerve; when a motor nerve is separated from its muscle (§ 72) the only change which we can appreciate in it is an electrical change. Similarly in the case of an afferent nerve, the central system is our chief teacher; in a bundle of afferent fibres isolated from the central nervous system in a posterior root of a spinal nerve for instance, the only change which we can appreciate is an electrical change. To learn the characters of afferent impulses we must employ the central nervous Cuap. 1.] THE SPINAL CORD. 917 system. But in this we meet with difficulties. In studying the phenomena of motor nerves we are greatly assisted by two facts. First, the muscular contraction by which we judge of what is going on in the nerve is a comparatively simple thing; one contraction differs from another by such features only as extent or amount, duration, frequency of repetition and the like, and all such differences are capable of exact measurement. Secondly, when we apply an artificial stimulus, such as an electric shock, directly to the nerve itself, the effects, so far as we at present know, differ in degree only from those which result when the nerve is set in action by a natural stimulus, such as the will. When we come, on the other hand, to investigate the phenomena of afferent nerves, our labours are for the time rendered heavier, but in the end more fruitful, by the following circumstances :—First, when we judge of what is going on in an afferent nerve by the effects which stimulation of the nerve produces in some central nervous organ, in the way of exciting or modifying reflex action, or modifying automatic action, or affecting consciousness, we are met on the very threshold of every enquiry by the difficulty of clearly distinguishing the events which belong exclusively to the afferent nerve from those which belong to: the central organ. Secondly, the effects of applying a stimulus to the peripheral end- organ of an afferent nerve are very different from those of applying the same stimulus directly to the nerve-trunk ; this may be shewn by the simple experience of comparing the sensation caused by bringing any sharp body into contact with a nerve laid bare ina ~ wound with that caused by contact of an intact skin with the same body. These and like differences reveal to us a complexity of impulses, of which the phenomena of motor nerves gave us hardly a hint. We shall further see in detail later on that our consciousness may be affected in many different ways by afferent impulses; we must distinguish not only sensory from other afferent impulses, but also different kinds of sensory impulses from each other. Certain afferent nerves are spoken of as nerves of special sense, and the nature of the afferent impulses passing along these special nerves together with the modifications of consciousness caused by arrival of these impulses at the central nervous system constitute by themselves a complex. and difficult branch of study. In some of the problems connected with the central nervous system we shall have to appeal to the results of a study of these special senses; but, on the other hand, a knowledge of the central nervous system is necessary to a proper understanding of the special senses; and on the whole it will be more convenient to study the former before the latter. We may, however, digress here to remark that the question whether an afferent impulse differs in itself from an efferent impulse is one of great difficulty. It is true that the electrical 918 SPINAL NERVES. [Boox m1. changes, which alone as we have said we can appreciate in an isolated piece of nerve, appear to be the same in both kinds of fibres ; in each the electrical change is propagated in both directions and possesses the same features. But it would be hazardous to insist too much on this. Moreover, we must remember that what we call a nervous impulse, especially one provoked by artificial stimulation, constitutes a gross change in the nerve fibre, and that changes of a finer, more delicate nature, such as cannot be shewn by the coarse methods used to detect a ‘nervous impulse, may take place in, and be propagated along, a nerve fibre. We shall have occasion immediately to point out that the condition of an afferent nerve fibre along its whole length is dependent on a nerve cell in the ganglion of the posterior root; the fibre when cut off from the nerve cell degenerates and dies. This means that in the intact fibre certain influences are propagated along the fibre from the cell in the ganglion to the peripheral endings of the fibre, that is to say in a direction the opposite of that taken by the ordinary centripetal nervous impulses; and it may be that in like manner in efferent fibres some influences are propagated centripetally from the peripheral endings to the central nervous system. Our knowledge of these influences is extremely limited ; but it is important to bear in mind the possibility of their occurrence. And we had this in view, when above, in speaking of efferent and afferent fibres, we used the phrase “usually carry impulses.” § 560. The proof that the afferent and efferent fibres which are both present in the trunk of a spinal nerve are parted at the roots, the efferent fibres running exclusively in the ventral or anterior root and the afferent fibres exclusively in the dorsal or posterior root, is as follows. When the anterior root is divided, the muscles supplied by the nerve cease to be thrown into contractions either by the will, or by reflex action, while the structures to which the nerve is distributed retain their sensibility. During the section of the root, or when the proximal stump, that connected with the spinal cord, is stimu- lated, no sensory effects are produced. When the distal stump is stimulated, the muscles supplied by the nerve are thrown into contractions. When the posterior root is divided, the muscles supplied by the nerve continue to be thrown into action by an exercise of the will or as part of a reflex action, but the structures to which the nerve is distributed lose the sensibility which they previously possessed. “During the section of the root, and when the proximal stump is stimulated, sensory effects are produced. When the distal stump is stimulated no movements are called forth. These facts demonstrate that sensory impulses pass exclusively by the posterior root from the peripheral to the central organs, and that motor impulses pass exclusively by the anterior root from the central to the peripheral organs; and so far as our knowledge Cuap. 1.| THE SPINAL CORD. 919 goes the same holds goad not only for sensory and motor but also for all centripetal and centrifugal impulses. An exception must be made to the above general statement, on account of the so-called “recurrent sensibility ” which is witnessed in conscious mammals, under certain circumstances. It some- times happens that when the distal stump of the divided anterior root is stimulated, signs of pain are witnessed. These are not caused by the concurrent muscular contractions or cramp which the stimulation occasions, for they persist after the whole trunk of the nerve has been divided some little way below the union of the roots above the origins of the muscular branches, so that no contractions take place. They disappear when the posterior root is subse- quently divided, and they are not seen if the mixed nerve trunk be divided close to the union of the roots. The phenomena are probably due to the fact, that bundles of sensory fibres of the posterior root after running a short distance down the mixed trunk turn back and run upwards in the anterior root, (being distributed probably to the pia mater) and by this recurrent course give rise to the recurrent sensibility. Further, certain experimental and histological results have been brought forward to shew that the posterior roots do or may contain efferent fibres carrying impulses to the plain muscular fibres such as those of the intestines and blood vessels; but the evidence for this is not at present sufficiently conclusive to render invalid the same general statement. § 561. Concerning the ganglion on the posterior root, we may say definitely that we have no evidence that it can act as a centre of reflex action ; nor have we any evidence that it can spontaneously give origin to efferent impulses and thus act as an automatic centre, as can the central nervous system itself. The bodies of the nerve-cells behave somewhat differently from the axis-cylinders at some distance from the cells, though, as we have seen, these are in reality processes of the nerve cells; thus the nerve cells in the ganglion are more sensitive to certain poisons (ex. gr. nicotin) than are the nerve fibres of the nerve trunk. But beyond this, our knowledge concerning the function of the ganglion is almost limited to the fact that it is in some way intimately connected with the nutrition of the nerve. As we have already (§ 83) said, when a mixed nerve trunk is divided the peripheral portion degenerates from the point of section downwards towards the periphery. The central portion does not so degenerate, and if the length of nerve removed be not too great, the central portion may grow downwards along the course of the degenerating peripheral portion, and thus regenerate the nerve. This degeneration is observed when the mixed trunk is divided in any part of its course from the periphery to close up to the ganglion. When the posterior root 1s divided between the ganglion and the spinal cord, the portion attached to the spinal cord degenerates, but that attached to the ganglion 920 .SPINAL NERVES. [Boox 111. remains intact.. When the anterior root is divided, the proximal portion in connection with the spinal cord remains intact, but. the distal portion between the section and the junction with the other root degenerates; and in the_ mixed nerve-trunk. many degenerated fibres are seen, which, if they be carefully traced out, are found to be motor (efferent) fibres. If the posterior root be divided carefully between the ganglion and the junction with the anterior root, the small portion of the posterior root left attached to the peripheral side of the ganglion above the section remains intact, as does also the rest of the root. from the ganglion to the spinal cord, but in the mixed nerve- trunk are seen numerous degenerated fibres, which when examined are found to have the distribution of sensory (afferent) fibres. Lastly, if the posterior ganglion be excised, the whole posterior root degenerates, as do also the sensory (afferent) fibres of the mixed nerve trunk. Putting all these facts together, it would seem that the nutrition or growth of the efferent and afferent fibres takes place in opposite directions, and starts from different nutritive or ‘trophic’ centres. The afferent fibres grow away from the ganglion either towards the periphery, or towards the spinal cord. The efferent fibres grow outwards from the spinal cord towards the periphery. The afferent fibre is essentially a process of a cell in the ganglion: the axis-cylinder of the fibre running in the root and that of the fibre running along the nerve-trunk, are divisions of the elongated process of a nerve cell. The axis- cylinder of the efferent fibre on the other hand is a process of a nerve cell in the spinal cord. In both cases the axis-cylinder degenerates and dies when it is separated from the part of the cell surrounding the nucleus; and the degeneration of the axis- cylinder entails the degeneration of its wrappings, the medulla, and so of the whole nerve. When an amoeba or other unicellular organism is so divided that one moiety retains the nucleus and the other is simply a portion of cell-body without a nucleus, it is observed that while the latter moiety speedily dies, the former moiety continues to live and may regenerate the whole body. Similarly the nerve fibre cut away from its nucleus dies, but the part retaining the nucleus continues to live and may re- generate the whole fibre. Hence though efferent and afferent fibres degenerate in different directions the cause of the degenera- tion is the same in the two cases. This difference in their mode of nutrition is frequently of great help in investigating the relative distribution of efferent and afferent fibres. When a posterior root is cut beyond the ganglion, or the ganglion excised, all the afferent nerves degenerate, and in the mixed nerve branches these afferent fibres, by their altered condition, can readily be traced. Conversely, when the anterior roots are cut, the efferent fibres alone degenerate, and can be similarly recognized in a mixed nerve tract. When the anterior Cuap. 1.] THE SPINAL CORD. 921 root is divided some few fibres in it do not, like the rest, de- generate, and when the posterior root is divided, a few fibres in the anterior root are seen to degenerate like those of the posterior root; these appear to be the fibres which give to the anterior root its “recurrent sensibility.” In the case of certain spinal nerves at all events, it has also been ascertained that when the posterior root is divided, while most of the fibres in the part of the root thus cut off from the ganglion but left attached to the cord degenerate, some few do not. These few appear to have their trophic centre not in the ganglion, but in some part of the spinal cord itself; we shall refer to these later on. This method of distinguishing nerve fibres by the features: of their degeneration, called the “degeneration method,” or sometimes from the name of the physiologist who introduced it, the “ Wallerian method,” has proved of great utility. Thus: in the vagus nerve which is composed not only of fibres which spring from the real vagus root but also of fibres proceeding from the spinal accessory roots, the two may be distinguished by section of the vagus and spinal accessory roots respectively. We shall presently see that this method may be applied to the differentiation of tracts of fibres in the brain and spinal cord. SEC. 2. THE STRUCTURE OF THE SPINAL CORD. § 562. Lying within the vertebral canal the spinal cord is protected by its ‘membranes, the dura mater, the arachnoid membrane and the pia mater. The consideration of the arrange- ment of these membranes and of the structure of the dura mater and arachnoid we will leave until we come to speak of the vascular and lymphatic supplies of the central nervous system ; the histo- logy of the pia mater may more fitly come with that of the spinal cord itself. Along its whole length from its junction with the bulb to its termination in the filum terminale the spinal cord, while possessing certain general features, is continually changing as to special features. It will be convenient to study first the general structure of some particular part, for instance the middle of the thoracic (dorsal)! region, and afterwards to point out the special features which obtain in the several regions. A transverse vertical section of either a fresh or a hardened and prepared spinal cord at the thoracic region possesses an outline which is roughly speaking circular. In the middle of the anterior or ventral surface is a vertical fissure, the ventral or anterior fissure (Fig. 96, A. F.) running some way across the thickness of the cord from the ventral towards the dorsal surface. Opposite to it on the posterior or dorsal surface is a corresponding deeper but narrower, dorsal or posterior fissure (Fig. 96, P. F.) which, however, as we shall see, differs materially in nature from the 1 Tt is very desirable to use the terms ‘dorsal’ and ‘ventral’ for the parts of the cerebro-spinal axis which lie respectively near the dorsal or back part, and the ventral or belly part of the body, instead of the terms posterior and anterior; but if this is done, the use of the word dorsal to denote the region of the cord between the lumbar and cervical regions is apt to lead to confusion; hence the introduction of the word thoracic. If this use of dorsal and ventral be adhered to, before and behind, above and below, may conveniently be used to denote nearer the head and nearer the tail (or coccyx) respectively; anterior and posterior may also be used in the same sense. Cuap. 1] THE SPINAL CORD. 923 W/ost.cod, Q Fic. 96. A Transverse DorsoventraL SEcTION oF THE Spina Corp (Human) at THE Leven or THE Sixt THoracic (Dorsau) Nerve. (Sherrington.) Magnified 15 times. One lateral half only is shewn. The large conspicuous nerve-cells (drawn from actual specimens) are shaded black to render their relative size, shape and position more obvious; the outline of the grey matter has been made thick and dark in order to render it conspicuous. ‘ A.F. anterior (ventral) fissure. P.F. posterior (dorsal) fissure. c.c. central canal. ¢.g.s. central gelatinous substance. A.r. anterior (ventral) root, P.r. lateral (or intermediate) bundle, P.r’. median bundle of posterior (dorsal) root of spinal nerve, p’, p” fibres of posterior (dorsal) root passing p’, indirectly through the substance of Rolando, p's, directly into grey matter. a.g.c. anterior (ventral) grey commissure. p.g.c. posterior (dorsal) grey commissure, a.c. anterior (ventral) white commissure. ant. col. anterior (ventral) column. lat. col. lateral column. ost. col. posterior (dorsal) column. s.g. the substance of Rolando. s. septum marking out the external posterior (dorsal) column or column of Burdach, e.p., from the median posterior (dorsal) column or column of Goll, m.p. 1. cells of the anterior (ventral) horn. 2. marks position of a group of small cells of considerable constancy and extent in the mammalian cord, the “middle cells.” 3. posterior (dorsal) vesicular column or vesicular cylinder, 924 STRUCTURE OF SPINAL CORD. {Book M1. or column of Clarke; the area of the cylinder is defined by a dotted line. 4. cells of the intermedio-lateral tract or lateral horn. 6.— cells of the posterior (dorsal) horn. 7. cells of the anterior ventral cervix peculiar to this and adjoining segments of the cord. y. a tract of fibres passing from the vesicular cylinder to the lateral column. anterior fissure, and ought to be called a septum rather than a fissure. Between the two fissures the substance of the cord is reduced to a narrow isthmus uniting the two lateral halves, which in a normal cord are like each other in every respect. In the middle of the isthmus lies the section of a small canal, the central canal (Fig. 96, c.c.), which is all that remains of the relatively wide neural canal of the embryo. Each lateral half consists of an outer zone of white matter surrounding, except at the isthmus, an inner more or less crescentic, or comma shaped mass of grey matter. The convexity of each crescent is turned towards the median line of the cord, the two crescents being placed back and back and joined together by the isthmus just spoken of. The somewhat broader anterior extremity of the crescent, or head of the comma, is called the ventral (anterior) cornu or horn; and the narrower posterior extremity of the crescent, or tail of the comma, is called the dorsal (posterior) cornu or horn. The part by which each horn is joined on to the middle part of the crescent is called the cervix, ventral and dorsal respectively. The isthmus joining the backs of the two crescents, like the crescents themselves, consists, for the most part, of grey matter, the band running posterior or dorsal to the central canal being called the dorsal (posterior) grey commissure (Fig. 96, p. g. ¢.), and the band running anterior or ventral to the canal being called the ventral (anterior) grey commissure (Fig 96, a. g.c.), The dorsal (posterior) fissure touches the dorsal (posterior) grey commissure, but the ventral (anterior) grey commissure is separated from the bottom of the ventral (anterior) fissure by a band of white matter, called the ventral (anterior) white commis- sure (Fig. 96, a. c.). If the section be taken at the level of the origin of a pair of spinal nerves, it will be seen that the ventral (anterior) root, piercing the white matter opposite the head of the comma in several distinct bundles (Fig. 96, A.r.), plunges into the ventral (anterior) cornu, while the dorsal (posterior) root (Fig. 96, P.r., P.r.), having the appearance of a single undivided bundle, passes, in part at least, into the dorsal (posterior) horn. Both roots are dispersed lengthways along the cord, the hinder roots of one nerve being close to the foremost roots of the nerve below, but it is only the ventral (anterior) roots which are dispersed sideways. The com- pact bundle of the dorsal (posterior) root divides, with tolerable sharpness, the white matter in each lateral half of the cord into a dorsal (posterior) portion lying between the dorsal (posterior) fissure and the dorsal (posterior) root (Fig. 96, post. col.), which: portion Cuap. 1.] THE SPINAL CORD. 925 since, as we shall see, it runs in the form of a column along the length of the cord, is called the posterior column, and into a portion lying to the outside of the dorsal (posterior) root between it and the ventral (anterior) fissure, called the antero-lateral column. This latter may be considered as further divided, by the entrance of the ventral (anterior) roots into a lateral column (Fig. 96, lat. col.) between the dorsal (posterior) root and the most external bundle of the ventral (anterior) root, and into an anterior column (Fig. 96, ant. col.) between the ventral (anterior) fissure and the most external bundle of the ventral (anterior) root. The part traversed by the bundles of the ventral (anterior) root, as they make for the ventral (anterior) horn, accordingly belongs to the anterior column; but some writers speak of the anterior column as lying between the ventral (anterior) fissure and the nearest bundle of the ventral (anterior) root, thus making the region of the ventral (anterior) root belong to neither anterior nor lateral column. And indeed the distinction between the anterior and the lateral column is to a great extent an artificial distinction. § 563. The ‘white matter’ consists almost exclusively of medullated fibres supported partly by connective tissue and partly by a peculiar tissue known as newroglia, of which we shall presently speak. The fibres are of various sizes, but many of them are large, and in nearly all of them the medulla is conspicuous. They run for the most part longitudinally, so that in transverse sections of the cord nearly the whole of the white matter appears under the microscope to be composed of minute circles, the trans- verse sections of the longitudinally disposed fibres, imbedded in the supporting structures. When examined by ordinary modes of preparation these longitudinal medullated fibres of the white matter, though they may occasionally be seen to bifurcate appear, on the whole, to run an undivided unbranched course; but a special mode of preparation has revealed the fact that they give off very fine lateral branches. This method known as that of Golgi, of which there are several modifications, consists in so treating the tissue with a silver salt and other reagents that while the mass of the tissue is rendered clear and transparent the course of the fibres, and especially of the very fine fibres is marked out by black lines consisting of reduced and deposited silver. By this method we learn that the longitudinal fibres of the white matter give off, generally at right angles, exceedingly fine branches, known as collaterals, which running transversely pass into the grey matter and there end in a manner to be described. Owing to the relative abundance of the white refractive medulla, the white matter possesses in fresh specimens a characteristic opaque white colour; hence the name. The grey matter from the relative scantiness of medulla has no such opaque whiteness, is much more translucent, and in fresh specimens has a grey or rather pinky grey colour, the reddish tint being due to the 926 STRUCTURE OF SPINAL CORD. [Book 11. presence partly of pigment and partly of blood, for the blood vessels are much more abundant in the grey matter than in the white. The pia mater which closely invests the cord all round consists of connective tissue, fairly rich in elastic elements and abun- dantly supplied with blood vessels; it is indeed essentially a vascular membrane and furnishes the nervous elements of the cord with their chief supply of blood. It sends in at intervals partitions or septa of the same nature as itself radiating towards the central grey matter. The narrow dorsal (posterior) fissure is completely filled up by a large septum of this kind, indeed as we have said is in reality a large septum not a fissure; but the ventral (anterior) fissure is too wide for such an arrangement ; the whole membrane dips down into this fissure, following the surface of the cord and being reflected at the bottom. From these primary septa, secondary finer septa still composed of ordinary fibrillated connective tissue, carrying blood vessels, branch off; but these are soon merged into the peculiar supporting tissue called, as we have said, neuroglia. This consists in the first place of small branching cells, lying in various planes. The branching is excessive, so that the body of the cell is reduced to very small dimensions, indeed at times almost obliterated, the nucleus disappearing while the numerous branches are continued as long fine filaments or fibres pursuing a devious but for the most part a longitudinal course. In the second place these cells and fibres or filaments are im- bedded in a homogeneous ground substance. Relatively to the fibres and ground substance the bodies of the cells (which are called Deiter’s cells), especially bodies such as bear obvious nuclei, are very scanty; hence in sections, especially in transverse sections, of the cord the neuroglia has often’a dotted or punctated appearance, the dots being the transverse sections of the fine lon- gitudinally disposed fibres imbedded in the ground substance. Examined chemically the neuroglia is found to be composed not like connective tissue of gelatine, but of a substance which appears to be closely allied to keratin, the chief constituent of horny epidermis, hairs and the like, § 435, and which has therefore been called neurokeratin, (see also § 68). And indeed this neuroglia, though like connective tissue a supporting structure, is not, like connective tissue, of mesoblastic, but of epiblastic origin. The walls of the neural canal of the embryo which are transformed into the spinal cord of the adult consist at first of epithelial, epiblastic cells; and while some of these cells become nervous elements, others become neuroglia. The epithelial cells which are destined to form neuroglia become exceedingly branched, while their originally protoplasmic cell-substance becomes transformed to a large extent into neurokeratin. The neuroglia fills up the spaces between the radiating larger septal prolongations of the pia mater and the finer branched septa Cuap. 1.] THE SPINAL CORD. 927 which starting from the larger ones carry minute blood vessels into the interior of the white matter. In these spaces it is so arranged as to form delicate tubular canals, of very variable size, running for the most part in a longitudinal direction. Each of these tubular canals is occupied by and wholly filled up with a medullated nerve fibre of corresponding size. A medullated nerve fibre of the white matter of the spinal cord resembles a medullated nerve fibre of a nerve (§ 68) in being composed of an axis-cylinder and a medulla; but it possesses no primitive sheath or neurilemma. This is absent and indeed is not wanted; the tubular sheath of neuroglia affords in the spinal cord (and as we shall see in the central nervous system generally) the support which in a nerve is afforded by the neurilemma. Nodes are, according to most authors, absent, but some say they are present. The white matter of the cord consists then of a more or less solid mass of neuroglia, having the structure just described, which is permeated by minute canals, some exceedingly fine and carrying very fine fibres, 24 or even less, others larger and carrying fibres up to the size of 154 or even more. This mass is further broken up into areas by the smaller and larger vascular connective-tissue septa with the edges and endings of which the neuroglia is continuous. Most of the nerve-fibres, as we have said, run longitudinally and in a transverse section of the cord are cut transversely; as we shall see fibres, more especially very fine fibres and in particular ‘collateral’ fibres, are continually passing into and out of the white matter, and in so doing take a more or less transverse course; but by far the great majority of the readily visible fibres, and the neuroglia canals in which these lie run in a longitudinal direction. On the outside of the cord below the pia mater the neuroglia is developed into a layer of some thickness from which nerve fibres are absent; this is often spoken of as an inner layer of the pia mater; but being neuroglia and not con- nective tissue is of a different nature from the pia mater proper. A layer of this superficial neuroglia also accompanies the larger septa, and a considerable quantity is present in the large septum called the dorsal fissure. The pia mater carries not only blood vessels but also lymphatics ; of these however we shall speak when we come to deal with the vascular arrangements of the whole of the central nervous system. § 564. In the grey matter we may distinguish the larger, more conspicuous nerve-cells and the rest of the grey matter in which these cells lie. We have already (§ 99) described some of the general features of these nerve-cells; but they must now be dealt with in greater detail. Our knowledge of the characters of these cells has of late years been greatly extended by the apph- cation of the silver method of Golgi and also of the methylene-blue method. When methylene-blue is injected into a living animal, certain cells take up and are stained by the colouring matter, and 928 STRUCTURE OF GREY MATTER. [Boox 11. by this means are clearly marked out from the other elements of the tissues which do not so stain. Among the cells which so stain, are the cells of the central nervous system. It is necessary for the reaction that the cells should be living; they do not take up the methylene-blue and do not stain in the same way when they are dead. The silver method and the methylene-blue method applied to the central nervous system give very concordant results and have led to the following conclusions concerning the features of the nerve-cells belonging to it. } The body of each cell (Fig. 97) is prolonged into processes of two kinds. The one kind of process, and in at least the vast majority of cells only one such process is present, becomes the axis-cylinder of a nerve-fibre and is spoken of as the axis-cylinder process; it has also been called the newrawon or the azon. Asa rule this kind of process runs a long course without dividing, though while within the central nervous system it may give off collaterals; but in some instances it may divide into a large number of branches at no great distance from the body of the cells of which it is a process. A typical axon is that of one of the cells in the ventral horn, which becoming the axis-cylinder of one of the fibres of the ventral root runs an undivided course until it reaches a skeletal muscle, in which after some division, it ends in certain end-plates of the muscle. The other kind of process, and a cell usually has several such, divides very rapidly, at no great distance from the nucleus of the cell, in a dendritic fashion, into a number of fine branches which appear to end abruptly, without becoming actually continuous with any other structures. It has been suggested that the pro- cesses of this kind do not carry out nervous actions but serve simply for the nourishment of the cell; hence they have been by some called protoplasmic processes. But there is no real foundation for this view; the evidence is distinctly in favour of their taking a share in nervous actions, and it is better to adopt another term which has been used, dendritic processes or, shortly, dendrites. Hence a typical nerve cell of the central nervous system may be described as consisting of a body surrounding the nucleus and prolonged into processes of which one is the axis-cylinder process, neuraxon, axon, and the others are dendrites. It will be convenient to distinguish by a separate name between the processes whether axon or dendrite, and the part from which these processes start, namely the body of the cell surrounding the nucleus; the latter might be called the perikaryon'. The axon if it leaves the spinal cord ends in one or more end- plates or in other terminal organs. If, as is the case with a large number of cells, the axon continues to run and finally ends in the central nervous system, its mode of termination as well as that of 1 rept and xapvoy nucleus. Cuap, 1] THE SPINAL CORD. 929 a the collaterals to which it may give rise is in the form of an arborescent tuft, which is applied to the body or dendrites of some other cell. So far as our present knowledge goes we are led to think that the tip of a twig of the arborescence is not continuous with but merely in contact with the substance of the dendrite or Fic. 97, Dracram or a Neuron wits Psrraryon, Denprrrus (d.d.d.) anp Axon. The perikaryon contains nucleus (x), pigment (p) and chromatic substance (chr), Note the absence of the latter from the axon. The axon acquires a myelin sheath (a’), then, outside the cord, a primitive sheath (a”). A, is the termination of the axon of another cell approaching close to the ‘perikaryon and dendrites of the neuron figured. cell-body on which it impinges. Such a special connection of one nerve-cell with another might be called a synapsis?. There are reasons for believing, though the matter is not one definitely proved, that in the ordinary action of the cell, nervous impulses pass along the axon centrifugally from the cell, and along ‘the dendrites centripetally to the body of the cell surrounding 1 From ody and drrw clasp. F, IL 60 930 . STRUCTURE OF GREY MATTER. [Boox m1. the nucleus. Hence we may suppose that nervous impulses or influences sweeping along the axon of one cell are brought to bear through the terminal arborisation of the axon or that of a collateral, on the dendrites of another cell, setting up in those dendrites nervous changes, which passing to the body of that cell issue in turn along its axon. And it has been suggested that the lack of continuity between the material of the arborisation of the one cell and that of the dendrite (or body) of the other cell offers an opportunity for some change in the nature of the nervous in- fluence as it passes from the one cell to the other. But this must be regarded at present as a useful suggestion rather than as a definitely proved truth. So far as we know, these features, namely the possession of an axon on the one hand and of dendrites on the other, are common to all the nerve-cells of the central nervous system; they may be more readily recognised in the larger, more conspicuous cells but are present also in the smaller ones. The cell-substance of a nerve-cell, large enough to be studied conveniently, presents an appearance which may be spoken of under the general term ‘granular’; it often however bears marks of fibrillation, the fibrille seeming to sweep into or out of the processes. When treated with staining reagents the whole of the cell-substance does not stain alike; there are usually in the cells different kinds of material, staining differently, and sometimes this is conspicuously the case. The exact appearances and staining reactions of the cell-substance vary much in different specimens and appear to depend on the circumstances affecting the cell. Among these circumstances must be placed the previous activity or quiescence of the cell; but to this point we shall return in another place. Such nerve-cells form, however, a part only, and in most regions of the cord the smaller part, of the whole grey matter. In a transverse section from the thoracic region (Fig. 96) a few only of these nerve-cells are seen in the whole section, and though they appear more numerous in sections from the cervical and especially from the lumbar regions (Figs. 99, 100), yet in all cases they occupy the smaller part of the area of the grey matter. The larger part of the grey matter consists, besides a neuroglia sup- ‘porting the nervous elements, of nerve filaments running in various directions and forming, not a plexus properly so called, but an interlacement of extreme complexity. The constituents of this nervous tangle as seen in a transverse section may be briefly described as follows. It consists in part of the terminal portions of the dendrites of nerve-cells; these cannot in a section be traced distinctly very far from the body of the cell, they are lost to view in the tangle. It will be understood of course that in a section a large part of this constituent of the grey matter will belong to cells whose bodies are not seen in the section, since these lie either Cuap. 1.] ‘ THE SPINAL CORD. 931 above or below the plane of the section, their dendrites alone, and only some of these passing into the section. To these terminal portions of dendrites of nerve-cells are attached the terminal arborisations of certain axons, belonging to célls which as we have said may be a long way off. These dendrites and the arborisations in contact with them both form as it were the basis of the tangle of grey matter; but they form only a part and indeed a small part of it. To this we must add a number of naked axis-cylinders, axons, of various sizes, some large, some quite small, running in various directions ; of these some are about to end immediately in arborisations, others are simply sweeping through the portion of grey matter under view, on their way to some other part of the grey matter or to the white matter. The above are all non- medullated nervous elements. There are also present, in relatively large numbers, the particular fine medullated fibres of which we spcke above; of these, which also run in various directions, some are collaterals, and while some are soon about to end, losing their scanty medulla, in arborisations lying in the section, or close to it, others are on their way to other, it may be, distant connections. Lastly there are present a certain number of ordinary medullated. fibres, some of even large size; these running also in various direc- tions may be considered as simply passing through the grey matter under view ou their way to other parts. All these several elements, some being terminal and forming a basis, others coming in to end in that basis, and yet others simply passing through it on their way elsewhere, all intricately interwoven, all supported in a bed of neuroglia, make up with the obvious well-defined nerve-cells what we call the grey matter. It should be added that besides the nerve-cells spoken of above, which, though of various sizes, are all large enough for their features to be readily recognized, a number of other cells of very small size, some of which at all events must be regarded as true nerve-cells, are present in the grey matter. The neuroglia in which all these structures, nerve-cells, fine medullated nerve-fibres, naked axis-cylinders and fine filaments, are imbedded is identical in its general characters with that of the white matter, but, as naturally follows from the nature of the nervous elements which it supports, is differently arranged. In- stead of forming a system of tubular channels it takes on the form of a sponge-work with large spaces for the larger nerve-cells and fine passages for the nervous filaments. At the junction of the grey matter with the white matter, the neuroglia of the one is continuous with that of the other, and the connective-tissue septa of the latter run right into the former; the outline of the grey matter is not smooth and even, but broken by tooth-like processes due to the septa. Since, as we have just said, some of the true nerve-cells are very small, and since the nerve filaments like the neuroglia fibres are very fine and take like them an irregular course, it often becomes very difficult in a section to determine 60—2 932 STRUCTURE OF GREY MATTER, [Boox 1. exactly which is neuroglia and which are nervous elements. The neuroglia cells may however be distinguished perhaps from the smaller nerve-cells by their nuclei not being so conspicuous or so. relatively large as in a nerve-cell, and by their staining differently. : The grey matter then may be broadly described as a bed of neuroglia, containing a certain number of branching nerve-cells, for the most part though not exclusively large and conspicuous, but chiefly occupied by what is not so much a plexus as an intricate interweaving of nerve filaments running apparently in all directions. It may be added that the grey matter is well sup- plied with blood vessels, these being in it, as stated above, relatively much more numerous than in the white matter. § 565. The central canal is lined by a single layer of columnar epithelial cells, which are generally described as bearing cilia; but it is not certain that the processes which may be seen project-. ing from the surfaces of the cells are really cilia. These epithelial cells rest not on a distinct basement membrane but on a bed of. neuroglia, free apparently or nearly so from nervous elements, . which surrounds the central canal and is sometimes spoken of as the substantia gelatinosa centralis (Fig. 96, c. g. s.). The attached bases of the epithelial cells are branched or taper to a filament, and become continuous with the branched cells or fibres of the neuroglia below. As we said above the neuroglia elements are transformed epithelial cells; and the continuity of the cells, which retaining the characters of epithelial cells form a lining to. the canal, with the cells which have become branched and lost their epithelial characters indicates the epithelial origin of the latter. The central canal with the surrounding area of neuroglia forms the central part of the isthmus uniting the two lateral halves of the cord. Dorsal (posterior) to this central mass lies the posterior grey commissure (Figs. 96, p. g.c., 99, 100), composed chiefly of fine filaments running transversely, and ventral (anterior) to it lies first the thinner anterior grey commissure (Figs. 96, a. g. c., 99, 100) of a similar nature, and then the relatively thick white commissure (Figs. 96, a. c., 99, 100) which is formed by medullated fibres crossing over from one side of the cord to the other, and thus constitutes a decussation of fibres along the whole length of the cord. On each side, the central mass of neuroglia of which we are speaking gradually Le into the central grey matter of the corresponding lateral alf. The end or head (caput) as it is frequently called of the dorsal,: posterior horn is occupied not by ordinary grey matter, but by a peculiar tissue, the substantia gelatinosa of Rolando, which forms a sort of cap to the more ordinary grey matter differing in size and shape in different regions of the cord. Cf. Figs. 96, 99, 100 sg. In carmine and some other modes of preparation it is ‘Cap. 1.] THE SPINAL CORD. 933 frequently stained more deeply than is the ordinary grey matter, and in such preparations is very conspicuous. It may fe described as consisting of a somewhat peculiar neuroglia traversed by fibres of the dorsal, posterior root, and containing a large number of cells, which, for the most part small, the cell-bodies being small relatively to the nuclei, are not all alike, some being probably nervous and others not. It takes origin from the cells forming the immediate walls of the embryonic medullary canal. In the embryo, this canal is relatively wide, though compressed from side to side, and in transverse sections of the medullary tube appears at a certain stage as a narrow oval slit placed vertically, and reaching almost from the dorsal to the ventral surface. The dorsal part of this long slit is later on closed up by the coming together of the walls and the obliteration of the greater part of the cavity, leaving the ventral part to form a circular canal, which by the development of the ventral columns assumes the central position. During this closure of the dorsal part of the canal a mass of the cells lining A B c Fic. 98. DracRaM To ILLUSTRATE THE NATURE OF THE Suznstance or Rowanvo. The figures are purely diagrammatic and are not drawn to the same scale. In all three figures the grey matter is shaded with fine lines and the white matter with dots. A, transverse section of the lower end of the conus medullaris in man. e. epithe- lium lining the medullary canal. «. lateral expansion of the canal. B, transverse section of the spinal cord of the calf in the lower thoracic region. r. substance of Rolando, c. central canal. C. transverse section through mid thoracic region of cord in man. +. substance of Rolando. the canal is cut from the rest on each side, and during the subse- quent growth takes up a position at the end of the dorsal horn. Hence, though it never apparently contains any cavity, the sub- stance of Rolando may be regarded as an isolated portion of the walls of the medullary canal, which has undergone a development somewhat different from that of the portion which remains as the lining of the central canal. Traces of this origin may be seen even in the adult. Thus in the lower end of the cord, in what we shall speak of presently as the conus medullaris, the central canal widens out dorsally, and in section (Fig. 98, A) presents on each side a bay , stretching out towards the position of the dorsal horn. At this region of the cord, though both white and grey matter are developed on the ventral surface, the posterior columns 934 THE SUBSTANCE OF ROLANDO. [Boox 111. C.P.T. Fie, 99. Transverse DorsoventraL Section or Spinau Corp (Human) at THE LeveL or THE SixtH Cervican Nerve. (Sherrington.) This is drawn on the same scale as Fig. 96, that is, magnified 15 times. r. f. l. lateral reticular formation. 1. f. p. posterior reticular formation. pp’. fine fibres of lateral bundle of the posterior root; p”, p’”’ fibres of median bundle Cuap, 1] THE SPINAL CORD. 935 of posterior root, entering grey matter from external posterior column. x. grey matter of posterior horn. Sp. a. bundles of fibres belonging to the spinal accessory nerve and issuing from the cell-group in the ventral horn marked Sp. a.; in the lateral reticular formation they are seen cut transversely. . is a natural septum of connective tissue marking out the cerebellar tract C.7. from the crossed pyramidal tract C.P.T. z.s. zona spongiosa. 2a, 8, lateral cells of the anterior horn. 5. Cells in the region of the lateral reticular formation. The other letters of reference are the same as in Fig. 96. do not meet on the dorsal surface, but leave the central canal covered only by tissue which perhaps may be called neuroglia, but is of peculiar nature and origin. In the calf, in a part of the thoracic region the substance of Rolando is not confined to the tip of the dorsal, posterior horn, but is continued to meet its fellow in the middle line. Fig. 98, B. If we imagine the dorsal portion of the canal of A to be cut off from the ventral portion, its cavity to be obliterated, and the lining epithelium with some of the sur- rounding elements to undergo a special development, the condition in B is reached by the growth of the posterior columns. From B, the transition to the normal state of things as in 98, C, is a very slight one. The extreme dorsal tip of the horn being of a more open texture than the substance of Rolando is sometimes called the zona spongiosa. § 566. The grouping of the nerve-cells. The nerve-cells, at all events the cells which are large enough to be easily and without doubt recognized to be nerve-cells, form, as we have seen, only a part of the grey matter, and in some parts of the cord, in the thoracic region for instance, are so sparse that in a section of the spinal cord in this region thin enough to shew its histological features satisfactorily, the bodies of a few only of such cells are visible (Fig. 96); the greater part of the grey matter consists not of the bodies of conspicuous nerve-cells, but of a mass of fibres and fibrils passing apparently in all directions. In the cervical (Fig. 99) and especially in the lumbar (Fig. 100) regions the nerve-cells are both absolutely and relatively more abundant; but even in a section taken from the lumbar region the nerve- cells, all put together, form the smaller part of the whole area of grey matter. Moreover, in respect of the number of cells all the sections of even the same region of the cord are notalike. Seeing that the cord may be considered as growing out of the fusion of a series of paired ganglia, each ganglion corresponding to a nerve, cf. § 96, we may fairly expect to find the fusion not complete, so that the nerve-cells would appear more numerous opposite a nerve than in the middle between two nerves. In some of the lower animals this arrangement is most obvious, and there are some reasons for thinking that even in man the nerve-cells are metamerically increased at the level of each nerve. Even when casually observed it is obvious that the nerve-cells are not scattered in a wholly irregular manner throughout the grey matter, being for instance much more conspicuous in the ventral 936 THE NERVE-CELLS OF THE CORD. [Book 11. horn than elsewhere; and more careful observation allows us to arrange them to a certain extent in groups. The cells of the ventral horn are for the most part large and conspicuous, 674 to 13854 in diameter, branch out in various directions, and present an irregular outline in sections taken in different planes. We have reason to think that every one of them possesses an axis-cylinder process which, in the case at all events of most of the cells, passing out of the grey matter becomes a fibre of the adjacent anterior root. They are obvious and conspicuous in all regions of the cord, though much more numerous and individually larger in the cervical and lumbar enlargements than in the thoracic region. We may further, with greater or less success, divide them into separate groups. ahs In the cervical and lumbar regions a fairly distinct group of cells is seen lying on the median side of the grey matter close to the anterior column (Figs. 99, 100, 1). This may be called the median growp. It appears also in the thoracic region (Fig. 96, 1); indeed the question arises whether all the cells of the ventral horn in this region do not belong to this group. The other cells so conspicuous in the lumbar and cervical enlargements, and therefore probably in some way associated with the limbs, may be spoken of as forming altogether a lateral group or limb group; but we may, though with some uncertainty, subdivide them into two or three groups. Thus in the lumbar region a group of cells (Fig. 99, 2) lying near the lateral margin of the more dorsal part or base of the horn may be distinguished, as a dorso-lateral subgroup, from the cells occupying the ventral lateral corner of the horn and forming a ventro-lateral subgroup (Fig. 100, 28); and the same distinction, though with less success, may be made in the cervical region (Fig. 99). Further, we may perhaps in both regions distinguish a group of cells placed in the lumbar region in the middle of the horn as a central subgroup (Figs. 99, 100, 2a). But, in all cases, the separation of these cells, which we have spoken of as a whole as lateral cells, into minor groups, is far less distinct than the separation of the median group from these lateral cells, especially if we admit that in the thoracic region, the median group 1s alone clearly represented. In the thoracic region a group of rather smaller cells is seen at the base of the anterior horn, near to the junction with the isthmus (Fig. 96,7). In the cervical and lumbar region these cells are very scanty (Figs. 99, 100, 7). The cells of the dorsal horn contrast strongly with those of the anterior horn in being few, and for the most part small. They are branched; and, like the cells of the ventral horn, each possesses an axis-cylinder process, though this is not easily de- termined without special preparation ; the processes do not run out to join the dorsal root as do the corresponding processes in the ventral horn and therefore are not so readily seen. These cells Cuap. 1.] THE SPINAL CORD. 937 occur in all regions of the cord, and appear to be arranged into two more groups. The lateral margin of the dorsal horn, at about the middle or neck of the horn, is along the whole length of the > Fie. 100. Transversz DorsoventraL SEcTion or THE Spina Corp (Humay) at THE Leven or THE Tuirp Lumpar Nerve. (Sherrington.) This is drawn to the same scale as Figs. 96, 99 and in the same way except: that the outline of the grey matter is not exaggerated. Pr’. median, Pr. inter- mediate, Pr”. lateral bundles of posterior roots. The region comprised under m.t. is the marginal zone or Lissauer’s zone. The other letters of reference are the same as in 96 and 99. The three figures 96, 99, 100 are intended to illustrate the main differential features of the thoracic, cervical, and lumbar cord. 938 THE NERVE-CELLS OF THE CORD. [Book 11. cord, but especially in the cervical region, much broken up by bundles of fibres passing in various directions and forming an open network, called the lateral reticular formation (Figs. 99, 100, r. ff lat.). In all regions of the cord a number of cells are found associated with this reticular formation, forming the group of the lateral reticular formation (Figs. 99, 100, 5). In all regions of the cord also a group of cells (Figs. 96, 99, 100, 6) is found in that part of the horn where, a little ventral to the substance of Rolando, the uniform field of grey matter is broken up into a kind of network by a number of bundles of white fibres running in various direc- tions. This network has also been called a reticular formation, and has received the name of posterior reticular formation (Figs. 98, 99, r. fp.) to distinguish it from the lateral reticular forma- tion just mentioned ; the two however in some regions (see Fig. 96) join each other, and thus cut off a ventral portion of the dorsal horn containing nerve-cells from a dorsal portion, in Figs. 98, 99, in which no obvious or conspicuous nerve-cells are present. The groups of cells just mentioned with the restrictions and modifications spoken of occur along the whole length of the cord; but the group of cells to which we must now call attention is almost confined to a special region of the cord, or at least is but feebly represented elsewhere. In the thoracic region, especially in the lower thoracic region (we shall return to the limits of the group later on) at the base of the dorsal horn (Fig. 96, 3) just ventral to the curve formed by the posterior grey commissure as this bends dorsally to jom the dorsal horn, is seen on each side of the cord a conspicuous group of cells known as Clarke’s column or the posterior vesicular column or vesicular cylinder. The cells composing this group, though varying in size at different levels, are rather large cells, and are for the most part fusiform, with their long axis placed lengthways along the cord, so that in transverse sections they often appear to have a rather small round body. They are surrounded by and as it were imbedded in a mass of fine fibres, the area of which is indicated by a dotted line in Fig. 96. Also conspicuous in the thoracic region is another group of cells lying on the outer side of the middle of the grey matter at about the junction of the ventral and dorsal horns. This is known as the intermedio-lateral tract and is sometimes called the lateral horn (Fig. 96, 4). The cells composing it are somewhat small spindle-shaped cells with their long axis placed transversely. The group is conspicuous as we have said in the thoracic regions ; it may be recognized in the lumbar region (Fig. 100, 4), but in the cervical region becomes confused with the most dorsally placed or lateral subgroup of the ventral horn. We shall however have to return to these groups of cells when we come to speak of the differences between the several regions of the cord. Cuap. 1.] THE SPINAL CORD. 939 § 567. The tracts of white matter. At first sight the white matter of the cord appears to be of uniform nature. We can use the nerve roots to delimitate the anterior, posterior and lateral columns, but we appear to have no criteria to distinguish parts in each column. In the cervical and upper thoracic regions of the cord, a septum (Fig. 96, s.) in the posterior column, somewhat more conspicuous than the other septa, has enabled anatomists to distinguish an inner median portion, the median posterior column, commonly called the postero-median column or column of Goll (Fig. 96, m. p.), from an outer lateral portion, the external posterior column, commonly called the postero-external column or column of Burdach (Fig. 96, e. p.), the lateral part of which, nearer the grey matter, has, for reasons which we shall see later on, been called the posterior root-zone. But beyond this neither the regular septa nor other features will enable us to distinguish one part of the white matter as different in nature from another. Nor have we better success when with the scalpel we attempt to unravel out the white matter into separate strands. Nevertheless we have convincing evidence that the white matter is arranged in strands, or tracts, or columns, which have different connections at their respective ends, which behave differently under different circumstances, which we have every reason to believe carry out different functions, but which cannot be separated by the scalpel because each of them is more or less mixed with fibres of a different nature and origin. The evidence for the existence of these tracts is twofold. One kind of evidence is embryological in nature. When a nerve fibre is bemg formed in the embryo, either in the spinal cord or elsewhere, the essential axis-cylinder is formed first and the less essential medulla is formed later. Now when the develop- mental history of the spinal cord is studied it is found that, in the several regions of the cord, all the fibres of the white matter do not put on the medulla at the same time. On the contrary, in certain tracts, the medulla of the fibres makes its appearance early, in others later. By this method it becomes possible to distinguish certain tracts from others. Another kind of evidence is supplied by facts relating to the degeneration of the fibres of the white matter. We have seen (§ 561) that the degeneration of a nerve fibre is the result of the separation of the fibre from its trophic centre, and that while the trophic centre of the afferent fibres is in the ganglion on the posterior root, that of the efferent fibres is in some part of the spinal cord. In the case of the efferent fibres the degeneration might be spoken of as descending from the spinal cord to the muscles or other peripheral organs. In the case of the afferent fibres of the trunk of the nerve, the degeneration is also one descending from the ganglion down to the skin or other peri- pheral organ. When however the section is carried through the 940 THE TRACTS OF WHITE MATTER. [Book 111. posterior root of a spinal nerve, the degeneration takes place in the part of the nerve between the section and the spinal cord, it runs up from the section to and into the spinal cord, and may therefore be called an ascending degeneration. Thus we may say that when a nerve trunk or when a nerve root is cut completely across, all the fibres which are thereby separated from their trophic centres degenerate. When the nerve trunk is divided all the tibres below the section undergo descending degeneration. If the ventral root be cut across, all the fibres of the root below the section undergo descending degeneration. If the dorsal root be cut across, all the fibres of the root above the section undergo ascending degeneration with the exception of certain fibres which do not degenerate at all, and of which we shall speak later on. When the spinal cord is cut across, for instance in the thoracic region, all the fibres of the white matter do not degenerate either in the part of the cord above the section or in the part below. Some fibres, and indeed some tracts of fibres degenerate, and some do not. Further, some tracts degenerate in the cord above the section, and thus undergo what has been called an ascending degeneration; other tracts degenerate in the cord below the section, and thus undergo what has been called a descending degeneration. These terms must however be used with caution. When a nerve trunk is cut across, the degeneration actually descends, in the sense that the progress of the degenerative changes may be traced downwards; they begin at the section and travel downwards at a rate sufficiently slow to permit a difference being observed between the progress of degeneration at a spot near the section and that at one farther off. After section of or injury to the spinal cord, however, it is not possible to trace any such progress either upwards or downwards; in the tracts both above and below the section or injury, degeneration either begins simultaneously along the whole length of the degenerating tract, or progresses along the tract so rapidly that no differences can be observed as far as the stage of de- generation is concerned between parts near to and those far from the section or injury. When, for instance, the cord is divided in the cervical region, subsequent examination of the tracts of so-called descending degeneration shews that the de- generation is as far advanced in the lumbar region far away from the section as in the cervical region just below the section. Applied to the spinal cord, therefore, the term descending de- generation means simply degeneration below the seat of injury or disease, ascending degeneration means simply degeneration above the seat of injury or disease. We may add that the histological features of the degeneration of fibres in the spinal cord are not wholly identical with those of the degeneration of fibres in a nerve trunk. Thus, the neurilemma with its nuclei being absent from the fibres of the cord, no proliferation of nuclei Ouap. 1.] i THE SPINAL CORD. 941 takes place; the axis-cylinder and medulla simply break up, are absorbed and disappear. Similar degenerations, ascending, or descending, or both, are seen when the section is not carried right through the whole cord, but particular parts of the cord are cut through or simply injured. And similar degenerations occur as the consequences of disease set up in parts of the cord. In this way the results of sections of or of other injuries to or of diseases of the spinal cord have enabled us to mark out certain tracts of the white matter as undergoing degeneration and others as not, and moreover certain tracts as undergoing descending and others as undergoing ascending degeneration. Further, the delimi- tation of tracts of white matter by the process of degeneration agrees so well with the results of the embryological method as to leave no doubt that the white matter does consist of tracts which differ from each other in nature and in function. The several tracts thus indicated vary in different regions of the cord. They may be broadly described as follows. I. Descending tracts, that is to say, tracts which undergo a descending degeneration in the sense noted above. The most important and conspicuous is a large tract (Fig. 101, cr. P.) occupying the posterior part of the lateral column, coming close upon the outer margin of the dorsal horn, and for the most part not reaching the surface of the cord. We shall have to return to this tract more than once, and may here simply say that it may be traced along the whole length of the cord from the top of the cervical region to the end of the sacral region, that it may be also traced right through the brain and indeed begins at the surface of the cerebral hemispheres, and that it enters the cord from the brain through the structures called the pyramids of the bulb, which we shall study later on. These pyramids cross over or decussate as they are about to pass into the cord, forming what is known as the decussation of the pyramids, and the tract of fibres in question shares in this decussation. Hence this tract is called the crossed pyramidal tract or more simply the pyramidal tract. It is no less distinctly marked out by the embryological method. The fibres forming this tract acquire their medulla later than do all other fibres of the cord; in the human embryo the medulla does not appear in them until about the end of the ninth month. See Fig. 102. . A smaller, less conspicuous descending tract occupies the median portion of the anterior column (Fig. 100, d. P.). This is not only much smaller but also much more variable than the crossed pyramidal tract, is not present in the lower animals, being found in man and the monkey only and being better developed in man than in the monkey, and reaches a certain way only down the spinal cord, generally coming to an end in the thoracic region. It too comes down from the pyramid, and 942 THE TRACTS OF WHITE MATTER. ([Boox 111. is a continuation of that part of the pyramid which unlike the rest does not decussate in the bulb; thus the tract which coming down from the left side of the brain runs in the left pyramid in Fic. 101. DiaGRaAM TO ILLUSTRATE THE GENERAL ARRANGEMENT OF THE SEVERAL Tracts oF WaitE Matter IN THE SprnaL Corp. The section is taken at the level of the fifth cervical nerve. The relations of the tracts in different regions of the cord are shewn in Fig, 107. The descending tracts, tracts of descending degeneration, are shaded with lines (figure A); the ascending tracts, tracts of ascending degeneration, are shaded with dots (figure B). cr.P. crossed pyramidal tract, or more shortly pyramidal tract. d.P. direct pyramidal tract, shaded on the side opposite to that on which cr.P. is shaded, in order to indicate the difference of the two as to crossing. P’, idiolateral degeneration in pyramidal tract. desc.l. the antero-lateral descending tract. The area, shaded, and marked d.c. in A, but left unshaded in B, is the small descending tract or rather patch mentioned in the text as observed, in certain regions of the cord, in the external posterior column e.m. C.b. cerebellar tract. p.m. or s.lr. and ¢.r. together indicate the median posterior tract or tract of fibres of the dorsal roots, c.r. representing, as is explained more fully in the text, the brachial and s.lr. the sacral, lumbar and thoracic roots. asc.a.l. the antero-lateral ascending tract. The small area at the tip of the posterior horn, marked L, is the posterior marginal zone or Lissauer’s zone. the bulb, passes down into the left anterior column of the cord. Hence this smaller tract is called the direct pyramidal tract. Moreover it has been observed that an injury, which gives rise to descending degeneration in the crossed and direct pyramidal tracts, for instance a lesion of one cerebral hemisphere, also causes descending degeneration in a number of scattered fibres which lie within an area having the same situation as that of the crossed pyramidal tract in the lateral column except that it is not on the crossed side, contralateral, but like the direct pyramidal tract on the same side, ¢dzolateral (Fig. 101, p’). Hence what we may in its entirety call ‘the great pyramidal tract’ as it passes from the brain to the spinal cord divides at the pyramids into three parts. One part, and that by far the greater part, crosses over to the other side, takes a contralateral course, and becomes the crossed (lateral) pyramidal tract, a second smaller part remains on the same side, takes an idiolateral course and becomes the direct (ventral) pyramidal tract, while the remaining third also Cuap. 1.] THE SPINAL CORD. 943 takes an idiolateral course, but runs in the lateral column and becomes the uncrossed lateral pyramidal tract, the fibres com- posing it being scanty and scattered. Fic. 102. Diagram sHEWING THE TRACTS IN THE Human Spina, Warre Marrer AS REVEALED BY THE DATE or MYELINATION or THE NERVE FIBRES. The numbers indicate the length of the embryo in centimeters at the date of myelination of the tract on which each number is placed. The earliest tracts to develope are left white; the latest, the pyramidal, is darkest (50) and its myelina- tion only occurs after birth. Spongy and gelatinous grey matter are represented by black and stippled fields respectively. It will be noted that if figures A and B in Fig. 103 are combined the area of scantiest degeneration in the combined figure resembles a good deal the area left white in this figure, because short paths develope earliest. The pyramidal tract is the most conspicuous and important descending tract, but names have been given to two other de- scending tracts. One, known as the antero-lateral descending tract, is a large tract placed in the antero-lateral column, and seen in section (Fig. 101, desc. J.) as an elongated area stretching from the pyramidal tract towards the anterior column and reaching at times as far as the ventral fissure. The area is large, however, because the tract is very diffuse, that is to say, the fibres with descending degeneration, or fibres which degenerate below the section or injury, are very largely mixed up with fibres which do not so degenerate ; in this respect this tract contrasts with the crossed pyramidal tract, which is to a much greater extent composed of fibres with descending degeneration, though even in it there are a considerable number of fibres which do not degenerate. Indeed this antero-lateral descending tract is so diffuse that it hardly deserves to be called a tract. The other is a small, narrow, comma-shaped tract (Fig. 101, «), situated in the middle of the external posterior column but limited to the cervical and upper thoracic regions, and has been called the “descending comma tract.” But the degeneration reaches a short way only below the section or injury, and the group of fibres thus degenerating can hardly be considered as forming a tract comparable to the other tracts. The area 944 THE TRACTS OF WHITE MATTER. [Book 11. is in part composed of the descending portions of certain fibres cof the dorsal root, which, as we shall see, divide soon after entering the cord, into an ascending and a descending portion. But it contains also fibres which, starting from cells lying within the cord itself, take a descending course. : Il. Ascending tracts, that is to say, tracts in which the degeneration takes place above the section or injury. __ A conspicuous ascending tract of a curved shape (Fig. 101, C.b.) occupies the outer dorsal part of the lateral column lying to the outside of the crossed pyramidal tract, between it and the surface of the cord. It appears to begin in the upper lumbar region, being said to be absent from the lower lumbar and sacral cord, and may be traced upwards increasing in size through the thoracic and ‘cervical cord to the bulb. In the bulb it may be traced into the restifurm body or inferior peduncle of the cere- bellum, and so to the cerebellum; for the restiform body serves, as we shall see, in each lateral half of the brain, as the main connection of the cerebellum with the bulb and spinal cord. Hence this tract is called the cerebellar tract or sometimes the direct cerebellar tract. The medulla appears in the fibres of this tract at about the begianing of the seventh month. A second important ascending tract occupies the median portion of the posterior columns (Fig. 101, c.r., s.lr.), and so far coincides with what we described above as the median posterior column, in the upper regions of the cord, that it may be called the median posterior tract; it extends along the whole length of the spinal cord, varying at different levels in a manner which we shall presently study, and ending above in the bulb. A third ascending tract, called the ascending antero-lateral tract, or tract of Gowers, occupies (Fig. 101, ase. a. l.) the outer ventral part of the lateral column. It has somewhat the form of a comma, with the head filling up the angle left between projecting portions of the cerebellar and pyramidal tracts, and the tail stretching away ventrally along the outer margin of the lateral column outside the antero-lateral descending tract, the end of the tail often reaching to the ventral roots. It may be traced along the whole length of the cord, but is not so distinct and compact a tract as the two ascending tracts just mentioned; the fibres with ascending degeneration, that is to say, the fibres degenerating above the section or seat of injury, are very largely mixed with fibres of a different nature and origin. The medulla appears in the fibres of this tract at a relatively late period, during the eighth month. We may further remark that these several tracts differ from each other, in some cases markedly, as to the diameter of their constituent fibres. Thus the cerebellar tract is composed almost exclusively of remarkably coarse fibres. The median posterior tract, on the contrary, is made up of fine fibres of very equable size, _ Cuar. 1.] THE SPINAL CORD. 945 while the fibres of the antero-lateral ascending tract are of a size intermediate between the other two. The pyramidal tract on the other hand is made up of fibres of almost all sizes mixed together. The tracts then which are thus marked out are, as descending tracts, the crossed and the direct pyramidal tracts, with the less distinct or important antero-lateral descending tract: and, as ascending tracts, the cerebellar tract the median posterior tract and the less distinct antero-lateral ascending tract. If we suppose all these tracts taken away there is still left a considerable area of white matter, namely, nearly the whole of the external pos- terior column, the external anterior column, including the region traversed by the bundles of the ventral roots, and that part of the lateral column which lies between the antero-lateral descending tract and the crossed pyramidal tract on the outside and the grey matter on the inside. From this area of white matter we may put on one side at present the external posterior column because, as we shall see, this column especially in its lateral part is largely composed of fibres of the dorsal root which leave it almost at once to pass to the grey matter; hence the alternative name of posterior root-zone. We may similarly leave for the present the small zone of white matter composed of very fine fibres known as the marginal zone or Lissauer’s zone (Fig. 101, Z.), lying dorsal and lateral to the tip of the dorsal horn and in the lower regions reaching to the outside of the cord; for this too belongs to the fibres of the dorsal root. If we take these parts away there remains an area of white matter immediately surrounding the grey matter on its lateral and ventral aspects. This area is characterised by the feature that no tracts of either descending or ascending degeneration can be traced in it after section of or injury to the cord. This feature is due to the fact that the area is composed of fibres which starting from cells within the cord run for a very short distance only in the white matter and soon end in connection with some other cells also within the cord. Such fibres do degenerate, like other fibres, when cut away from their trophic cells, but since they run a very. short course their degeneration can be traced for a short distance only from the section or injury causing their degeneration. If we examine sections of a spinal cord immediately above or below the level at which the cord has been cut across or injured (Fig. 103) we find a considerable number of degenerated fibres more or less scattered over the area in question ; these are fibres, ascending or descending, which the section or injury has separated from their trophic cells. As in our examination we passed upwards or down- wards from the level of the section or injury we should find that these degenerated fibres rapidly diminished in number and finally disappeared. The absence of degeneration in the area in question at a distance from the level of a section or injury is not due as was once thought to the fibres of the area not degenerating when cut away from their trophic centres or to their possessing second or F, III. 61 946 THE TRACTS OF WHITE MATTER. [Boox 11. vicarious trophic centres, but simply to their running for a short distance only in the area. The area is composed of fibres providing short intraspinal paths from the grey matter of one part to some grey matter not far off, and hence contrasting strongly with the fibres forming the tracts of descending and ascending degeneration spoken of above, which pass to or from the cord from or to structures outside the cord itself and run long distances. The fibres composing the area acquire their medulla before any of the tracts mentioned above. It may be added that some of the fibres Fic. 103. A, Betow TraNnsEctTion. B, Asove TransEction. Fic. 103. Caarts oF CROSS-SECTIONS OF THE SPINAL CORD (Monkey) showing the topography of the degeneration consequent on a transection carried out at the 10th thoracic level. A, one segment below the transection ; B, one segment above. _ Cross-hatching means ‘‘absolute” degeneration, i.e. less than 1°/, fibres remain- ing sound. Simple hatching /// means “severe” degeneration. xx means “slight” degeneration. Numerals 1, 2, indicate the places of scattered single or double fibres observed to be in degeneration. The spongy grey matter is shown as a black field, the gelatinous grey and Clarke’s column (CC.) are stippled. In A, the degenerations being below the injury are called descending. In B, the degenerations being above are ascending; note that th i CC. as well as the white matter. ‘ Ea Cuap. 1.] THE SPINAL CORD. 947 forming the descending comma tract are like those of the area in question, short, intraspinal fibres. Fig. 103 C. From the same experiment as the preceding and drawn to the same scale but at the 2nd cervical level. The same signs are used for denoting the degeneration. The patch of degeneration in the dorsal column is still ‘‘absolute” in character, but its area measures less than half that in B. The long tract of ‘severe” degeneration in the dorsal part of the lateral column is the direct cerebellar tract, which in monkey is partly shut off from the free edge of the column by fibres of the pyramidal tract. In man thisis not so. Vide Fig. 101. Compare these three charts with the diagrams given of the spinal tracts in Fig. 107. 61—2 948 THE TRACTS OF WHITE MATTER. [Book 111. § 568. It may be as well perhaps to insist here once more, that when these several tracts or the fibres running in the tracts are spoken of as ascending or descending, what is meant is that the degeneration takes place above the section or seat of injury or — disease in the one case, and takes place below in the other. it has been supposed by many that the nervous impulses which these fibres severally carry, travel in the same direction as that taken by the degeneration, that the ascending tracts carry impulses from below upward, that is to say, carry impulses which arising from peripheral organs pass to various parts of the spinal cord or of the brain, that they are, in other words, channels of afferent impulses, and that conversely the descending tracts carry efferent impulses. To this view is often added as a corollary, that the tracts which do not degenerate at all carry impulses both ways, and hence cannot be considered as either afferent or efferent channels but simply as communicating channels. Upon this it may be remarked that impulses do not necessarily travel in the same direction as the degeneration; when a spinal nerve trunk is divided the afferent fibres as well as the efferent fibres both degenerate in a descending direction towards the periphery, though the former carry impulses in the other direction. Hence the direction of degeneration is no proof of the direction in which impulses travel; moreover, as we have seen, deyeneration does not actually travel along the fibres of the spinal cord in the same way that it does along the fibres of a nerve trunk. It may be that the descending tracts do carry impulses in a descending direction, that is, efferent impulses, and that the ascending tracts serve to carry afferent impuises; but the proof that they do thus respectively act must be supplied from other facts than those of degeneration. Moreover, we shall have to return to these ascending and descending tracts and to study their behaviour along the length of the cord before we can use the facts concerning them as a basis for any discussion as to their functions. § 569. The connections of the nerve-roots. If following a common view we regard the spinal cord as resulting from the fusion of a series of segments or metameres, each segment, represented by a pair of spinal nerves, being a ganglionic mass, that is to say a mass containing nerve-cells with which nerve- fibres are connected, we should expect to find that the fibres of a spinal nerve soon after entering in, or before issuing from the spinal cord are connected with nerve-cells lying in the neighbourhood of the attachment of the nerve to the cord. And this to a certain extent is actually the case, more especially in respect to the issuing nerve-fibres; but as regards the entering nerve-fibres such an arrangement is obscured by other developments. With regard to the ventral root, there can be no doubt that a very large proportion of the fibres in the root are continuations of the axis-cylinders or axons of the large cells in the ventral horn of the Cuap. 1.] THE SPINAL CORD. 949 same side. The fibres which can thus be traced are of large diameter, and appear to be chiefly if not exclusively motor fibres for the skeletal muscles. But the ventral root contains other than motor fibres for the skeletal muscles, vaso-motor fibres for instance, secretory fibres and others; and these probably have a different origin. And indeed some of the fibres of the root arise not from large but from small cells in the ventral horn, while others have been traced through the ventral horn, on the one hand towards the dorsal ~ horn and on the other hand towards the lateral column; others again are found to pass through the ventral horn of their own side to the bottom of the ventral fissure where, crossing over to the other side and thus forming part of the white commissure, they appear to proceed to the ventral horn of the other side. We cannot at present make any positive statement as to the real origin and exact nature of these fibres which thus upon entering the cord pass by the cells in the ventral horn without joining them, though those which cross by the white com- missure are supposed to take origin in the cells of the ventral horn of the other side; it is sufficient for our present purposes to remember that while a large number of the fibres of the ventral root, presumably those supplying the skeletal muscles, take origin in the cells of the ventral horn, shortly before they issue from the cord, others have some other origin. And similarly we have reason to think that all the cells in the ventral horn do not send out axis-cylinder processes to join the ventral roots of the same side. We may however regard a large number at all events of the cells of the ventral horn, at the level of as well as a little below and a little above the level of the exit of any particular ventral root, as constituting a sort of nucleus of origin for the larger number of the fibres, and those most probably the skeletal motor fibres, of that ventral root. The dorsal root enters the cord not in several bundles laterally scattered as does the ventral root, but in a more compact mass. This mass however consists of at least two distinct bundles, which upon their entrance into the cord take different courses. One bundle, the larger one, lying to the inner or median side of the other, consisting of relatively coarse fibres, and called the median bundle (Fig. 99, P.r’), passes obliquely into the lateral part of the external posterior column, which, as we have said, is in con- sequence often spoken of as the posterior root-zone. The other smaller bundle placed to the outside of the former, and called the lateral bundle (Fig. 99, P.r), may be again divided into an tnter- mediate bundle (Fig. 100, Pr) lying next to the median bundle, and into a still more lateral bundle (Fig. 100, Pr”). The former, consisting also of coarse fibres, plunges directly through the sub- stance of Rolando at the extremity of, and so into the grey matter of the horn, where the fibres in part at least changing their direction run longitudinally in the grey matter in bundles known 950 THE NERVE-ROOTS. | [Book ul. as “the longitudinal bundles of the dorsal horn,” Figs. 99, 100 r. f.p. The small most external or lateral portion of the lateral bundle, consisting of fine fibres and sometimes spoken of as the lateral bundle, on entering the cord at once take a longitudinal direction, and thus forms the thin layer of fine fibres, the posterior marginal zone or Lissawer’s zone, indicated in Fig. 100 by m. t., which lies between the actual extremity of the horn and the surface of the cord, and in the upper regions of the cord (cf. Fig. 99, p’), runs some way upward on the lateral margin of the horn between ° the grey matter and the crossed pyramidal tract. Thus much may be learnt by the ordinary methods of pre- paration. The degeneration method further teaches us, as we shall presently see, that many of the fibres of the median bundle after running for a certain distance in the lateral part of the posterior column pass into the median part and go to form the median posterior tract; some of these fibres from each root, as we shall also see, may be traced in this tract along the whole length of the cord until they end in the bulb, some few passing still higher up. The special silver and methylene-blue methods mentioned above (§ 564) have also taught us the following: A fibre of the dorsal root (and so far as we can judge each of its fibres) on entering the cord divides into two, one division running forward towards the head, the other backward towards the hind end, the former however having the longer course. Each fibre thus arising by division gives off along its course collaterals, the terminations of the collaterals and of the fibre itself being in the form of arborescences attached to the body of the processes of some or other cells. Some of the fibres thus end in connection with the cells of Clarke’s column (vesicular cylinder) of the same side, others in connection with cells in the substantia gelatinosa also of the same side. Of those fibres which run in the median posterior tract a large number end in connection with certain cells in the spinal bulb. And the endings in connection with these three sets of cells, those of Clarke’s column, of the substantia gelatinosa, and of the spinal bulb, all of the same side, may be regarded as the main endings of the fibres of the dorsal root. But we have reason to think that some of the fibres make connections with the (motor) cells of the ventral horn of the same side (and so provide a direct mechanism for certain simple reflex movements), while others end in connection with cells lying in the grey matter of the other side of the cord. But we shall have to return to these matters later on in their appropriate place. §570. The Special Features of the several regions of the Spinal Cord (in Man). The cord begins below in the slender filament called the filum terminale, which lying in the vertebral canal, in the midst of the mass of nerve-roots called the cauda equina, rapidly enlarges at about the level of the first lumbar vertebra into the conus medullaris. This may be regarded as the beginning of the lower Cap, 1] THE SPINAL CORD, 951 portion of a fusiform enlargement of the cord known as the lumbar swelling, which reaches as high as about the attachment of the roots of the twelfth or eleventh thoracic nerve at the level of the eighth thoracic vertebra, the broadest part of the swelling being about opposite the third lumbar nerve. Above the lumbar swelling, through the thoracic region the somewhat narrowed cord retains about the same diameter until it reaches the level of the first or second thoracic nerve opposite the seventh cervical vertebra where a second fusiform enlargement, the cervical swelling, broader and longer than the lumbar swelling, begins. The broadest part of the cervical swelling is about opposite to the fifth or sixth cervical nerve; from thence the diameter of the cord becomes gradually somewhat less until it begins to expand into the bulb, but even in the highest part is greater than in the thoracic region. The sectional area of the cord increases therefore from below upwards, but not regularly, the irregularity being due to the lumbar and cervical swellings. The extremity of the filum terminale is said to consist entirely of neuroglia closely invested by the membranes, even the central canal being absent. A little higher up the central canal begins, and nerve-cells with uerve-fibres make their appearahce in the neuroglia ; thus a kind of grey matter covered by a thin super- ficial layer of white matter is established. We have already referred to the peculiar features of the lower end of the conus, § 565; but higher up the canal becomes central and small, the posterior columns are developed, and the grey matter contains more nervous elements and relatively less neuroglia, becoming in fact ordinary grey matter. From thence onward to very near the junction with the bulb, where transitional features begin to come in, the spinal cord may be said to have the general structure previously described. The sectional area of the white matter increases in absolute size and on the whole in a steady manner from below upwards. In other words, in a section at any level, the number of longi- tudinal fibres forming the white matter is greater than the number at a lower level, and less than the number at a higher level; for any difference which may exist in the diameter of the individual fibres is insufficient to explain the differences in the total sectional area of the white matter. If we were to measure in man the sectional area of each of the spinal nerves as it joins the cord, and to add them together, passing along the cord from below upwards, the results put in the form of a curve would give us some such figure as that shewn in Fig. 104; the area gained by adding together the sectional areas of the nerves increases in a fairly steady manner from below upwards. The curve of the sectional area of the white matter of the cord taken from below upwards would be very similar, but if anything more regular. It must be understood however that the dimensions of 952 THE FEATURES OF DIFFERENT REGIONS. [Book 111. the areas would not be the same in the two cases. The sectional area of the white matter at the top of the cervical region, though "vuvtevvon Pxexi X KVINVI VE V WV Evin vv vein ot Fia. 104. Diacram sHEWING THE UNITED SECTIONAL AREAS OF THE Spinal NERVES, PROCEEDING FROM BELOW UPWARDS. In this as in the succeeding figures 102—3, —5, —6, —7, all of which refer to man, the left-hand side represents the bottom of the cord and the right-hand the top of the cord, the numerals indicating successively the sacral, lumbar, thoracic and cervical nerves. The several figures are not drawn to the same scale. greater than anywhere lower down, is far less than the united sectional area of all the nerves below that level. The white matter is not formed by all the fibres from the nerves which join the spinal cord continuing to run along the cord up to the brain ; as we have seen, some at least of the fibres end in the grey matter. Nevertheless the white matter in passing up the cord appears to receive a permanent addition at the entrance of each nerve. We may infer that each nerve has a representative of itself starting from the level of its entrance and running up to some part of the brain. Whether the fibres thus representative of the nerve are continuations of the very fibres of the nerve itself, or are new fibres starting from some relay of grey matter, with which the fibres of the nerve are also connected, is another question. § 571. The grey matter in contrast to the white matter shews great variations in area along the length of the cord (Fig. 105). From the entrance of the coccygeal nerve upwards the area 0 f vwvnwttvwin ow t xxx KX VIIVIVE VV i ob VVIVE Vive ono Fic. 105. Diagram sHEWING THE VARIATIONS IN THE SECTIONAL AREA OF THE GREY MATTER OF THE SPINAL CoRD, ALONG ITS LENGTH. increases very rapidly, reaching a maximum at about the level of the 5th lumbar nerve. It then rapidly decreases to about the level of the 11th thoracic nerve, maintains about the same dimensions all through the thoracic region, and begins to increase again at about Cuap. 1] THE SPINAL CORD. 953: the level of the 2nd thoracic nerve. Its second maximum is reached at about the level of the Sth or 6th cervical nerve, after which the area again becomes smaller, remaining however at the upper cervical region much larger than in the thoracic region. The meaning of these variations becomes clear when we turn to Fig. 106, which shews in a similar diagrammatic manner the A ee one vw wu tivv id tx xX KVM ti Q Fie. 106. Diagram sHEWING THE RELATIVE SECTIONAL AREAS OF THE SprnaL NERVES, AS THEY JOIN THE SPINAL Corp. sectional areas of the several spinal nerves. It will be observed that the increase and decrease of the sectional area of the grey matter follow very closely the increase and decrease of the quantity of nerve, that is to say, neglecting differences in the diameter of the fibres, in the number of nerve-fibres passing into the cord. The sectional areas of the Ist and 2nd sacral, 4th and 5th lumbar ~ nerves are very large, and opposite to these the sectional area of the grey matter of the cord is very large also; the enlargement of grey matter which is the essential cause of the lumbar swelling is correlated to the large number of fibres which enter and leave the cord at this region to supply chiefly the lower limbs. Similarly the enlargement of grey matter which is the essential cause of the cervical swelling is correlated to the large number of fibres which enter and leave this region of the cord to supply chiefly the upper limbs. In the thoracic region, where the number of fibres entering and leaving the cord is relatively less, the sectional area of the grey matter is also less. Since the attachments of the several spinal nerves are not exactly equidistant from each other along the length of the cord, the sectional area is not an exact measure of bulk; the total bulk of grey matter for instance belonging to two nerves which enter the cord close together is less than that of two nerves giving rise to the same sectional area of grey matter as the former two but entering the cord far apart from each other. Still the error which may be introduced by taking sectional area to mean bulk is, for present purposes at all events, so small that we may permit ourselves to say that in the successive regions of the spinal cord the bulk of grey matter in any segment is greater or less according to the size of the nerve (or pair of nerves, right and left) belonging to that segment. From this anatomical fact we appear justified in drawing the conclusion that at all events a great deal of the grey matter of the spinal cord may be considered as furnishing a nervous mechanism, with which the efferent fibres of each spinal nerve just before they leave the cord, and the afferent fibres soon after they join the cord, are more immediately connected. This need not mean that 954 THE FEATURES OF DIFFERENT REGIONS. [Boox 11. the whole of the grey matter is thus directly connected with and thus rises and falls with the fibres of the nerves; it might mean that there is a sort of core of grey matter, which maintains a uniform bulk along the whole length of the cord and serves as a basis which is here more and there less swollen by the addition of the grey matter more immediately connected with the fibres of the nerves. This question the method which we are now using cannot settle. § 572. Owing to these different rates of increase of the grey and white matter respectively along the length of the cord, we find that in sections of the cord taken at different levels the appearances presented vary in a very distinct manner. This is strikingly shewn by comparing Figs. 96,99 and 100. At the level of the third lumbar nerve (Fig. 100) the grey matter is very large, reaching, as we have seen, its maximal sectional area at about this point, so that although the area of white matter is not very great the whole area of the cord is considerable. At the level of the sixth thoracic nerve (Fig. 96), in spite of the white matter having very decidedly increased, the grey matter has shrunk to such very small dimensions, that the total sectional area of the cord has markedly diminished. At the level of the sixth cervical (Fig. 99) the grey matter has again increased, reaching here as we have seen its second maximum; the white matter has also further increased, and that indeed very considerably, so that the total area of the cord is much greater than in any of the lower regions. Further details of the varying size of the white matter and of the grey matter at different levels are also shewn in the series given in Fig. 107. In these, combined with the three figures just referred to, it will be observed that the serial increase and decrease of the grey matter does not affect all parts of the grey matter alike, so that the outline of the grey matter changes very markedly in passing from below upwards. In the coccygeal region each lateral half is a somewhat irregular oval, and in the sacral region, Fig. 107, Sac, the differentiation into ventral and dorsal horns is still very indistinct. In the lumbar region the two horns are sharply marked out, though both the dorsal and ventral horns are broad and more or less quadrate. In the thoracic region the decrease of grey matter has affected both horns, so that both are pointed and slender, while the junction between them has not undergone so much diminution, so that what has been called the lateral horn is relatively conspicuous. In the cervical region the returning increase bears much more on the ventral horn which again becomes large and broad, than on the dorsal horn which still remains slender and pointed. Taking the form of the grey matter in the thoracic region as the more typical form of the grey matter we may say that while the increase on the lumbar swelling bears equally on the ventral and dorsal horns, that in the cervical region bears chiefly on the ventral horn. Cuap. 1.] THE SPINAL CORD. 955 _ Now we have no reason to suppose that either centripetal impulses reach the lumbar spinal cord in greater numbers from the lower limbs, or along any of the nerves joining this part of the cord, or that those which do reach it are of a more complex nature than is the case with the centripetal impulses reaching the cervical cord along the nerves of the upper limbs. The increase of grey matter in the dorsal horns is therefore not correlated to any increase in the number or complexity of the centripetal impulses reaching the cord; and we may, provisionally, conclude that at least a large part of the grey matter in the dorsal horn is not specially concerned in any elaboration or transformation of centripetal impulses immediately upon their arrival at the cord. Indeed we have seen that while there is ample evidence to connect the nerve-cells, and therefore presumably the grey matter in general of the ventral horn with the centrifugal motor fibres of the ventral root, there is no corresponding evidence as to any large _ immediate connection of the centripetal fibres of the dorsal root with the nerve-cells or indeed any other part of the grey matter of the dorsal horn. We may add that, as we shall point out later on, so essential is the concurrence of appropriate centripetal impulses to the due carrying-out of complex coordinate motor or centrifugal impulses, that we can scarcely expect to find any increase in the nervous mechanisms devoted to the purely motor function of carrying out motor impulses without a corresponding increase in the nervous mechanisms belonging to the centripetal impulses, by means of which those motor impulses are guided and coordinated. Hence, were the latter nervous mechanisms restricted to the dorsal horns we should expect to find a greater parallelism than does actually exist between them and the ventral horns. § 573. The changes in the area of grey matter illustrated by the statements and diagrams given above refer to the grey matter as a whole, that is, not only to nerve-cells, but also to strands and networks of nerve fibres and nerve fibrils, and indeed include to a certain extent neuroglia. We have seen § 566 that we are able to distinguish certain large and conspicuous nerve-cells in the grey matter and to arrange these into groups. The grey matter contains many other small nerve-cells, which we are not able at present to name or arrange, but whose existence must always be borne in mind. Confining ourselves now however to the groups of larger, more conspicuous nerve-cells, we find that, broadly speaking, the chief differences which can be observed in the cells of the ventral horn along the length of the cord are that in the thoracic region the nerve-cells of the ventral horn are few, and relatively small, while in the cervical and lumbar region, especially in the latter, they are numerous and large. Tt is not easy, even if possible, to distinguish in the thoracic region the several groups of cells marked in Figs. 99, 100 as 2a, 8, y: the median group (Figs. 99, 100, 1), indeed seems to be the only group 956 THE FEATURES OF DIFFERENT REGIONS. [Book 111. present in the mid-thoracic region (Fig. 96,1). The group of the dorsal horn (Figs. 96, 99, 100, 6) appears to be about the same in all regions. With two other groups of nerve-cells striking differences are seen in different regions. The vesicular cylinder, for instance (Fig. 96, 3), is most conspicuous in the thoracic region. It may be said to reach from about the 8th cervical nerve to the 3rd lumbar nerve, being perhaps most developed in the lower thoracic and upper lumbar region. It is absent in the cervical region above the 7th or 8th cervical nerve, and in the lumbar region below the 3rd lumbar nerve; but a similar group of cells is Cuap. I.] THE SPINAL CORD, 957 sr. tn.dr, as Sac. Fic. 107. Diagram ILLUSTRATING SOME OF THE FEATURES oF THE Spina Corp AT DIFFERENT LEVELS, (Sherrington.) All the figures are drawn to scale, and represent the cord magnified four times. They shew the differences at different levels in the shape and size of the cord, in the outline of the grey matter, and in the relative position of the anterior and posterior fissures, and also shew the variations at different levels of the several ‘tracts’ of the white matter. C, at the level of the second cervical nerve, C, of the fifth cervical, Cg of the eighth cervical. D, of the second thoracic, D, of the fifth thoracic, L, of the first lumbar, L, of the fifth lumbar, and Sac. of the second sacral nerve. The shading of the tracts is a little different from that in Fig. 101. In the median posterior column of D, the areas of fibres coming from the sacral nerves s.r. and lumbar nerves l. 7. are distinguished from the area, d. r., of fibres belonging to the thoracic nerves. In Cg, no distinction is made between any of these sets of fibres; in L, only fibres of sacral nerves are represented; in L, D, D,, the more dorsal small portion corresponds to sacral fibres and the next to lumbar, or lumbar thoracic nerves. present opposite the 2nd and 3rd cervical nerves; a group of more doubtful likeness is seen in the sacral region below; and 958 THE FEATURES OF DIFFERENT REGIONS. [Book 11. the column is said to have a representative in the bulb above the spinal cord proper. It seems natural to infer that the cells forming this vesicular cylinder are connected neither with the ordinary somatic motor fibres governing the skeletal muscles, nor with the ordinary-afferent sensory, somatic fibres coming from the skin and elsewhere, but in some way with some special sets of fibres; on this point however no authoritative statement can as yet be made. ; The lateral horn or intermedio-lateral tract, Fig. 96, 4, 1s also most conspicuous in the thoracic region. In the lumbar region it is lost or traced with great difficulty, and in the cervical region seems to be merged into the most dorsally placed division of the lateral group of the cells of the ventral horn. It is possible that this group represents in the limbless thoracic region the cells which are developed into the great lateral group of the ventral horn in the regions of the limbs. j § 574. The white matter as we have seen increases in sectional area with considerable regularity from below upwards. If instead of a diagram of the increase of the whole white matter, we construct in a similar way diagrams of the ventral, dorsal and lateral columns respectively we find that while the sectional area of the lateral column (Fig. 108) increases with some considerable Fie. 108. Di1acRAM SHEWING THE VARIATIONS IN THE SECTIONAL AREA OF THE LATERAL COLUMNS OF THE SPINAL Corp, ALONG ITS LENGTH. regularity from below upwards, though not so regularly as does the whole area of white matter, both the ventral (Fig. 109) and | vv tv Wa Wt xx X XVII VY OW ot ob vviVE VowWwom nt Fic. 109. DiacraM SHEWING THE VARIATIONS IN THE SECTIONAL AREA OF THE ANTERIOR COLUMNS OF THE SPINAL CorRD, ALONG ITS LENGTH. the dorsal (Fig. 110) columns agree to a certain extent with the grey matter in shewing a decided increase in both the lumbar and the cervical swellings. We may, provisionally at least, infer from this that, while considerable portions of both the ventral and Cuap. 1] THE SPINAL CORD. 959 the dorsal columns are like the adjoining grey matter in some way or other concerned in the exit and entrance of efferent and iS ; ee ees Vw tp v vow ex xX Xv VOW oo woiviiviwu vvmuo Fic. 110. DiaeRam sHEWING THE VARIATIONS IN THE SECTIONAL AREA OF THE POSTERIOR COLUMNS OF THE SPINAL CORD, ALONG ITS LENGTH. afferent fibres, the larger portion of the lateral column is concerned in the transmission of impulses to and fro, between the local mechanisms below, immediately connected with the several spinal nerves, and the brain above. This conclusion seems incidentally confirmed (though these diagrams must not be strained to carry detailed inferences) by the sudden increase of the lateral column above the lumbar swelling, as if the large mass of nervous mechanism for the lower limbs concentrated in this region demanded a sudden increase in the number of fibres connecting it with the brain above. This more or less continuous increase of the lateral column partly explains the change of form in the general outline of the transverse section of the cord which is observed in passing upwards from the lower to the higher regions. In the coccygeal, sacral and lumbar regions the outline, though varying somewhat chiefly owing to the disposition of the grey matter, is on the whole circular. As the thoracic passes into the cervical region, the increase of the lateral columns increases the side to side diameter so much that the section becomes oval, and in the cervical swelling itself this increase of the side to side diameter out of proportion to the dorso-ventral diameter is very marked. The actual outline of the whole transverse section is however determined also to a certain extent by the changes of form of the grey matter. The cord moreover undergoes along its length a change which is not very clearly indicated in the diagrams Figs, 108, 110. By comparing the series of transverse sections given in Fig. 107 it will be seen that the relative position of the central canal shifts ‘along the length of the cord. In the sacral and lumbar regions the central canal is nearly at the centre of the circle of outline, and the dorsal and ventral fissures are nearly of equal depth. Even in the upper lumbar region, and still more in the thoracic region, the position of the central canal is shifted nearer to the ventral surface, so that the dorsal fissure becomes relatively longer, deeper, than the ventral. This shifting goes on through the cervical region up to about the level of the 2nd cervical nerve, where it is arrested by the beginning of the changes through which the spinal cord is transformed into the far more complicated bulb. “960 THE FEATURES OF DIFFERENT REGIONS. [Book 111. This lengthening of the dorsal fissure indicates an increase in the dorso-ventral diameter of the dorsal columns, and this, not being accompanied by a compensating diminution of the side to side diameter, shews in turn that the dorsal columns undergo an increase in passing upwards. From this we may add to the provisional conclusion just arrived at with regard to the lateral columns, the further conclusion that some part of the dorsal columns also is concerned in transmitting impulses, in a more or less direct manner, between the various regions of the cord below and the brain above. The ventral columns do not increase in the same marked manner, though over and above the increase due to the lumbar and cervical swellings, a continued increase may be observed, especially in the upper cervical region; it is in this upper region that the direct pyramidal tract is best developed. § 575. The provisional conclusions at which we have arrived are further, to a certain extent at least, confirmed and extended by a study of the behaviour at the several regions of the cord of the special tracts of white matter described in § 567. 5 The pyramidal tract, that is to say, the crossed pyramidal tract entering the spinal cord above from the pyramid is very large in the cervical region, having the form and situation shewn in Fig. 107,C,C,C,. From thence downward it diminishes in size, the diminution being especially rapid in the lumbar swelling, Fig. 107, L,, where the tract being no longer covered in by the cerebellar tract comes to the surface of the cord; but it may be traced by the degeneration method down as far as the coccygeal region, and indeed appears to be coexistent with the issue of spinal nerves from the cord. Diminution of the tract means a lessening of the number of fibres; and since we cannot suppose that any of the fibres come suddenly to an end in the tract itself we are led to infer that along the cord, from above downwards, fibres are successively leaving the tract and passing to some other part of the cord. We seem further justified in concluding that the fibres which thus successively leave the tract go to join the series of local nervous mechanisms with which the spinal nerves communicate, as we have seen reason to believe, upon their entrance into the cord. Indeed, as we shall see later on, we have reason to think that the nervous mechanisms which the fibres in ‘question join are those belonging to the motor fibres of the ventral roots; the fibres of the pyramidal end by forming synapses with the cells whose axons go to form the fibres of the ventral roots. This pyramidal tract does not begin in the pyramid, but may be traced through the lower parts of the brain right up to special areas in the cortex or surface of the cerebral hemispheres; and very strong reasons may be brought forward in support of the view that the fibres of this tract are fibres which carry impulses from the cortex to successive portions of the spinal cord, and there give rise to efferent impulses which pass to Cuap. 1] THE SPINAL CORD. 961 appropriate skeletal muscles. The tract, therefore, is not only a descending tract by virtue of the mode of degeneration, but may be spoken of in a broad sense as a tract of efferent impulses descending from the cerebral cortex; and indeed it is maintained that it is the channel of the particular kind of efferent impulses which we shall speak of as voluntary or volitional impulses. We may add that as the tract passes along a path which we shall subsequently describe, from the cerebral cortex through the lower parts of the brain to the pyramid, it gives off fibres to mechanisms connected with several of the cranial nerves, much in the same way that it gives off fibres to those connected with the spinal nerves. We may therefore picture to ourselves this pyramidal tract as starting in the form of a broad sheaf of fibres from a certain district on the surface of one of the cerebral hemispheres. Putting aside for the present any possible increase of the number of fibres by division (though we have reason to think that this does to a certain extent occur) we may regard the tract as being at its maximum at its beginning in the cortex. As it descends to the decussation of the pyramids in the bulb it loses a certain number of fibres, which pass off to the cranial nerves. Having crossed and entered into the lateral column of the cord it continues to give off fibres which make connections with the nerve-cells giving rise to the ventral roots of the spinal nerves, probably of each in succession, and so goes on its way down the cord continually diminishing until the last remaining fibres are given off to the last coccygeal nerve. When degeneration is set up along this tract, as may be done, by injuries to particular areas of the cerebral cortex, the main mass of degenerated fibres, after crossing over from one side of the cerebrospinal axis to the other in the decussation of the pyramids at the lower end of the bulb, during its further progress down the spinal cord, keeps to the side to which it has crossed right down to the end. Hence, as we have said, it is called the crossed pyramidal tract. The main mass of fibres, the degene- ration of which has been started by injury to the left side of the brain, crosses over to the right side of the spinal cord and runs down the lateral column of the right side to the end of the cord. Nevertheless some of the fibres of the pyramid pass directly to the lateral column of the same side, forming the area of scattered fibres shewn in Fig. 107 P’. The direct pyramidal tract (Fig. 107, dP), except that it does not cross at the decussation of the pyramids, is otherwise similar to the crossed pyramidal tract, and indeed is a part of the same strand to which the crossed tract belongs. When degeneration in this tract is started by injury to particular areas of the cerebral cortex, say on the left half of the brain, the degeneration may be traced through the left ventral pyramid, and so to the left F, III. 62 | 962 THE FEATURES OF DIFFERENT REGIONS. [Book 111. median ventral column of the spinal cord. The direct tract 1s never so extensive or marked as the crossed tract, does not reach so far down, is much more variable both in length and in sectional area, and, as we have said, is almost confined to man. Diminishing as it descends it may be said to cease in the middle thoracic region, Fig.107,D;D;. Taking an average we may say that, of the whole strand running in the pyramids above the decussation, about three-fourths of the fibres go to form the crossed and about one- fourth to form the direct tract. We shall see later on that the impulses coming down along the united tract in the brain may, broadly speaking, be said to cross over wholly from one side to the other before they reach the skeletal muscles, so that the impulses passing along fibres in, say, the left pyramid, reach the muscles of the right limbs and right side of the body whether the fibres cross over at the decussation to form the crossed or remain on the same side to form the direct pyramidal tract. We are therefore led to infer that the fibres in the direct tract, as they pass down the cord, cross over in the cord itself before they make connections with the cells belonging to the ventral roots. Probably the crossing is effected by means of some of the decussating fibres which form the ventral white commissure. A part only, indeed a small part, of the commissure can serve this purpose; most of the fibres of the commissure, and in the lower regions of the cord, where the direct tract no longer exists, all the fibres must have some other functions. — Some of the fibres of this great pyramidal tract, leave the tract, as we have said, to join some of the cranial nerves before the pyramids of the bulb are reached; and the impulses passing along these fibres also cross over to the opposite side before they issue along the cranial nerves. Hence we infer that these fibres decussate above the decussation of the pyramids just as those of the direct tract decussate below it. So that of the whole strand as it leaves the cerebral cortex, while the main mass of fibres crosses over at the decussation of the pyramids, the rest of the fibres cross the middle line in succession from the level of the third cranial nerve to the level of the lower limit of the direct tract ; below the decussation of the pyramids the crossing takes place by means of the ventral commissure of the cord, above the decussation by means of what we shall later on learn to speak of as the raphe of the bulb, or by structures corresponding to this higher up. § 576. The cerebellar tract (Fig. 107, Cb) is as we have seen a tract of ascending degeneration ; the degeneration in it makes its appearance above the section or the seat of other injury of the cord, It begins somewhat suddenly at the level of the second lumbar nerve, being absent at least as a distinct tract below; injury of the cord at the level of the middle and lower lumbar nerves leads to no marked tract of degeneration (though possibly scattered single fibres may degenerate), while injury higher up Cuap. 1.] THE SPINAL CORD. 963 does. The tract lies, as we have said, close to the surface of the cord in the dorsal part of the lateral column just outside the crossed pyramidal tract, and while varying somewhat in the shape of its section from level to level remains throughout a somewhat narrow crescentic patch. At the top of the spinal cord it passes, as we have said, from the lateral columns into the restiform bodies of the bulb, and so to certain parts of the cerebellum. When the section or lesion is limited to one side of the cord, the degeneration is similarly limited to the same side, and that along its whole course up to the cerebellum ; there is no evidence of any of the fibres decussating in the cord. The area of the tract increases from below upward. This has been determined by the embryological method, by noting the appearance of the medulla in the fibres, as well as by comparing the extent of the degeneration following upon a section high up in the cord with that following upon a section lower down. From this we infer that the fibres composing the tract must start successively from other parts of the cord along its length, that is to say, the tract must be fed by fibres coming from other structures in the cord. On the other hand, it is found that the degenerated | area following upon a section or injury diminishes as it is traced upward ; when, for instance, a section is made in the mid-thoracic region the area of degeneration in the tract is greater immediately above the section than it is higher up, say in the cervical region. From this we are led to infer that though the tract is successively fed along its course by fibres coming from other parts of the cord, some of the fibres entering the tract, though like their companions undergoing an ascending degeneration, do not like them continue in the tract right up to the cerebellum, but pass off to other parts of the cord on their way upward. This, however, is equivalent to saying that the tract is not a pure or homogeneous one, but consists of at least two sets of fibres, only one of which is continued on to the cerebellum and strictly deserves the name of ‘cerebellar.’ It may perhaps here be mentioned that while the fibres composing the tract are as a whole conspicuously coarse, large tibres, with these there are mingled, especially in the thoracic region, a number of much finer fibres; but these apparently undergo a descending not an ascending degeneration and do not therefore really belong to the tract; they may be fibres which have strayed from the pyramidal tract. ; Unlike the case of the median posterior tract of which we have next to speak, no degeneration, at least in the lumbar and thoracic regions, appears in the cerebellar tract after section merely of the roots of the nerves; to produce the degeneration the cord itself must be injured. From this we may infer that the tract is not fed directly by the fibres of the posterior roots. And there is increasing evidence that the tract is fed by fibres coming from the 62—2 964 THE FEATURES OF DIFFERENT REGIONS, [Book ul. vesicular cylinder, that these cells send out axons which passing laterally become the fibres of the tract. From the fact that the degeneration taking place in it is an ascending one, it 1s supposed that the tract is the channel for ascending, that 1s to say, na broad sense, afferent impulses. And considerable interest attaches to the fact that these impulses should be carried, not to the cerebrum but to the cerebellum. Our knowledge on this point, however, is still imperfect, and what can be said in the matter had better be said later on. . § 577. The median posterior tract is the other conspicuous tract of ascending degeneration; it also is supposed to be a channel for ascending, afferent impulses; and this view is rendered almost certain by the intimate relations of the tract to the fibres of the posterior roots. : In dealing so far with the tracts of degeneration in the spinal cord we have always spoken of the degeneration as being the result of lesions of the spinal cord itself. Experiments on animals, however, and clinical experience have shewn that division or injury of the fibres of the dorsal roots is followed by tracts of degeneration in the spinal cord, though no damage whatever may have been done to the substance of the cord itself. These tracts make their appearance in the median posterior columns, the exact path and limits of the degeneration differing with the different spinal nerves. The results of the division of different groups of nerves are so instructive that we may dwell upon them in detail. If the dorsal roots of two or three lumbar nerves (on one side) be divided, an examination of the cord, after an interval long enough to allow degeneration to be well established, will bring to hight the following features. The divided roots will be found to have degenerated right up to their entrance into the cord.