THE ASCENT OF PREHISTORIC MAN MANS CULTURAL STAGES AND PHYSICAL DEVELOPMENT TO THE END OF THE OLD STONE AGE [DIAGONAL LINES INDICATE RACIAL PERIODS. DOT SHOWS CONiECTURED DAW OF EXTINCTION] ICE ACES BC POST-OLACIAL ■3.d INTERGLACtAL 2ni INTEfi-GLAClAL PR,E-PALyEOLITHIC on. EOLITHIC ? PLEISTOCENE The Brain from Apt M Man by Frederick Tilnty Spf^ tb^y^ ■ ■t6 Copyright iglS by Paul B. Hoeber, he., N. Y. PLIOCENE "> THE BRAIN FROM APE TO MAN THE BRAIN FROM APE TO MAN A CONTRIBUTION TO THE STUDY OF THE EVOLUTION AND DEVELOPMENT OF THE HUMAN BRAIN BY FREDERICK TILNEY, ph.d.,m.d. Professor of Neurology, Columbia University With Chapters on the Reconstruction of the Gray Matter in the Primate Brain Stem by HENRY ALSOP RILEY, a.m.,m.d. Associate Professor of Neurology, Columbia University foreword by HENRY FAIRFIELD OSBORN, sc.d.,ll.d. Research Professor of Zoology. Columhia University 557 illustrations, many in color VOLUME ONE PAUL B . HOEBER • I^c NEW YORK ^ MCMXXVIII COPYRIGHT 1928 BV PAUL B. HOEBER, INC. ALL RIGHTS RESERVED - PUBLISHED MAY I928 PRINTED IN THE UNITED STATES OF AMERICA IN APPRECIATIVE ADMIRATION THIS BOOK IS DEDICATED TO PROFESSOR HENRY FAIRFIELD OSBORN UNDER WHOSE INSPIRED LEADERSHIP THE DIM PATHWAYS OF EVOLUTION AND THE OBSCURE APPROACHES TO THE FUTURE HAVE BECOME MORE ACCESSIBLE TO THOSE SEEKING THE TRUTH HP PREFACE "^HE study of the brain in all of its evolutionary aspects is a task requiring the hibors of many investigators. Such studies are urgently needed because the questions surrounding this subject are assuming increasing prominence in modern thought. No single work may even aspire to a satisfactory completeness in dealing with the entire problem; therefore only certain aspects of it are approached in these volumes. Since brain power admittedly is the secret of human success, the brain itself must be the pioneer hewing the path of man's progress; hence the genesis of this dominant organ is a vital issue which calls for most intensive research. The answers to the cjuestions "whence came the human brain, and how" have become indispensable to modern understanding. Their value lies in the light thrown by them upon the course of man's long ascent and in their searching penetration through the dense veil which hides his future. Upon directing our attention toward this future, there are many reasons for asking whither we are trending as a race. Does the path lead forward through repetitious ancient cycles of disaster, toward wars and revolutions, toward civic frustrations amid the preparations for future war in tunes of peace, toward further stupidities in the management of social organizations, toward grosser misconceptions of man and humanity, toward greater depths of superstitious delusion and human unworthiness with their full and final penalty of ultimate extinction? Catastrophe such as this has not infrequently overtaken many races of men long since extinct. It is possible, however, that the supreme organ which created what there is of human succeeding has still further benefits to confer. And yet no satisfactory appraisal of these possibilities could be attempted without a thorough study of the processes by which the brain came into existence. To this study must be added some comprehension of the viii PREFACE manner in which this master organ has progressively adapted itself for the effective distribution of its predominant powers. These volumes dealing with the evolution of the brain of primates represent approximately fifteen years of preparation, collection and study. They emiDody the neurological evidence based upon structural and behav- ioral studies of Lemur, Tarsius, Marmoset, HowHng Monkey, Baboon, Macacus, Gibbon, Orang, Chimpanzee, Gorilla and Man. In this investi- gation, it has been my good fortune to have access to the unusually- com- plete collection of vertebrate brains gathered under the direction of Prof. George S. Huntington in Columbia University. Still more fortunate w^as my privilege of studying for many years under his personal supervision. Indeed, most of this work is the direct result of his inspiration, instruction and guidance. Each page bears some affectionate memory of his influence, and the entire effort could have no finer reward than to be, if only in some small way, a pupil's tribute to his great master. To Prof. Henry Fairlield Osborn I am also deeply indel^ted for his gracious counsel and his generosity in putting at my disposal many of the valuable collections of the American Museum of Natural History. The New York Zoological Garden and the New York Aquarium have both been con- stant and liberal contributors of valuable material. Many fine brain speci- mens, particularly of the great anthropoid apes, were presented to me by the late Mr. Carl Akeley after his last African expedition. One of the most valuable specimens in the collection is the brain of the young gorilla, John Daniel i, an animal \\ hich lived for a relatively long time in captivity and offered an exceptional opportunity for the study of its behavioral reactions under the influences of domestic life. It gives mc especial pleasure to acknowledge my gratitude to President Nicholas Murray Butler and t(^ Dean William Darrach for their interest in this research and particularly for having made possible the photographic reproductions. PREFACE ix The purpose of this work is to assemble and discuss the evidence of evohition contained in the brains of primates. The present vokimes are the first part of a cerebral survey subdivided as follows: I. The Brain from Ape to Man. II. Brain Evohition from Mammals to Man. III. The Brain from Fish to Man. The treatise setting forth this evidence of cerebral evohition is unavoid- ably lengthy. It necessarily contains a large amount of technical detaiL The full presentation of the facts, however, has been deemed essential in order to substantiate the arguments and conchisions ch'awn from them. On the other hand, the convenience of those who would not be burdened by the tedium of detailed recitals has not been overlooked. The text has been arranged in five parts. The first part recounts the observations made upon the Lower Primates; the second, upon the Inter- mediate Primates; the third, upon the three Great Anthropoids; the fourth, upon Man; and the fifth summarizes all of the evidence adduced in the preceding parts. At the end of the first, second and third parts are sectional summaries including interpretations and discussions of the facts as they bear upon the correlative evolution of structures in the brain stem and the behavioral development of the animals described. In the hfth part a summary in conclusion discusses the evidence afforded by the entire group of primates. It gives the author's views concerning the significance of the evolutionary process which, it is believed, reveals itself clearly in this order of mammals. In dealing with the technical problems involved in a discussion of the brain, it is recognized that many features might well have been treated more extensively. This recognition applies notably to the cerebral hcmi- X PREFACE spheres, especially the cerebral cortex. It is doubtless unfortunate that such a condition exists, but an adequate treatise on the neopallium alone would necessitate the addition of at least several vohimes to the present work. The chief emphasis m this description and analysis has been laid upon the brain stem, an element in the cerebrum which has hitherto been too little dwelt upon. In the brain stem are included the medulla oblongata, the cerebellum, the pons Varolii, the midbrain and the interbrain. Even of these parts the more intimate details concerning the structure and evolution of the cerebellum and of the interbrain have not been discussed in extenso. These regions of the primate brain are at present being studied by my colleague. Prof. Henry Alsop Riley, and will shortly appear as separate publications. In submitting the brain stem to study, several methods of analysis have been employed. Not only have the gross appearances in this part of the neuraxis been analyzed, but careful microscopic studies of serial sec- tions of the brain have been made at the following critical levels: Level of the pyramidal decussation Level of the caudal extremity of the dorsal sensory nuclei Level of the caudal extremity of the inferior olivary body Level through the middle of the inferior olivary body Level of the vestibular nuclei Level of the cerebellar nuclei Level of the inferior portion of the pons Varolii and the emergence of the sixth cranial nerve Level through the middle of the pons Varolii Level of the emergence of the fourth cranial nerve (nervus trochlearis) Level of the inferior colliculus Level of the superior colliculus Level of the optic chiasm Level of the anterior commissure. PREFACE xi In addition to these microscopic studies, reconstructions according to the Bourne method have been employed. Such reconstructions, made at a magnification of lo or 12 diameters, reveal the intimate characters and dmiensional relations of the gray matter. By no other means is it possible to acquire so comprehensive a conception of the complex nature of the internal structure in the brain stem. Dr. Riley has devoted himself to this \\()rk for a number of years and as a result has produced a series of most illuminating reconstruction models which present this aspect of the subject so as to disclose beyond all question the major nuclear movements in the evolutionary process of the primate brain. I am indebted to Dr. Riley not merely for the production of this collection of unsurpassed models, but equally for the careful descriptions which he has given of them in the eleven chapters dealing with this phase of the subject. For the purpose of further checking the proportional relations of the various structures m the brain stem, several methods of mensuration have been employed. On the basis of these measurements, two series of coefTicients were established: first, planimctric coefficients in which the transverse proportions of a given structure were estimated in relation to the entire cross section by means of a planimeter. This method produced a series of figures which afford a basis of metric comparison of homologous structures. For the painstaking work in these protracted planimetric calculations I am indebted to Mrs. Seymour Basch, whose repeated measurements and remeas- urements of each structure required the most exacting application and produced results which are probably as close an approximation to actual facts as may be obtained. Second, estimates for longitudinal coefficients were made with the purpose of determining the relative length of each structure to the entire length of the brain stem. While it is probably true that no method of mensuration may express adequately the relative proportions of the several structures considered xil PREFACE in the brain stem, it seems fair to presume that by means of these two sets of calculations a fair degree of accuracy has been attained. Even though every method which reasonably could be appHed to the material has been used in controlling these observations, the fact still remains that this study lacks detailed investigation of the cellular elements of the various nuclear components. The addition of this detail must be left for later consideration, as the inclusion of such studies would of necessity too greatly expand the size of the present work. In dealing with the brain stem the treatment has not been that of an atlas, but rather that of a critical review with especial emphasis upon the structures having the greatest evolutional significance. In the selection of such structures two groups were recognized : First, those more plastic elements of recent acquisition which have been especially susceptible to adaptive influences. Second, the more inflexible, archaic components of the brain stem, which because of their great antiquity have acquired fundamental stability and respond but little to the influences of adaptation. Chief attention is directed to the first group from which a surprisingly rich harvest of evidence has been gathered. The addition of the chapters dealing with the cultural phases of human development and the brain of prehistoric man was made upon the suggestion and with the assistance of Professor Osborn. Prof. William K. Gregory of Columbia University has given me most generous assistance and I am especially grateful to him for his careful read- ing and criticism of the concluding chapters. I also wish to express my appreciation to Miss Christine D. Matthews of the American Museum of Natural History for her revisions in the tabu- lation showing the fossil remains of prehistoric man. Throughout the entire preparation of the manuscript I have had the PREFACE xiii able and sympathetic assistance of Mrs. Alice G. Margulies, whose eflorts in assembhng the bibliography have provided valuable addenda for those desiring further references to the literature. To make due acknowledgment for all of the assistance I have received would necessitate setting forth a long list of my friends and associates. Among these, however, I cannot fail to mention Mr. Frank N. Doubleday, whose invaluable advice has led to many important revisions in and addi- tions to the text. It also gives me great pleasure to mention the indispensable services of Miss Regina linger in preparing the serial sections of the primate brains upon which this study is based. Miss Unger has for a number of years been engaged in the technical production of brain series, particularlv Pal-Weigert preparations, which constitute one of the most highly valued accessions of the Neurological Department in Columbia University. Without these speci- mens, neither the microscopic nor reconstruction studies would have been possible. In the preparation of the illustrations and text for publication, Miss Florence Fuller of the publishers' staff has given most enthusiastic and helpful attention. And, finally, to my publisher, Mr. Paul B. Hoeber, I wish to acknowl- edge my deep indebtedness for his generosity and for the encouragement and unfailing inspiration which he has given me at all times. F. T. New ^'ork, N. Y. March, 1928. T FOREWORD "^HE discoveries in astronomy during the last decade have not only widened the boundaries of our own universe but have revealed the existence of universes far beyond our own. These discoveries prove that in our own and the outer universes exactly the same physical and chem- ical principles prevail, namely, gravitation, heat, hght, the genesis of new compounds, development from nebulous and active phases mto dark and inert phases of death. Marvelous as are these recent discoveries in astronomy, they are becom- ing comprehensible because of the uniformity of the laws and principles revealed to man through centuries of research. In brief, physics, astronomy and chemistry are ahke coming within the field of exact science capable of measurement, calculation, prediction and prophecy. What a contrast is presented in the biological sciences, ancient and modern ! With a wide circle of astronomic friends and with the most intense admiration for the achievements of astronomy and pure mathematics, I yet believe that their problems are not nearly so difficult or so baffling as our problem. In anatomy, in physiology, in pathology, in heredity we have not yet reached even the threshold of exactitude. With increasing energy, refinement and ingenuity, we know all the organs revealed in com- parative and human anatomy, in both their grosser and their finer structure. We know also the history of the rise of many of these organs in the course of past time and what their functions and relations are, but there is always the Great Beyond of the unknown, and perhaps unknowable, which is summed up in the word life. Of all incomprehensible things in the universe Man stands in the front rank, and of all incomprehensible things in Man the supreme difficulty centers in human intelligence, human memory, human aspirations, human powers of xvi FOREWORD discovery, research, and conquest of obstacles. The approach to this unknown field of future human advance — the seat of the human mind and the con- stitution of the human mmd — is along the great paths of human and com- parative anatomy and of human and comparative psychology. This volume contains the basis of what to our Icnowledge is the first profound study of the genesis of the intimate or internal structure of the human brain in comparison with the brains of animals more or less nearly related to man. It is a summary of Frederick Tilncy's lifework along largely new and original paths, pursued with the most unremitting intelligence and energy and yielding a result of exceptional breadth, precision and exactitude which affords a new and strong ground on which neurologists, psychologists, pathologists, and students of animal and human behavior may advance further into the unknown. May we not add a further A\<)rd of welcome to this splendid mono- graph at a time when we are mourning the loss of George Sumner Huntington, who laid the foundations of comparative anatomy for the School of Anatomy of Columbia University and who unfortunately passed away without witness- ing the consummation of the research and endeavors of one of his most distinguished students, namely, the linking of Man in all his parts and functions with the long lines of his ancestry. Henry Fairfield Osborn New York, N. Y. •*■ March, 1928. CONTENTS VOLUME I Page Preface vii Foreword xv List of Illustrations xix Introduction. The Primates: Lemurs, Monkeys, Apes and Man, Their Place in Nature i Part I. The Lower Primates Introduction 21 Chapter I. Lemur Mongoz, Its Brain and Behavior 23 II. Reconstruction of the Gray Matter in the Brain Stem of Lemur Mongoz . 73 III. Tarsius Spectrum, Its Brain and Behavior 85 IV. Reconstruction of the Gray Matter in the Brain Stem of Tarsius Spectrum 135 V. Callithrix Jacchus, the Marmoset, Its Brain and Behavior 153 VI. Reconstruction of the Gray Matter in the Brain Stem of Callithrix Jacchus 183 VII. Mycetes Seniculus, Its Brain and Behavior 191 VIII. Reconstruction of the Gray Matter in the Brain Stem of Mycetes Seniculus 233 IX. Comparative Summary of Structures Having Evolutional Significance in the Brain Stems of the Lower Primates 243 Part II. The Intermediate Primates Introduction 287 Chapter X. Papio Cynocephalus, the Common Dog-Headed Baboon, Its Brain and Behavior 289 XL Reconstruction of the Gray Matter in the Brain Stem of Papio Cynocephalus 335 XII. Pithecus Rhesus, Macacus Rhesus, Its Brain and Behavior 349 XIII. Reconstruction of the Gray Matter in the Brain Stem of Pithecus Rhesus 391 XIV. Hylobates Hoolock, the Gibbon, Its Brain and Behavior 405 XV. Reconstruction of the Gray Matter in the Brain Stem of Hylobates Hoolock 447 XVI. Comparative Summary of Structures Having Evolutional Significance in the Brain Stems of the Intermediate Primates. . . . , 457 3039^ J xviii CONTENTS VOLUME II Part III. The Higher Anthropoids Pace Introduction 477 Chapter XVII. Simia Satyrus, the Orang-Outang, Its Brain and Behavior 479 XVIII. Reconstruction of the Gray Matter in the Brain Stem of Simia Satyrus. . 533 XIX. Troglodytes Niger, the Chimpanzee, Its Brain and Behavior 545 XX. Reconstruction of the Gray Matter in the Brain Stem of Troglodytes Niger 609 XXI. Troglodytes Gorilla, Its Brain and Beha\ior 623 XXII. Reconstruction of the Gray Matter in the Brain Stem of Troglodytes Gorill 685 XXIII. Comparative Summary of Structures Having E\olutional Significance in the Brain Stems of the Higher Anthropoids 699 Part IV. Man Introduction 729 Chapter XXIV. From Primitive to Modern Man 731 XXV. The Brain of Modern Man 777 XXVI. Reconstruction of the Gray Matter in the Human Brain Stem 837 XXVII. The Brain of Prehistoric Man 861 XXVIII. Man — Past, Present and Future 925 Part V. Evolutional Modifications of the Primate Cerebrum Culminating IN the Human Brain Introduction 939 Chapter XXIX. The Significance of the Structural Homogeneity and Specific Modifications in the Primate Brain. Their Relations to the Progressive Adaption of Behavior 941 XXX. The Internal Structure of the Brain Stem of the Primates. Its Evolutional Modification in Relation to the Development of Behavior. Essential Similarities in Internal Elements 993 References for Further Reading 1047 Inde.x to Volumes I and II 1077 LIST OF ILLUSTRATIONS Figure Pace 1. Brains of lower vertebrates compared with the human brain 4 2. Brains of mammals compared with the human brain 5 3. Brains of apes compared with the human brain 6 4. Comparison of slceletal structures, from fish to man 8, 9 5. Lemur group in Madagascan habitat 25 6. 7. Two views of Lemur mongoz 27 8-1 1. Hand and foot of Lemur mongoz. . 30, 31 12-15. Hand and foot of Lemur potto 32, 33 16. Dorsal surface of brain, Lemur mongoz 34 17. Base of brain, Lemur mongoz 35 18. Left lateral surface of brain. Lemur mongoz 36 19. Right lateral surface of brain. Lemur mongoz 37 20. Ventral surface of brain stem. Lemur mongoz 38 21. Dorsal surface of brain stem. Lemur mongoz 39 22. Lemur mongoz. Level of the pyramidal decussation 42 23. Lemur mongoz. Level of caudal extremity of inferior olive 46 24,25. Lemur mongoz. Level through middle of inferior olive 48,49 26. Lemur mongoz. Level of the vestibular complex 50 27. Lemur mongoz. Level of the cerebellar nuclei 52 28. Lemur mongoz. Level of emergent fibers of sixth nerve 55 29. Lemur mongoz. Level of caudal extremity of pons Varolii 57 30. Lemur mongoz. Level of the middle of the pons Varolii 60 31. Lemur mongoz. Level of emergence of trochlear nerve 61 32. Lemur mongoz. Level of the inferior colliculus 62 33. Lemur mongoz. Level of the superior colliculus 66 34. Lemur mongoz. Level of the optic chiasm 69 35. Lemur mongoz. Level of the anterior commissure 70 36. Ventral surface of gray matter of brain stem. Lemur mongoz (color) 75 37. Dorsal surface of gray matter of brain stem. Lemur mongoz (color) 77 38. Lateral surface of gray matter of brain stem. Lemur mongoz (color) 79 39. 40. Two views of Tarsius spectrum 86 41-44. Hand and foot of Tarsius spectrum 88, 89 45. Dorsal surface of brain of Tarsius spectrum 95 46. Base of brain, Tarsius spectrum 96 47. Right lateral surface of brain, Tarsius spectrum 98 48. Ventral surface of brain stem of Tarsius spectrum 100 49. Dorsal surface of brain stem of Tarsius spectrum loi 50. Tarsius spectrum. Level of the first cervical segment 102 XIX XX LIST OF ILLUSTRATIONS Figure 51. Tarsius spectrum. Level of the pyramidal decussation 52. Tarsius spectrum. Level of the dorsal sensory nuclei 53. Tarsius spectrum. Level of caudal tip of inferior olive 54. Tarsius spectrum. Level through middle of inferior olive 5y. Tarsius spectrum. Level of the vestibular complex 56. Tarsius spectrum. Level of caudal extremity of trapezoid body 57. Tarsius spectrum. Level of the cerebellar nuclei 58. Tarsius spectrum. Level of the trochlear emergence 59. Tarsius spectrum. Level of the inferior colliculus 60. Tarsius spectrum. Level of the superior colliculus 61. Tarsius spectrum. Level of the optic chiasm 62. Tarsius spectrum. Level of the anterior commissure 63. Ventral surface of the gray matter of the brain stem, tarsius spectrum (color) . . 64. Dorsal surface of the gray matter of the brain stem, tarsius spectrum (color) . . . 65. 66. Lateral surface of the gray matter of the brain stem, tarsius spectrum (color) 141, 67, 68. Callithrix jacchus (marmoset) 69-72. Hand and foot of marmoset 156, 73. Dorsal surface of brain, Callithrix jacchus (marmoset) 74. Base of brain, Callithrix jacchus (marmoset) 75. Left lateral surface of brain, Callithrix jacchus (marmoset) 76. Right lateral surface of brain, Callithrix jacchus (marmoset) 77. Ventral surface of brain stem, Callithrix jacchus (marmoset) 78. Dorsal surface of brain stem, Callithrix jacchus (marmoset) 79. Marmoset. Level of the pyramidal decussation 80. Marmoset. Level of the dorsal sensory nuclei 81. Marmoset. Level of the tip of the inferior olive 82. Marmoset. Level through the middle of the inferior olive 83. Marmoset. Level of the vestibular nuclei and tuberculum acusticum 84. Marmoset. Level of the cerebellar nuclei 85. Marmoset. Level at the middle of the pons Varolii 86. Marmoset. Level of the inferior colliculus 87. Marmoset. Level of the superior cerebellar peduncular decussation 88. Marmoset. Level of the superior colliculus 89. Marmoset. Level of the optic chiasm 90. Marmoset. Level of the anterior commissure 91. Ventral surface of the gray matter of the brain stem, Callithrix jacchus (color). . 92. Dorsal surface of the gray matter of the brain stem, Callithrix jacchus (color) . . 93. Lateral surface of the gray matter of the brain stem, Callithrix jacchus (color) . . 94. Habitat group of Mycetes seniculus, the red howling monkey 95. 96. Two views of Mycetes seniculus, showing prehensile tail 97, 98. Two views of Mycetes seniculus, the red howling monkey 99. Habitat group of Ateles ater, the spider monkey 100. Ateles ater, the spider monkey, showing prehensile tail Page 04 LIST OF ILLUSTRATIONS xxi Figure P^ce 101-104. Hand and foot of Mycetes seniculus 198, 199 105-108. Hand and foot of spider monkey 200,201 109,110. Distal extremities of prehensile tails, Mycetes seniculus and Ateles ater. . . 203 111. Dorsal surface of brain, Mycetes seniculus 204 112. Base of brain, Mycetes seniculus 205 113. Left lateral surface of brain, Mycetes seniculus 206 114. Right lateral surface of brain, Mycetes seniculus 207 115. Ventral surface of brain stem, Mycetes seniculus 208 116. Dorsal surface of brain stem, Mycetes seniculus 209 117. Mycetes seniculus. Level of the pyramidal decussation 211 118. Mycetes seniculus. Level of the caudal extremity of the inferior olive 215 119. Mycetes seniculus. Level through the middle of the inferior olive 217 120. Mycetes seniculus. Level of the nucleus ambiguus 218 121. Mycetes seniculus. Level of the cerebellar nuclei 220 122. Mycetes seniculus. Level of the vestibular nuclei 221 123. Mycetes seniculus. Level of emergence of sixth nerve 222 124. Mycetes seniculus. Level through middle of pons Varolii 224 125. Mycetes seniculus. Level of the inferior colliculus 226 126. Mycetes seniculus. Level of the superior colliculus 228 127. Mycetes seniculus. Level of the optic chiasm 229 128. Mycetes seniculus. Level of the anterior commissure 230 129. Ventral surface of the gray matter of the brain stem, Mycetes seniculus (color) . 235 130. Dorsal surface of the gray matter of the brain stem, Mycetes seniculus (color) . . 237 131. Lateral surface of the gray matter of the brain stem, Mycetes seniculus (color) . 239 132. Habitat group. Baboon at the water hole 290 133. Papio cynocephalus (baboon) 291 134-137. Hand and foot of Papio cynocephalus 292,293 138. Dorsal surface of brain, Papio cynocephalus 296 139. Base of brain, Papio cynocephalus 297 140. Right lateral surface of brain, Papio cynocephalus 298 141. Left lateral surface of brain, Papio cynocephalus 299 142. Ventral surface of brain stem, Papio cynocephalus 300 143. Dorsal surface of brain stem, Papio cynocephalus 301 144. Baboon. Level of the pyramidal decussation 306 145. Baboon. Level of caudal extremity of nucleus of Burdach 309 146. Baboon. Level of the caudal extremity of the inferior olivary nucleus 311 I4-". Baboon. Level through the middle of the inferior olive 313 148. Baboon. Level of the vestibular nuclei 316 149. Baboon. Level of the cerebellar nuclei 319 150. Baboon. Level of emergent fibers of sixth nerve and caudal fibers of pons. . . 321 151. Baboon. Level at middle of pons, showing entering trigeminal fibers 324 152. Baboon. Level of inferior colliculus showing emergence of fourth nerve 326 153. Baboon. Level of the inferior colliculus 327 xxii LIST OF ILLUSTRATIONS Figure Page 154. Baboon. Level of the superior colliculus 330 155. Baboon. Level of the optic ehiasm 332 156. Baboon. Level of the anterior commissure 333 157. Dorsal surface of the gray matter of the brain stem, Papio cynocephalus (color) . 33" 158. Lateral surface of the gray matter of the brain stem, Papio cynocephalus (color) . 339 159. Ventral surface of the gray matter of the brain stem, Papio cynocephalus (color) . 341 160. Macacus rhesus. Full-grown monkey and young 351 161-164. Hand and foot of ALacacus rhesus 352,353 165. Dorsal surface of brain of Macacus rhesus 358 166. Base of brain of Macacus rhesus 359 167. Left lateral surface of brain, Macacus rhesus 360 168. Right lateral surface of brain, Macacus rhesus ■ 361 169. Ventral surface of brain stem, Macacus rhesus 362 170. Dorsal surface of brain stem, Macacus rhesus 363 171. Macacus. Level of the pyramidal decussation 366 172. Macacus. Level of caudal limit of dorsal sensory nuclei 36^ 173. Macacus. Level of caudal limit of inferior olivary nucleus 369 174. Macacus. Level through middle of inferior olivary nucleus 3~2 175. ALicacus. Level of the vestibular nuclei 3"3 176. Macacus. Level of the cerebellar nuclei 376 177. Macacus. Level of the emergence of the sixth nerve 378 178. Macacus. Level through the middle of the pons Varolii 381 179. Macacus. Level of the emergence of the trochlear nerve 382 180. Macacus. Level of the inferior colliculus 384 181. Macacus. Level of the superior colliculus 386 182. Macacus. Level of the optic chiasm 387 183. Macacus. Level of the anterior commissure 388 184. Ventral surface of gray matter of brain stem, Pithecus rhesus (co/or) 393 185. Dorsal surface of gray matter of brain stem, Pithecus rhesus (co/or) 395 186. Lateral surface of gray matter of brain stem, Pithecus rhesus (co/or) 397 187. 188. Hylobates syndactylus (gibbon) 406,407 189-192. Hand and foot of Hylobates syndactylus 408,409 193. Dorsal surface of brain, Hylobates hoolock 414 194. Base of brain, Hylobates hoolock 415 195. Left lateral surface of brain, Hylobates hoolock 416 196. Right lateral surface of brain, Hylobates hoolock 417 197. Ventral surface of brain stem, Hylobates hoolock 418 198. Dorsal surface of brain stem, Hylobates hoolock 419 199. Gibbon. Level of the pyramidal decussation 422 200. Gibbon. Level of the caudal extremity of the inferior olive 425 201,202. Gibbon. Level through the middle of the inferior olive 428,429 203. Gibbon. Level of the vestibular nuclei 43 1 204. Gibbon. Lc\el of the cerebellar nuclei 433 LIST OF ILLUSTRATIONS xxiii Figure Page 205. Gibbon. Level of the emergence of the sixth cranial nerve 436 206. Gibbon. Level through the middle of the pons 437 207. Gibbon. Level of the emergence of the trochlear nerve 438 208. Gibbon. Level of the inferior colliculus 440 200, 210. Gibbon. Level of the superior colliculus 442, 443 211. Gibbon. Level of the optic chiasm 444 212. Gibbon. Level of the anterior commissure 445 213. Ventral surface of gray matter of brain stem, Hylobates hoolock (cofor) 449 214. Dorsal surface of gray matter of brain stem, Hylobates hoolock (co/or) 451 215. Lateral surface of gray matter of brain stem, Hylobates hoolock (color) .... 453 216. Habitat group, orang-outang, Sadong River, Borneo 481 217. Orang-outang, erect posture, showmg disproportion of arms and legs 482 218. Orang-outang, showing anthropomorphous tendencies in head and face 483 219-222. Hand and foot of the orang-outang 484, 485 223A and B. Dorsal surface of brain, orang-outang 488, 489 224A and B. Base of brain, orang-outang 490, 491 225A and B. Left lateral surface of brain, orang-outang 494, 495 226a and b. Right lateral surface of brain, orang-outang 496, 497 227. Ventral surface of brain stem of orang-outang 500 228. Dorsal surface of brain stem, orang-outang 501 229. Orang-outang. Level of the pyramidal decussation 502 230. Orang-outang. Level of the caudal extremity of the dorsal sensory nuclei .... 505 231. Orang-outang. Level of the caudal extremity of the inferior olive 508 232. Orang-outang. Level through middle of inferior olive 511 233. Orang-outang. Level of the vestibular nuclei 514 234. Orang-outang. Level of the cerebellar nuclei 516 235. Orang-outang. Level near caudal limits of pons, emergent sixth nerve fibers. . 518 236. Orang-outang. Level of the middle of the pons Varolii 521 237. Orang-outang. Level of emergence of trochlear nerve 522 238. Orang-outang. Level of the inferior colliculus 524 239. Orang-outang. Level of the superior colliculus 526 240. Orang-outang. Level of the optic chiasm 530 241. Ventral surface of gray matter of brain stem, Simla satyrus (color) 535 242. Dorsal surface of gray matter of brain stem, Simla satyrus (color) 537 243. Lateral surface of gray matter of brain stem, Simla satyrus (color) 539 244. Habitat group, chimpanzee 54-7 245. 246. Chimpanzee Susie 548, 549 247-250. Hand and foot of chimpanzee 552, 553 251A and B. Dorsal surface of brain, chimpanzee 560, 561 252A and B. Base of brain, chimpanzee 562, 563 253A and B. Left lateral surface of brain, chimpanzee 568, 569 254. Ventral surface of the brain stem, chimpanzee 5-2 255. Dorsal surface of brain stem, chimpanzee 5-3 xxiv LIST OF ILLUSTRATIONS Figure Pace 256. Chimpanzee. Le\cl of the pyramidal decussation 577 257. Chmipanzee. Level oT caudal extremity of interior oIi\e 579 258. Ciiimpanzee. Level through the middle of the inferior olive 581 259. Chimpanzee. Level of the vestibular nuclei 584 260-262. Chimpanzee. Level of the cerebellar nuclei 587, 588, 589 263. Chimpanzee. Level of caudal limit of pons, showing emergent sixth nerve fibers 593 264. Level through pons, showing emerging sixth nerve fibers 594 265. Chimpanzee. Level through the middle of the pons Varolii 596 266. Chimpanzee. Level of the emergence of the fourth nerve 597 267. Chimpanzee. Level of the inferior colliculus 598 268. Chimpanzee. Level of the superior colliculus 602 269. Chimpanzee. Level of the lateral geniculate body 604 270. Chimpanzee. Level of the optic chiasm 606 271. Chimpanzee. Level of the anterior commissure 607 272. Ventral surface of gray matter of brain stem, Troglodytes niger (co/or) 611 273. Dorsal surface of gray matter of brain stem, Troglodytes niger (co/or) 613 274. Lateral surface of gray matter of brain stem, Troglodytes niger (color) 615 275. Gorilla group 624 276. Large male gorilla killed in the African Congo 625 277. Gorilla gorilla 627 278. Bronze statue of gorilla and woman 628 279. John Daniel in an amiable attitude 629 280. Six-year-old gorilla, John Daniel 633 281. Cast of gorilla taken in last hunt of Mr. Carl Akeley 634 282. ^'oung gorilla, John Daniel, showing breast-beating act 636 283. Adult male gorilla 639 284-287. Casts of hand and foot of the gorilla, John Daniel 640,641 288, 289. Casts of hand and foot of an adult male gorilla 642 290, 291. Casts of the hand and foot of the gorilla, John Daniel 643 292A and B. Dorsal surface of brain, gorilla 646, 647 293A and B. Base of brain, gorilla 648, 649 294A and B. Left hemisphere of brain, gorilla 650, 651 295. Ventral surface of brain stem, gorilla 654 296. Dorsal surface of brain stem, gorilla 655 297. Gorilla. Level of the pyramidal decussation 658 298. Gorilla. Level of the dorsal sensory nuclei 659 299. Gorilla. Level of caudal extremity of inferior olive 661 300. Gorilla. Level of the middle of the inferior olive 664 301. 302. Gorilla. Level of the vestibular nuclei 668, 669 303,304. Gorilla. Level of the cerebellar nuclei 670,671 305. Gorilla. Level of the emergence of the sixth cranial nerve. 673 306. Gorilla. Level through the middle of the pons Varolii 675 307. Gorilla. Level of the emergence of the trochlear nerve 677 LIST OF ILLUSTRATIONS xxv Figure Page 308. Gorilla. Level of the inferior colliculus 678 309. Gorilla. Level of the superior colliculus 680 310. Gorilla. Level of the optic chiasm 683 311. Gorilla. Level of the anterior commissure 684 312. Ventral surface of gray matter of brain stem, Troglodytes gorilla (co/or) .... 687 313. Dorsal surface of gray matter of brain stem, Troglodytes gorilla (co/or) .... 689 314. Lateral surface of gray matter of brain stem, Troglodytes gorilla (co/or) .... 691 315-318. Four extinct races of prehistoric man 732 319. Prof. Osborn's estimates of man's antiquity, industries, arts and races 734 320. Restoration of Pithecanthropus compared with human and anthropoid skulls . . 736 321. Restorations of Heidelberg and Piltdown man by Professor McGregor 739 322. The Neanderthal race 741 323. The Cro-magnon race 742 324. Stone implements representing the several stages of Paleolithic culture 744 25. "Eoliths" from Piltdown, Sussex 746 326. Contrasts between implements of the Paleolithic and Neolithic Ages 748 327. Evolution of lance point through several stages of the Old Stone Age 750 328. 329. Manufacturing flints 752, 753 330. Implements and ornaments typical of Upper Paleolithic Age 755 331. Australian native 768 332. African pygmy group from the Belgian Congo 769 333. African negro 770 334. American Indian 772 335. Eskimo 773 336. Group of British scientists discussing the Piltdown skull 774 337A and B. Dorsal surface of brain. Homo sapiens 780, 781 338A and B. Base of brain. Homo sapiens 784, 785 339A and B. Left lateral surface of brain. Homo sapiens 792, 793 340. Ventral surface of brain stem. Homo sapiens 796 341. Dorsal surface of brain stem. Homo sapiens 797 342. Man. Level of the pyramidal decussation 801 343. Man. Level of caudal extremity of dorsal sensory nuclei 805 344. Man. Level of caudal extremity of inferior olivary nucleus 807 345. Man. Level through middle of inferior olivary body 809 346. 347. Man. Level of the vestibular nuclei 812, 813 348. Man. Level of the cerebellar nuclei 816 349. Man. Level of inferior portion of pons and emergence of sixth nerve 818 350. Man. Level of the middle of the pons Varolii 821 351. Man. Level showing the emergence of the fourth or trochlear nerve 825 352,353. Man. Level of the inferior colliculus 828, 829 354. Man. Level of the superior colliculus 831 355. 356. Man. Level of the optic chiasm 833, 834 357. Man. Level of the anterior commissure 835 xxvi LIST OF ILLUSTRATIONS Figure Page 358, 359. Lateral surface of gray matter of brain stem, Homo sapiens (color) . . 839, 841 360. Dorsal surface of gray matter of brain stem, Homo sapiens (color) 843 361. Ventral surface of gray matter of brain stem. Homo sapiens (co/or) 845 362-365. Four views of an endocranial cast of a modern human skull 863 366-3"!. Six views of the endocranial cast of Pithecanthropus erectus (Javan Ape- man) 870 372-374. Comparison ot endocranial casts of gorilla, Pithecanthropus and Homo sapiens ... 8~3 3"5. Functional localization of the brain outlined upon the left hemisphere of the endocranial cast of Pithecanthropus erectus 880 376-380. Five views of the endocranial cast of Eoanthropus dawsoni (Dawn man of Piltdown) 885 381. Functional localization of the brain outlined upon the left hemisphere of the endocranial cast of Eoanthropus. (Piltdown.) 890 382-387. Six views of the endocranial cast of Homo neanderthalensis 894 388. Neanderthal flint workers 896 389-393. Five views of the endocranial cast of the La Quina skull 900 394-399. Six views of the endocranial cast of the Gibraltar skull 905 400. Functional localization of the brain outlined upon the left hemisphere of Homo neanderthalensis (La Chapelle aux Saints) 907 401-406. Six views of the endocranial cast of Homo rhodcsiensis (Rhodesian man) . . 910 407. Functional localization outlined on left hemisphere of Homo rhodesiensis . . . 914 408-413. Six views of the endocranial cast of Homo sapiens of the Predmost race . . 919 414. Functional localization of the brain outlined upon the left hemisphere of the Predmost endocranial cast 921 415. Cro-magnon artists making fresco in cave of Font-de-Gaume 927 416. Neolithic men 931 41-7-422. Comparison of the endocranial casts of Pithecanthropus, Piltdown, Rho- desian, Neanderthal and Predmost with modern Homo sapiens 934 423-426. Size and configuration of the dorsal and lateral surfaces of the human brain compared with those of Lemur mongoz 946 427-430. Size and configuration of the dorsal and lateral surfaces of the human brain compared with those of Callithrix jacchus, the marmoset 949 431-434. Size and configuration of the dorsal and lateral surfaces of human brain com- pared with those of Mycetes seniculus 953 435-438. Size and configuration of the dorsal and lateral surfaces of the human brain compared with that of Papio cynocephalus, the baboon 956 439-442. Size and configuration of the dorsal and lateral surfaces of the human brain compared with those of Pithecus rhesus, the macacus 962 443-446. Size and configuration of the dorsal and lateral surfaces of the human brain compared with those of hylobates hoolock, the gibbon 967 447-450. Size and configuration of the dorsal and lateral surfaces of the human brain compared with those of Simia satyrus, the orang-outang 9"3 451-454. Size and configuration of the dorsal and lateral surfaces of the human brain compared with those of Troglodytes niger, the chimpanzee 979 455-458. Size and configuration of the dorsal and lateral surfaces of the human brain compared with those of Gorilla gorilla 987 459. Principal primate horizons, showing evolutional expansion of neopallium (color) 989 LIST OF ILLUSTRATIONS xxvif Figure Paci- 46o-4~o. Cross scc-tioii at le\cl ol pyramiiial dccussatioti in tlic comparative primate series ggf,^ gg-7 471-481. Cross section at the level of the caudal extremity of inferior oli\e in the com- parative primate series 1000, 1001 482-492. Cross section at the level of the middle of the inferior oli\e in the com- parative primate series 1004, 1005 493. Graphs constructed on the basis of the planimetric indices 1009 494-504. Cross section at the le\-el of the \ estibular nuclei in the comparati\-e primate series 1012, 1013 505-515. Cross section at the le\el of the emergence of the trochlear ner\e in the com- parative primate series 1016, loi 7 516. Graph based on planimetric indices of the pontile nuclei 1018 517-527. Cross section at the level of the inferior colliculus in the comparati\e primate series 1022, 1023 528-538. Cross section at the le\el of the superior colliculus in the comparati\e pri- mate series 1026, 1027 539. Graph based on planimetric indices of cerebral peduncle 1028 INTRODUCTION THE PRIMATES: LEMURS, MONKEYS, APES AND MAN THEIR PLACE IN NATURE THE BRAIN FROM APE TO MAN INTRODUCTION Relative Importance oj the Braifi as Evolutinjtal Evidence. Its Signijicance Compared with Other Parts oj the Body ^ HE brain is conceded to be the master organ of the body, the regulator of life, the source of human progress. In this capacity how has it contributed to the evolution of mankind? Has it, as some authorities assume, passed through suc- cessive evolutional stages to reach its highest development? Both of these questions imply the acceptance of the Evolutionary Theory. If this hypothesis be correct, the human brain should prove an important witness in its favor. It may be expected to retain conclusive evidence of the evolutional process in at least three particulars: 1. It should manifest signs of its primitive origin from the lowest vertebrates (Fig. i). 2. It should bear identifying marks of intimate association with animals of its own class, the mammals (Fig. 2). 3. It should also have many specific details in common with members of the Primate Order to which man belongs, together with lemurs, tarsiers, monkeys and apes (Fig. 3). A comparison of the human brain with that of its primate coordinates may thus become of utmost importance by disclosing the exact structural basis of such evolution as has occurred in man from his early prehuman beginnings. ALLIGATOR OSTRICH FIG. I. BRAINS OF LOWER VERTEBRATES COMPARED WITH THE HUMAN BRAIN. [4] horse: tllphant fig. 2. brains of mammals compared with the human brain. [5] CHIMPANZEE GORILLA FIG. 3. BRAINS OF APES COMPARED WITH THE HUMAN BRAIN. [6] INTRODUCTION 7 Other Body Tissues Which Re\eal Evolutionary Kinship the blood The actual existence of evolutionary kinship has ah^eady been revealed by many other organs and tissues of the body. The blood, for example, has been most outspoken in this respect. Human blood scrum injected into the rabbit produces what is known as anti-buman scrum. This latter serum affords an exceedingly delicate test for iiuman blood, either fresh or in the form of old and dried clots. If the blood tested be that of a domestic animal, such as the horse, sheep, pig or fowl, no reaction results from this scrum. Anti-horse serum, anti-sheep serum, anti-pig serum and anti-fowl scrum may be prepared in a similar manner. These are sensitive agents for detect- ing the presence of horse, sheep, pig or fowl blood. An anti-serum may also be made for all animals and tests thus provided to detect the blood of similar species. Investigations have shown that these blood tests may be employed to determine the degree of relationship existing between different kinds of animals. A prompt and often strong reaction is obtained only from the blood of the same species, while the blood of closely allied species, such as that of the horse and donkey, gives a weaker, slower precipitation. These precipitin tests support the supposition that there is some distant relation between the old-world monkeys, apes and man. They also make clear a less intimate relationship between the new-world monkeys and the human stock. The lowest animals classified among the primates, the lemurs, give no sign of blood relationship with man, although there is some indication of such a kinship with the lower new-world monkeys. Tarsius, however, is an exception to this rule. It bears a closer blood relation to man, simia and gib- bon than it does to macacus, the new-world monkeys or the cat. Hcmal tests demonstrate a relationship among all carnivora closer than exists between carnivores and ungulates, while cetacean forms (whales and c i^jgefh. FIG. 4. COMPARISON OF \ERTEBRATES SHOWING THE ESSENTIAL SIMILARITIES IN THE SKELETAL STRUCTURES, FROM FISH TO MAN. [9] 10 INTRODUCTION porpoises) are more closely allied to cud-chewing animals (ruminants) and pigs. Blood reactions indicate that a strong interrelationship characterizes all reptilian forms, although there is some serial grading in these afiinities as among turtles, crocodiles, lizards and snakes respectively. Avian blood tests show a striking similarity in the hemal constituency of all birds. This fact stands out in contrast to the mammalian blood which has a specific variability in the intensity of its reactions dependent upon the order and family of the animals tested. In general, the precipitin blood tests are among the most convincing proofs put forward in support of the evolutionary hypothesis. THE BONY SYSTEM The bony system of the body (Fig. 4) is also an illuminating recorder of evolutional kinship. The skeleton of the fore- and hindlimbs sheds much light on the modifications connected with the locomotor apparatus. In accord- ance with the limb specialization as fins, paddles, wings, hoofs, paws, claws, hands or feet, the general propinquities in kinship may to a certain extent be estimated. The size and shape of the skull are equally significant in deter- mining an exact classilication of species. THE MUSCULAR SYSTEM Another significant index of a generalized ground-plan in vertebrate organization is the muscular system. This system, while it demonstrates the great stability underlying the motor apparatus, discloses a flexibility capable of providing for the special needs of aquatic, amphibian, terrestrial and aerial life. It thus reveals propinquities in kinship amongst animals which have adjusted their locomotor apparatus to similar or closely allied conditions of environment. INTRODUCTION ii THE HEART AND LUNGS The heart and the kings afford important evidence of vertebrate relation- ship and evolution. The increase in specialization of the respiratory appara- tus, from the gill stages of the fish to the conditions of the mammalian puhrionary organs, is a most rehable means of distinction between classes and orders. Embryogenesis as Evidence of Evolutionary Kinship And iinally, embryogenesis, coming as a cuhiiination of reproductive activity, discloses the fact that all vertebrates — fishes, amphibians, reptiles, birds and mammals — however different their form, habitat, mode of Ufe and behavior — arc cast in a structural mold of development hiid down in accord with a common plan. Each department of structural organization in the body contributes adequate testimony of a common ancestry. But no one of them portrays more than a single phase of vertebrate adaptation. Thus the blood repre- sents the metaboHc and biochemical adjustment of the animal. The osseous system and the muscles are indicative only of motor and locomotor capabili- ties. The teeth reveal the feeding habits and part of the protective mecha- nism. The genito-urinary system bears witness to the variety of excretory differentiation and the type of reproductive specialization. Embryogenesis summarizes the process of structural unfolding and thus discloses the general morphological plan of organization. The Brain as the Most Comprehensive Record OF AN Evolutional Process The reasons why the brain contains the most comprehensive record of the evolutional process are readily perceived. As an organ its mllucnces pervade and dominate all other systems of the body. It is the great trans- 12 INTRODUCTION former of energy which integrates all parts into a harmoniously acting machine. It interprets ail impressions from without. It projects all impulses Irom withni. In its dual capacity of servant and master the brain has been peculiarly susceptible to the inlluence of that combination of factors described by Professor Osborn in his tetraplastic theory of evolution. The cerebrum has felt more, perhaps, than any other organ, the etlects of action, reaction and interaction. It has responded more extensively to its inorganic and vital environment because it comprises the most highly ditlerentiated tissue of the organism. It has been ecjually sensitive to the inlluence of hered- ity-chromatin and lile (ecological) environment. The Comparison of the Human Brain with THE Brains of Other Primates A structural comparison ol the human brain with the brains of other members ot the primate order illustrates in detail the progressive modifica- tions in this evolutionary process. In this comparison the brain ol man for many obvious reasons is placed at the head of this order. It has been com- pared with the brains of Troglodytes gorilla (gorilla), Troglodytes niger (chimpanzee), Simla satyrus (orang-outang), Hylobates hoolock (gibbon), Papio cynocephalus (baboon), Macacus rhesus (common old-world monkey), Mycetes seniculus (South American howling monkey), Callathrix jacchus (marmoset), Tarsius spectrum (tarsier) and Lemur mongoz (lemur). These species comprise representatives of the highest and lowest primates. The lemur represents the transition from some mammalian form which was predominantly arboreal. The weight, size and celerity of this animal adapt it especially to a lofty arboreal habitat. In most of the new-world monkeys the development of a prehensile tail greatly extends the range of motor activity. Due to increasing size and weight of body, the larger apes have shown a progressive tendency to desert their INTRODUCTION 13 more inaccessible arboreal retreats and approach nearer to the ground. The transition IVom semi-arboreal habitat to completely terrestrial life has culminated m the development of the erect posture and plantigrade loco- motion with tree climbing but an incident in bipedal adjustment. CERTAIN FUNDAMENTAL ADJUSTMENTS OF BEHA\TOR In making the comparison of these primate brains, the chief object has been to note the structural changes whereby the evolutional process has been advanced. Certain fundamental components of behavior have been simultaneously studied in relation to these structural modifications. These components include the following: The development of the prehensile tail, its recession and linal dis- appearance The progressive adaptation of the hand for manual performances other than those concerned in locomotion The adjustment of simultaneous movements in the eyes, head and hands necessary to the execution of skilled acts The gradual assumption of the erect posture The increase of volitional control of the arms and the legs The amplification ot postural regulation in the body and extremities The development and perfection of binocular vision The progressive readjustments of visual, auditory and equilibratory reactions in passing from arboreal to ground-living habits The modification of the automatic associated movements induced by such adjustments. In comparing these species one with another, it is possible to observe the reflection of certain behavioral adjustments as they are mirrored by the organic constituents of the brain. In some cases, the range of variation in these behavioral reactions is so wide as to create most outspoken differences. 14 INTRODUCTION Again, in closely allied species the contrast is not pronounced or convincing; but by comparing members in the primate group standing phyieticaily far apart from each other, the essential differences between them gather the full force of their cvohitional significance. They leave no room to doubt that a progressive functional development has gone hand in hand with impressive evohitional modifications in the structure of the brain. Bonds of Kinship among the Primates No attempt has been made to arrange the species in exact serial order. Such classification would be open to numerous objections. A brief panoramic view of the primates may, however, disclose the Inroad bonds of kinship which hnt: them together in the foremost place of the vertebrate phylum. One feature in this panorama is especially noteworthy. There has been a strong tendency among the vertebrates to take refuge in the trees, either to escape predacious contemporaries or otherwise to foster the opportunity to live. Its obvious purpose is to increase the measure of safety by extending the radius of retreat and making ascent from the ground a real biological advantage. modifications induced by tree-dwelling With the advent of the primates, this arboreal tendency was diverted into still another channel. Here it took the turn of affording an almost com- plete permanency of abode rather than a refuge in time of emergency. Such a change to more or less permanent tree-dwelling could but induce far- reaching modifications. Not only did this life create a new type of habitat; it at once enforced a new mode of transit over the leafy highways of the tree tops. In order to obtain proper adjustment for such transportation, both hand and loot acquired the qualities of prehensile organs. Locomotion of this kind eventuated in the development of quadrumanal characters, with the foot INTRODUCTION 15 departing from its more ancient patterns of hoof, paw or claw and assuming the nearly complete grasping powers of the hand. The tail also, in certain instances, participated in this specialization for arboreal locomotion. These physical modifications actuated by tree life were quite as emphat- ically augmented by psychic influences arising from the new habitat. The background of the lives of almost all of the primates has played a dominant role in molding their behavior. The primeval forests and jungles, in their perpetual semi-darkness, created a domain whose subtle powers made them- selves felt in all primate reactions. It might be that the forest stood on the edge of a wide plain with a clear opening from which to look across toward the distant hills. The psychological effect of these factors, it must be clear, determined certain decisive attitudes on the part of the animals toward their environment. For example, there could not fail to be an alluring temptation in the green plain and its expansive freedom. To venture into this open space, however, the early primates required mechanisms \\hich they did not as yet possess. Certain dangers lurked in and over the plain. Predatory creatures of every kind were there — reptile, mammal and bird — lying in wait for just such an excursion. For the time, at least, the semi-darkness was safer with its limited view and real security in the upper highways out of reach of these preying enemies. THE LEMURS AND TARSI ERS, SHOWING THE EARLIEST MODIFICATIONS OF PRIMATE STRUCTURE With the lemurs, perhaps, there came the first modifications of structure leading to the more conspicuous groups of the primate kind. These animals, with their slender, furry bodies, their fox-like heads, their widely separated eyes and bushy tails, showed in both hands and feet the beginning differen- tiation of lingers and toes. This adaptation marks the transition from some lower form of mammal to the primates. It was in an epochal modification i6 INTRODUCTION such as this that the primate order had its origin. Then began a tenancy of the trees which profoundly influenced these animals as they passed through their many and varied adaptations. Tarsius, in many respects even more than the lemurs, illustrates the eflects ot these new mlluences. THE CEBIDAE FORESHADOWERS OF A NEW RACE It is dillicult to discern the exact point at which the Hapalidae, most familiarly represented by the diminutive marmosets, took departure irom the early beginnings of this primate line. Their appearance on the scene was probably a retrogressive step in the development of the new-world monkeys. Their small size, their lack oi power both m body and brain, would scarcely permit of a dominating ad|ustment to then- envnonment. In many respects their organization was inferior to that of the lemurs. This is particularly true in their manual ditTercntiation, since the development of their fore- and hind- limbs tended much more toward claws than hands. Nevertheless, for all their insignificance, there was that about them which suggested the coming of a new race. The configuration of the head, the expression of the face, the relation of the eyes, the shape of the nose, the position of the mouth, all were prophetic of the more definitely ape-like tribes which were to follow. The arrival of the new-world monkeys at length intro- duced all of the major simian characters. Throughout the large family of the Cebidae, quadrumanal development is well established. Most of the members of this group have also acquired prehensile tails. INFLUENCES ACTIVATING MODIFICATION WHICH PRODUCED OLD-WORLD CONGENERS OF PRIMITIVE SIMIANS What influences activated the modification which produced the old- world congeners of these South American simians is not clearly understood. It may have been a progressive tendency to increase in weight, added to the INTRODUCTION i- waning importance of the tail. In any event, the old-world monkeys do not develop prehensile functions ni their tails, and many of them, like the baboons, have ventured out of the trees to hve more upon the ground. Most of the kxtter show a distinct dog-hkc dc\'elopment in mode of hfe, in size and in form of body. In the gibbons for the first time is estabhshed the abihty to walk upright, but with these animals forest life is still predominant in its inlluence. By means of their long arms they are capable ot swinging from branch to branch with a marvelous agility. The absence of the tail in them, and the partial assumption of the erect posture, are most significant as fore- runners of developments in the larger anthropoids. In these latter a great increase of weight enforced the habit of living nearer to the ground. The first of these large anthropoids, the orang-outang, still shows its ancestral adherence to the forest, although it is able at times to stand and run. The chimpanzee, while it has great cleverness in climbing, seeks the ground oftener and has learned to walk upon all fours, using the knuckles of the hand as a support in this act. Finally, the gorilla, often attaining the great weight of nearly four hundred pounds in adult life, has been compelled by this reason to seek the support of the earth and only at times makes use of the heavier portions of the trees for refuge or retreat. Still, so indomitable is the arboreal tendency', even in these great animals, that they have made but imperfect terrestrial adaptation and also at the same time they are onlv partially suited to tree life. Were it not for their prodigious strength, their limited arboreal activities would be even further curtailed. All of the man-like apes are capable of the upright posture, of standing and walking on the hind legs but in an awkward, inefTicient manner. This is due to the fact that the foot in all three of these animals maintains predomi- nant characters of the hand and affords at best an imperfect basis for bipedal locomotion. Yet, notwithstanding such limitations as long, dragging arms and inadequate feet, the great anthropoids are able to venture beyond the limit i8 INTRODUCTION of their forest home. They are partially equipped to cope with some of the dangers hirking without. Their forehmbs are in part freed from the responsi- bilities of locomotion and so made available for acts of self defense and even, to a considerable degree, for exploration. Actual progress in the direction of human specialization had its l^egin- nings in this recession of superarboreal tendencies and the estabhshment of a terrestrio-arboreal mode of life. Such a gradual recession may be seen in this panorama of the primates. It is apparent in the first indecisive yet promising steps that led the anthropoid out of his ancient forest dominion toward the inviting plain. And fnially, with the complete recession of arboreal life, there began that long journey destined to lead over every sea and into every land, until no region of the earth remained for further conquest; until the complete acquisition of the upright posture and the full development of the hand had more and more bent the forces of nature to the designs of the races of man. Br.\in Development Parallelixg the Progressive Development of Beha\ ior Step by step the brain has kept pace with these progressive alterations. Its newer, recently acquired portions reveal a much more conspicuous response to this progress than its less plastic, more archaic elements. But old parts and new alike, according to their varying degrees of susceptibility, bear the certain imprint of adaptive change. Among the primates the bio- logical formula determined by quadrumanal specialization and arboreal habitat has worked itself out through graded stages from lemur to man. The evolutional process of this gradual transition is disclosed by structural modifications of the nervous system. Beginning with the Lower Primates, such as the tarsiers, the advance may be traced through many intermediate phases of cerebral development to its ultimate goal in the human brain. PART I THE LOWER PRIMATES INTRODUCTION TO PART I 1 "^HOSE mammalian forms occupying the upper extremity of the vertebrate phylum have, since the time of Linnaeus, been known as Primates, or chief of mammals. It was Linnaeus who, recognizmg the anthropomorphous characteristics of this large group, created the term to designate their order. He further subdivided them into Humans, Simians and Prosimians. The Suborders in the Order of Primates At present the order of primates is arranged in three suborders: (i) Lemuroidea, (2) Tarsioidea and (3) Anthropoidea. The first of these includes all of the prosimian forms of the lemur kind, while the third comprises all families of actual simians and man. According to this arrangement the tarsiers occupy an intermediate position between the other two suborders. The following are the accepted families of these three suborders: Suborder Lemuroidea: Suborder Anthropoidea: Daubentonidae Hapalidae Nycticebidae Cebidae c T J -r • -J Lasiopvgidae buborder larsioidea: ''° T. ••I Hylobatidae 1 arsiidae -^ Simiidae Hominidae In this discussion a slight rearrangement has been made to serve the purposes of presentation and at the same time to observe the dictates imposed by certain morphological similarities in the brain. Accordingly, the primates are here considered in four distinct divisions: First, the lower priinates which comprise the lemurs, tarsiers and all of the new-world monkeys (including lUJ L ( 8 P ^ p V, 22 INTRODUCTION TO PART I marmosets and the Cebidae). The second group, or iiiterrnediate primates, includes all of the old-world Catarrhine monkeys with the exception of the three great anthropoid apes. The third group, called the higher anthropoids, includes the orang, the chimpanzee and the gorilla; while the sole occupant of the fourth group is Homo sapiens, although reference is made to certain features in the development of the several races of prehistoric man. HP Chapter I LEMUR MONGOZ, ITS BRAIN AND BEHAVIOR Its Position among the Primates; Measurements and Brain Indiees; Surface Appearance 0/ the Brain; Internal Structure of the Brain Stem in Cross Section "^HE lemurs, so called because of their nocturnal and ghostly habits, represent the lowest level of primate organization. In all, there are some fifty species of these animals referable to seventeen genera. Thirty-six of these species are indigenous to Madagascar and its small adjacent islands. The remaining species have their habitat in Ethiopian and oriental regions. It is a fact of much interest that the rest of the world is quite devoid of these animals, although fossil remains indicate that the lemurs were much more widespread throughout the globe in some earlier geological period. External Appearance of the Lemur (Figs. 5, 6 and 7) The external appearance of the lemurs justifies the establishment of a suborder to contain these animals. They are readily distinguished from the apes and monkeys constituting the higher suborder of primates. One impor- tant distinguishing feature is the head, which in lemurs is more like that of the fox and is drawn out into a long muzzle. The face lacks the humanoid expression characteristic even of the lower apes. The tail, which may be long, is not prehensile; cheek pouches do not develop, nor do integumental callos- ities occur, although all these characters are frequently met with in the apes. The lemurs have certain striking resemblances to other primates, especially in the opposable fingers and toes, flattened digits and pectoral mammae. In all lemurs the second toe is provided with a sharp nail entirely 24 THE LOWER PRIMATES unlike the other fingers and toes; w hile in tarsias the third toe also is furnished with a similar nail (Figs. 9 and 11). Certain important divergences exist between the suborders of primates, which involve the digits of the feet and hands. In lemurs the thumb and great toe are always well developed, while the second or third digit constantly manifests some abnormality; as, for example, a remarkable elongation of the third digit in Chiromys, or the complete absence of the index in Potto (Figs. 12 and 13). In the Anthropoidea, on the other hand, it is the hallux and the pollex \\hich are subject to most marked variation. In size the lemur is about equal to the domestic cat. The fur is thick and often woolly in texture. The eyes are large and prominent, the ears long, with tufts of hair on their upper portions. The arms are not c]uite as long as the legs. The tail, usually about half as long as the body, is sometimes inclined to be bushy. Fleshy pads appear on the palms of the hands and feet as well as upon the palmar surface of the fingers, thus permitting the animal to grasp the branches of trees with great tenacity. The Lemur's Habits in the W ild State Little is known of the lemur's habits in the wild state. It is not strictly nocturnal, for some species are seen in search of food during the day as well as at night. They go in troupes often consisting of many individuals. They are very noisy and live in the forests. One species alone frequents rocky places destitute of trees; namely. Lemur catta. The animals live upon fruits of various kinds, insects, birds' eggs and birds themselves when they can catch them. During the heat of the day they sleep with the head placed beneath the arm and the tail curled about the neck. When walking they go upon the hands and the feet both on the ground and in the trees, the tail being carried above the body in the manner of a balancing or steering organ. Their f )ur limbs are primarily used in locomotion and only to a limited Courtesy, AmtTuan Museum of Natural History FIG. 5. LEMUR GROUP IN MADAGASCAN HABITAT. 26 THE LOWER PRIMATES degree serve the purposes of other volitional acts. The simultaneous move- ments in their eyes, head and hands necessary to the execution of skilled acts show a much more hmited range of adaptation than in some of the higher apes. The tendency to assume the erect position or to accjuire a sitting posture, even in some partial degree, is not pronounced. Their development of binocular vision is much less advanced than m many of the Anthropoids. Activities of Lemur Mongoz Tlie first specimen of the Primates considered in this series is one of the subfamily Lemurinae, Lemur mongt)z. This animal is distributed along the northwest coast of Madagascar, from Baly to Marinda, and is also found in the islands of Anjouan, Comorro, Mohilla and Nossi-Be. It inhabits the forests and goes in large troupes, keeping to the uppermost branches of the highest trees. Its agihty in leaping from tree to tree is most remarkable and so rapid that it can with diflicuity be followed by the eye. Hunters say that it is easier to kill a bird on the wing than a lemur when leaping. If pursued, it has a habit of dropping suddenly from the topmost branches into the bushes, giving the hunter the impression that the animal has been killed. This impression, however, is soon dissipated upon seeing the lemur in another tree at a considerable distance from the spot where it fell. In the wild state the animals subsist largely upon bananas, but they are also fond of the brains of birds which they are most skillful in capturing. After fracturing the bird's skull with the teeth, as they might a nutshell, they suck the brain out of the brain-case. The lemur, however, does not eat the rest of the bird. From this description of its activities, the great agility of the animal is evident, as is also the fact that it lives in an environment requiring the utmost FIGS. 6 AND 7. Courtesy, American Museum of Natural History TWO VIEWS OF LEMUR MONGOZ. [27] 28 THE LOWER PRIMATES nicety in balancing and the greatest precision in all movements. Measurements and Indices of Lemur Mongoz The measurements of Lemur mongoz are: Total length 876 to 906 mm. Skull in its occipito-nasal diameter 76 to 82 mm. Skull in its bitemporal diameter 27 to 31 mm. Width of the brain case 35 mm. The dimensions of the brain, including the cerebellum and brain stem, are 45 mm. longitudinally and 32.5 mm. transversely. Total weight of the brain 18 gms. Total water displacement 17 c.c. Weight of the forebrain 14.5 gms. (including endbrain and interbrain) Weight of the midbrain i gm. Weight of the hindbrain 2.5 gms. Upon the basis of these figures the following encephalic indices have been computed for the several divisions of the brain: Forebrain index 81 per cent Midbrain index 5 per cent Hindbrain index 14 per cent A forebrain index of 81 per cent definitely aligns this animal with those forms in which the inception of manual development is already under way. The highest forebrain index of what may be called the submanual group, such as the dog, cat and horse, is 80 per cent. The lemur marks the transi- tion line between the manual and submanual groups. LEMUR MONGOZ 29 Surface Appearance of the Brain in Lemur Mongoz THE FISSURAL PATTERN Since it is the purpose of this comparative review to deal with the structural evidence of an adaptive unfolding in the brain, it will be impossible to present or discuss the morphological details with that completeness which Jiiight be expected in an atlas on the subject. Especial emphasis is laid upon structural features deemed to possess the greatest evolutional significance. On the other hand, the effort has been made to present the necessary topo- graphical correlations appearing in the several different levels of the brain. Thus, certam prominent features on the external surface of the brain appear to have such decisive evolutional significance as to require extended consideration. Other features are, so to speak, structurally incidental and are mentioned largely for purposes of identification and topography. The type of the cerebral hemisphere of Lemur mongoz (Eigs. 16 and 17) is gyrencephalic. It presents a relatively simple fissural pattern. There is a general tendency for the major fissure lines to arrange themselves about the Sylvian sulcus, as is the rule in many of the gyrencephalic mammals of the lower orders. On the other hand, the circumsylvian disposition of the sulci in lemur shows a distinct tendency to depart from that concentric arcuate arrangement so characteristic, for example, of the carnivores. The angulation of the fissure of Sylvius in lemur with the base line of the brain is between s^i^° and 60°. The definite circumferential arrangement of the other fissures which surround the lissure of Sylvius disappears to the extent that there may be found only a remnant of the postsylvian sulcus in the form of the sulcus parallelus or superior temporal fissure. The sulcus lateralis also has lost much of its circumsylvian disposition and now exists as the main portion of the interparietal fissure. A well-defined sulcus in the area of transition between the frontal and orbital surfaces constitutes the fronto-orbital sulcus 30 THE LOWER PRIMATES and indicates in a general way the position of the inferior frontal sulcus of the higher primates. A faint indentation on the boundary between the parietal and the frontal lobes marks the position of the sulcus centrahs, but no Courlc.sv. American Museum of Na!ural Hislory FIGS. 8 AND 9. HAND AND FOOT OF LEMUR MONGOZ. Left. Palmar surface of hand showing digitation, palmar creases, phalangeal pads and opposable thumb. Right. Plantar surface of foot showing plantar creases, digitation, long toes, plantar pads, distal phalanges and opposable great toe. lissural marking corresponding to the sulcus simiarum or any occipital marking on the lateral aspect of the brain is apparent. In fact, such expansion as has occurred in the differentiation of the occipital lobe confines itself almost exclusively to the mesial surface of the hemisphere. This primitive development of the occipital lobe leaves a large portion of the cerebellum still uncovered bj' the overhanging cerebral hemisphere. It thus determines a condition intermediate between the cerebral development in all the monkeys and apes (in which the cerebellum is completely overhung by the cerebral hemisphere) and in lower mammals such as the carnivores (in which the cere- LEMUR MONGOZ 31 bellum is not so covered). The hemisphere of lemur is in many respects intermediate and transitional between the lower mammals and the higher primates (Fig. 18). Courtesy, American Museum oj Natural Historic FIGS. 10 AND II. HAND AND FOOT OF LEMUR MONGOZ. Left. Dorsum of hand showing discrete digitation with tendency to syndactyle. Right. Dorsum of foot showing toe-nails, digitation with marked syndactyle, and opposable great toe. CONVOLUTIONS, LOBES AND OTHER SURFACE CHARACTERISTICS In the convohitions of the lemur brain only the first tendency may be seen to develop that gyral arrangemnct which is characteristic of the apes. The lobation of the hemispheres is correspondingly limited, the boundary between the frontal and parietal lobes existing as a faint indentation indi- cating the inception of the sulcus centralis. The division between the parietal and temporal lobes is more clearly defined in the boundary line established by the Sylvian fissure. The temporal lobe has shown but slight advance over what is charac- teristic of the carnivores, with the possible exception of the disappearance of 32 THE LOWER PRIMATES the rhinal fissure on the lateral aspect, and a sHght further protrusion down- ward of the tip of the temporal lobe. No actual distinction of an occipital lobe can be made upon the lateral surface although the markings on the mesial 4^k^ , tf Courtesy, American Museum oj Natural History FIGS. 12 AND 13. HAND AND FOOT OF LEMUR POTTO. Left. Palmar surface of liand showing rudimfntary development of index linger, palmar creases and opposable thumb. Right. Plantar surface of foot showing rudimentary second toe and opposable hallux. surface show that this specialization of the neopallium has already begun to make definite expansions toward the occipital pole of the hemisphere. On the orbital surface of the brain the two orbital concavities are fairly well marked. This surface has a general obliquity outward, which increases the prominence of this concavity and permits the orbital plane to pass over into the frontal surface without sharp angulation in the region of transition. The interorbital keels are well marked. The olfactory bulb is large and detachable for a certain distance, although the olfactory tract is short and not detachable. The latter shows its greatest development in its lateral roots LEMUR MONGOZ 33 extending backward into the Sylvian fossa and exposing a trigonum olfac- toriuni larger than in the higher primates. In the occipital region there is a deep occipital concavity, particularly about the midline, where the superior Courusy. Ameruan Museum o/ Natural History FIGS. 14 AND 15. H.\ND AND FOOT OF LEMUR POTTO. Left. Dorsum of hand showing finger-nails and moderate syndactyle. Right. Dorsum ol loot. Syndactyle not so conspicuous as in Lemur mongoz. vermis of the cerebellum is lodged in what may be called the postsplenial fossa. The cerebellum in its tentorial surface presents a prominent superior vermis which appears as a ridge-pole in this sharply gabled surface. Its lateral extension is short because of the limited cerebellar hemisphere. The occipital surface of the cerebellum shows the vermal portion as the most pronounced structure in this area, with two rather insignificant lateral extensions forming the hemispheres (Fig. 18). The cerebellum of the lemur shows none of the tendency to form a vallecula into which the vermis gradualK" sinks on account of the increasing 34 THE LOWER PRIMATES prominence of the lateral cerebellar lobes. In other words, the vermis cerebelli is equally as conspicuous as the lateral lobes. In the tentorial surface, the folial sulci extend without interruption from '^%.'> FIG. 16. DORSAL SURFACE OF BRAIN, LEMUR MONGOZ. [Actual length, 43 mm.] Key to Diagram, c. Sulcus Centralis; i-is. silvii. Fissure of Sylvius; sllc. temp, sup., Sulcus Temporalis Superior. the vermis to the lateral lobes. On the occipital surface, however, two lateral paramedian sulci interrupt the course of the interfolial grooves, as they pass from the vermis to the lateral lobes. THE BRAIN STEM The markings on the several surfaces of the brain stem are not so decisive as in many other species of primates (Fig. 20). The Oblongata. The oblongata upon its ventral surface presents a ventromedian sulcus and two ventrolateral sulci. In the more cephalic por- tion of the ventral surface the ventromedian sulcus is flanked on either side by the pyramids which, in turn, are separated from the inferior olivary eminences by the ventrolateral sulci. The pyramids become progressively less well marked as they approach their caudal extremity, at which point LEMUR MONGOZ 35 there are faint indications of the pyramidal decussation. Some crossing pyram- idal strands remain close to the surface and these partially interrupt the ventromcdian sulcus at this level. FIG. 17. BASE OF BRAIN, LEMUR MONGOZ. (White dots are due to salt precipitation in fixative.) The rather feeble relief both of the pyramids and the inferior olives is taken to indicate a comparatively low organization in spheres of action which these two structures represent. In the case of the relatively insignificant pyramids the cortico-spinal connection between the motor cortex and spinal cord must be regarded as correspondingly small and volitional control of motion but poorly developed; likewise, in the case of the inferior olive, the lack of surface prominence suggests a relatively low degree of organization in the reflex control of simultaneous movements in eyes, head and hands, and in the facilitation of the coordination of all skilled learned performances. The cephalic extremity of the oblongata on its ventral surface comes to an abrupt termination in the slight elevation produced by the pons Varolii, 36 FHE LOWER PRIMATES the line of demarcation between the pons and the oblongata giving rise to the bulbopontile sulcus. At the point where the ventromedian sulcus meets the bulljopontile iw^ S; FIG. 18. LEFT LATERAL SURFACE OF BRAIN, LEMUR MONGOZ. [Actual Length, 45 mm.] Key to Diagram, c, Sulcus Centralis; SULC. orb., Sulcus Orbitalis; si lc. temp, sup.. Sulcus Temporalis Superior. sulcus, there is a small bHnd pocket underlying the pons, the foramen cecum posticum. In the sulcus between the inferior olive and the pyramid the twelfth nerve emerges from the oblongata (Fig. 20). The Pons Varolii. Continuing further cephalad, the l)rain stem is characterized by the presence of a relatively narrow and flat transverse band, the pons Varolii, lying in front of which is the optico-peduncular space. This space is bounded cephalically by the optic chiasm and optic tracts, and caudally by the convergent fdjcrs of the cerebral peduncles. It contains the tuber cinereum, the region of attachment of the infundibular stalk, the postinfundibular eminence and the mammillary bodies. A fairly well-pronounced median groove in the pons marks the position of the basilar artery. In general, the degree of prominence attained by the pons Varolii has been regarded as indicative of neopallial development. The pons provides ultimate connection between the cerebral cortex and cerebellum, and is LEMUR MONGOZ 37 necessary to the coordinative regulation of skilled movements. The small size of the pons in lemur thus suggests a poor capacity on the part of the animal to acquire or execute even a comparatively snnple series of skilled acts. FIG. U). RIGHT LATERAL SURFACE OF BRAIN, LEMUR MONGOZ. [Actual Length, 45 mm]. Kev to Diagram, c, Sulcus Centralis; obl.. Oblongata; sulc. orb.. Sulcus Orbitalis; sulc. temp, sup.. Sulcus Temporalis Superior. The Dorsal Surface of the Brain Stem. The dorsal surface of the brain stem in its axial portion is concealed by the overlapping cerebellum which presents a vermis with t\\o small but well-marked lateral lobes. These lobes are clearly separated from the vermis by two distinct lateral paramedian sulci. The small cerebellum of lemur emphasizes its limited range of skilled performances, especially as regards skilled acts of the upper extremities in purposes other than those essential to locomotion (Fig. i6). When the cerebellum is removed by section through its peduncles, the dorsal aspect of the brain stem in its axial portion is revealed. The oblongata presents its typical ventricular and infraventricular portions. In the infra- ventricular area the dorsomedian seam indicates the line of fusion between the two alar plates of the bulb. The eminences of the clava and cuneus are not prominent. In the ventricular portion of the oblongata the gradual divarication ot the alar plates exposing the floor of the fourth ventricle follows the same general outline as in other mammals. The inferior triangle of the 38 THE L0\\ ER PRIMATES fourth ventricle is bounded laterally by a fairly prominent clava but a much less conspicuous cuneus. Both of these structures are most prominent in their caudal portions. At their cephaHc extremities they become less conspic- FIG. 20. \ENTRAL SURFACE OF BRAIN STEM, LEMUR MOXGOZ. [Actual Length, 35 mm.) Key to Diagram, cereb. peduncle. Cerebral Peduncle; optico.-ped.-space, Opticopeduncular Space; PYR. dec. Pyramidal Decussation; trap, body, Trapezoid Body. uous as elevations. The decrease in surface rehei is more marked in the cuneus than in the clava (Fig. 21). The relatively slight development of the cuneus may be interpreted as due to a sensory representation of the forehmb which, as yet, has gained no marked prominence. The cuneus itself consists of nerve fibers and cells related to the transmission of sensory impulses arising in the upper trunk and forelimb. The types of sensory impulses passing over this conduction path pertain to the discriminative quality of sensibility, i.e., tactile, muscle. LEMUR MONGOZ 39 joint and posture discrimination. Marked prominence of the cuncus is signilicant particularly of highly developed discriminative sensibility in the hand. Conversely, a lack of such prominence indicates an upper extremity FIG. 21. DORSAL SURFACE OF BRAIN STEM, LEMUR MONGOZ. [Actual Length, 35 mm.] Key to Diagram, d. med. fis., Dorsomedian Fissure; d.m.s., Dorsomedian Sulcus; inf. coll., Inferior Colliculus; SUP. CER. ped., Superior Cerebellar Peduncle; sup. coll., Superior Colliculus; tub. tr., Tuber- culum Trigemini. and hand still mainly employed in locomotion. Of the two dorsal columns in lemur, the clava and cuneus, the former seems to be structurally emphasized because it represents the path of sensory conduction not only from the leg but also from the important steering and balancing organ, the tail. The cuneus in lemur is indicative of a forelimb as yet wholly inadequate for advanced manual adaptation. A fairly well-defined dorsolateral sulcus extends upward upon either side of the dorsomedian seam, becoming most prominent in the region between 40 THE LOWER PRIMATES the cuneus and the chiva. Other markings on the dorsal surface, as well as upon the lateral surface of the oblongata, are faint and may only with difficulty be identified. The Hindbrain. The axial portion of the metencephalon presents upon its dorsal surface the cephahc continuation of the fourth ventricle, bounded upon either side by the middle and superior cerebellar peduncles. The markings on the floor of the fourth ventricle are not easy to discern. The median sulcus is prominent, running from the region just i^elow the obex cephalad to the beginning of the iter. Well-defined striae acusticae cross the floor of the ventricle at the level of the lateral recesses. Their direction from their point of entry toward the midline is almost at right angles to the median sulcus in the floor of the ventricle. Some fibers, however, have an obliquity caudad from their point of entrance toward the midline. In the inferior triangle of the fourth ventricle a shallow depression upon either side of the median sulcus indicates the position of the nucleus hypoglossus, lateral to which is a faintly marked fovea vagi. It is impossible to discover any distinctly demarcated region corresponding to the area postrema or area plumiformis. The cephalic triangle of the fourth ventricle is devoid of distinct mark- ings, although immediately above the striae acusticae may be seen a slight elevation, the eminentia abducentis. There is no evidence of a locus coeruleus. The Midbrain. The dorsal aspect of the midbrain presents the char- acteristic cfuadrigeminal plate with the median sulcus intersected at right angles by the intercollicular sulcus. At the cephalic extremity of the median sulcus there is a triangular expansion, the fovea pinealis, which lodges the epiphysis cerebri. The superior colliculi are somewhat larger than the inferior colliculi, but both collicular elevations are well-defined and prominent tectal features. LEMUR MONGOZ 41 Near the cephalic extremity of the midbrain on the lateral aspect of the diencephalon are the mesial and lateral geniculate bodies. The surface prominences on the ventral aspect of the midbrain, due to the presence of the two cerebral peduncles, present a relatively low relief. This fact is dependent upon the development in this animal of a small pyramidal system. It also indicates that the fibers coming from the cerebral cortex to reach the cerebellum, in the interest of maintaining the proper coor- dination of the animal's limited skilled movements, are not numerous. The markings upon the lateral aspect of the midbrain, with the excep- tion of the mesial geniculate body, are not pronounced. In fact, the entire relief of the structures appearing upon the mesencephalic surfaces, with the exception of those features already mentioned, conveys the impression that this is a particularly generalized portion of the neuraxis devoid of many of the striking features characteristic of the higher forms. Internal Structure of the Brain Stem in Lemur Mongoz Some features which have a certain degree of prominence in surface outline assume more significant proportions when seen in cross sections of the brain stem. The sections of the axis in the lemur about to be discussed provide a survey of the internal structure at all of its critical levels. These descriptions begin with the sections at the lower end of the stem and proceed to higher levels of the more expanded portions of the brain. LEVEL OF THE PYRAMIDAL DECUSSATION (FIG. 22) At the level of the pyramidal decussation the outstanding feature of the oblongata is the crossing of the pyramidal fibers (Pyx) with the consequent separation of the ventral gray column ( Ven) from the central gray matter (Cen) by the pyramidal fasciculi (Py). 42 THE LOWER PRIMATES The pyramidal bundles forming this decnssation arc relatively narrow, in this respect bearing out the impression gained from the surface relief of the pyramid which indicates a limited capacity for volitional control over FIG. 22. LEMUR MONGOZ. LEVEL OF THE PYRAMIDAL DECUSSATION. CB, Column of Burdach; cen. Central Gray Matter; CG, Column of Goll; dt, Deiterso-spinal Tract; fle. Dorsal Spinocerebellar Tract; cow, Ventral Spinocerebellar Tract; nr, Nucleus of Rolando; pv. Pyramidal Tract, Pyramidal Fibers; pvx. Pyramidal Decussation; ref, Reticular Formation; rst. Rubrospinal Tract; SPT, Spinothalamic Tract; trd. Descending Trigeminal Tract; ven, Ventral Gray Matter. [Accession No. 147.* Section 57. Actual Size, 9X5 mm. In all actual measurements fractions of the millimeter have been disregarded.] the animal's voluntary muscles. As a corollary it may be inferred that the small pyramidal system bespeaks a small ability on the part of the animal to acquire learned reactions. From observation of lemurs in captivity, it is apparent that they learn little more, either by imitation or repetition, than in the free state. *The numbers mentioned at the end of each caption in this and the following cross sections refer to the Study Collection of the Columbia University Neurological Department. LEMUR MONGOZ 43 Another feature is the appearance of the caudal extremity of the nucleus of Goil together \\'ith an extensive bundle t)f frbers which sur- rounds it and forms the column of Goll (CO). This broad field of myelinized axons makes a long ascent from the spinal cord, arising in those levels which represent the dermatomes and proprioceptors of the tail and lower extremity. In lemur, the nucleus of GoII and the surrounding fasciculus have their special significance in the fact that the adjacent bundle of libers m the column of Burdach is relatively more extensive. From this it may be inferred that the parts of the body represented by the column of Burdach possess more functional prominence than the parts represented by the more mesial collection of fibers in Goll's column. This idea gains support from the fact that the column of Burdach represents the upper extremity. In lemur the manual differentiation of the forelimb is still far from complete. Although all of the principal elements entering into the formation of the hand are present, the upper extremity is in a distinctly low manual stage. The disparity in size of these two columns speaks in favor of a less highly developed functional adaptation in the use of the hindlimb and the tail than in the forelimb and hand. Another feature in the dorsal region of the section is the marked expan- sion of the substantia gelatinosa trigemini (nucleus of Rolando) (NR) which serves as a relay station for all fibers bearing impulses from the trige;TiinaI areas of the face and head.. This innervation includes regions anterior to the interparietal line drawn across the vertex of the skull from one external auditory meatus to the other. In an animal which has not yet developed a hand to serve in capturing prey for food, the head, and partic- ularly the mouth, affords an important organ of offense and defense. Such an organ must be provided with an adequate sensory apparatus to guide it in these essential efforts of life. Its sensory role in directing the course of the animal as it makes its way through the leafy parts of the trees or through L_i LIBRARY -r, 44 THE LOWER PRIMATES the underbrush supplements the sense of sight and thus facihtates passage amid obstacles in the environment. For this reason the head and face are amply provided with a sensory equipment. The entire dorsal field of the section is fundamentally sensory in its significance. Thus a hne ch-awn transversely through the central canal bounds a territory which represents the animal's capacity in discriminative sensibility for the whole body. This territory may be spoken of as the dorsal field of sensory discriminuUon. It is the area ot the central nervous system which affords the best index of an animal's capacity in discriminative sensi- bility. Beginning at the more lateral portion of this sensor^' field are the descending tract of the fifth nerve (Trd) and the substantia gelatinosa (NR). These structures represent the head and face. The sensory representation of the neck and arm lies contiguous to the substantia gelati- nosa (NR), while occupying a mesial position, adjacent to the dorso- mesial septum, is the area concerned in sensory transmission from the tail and the leg ( CG ). Analysis of this sensory field in lemur indicates that innervation is more generously provided for the leg, tail, face and head than for the forelimb with its slightly ditlerentiated hand. The type of sensibility which this area serves is preeminently discriminative. It is particularly involved in finer adjustments of body posture and contact relations with the outside world. Hence it not only plays an important part in afl'ording discrimination in the analysis of elements encountered in the environment, but must also be regarded as the sensory substratum upon which the complexity of the skilled reactions depends. Other structures in this level need mention for topographical identifica- tion. Among these are the ventral gray column (Ven) which, because of the decussating bundles of the pyramid ( Pyx), has been detached from the central gray matter (Cen), the latter now lying in contact with the median line and surrounding the small central canal. Dorsolateral LEMUR MONGOZ 45 to the ventral gray column are the scattered bundles of the pyramidal tract ( Py) as they begin to assemble in the position occupied by them in their descent through the oblongata and into the spinal cord. On the periphery of" the section is a narrow band of medullary substance, the circumferential zone, which contains the two large ascending spinocerebellar tracts (Gow, Fie). Mesial to this zone and bordering upon the gray matter of the ventral gra\- column is the intermediate zone which in its ventral portion contains the two descending Deiterso-spinal tracts (DT) and, in its lateral portion, the spinothalamic and rubrospinal tracts (Spt, Rst). LEVEL OF THE CAUDAL EXTREMITY OF THE INFERIOR OLIVE (fIG. 23) At the level of the caudal extremity of the inferior olive the general appearance of the section has changed considerably. The chief altera- tion is due to the presence in the ventral field of a new collection of gray matter. This is the inferior olivary nucleus (10) which lies im- mediately dorsolateral to the liber bundles constituting the pyramid (Py). Dorsal to the olive is a smaller mass, the dorsal accessory olive. Situated in front of the central gray matter (Cen) are fibers (Fai) arising in the dorsal nuclear masses, which pass inward and forward to the midline where they undergo decussation. At this point they turn upward and constitute the mesial fillet. These decussating axons are internal arcuate fibers. Most of them take origin in the nucleus of Goll (NG) and contribute to the formation of the lowest portion of the mesial fillet which here represents the lower extremity and tail. The pyramid (Py) appears as a compact bundle occupying the most ventromesial portion of the section. Its transverse and anteroposterior diameters give it the proportion of about i to 1 1 to the entire section. This ratio conveys some impression as to the relative importance of the pyramidal system in the 46 THE LOWER PRIMATES regulation ol volitional movements. The dorsal eoiumns of'Goll and Buidaeh (CG, CB) are considerably increased in extent due to the presence in them of nuclear masses constituting the nucleus of Goll (NG) and the FIG. 23. LEMUR MONGOZ. LEVEL OF CAUD.\L EXTREMITY OF INFERIOR OLIVE. CB, Column of Burdach; cen, Central Gray Matter; CG, Column of GoII; dt, Deiterso-spinal Tract; fai. Internal Arcuate Fibers; fle. Dorsal Spinocerebellar Tract; cow. Ventral Spinocerebellar Tract; lo. Inferior Olive; NB, Nucleus of Burdacli; ng. Nucleus of Goll; nr. Nucleus of Rolando; pv, Pyramidal Tract and Fibers; ref, Reticular Formation; rst. Rubrospinal Tract; spt. Spinothalamic Tract; trd, Descending Trigeminal Tract. [Accession No. 147. Section 75. Actual Size, 10X6 mm.] nucleus of Burdach (NB). It is evident that the nucleus of Goll is smaller than that of Burdach. This appearance bears out the point already made as to the sensorj^ predominance of the more lateral of the two bundles in the dorsal field. The fibers representing the hand and arm are more numerous and constitute a more conspicuous bundle than those representing the leg. In a similar manner, the substantia gelatinosa of Rolando (,NR), as well LEMUR MONGOZ 47 as the descending tract of the fifth nerve (Trd), stand out as prominent elements and add to the preeminent sensory character of the dorsal field. Dorsal to the pyramidal tract and ventral to the fillet is an important bundle of fillers, namely the posterior longitudinal fasciculus, which is concerned with the automatic rellcxes for the regulation of certain primitive movements. Lateral to the fasciculus longitudinalis posterior is a considerable bundle representing the Dciterso-spinal tracts (DT), whose function is the conduction of those impulses necessary to the adjustments ol equilibrium. LEVEL THROUGH THE NHDDLE OF THE LNFERIOR OLI\ E (FIGS. 24 AND 25) At the level through the middle of the inferior olive, the outstand- ing feature is the appearance of an irregular mass of gray matter lying in the ventral iield near the pyramid. This is the inferior olivary nucleus (10). Its significance has already been touched upon in relation to the. automatic regulation of simultaneous movements of the head, eyes and hand and to the facilitation of the coordination of all skilled learned perform- ances. This structure has a direct connection, through the central tegmen- tal tract, with the oculomotor nuclei and, through the olivary fasciculus of Helweg, with the cervical levels of the spinal cord. By means of many arcuate fibers it is also connected with the cerebellum. \\ hilc there is no diOiculty in identifying this structure in lemur, it is smaller and much more indefinite in outline than in most of the other species considered. The lemur thus appears to be provided with but a small amount of automatic regulation of simultane- ous movements of hands, head and eyes. Mesial to the olive and adjacent to the midline of the section is the mesial fillet (Mf). This structure represents the continuation of the discriminative sensory pathway, being made up largely of fibers for sensation in the extremities. The substantia gelatinosa (.NR) and the accompanying descending trigeminal tract 48 THE LOWER PRIMATES (Trd) are still prominent, while the dorsal columns of Goll and Burdach again give evidence that the more mesial of these fasciculi, representing the leg and tail, is larger than the lateral one representing the upper extremity. FIG. 24. LEMUR MONGOZ. LEVEL THROUGH MIDDLE OF INFERIOR OLIVE. CB, Column of Burdach; do. Dorsal Accessory Olive; dt, Deiterso-spinal Tract; fle. Dorsal Spinocerebellar Tract; cow. Ventral Spinocerebellar Tract; 10, Inferior Olive; mf. Mesial Fillet; nb. Nucleus of Burdach; NBL, Nucleus of Blumenau; ng, Nucleus of Goll; nhv. Hypoglossal Nucleus; nr. Nucleus of Rolando; N12, Hypoglossal Nerve; pd, Predorsal Bundle; pl, Posterior Longitudinal Fasciculus; py. Pyramid; ref. Reticu- lar Formation; rst. Rubrospinal Tract; spt. Spinothalamic Tract; trd, Descending Trigeminal Tract; vo, Ventromesial Accessory Olive. [Accession No. 147. Section 102. Actual Size, 11 X 5 mm.) LEVEL OF THE VESTIBULAR NUCLEI (FIG. 26) At the level of the vestibular nuclei some noteworthy changes have occurred. They result primarily from the introduction of two additional sensory systems. One of these additional elements in the sphere of sensation, namely, the tuberculum acusticum (Tub) is related to the sense of hear- ing; the other, connected with the function of balancing, is the vestibular LEMUR MONGOZ 49 area. The tuberculum acusticum receives the fibers which arise in the cochlear portion of the internal ear. In lemur it is particularly h\rge, thus indicating a highly developed auditory sense. FIG. 25. LEMUR MONGOZ. LEVEL THROUGH MIDDLE OF INFERIOR OLI\E. CB, Column of Burdach; do. Dorsal Accessory Olive; fle, Dorsal Spinocerebellar Tract; Gow, Ventral Spinocerebellar Tract; lo. Inferior Olive; mf. Mesial Fillet; nb, Nucleus of Burdach; NFS, Nucleus Fasciculus Solitarius; ng, Nucleus of Goll; nhv. Hypoglossal Nucleus; nr, Nucleus of Rolando; nvd. Dorsal Vagal Nucleus; N12 Hypoglossal Nerve; pd, Predorsal Bundle, pl. Posterior Longitudinal Fasciculus; pv, Pyramid; REF, Reticular Formation; rst, Rubrospinal Tract; spt, Spinothalamic Tract; trd. Descending Trigeminal Tract; vo, Ventromesial Accessory Olive. (Accession No. 147. Section 1 17. Actual Size 1 1 X 4 mm.] In the region formerly occupied by the tracts and the nuclei for the conduction of impulses coming from the skin, the joints, the muscles and the bones of the extremities, there now appear two groups of cells and fibers for the reception of impulses arising in the proprioceptive organs of the vesti- bule; i.e., the semicircular canals, utricle and saccule. These highly specialized parts of the internal ear play a fundamental role in equilibrium. Near the midline, in the dorsal field, immediately beneath the lloor of the fourth 50 , THE LOWER PRIMATES ventricle, is the trianiiular nucleus of Schwalhe (NSc), and lateral to this a large-celled nucleus known as Deiters' nucleus (ND). Together they constitute the vestibular area. Dispersed among the cells of Deiters' nucleus FIG. 26. LEMUR MONGOZ. LEVEL OF THE VESTIBULAR COMPLEX. CTT, Central Tegmental Tract; dt, Deiterso-spinal Tract; gow. Ventral Spinocerebellar Tract; icp, Inferior Cerebellar Peduncle; mf, Mesial Fillet; nd, Nucleus of Deiters; nfs. Facial Nucleus; nr. Nucleus of Rolando; NSC, Nucleus of Schwalbe (Triangularis); n8. Auditory Nerve; pd, Predorsal Bundle; pl. Posterior Longi- tudinal Fasciculus; py, Pyramid; ref. Reticular Formation; rst, Rubrospinal Tract; spt, Spinothalamic Tract; trd. Descending Trigeminal Tract; tub, Tuberculum Acusticum. (Accession No. 147. Section 138. Actual Size, 14 X 4 mm.] are many scattered bundles of nerve fibers representing essential connections of this nucleus. The relative dimensions of the two vestibular nuclei to the rest of the cross section are of especial internst. They provide a basis for estimating the degree of balancing function possessed by the lemur, whose life is spent primarily in the loftiest parts of trees and whose locomotion is adapted to the constantly varying movements of the tree tops. The animal is able to execute difficult locomotor feats with great precision and finds as much security in its balancing under these most difFicult circumstances as the quadruped does upon terra lirma. It is cjuite in keeping, therefore, with the inherent behavioral requirements, to find the central representation of LEMUR MONGOZ 51 this important proprioceptive system so highly developed in these tree-Hving animals. Situated between the tubereuhini acusticum (Tub) and Deiters' nucleus is the restiform body (ICP) which represents an aggregation of ascending fibers from the spinal cord and oblongata on their way to the cerebel- him. These ascending cerebellar fibers come from several different sources and serve the purpose of conveying to the cerebelhim afferent impulses necessary, as it were, to orient the cerebelhim m its activity of transmitting to the muscles those impulses essential to coordination. An exact adjustment of action exists between the various groups of muscles in the body in order to maintain such coordination. Since the cerebellum is specialized to regulate the factors entering into this function, it naturally follows that the organ must at all times be in possession of information concerning the varying tensional status of all the muscles of the body. Were this not the case it would be impossible for the cerebellum to transmit to the muscles the ten- sional control essential to the execution of ail muscular acts. Thus the sig- nificance of the afferent cerebellar connections as indicated by the restiform body may be clearly discerned. The degree of development of this structure discloses the extent to which the animal is dependent upon its coordinating mechanism. LEVEL OF THE CEREBELLAR N'UCLEI (PIG. 2~) At the level of the cerebellar nuclei certain structures which fur- nish a criterion concerning the functional capacity of the cerebellum make their appearance for the first time. These are the cerebellar nuclei. Only a portion of the cerebellum is shown in this cross section. Structurally, the organ consists of a central division, the vermis (Ver), and two lateral lobes or hemispheres. The beginning of the lateral lobe is indicated in the cross section. Immediatelv mesial to the mass of medullarv substance 52 THE LOWER PRIMATES of the lateral lobe is a large, irregular and rather diffuse collection of cells, the nucleus dentatus (Ndt). From this nucleus arise the fibers which make their way out of the cerebeUum, providing the efferent pathway for FIG. LEMUR MONGOZ. LEVEL OF THE CEREBELLAR NUCLEI. CIR, Juxtarcstiform Body; ctt. Central Tegmental Tract; icp, Inferior Cerebellar Peduncle; lf, Lateral Fillet; nd, Nucleus of Dciters; ndt. Dentate Nucleus; nf, Facial Nucleus; nfg, Nucleus Fastigii; nod, Vermis Cerebelli; nr. Nucleus of Rolando; nsc, Nucleus of Schwalbe (Triangularis); ntr, Trapezoid Nucleus; N7, Facial Nerve; n8. Auditory Nerve; pd, Predorsal Bundle; PL, Posterior Longitudinal Fasciculus; py. Pyramid; ref, Reticular Formation; rst. Rubrospinal Tract; scp, Superior Cerebellar Peduncle; TRD, Descending Trigeminal Tract; trp. Trapezoid Body; ver, Vermis. [Accession No. 147. Section 163. Actual Size, 15 X 10 mm.] cerebellar impulses which regulate the coordinative control ut the muscles. The relative size of the dentate nucleus is consequently of much significance, as it indicates to what degree the cerebellum contributes to the tunction of LEMUR MONGOZ 53 coordination. TIic nucleus may be, as in this instance, an irregular, diffuse mass of gray matter, or it may present itself as a complexly convoluted structure whose convokitions serve to increase its functional capacity. An animal having a small dentate nucleus should be capable of a relatively limited range of highly coordinated acts. It is of interest to note that in lemur this nucleus is neither well defined nor extensive in size, from which it may be inferred that the degree of coordinative control dependent upon the cerebellum is relatively less in this animal than in some of the higher primates. Mesial to the dentate nucleus is another aggregation of nerve cells. This is the nucleus Jastigii (Nfg). By means of fibers constituting the juxtarestiform body (Cjr), it is connected with those nuclei in the floor of the fourth ventricle which receive impulses from the semicircular canals, utricle and saccule. The nucleus fastigii is, therefore, a structure intimately concerned with the function of balancing and may be regarded as one of the higher elaborating stations for equilibratory control. Its compara- tively large size in lemur has a significance similar to that of the large nuclei of Schwalbe and Deiters, unquestionably indicating the great need on the part of the animal for a highly organized balancing control. It is presumed that functionally the vermis (Ver) is concerned with the maintenance of coordinative control of the axial and paraxial musculature of the body. This is a portion of the cerebellum which manifests the greatest phyletic constancy, whereas the lateral lobes, because they are functionally related to the muscles of the extremities, vary according as the upper and lower extremities arc capable of more or less complex perform- ances. Another feature at this level is the trapezoid body (Trp), and adjacent to it the trapezoid nucleus (Ntr). The trapezoid body is situated immediately dorsal to the pyramid (Py ) and consists of a complex system of decussating fibers. These crossing libers represent the decussation in the 54 THE LOWER PRIMATES secondary auditory pathway. They provide the means for the convey- ance of impressions received by the cochlea of the internal ear to the higher auditory centers. Immediately lateral to the trapezoid body is a dense bundle of fibers, the lateral fillet (Lf ), constituting the secondary pathway for auditory impulses. The nucleus of the seventh nerve which supplies motor innervation to the facial muscles (Nf) and a spray of fibers, the first part of the seventh nerve, w^hich extends dorsomesially toward the lloor of the ventricle, are also features of this level. LEVEL OF THE EMERGENT FIBERS OF THE SIXTH NERVE (FIG. 28) At the level of the emergent fibers of the sixth nerve, some minor changes have occurred which are noteworthy as preparing the way for the marked transition observed in the next succeeding sections. One significant feature is the appearance of these emergent fibers of the abducens nerve, the sixth nerve of the cranial series, which arises in a nucleus situated near the midline beneath the iloor of the fourth ventricle. These emergent fibers proceed forward and outward, penetrating the corpus trapezoid- eum in their course and finally emerging at the junction of the ventro- lateral and bulbopontile sulci. The abducens nucleus supplies the external rectus muscle of the eyeball and has special significance in that it sets the pace for all movements of lateral gaze involving both eyeballs. The appear- ance of the superior cerebellar peduncle, taking origin in the dentate nucleus, adds another feature to this level. This bundle represents the discrete aggregation of nerve fibers by means of which impulses arising in the cere- bellum leave this organ on their way to the several levels of the neuraxis. The^^ have their ultimate distribution to the muscles in the interest of proper coordination. The superior cerebellar peduncle is the one great path- way out of the cerebellum; hence its relative size furnishes a valuable index of cerebellar function. Ancnher peduncular structure connected with LEMUR iMONGOZ ^^ the cerebellum also appears at this level, the middle ccrthellar peduncle. The significance of this peduncle becomes more evident at the higher levels of the stem. Functionally it is the final connecting link between the cerebral FIG. 20. LEMUR MONGOZ. LEVEL OF EMERGENT FIBERS OF SIXTH NERVE. CEN, Central Gray Matter; ctt, Central Tegmental Tract; GOW, Ventral SpInocerebellarTract; icp. Inferior Cerebellar Peduncle; nr. Nucleus of Rolando; n6, Abducens Nerve; pd, Predorsal Bundle; PL, Posterior Longitudinal Fasciculus; pv, Pyramid; ref, Reticular Formation; rst. Rubrospinal Tract; so, Superior Olive; SPT, Spinothalamic Tract; trd, Descending Trigeminal Tract; trp. Trapezoid Body. [Accession No. 147. Section 173. Actual Size, 15 X 5 mm.] hemispheres and the lateral lobes of the cerebellum. The inferior cerebellar peduncle has begun to spread out its fasciculi like a fan as it enters the medullary substance of the cerebellum immediately lateral to the dentate nucleus. Much significance attaches to the physiological importance of these three cerebellar peduncles. Collectively they constitute a reliable index for estimating the degree of coordination possessed by the animal, and thus reveal the extent to which the more complex muscular performances have been developed. The size of the inferior cerebellar peduncle provides a 56 THE LOWER PRIMATES scale for measuring the relative amount of atlerent inflow to the cerebellum. The superior cerebellar peduncle gives a similar opportunity regarding the relative outflow from the cerebellum, while the middle cerebellar peduncle furnishes the means of estimating the degree of communicational capacity between the cerebral cortex (where all courses of sustained volitional action take origin) and the cerebeHum which coordinates the movements of such action. LEVEL OF THE CAUDAL EXTREMITY OF THE PONS \'AROLII (fIG. 29) At the level of the caudal extremity of the pons Varolii, several marked changes have occurred. Most conspicuous among these is the appearance of a fairly wide band of transverse fibers extending across the ventral surface of the brain stem and constituting the stratum superficiale of the pons Varolii. This layer has now become the most ventral element in the neuraxis. It lies in front of the pyramidal fibers (Py) which, in con- sequence, have lost their surface position and are no longer seen in relief along the ventral aspect of the stem. The appearance of the pyramidal tract itself is also altered in such a manner that it no longer maintains its compact solidarity. It now appears as a collection of scattered bundles. This dissemina- tion of the pyramidal fasciculi is due to a number of transverse pontile fibers which weave themselves among the descending pyramidal bundles and thus constitute a complex layer of the pons Varolii known as the stratum com- plexum. The appearance in this stratum of numerous nerve cells still further complicates the structure. These cellular elements form an irregular mass of gray matter of considerable size scattered amidst the pyramidal and transverse pontile fibers. They constitute the pontile nuclei (PN). From the degree of development, both in the pontile nuclei and the transverse pontile fibers, inference may be drawn regarding the range of skilled volitional movements with which the animal is endowed. The anatomi- LEMUR MONGOZ 57 cal and physiological reasons for tliis inference are not far to seek. Such fibers as constitute the transverse fasciculi of the pons have their origin in the cerebral cortex of the frontal, parietal, occipital and temporal lobes. Descend- FIG. 29. LEMUR MONGOZ. LEVEL OF CAUDAL EXTREMITY OF PONS VAROLIL CEN, Central Gray Matter; Gow, Ventral Spinocerebellar Tract; mcp. Middle Cerebellar Peduncle; nbe, Nucleus of Bechterew; nr, Nucleus of Rolando; N5, Trigeminal Nerve; n6, Abducens Nerve; N7, Facial Nerve; pd, Predorsal Bundle; pl. Posterior Longitudinal Fasciculus; pn. Pontile Nuclei; pns. Pons; py, Pyramid; ref, Reticular Formation; rst, Rubrospinal Tract; scp, Superior Cerebellar Peduncle; so, Superior Olive; spt, Spinothalamic Tract; trd, Descending Trigeminal Tract; trp, Trapezoid Body; tur, Tractus Uncinatus of Russel (Hook Bundle); ver, Vermis. [Accession No. 147. Section 188. Actual Size, 15X7 mm.] ing from these origins they make their entrance into the cerebral peduncle and ultimately reach the pons. At this point they alter their direction from the vertical to the horizontal plane, thereafter pursuing a transverse course. Many of these fibers end in the pontile nuclei of the same side for a relay here. The relaying axons then undergo decussation across the midline and eventualh' enter the middle cerebellar peduncle (Mcp). Some pallio- 58 THE LOWER PRIMATES pontile fibers, however, cross the midline and recei\e their rehiy in the contralateral pontile nuclei whose axons in turn pass into the middle cere- bellar peduncle. This peduncle extends into the cerebellum and its fibers finally ramify among the various lobules constituting the lateral cerebellar lobes. By means of the pallio-pontile fibers the major functional divisions of the cerebral hemispheres establish direct communication with the lateral lobes of the cerebellum. Experimental and clinico-pathological observations warrant the opinion that these connections are essential to proper coordi- nation in the more complex motor performances of the animal. The pathway between the cerebral cortex and the cerebellum, in a functional sense, parallels the conduction tract which conveys impulses necessary to the voluntary control of action. The latter pathway provides for the trans- mission of nervous energy involved in the purpose and pattern of the act to be performed. This includes the incentive, initiation, design, direction and ultimate inhibition of the act. The pallio-pontile connections provide for concurrent impulses which regulate the coordinative and postural attributes necessary to the execution of such voluntary performances. The slight degree of development in the transverse fibers of the pons, as well as in the pontile nuclei and the middle cerebellar peduncle in lemur, points conclusively to a motor organization capable of but a limited range and variety of the more highly synthetized voluntary performances. This fact is corroborated by the rather feeble development of the pyramidal system. In this sense the pons Varolii and its several constituents may be accepted as an index of the extent to which the cerebral cortex has developed. They provide a structural basis for estimating the range of adaptation and degree of volitional adjustment of which the animal is capable. The position of the cerebellum is similar to that of the lower levels, and the cross section shows especially well the medullary vestibule consist- ing of all the fibers assembled from the middle and inferior cerebellar pedun- LEMUR MONGOZ 59 cles, which near its center contains the much reduced cephah'c extremity of the nucleus dentatus. At a considerable distance ventrally from the dentate nucleus is the superior cerebellar peduncle (Sep). At the lateral extremity of the section and penetrating the superficial pontile fibers, the fdth cranial nerve (N5) makes its way into the brain stem. These fibers constitute the motor and sensory roots of the trigeminal nerve, the motor root of which may be traced to its nucleus of origin close in the angle of the fourth ventricle, the nucleus masticatorius of the lifth nerve supplying the muscles of mastication. LEVEL AT THE MIDDLE OF THE PONS VAROLII (FIG. 30) At the level at the middle of the pons Varolii this structure attains its full dimensions and displays its three major layers, the stratum super- ficiale, the stratum profundum and the intermediate stratum complcxum. Scattered among the transverse fibers of the stratum complcxum are the much separated bundles of the pyramidal system (Py) and also the accumulated mass of gray matter constituting the pontile nuclei (PN). The fibers emerging from the pontile nuclei become collected both in the superficial and deep layers of the pons and pass laterally to enter the bundle constituting the middle cerebellar peduncle (Mep). As already indicated, the pontile nuclei, together with the libers entering into and forming the middle cerebellar peduncle, may be accepted as evidence con- cerning the degree of cooperative innervation existing between the portions of the brain having to do with skilled movements and the coordination of such movements. The relatively small size of these structures in lemur points to a correspondingly limited range of skilled performances. That portion of the axial stem, which comprises the several layers of the pons and contains the pontile nuclei together with the scattered fibers of the pyramidal fasciculi, constitutes what is known as the basis, whose 6o THE L0\\ ER PRIMATES dorsal boundary is created by a narrow band of transversely disposed fasciculi forming tlie mesial fdlet (Mf). This line of boundary indicates the division between the basis po7itis situated ventrally and the teg77ie7%tum FIG. 30. LEMUR MONGOZ. LEVEL OF THE MIDDLE OF THE PONS VAROLII. CEN, Central Gray Matter; ctt, Central Tegmental Tract; cow, Ventral Spinocerebellar Tract; lf, Lateral Fillet; mf, Mesial Fillet; mcp, Middle Cerebellar Peduncle; nbe, Nucleus of Von Bechterew; nr, Nucleus of Rolando; pd, Predorsal Bundle; pl, Posterior Longitudinal Fasciculus; pn". Pontile Nuclei; pv. Pyramid; REF, Reticular Formation; rst, Rubrospinal Tract; scp, Superior Cerebellar Peduncle; spt, Spinothalamic Tract; trp, Trapezoid Body; tur, Tractus Uncinatus of Russel (Hook Bundle). (Accession No. 147. Section 208. Actual Size, 14 X 5 mm.] po7-itis, situated dorsally. In the dorsolateral aspect of the section are three conspicuous tracts, the ventral spinocerebellar tract (Gow) making its way toward the vermis, ventral to which is the tractus unci?ja/us of Russel (Tur), while immediately in front of this is the superior cerebellar peduncle (Scp). In the angle between the superior cerebellar peduncle LEMUR MONGOZ 6i and the central gray matter (Cen), immediately beneath the floor of the fourth ventricle, is a small collection of medium-sized nerve ceUs con- stituting the nucleus of von Bechterew (NBe) which is one of the group of FIG. 31. LEMUR MONGOZ. LEVEL OF EMERGENCE OF TROCHLEAR NERVE. CEN, Central Gray Matter; ctt. Central Tegmental Tract; lf, Lateral Fillet; mf. Mesial Fillet; N4, Trochlear Nerve; PL, Posterior Longitudinal Fasciculus; pn, Pontile Nuclei; pns. Pons; py, Pyrarnid; ref, Reticular Formation; rst. Rubrospinal Tract; scp, Superior Cerebellar Peduncle; scpx. Crossing of the Superior Cerebellar Peduncle; spt, Spinocerebellar Tract. [Accession No. 147. Section 245. Actual Size, 11X6 mm.] vestibular nuclei. Ventral to this nucleus are the ascending root fibers of the trigeminal nerve known as the tractus mesencephalici trigemini. A large collection of small nerve cells lies at the border of the tegmentum opposite the middle cerebellar peduncle. This is the dorsal nucleus of the lateral fillet. 62 THE LOWER PRIMATES LEVEL OF THE INFERIOR COLLICULUS (fIG. 32) At the level of the inferior collicuius, the appearance of certain strikino; features alters the external conlicruration of the stem. Among these FIG. 32. LEMUR MONGOZ. LEVEL OF THE INFERIOR COLLICULUS. CEN, Central Gray Matter; ctt. Central Tegmental Tract; cp. Cerebral Peduncle; ic, Inferior Collicuius; IT, Aqueduct of Sylvius; lf. Lateral Fillet; mf. Mesial Fillet; N4, Trochlear Nerve; pl, Posterior Longitudinal Fasciculus; ref. Reticular Formation; rst, Rubrospinal Tract; sbn, Substantia Nigra; spt, Spinothalamic Tract; tmt, Tractus Mesencephalici Trigemini; xscp, Crossing of Superior Cerebellar Peduncle. [Accession No. 147. Section 255. Actual Size, 11 X 7 mm.] LEMUR MONGOZ 63 features may be noted the narrow ing of the ventricular space to form the beginning of the Sylvian acjueduct and the appearance of two large nuclear masses situated in the dorsolateral portion of the held. These structures constitute the inferior collicuh ( IC), distinguishing characteristics of the quadrigeminal plate in the midbrain. Ventrally the transverse continuity of the pons Varolii is interrupted by the disappearance of the decussating pon- tile fibers, causing a sulcus to appear on the ventral aspect of the stem which marks the beginning of the cerebral peduncles (CP) and interpeduncular space. The inferior colliculi are significant because they represent primordial receiving stations for the sense of hearing. In many of the lower forms of vertebrates they are the chief centers for auditory sense. In most of the higher forms, especially in mammals, they have delegated much of their original dominance in auditory function to cortical areas of the cerebral hemispheres. Their conspicuous size in lemur indicates that they have here retained much of their primordial significance. They appear to provide a correlating center for auditory impulses essential in determming rapid auto- matic motor responses incited by auditory stimuli. While these lower centers of hearing have lost much of their autonomy during the process of telenceph- alization (that is, the progressive advancement of higher synthetic control to the endbrain), they have not entirely surrendered their auditory func- tions. Through the inferior colliculi the animal is able to respond to auditory stimuli by adequate motor reactions without submitting these stimuli of hearing to the supervision of the higher and more deliberative centers of the cerebral cortex. The fundamental need in the life of these annuals for an apparatus which reacts immediately to threatening sounds by automatic movements of escape or attitudes of defense may readily be understood. At the same time, this very tendency for auditor^' stimulation to be short- circuited in the interest of producing immediate and definitely crystallized automatic reactions cannot fail to create a certain degree of limitation in the 64 THE LOWER PRIMATES adaptability of motor responses excited by auditory stimuli. It does, as it were, deprive the reaction of a deliberative quality, a period of latency and reflectioa in which the acts in response to sound may be made more complete, more precise and more effectiveh' adjusted to the several alternatives of action developing in a given situation. Lateral to the inferior colliculus is the brachium conjunctivum posticum which constitutes a connecting link in the auditory pathway to the mesial geniculate body. Ventral to the colliculus is the lateral fillet (Lf), making its entrance into the primordial receiving station for the sense of hearing. A large mass of transversely disposed fibers sweeping toward the midline comprises the two major divisions of the superior cerebellar peduncle (XScp) now about to undergo decussation preparatory to entering the red nucleus. The pyramidal tracts, together with the descending fibers which con- stitute the pallio-pontile tracts, are situated along the ventral aspect of the axis in a discretely collected bundle of fibers (CP ). A few of the more cephalic transverse fibers of the pons, together with some of the pontile nuclei, are shown ventral to the pyramidal fibers and the fibers of the pallio-pontile system. Dorsal to the pyramidal fibers and stretching trans- versely across the section from the midline to the periphery is a mass of gray matter containing cells of several sizes and constituting the caudal extremity of an important nucleus known as the substantia nigra (Sbn). It is presumed that this nuclear mass is essential to the regulation of automatic associated movements. Since it assumes such striking propor- tions in lemur, the inference seems warranted that this type of motor reaction is especially characteristic of these animals. The central gray matter (Cen) has become greatly enlarged and surrounds the caudal extremity of the acjueduct of Sylvius, the roofplate of which is formed by the superior medullary velum which supports the cephalic extremity of the vermis LEMUR MONGOZ 65 cerebclli. At the lateral extremity of the central !j;ra\ matter are several scattered bundles of nerve fibers, the meseiicephalic root 0/ the trigeminal nerve (Tmt), while along the ventral border of the central gray matter are two or three small bundles of libers, the descending fasciculi of the trochlear nerve (N4). LEVEL OF THE SUPERIOR COLLICULUS (fIG. 33) At the level of the superior collicuhis, prominent tectal structures of the mesencephalon make their appearance. They are the much reduced remnants of the optic lobes which form the most conspicuous elements in the midbrain of the lower classes of vertebrates. Here they retain a certain degree of stratification reminiscent of the optic lobes. Three distinct layers or strata of alternating cell and fiber distribution may be discerned microscopically in the area indicated as stratum griseum superficiale. Still other layers may be differentiated in this tectal region of the mid- brain. The optic lobes have been progressively superseded in the impor- tance of their visual function as the cortex of the cerebral hemispheres expanded and became more intimately concerned in those higher associational syntheses necessary for a fuller sensory adjustment to the external world. This supersedence on the part of the cerebral cortex involves not only the sense of vision and the sense of hearing, but quite as much the general body sense in all those various modalities which are essential to the produc- tion of highly organized volitional movements. That all of these specialized forms of sensation, including those which put the animal in touch with elements in its environment more or less remote from its own body, as well as those requiring actual contact with the body surfaces, have in their more primitive states been represented by parts of the brain less highly organized than the cerebral cortex, there can be no doubt. By a slow and gradual process, proceeding step by step from species to species, and onlj- incompletely 66 THE LOWER PRIMATES consummating its progress as it sought new sensory liclds for lurther expan- sion, the functions related to sensibility have attained their fullest representa- tion in the pallium of the cerebral hemispheres. This stepping-up process FIG. 33. LEMUR MONGOZ. LEVEL OF THE SUPERIOR COLLICULUS. CEN, Central Gray Matter; cp. Cerebral Peduncle; ctt, Central Tegmental Tract; mgb, Mesial Geniculate Body; mf, Mesial Fillet; noc. Nucleus Oculomotorius; nru, Nucleus Ruber; pd, Predorsal Bundle; pl, Posterior Longitudinal Fasciculus; ref, Reticular Formation; sbn, Substantia Nigra; sc, Superior Colliculus. [Accession No. 147. Section 300. Actual Size, 19X9 mm.] from a lower, less highly organized region to a higher and more expansible territory in the end-brain, is spoken of as telencephalization. Yet in no case has this transference upward been made at one such stride that the entire allegiance of a certain form of sensibility is advanced from its more lowly sphere of structural organization to its new domain in a higher region. The gradual projection from lower to higher centers is clearly demonstrated in the phyletic series of the vertebrates. Nowhere is there a more marked instance LEMUR MONGOZ 67 of this partial and graded telcncephalization than in the case of the optic lobes which ultimately give over their dominance in visual function to the occipital area of the cerebral hemispheres. Even within the limited range of the primate order there is still evidence of this slow transference from a highly organized visual region in the midbrain to the more highly developed visual centers of the occipital lobe. The stratification already noted in the superior collicular region which retains, even in this high form of mammal, at least ten of the original fourteen cortical layers of the optic lobe, indicates that tclencephalization has by no means attained its full expression in lemur. It must be inferred, therefore, that some at least of the primordial visual function is vested in these still highly differentiated tectal structures of the midbrain. This supposition is further borne out by the large number of optic fibers which end in the superior colliculi. Some ol these fibers are shown immediately lateral to the stratum griseum superficiale. Another feature at this level of the brain stem is the oculomotor nucleus (Noc). This nucleus gives rise to the third cranial nerve whose fibers supply all but two of the muscles which move the eyeball within the orbit. It also innervates the intrinsic muscles of the eye, the constrictor iridis and the muscle of the ciliary body which regulates the convexity of the lens. In addition, the oculomotor nucleus sends fibers to the levator palpe- brae muscle which produces elevation of the upper eyelid. This portion of the midbrain therefore is preeminently related to the visual sense. Not only is this true as to the structures concerned with the actual receipt of visual impressions, but quite as much in the transmission of motor impulses requisite to the muscular adjustments of the eyes in visual fixtaion and in visual pursuit of objects. The fact that this oculomotor nucleus (Noc) shows a striking simplicity in its development indicates a relatively low degree of oculomotor organization. It implies that the lemur's vision is as yet only partially binocular; that in the main the animal is still using its eyes 68 THE LOW ER PRIMATES as far-distance receptors. The adiustment of ocular movement is as yet so incomplete that the animal has not learned the many advantages arising from a more exact stereoscopic vision for objects near at hand. Ocular con- vergence brings the visual axes into such a position as to make possible the more discriminating scrutiny of objects near the animal. It secures the more exact effects of perspective and proportion characteristic of complete binocu- lar vision. The relatively small size of the oculomotor nucleus in lemur indicates an imperfectly developed binocular vision, while the lack of internu- clear commissures implies that the muscles of the two eyes have not as yet acquired the intimate intermuscular cooperation characteristic of animals which have developed a high degree of binocular fusion and stereoscopic vision. Another feature is the appearance of the red nucleus (NRu), a col- lection of nerve cells in which at least two great systems ot fibers receive relay. One of these systems is especially concerned in the efferent conduction of impulses arising in the cerebellum and destined for distribution in the lower levels of the axis. The nucleus ruber (NRu) in lemur is a relatively small structure. Its size is in proportion to the correspondingly small number of fibers forming the superior cerebellar peduncle. This small red nucleus sig- nifies a limited functional capacity for the distribution of impulses essential to the coordinative control, more particularly the coordinative control of complex motor reactions in the upper and lower extremities. The two cerebral peduncles (CP) extend along the ventral surface and are becoming more divergent as each approaches its corresponding cerebral hemisphere. At the basis of the peduncle are the collected fibers of the pyramidal system and also of the pallio-pontile system. Immediately dorsal to these bundles of fibers is an extensive mass of gray matter, the substantia nigra (Sbn), the significance of which has already been dis- cussed in connection with the control of certain automatic associated move- LEMUR MONGOZ 69 mcnts of a primordial character largely vested in the midbrain. Immediately dorsal to the substantia nigra is the mesial fillet (Mi), while occupying positions in front of the oculomotor nuclei are two decussations. One of FIG. 34. LEMUR MONGOZ. LEVEL OF THE OPTIC CHIASM. CIN, Internal Capsule; cph. Corpus Hypothalamicum; fdp. Descending Pillar of Fornix; gh. Ganglion Habenulae; glp. Globus Pallidus; mf, Mesial Fillet; nl, Nucleus Lateralis Thalami; nm, Nucleus Mcdialis Thalami; opt, Optic tract; OPX, Optic Chiasm. [Accession No. 147. Section 363. Actual Size, 25 X 15 mm.] these is the dorsal decussation of Mcjnert through which the emer- gent fibers of the third nerve make their way toward the surface. Ventral to the dorsal decussation of Meynert is the smaller decussation of Forel. These decussations represent respectively the crossing of the fibers arising in the reticular formation of the midbrain to enter into the predorsal bundle (PD) and the decussating descending fibers arising in the red nucleus to form the rubrospinal tract. 70 THE LOWER PRIMATES LEVEL OF THE OPTIC CHIASM (fIG. 34) At the level of the optic chiasm the decussation in the optic pathway is "^the identifying feature (Opx). Another salient element is the optic FIG. 35. LEMUR MONGOZ. LEVEL OF THE ANTERIOR COMMISSURE. AC, Anterior Commissure; ciN, Internal Capsule; fdp, Descending Pillar of Fornix; for, Fornix; gh. Ganglion Habenulae; glp. Globus Pallidus; nm. Nucleus Medialis Thalami; nli, Nucleus Lateralis InternusThalami; NCA, Caudate Nucleus; put, Putamen; vq. Fasciculus of Vicq d'Azyr. [Accession No. 147. Section 417. Actual Size, 20 X 10 mm.) thalamus which represents the last great Jelay station in the pathway of all types of sensibility with the exception of olfactory sense. It com- pletes the conduction pathway of body sensibihty. Dorsal to the optic chiasm are crossing fibers which constitute the supra-optic commissure of Meynert. Immediately dorsal to this is a large mass of gray matter con- taining cells for the most part of the motor type and forming the globus LEMUR MONGOZ 71 pallidus (GIp). This is one of the most important portions of the end- brain. Being especially concerned with the regulation of automatic associated movements, it constitutes an outstanding division of the basal telencephahc ganglia. Resting upon and dorsal to the globus pallidus is a fairly large mass of myehnatcd nerve fibers representing axons which arise in the motor cortex and descend as the pyramidal system. Associated with these arc the many fibers which form the main part of the pallio-pontile system. The mass of fibers situated between the thalamus and the globus pallidus con- stitutes a portion of the internal capsule (Cin). In contact with the dorsal aspect of the internal capsule is a mass of gray matter surrounded by a considerable capsule of myehnized fibers. This is the corpus hypothalamicum (Cph) which contains the hypothalamic nucleus of Luys and the adja- cent medullary substance comprising the tivo fields of Forel known as Hi and H2. Mesial to the corpus hypothalamicum is the fasciculus of Vicq d' Azyr and ventral to this a bundle of small, weakly staining fibers, the descend- ing pillar of the fornix ( Fdp). LEVEL OF THE ANTERIOR COMMISSURE (fIG. 35) At the level of the anterior commissure (AC) the section illustrates the cephalic limit of the brain stem, the last remaining structure of which is the anterior portion of the optic thalamus. Other structures of topo- graphical interest at this level are indicated in the caption. Chapter II RECONSTRUCTION OF THE GRAY MATTER IN THE BRAIN STEM OF LEMUR MONGOZ Y' I ^HE impression conveyed by a survey of cross sections of the brain I stem does not give any such realistic idea of the nuclear masses as H that obtained from a study of reconstructions by the Bourne method. Such reconstructions of the gray matter are here employed to make more comprehensible the proportions of those nuclear structures which bear the most significant evidence of evolutional unfolding in the brain stem. It is to be noted, however, that these reconstructions do not disclose the dmien- sions or rehitions of the nuclear masses which occupy positions within the reticular formation. It is equally notewortliy that these deeper lying celhilar collections constitute the more archaically fixed and least variable elements in the composition of the axial graj' matter, while the superficial nuclear aggre- gations represent the recently acquired, more plastic structures of the stem. The tri-dimensional demonstration of the gray matter afforded by recon- structions facilitates the visualization of these elements and establishes for each structure a discrete mf)rphoIogical entity of its own. In the following descriptions, only the structures seeming to have salient evolutional significance have been selected for discussion. Any complete anatomical analysis of the constituents of the brain stem belongs more properly to the realm of an anatomical atlas. The structures to be considered here include: the dorsal sensory nuclei, the mferior olivary nucleus, the reticu- lar formation, the pontile nuclei, the vestibular nuclei, the cochlear nucleus, the inferior and superior colliculi, the substantia nigra and the red nucleus. The Dorsal Sensory Nuclei In lemur the sensory nucleus of Goll begins as a slender extension from the central gray column close to the midline and separated from 73 74 THE LOWER PRIMATES its fellow of the opposite side by the dorsal septum. The base remains narrow while the distal portion presents a tendency to swing and spread laterally. At a sHghtly higher level the first indication of the sensory nucleus of Burdach appears as a flat, bilateral thickening in the dorsolateral portion of tlie central gray cohmin. The two nuclei then extend dorsally with the same lateral swing which is characteristic of the dorsal gray masses and of the substantia gelatinosa Rolandi preparatory to the opening of the fourth ventricle. As these nuclei of GoII and Burdach increase in size, they develop at their dorsal extremities overhanging masses of nuclear material, almost arboreal in form, but limited to the lateral aspects of the main trunks of the nucleus. The more lateral of these nuclear appendages forms the external nucleus of Burdach. Its Icaf-hke appearance is produced by the breaking-up of the nuclear material by the bundles of fibers of the column of Burdach which are here seeking their cells of relay. The substantia gelatinosa of Rolando which forms the cap surmounting the dorsal horns passes insensibly into the substantia gelatinosa trigemini with a marked lateral inclination in accordance with the common lateral swing of these nuclear columns, until it almost reaches the lateral meridian. As this swing takes place, the substan- tia gelatinosa Rolandi, slender in the spinal cord, rapidly increases in size to become the substantia gelatinosa trigemini of the oblongata. These three dorsal columns of gray matter expand and diverge in almost parallel para- bolic rows. The substantia gelatinosa trigemini approaches the lateral sur- face of the cord and, losing its intimate contact with the reticular formation, extends upward in a position so constant that it may be used as an orient- ing structure in the study of the stem. The nuclear gray column reaches upward to the midpontine level where dorsomesial to it there appears the motor nucleus of the trigeminus nerve. At the midolivary level the sub- stantia gelatinosa trigemini presents a definite constriction which has been RECONSTRUCTION OF LEMUR MONGOZ 75 termed the "waist of the trigeminal nucleus." At its cephalic extrem- ity the nucleus increases in all its diameters as it prepares to meet the entering mass of fibers constituting the dorsal root of the Gasserian gan- FIG. 36. VENTRAL SURFACE OF GRAY MATTER OF BRAIN STEM, LEMUR MONGOZ. Key to Diagram, cochlear, Cochlear Nucleus; inf. olive. Inferior Olive; lat. gen. body. Lateral Genicu- late Body; meso-cen. body. Mesial Geniculate Body; pontile, Pontile Nuclei; ret. form.. Reticular Formation; subst. nigra. Substantia Nigra; sup. olive, Superior Olive; vent, gray col.. Ventral Gray Column. ghon. The nucleus extends somewhat above the entering fibers in order to afford relay cells for the short ascending arms of the incoming axons. The Inferior Olivary Body The reconstruction of the inferior olivary body in lemur consists in large part of the dorsal and ventral accessory olivary nuclei. The main mass of the inferior olive seems to be a later addition, developing in a position between the two accessory nuclei as a new structure presenting in the higher forms a saccular fundus and two branches. 76 THE LOWER PRIMATES The ventral accessory (paleo-olivc) nucleus appears somewhat below the mid-decussational level of the oblongata as an oval mass directed mesially and shghtly dorsally. It presents no secondary pHcations and after appearing as a small oval collection of nerve cells it extends transversely to form a flattened band of nuclear material. Mesially it fuses with the extremity of the dorsal accessory nucleus. The dorsal accessory nucleus appears as a round nuclear accumulation imbedded in this ventral surface of the reticuhir for- mation. It spreads rapidly into a thin lamina which fuses with the mesial extremity of the ventral accessory ohve. The chief oHvary nucleus appears as a loop between the lateral extremities of the accessory olivary nuclei with which it fuses. There are no reduplications in this loop. The Reticular Formation In the reconstruction, the reticular formation presents an extensive mass of nuclear material traversed by great numbers of scattered nerve libers. It is roughly quadrilateral in shape and forms the main portion of the tegmentum of the oblongata, pons and midbrain. In it develop the various nuclei of termination and origin of the medullary and pontile cranial nerves. Upon its surface it presents the indentations made by the various ascending and descending tracts of the brain stem; while at various levels it is pierced by the wide swinging bundles of decussating fibers constituting the mesial and lateral fillets and the inferior cerebellar peduncles. It forms a matrix along the mesial septum for the posterior longitudinal fasciculus, the predorsal bundle and other longitudinal fasciculi. The reticular formation, in affording passage to the various white fiber tracts, seems to serve as a semi-fluid medium which surrounds these tracts on all sides, supporting them in a soft, gelatinous matrix. It begins at about the level of origin of the decussation of the pyramidal tracts and, extending ventrally, dorsally and laterally, receives the cephalic termination of the RECONSTRUCTION OF LEMUR MONGOZ 77 ventral gray column. It allords attachment to the nuclei of GoII and Burdach dorsally and to the substantia gelatinosa trigemini laterally, which, however, becomes separated from the reticular formation at a higher level by the FIG. 37. DORSAL SURFACE OF GRAY MATTER OF BRAIN STEM, LEMUR MONGOZ. Key to Diagram, dors, cochl.. Dorsal Cochlear Nucleus; inf. coll., Inferior Colliculus; lat. gen. body. Lateral Geniculate Body; meso-gen.. Mesial Geniculate Body; nucl. of burdach. Nucleus of Burdach; NUCL. OF deiter, NucIcus of Deiters; nucl. of goll, Nucleus of GoII; ret. form., Reticular Formation; SUBST. gelat.. Substantia Gelatinosa; subst. gel. rolando. Substantia Gelatinosa of Rolando; vent, cochl.. Ventral Cochlear Nucleus. entering vestifiular and cochlear nerves and the lateral fillet. Arising from the dorsal and ventral cochlear nuclei of both the homo- lateral and contralateral nuclear complexes, the fibers of the trapezoid body develop as a boundary between the reticular formation and the deep layer of the pontile nuclear masses. This body lies in a recess in the ventral surface of the reticular formation. Passing laterally from the superior olive, the trapezoid body forms the lateral fillet which then skirts the edge of the reticular formation, piercing it in some areas as its fibers pass upward to end 78 THE LOWER PRIMATES in the inferior colliculus. From the inferior colliculus arises the inferior brachium which occupies a groove on the lateral surface of the mesencephalic reticular formation until it reaches and ends in the mesial geniculate body. The reticular formation is most extensive at the level of the inferior olivary nuclei which develop in its ventral portion. It forms the main mass of the gray matter of the brain stem. Between the levels of the vestibular complex and the inferior colliculus, the reticular formation sends out a long, slender process on the lateral surface of the pons and midbrain to form an enveloping layer over the superior cerebellar peduncle. The reticular formation contains the ventral portions of the twelfth, sixth, fourth and third nerve nuclei, while the dorsal portions of these nuclei project into the central gray matter. In the upper medullar}' and lower pontile regions the reticular formation presents the specialized condensations forming the vestibular nuclei. In the mesencephalon the reticular formation approaches but docs not come into actual fusion with the substantia nigra. Laterally it forms a support for the mesial geniculate body, while dorsolaterally it becomes continuous with the inferior and superior colliculi. At its cephalic extremity it is con- tinuous with the zona incerta of the diencephalon and fuses with the thalamic gray matter. The Pontile Nuclei Reconstruction of this mass of gray matter provides one of the most striking differential features in the brain stem of primates. This nuclear aggregation, forming with the pallio-spinal and pallio-pontile tracts the basilar portion of the metencephalon, presents a relatively simple arrange- ment. The pontile nuclei begin rather suddenly at the trapezoid level and show but little tendency to expansion caudally into the ob ongata. The RECONSTRUCTION OF LEMUR MONGOZ 79 nuclear mass is divisible into a superficial, ventral layer and a deep, dorsal layer, both of which are conlluent laterally and mesiaily in an area surround- ing the descending paUio-pontile and palho-spinal tracts. The pyramidal FIG. 30. LATERAL SURFACE OF GRAY MATTER OF BRAIN STEM, LEMUR MONGOZ. Key to Diagram, inf. olive, Inferior Olive; lat. gen. body, Lateral Geniculate Body; meso-gen. body. Mesial Geniculate Body; nucl. of burdach. Nucleus of Burdach; nucl. of deiters and n. of DEiTERS, Nucleus of Dcitcrs; ret. form.. Reticular Formation; subst. gel. rolando, Substantia Gelatinosa of Rolando; subst. nigra, Substantia Nigra; sup. coll., Superior Colliculus; vent, gray col.. Ventral Gray Column. tract is separated into fasciculi by the transverse pontile fibers only to a limited extent and in the main appears as a single heavy mass of fibers. The pontile nuclear masses are large and their continuity is not materially interrupted by the decussating ponto-cerebellar fibers. The deep layer of the nuclear mass is continuous at its cephalic extremity with the substantia nigra. Marked condensations of nuclear material appear laterally and mesiaily. They connect the superficial and deep layers of the pontile nucleus and form lateral and mesial buttresses for the substantia nigra. 8o THE LOWER PRIMATES The Vestibular Nuclei In reconstruction this mass of gray matter first makes its appearance as the large nucleus of Deiters at a point a little above the mid-decussational pyramidal level. It appears as a small, wedge-shaped mass of gray matter located between the upper extremity of the nucleus of Burdach, the nucleus of Goll and the central gray matter. It also j^articipates in the general lateral swing occasioned by the opening of the fourth ventricle and rapidly increases in size. It occupies a position beneath the central gray matter and in relation with the ccphahc extremity of the nucleus of Burdach. In the mid-ventricular region the triangular nucleus of Schwalbe makes its appear- ance mesial to the nucleus of Deiters and in relation with the central gray matter. The nucleus of Deiters gradually dwindles as it extends cepha- lad, wliilc the triangular nucleus is continued upward as a large, irregular mass in the lateral reticular formation dorsal to the substantia gelatinosa trigemini. Dorsally and cephalically the nucleus of von Bechterew appears as a small, indefinite mass of gray matter in the lateral wall of the fourth ven- tricle close to the thin subependymal layer of central graj^ matter. The Cochlear Nuclei The reconstruction of this nuclear mass shows a large ventral cochlear nucleus which extends for a considerable distance into the lower pontile regions of the brain stem. It is separated from the ventrolateral angle of the tegmentum of the stem by the collected mass of the middle and inferior cerebellar peduncles and the descending trigeminal tract. It is connected with the stem only by the fibers of the cochlear nerve. The large dorsal cochlear nucleus is situated at the extreme dorsolateral angle of the tegmentum in the extremity of the lateral angle of the fourth ventricle. It begins below the recess and extends about an ccjual distance above the recess. It is limited in extent by the pontile peduncular fibers but RECONSTRUCTION OF LEMUR MONGOZ 8i extends for some distanee mesially under the subependymal layer of the central gray matter of the fourth ventricle. It lies immediately dorsal to the vestibular nuclei, separating this mass from contact with the gray matter of the lloor ol the fourth ventricle. Between the ventral and dorsal cochlear nuclei are connecting strands of nuckuir material as well as the root fibers of the cochlear ner\e and secondary libers of the cochlear tract. The Colliculi In reconstruction, these masses of gray matter are about equal in size and extent. They appear as rounded elevations on the tectum of the midbrain, occupying the entire width of the dorsal aspect of the axis. They are contin- uous laterally with the dorsal extension of the reticular formation of the mesencephalon. Mesially and dorsally they are continuous across the midline with each other by means of the dorsal gray matter which serves as a matrix for the superior and inferior collicular commissures. The colliculi are sepa- rated from the central gray matter by these collicular commissures and by the peripheral fiber condensation which is disposed as a fringe along the line of contact between the central gray matter and the reticular formation. The inferior colliculus is sharply separated from the large superior colliculus by a narrow, finger-like process sent out by the mesencephalic reticular formation, while the superior colliculus merges insensibly into the subpineal region of the epithalamus. The surface of the mesencephalic reticular forma- tion is traversed in this region by the passage of the two brachia connecting the colliculi with the geniculate bodies. The Substantia Nigra As reconstructed, this mass of gray matter appears as a gradual trans- formation in the deep layer of the pontile nuclear mass, assuming its charac- teristic coloration and form at the junction of the isthmus and midbrain. It 82 THE LOWER PRIMATES is supported not only by the deep pontile layer, but also rests upon the two buttresses which form laterally and mesially in the pontile gray matter. It is dense and heavy in lemur and is connected mesially to its fellow of the opposite side by means of the interpeduncular gray matter which appears to be quite undiflerentiatcd. This interpeduncular gray matter is continuous with the structures forming the hypencephalon. In the lateral portion of the substantia nigra there develops a discrete nucleus containing many tangled nerve fibers which pass dorsally into the tegmentum. The substantia nigra gradually attenuates and disappears. It is not continuous with any definite gray mass in the thalamic region of the brain. The Nucleus Ruber The reconstruction of this nuclear mass, so predominant in sections of the mesencephalon in man and the higher primates, is relatively insignificant in size. The nucleus is located in the most cephalic portion of the mesenceph- alon extending into the reticular formation of the diencephalon. It is relatively indistinct and poorly demarcated from the surrounding tissue by the encapsulating fibers of the superior cerebellar peduncle. The Central Gray Matter In the upper regions of the spinal cord this nuclear mass is roughly quadrilateral, receiving the bases of the two dorsal gray columns and ventro- laterally the two ventral gray columns. Passing upward, the central gray matter receives the bases of the developing nuclei of Goll and Burdach and then gives ofl" dorsally a narrow, tongue-like extension which passes along the dorsal median septum. As the caudal extremity of the fourth ventricle is approached, the entire central gray matter migrates dorsally, carrying the central canal with it, and as the dorsal gray columns begin to diverge, the quadrilateral body of gray matter gradually flattens dorsoventrally and RECONSTRUCTION OF LEMUR iMONGOZ 83 the central canal emerges dorsally into the fourth ventricle, thus disposing the central gray matter as two lateral halves connected across the midhne. This lateral displacement continues until at last the gray matter becomes flattened out in an ahiiost transverse plane as the floor of the fourth ventricle. From the edges of the lateral walls (;f the fourth ventricle the ependymal lining continues across the roof of the fourth ventricle which caudaliy is formed by the inferior medullary velum. In this locality the roof is invaginated by the chorioidal plexuses of the fourth ventricle and here the layer of central gray matter, which at best is extremely attenuated on the roof of the fourth ventricle, practically ceases to exist. In the midpontile portion of the fourth ventricle the cerebellum enters into the roof of the ventricle and cephalad to the cerebellum the superior medullary velum contains the superior cerebellar peduncles. Approaching the upper portion of the metencephalon the lateral walls gradually become convergent until they meet just caudad to the inferior colliculus. At this point the fourth cranial nerves decussate in this reconstituted roof of the ventricular cavity. The lateral walls of the fourth ventricle are formed from below upward by the nuclei of the columns of Goll and Burdach, the nuclei of Deiters and Schwalbe and the inferior cerebellar peduncle. The lateral walls fail at the lateral recesses but appear again as the middle cerebellar peduncle. Cephalically the superior cerebellar peduncles lie in the rapidly converging lateral walls of the cephalic portion of the fourth ventricle. The floor of the fourth ventricle is comparatively smooth and presents but little modelling. The underlying nuclear masses and hber tracts produce scarcely any impression on the floor which is stretched evenly from side to side and from below upward. It is in general diamond-shaped, beginning from the opening of the central canal caudaliy, spreading to its greatest width at the level of the lateral recesses and then rapidly narrowing as the aqueduct of Sylvius is approached. 84 THE LOWER PRIMATES In the oblongata the rounded dorsal surface of the hypoglossal nucleus passes backward and invades to a limited extent the central gray matter. In the metencephalon the dorsal surface of the abducens nerve passes some- w hat further dorsally, while in the mesencephalon the trochlear and oculo- motor nuclei extend deeply into the central gray matter. The central gray matter in the regions of the mesencephalon forms a complete investment for the central canal and as the diencephalon is approached, it becomes contin- uous with the central gray matter underlying the ependymal lining of the ventricle of the diencephalon. Dorsally at the junction of the tectal region with the habenular region, the central gray matter is penetrated by the habenular and the posterior commissures. Chapter III TARSIUS SPECTRUM, ITS BRAIN AND BEHAVIOR Its Position among the Primates; Measurements and Brain Indices; Surface Appearance oj the Brain; Internal Structure oj the Brain Stem TARSIUS occupies a unique position among the primates. It has been singled out by eminent authorities to carry upward the line of human derivation from some lower mammalian form. In this role it deserves closest scrutiny regarding its structure and behavior. Appearance and Behaxior of Tarsius In size, the animal is about as large as a small squirrel. It is peculiar in appearance because of its closely set, protruding eyes, its long tufted tail, its protruding ears, and the disc-like pads upon the ends of its fingers and toes. The tarsiers inhabit some of the Malay Islands. They are noted for two curious habits: they can rotate their heads until they look directly back- wards; and they leap with astonishing speed among the trees from bough to bough in pursuit of insects. Mr. Le Gros Clark has recently given the most detailed description of the tarsier's behavior in captivity. He discusses the affinities of tarsius to lemurs and the insectivores on the one hand, and the anthropoids on the other. In this discussion he recalls the opinion of the Royal Zoological Society of London, previously expressed, to the effect that whatever its definitive allocations, tarsius should be placed in a suborder of primates (Tarsioidca) which is intermediate between L.emuroidca and Anthropoidea. 8s 86 THE LOWER PRIMATES Clark's observation on tarsius covers a period of three years in Sarawak. During this time he was never fortunate enough to see one of the animals in the wild state. Such specimens as he observed were captured by the Djaks Courtesy, American Museum of Natural History FIGS. 39 AND 40. TWO VIEWS OF TARSIUS SPECTRUM. while felling trees. The animal was easily caught during the daytime. It acted almost stupidly, as though its diurnal vision was imperfect. In the main, it made most ineffectual efforts to escape, at the most pivoting on a branch in such a way as to put the latter between itself and its pursuer. It lives in the jungle of secondary type, particularly in a low country, and for the most part passes its time clinging in a vertical position to branches TARSIUS SPECTRUM 87 of trees and underbrush. The manner in which it supports itself in this vertical posture is interesting and peculiar. With the lingers and toes it holds the branch in prehensile grasp, meantime pressing its long slender tail against the branch very much in the manner of a spring. If the tail is pulled away from its support, tarsier tends to sag downward. The tail, however, is in no sense prehensile, nor is it used, with the exception above mentioned, in other functions than as a balancing and steering organ during locomotion. Tarsius is humanoid in reproducing a single offspring at a time. It is not gregarious, as many of the other primates are, but goes in pairs. After the breeding season the female and her young usually live alone together. There is no evidence that these animals build nests, or even live in the holes of trees. \\ hen sleeping, the head sinks downward, much as that of an old man asleep in his chair. Often the young tarsius will perch upon the mother's head while she is thus sleeping, and in this position fall asleep itself. This is an interesting motor combination, plainly showing that the mechanism for maintaining the vertical clinging posture operates perfectly even though the animal sleeps. The general behavior of tarsius seems extremely stereotyped. It is capable of but little acquisition under training, and in captivity is apparently unable to make new adaptations. In spite of its enormous eyes, it has diffi- culty in securing its food during the daytime, grasping awkwardly at grass- hoppers or other insects oflered to it. This no doubt is due to the fact that its visual apparatus is specialized for nocturnal hunting, and also because the retina possesses no macula. Characteristic in the motor activity of the tarsier are its marvel- ous leaps from branch to branch, which are so swift as almost to defy the human eye. It is said that the animal is able to capture small insects on the wing in its leaps. On the ground it leaps also like a frog, but awkwardly. When it lands it sprawls, and then hops away again. Only occasionally does 88 THE LOWER PRIMATES it walk and then its gait is ineffective and ungainly. Insects, especially grass- hoppers, are the main staple of its food supply, but it is also partial to milk and drinks water. Cour/esv. American Museum oj Natural History FIGS. 41 AND 42. HAND AND FOOT OF TARSIUS SPECTRUM. Left. Palmar surface of hand showing rudimentary development of the palm, palmar pads, pronounced digitation, disc-like specialization on the distal phalanx of each finger. Right. Plantar surface of foot showing rudimentary sole and heel, long hallux partially opposable, and disc -like specialization on the end of each toe. Tarsius performs its toilet much as a cat does. When it comes to the hind legs it grasps one and then the other with its hands, and thus keeps itself scrupulously clean. It is not known to make vocal sounds indicative of fear or anger, and only on rare occasions has it been heard to squeak, most particularly when young. It is probably not the case that the mother carries her offspring with her teeth in the manner of a cat. This act Clark has never observed. As with many other primates, the infant tarsius grasps and clings TARSIUS SPECTRUM 89 to the hair on the abdominal wall of the mother. The eyes are open at birth, and many reaetions appear at once which are often long delayed in such animals as the rat, cat, dog and higher primates. Courtesy. American Museum oj Natural Historv FIGS. 43 AND 44. HAND AND FOOT OF TARSIUS SPECTRUM. Left. Dorsum of hand showing disc-like specialization on finger tips, with claws instead of nails. Thumb partly opposable. Right. Dorsum of foot. Each toe shows the marked disc-hkc specialization on the distal phalanx; the great toe is short with little or no opposability. The contention that tarsias may be in the direct hne of human ancestry is borne out by its blood tests. Clarlv has shown that tarsius has a definite blood relationship with man, chimpanzee and gibbon. On the other hand, the blood reactions are negative with macacus, nycticebus, the cat and the squirrel. The hands are pecuhar for their long and slender lingers and short thumbs; the nails resemble claws. The feet are long, the toes slender; the index and middle digits have true claws. The palms of the hands and soles of the feet are provided with arch-like pads. 90 THE LOW ER PRIMATES In a personal communication, Mr. Harry Raven tells of hunting tarsius both in Dutch Borneo and Celebes. The animals were usually in second growth of the forest, or the scrubby forest along river I^anks. They are entirely nocturnal and frequently found in clumps of bamboo or in vines that sur- round the trunks of large forest trees. He hunted at night with a jacklight. The first time he saw tarsius, two or three of them were together and he caught the reflection of the eyes of one, shot and wounded it. When it was picked up it squeaked, a sharp, piercing squeak. The other two answered the call and when the wounded one was shaken it would repeat the squeaking until the others came up close. The animals are extremely active, probably the quickest jumpers of all mammals. When they are grasping a small branch, they can twist their heads in the direction they are going to jump, so quickly that it is almost impossible to see it — more cjuickly than the eye can follow. It is as though they were looking in one direction and jumping in another, they turn their heads with such great speed. In captivity they are pugnacious and cannot be tamed, although it is difhcult to keep them for long. In habits tarsius is close to the galago of South Africa, which lives in thickets and dense forests; in fact, their habits are nearly identical. Affinities of Tarsius to Other Primates G. Elliot Smith, who has given much attention to the aiFinities of the primates, says: " It is well to recall the fact that the brain of tarsius exhibits decisive evidence of the lemuroid status in the calcarine region, in the Sylvian fissure and in numerous traits which have been enumerated. In the degree of caudal extension of its hemispheres, it is even further removed from the insectivora and more pithecoid than the lemurs. But the evidence of cerebral anatomy lends no more support than I believe the structure of the rest of the bod}' does to the view that the approximation of tarsius to the apes implies its separation from the lemurs. So far as its brain is concerned, Tarsius is a TARSIUS SPECTRUM 91 lemur of the lemurs, to use an expression of Professor Howe's. It is certainly more nearly related to the apes than most other lemurs. But on the other hand, all the apes and lemurs are linked by a much closer bond of aflinity, the one to the other, than are any of them to the other mammals. Tarsius is unquestionably the most primitive living primate." A. A. W. Hubrccht, on the other hand, maintains that "tarsius is not a lemur at all. It should never have been placed among the lemurs. Its position is somewhere between an unknown type of inscctivores and oiu- modern monkeys and man." Memoirs published within recent years by Forsyth-Major, Earle and Standing have made it perfectly clear that the demonstration of the afTmities of tarsius to the apes does not in any way affect the recognition of the fact that it is at least as nearly related to the lemurs. So that Hubreeht's proposal to restrict the term primates to tarsius and the apes lacks any adequate justification. G. Elliot Smith speaks again even more emphatically concerning the position of tarsius. He has, in fact, come to the conclusion that tarsius is much more primitive and at the same time distinctly more pithecoid than the lemurs. He believes that the primate stem flowed from its source among a group of tarsius-like mammals. The apes and the lemurs were merely diver- gent branches of this stem and the latter, the lemurs, as a suborder although definitely specialized in structure, remained nearer to the Tarsiidae than to the apes. The primates, he asserts, consist of three divergent phyla which have all departed in varying degrees and in different ways from their original com- mon ancestor which must have been a creature in many respects like tarsius, but more macrosmatic and possessed of a small and less highly specialized visual cortex. \\'oolIard, on the basis oi an exhaustive anatomical study, con- 92 THE LOWER PRIMATES eludes that tarsius is a lemur anneetant to the early Eoeene primitive placentals. Standing at the base of the primate stem, it reaches forth to the simian forms and is anneetant to the Anthropoidea. In stating his views concerning the allinities of tarsius, E. D. Cope claimed that "the genus Ana])t()m(ir})l.nis is the most simian lemur yet dis- covered and probably represents the family from which the anthropoid monkeys and men were derived. The animal was nocturnal in its habits and had a number of resemblances to Tarsius which is perhaps its nearest ally among the lemurs." Mathew and Granger both are in general accord with this view when they say that "There are several characters in addition to the larger braincasc in which the skull of Tarsius is more modernized than that of the Lower Eocene Ana])t(>mi)rpl.ms, but in some other genera of this group the dentition is much nearer to Tarsiiis and the skull construction may likewise have been nearer." In regard to tarsius, Earle likewise believes that this is evidently a type nearly between the lemurs and the apes, but with many essential char- acters belonging to the former group. Some of its anthropoid characters are nascent, so to speak. They are Just developing, and as in the case of the orbit of tarsius, it is not yet fully differentiated into the higher type of true anthro- poids. The anthropoids diverged from a lem urine stock probably not earlier than the Upper Eocene. This deduction is supported by the fact that the first lemurs to appear are insectivorous in their allinities. Sonntag also believes that the Eocene Anaptomorphidae gave rise to tarsius and the monkeys arose from a tarsioid ancestor. Wood-Jones, however, has assumed what is perhaps the most radical attitude in holding that tarsius "like man shows primitive cranial architec- ture. His kidney is formed on human lines, his aortic arch is arranged as in man, and in a w ord he shows with man the basal mammalian simplicity of the primate group. He is a most highly specialized little creature on his own cur- TARSIUS SPECTRUM 93 ious lines and yvt he retains with man a host ot those astonishingly primitive features that plaee this odd eouple at the base of the primate stem. He lingers today a specialized primitive primate nearer akin to man than any other ani- mal known to the zoologist. Tarsius dates right back in the form of Aitapto- morpbus to the base of the Eocene period and at that astonishingly early epoch he had ah-eady gained his own peculiar specializations. His companion in primitiveness — homo — has his own specialization." The paleontological evidence reviewed by Professor Gregory seems to be against Wood-Jones' view that the existing tarsius is the nearest living relative of man. "Tarsius may well parallel the human condition in construc- tion of the placenta and in a few other points noted by Wood-Jones, but its relationships to man arc plainly very indirect and must be traced backward along gradually converging lines to the primitive Tarsioid stocks which gave rise at different times and at different places to the higher groups of Tarsius. " "Neither tarsius itself nor its own Eocene relatives mentioned above appear to be directly ancestral either to the platyrrhirie or to the catanhine divisions of the Anthropoidea. Nevertheless, tarsius parallels the higher primates in so many characters of the brain, skull, reproductive organs and other parts that a very remote common ancestry of the three suborders seems highly probable." Measurements of Tarsius The average measurements of tarsius are: Body length 14Q mm. Tail 208 mm. Head length 40 mm. Head breadth 32 mm. Upper extremity 109 mm. Lower extremity 177 mm. Total weight 923 gms. 94 THE LOWER PRIMATES Surface Appearance of the Brain in Tarsius The cerebral hemisphere of tarsiers is hssencephalic, the iissure of Sylvius lacing the sole indication of sulcal marking. In general contour the brain of tarsius is elongate, its greatest length being 21 mm., its greatest width 20 mm. The length is further augmented by the addition of a pro- nounced bulbus olfactorius which projects forward for a consideralale distance beyond the frontal region. In this regard the tarsial brain is as primitive as many of the lower mammals and even approaches reptilian conditions. None of the primates possesses a bulbar development comparable in prominence to that of tarsius. In fact, it is in exactly this olfactory detail that all of the Anthropoidca manifest that marked involution significant of a progressive microsmatic development. THE LISSENCEPHALIC CHARACTER OF THE CEREBRAL CORTEX The Sylvian fissure, although clearly distinguishable, has none of those features which characterize it in the apes and man. It unquestionably corre- sponds to that suprarhinal fold which forms the mammalian pseudo-Sylvian fissure. Lobation of the tarsicr's hemisphere is almost negligible. The Sylvian indentation separates a poorly developed frontal lobe from a feebly delin- eated temporal area. The occipital pole has extended caudad so that the cerebral hemisphere covers a large portion ot the tentorial surface of the cerebellum. This is a definitely pithecoid character. On the other hand, that total absence of boundary between the occipital and parietal as well as between the parietal and frontal areas bespeaks a neopallium of such pronounced generalization that it might well serve for the lowest of mammalian forms. With the exception of the marmoset, no such Hssencephalic condition of the cerebral cortex is encountered among the primates. In the Hapalidae, how- TARSIUS SPECTRUM 95 ever, the lack of fissures in the neopallium may be attrilmted to a retrograde process in the cortex which, according to one interpretation at least, repre- sents a secondary degenerate stage produced during the evolution of the FIG. 45. DORSAL SURFACE OF BRAIN OF TARSIUS SPECTRUM. [Actual Length, 21 mm.] Key to Diagkam. cerebel.. Cerebellum; front, lobe. Frontal Lobe; obl.. Oblongata; olf. bulb. Olfactory Bulb; sylv. fiss., Sylvian Fissure. Cebidae. The simple condition of the hemispheres of tarsius would seem, on the other hand, to represent the retention of definitely primitive characters in a brain in many other respects affected by marked progressive tendencies. ORBITAL CONCAVITIES AND OLFACTORY BULB The basal surface of the hemisphere presents two ill-defined orbital con- cavities although the orbits in tarsius are extremely large. The lack of prom- inence of these concavities is due to the fact that the frontal lobe is poorly developed. The olfactory bulb is large, protruding rostrad in front of the 96 THE LOW ER PRIMATES frontal pole and passing backward into a short stubby olfactory tract which terminates in a prc^ninent tubercukini ollactorium. This protraction of the bulb is not found in lemurs. The tLibercuhim is embraced by the olfactory OLFACTORY NRR,Vg HIG. 46. BASE OF BRAIN, TARSIUS SPECTRUM. [Actual Length, 2i mni.| Striae and bounded posteriorly by a well-defined perforated space and diag- onal band of Broca. The two olfactory bulbs and tracts together constitute a most pronounced intcrorbital keel. The olfactory bulb is further identified in tarsius by its connection with the olfactory portion of the nasal mucosa through a single olfactory nerve. The lila olfactoria, undoubtedly because of the large size of the orbits, have coalesced to form a single strand, although near the entrance into the olfac- tory bulb there is some evidence of the spreading-out characteristic of other primates. TARSIUS SPECTRUM 97 THE OPTIC CHIASM The optic chiasm is Hat and broad and gives the impression of generally greater thickness than in other forms, due to the fact that almost all of the optic fibers undergo decussation. Very few optic fibers in tarsius are uncrossed. The interpeduncular space is small, bounded by feebly developed cerebral peduncles, while the temporal lobes are broad and Hat, each filling a more or less irrcguhir quadrate area in the lateral portion of the middle fossa. THE VENTRICLES AND THE VISUAL CORTEX The ventricular system of the hemispheres is particularly significant, inasmuch as the lateral ventricle develops a posterior horn extending into the occipital region, which is not the case in any of the lemurs. The ventricular extension of the olfactory peduncle characteristic of many mammals is, however, being obliterated. The visual cortex of the cerebral hemisphere shows a marked increase as compared with lower mammals and even the lemurs. It is easily detected by the naked eye and occupies nearly one-third of the entire neopallium. THE CEREBELLUM The cerebellum is notable as indicating those lateral expansions which in higher primates become the most important part of the organ, namely, the lateral cerebellar lobes. Tarsius in many respects is an excellent proto- type in most simple terms for the subsequent extensive development of the primate cerebellum. Its tentorial surface lies at an angle of about 70'^ with the axis of the stem, thus showing the early tendencies which eventually carry this surface more nearly into the horizontal. The occipital lobe covers most of the tentorial surface which consists largely of the superior \-ermaI 98 THE LOWER PRIMATES portion of the cerebellum. A well-defined llocculus and parallocculus are attached along the region of confluence with the axis. The occipital surface also occupies a vertical position and as yet has not FIG. 47. RIGHT LATERAL SURFACE OF BRAIN, TARSIUS SPECTRUM. [Actual Length, 21 nim.l been tipped backward, as is the case in higher primates. This fact accounts for the exposed position of the uvula and nodule which have not yet under- gone that introversion which produces their ultimate intraventricular relation characteristic of most of the anthropoids. The fissures of the cerebellum have an almost diagrammatic clearness in producing the three major subdivisions of the cerebellum. The lissura prima appears upon the tentorial surface and extends outward from the vermis upon the lateral lobe without sulcal interruption. This is likewise true of several other fissures of this surface. The fissura secunda appears in relation with the occipital surface but, as in other mammals, it does not extend out upon the lateral expansion of the cerebellum, being interrupted by the presence of a definite paramedian sulcus. All things considered, the cerebellum of tarsius is the most primitive of all primates, and yet it foreshadows in so many notable details the future development of this organ in apes and man that it must be regarded as inherently anthropoid. TARSIUS SPECTRUM 99 The Brain Stem in Tarsius the ventral surface The ventral appearance of the brain stem in tarsius is peculiar because of the relatively small size of the pons Varohi, the pyramids, the inferior olivary eminences and the cerebral peduncles. All of these are evidences of a low degree of development in the neokinetic organization of the animal. Its vohmtary control of motor activity, its dispensation of coordinative control of the muscles, its measure of simultaneous regulation in the move- ments of the head, eyes and hands, could not be other than extremely simple and generahzed on the basis of these structures. The pyramids appear as two narrow bands extending caudad from the lower border of the pons toward the upper cervical region of the spinal cord. The pons itself is a narrow, flat band of very httle surface prominence. Caudal to it the corpus trapezoideum occupies a transverse position upon the surface of the axis. Tarsius is the only primate in which the corpus trapezoideum is entirely exposed in this position. This fact does not imply any greater organization in the auditory apparatus of the animal, but does denote how poorly the pons Varohi has developed. The ohve makes a very small protuberance lateral to the pyramid and is separated from it by a wide area hardly justifying the term preohvary sulcus. The emergent fibers of the hypoglossal nerve, however, make their appearance on the surface in close relation to the olivary eminence. Two small peduncles form the caudal boundary of a limited optico-peduncular space whose cephalic boundary is provided by the massive optic chiasm and tracts. THE DORSAL SURFACE The dorsal surface, upon removal of the cerebellum, shows a poorly defined clava, cuneus and tuberculum trigemini. The floor of the fourth 100 THE LOWER PRIMATES ventricle has its usual boundaries but shows in a most indefinite way these surface markings which characterize it in Anthropoidea. This dorsal surface of the brain stem, however, assumes great prominence because of the develop- FIG. 48. VENTRAL SURFACE OF BRAIN STEM OF TARSIUS SPECTRUM. [Actual Length, 16 mm.] Key to Diagram, cerebr.-peduncle, Cerebral Peduncle. mcnts in the mesencephahc roofplate. Here the mfenor colliculus and mesial geniculate body are larger and better defined than in all other primates. The superior colliculus attains dimensions almost warranting the designation of optic lobe, but in any event much more conspicuous than in lemurs, monkeys, apes or man. Thus, in the feeble development of certain features whose progressive expansion characterizes the line of evolutional development in the primates, tarsius appears to be an expression of this process m its simplest terms. It indicates this primitiveness quite as much in the superior and inferior colliculi of the midbrain. In spite of the fact that the animal has made such notable TARSIUS SPECTRUM loi gains ill the telenccphalization of vision, the primordial receiving centers for this sense still maintain a high degree of prominence in the midbrain. This primitive feature becomes progressively more prominent in descending FIG. 49. DORSAL SURFACE OF BRAIN STEM OF TARSIUS SPECTRUM. [Actual Length, 16 mm.] Key to Diagram, dorso-med. fissure, Dorsomedian fissure; d. m. septum, Dorsomcdian Septum; sup. CEREBL. PEDUNCLE, Superior Cerebellar Peduncle; tub. trigem. Tuberculum Trigemini. the vertebrate scale to its convergence upon the reptilian basis of mammalian differentiation, at which level it reaches its full expansion in the optic lobes. Internal Structure of the Brain Stem The impression obtained from a microscopic review of the brain stem is that of a primitive mammalian organization. The structural details are, so to speak, still in the crude. They have none of that refined defmition which dis- tinguishes the higher primates and man. All of the topographical features 102 THE LOWER PRIMATES throughout the stem show an indecisive differentiation. This feature is partic- ularly noted in the cranial nerves whose emergent fibers by comparison with other primates seem coarse and heavy. The nuclear territories and tract FIG. TARSIUS SPECTRUM. LEVEL OF THE FIRST CERVICAL SEGMENT. CEN, Central Gray Matter; cb, Column of Burdach; CG, Column of Goll; dg, Dorsal Gray Column; dt, Deiterso-spinal Tract; fle. Dorsal Spinocerebellar Tract; cow, Ventral Spinocerebellar Tract; py, Crossed and Uncrossed Pyramidal Tract; ref, Reticular Formation; rst, Rubrospinal Tract; spt, Spinothalamic Tract; ven, Ventral Gray Column. [Accession No. 210. Section 26. Actual Size, 3X2 mm.] regions ahke are indefinite in their boundaries. Some noteworthy speciahza- tions appear in particuhir mechanisms, such, for example, as the pronounced expansion of the ventral gray cohimn connected with the origin of the spinal TARSIUS SPECTRUM 103 accessory nerve, and the large dimensions of the vestibular area. These features will be especially dealt with in the interpretation of their physiologi- cal significance. LEVEL OF THE PYRAMIDAL DECUSSATION (piG. yl) At this level the characteristic feature is the crossing from one side to the other of pyramidal fibers (Pyx). This is much less regular than in other primates. The crossing strands arc somewhat indiscriminate in their disposi- tion. The bundles tend to interlace and have little of that successive crossing first from one side and then from the other, notable in higher primates. There is a suggestion, particularly at the caudal end of the decussation, that some fibers may make their way into the dorsal columns after the manner of certain lower mammals. This observation needs experimental confirmation. Pal- Weigcrt preparations scarcely more than suggest this possibility. In all details the pyramidal crossing seems more primitive than in any other pri- mates. If some fibers actually do enter the dorsal columns, the conception of tarsius as a much generalized intermediate form gains further support. The central gray matter (Cen) is large and rectangular in outline, con- taining near its center the central canal. Its greatest diameters are in the antero-posterior direction. It is connected with a large dorsal gray column by a cervix of somewhat irregular outline, but separated from the ventral gray column by the crossing pyramidal fibers which assume a juxtagriseal position in dense bundles after they have decussated. The large size of the ventral gray column (Ven) is notable, particularly as there appear to arise in it many fibers which take a course backward and outward, characteristic of the spinal accessory nerve. These fibers constitute a nerve of much larger size and generally more conspicuous than in any of the apes. This fact, in conjunction with the great prominence of the ventral gray column, suggests the probable specialization in connection with one of the peculiar habits of 104 THE LOWER PRIMATES tarsius, namely, the turning of its Iiead completely around so that it can look backward as well as to the front. The animal moves its head rather than its eyes in its visual pursuit of objects. This requires a highly speciahzed cervical FIG. 51. TARSIUS SPECTRUM. LEVEL OF THE PYRAMIDAL DECUSSATION. CB, Column of Burdach; cen, Central Gray Matter; CG, Column of Goll; dt, Deiterso-spinal Tract; fle, Dorsal Spinocerebellar Tract; cow, Ventral Spinocerebellar Tract; nb. Nucleus of Burdach; nr. Nucleus of Rolando; py. Pyramid; pyx. Pyramidal Decussation; ref, Reticular Formation; rst. Rubrospinal Tract; SPT, Spinothalamic Tract; trd, Descending Trigeminal Tract; ven, Ventral Gray Matter. [Accession No. 210. Section 65. Actual Size 4X3 mm.] musculature, which no doubt determines the prominent spinal accessory feature in this region of the brain stem. TARSIUS SPECTRUM 105 The dorsal field consists of a small column of GoII (CG),a large column of Burdach (CB) and an extensive substantia gelatinosa (NR). Adjacent to the latter is a wcll-dcfincd descending trigeminal tract (Trd). The separation of the dorsal gray cohimn is less pronounced than in many of the higher forms. The general proportions of the elements entering into this field suggest at once a tail and lower extremity which give rise to a sensory influx much k^ss than that from the hand and arm. It is probably the case that in tarsius differentia- tion of the upper extremity is more effective than of the lower extremity. The great speed and accuracy with which the hands are employed in the capture of insects, and at the same time for prehensile purposes in ahghting after its remarkable leaps, would seem to imply a specialization in the upper extremity much superior to that in the hind limbs. The tail has no prehensile characteristics, and while it acts in a supple- mentary manner for supporting the clinging posture of the animal, its activi- ties indicate no great increment of sensory inllux. The size of the nucleus of Rolando is indicative of facial innervation, which is essential to the animal in guiding its locomotion. The lateral white column contains in its circumferential zone the spino- cerebellar tracts (Fie, Gow) while in the intermediate zone are the rubro- spinal (Rst), Dciterso-spinal (DT) and spinothalamic tracts (Spt).The pyramidal status denotes a behavioral complex consisting of relatively few components and extremely poor in its pattern for skilled acts. CAUDAL EXTREMITY OF THE DORSAL SENSORY NUCLEI (fIG. 52) Here the chief feature is the appearance of the three nuclear structures representing discriminative sensory transmission from the body. The nucleus of Goll (NG) is well defined but shows no evidence of the median unpaired nucleus of Bischoff. This deficiency- in tarsius harmonizes with the fact that the animal's tail is not highly specialized, although it acts in spring- io6 THE LOWER PRIMATES like capacity, aiding the animal to cling in vertical position. The tail has no prehensile qualities but serves in the general capacity of a steering and balancing organ much as in the tailed monkeys of the old world. FIG. y2. TARSIUS SPECTRUM. LEVEL OF THE DORSAL SENSORY NUCLEI. CEN, Central Gray Matter; CB, Column of Burdach; CG, Column of GoU; dt, Deiterso-spinal Tract; fle. Dorsal Spinocerebellar Tract; cow. Ventral Spinocerebellar Tract; mf, Mesial Fillet; nb. Nucleus of Burdach; ng, Nucleus of Goll; nr. Nucleus of Rolando; pv, Pyramid; ref, Reticular Formation; rst. Rubrospinal Tract; spt, Spinothalamic Tract; trd. Descending Trigeminal Tract. [Accession No. 210. Section 72. Actual Size, 6X4 mm.] The nucleus of Burdach (NB) is considerably larger than the nucleus of GoII. This fact gains considerable weight wlicn tlie cohuiins of Burdach and Goll are contrasted. Tarsius seems to possess a mechanism for sensory conduction more extensive in the representation of its hands and arms TARSI us SPECTRUM 107 than of its feet, legs and tail. In contrast with the lemur and marmoset, this relation approaches nearer to the more highly developed anthropoids. The explanation, however, depends probably not so much upon the high speciah'za- tion in the upper extremities, as upon the low speciahzation in the lower extremities. Tarsius uses the hind legs much as does a frog. It leaps from branch to branch and hops upon the surface of the ground. The specializa- tion of the legs is thus not particularly advanced for purposes of locomotion. It is inferior to that in the forelimbs for such acts as the animal develops in connection with procuring lood. The nucleus of Rolando (NR) is particularly prominent as a sensory element. This is also true of the descending trigeminal tract (Trd), from which it is obvious that the sensory innervation of the head and face plays a conspicuous part in directing the animal's locomotion. The head and face are especially provided with vibrissae about the chin, mouth, and beneath the nose. Supraorbital vibrissae also exist. The central gray matter (Cen) occupies a position in the center of the section but has shown considerable migration dorsad, thus manifesting the general tendency to assume its characteristic position in the floor of the fourth ventricle. The ventral gray column has entirely disappeared, and its position is occupied by a continuous irregular mass representing the formatio retic- ularis grisea (Ref). Contiguous with the latter on its lateral aspect is the intermediate medullary substance which contains the rubrospinal (Rst), spinothalamic (Spt) and Dciterso-spinal (DT) tracts. On the periphery is the circumferential zone which contains the ventral (Cow) and dorsal (Fie) spinocerebellar tracts. Immediately adjacent to the ventromedial sulcus is a dense mass of decussating axons com- posed of internal arcuate fibers arising in the nucleus oi Goll (NG). These decussating fibers form the lower portion of the mesial iiilet (Mf). io8 THE LOWER PRIMATES LEVEL OF THE CAUDAL EXTREMITY OF THE INFERIOR OLIVE (fIG. 53) At this level the lower tip of the inferior olive makes its appearance nO). An important fact concerning this structure in tarsius is its indecisive FIG. TARSIUS SPECTRUM. LEVEL OF CAUDAL TIP OF INFERIOR OLI\'E. CEN, Central Gray Matter; fle, Dorsal Spinocerebellar Tract; do, Dorsal Accessory Olive; dt, Deiterso- spinal Tract; cow, Ventral Spinocerebellar Tract; 10, Inferior Olive; mf, Mesial Fillet; nb, Nucleus of Bur- dach; ng, Nucleus of Goll; nhy, Hypoglossal Nucleus; nr, Nucleus of Rolando; nvd, Dorsal Vagal Nucleus; Nio, Vagus Nerve; N12, Hypoglossal Nerve; pd, Predorsal Fasciculus; pl. Posterior Longitudinal Fasciculus; PY, Pyramid; ref. Reticular Formation; rst. Rubrospinal Tract; spt, Spinothalamic Tract; trd, Descending Trigeminal Tract. |Acccssion No. 2 to. Section 95. Actual Size 6X3 mm.) outline. Its diminutive size as compared with that of other primates is also notable. Mesial to the olive is the pyramid (Py) which likewise is small. Dorsomesial to the pyramid is the mesial fillet (Mf), still receiv- TARSIUS SPECTRUM 109 ing some contributions from the internal arcuate libers arising in the nucleus of Burdach (NB). By contrast, the extensive size of the posterior longitudinal fasciculus (PL) and of the predorsal bundle ( PD) denotes the importance of these colhculo-spinal and other midbrain connections. The central gray matter (Cen) is oval in outline and still more dorsal in position. It contains in its center a much enlarged central canal, while in its ventromesial area is the nucleus hypoglossus (Nhy). This nucleus has none of the discrete distinction in its boundaries notable in the higher primates. The emergent libers passing from it appear in larger and coarser bundles than is the case in any other species examined. In a position dorsal to the central gray matter are the chief nuclei of the dorsal sensory field. Their relation discloses the preponderance of the nucleus of Burdach (NB) as compared with the nucleus of Goll (NG). The large size of the nucleus of Rolando (NR) and of the descending trigeminal tract (Trd) calls attention to the important role of the head and face as a sensory director of the animal's locomotion. The outline of the dorsal aspect of the section shows an increasing con- cavity due to the presence of the uvula cerebelli in this position. The fibers of the spinal accessory nerve pass ventrolaterally through the nucleus of Rolando (NR) toward then- pomt of emergence. The reticular torma- tion (Ref) occupies the greatest portion of the section at this level. In its lateral periphery is the nucleus ambiguus from which the emergent fibers of the vagus nerve pass backward and inward toward the central gray matter in the first part of their intramedullary course. The reticular formation is further specialized to form the nucleus funiculus lateralis and is penetrated by many long internal arcuate fibers. It is noteworthy that these latter elements have none of the conspicuous appearance charac- teristic of higher primates. They, like the mesial fillet and the pyramid of no THE LOWER PRIMATES tarsfus, indicate a less highly organized condition in discriminative sensi- bility and vokintary motor control. It seems obvious from the known behavior of the animal that its range of skilled movements is comparatively limited. From these structures in the brain stem it is permissible to conclude that tarsiers in general accjuire but little increase in their motor attainments under the influence of training. This level might well pass for the corresponding region in the cat or rabbit; in fact, its components have a close resemblance to marsupial organization. The medullary substance in the intermediate zone contains the rubrospinal and spinothalamic tracts (Rst, Spt), while on the circumference are the two ascending cerebellar fascicles (Fie, Gow). A small collection of gray matter dorsal to the main body of the olive is the dorsal accessory olivary nucleus (DO). LEVEL THROUGH THE MIDDLE OF THE INFERIOR OLIVARY NUCLEUS (fIG. 54) At this level the inferior olive is conspicuous because of its structural inferiority as compared with other species. Tliis nucleus in tarsius consists of an aggregation of gray matter, so poorly differentiated from the formatio reticularis that it might appear as an intrinsic part of this formation. The portion of the nucleus which may be discerned appears to correspond to the mesial accessory olive in other forms. A very small mass of cells lateral to this structure represents the inferior olive itself. Thus the most conspicuous portion of the nucleus seems to represent what is usually described as the paleo-olive. Compared with either the cat or the rabbit, the olive in tarsius is distinctly more primitive, and this certainly is the case in comparison with all other primates. Judged alone by the criteria of this structure, it would seem that the tarsier occupies a low position in the organization of its central nervous system; one which might, however, serve most advantageously as a fairlv unbiased foundation for extensive further modifications. In the inter- TARSIUS SPECTRUM iii pretation of olivary fLinction, tarsius is an important witness concerning the activity of this nucleus. If, as is assumed to be the fact, the olivary nucleus is essential to simultaneous coordination of the hands, eyes and head in complex FIG. 54. TARSIUS SPECTRUM. LEVEL THROUGH MIDDLE OF INFERIOR OLIVE. CB, Column of Burdach; cen, Central Gray Matter; fle, Dorsal Spinocerebellar Tract; cow, Ventral Spino- cerebellar Tract; 10, Inferior Olive; mf. Mesial Fillet; nb, Nucleus of Burdach; NFS, Nucleus Fasciculus Solitarius; ng, Nucleus of Goll; nhy, Hypoglossal Nucleus; nr, Nucleus of Rolando; nvd, Dorsal Vagal Nucleus; N12, Hypoglossal Nerve; pd, Predorsal Bundle; pl, Posterior Longitudinal Fasciculus; py, Pyramid; REF, Reticular Formation; rst, Rubrospinal Tract; spt, Spinothalamic Tract; trd, Descending Trigeminal Tract. [Accession No. 210. Section log. Actual Size, 7X4 mm.] skilled acts and helps to facilitate the coordination of all skilled learned performances, its inferior development in the tarsier imphes a low develop- ment of such activity. 112 THE LOWER PRIMATES It has already been noted that the animal is able to turn its head com- pletely around so that it may in eflcct look both forward and backward. Such rotation is due to the fact that the animal moves its eyes relatively little in the visual pursuit of objects. If it wishes to cover its visual field it does so in great measure by movements of the head. This predominance of ccphalo- gyric action over ocular movement may account for the extensive development of the ventral gray column in those cervical regions of the spinal cord in which the spinal accessory nerve takes origin. Thus may be explained the preeminence of the neck muscles in acts of visual pursuit, as well as the incon- siderable development of the inferior olive. From the behavior of tarsius it is clear that the animal does not employ the muscles of the eyes, hands and head in many acts requiring intimate cooperation. It is essentially a noctur- nal hunter, and this fact in itself limits its manual activities to a somewhat restricted range of performance. The remarkable dexterity of the tarsier in capturing insects on the wing, as it is said to do, probably represents the acme of its skilled achievement. This no doubt is a highly specialized act, but it is no more complex than that manifested by many birds which capture their prey while on the wing. Such development of skill would not necessarily make great demand upon the olivary mechanism. Two other facts support the view that tarsiers do not require particular exactitude in the simultaneous control of the head, eye and hand muscles, i.e., the absence of retinal macula and the more or less complete decussation of fibers in the optic chiasm. Both of these important conditions imply the absence of stereoscopic vision, as they also indicate the lack of those visual specializations essential to the development of highly skilled acts. The indispensable relation of simultaneous coordina- tion of head, eye and hand in such acts as handwriting is obvious. In acts of this kind the eyes follow the hand, and the head cooperates with the ej^es in this exact visual guidance. Tarsius is capable of no reactions comparable in any sense to such a highly developed skilled performance. It manifests no TARSIUS SPECTRUM 113 motor pattern which necessitates simultaneous association of the ocuhir, cervical and brachial musculature. It is not surprising in this hght that the inferior olive is poorly developed in tarsius and in marked contrast to the proportions and conhguration of higher primates. The other features at this level serve to bear out the observations ah-eady made concerning the motor and sensory organization, as these functions are ilhistrated I^y the small size of the pyramid and mesial fillet. The great importance of the animal's automatic associated movements is indicated by the size of its midbrain bundles, the posterior longitudinal and predorsal fasciculi (PL.PD). The central gray matter has assumed its position in the floor of the fourth ventricle, and above the central canal a massive obex marks the point of ventricular transition. The nucleus of the hypoglossal nerve has all the indecisiveness in boundary characteristic of lower levels. Its emergent fibers are coarse and heavy. Lateral to this nucleus is the dorsal vagal nucleus (Nvd). In the dorsolateral angle of the central gray matter is the nucleus of the fasciculus solitarius (Nfs). The nuclei of Goll and Burdach (NG, NB) are both prominent and partially invested by fibers forming their corresponding columns. The nucleus of Burdach is remarkable in tarsius for the very high development of the ancillary nucleus of Blumenau. The nucleus of Rolando (NR) and the descending trigeminal tract (Trd) are further removed from the periphery due to the moving of the dorsal spinocerebellar tract into a more dorsal position. The reticular formation still occupies the largest portion of the section and is characterized by its diffuse arrangement (Ref). In its ventrolateral portion may be discerned the nucleus funiculus lateralis. LEVEL OF THE VESTIBULAR NUCLEI (fIG. 55) At the level of the vestibular nuclei the dorsal sensory field has changed in character, as is the rule in all primates. It still represents proprioceptors, 114 THE LOWER PRIMATES but of a highly specialized order. This field, formerly occupied by the nuclei of Goll and Burdach, has given place to the nucleus of Deiters (ND) and the nucleus of Schwalbe (NSc), both of which have acquired remark- FIG. jj. TARSIUS SPECTRUM. LEVEL OF THE \'ESTIBULAR COMPLEX. CTT, Central Tegmental Tract; dt, Deiterso-spinal Tract; GOw, Ventral Spinocerebellar Tract; ICP, Inferior Cerebellar Peduncle; mf, Mesial Fillet; nab, Nucleus Abduccntis; nd, Deiters' Nucleus; nf, Facial Nucleus; NR, Nucleus of Rolando; nsc, Nucleus of Schwalbe; pd, Predorsal Bundle; fl, Posterior Longitudinal Fasciculus; pv. Pyramid; ref, Reticular Formation; rst. Rubrospinal Tract; spt. Spinothalamic Tract; TRD, Descending Trigeminal Tract. [Accession No. 310. Section 140. Actual Size 7X3 mm.] able dimensions as compared with other primates. The Dcitersal area in tarsius is larger than in the cat, rabbit, kangaroo, horse, dog, or any of the primates. The triangular nucleus of Schwalbe, while large, is perhaps not so striking in comparison with other forms. To fmd such extensive development in the balancing mechanism of tarsius implies that the animal requires a most highly efficient apparatus for equilibrium. The behavior of tarsius bears out this supposition. There is nothing perhaps in its postural requirements while at rest which may sug- TARSIUS SPECTRUM i in- gest extensive equilibratory needs; but the long, swift leaps which it makes, not unlike volplaning, would almost certainly require a capable mechanism for maintaining balance. Compared with other primates, tarsius may be considered c[uite alone in the specialization of this peculiar llight-like type of locomotion. In the case of lemur and marmoset, the leaping propensities have a different character, appearing to be intermediate steps in a continuous process of passing from one point of support to another. In this act, all four extremities and the tail participate. The gibbon is another example of flight-like passage through the trees, but in the case of this latter primate, the arms are the principal means of locomotion in the long swings from branch to branch. Tarsius, on the other hand, executes its leaps more in the nature of long dives or upward hops in which it glides through the air for considerable distances in order to reach its next objective. If it is true that in these dives it captures insects on the wing, then the delicate balancing needed for its locomotion is all the more apparent. The great prominence of the vestibular area must depend upon some such specialization as this in tarsius, although it is to be regretted that no authentic statements are forthcoming concerning the actual equilibratory development of this animal. It is remarkable and noteworthy, however, that tarsius leads all the primates, and for that matter most of the mammals, in the high specialization of its vestibular nuclei. At this level the fourth ventricle is widely open and the lateral recesses communicate with it over its dorsolateral angle. The uvula lills the entire ventricular cavity. On the lateral margin of Deiters' nucleus (ND) are the collected bundles of the inferior cerebellar peduncle (ICP), while ventral to the nucleus is the substantia gelatinosa of Rolando (NR). Along the outer border of the latter structure lies the descending trigeminal tract (Trd). The rubrospinal ( Rst) and spinothalamic tracts ( Spt) occupy a position upon the outer edge of the formatio reticularis ( Ref ). The posterior ii6 THE LOWER PRIMATES longitudinal fasciculus (PL) has been augmented by the addition of fibers from Deiters' nucleus, whose course may be traced inward and back- ward from this nucleus. Some of the caudal fibers of the vestibular division FIG. j6. TARSIUS SPECTRUM. LEVEL OF CAUDAL EXTREMITY OF TRAPEZOID BODY. COCH, Cochlear Fibers; ctr, Trapezoid Body; ctt, Central Tegmental Tract; dt, Deiterso-spinal Tract; cow. Ventral Spinocerebellar Tract; icp. Inferior Cerebellar Peduncle; mf, Mesial Fillet; nd, Deitersal Area; n8. Auditory Nerve; nfs. Genu of Facial Nerve; nr, Nucleus of Rolando; nsc, Nucleus of Schwalbe; pd, Predorsal Bundle; PL, Posterior Longitudinal Fasciculus; py, Pyramid; ref. Reticular Formation; rst. Rubrospinal Tract; so, Superior Olive; spt, Spinothalamic Tract; trd. Descending Trigeminal Tract; tub, Tuberculum Acusticum. [Accession No. 210. Section 153. Actual Size 7X3 mm.] of the eighth nerve pass through the descending trigeminal tract and the substantia gelatinosa to enter Deiters' nucleus. A large nuclear structure on the periphery of the reticular formation gives rise to the first portion of the facial nerve. This is the nucleus facialis (Nf). The fibers arising in it pass backward and inward to\\ard the lloor of the fourth ventricle in the form of a heavy spray. This nucleus in tarsius is nearer the periphery of the section than in any of the other primates and the fibers constituting the first portion of the facial nerve are coarser than observed in other species. TARSIUS SPECTRUM 117 Both of these conditions are in closer accord with the subprimate mammals than with the Anthropoidea. Mesial to the facial nucleus is a disseminated collection of axons reach- ing backward toward Deiters' nucleus. These form the beginning of the Deiterso-spinal tracts (DT). Immediately dorsal to the posterior longi- tudinal fascicuhis is the caudal extremity of the nucleus abducentis (Nab). The predorsal bundle (PD) is large and indicates to what degree tlie annual depends on its automatic movements. This observation becomes more significant in view of the small size of the pyramid which calls attention to the relatively meager control of voluntary movement possessed bj" tarsius. The mesial fdlet (Mf) occupying a position dorsal to the pyramid, denotes a low degree of discriminative sensibihty in the animaL All of these facts collectively signify an animal of a simple motor organi- zation and extremely limited in such behavior as is conditioned by learning and imitation. LE\'EL OF THE CEREBELLAR NUCLEI (fIG. 57) Characteristic features at this level are the appearance of the heavy bundles constituting the intramedullary portions of the sixth and seventh cranial nerves (N6, N7). The fibers of the vestibular division of the eighth nerve make their way to Deiters' nucleus between the restiform body (ICP) and the descending trigeminal tract (Trcl). Some fibers of the acoustic division ( N8 ) of the eighth nerve appear in relation to the tubcrcu- him acusticum (Tub). The fourth ventricle is still further reduced in size. Its roof is formed by the median portion of the cerebeHum in which is situated the roof nucleus (Nfg). Heavy bundles of fibers entering the juxtarestiform body pass backward and inward from Deiters' nuckms to the nuck'us of the roof. Lateral to these fibers in the median vestibule of the cerebelkim is a collection of gray matter forming the nucknis dentatus and ii8 THE LOWER PRIMATES the nucleus emboliform Is ( NDt). It is impossible to discern distinctive bound- aries between these two nuclear masses. Even by means of reconstruction these nuclei do not disclose any distinct individualization. It is hence deemed FIG. 57. TARSIUS SPECTRUM. LEVEL OF THE CEREBELLAR NUCLEL CJR, Juxtarestiform Body; ctr, Trapezoid Body; dt, Deitersal Tract; Govv, Ventral Spinocerebellar Tract; icp. Inferior Cerebellar Peduncle; mf, Mesial Fillet; nd, Deitersal Area; ndt, Cerebellar Nuclei, Lateral Group; NFG, Cerebellar Nuclei, Mesial Group; nr. Nucleus of Rolando; nsc, Nucleus of Schwalbe; n6, Abducens Nerve; N7, Facial Nerve; n8. Eighth Nerve; pl, Posterior Longitudinal Fasciculus; pd, Predorsal Bundle; py. Pyramid; ref, Reticular Formation; rst. Rubrospinal Tract; so, Superior Olive; spt. Spinothala- mic Tract; trd, Descending Trigeminal Tract; tub, Tuberculum Acusticum. [Accession No. 210. Section 163. Actual Size 8X5 mm.) advisable to designate this collection as the dentate mass. The outstanding feature regarding it is its lack of definition and the entire absence of any tendency toward convokition which characterizes this nucleus in the higher primates. In tarsius the dentate nucleus is small and, as might be interred from it, the lateral lobes of the cerebellum are poorly developed. This con- dition indicates an animal provided with a low degree of eoordinative control, TARSIUS SPECTRUM 119 a fact which permits of the conclusion that the extremities, and particularly the upper extremities, are capable of the simplest motor patterns only This index of motor organization is an added argument supporting the view that tarsius is but nieagerly endowed in behavioral reactions. Adjacent to the ventromedian sulcus of the fourth ventricle, and beneath the ventricular floor, is the dense circular bundle representing the second portion of the seventh nerve (Ny). Lateral to this facial bundle are the heavy fibers of the abduccns nerve (N6), which make then- way directly forward toward the trapezoid body (Ctr). They arise in the abducens nucleus which is situated ventral to the triangular nucleus. It is of interest in this connection to note the diffuse character of the triangular nucleus. Its boundaries have none of the definition characteristic of the higher species. Many fibers traverse it, giving the impression of a nuclear territory which has not yet thoroughly established its own autonomy. The origin of the Deiterso-spinal fibers (DT) is clearly seen. The restiform body (ICP), the substantia gelatinosa of Rolando (NR) and the descending trigeminal tract (Trd) all occupy their usual positions. The trapezoid body (Ctr), with its large superior olive (So) and its extensive decussation, indicates the degree of development in auditory conduction. The reticular formation(Ref) is extensive and contains no discretely ditler- entiated territories. Adjacent to the raphe, the posterior longitudinal and predorsal bundles (PL, PD) appear in the unusually large dimensions characteristic of tar- sius. The mesial fillet (Mf) is still partially obscured by the crossing fibers of the trapezoid decussation (Ctr), while the relatively small size of the pyramid (Py) is decisively shown in comparison to the rest of the section. It is noteworthy that the entire trapezoid body remains without being submerged by pontile fibers. Tarsius is the only one of the primates in which this is the case. Although the corpus trapezoideum is only partially 120 THE LOWER PRIMATES exposed in lemur and marmoset, the pons Varolii is so poorly developed in tarsius as to leave this auditory decussation wholly uncovered. This fact again speaks in favor of the low development attained by tarsius in the organization of its skilled performances. The summary of features in this region of the axis clearly calls attention to the large size of both the auditory and balancing mechanisms of the animal. It is equally emphatic in denoting the relatively low development of voluntary control possessed by tarsius. It clearly indicates the ample provi- sion made for that more fundamental regulation of motion ailorded by the posterior longitudinal fasciculus andpredorsal bundle. Attention is also called to the rather coarse appearance of both emergent and intersegmental fibers which are in such contrast to the finer architectonics of the higher species. LEVEL OF THE EMERGENCE OF THE TROCHLEAR NERVE (fIG. 58) At this level the main features of the posterior isthmus are apparent. The roof plate of the fourth ventricle is here formed by the superior medul- lary velum upon which rests the lingula of the cerebellum. Passing through the velum are the decussating fibers of the trochlear nerve (N4) which emerges from this region of the brain stem. The tourth ventricle is much reduced in size as it approaches the caudal orifice of the aqueduct. The gray matter in its Hoor (Cen) shows no specialization, and ventral to it are the dense bundles of the posterior longitudinal fasciculus (PL) and thepredorsal bundle (PD). Lateral to the central gray matter (Cen) are the axons con- stituting the mesencephalic root of the trigeminal nerve, outside of which are the fasciculus uncinatus of Russel (hook bundle) (Tur) and some fibers of the vcntrospinal cerebellar tract (Gow). In the dorsolateral position of the section are the fibers forming the lateral lillet (Lf) now ascending on its way to the inferior'colliculus. The fibers of the superior cerebellar peduncle (Sep) occupy a position mesial to the hook bundle of Russel. Some hori- TARSIUS SPECTRUM 121 zontal fibers are already in process of crossing toward the midline in a position dorsal to the posterior longitudinal fasciculus (PL). This forms part of the dorsal decussation of the superior cerebellar peduncle. FIG. 58. TARSIUS SPECTRUM. LEVEL OF THE TROCHLEAR EMERGENCE. CEN, Central Gray Matter; ctt. Central Tegmental Tract; Gow, Ventral Spinocerebellar Tract; lf, Lateral Fillet; mcp. Middle Cerebellar Peduncle; mf. Mesial Fillet; nr, Nucleus of Rolando; ntr. Trapezoid Nucleus; N4, Trochlear Nerve; pd, Predorsal Bundle; pl. Posterior Longitudinal Fasciculus; pns, Pons; py, Pyramid; REF, Reticular Formation; scp, Superior Cerebellar Peduncle; trd. Descending Trigeminal Tract; tur, Tractus Uncinatus of Russel (Hook Bundle). [Accession No. 210. Section 195. Actual Size 9X5 mm.] The fibers constituting the ventral peduncular decussation have already moved into a position nearer the midline. This latter disposition in the decus- sation of the superior cerebellar peduncle is characteristic of all of the pri- mates. The arrangement of the more dorsal fibers in tarsius, however, corresponds more to the conditions in the cat and the rabbit in which the dorsal decussating axons from the superior cerebellar peduncle tend to take up positions dorsal to the bundles of the posterior longitudinal fasciculus. 122 THE LOWER PRIMATES In another respect the superior cerebellar peduncles show primitive characteristics in tarsius. This appears in the fact that the crossing of this cerebellar connection begins much lower down than is the case in any of the other primates. As in other instances, the peduncle crosses in t\A o divisions, l)ut the more dorsal division decussates earlier in tarsius than in other forms, leaving a dense ventral bundle in the lateral portionof the reticular formation. These more ventral fibers eventually move inward to complete the superior peduncular decussation. Especial importance attaches to this division of the decussation, as it represents a less highly organized cerebellar connection than in any of the other primates. The ventral portion of the section contains a small collection of pontile libers and nuclei. The number of the crossing fibers in the pons is so limited that it is diflicult to distinguish the three typical strata of this portion of the stem. On the other hand, there is a slight separation of the pyramidal fibers by transverse axons passing through them in the pons region. Pyramidal dissemination, however, has none of those characteristic features prominent in all of the other primates. The mesial fillet ( M f ) occupies a position immediately dorsal to the pyramid. In the lateral portion of the section is the upper extremity of the trapezoid nucleus (Ntr). In general appear- ance this level is notable for the unusually meager representation of the pontile nuclei and the primitive manner in which the superior cerebellar peduncle undergoes its decussation. LEVEL OF THE INFERIOR COLLICULUS (fIG. 59) Here, the special features are the marked development of the tectum (IC) and the pons (Pns). This latter character of the brain stem in tar- sius deserves particular consideration. In several ways it differs from the corresponding structure in all other primates. The lowermost transverse fibers of the pons Varolii do not make their appearance until the level of the FIG. 59. TARSIUS SPECTRUM. LEVEL OF THE INFERIOR COLLICULUS. CEN, Central Gray Matter; ctt, Central Tegmental Tract; ic. Inferior Colliculus; lf, Lateral Fillet; mf, Mesial Fillet; mcp, Middle Cerebellar Peduncle; ntr, Nucleus Troclilearis; pd, Predorsal Bundle; PL, Posterior Longitudinal Fasciculus; pns, Pons; pv. Pyramid; ref, Reticular Formation; rst, Rubrospinal Tract; spt, Spinothalamic Tract; tmt, Mesencephalic Root of Fifth Nerve; xscp. Decussation of Superior Cerebellar Peduncle. [Accession No. 210. Section 205. Actual Size 9X9 mm.) [123I 124 THE LOWER PRIMATES trochlear decussation is reached. In other words, the bulbopontile sulcus holds a much more cephalic position than in other species. It is even higher than in the cat and most rodents. This position of the sulcus clearly indicates the inferior development of the pons VaroHi. The internal pontile structure, however, does not depart from the type of organization characteristic of it elsewhere. In it may be discerned the three typical strata. A pecuharity in their arrangement is the fact that the stratum superlicialc does not, as in other species of primates, appear as the most caudal element in the pons. Fibers from the stratum complexum make their way across the stem in the direction of the middle cerebellar peduncle before there is evidence of the more superficial transverse axons. This gives rise to an unusual appearance, in that the pyramid maintains its ventral position m the stem although pon- tile fibers are ah'cady present. It is usually the case that in the caudal por- tion of the pons, superficial transverse fibers cross in front of the pyramid and thus exclude the latter from a position on the surface. This arrangement of fibers makes the pyramid in tarsius appear much longer than in other species, while the pons is actually much shorter. Nor is the peculiarity of the pontile fibers limited alone to this late appearance of the superficial stratum. Such decussation as the pontile fibers do make, exerts a feeble influence upon the long suprasegmental systems. The pyramid, for example, which in all other primates becomes separated into more or less scattered bundles, shows little of such tendency in tarsius. The manner in which the pyramidal system maintains its integrated constituency and seems in this way to force the pontile fibers to sweep around rather than through it, is another fact emphasizing the low organization of the pons Varolii in tarsius. The marked tendency of the mesial fillet to stretch out transversely in the pons so as to form the boundary between the tegmentum and the basis, is almost entirely absent in tarsius. The fillet maintains its position relatively near the median line, where it appears as a dense bundle. All of these phenom TARSI us SPECTRUM 125 ena, as they affect respectively the appearance of the pyramid and the mesial fillet, denote the slight inlluence exerted by the pontile libers upon the course of such suprasegmental systems. Such a pons as that of tarsius could not belong to an annual with more than a meager neokinetic endowment. As a behavioral index it gives a rating lower than the Felidac and about equal to some of the rodents. A capacity so low as this implies a low cerebral organization which might well serve as the stepping stone in the progressive ascent from lower mammalia toward the primates. The surface appearance of the mesencephalic tectum strongly suggests that the inferior colliculus is a functionally important structure. Its general and microscopic characters support this view. It is pronounced in size even when compared with many subprimate forms. Mensuration also indicates that the inferior colliculus plays an important role in the sense of hearing. That part of the animal's behavior dictated by auditory stimuli is characterized by prompt reflex responses. Tarsius consumes little time in reflecting upon the nature of sounds which it hears. It employs such stimuli with almost automatic direct- ness to produce such reactions as guarantee its immediate safety or pro- voke fundamental activities. Furthermore, these auditory stimuli call forth at best only a most limited series of motor reactions. This conception is substantiated by the meager development of auditory areas in the temporal lobe of the brain, as well as by the auditory suprasegmental connections between the midbrain and the endbrain. Tarsius also retains in the microscopic appearance of its inferior collic- ulus much of that organization characteristic of lower forms. In the cortex mesencephalica posterior it is possible to identify nine distinct strata. This arrangement harks back to the lower vertebrates, so that the large size of the inferior colliculus, in conjunction with its architectonic specialization signi- fies a retention of the primordial midbrain control over the function of hearing. 126 THE LOWER PRIMATES The central gray matter is quadrilateral in outline surrounded by the collicular eminences and contains a central canal considerably elongated in the dorsdventral direction. It is of interest in this connection to note a numl^er of small diverticula connected with the central canal. Such appear- ances are common in the adults of many subprimate mammals, but become less conspicuous in the higher members of the primate group. Ontogenetically, these evaginations are boubtless related to those extensions of the Sylvian aqueduct observed in manjf Io\\er mammals and connected w ith the de\'elop- ment of both the inferior and superior colliculi. Traced further back they are doubtless remnants of the phyletically ancient ventricles of the optic and auditory lobes of the midbrain. In the ventral aspect of the gray matter is the trochlear nucleus (Ntr), which is notable because of its indefinite boundaries and the profusion of fibers interspersed throughout its entire mass. It is a nuclear collection of unusual size, suggesting that the fourth nerve in tarsius must play an impor- tant role. In conjunction with the closely set, protruding eyes, it may be that convergence of the visual axes has assumed great physiological prominence in these animals. Their acquisition of binocular vision has doubtless already set on foot those specializations which culminate in the development of stereoscopic function. On the other hand, the absence of a retinal macula might call the validity of this view in question. However this may be, it is clear that the tarsiers need and employ the superior oblique muscle of the orbit in such a manner as to require a large nucleus for its innervation. The course of the trochlear nerve is of interest because in its emergence it departs somewhat from the course pursued in other primates. In emerging from its nuclear origin, it passes immediately dorsad following a path restricted to the central gray matter. It has no descending portion character- istic of members of this group. It leaves the dorsal rather than the dorso- mesial aspect of its nucleus. Its entire course gives the impression of TARSIUS SPECTRUM 127 shortening due to compression in tlie stem, particularly about the region of the posterior isthmus. Suf^jacent to the trochlear nucleus are the dense bundle of the posterior longitudinal fasciculus (PL) and the predorsal bundle (PD). On the periphery of the central gray matter is the mcsenccphahc root of the fifth nerve (Tmt). The reticular formation at this level is extensive, but its details are considerably obscured by the decussation of the superior cere- bellar peduncle (XScp). LEVEL OF THE SUPERIOR COLLICULUS (FIG. 6o) At this level certain features peculiar to Tarsius become apparent. Most conspicuous among these is the unusually large size of the entire tectal region. Yet, in spite of exceptional dimensions, the superior colliculus (SC) presents less stratigraphical specialization than might be expected from its size. The colliculus as a whole appears to be a region extremely rich in cellular elements. In contrast to the large proportions of the colliculus, the extremely small size of the cerebral peduncle (CP) is another striking feature of the midbrain. Considered together, they impart to this segment of the tarsial stem a characteristic appearance different from all other primates. The small size of the cerebral peduncle is dependent primarily upon the meagre contributions made by the neopallium to the pallio-pontile sys- tem. It is also dependent upon the relatively small volume of the pyramidal system. Thus, the peduncle is indicative of an animal whose behavioral patterns are extremely rudimentary and limited. It is another fact confirming the opinion that tarsius has little capacity in the acquisition of learned skilled acts and that its biological formula in adaptive reaction is determined largely by its automatic associative combinations. 128 THE LOWER PRIMATES If comparison were made between tarsius and some of the lower mammalian forms — carnivores, rodents and even marsupials — on the basis of the cerebral peduncle, the tarsier would be forced into a position FIG. 60. TARSIUS SPECTRUM. LEVEL OF THE SUPERIOR COLLICULUS. CEN, Central Gray Matter; ctt. Central Tegmental Tract; cp. Cerebral Peduncle; mf. Mesial Fillet; mgb. Mesial Geniculate Body; N3, Oculomotor Nerve; nru, Nucleus Ruber; noc. Oculomotor Nucleus; pd, Predorsal Bundle; PL, Posterior Longitudinal Fasciculus; ref. Reticular Formation; sbn, Substantia Nigra; sc, Superior Colliculus; spt, Spinothalamic Tract. [Accession No. 210. Section 250. Actual Size 9X7 mm-1 of relative inferiority. It may perhaps, to some extent at least, excel certain of the marsupial species. Its primitiveness is again emphasized by the extreme dimensions of the superior colhcuhis. This structure, because of its size, by TARSIUS SPECTRUM 129 permitting a certain degree of latitude, might be considered an "optic lobe." The disposition of the central canal in this midbrain region would carry further suggestiveness in this line of interpretation. The large lateral diverticulum extending in the direction of the collicuhis is strongly reminis- cent of the hiteral mescncephaHc extension characterizing the brain of many lower vertebrates. One notable advance in the brain of tarsius toward primate differ- entiation is the definite progress which it has made in extending the visual cortex. On the other hand, it is improbable that such advance has gone so far as to relieve the superior colhculus of much of its primitive visual func- tion. Such a view attributes to the optic portion of the mesencephalic roof a large retention of activities related to vision, a condition which again declares the primitive organization of the bram in this animaL Accompanying these strikingly primitive characters in the cerebral peduncle and the superior colhculus are other features at this level which bear testimony of similar moment. The substantia nigra (Sbn), for example, is an even more extensive structure than it is in lemurs. It thus stands out in contrast to this structure in all others of the primate order. The reticular formation (Ref) likewise is more conspicuous and much more diffuse. In it may be distinguished a poorly defined nucleus ruber ( NRu) of relatively small size. Along the ventrolateral border of the substantia nigra are the two divisions of the mesial fillet (Mf), the ventral portion lying in close proximity to the emergent fibers of the nerve, the dorsal portion border- ing the brachium geniculatum and lying ventral to the spinothalamic tract (Spt). The central gray matter (Cen) is more extensive and also more pyriform than in other primates. It contains in its ventral apex the dorsal and ventral divisions of the oculomotor nucleus (Noc) which is charac- terized by the fact that but few fibers enter the oculomotor decussation. This latter observation is in harmonv with the statement made l^y the late 130 THE LOWER PRIMATES Dr. John Hunter to the effect that the nucleus of Perha, although present in tarsius, is extremely small. The inference to be drawn from these cor- related facts again indicates an animal poorly equipped in the neural mech- anisms necessary for binocular stereoscopic vision. Hence in the organization of visual function the tarsier occupies a position below the lemurs and new-world monkeys. The importance of that ocular advance which distinguishes the primates from all other mammalian orders cannot be overestimated. It has been a momentous factor in the e\'olutionary process. What effects must have been produced in consequence of forward-looking eyes — eyes so related to each other that both visual axes could be directed upon an object cither nearby or at a distance, eyes which no longer looked more or less independently to one side or to the other — it is difficult to estimate in measurable terms. In the final outcome of constructive development this new ocular relation must have borne in the weightiest manner upon that enormous super- structure of skilled movements which has had its supreme expression in the achievements of man. Whether tarsius represents the earliest hesitating steps in this direction, or whether it has made real progress over some even simpler prototype in visual organization, is a question beside the point. This animal is illustrative of one of those early steps, if not the earliest, toward that inestimable consummation of visual activity in the acquisition of skilled movements which at length came to be the distinguish- ing feature in the neokinetic progress of the primates. Lateral to the nucleus of the third nerve is an extensive fasciculus longitudinalis posterior (PL) whose ventral extremity borders upon the large collection of fibers constituting the predorsal bundle (PD). In the concavity of this extensive collection of axons is the central tegmental tract (Ctt) now drawn closely in toward its ultimate position of contact with the central gray matter. Bordering this substance in its more dorsal TARSIUS SPECTRUM 131 region is the ascending trigeminal tract. Arcuate filsers passing inward from tlie region of this latter tract sweep forward to decussate in the dorsal decussation of Meynert. Decussating fibers occupying a still more ventral FIG. 61. TARSIUS SPECTRUM. LEVEL OF THE OPTIC CHIASM. CEN, Central Gray Matter; ctt. Central Tegmental Tract; cp, Cerebral Peduncle; lgb, Lateral Geniculate Body; mf. Mesial Fillet; mgb, Mesial Geniculate Body; nli, Nucleus Lateralis Internus Thalami; opt. Optic Tract; opx, Optic Chiasm; rem, Tractus Retroflexus of Meynert; \'3, Third \'cntricle. [Accession No. 210. Section 261. Actual Size 10 X 10 mm.] 132 THE LOWER PRIMATES position constitute the decussation of Forel, many of whose axons appear to take origin in the red nucleus (NRu), cross the midline and enter into that descending bundle which constitutes the rubrospinal tract. On the FIG. 62. TARSIUS SPECTRUM. LEVEL OF THE ANTERIOR COMMISSURE. AC, Anterior Commissure; cin. Internal Capsule; cix, External Capsule; fdp, Descending Pillar of Fornix; FOR, Fornix; len. Lenticular Nucleus; XCA, Nucleus Caudatus; nep. Neopallium; th, Tlialamus. [Accession No. 210. Section 305. Actual Size 15 X 7 mm.) htteral periphery of the section and ventral to the superior colhcukis is the mesial geniculate bodv (IVlgb). LE\EL OF THE OPTIC CHIASM (FIG. 61) At the level of the optic chiasm the bram stem is seen approaching its caudal extremity. The third ventricle is here Hanked upon either side by the massive thalamic groups of nuclei, and the roof formed by some fibers TARSIUS SPECTRUM 133 of the posterior commissure. Other elements of interest at this level are indicated by letters in the caption. LEVEL OF THE ANTERIOR COMMISSURE (fIG. 62) At the level of the anterior commissure the brain stem has come to its caudal termination. The structures at this level are indicated by letters specified in the caption. Chapter IV RECONSTRUCTION OF THE GRA\^ MATTER IN THE BRAIN STEM OF TARSIUS SPECTRUM "^HE reconstruction of the gray matter of tarsius presents a constitu- tion more elemental in character than that encountered in any other species of the primate series. In general contour the reconstruction is distinctly pyriform, beginning with the small dimensions at the most cephahc of the cervical levels and expanding rapidly in the medulla, mamtaining a uniform width in the pons and midbrain, but presenting at the upper portion of the midbrain near its junction with the diencephalon a marked increase in its lateral development. Viewed from the side the reconstruction has a singularly Hat appearance on the dorsal surface which continues upward to the junction of the metencephalon with the mesencephalon. The ventral contour shows a gradual but continued increase in the dorsoventral dimension from the medulla to the isthmus. At the junction of the metencephalon with the mesencephalon, the dorsoventral diameter of the reconstruction approx- imately doubles itself by the sudden appearance of the colliculi which rise dorsally in a precipitous, almost palisade-likc outgrowth from the dorsal sur- face of the neuraxis. The collieuli almost immediately assume their extreme vertical diameter, producing a right angle transition between the caudal limit of the collicular plate and the plane presented by the floor of the fourth ventricle. This space, empty in the reconstruction, is occupied by the mass of the cerebellum which lies in contact with the collicular plates throughout the major portion of its cephalic surface. The geniculate bodies make their appearance at a level unusually low in the brain stem, taking on recognizable form on the lateral aspect of the mesencephalon. The mesial geniculate body, developing first at a point 136 THE LOWER PRIMATES opposite the middle of the inferior eolliculus, continuing upward and increas- ing gradually in widtli is joined by the lateral geniculate which rapidly develops into a large mass applied to the lateral surface of the midbrain. The superior eolliculus is unusually large and appears as a direct continuation upward of the collicular plate, presenting a flat plateau-like structure which extends to the upper limit of the reconstruction. The ventral surface presents a marked development of what corresponds to the interpenduncular graj- matter, in this case, however, protruding between and ventral to the pedun- cles. The interpeduncular gray matter continues forward as a protrusion which extends vcntrally from the surface of the brain stem for a considerable distance. The major portion of the brain stem is formed by the reticular formation which has a particularly undifferentiated appearance and serves as a support for the more specialized structures of the stem which develop upon its several surfaces. The presence of a small quantity of reticular formation in the angle between the ventral and dorsal gray horns, indicates that at the lowest levels of the reconstruction the decussation of the pyramidal tract has already begun. The first sections of the reconstruction correspond to the lowermost limit of the medulla oblongata and have an appearance almost typically that of the spinal cord, with the exception of the small mass of the reticular for- mation already mentioned. The central gray matter is distinctly V-shaped in appearance, presenting definite ventral gray horns and dorsolateral exten- sions with a distinct cap of gray matter, the substantia gelatinosa of Rolando. For a short distance the substantia gelatinosa of Rolando continues upward on both sides of the midline separated by a rather deep groove in which lie the dorsal white columns of the spinal cord and medulla oblongata. The dorsal horns, however, soon show a tendency to deviate outward, the tips of the horns first turning laterally and then somewhat ventrally, as the dorsal gray horns separate themselves one from the other. The ventral gray column RECONSTRUCTION OF TARSIUS SPECTRUM 137 presents a typical appearance, is rather irregular in contour and rapidly diminishes in size. The reticular formation appearing in the angle between the dorsal and FIG. 63. VENTRAL SURFACE OF THE GRAY MATTER OF THE BRAIN STEM, TARSIUS SPECTRUM. Key to Diagram, cochl. com.. Cochlear Complex; inferior oliv. nuc, Inferior Olivary Nucleus; mesial GENic, Mesial Geniculate Body; ret. form., Reticular Formation. ventral gray cokimns gradually increases in size and assumes the position which is being vacated by the ventral gray column. The central gray matter continues upward relatively unchanged until the point at which the nuclei of Rolando begin to diverge one from the other. At this point the ventral gray matter rather rapidly approaches the surface and becomes somewhat flattened out from side to side. The Dorsal Medullary Nuclei The nucleus of Goll is the first of the medullary nuclei to appear with, of course, the exception of the nucleus of Rolando. This nucleus appears a short 138 THE LOWER PRIMATES distance above the point at which the nuclei of Rolando begin to diverge one from the other. The nucleus appears on cither side of the midline as a small protrusion of gray matter from the dorsal surface of the central gray matter. These rehitively insignificant dorsal protrusions He close to the midline and increase very slightly in size as they continue upward. As the opening of the fourth ventricle is approached, these nuclei assume somewhat larger propor- tions and form a part of the lateral wall of the ventricle, extending laterally but never raising themselves to any extent from the surface. The nuclei oi Goll continue upward to a point only slightly above the opening of the fourth ventricle in the lateral ventricular wall, and come to a rather abrupt termination. The nucleus of Burdach appears at a higher level than the nucleus of Goll, as a Hat sessile elevation on the dorsal surface of the central gray matter lateral to the nucleus of Goll. It slowly but gradually increases in size, diverg- ing rather markedly from its fellow of the opposite side as the fourth ven- tricle opens. It extends upward to about the midventricular level. In the upper portion of its course it arrives at a certain degree of independence from the wall of the ventricle and rises dorsally to a moderate degree in a well- marked prominence in the dorsolateral angle of the brain stem. It rather suddenly terminates and its place is taken by the nuclei of the vestibular complex. The substantia gelatinosa Rolandi continues upward as a direct pro- longation of the substantia gelatinosa of the spinal cord into the medulla oblongata. These two ULiclei show the first indication of the opening of the fourth ventricle by their divergence one from the other, thus allowing the central gray matter to approach the dorsal surface of the oblongata. Con- tinuing to diverge, the nuclei assume considerable lateral proportions, are supported directly on their mesial aspect by the reticular formation, and finally reach the lateral meridian of the brain stem. From this point they RECONSTRUCTION OF TARSIUS SPECTRUM 139 continue along the lateral surface of the medulla oblongata, present a definite constriction in the lower portion of the metencephalon, the waist of the trigeminal nucleus. Thev then extend somewhat dorsailv and still further FIG. 64. DORSAL SURFACE OF THE GRAY MATTER OF THE BRAIN STEM, TARSIUS SPECTRUM. Key to Diagram, cen. gray matter, Central Gray Matter; COCH. com., Cochlear Complex; dors, gray MATTER, Dorsal Gray Matter; Nuc. CLAva, Nucleus Clava; lat. gen.. Lateral Geniculate Body; mes. gen.. Mesial Geniculate Body; Nuc. CUN., Nucleus Cuneatus; ret. form.. Reticular Formation; sub. gel. TRiGEMiNi, Substantia Gclatinosa Trigcmini; vestib. com.. Vestibular Complex. laterally to come to an end in an expanded upper extremity. In their course upward these nuclei, originally situated in an oblique direction from before, backward and outward, gradually turn upon a vertical axis until their long diameter is directed almost precisely dorsovcntrally. They terminate at the mid-metencephalic level above the upper limit of the vestibular complex. The Inferior Olivary Nucleus The inferior olivary nucleus appears at a level corresponding almost exactly \\ith the opening of the fourth ventricle as a narrow, somewhat crescentic lamina of gray matter, situated almost on the ventral surface of the oblongata, and applied to the reticular formation. The two nuclei are 140 THE LOWER PRIMATES separated, one from the other, by the pyramidal tracts which lie on the immediate ventral aspect of the brain stem. These nuclei continue upward in this same position, increasing to a moderate extent chiefly by an increment in their most ventral portion which assumes a somewhat chib-shaped appear- ance. There is but little indication of the development of the accessory and main olivary nuclei, the entire nucleus apparently consisting of a single lamina of gray matter. There is not the slightest indication of the formation of a fundus; neither is there any intimation of plication in the arrangement of the lamina. At its upper extremity the inferior olivary nucleus is the most ventral of all of the structures of the brain stem. The nucleus continues upward to a point somewhat below the level of the greatest width of the ventricle. It then rapidly diminishes and comes to an abrupt termination. The Reticular Formation This mass of nuclear material makes its first appearance m the lower- most level of the reconstructicMi as a small accumulation of gray matter situated in the angle between the dorsal and ventral gray columns. It con- tinues upward in this location, very gradually increasing in size, and replaces the ventral gray column through the merging of the latter into the undiffer- entiated reticular formation. From this point upward the reticular formation assumes the position of the major constituent of the brain stem, being rela- tively massive and supporting the various specializations which appear on its several surfaces. From the point of the disappearance of the ventral gray column, the reticular formation rapidly increases in size both laterally and ventrally being separated in the midline by the longitudinally coursing fiber bundles which form the raphe and by the inferior olivary nucleus which appears as a special condensation in the ventromesial angle of the formation. Above the level of the inferior olivary nucleus, the reticular formation of either side closely approaches the midline, the space occupied by the raphe RECONSTRUCTION OF TARSIUS SPECTRUM 141 being reduced to a mere slit between the mesial surfaces of the reticular for- mation extending backward to the central gray matter. The reticular for- mation on its lateral aspect forms a support for the substantia gelatinosa of FIG. 65. LATERAL SURFACE OF THE GRAY MATTER OF THE BRAIN STEM, TARSIUS SPECTRUM. Key to Diagram, interped. gray matter, Interpeduncular Gray Matter; Nuc. clava, Nucleus Clava; Nuc. CUN., Nucleus Cuneatus; font. Nnjc, Pontile Nucleus; ret. form.. Reticular Formation; sub. gelat. TRIG., Substantia Gelatinosa Trigemini; sub. nig.. Substantia Nigra; ven. gray col.. Ventral Gray Column. Rolando which lies in a bed excavated from its lateral surface. Dorsally the reticular formation is in close relationship with the central gray matter forming the floor of the fourth ventricle, the dorsal surface of the reticular formation supporting the central gray matter throughout its entire lateral extent. The reticular formation in the oblongata presents no definite special nuclear collections on its surface with the exception of the inferior olivary nucleus and the substantia gelatinosa of Rolando. At the junction of the oblongata with the metencephalon, the ventral surface of the reticular for- mation becomes irregular and recedes from the surface, giving place to the 142 THE LOWER PRIMATES nuclear collections which form the very poorly organized system of the pontile nuclei. Its ventral surface, partially covered by the irregular pontile nuclei, shows two grooves, one which proceeds laterally and dorsally in a somewhat spiral fashion about the lateral surface of the reticular formation toward the inferior colliculus, representing the course of the lateral fillet as this bundle of fibers seeks its nucleus of termination in the inferior colliculus. In its more ventral portion, largely covered n\cv and concealed from view- by the pontile nuclei, the reticular formation presents another groove which indicates the course of the mesial fillet, which in this form maintains an almost undeviating upward course in the ventromesial angle ol the formation throughout the entire length of the brain stem to end in the nuclei of the thalamus. In the upper half of the metencephalon the reticular formation is almost entirely covered by the pontile nuclei vcntrally, and in its lateral aspect in this region it presents a particularly irregular outline. As the reticular formation is followed into the mesencephalon, it can be seen to send a prolongation laterally and dorsally which passes backward in a rather massive lamina of gray matter into the space separating the superior and inferior col- liculi, extending dorsally and mesially in the space separating the two colliculi to join in the midline dorsally. In the mesencephalon the reticular formation is almost entirely obscured from view vcntrally by the fusion of the upper- most part of the pontile nuclei with the indifferent gray matter filling the interpeduncular space. Lateral to this there appears the substantia nigra which is separated from the ventral surface of the reticular formation by ascending and descending bundles of fibers. The lateral aspect of the reticular formation in the mesencephalon is entirely obscured by the exceptionally low appearance of the geniculate bodies which develop considerably below the mid-mesencephalic level. In the mesencephalon the only portions of the reticular formation which can be seen are those which form the dorsolateral expansions, extending into the space between the superior and inferior col- RECONSTRUCTION OF TARSIUS SPECTRUM 143 liculi, and a small vcntronicsial insinuation which appears between the mesial extremity of the substantia nigra and the interpeduncular gray matter. FIG. 66. LATERAL SURFACE OF THE GRAY ALATTER OF THE BRAIN STEM, TARSIUS SPECTRUM. Kev to Diagram, coch. complex, Cochlear Complex; inf. coll.. Inferior Colliculus; interped. gray MATTER, Interpenduncular Gray Matter; mes. gen.. Mesial Geniculate Body; nuc. cun., Nucleus Cuneatus; pontile nuc. Pontile Nuclei; ret. form.. Reticular Formation; sub. gel. trigem.. Substantia Gelatinosa Trigcmini; subst. nig., Substantia Nigra; ven. gray col.. Ventral Gray Column; vestib. comp.. Vestibular Complex. On the dorsal aspect of the reconstruction, the reticular formation only appears in the region immediately caudal to the isthmus, at which point its dorsolateral angle is seen between the central gray matter mesially, and the substantia gelatinosa of Rolando laterally. In this region, the reticular for- mation sends dorsally and mesially a thin prolongation which extends around and finally completely surrounds a tunnel which is occupied by the superior cerebellar peduncle. This peduncle in the tarsius is very small, insignificant and occupies a flattened, oval tunnel, directed upward, inward and forward. The superior cerebellar peduncle continues this course through the reticular formation of the mesencephalon, decussating across the midline to end in the 144 THE LOWER PRIMATES special tegmental nuclear condensation of the mesencephalon, the red nucleus. The reticular formation in the mid-mesencephaiic region extends around the central gray matter in the form of a thin lamina which separates the central gray matter from the inferior and superior colliculi, and forms a support for the ventrolateral angle of both colliculi. Above the level of the substantia nigra the reticular formation again appears between the interpeduncular gray matter and the lateral geniculate as a massive development, extremely irregular in its arrangement and extend- ing upward to become continuous with the nuclei and reticular formation of the diencephalon. The Pontile Nuclei The pontile nuclei appear at about the midvcntricular level as small, isolated collections of gray matter on the ventral surface of the brain stem. These collections of gray matter develop chielly near the midline, but scat- tered nuclear masses can be seen extending outward to form small discrete accumulations. Above the midvcntricular level, the pontile nuclei become somewhat larger and form a definite lamina of gray matter which assumes a fair degree of thickness and a relatively extensive lateral disposition, covering a small segment of the ventral aspect of the gray matter of the brain stem. This mass of pontile nuclei gradually develops out of the isolated groups of gray matter which form the pontile nuclei in the lower portions of the meten- cephalon. They rapidly extend laterally, continue upward and then rapidly diminish by drawing toward the midline, where they end in contact, if not by fusion with the undifferentiated interpeduncular gray matter. The pontile nuclei, where they assume any proportions at all, appear only as a simple lamina of gray matter forming the ventral contour of the gray matter of the brain stem. They show no tendency whatsoever to establish any degree of complexity and do not interlace with the bundles of the pyramidal tract which lie in a compact group dorsal to the middle of the pontile lamina. RECONSTRUCTION OF TARSIUS SPECTRUM 145 The Vestibular Nuclei A relatively short distance above the opening of the fourth ventricle there appears a specialization in the reticular formation which hcs between the nucleus of Burdach and the iloor of the fourth ventricle. This specializa- tion consists of groups of fibers, rather loosely arranged in a matrix of gray matter, thus presenting the typical appearance of the nucleus of the descend- ing root of the vestibular complex. This mass of gray matter enlarges in size, and very soon assumes tlie characteristic arrangement of the nucleus of Deiters. As it increases in size, the nucleus of Burdach coriTspondingly decreases, and as the nucleus of Burdach comes to an end, the nucleus of Deiters assumes its full proportions. In tarsius the nucleus of Deiters is a relatively large collection of gray matter, and occupies the outer third of the dorsal portion of the tegmentum directly ventral to the floor of the fourth ventricle. As the nucleus of Burdach terminates, the nucleus of Deiters comes to the surface and appears on the dorsolateral aspect of the neuraxial tegmen- tum. The increase in size of the gray matter in this region shows a very defi- nite effect upon the ascending nucleus of Rolando which is displaced to a distinctly ventral position by the increasing mass of the Deitersal complex. The nucleus reaches its maximum proportions at a point somewhat above the mid-ventricular level where, in its mesial portion, there develops a small area somewhat more homogeneous in appearance, which is the triangular nucleus of Schwalbe. This nucleus forms part of the lateral wall of the ven- tricle underlying the ependymal floor and is in contact laterally with the Deitersal complex. At this point there appear the cochlear nuclei in connec- tion with the entrance of the cochlear roots, and the dorsal cochlear nucleus overlies the nucleus of Deiters, hiding it from view. Having assumed its maximum proportions, the nucleus of Deiters rapidly diminishes and disap- pears, giving place to a rather large collection of reticular substance situated lateral to the central gray matter. As the nucleus of Deiters approaches the 146 THE LOWER PRIMATES surface it presents the typical tongue-shaped prolongation laterally and ventrally in the curve of which is disposed the ascending inferior cerebellar peduncle which at this point passes between the vestibular and the cochlear complex to enter the ccrcbenum. No delinite evidence of a nucleus of von Bechterew could be obtained in the sections. The Cochlear Nuclei These nuclei appear at the mid-ventricular level of the brain stem oppo- site the lateral recess of the fourth ventricle. This nuclear mass in tarsius presents a lateral and a mesial group rather than the generally accepted division of the cochlear nucleus into a ventral and dorsal division. The lateral division of the cochlear nucleus is a well-defined mass of gray matter which lies lateral to the recess of the fourth ventricle connected by strands of gray matter with the mesial cochlear nucleus. The lateral cochlear nucleus appears to be considerably more extensive than the mesial portion of the nucleus and differentiates as a rather definite mass of gray matter almost independent of the gray matter of the stem itself. There could be identified no definite trough- like form of the nucleus which is the usual arrangement seen in the higher pri- mates. The mesial cochlear nucleus is a rather poorly defined nuclear mass which is situated in the recess of the fourth ventricle and is immediately superimposed upon the Deitersal complex which underlies it. It does not extend upward or downward as far as the lateral division of the nucleus and is a relatively insignificant mass of gray matter. The Colliculi These masses of gray matter, together with the geniculate bodies, form the predominating and outstanding development of the gray matter in the brain stem of tarsius. They are relatively massive in appearance and occupy the tectal portion of the mesencephalon. The superior colliculi are very much RECONSTRUCTION OF TARSIUS SPECTRUM 147 larger than the inferior colliculi. The inferior collieuli ap])ear at the dorso- lateral angle of the brain stem at first as small rounded collections of gray matter. These expand enormously in succeeding sections, rapidly assuming their maximum proportions. They are situated lateral to a dorsal prolonga- tion of the central gray matter. Thej' are roughly oval in outline, the axis being directed somewhat from before, backward and inward. Having rapidly assumed their greatest dimensions, the colliculi continue upward for a short distance and then begin to contract. They are in relation laterally with a prolongation of the reticular formation which arises in the dorsolateral angle of the tegmentum, and sweeps around the colliculi sending a prolongation upward to interpose itself between the upper limits of the inferior colliculus and the lower limits of the superior colliculus. The colliculi are suppcjrtcd vcntrally by the reticular formation and mesially by the gray matter of the central formation. As seen in the cross sections the inferior colliculus corre- sponds fairly accurately \\ ith the general conformation of the inferior col- liculus in the whole primate series, being represented chiefly by a core of white matter, upon \\hich are superimposed successive layers of white and gray matter. The superior colliculus, however, presents a very different appearance, having a typical cortex with a core of medullary substance. The superior colliculus makes its appearance slightly above the point of termina- tion of the inferior colliculus, as a small, dorsomesially situated mass of gray matter. This rapidly enlarges in size, spreading laterally and presenting a ventral prolongation which continues forward laterally along the outer aspect of the dorsal portion of the brain stem. Central to the superior colliculus, there appears a prolongation of the reticular formation dorsally around the central gray matter which is situated in the center of the reconstruction. As the superior colliculus is followed upward it assumes greater and greater proportions, becomuig considerably thicker, and presents a very definite plateau-like appearance on its dorsal aspect, which turns at a sharp angle 148 THE LOWER PRIMATES ventrally and laterally into the lateral prolongation which continues forward toward the mesial geniculate body. The superior collicuhis continues upward to the point of junction of the mesencephalon with the diencephalon, where it rapidly contracts and comes to a termination. The Substantia Nigra This mass of gray matter appears at about the junction of the meten- cephalon with the mesencephalon in the ventromesial aspect of the I^rain stem, as a direct continuation upward of a thin lamina of reticular formation which is found to underHe the metencephahc pontile nuclei. The substantia nigra is situated obliquely from before, backward and outward, as a rather definite lamina of gray matter of mediocre dimensions. It continues upward in this position relatively unchanged, being in relationship mesially with the development of the interpeduncular gray matter, and dorsally with the reticular formation of the mesencephalic tegmentum. Laterally the sub- stantia nigra is in relationship with the origin of the mesial geniculate body and somewhat more cephalically, with the beginning of the lateral geniculate body. In the lateral portion of this lamina of gray matter there develops the lateral nucleus of the substantia nigra which has been found to be present in the brain stems of all of the primates. This forms a rather complicated tangle of nerve fibers arising from a definite nuclear formation, which seems to turn backward into the mesencephalic tegmentum. The substantia nigra con- tinues upward to a point somewhat above the beginning of the superior col- liculus, and then comes to an end being replaced by various developments of the reticular formation, which in this region assumes very considerable pro- portions and extreme complexity. The Central Gray Matter The central gray matter appears in the lowest levels of the model as a more or less v-shaped mass of nuclear material, presenting at its ventrolateral RECONSTRUCTION OF TARSIUS SPECTRUM 149 angles the ventral gray columns and at its dorsolateral angles the connections of the central gray matter with the substantia gelatinosa of Rolando. Con- tinuing upward, the central gray matter retains very much the same position, the connection between it and the ventral gray cokimn being cut across by the decussating fibers of the pyramidal tract. As the dorsal gray columns begin to diverge, the central gray matter migrates dorsally, becoming ilat- tened out, and then gives rise to the dorsal medullary nuclei, while to its ventral aspect is attached the reticular formation. Continuing upward the central gray matter gives rise to the nucleus of the column of Goll, and at a somewhat higher level, to the nucleus of the column of Burdach. As the floor of the fourth ventricle begins to open, the central gray matter comes to the surface and forms the lloor of the fourth ventricle which presents a shal- low depression, concave from side to side and from above downward. The gray matter of the floor of the fcnirth ventricle is spread out in a relatively thin sheet, and presents no particular modeling produced by the underlying structures of the floor of the fourth ventricle. It is continued outward into the lateral recesses and covers the nucleus of Dciters and the triangular nucleus of Schwalbe. It is encroached upon at the angle of the mesial coch- lear nucleus which extends considerably toward the midline. Having passed the level of the lateral recess of the fourth ventricle, the gray matter con- tracts toward the formation of the aqueduct of Sylvius, and the central canal is soon reconstituted by the development of the roof of the fourth ventricle and the lower portion of the aqueduct of Sylvius. It here is in direct relation- ship with the dorsal prolongation of the reticular formation which surrounds it on either side, and receives the superior cerebellar peduncle. Continuing upward, the central gray matter again forms a complete canal which is rather massive and presents walls of considerable thickness. The aqueduct itself is an elongated dorsoventral slit rather than an oval opening, and the central gray matter is continued backward in a long drawn-out projection I50 THE LOWER PRIMATES between the mesial aspects of the inferior coiliculi. Continuing upward in the mesencephalon, the central gray matter becomes more rounded about the ac|ucduct of Sylvius, being surrounded Idv a dorsal prolongation of the reticular formation which separates it from actual contact with the super- imposed superior coHicuIi. In the middle portion of the mesencephalon the central gray matter rapidly begins to extend ventrally forming a long slit-like structure which is surrounded by thick walls of gray matter which then become continuous with the interpeduncular gray matter. Contnniing upward, the central gray matter of the mesencephalon becomes continuous with the central gray matter of the diencephalon surrounding the slit-like third ventricle. The interpeduncular gray matter presents a rather massive development immediately above the pontile nuclei. This presents consider- able proportions and contains a central ventricle. Continuing upward, with a definite ventral evagination, the interpeduncular gray matter fuses with the ventral prolongation t)f the central gray matter as the junction of the mesen- cephalon with the diencephalon is approached. The interpeduncular gray matter presents at a relatively low level the condensations representing the mammillary bodies which appear at a level between the superior and inferior coiliculi. The Geniculate Bodies These bodies since they form a definite part of the brain stem of tarsius, are described in connection with the gray matter of the brain stem. The mesial geniculate body is the first one to appear and the first evi- dences of its existence are found caudal to the termination of the inferior colliculus. It appears as an oval mass of gray matter, situated lateral to the substantia nigra and directed outward and backward. It increases in size, shifts somewhat laterally and then dorsally and continues up as a rapidly increasing mass of gray matter in contact laterally with the ventral aspect RECONSTRUCTION OF TARSIUS SPECTRUM 151 of the lateral prolongation of the superior colHcuhis. It then becomes stretched out laterally in a relatively thin lamina of gray matter which continues upward into the diencephalon. The lateral geniculate body appears at a slightly higher le\el than the mesial geniculate body and is situated somewhat ventral to it. It forms the most prominent structure on the ventrolateral aspect of the bram stem. It consists of a relatively massive collection of gray matter, situated lateral and ventral to the mass of the mesial geniculate. It is disposed in an oblique direction from before backward and outward, and presents at its ventral extremity a series of prolongations which receive or give origin to masses of fibers. Continuing upward, the lateral geniculate, rapidly dmimishing, ends somewhat caudal to the upper limit of the mesial geniculate which extends further upward. Chapter V CALLITHRIX JACCHUS, THE MARMOSET, ITS BRAIN AND BEHAVIOR Its Positiun amansi the Primates; Measureynents cutd Brain Indices; Surface A})])earance of the Brain; Internal Structure of the Brain Stem in Cross Section Iow -J OWEST amono; the Anthropoidea arc the Hapalidac or marmosets. These animals belong to the group of new-world monkeys. They ^M inhabit South and Central America. Their digits are for the most part clawed, with the exception of the great toe which alone bears a Hat nail. The tail is long, bushy and rmgcd, a condition characteristic of many of the lower groups of mammals but not observed among the higher apes. Unlike most of the South American monkeys, the tail of marmoset is not prehensile. In size they are about as large as a small squirrel and are covered with a thick silky fur. Although naturally timid, they readily become accustomed to those with whom they are familiar in captivity. The female produces two or three young at a birth and in this respect is unlike other Anthropoidea. In facial appearance, in size and shape of head, the Hapalidae convey a much more ape-like impression than do the lemurs. On the other hand, their spe- cialization in limb, particularly with reference to the hand, seems hardly so far advanced as in the Lemuridae. The eyes are much closer together and separated by a flat, narrow nose, suggesting the possibility of some degree, at least, of binocular vision, although it seems probable that even in this respect, the animal's visual function has not attained a high degree of difl'erentiation. On the whole, the marmosets, because of their very small size, seem but retrograde steps in the dilTerentiation which culminates in the strikingly 154 THE LOWER PRIMATES outspoken ieatures of the simian tribes. This relatively large family of new- world primates, however, because of its convergences and divergences within this order of mammals, should afford interesting evidence in the structure of Courtesy, AmeTtcan Museum of Natural History FIGS. 67 AND 68. CALLITHRIX JACCHUS (MARMOSET). its central nervous system and more particularly in the bram. The species here described is Callithrix jacchus. The greater number of the species included in the genus Callithrix are natives of Brazil, one species formerly CALLITHRIX JACCHUS, THE MARMOSET 155 extending its range as far as Bolivia, another being indigenous to Colomljia. There are records of CaUithrix jacchus having also been found in the island of Marajo, lying between the mouths of the Amazon and Para Rivers. The marmoset has a black face with white spots. There arc cross-bands on the back and tail. The animal lives in the tree tops or smaller underbrush. The claws upon its feet and hands enable it to climb along the limbs and up the trunks of trees much in the manner of a squirrel. It has a cat-like agility but does not make long and daring leaps like the lemur. It often loses its hold upon the branches and falls from considerable heights to the ground without sustaining apparent injury. For this reason, and perhaps because of its small size, it is not in need of the extreme degree of prehensile power in either fore- or hindlimbs. It may also be because of this fact that its tail has failed to develop prehensile qualities. While in captivity it shows little tendency to acquire reactions which it does not already possess in the free state. It does not lend itself to training or the acquisition of tricks, as do many of the other anthropoid forms. The animal lives upon worms, insects and fruits. It is known also to invade the nests of birds and suck the eggs. Only exceptionally, however, does it prey upon bird-life, and in such exceptions it may occasionally be able to overpower one of the smaller birds or unpro- tected young. It has little of the acquisitive celerity manifested by the lemur. Measurements and Indices of Callithri.x Jacchus Total length of the animal 510 mm. Length of the tail 295 mm. Length of the foot 61 mm. Diameters of the skull Occipito-nasal 42 mm. Bitemporal 22 . 5 mm. Length of the brain case 35 mm. 1^6 THE LOWER PRIMATES Brain, including cerebellum and brain stem without meninges Longitudinal 31 m"!- Transverse -3 '''"'""'■ Courtesy, American Museum of Natural History FIGS. 69 AND 70. HAND AND FOOT OF MARMOSET. Left. Palmar surface of hand showing poorly developed pahn, rudimentary thumb, well-delined digitation, with claws instead of finger-nails. Right. Plantar surface of the foot showing imperfectly developed heel and sole, rudimentary great toe, claws instead of toe-nails. Total weight of the brain 6.2 gms. Total water displacement of the brain 6 c.c. Weight of forebrain 4-75 gms. Weight of midbrain o . 25 gms. Weight of hindbrain 1.2 gms. On the basis of these figures the following encephalic indices were computed for the several divisions of the brain: CALLITHRIX JACCHUS, THE MARMOSET 157 Forebrain index 80 . 5 per cent Midbrain index 0.5 per cent Hindbrain index 19.0 per cent Courtesy. American Museum oj Natural History FIGS. 71 AND 72. HAND AND FOOT OF MARMOSET. Left. Dorsum of hand showing poorly developed thumb, and claws instead of finger-nails. Right. Dorsum of foot showing rudimentary toe, claws instead of toe-nails. These indices align the animal in a definitely submanual group with a forebrain index slightly below that of the lemur and only slightly above the forebrain index characteristic of mammals possessed of paws and claws. SuRF.\CE Appearance of the Brain in Callithrix Jacchus THE FISSURAL PATTERN The surface appearance of the hemisphere in the marmoset gives the impression at first glance of a lissencephalic brain. Further inspection, how- ever, shows that at least three fissures constitute an indefinite lissural pattern THE LOWER PRIMATES in this animal. The Sylvian fissure is a prominent sulcus whose angulation with the base hne of the brain is slightly less than 50°, showing in this regard a general tendency to approach the condition of the higher primates. The FIG. 73. DORSAL SURFACE OF BRAIN, CALLITHRIX JACCHUS (MARMOSET). [Actual Length, 29 mm.] Key to Diagram, obl., Oblongata; sup. long, fiss., Suporior Longitudinal Fissure. Sylvian fissure, however, is relatively short and extends I3acl:^\•ard and upward for something less than half the distance of the entire lateral surface. Below and behind the Sylvian fissure is a slight indenture indicating the position of the superior temporal fissure, corresponding in its general position to the sulcus parallelus in the lemur's brain. A slight indenture above and in front of the Sylvian fissure indicates the position of the sulcus centralis which forms the faint boundary by means of which the limits between the frontal and parietal lobes may be established. There is no evidence of any parietal sulci or of any incisure which may be considered as the homologue of the sulcus simiarum. LOBATION The lobation in the marmoset brain, therefore, is only most rudimen- tarily outlined. Distinction may be made between the parietal, parieto- CALLITHRIX JACCHUS, THE MARMOSET 159 frontal and temporal regions by the boundary estalilished through the Sylvian fissure. No boundary exists which indicates the actual limits between the parietal and the frontal lobes except the faint indenture already indicated FIG. 74. BASE OF BRAIN, CALLITHRIX JACCHUS (MARMOSET). [Actual Length, 29 mm.] as the probable inception of the sulcus centralis which, however, is not a constant marking m the marmoset brain. In many species it does not appear at all. As no boundary line exists in the occipital region, it is impossible to describe the limits between the parietal and the occipital lobes. The latter lobe, however, has apparently expanded considerably in its general dimen- sions because the pole of the brain now completely overhangs the cerebellum and assumes more closely the general outline of this region in the higher primates. While it is possible to identify the general topography of the frontal, parietal, temporal and occipital lobes in the brain of marmoset, no reliable landmarks may be established as marking the boundaries of these four great hemispheral divisions. The impression conveyed by the survey of the brain in the Hapalidae is that even if this family has definitely entered the lists of the primate kind, its advance has been a most diffident one. While the hemis- phere shows the general outline characteristic of the primate endbrain, it has i6o THE LOWER PRIMATES none of the bold markings in the way of fissures and convohitions which ultimately become the identifying (Fig. 75) features of this part of the nervous system in primates. The marmoset brain may be considered as FIG. 75. LEFT LATERAL SURFACE OF BRAIN, CALLITHRIX JACCHUS (MARMOSET). [Actual Length, 31 mm.] Kev to Diagram, ob., Oblongata. something more than a mere transitional form. It must be taken to represent a transition already achieved from the lower mammals into the primate order but also manifesting certain retrograde changes. THE ORBITAL SURFACE On the orbital surface the brain has all of the typical markings of the lower primates. The two orbital concavities are well defined. The inception of these concavities seen in the lemur's brain is here carried to its ultimate development. The orbital surface now rests upon an expanded orbital plate of the frontal bone. The interorbital keels are also well defined and more marked than in the lemur brain. The olfactory bulbs, though fairly large, are less pronounced than in the lemur, and the olfactory tract is detachable as far back as the trigonum which is fairly prominent (Fig. 74). THE OCCIPITAL REGION OF THE BASAL SURFACE In the occipital region of the basal surface a well-marked occipital con- cavity exists which is most emphasized in and about the midline, particu- CALLITHRIX JACCHUS, THE MARMOSET i6i larl}' in the region of the postsplenial fossa. This concavity accommodates the upward protrusion of the cerebelhim. The cerebeHum upon its tentorial surface is almost completely overhung by the occipital pole of the hemisphere. FIG. 76. RIGHT LATERALSURFACE OF BRAIN, CALLITHRIX JACCHUS (mARMOSET). [Actual Length, 31 mm.] Key to Diagram, ob., Oblongata. The superior vermal portion of the cerebellum is more conspicuous than the lateral lobes which show but slight expansion. On the occipital surface the vermis is also the outstanding feature. It represents scarcely less than a third of this surface and stands out conspicuously against the poorly devel- oped lateral expansions of the cerebellum. The two paramedian sulci interrupt the passage of the interfolial fissures from the inferior vermis to the lateral lobe. No such sulcus, however, exists on the tentorial surface. While the cerebral hemispheres of marmoset assign the animal to a definite place among the primates, the simplicity of the brain indicates the relatively low position of the Hapalidae (Fig. 76). THE SURFACE MARKINGS OF THE BRAIN STEM The surface markings of the brain stem are in general much less dis- tinct than m the higher primates. In many respects, they are less impressive than in Lemur mongoz. The oblongata on its ventral surface presents a ventromesial sulcus and two faintiv marked ventrolateral sulci. Neither the l62 THE LOWER PRIMATES pyramids nor the inferior olivary bodies have any distinct surface relief, nor is it possible to detect any superficial indication of the pyramidal decus- sation. The relative insignificance of the pyramidal system indicates an fig. 77. ventral surface of brain stem, callithrix jacchus (marmoset). [Actual Length, 22 mm.] Key to Diagram, cer. ped.. Cerebral Peduncle; o.N., Optic Nerve; op. ch., Optic Chiasm, trap; BODY, Trapezoid Body; ventro med. sulcus, Ventromedian Sulcus. extremely limited range of volitional performances and in this respect places the animal even lower in the scale than the lemur. The bulbopontile sulcus is difficult to make out except in fresh specimens, and the pons itself is a flat, narrow band conveying the impression that the animal has not devel- oped a pallio-cerebellar connection capable of producing any large degree of coordinative control over skilled movements. On the dorsal aspect of the oblongata there is a slight mesial eminence indicating the presence of the column and nucleus of Goll. No similar lateral prominence adjacent to it gives surface indication of the column of Burdach. This fact seems to indicate that the tail and lower extremity are more prominent functional elements than the forelimb (Fig. 78). CALLITHRIX JACCHUS, THE MARMOSET 163 THE CEREBELLUM, THE FOURTH \ ENTRICLE AND THE NHDBRAIN The cerebellum is small, presenting a large median portion, the vermis, and two limited lateral expansions, the lateral hemispheres. The size of the fig. 78. dorsal surface of brain stem, callithrix jacchus (marmoset). [Actual Length, 22 mm.] Key to Diagram, d. m. fis., Dorsomedian Fissure; d. m. sept., Dorsomedian Septum; inf. coll., Inferior Colliculus; SUP. cer. ped., Superior Cerebellar Peduncle; sup. coll., Superior Colliculus; tub. trigem., Tuberoulum Trigemini; 4TH \ ent., 4th Ventricle. cerebellum, particularly of its lateral lobe, indicates a coordinating organ with a low degree of functional capacity. From this it may be inferred that the animal's attainment in complex voluntary movements is extremely limited. On removal of the cerebellum the fourth ventricle is revealed, bounded upon either side in its inferior triangle by an inconspicuous clava and in its superior triangle by the superior and middle cerebellar peduncles. i64 THE LOWER PRIMATES The floor of the ventricle, even with high magnification, shows few of the characteristic markings usually observed in this region of the brain. It is not possible to detect striae acusticae crossing the floor. The lateral reces- ses are narrow and the configuration of the rhombencephalon is much more compressed in its cephalo-caudal relations than is the case with the fourth ventricle of other prmiates. In the ventral region of the midbrain the cerebral peduncles may be observed, diverging as they approach the cerebral hemispheres and thus bringing to light the interpeduncular space. The cerebral peduncles, however, make but a faint relief on the ventral mesencephalic surface. On the dorsal aspect of the midbrain the inferior and superior colliculi are well marked and give the impression that the primordial stations for both vision and hearing have a high degree of representation in these primates. The insignificant development of the cerebral peduncles speaks again in favor of a low organization for volitional skilled acts con- trolled through the agency of the pyramidal system. Internal Structure of the Brain Stem in Callithrix Jacchus LEVEL OF the PYRAMIDAL DECUSSATION (FIG. 79) At this level two notable features are present, i.e., the decussation of the pyramidal tracts (Pyx) which has the effect of separating the ventral gray column (Ven) from the central gray matter (Cen), and the pres- ence of the caudal extremity of the nucleus of Goll ( NG ). This nucleus is surrounded by a massive tract of fibers, the column of Goll, which ascends from the lumbosacral and lower thoracic segments of the cord. From the sensory standpoint, it represents chiefly the tail and lower limb. Immediately adjacent and lateral to the tract of Goll is the tract of Burdach (CB) which, however, presents a size relatively equal in comparison ^\■ith the more mesial sensory pathway. Lateral to Goll's tract is the large substantia gelatinosa (NR) bordering upon which is the descending tract of the fifth CALLITHRIX JACCHUS, THE MARMOSET 165 nerve (Trd). This dorsal field of sensory discrimination, embracing the three major sensory territories of the body, the head and face, neck and upper extremity, leg and tail, is almost as conspicuous as m lemur. Its pro- FIG. 79. MARMOSET. LEVEL OF THE PYRAMIDAL DECUSSATION. CEN, Central Gray Matter; CB, Column of Burdach; CG, Column of Goll; dt, Deiterso-spinal Tract; fle. Dorsal Spinocerebellar Tract; cow, Ventral Spinocerebellar Tract; hel, Spino-olivary Tract of Helvveg; NB, Nucleus of Burdach; NC, Nucleus of Goll, nr. Nucleus of Rolando; pv, Pyramid; pvx, Pyramidal Decus- sation; TRD, Descending Trigeminal Tract; ven, Ventral Gray Matter; xpv. Crossed Pyramidal Tract. [Accession No. 146. Section 4. Actual Size 5X3 mm.] portional development of gray matter, as well as the size of the tracts, has practically the same connotation as in the lower form already described. Thus, sensory conduction from the leg and tail, head and face is less amply represented than that from the arm and neck. The significance of this rela- tive disproportion is obvious at a glance. The head, forearm and the hand are 1 66 THE LOWER PRIMATES functionally more prominent than the hind extremity with its poorly dif- ferentiated foot. This disproportion is less striking than in lemur, for although marmoset makes but limited use of its hand, its tail and hind leg FIG. 80. MARMOSET. LEVEL OF THE DORSAL SENSORY NUCLEI. CB, Column of Burdach; cen, Central Gray Matter; CG, Column of Goll; dt, Deiterso-spinal Tract; fle. Dorsal Spinocerebellar Tract; GOW, Ventral Spinocerebellar Tract; hel, Spino-olivary Tract of Helweg; NB, Nucleus of Burdach; ng, Nucleus of Goll; nr, Nucleus of Rolando; pv, Pyramid, pyx. Pyramidal Decus- sation; REF, Reticular Formation; rst, Rubrospinal Tract; spt. Spinothalamic Tract; trd. Descending Trigeminal Tract; yen, Ventral Gray Matter. [Accession No. 146. Section 8. Actual Size 6X4 mm.] manifest as much adaptive significance as in the case of lemur. That the discriminative type of sensibility has a low representation in marmoset, thus providing a meager substratum for the development of its skilled acts, is further substantiated by the fact that the pyramidal tract (Py) is a CALLITHRIX JACCHUS, THE MARMOSET 167 small bundle ol libers in this animal. This creates the impression that the pyramidal system in its relation to the control of voluntary movement plays but a small part in the animal's somatic behavior. FIG. 81. MARMOSET. LE\ EL OF THE TIP OF THE INFERIOR OLIVE. CEN, Central Gray Matter; dt, Deitcrso-spinal Tract; fle, Dorsal Spinocerebellar Tract; GOW, Ventral Spinocerebellar Tract; hel, Spino-olivary Tract of Helweg; 10, Inferior Olive; mf, Mesial Fillet; nb, Nucleus of Burdach; ng. Nucleus of Goll; nr, Nucleus of Rolando; pd, Predorsal Bundle; pl. Posterior Longitudinal Fasciculus; pv, Pyramid; ref. Reticular Formation; rst, Rubrospinal Tract; spt. Spinothalamic Tract; TRD, Descending Trigeminal Tract. [Accession No. 146. Section 22. Actual Size 8X5 mm.] LEVEL OF THE CAUDAL EXTREMITY OF THE INFERIOR OLIVE (FIG. 81) The appearance of the caudal limit of the inferior olivary body (10) adds another striking feature to the next level. This structure i68 THE LOWER PRIMATES occupies its usual position dorsal to tiie pyramidal bundles. It is of relatively small size and irregular outline. While it has nearly the same prominence as in lemur, in many respects it seems more poorly delined as a distinctive structure in the oblongata. Its edges are not so clearly demarcated. This holds true of the olivary body at all levels in the marmoset, in which it appears to be a poorly developed constituent indicating a low degree of organization m the olivary sphere of action. Its function, as already described, is related to the automatic control of simultaneous movements of the hand, head and eye in the performance of skilled acts. Certainly there are no acts executed by this animal which indicate the necessity of any very close coordinative cooperation between eye, head and hand. The pyramid (Py) here reveals its dimensions in the cross section and again signifies a conduction bundle with rather low capacity for dis- tributing volitional motor impulses. Dorsal to the pyramidal fibers and adjacent to the midline arc the first aggregations of iibers which enter into the mesial fillet (Mf). This bundle is formed by decussating arcuate fibers arising in the nuclei of Goll and Burdach which sweep forward across the midline where they take a cephalic course toward the higher levels of the brain. It furnishes some idea as to the extensivencss of the secondary sensory pathway. LEVEL THROUGH THE MIDDLE OF THE INFERIOR OLIVE (FIG. 82) At the level through the middle of the inferior olivary body, although this body appears in its greatest dimensions, it gives the impression of a poorly developed structure. Its relatively small size supports the supposi- tion that the coordinative control of simultaneous movements of head, eye and hand and the coordination of all skilled learned performances are com- paratively slight in this animal. The relative functional significance of the nucleus of Goll as compared with that of Burdach is obvious by the con- CALLITHRIX JACCHUS, THE MARMOSET 169 trast in size of these nuelei as they lie sibc by side in the dorsal portion of the section (NG, NB). The mesial fillet (Mf) has attained more prominence because of the addition to it of a larger number of arcuate fibers. FIG. 02. MARMOSET. LEVEL THROUGH THE MIDDLE OF THE INFERIOR OLIVE. DT, Deitcrso-spinal Tract; fle. Dorsal Spinocerebellar Tract; Gow, Ventral Spinocerebellar Tract; hel, Spino-olivary Tract of Hehveg; 10, Inferior Olive; mf. Mesial Fillet; nb, Nucleus of Burdach; ng. Nucleus of Goll; NHV, Hypoglossal Nucleus; nr. Nucleus of Rolando; nvd. Dorsal Vagal Nucleus; N12, Hypoglossal Nerve; pd, Predorsal Bundle; pl. Posterior Longitudinal Fasciculus; py, Pyramid; ref. Reticular Formation; RST, Rubrospinal Tract; spt, Spinothalamic Tract; trd, Descending Trigeminal Tract. [Accession No. 146. Section 37. Actual Size 8X4 mm.] LEVEL OF THE AESTIBULAR NUCLEI AND TUBERCULUM ACUSTICUM (fIG. 83) At the level of the vestibular nuclei and tuberculum acusticum, the cross sections are characterized by the wide separation of the lateral walls of the fourth ventricle and the appearance of the vermis cerebelli in relation with the ventricle. In the position formerly occupied by the nuclei of Goll and Burdach are two prominent nuclear masses intimately concerned 1 70 THE LOWER PRIMATES with afferent conduction. These nuclei are especially related to a type of con- duction which has to do with the maintenance of equihbriuin. They receive impulses from the receptive organs of the semicircular canals, utricle and FIG. 83. MARMOSET. LEVEL OF THE \'ESTIBULAR NUCLEI AND TUBERCULUM ACUSTICUM. CTT, Central Tegmental Tract; dt, Deiterso-spinal Tract; cow, Ventral Spinocerebellar Tract; icp, Inferior Cerebellar Peduncle; lo. Inferior Olive; mf. Mesial Fillet; nd, Deiters' Nucleus; nr, Nucleus of Rolando; NSC, Nucleus of Schwalbe; n8. Auditory Nerve; pd, Predorsal Bundle; pl, Posterior Longitudinal Fasciculus; FY, Pyramid; ref, Reticular Formation; rst, Rubrospinal Tract; spt. Spinothalamic Tract; trd. Descending Trigeminal Tract; tub, Tuberculum Acusticum. [Accession No. 146. Section 67. Actual Size 10X4 mm.] saccule. The two nuclei which have thus replaced the columns t)f Goll and Burdach are the nucleus triangularis of Schwalbe ( NSc) and the nucleus magnoccllularis of Deiters (ND). The size of these nuclei and their fiber connections with the vestibular mechanism indicates a high degree of organi- zation in the reflex acts connected with balancing. These vestibular nuclei are outstanding features in the section. In their general dimensions they cor- respond with the similar structures of lemur, a fact which would seem to justify the inlerence that the marmosets are as well equipped in balancing CALLITHRIX JACCHUS, THE MARMOSET 171 function as the Icmiiuis. The restiform bt)dy ( ICP), which represents the aggregation of ascending libers passing to the vermis of the cerebellum, affords a suggestive index of coordinative control. The development of such FIG. 84. MARMOSET. LEVEL OF THE CEREBELLAR NUCLEI. CJR, Juxtarestiform Body; ctt. Central Tegmental Tract; icp. Inferior Cerebellar Peduncle; mf. Mesial Fillet; nab, Abducens Nucleus; nd, Deiters' Nucleus; ndt, Dentate Nucleus; nfg, Cerebellar Nuclei, Mesial Group; NOD, Vermis Cerebelli; nr. Nucleus of Rolando; nsc. Nucleus of Schwalbe; n6, Abducens Nerve; N7, Facial Nerve; n8. Auditory Nerve; pd, Predorsal Bundle; pl, Posterior Longitudinal Fasciculus; PY, Pyra- mid; REF, Reticular Formation; so, Superior Olive; trd, Descending Trigeminal Tract; trp, Trapezoid Body. [Accession No. 146. Section 92. Actual Size 10 X 7 mm.] control in marmoset appears to be relatively less than in other primates, a supposition which finds support in the relatively low degree of organization in the cerebellum. 172 THE LOWER PRIMATES LEVEL OF THE CEREBELLAR N'UCLEI (fIG. 84) At the level of the cerebellar nuclei the intimate relation between the medulla oblongata and the cerebellum is clear, while an index to the degree of cerebellar control of motion is furnished in the presence of the main efferent cerebellar nucleus, the nucleus dentatus (Ndt). Occupying the most ventral position in the cross section is the pyramid (Py) immediately dorsal to which is the trapezoid body (Trp) made up of crossing fibers entering the secondary pathway for the conduction of auditory stimuli. The reticular formation (Ref ) occupies an extensive area along whose ventral border is the superior olivary body (SO), a way station in the pathway of hearing. LE\EL AT THE AUDDLE OF THE PONS XAROLH (FIG. 85) At the level at the middle of the pons Varolii this structure attains its maximum dimensions and consists of its three typical lajcrs, the stratum superiiciale, the stratum complexum and the stratum profundum. The stratum superficiale continues directly into the middle cerebellar peduncle (Mcp). The stratum complexum contains many transverse pontocerebellar fibers together with the scattered fasciculi of the pyramidal system (' Py ) and the pontile nuclei (Pn). The general dimensions of all ot the structures occupying the basis pontis afford a significant index of the animal's degree of adaptation. The relatively small size of the pyramidal system is clearly demon- strated. The equally small size of the pons, including the pontile contribution of fibers, which goes to make up the middle cerebellar peduncle, is significant of connections which provide but limited means for the animal's organiza- tion of a complicated motor adjustment. It may be inferred from this evi- dence that the marmoset is poorly endowed with motor activities having a high degree of adaptive flexibility and that the entire range of its motor effectiveness is limited to narrowly prescribed motion formulas. The CALLITHRIX JACCHUS, THE MARMOSET 173 animal docs not possess the celerity, the dexterity or the muscular power to adapt itself to an extensive natural environment. Neither has it the capacity to create out of its environmental conditions new coml^inations which FIG. 85. MARMOSET. LEVEL AT THE MIDDLE OF THE PONS VAROLII. CEN, Central Gray Matter; ctt, Central Tegmental Tract; cow. Ventral Spinocerebellar Tract; lf. Lateral Fillet; mcp. Middle Cerebellar Peduncle; mf. Mesial Fillet; N4, Trochlear Nerve; pd, Predorsal Bundle; PL, Posterior Longitudinal Fasciculus; pn, Pontile Nuclei; pv, Pyramid; ref. Reticular Formation; scp, Superior Cerebellar Peduncle; tvr, Tractus Uncinatus of Russel (Houk Bundle). (Accession No. 146. Section 123. Actual Size 9X5 mm.| would necessitate the development of a more complex motor organization. Hence it lives within a limited sphere of action whose requirements demand a relatively simple motor organization. LEVEL OF THE INFERIOR COLLICULUS (FIG. 86) At the level of the inferior colliculus the most significant modifica- tion is the appearance of this elevation (IC), the primordial relay station 174 THE LOWER PRIMATES for the sense of hearing. This structure in the marmoset shows a high degree of development which may be taken to indicate an auditory sense still having a large representation at this lower level in the brain. Here, as in FIG. MARMOSET. LEVEL OF THE INFERIOR COLLICULUS. CEN, Central Gray Matter; ic, Inferior Colliculus, lf, Lateral Fillet; mcp. Middle Cerebellar Peduncle; MF, Mesial Fillet; N4, Trochlear Nerve; pl, Posterior Longitudinal Fasciculus; pns, Pons, pv. Pyramid; ref. Reticular Formation; scp, Superior Cerebellar Peduncle. (Accession No. 146. Section 139. Actual Size 9X7 rnm.) CALLITHRIX JACCHUS, THE MARMOSET 175 the case of lemur, this primordial auditory organization appears to be in the interest of rapid automatic reactions in response to auditory stimuH. It provides for immediate readjustments of body posture against impending attack or other peril, the approach of which is indicated by sounds. Also, as in the case ot lemur, this provision for immediate reaction in response to sound imposes a delinite degree of hmitation in the range and scope of motor activity. It deprives the animal of certain dehberative and reflective ele- ments which furnish a period ol latency in the selection of alternative courses of action designed to meet emergencies which may develop. The superior cerebellar peduncle (Sep) appears as a crescent extending inward and forward preparatory to its decussation. The pyramid (Py) still presents a disseminated appearance and interspersed among its fasciculi are clusters of the pontile nuclei. Lateral to the mesial fillet (Mf ) is a long, sweeping mass of fibers on their way to the inferior colliculus, the axons of which constitute the lateral fillet (Lf ). Dorsolateral to the mesial fillet is a bundle of fibers constituting the rubrospinal tract and spinothalamic tract. The central gray matter (Cen) has greatly increased in size and now almost surrounds the small cavity constituting the caudal orifice of the Sylvian aqueduct whose roof is formed by the superior medullary velum and by the decussating fibers of the trochlear nerve. Between the two inferior colliculi, resting upon the decussation of the trochlear nerve, are the lingular folia of the cerebellum. LEVEL OF THE SUPERIOR CEREBELLAR PEDLINCULAR DECUSSATION (FIG. S"^) The next cross section is introduced to show the decussation of the superior cerebellar peduncle (^XScp) preparatory to its entrance into the red nucleus. The section selected discloses this decussation at its maximum and afl'ords another illustration of the relatively poor development of the cerebello-rubro-spinal connections, which implies a low degree of organiza- 176 THE LOWER PRIMATES tion in coordinative control. This section also shows the caudal extremity of a large mass of gray matter situated ventral to the mesial fdlet (Mf), namely, the substantia nigra (Sbn). This mass, as in lemur, assumes MARMOSET. LEVEL OF THE SUPERIOR CEREBELLAR PEDUNCULAR DECUSSATION. CEN, Central Gray Matter; cp, Cerebral Peduncle; ctt. Central Tegmental Tract; mf. Mesial Fillet; NOC, Oculomotor Nucleus; nru, Nucleus Ruber; N3, Oculomotor Nerve; pd, Predorsal Bundle; pl. Posterior Longitudinal Fasciculus; ref. Reticular Formation; sc, Superior CoIIiculus; sen. Substantia Nigra; xscp. Decussation of the Superior Cerebellar Peduncle. [Accession No. 146. Section 162. Actual Size 9X7 mm.] large proportions. If the substantia nigra is concerned with the control of certain automatic associated movements, as has been presumed by manj^ CALLITHRIX JACCHUS, THE MARMOSET 177 authorities, then it is fair to believe that the marmoset is possessed of a high degree of automatic associative control. Such a presumption would accord well with its recognized inferiority in the development of volitional control of movement. The animal, being poorly provided with such control, would of necessity require a considerable degree of automatic and associative regu- lation to carry on the performances essential to its natural motor adjust- ments. The probability of this supposition is further attested by the large size of the basal ganglia in the forebrain. These structures, comprising the globus pallidus and caudate nuclei, are usually regarded as part of the mechanism essential to the control of automatic associated movements. The pyramidal nerve fibers are now collected in a compact bundle accom- panied by other axons representing the pallio-ponto-cerebellar system. A large mass of decussating libers dorsal to the central gray matter constitutes the commissure of the superior colliculi. The central gray matter (Cen), much enlarged in size, surrounds the small aqueduct of Sylvius. Ventro- mesially it contains the nucleus oculomotorius (Noc). A superficial layer of gray matter forms the stratum griseum superhciale of the superior colliculus, while lateral to this the brachium conjunctivum posticum is extending forward to the mesial geniculate body. LEVEL OF THE SUPERIOR COLLICULUS (FIG. 88) At the level of the superior colliculus the most distinguishing feature is the appearance of the tectal plate of the midbrain whose surface relief has previously been referred to as particularly prominent. The superior colliculus itself presents a definite degree of stratification reminiscent of the optic lobes in the birds, reptiles, amphibia and fish. The most superficial layer consists of the stratum griseum superficiale. This part of the brain still retains some of its original dominance in visual function. \\ hile it is true that much or most of the higher visual syntheses have their representation 178 THE LOWER PRIMATES in the cerebral cortex of the occipital lobe, it is probably true also that the superior coliicukis maintains its primordial arrangement in the interest of certain primitive and immediate rellex reactions in response to visual stimuh. FIG. »». MARMOSET. LEVEL OF THE SUPERIOR COLLICULUS. CEN, Central Gray Matter; cp. Cerebral Peduncle; crx, Central Tegmental Tract; mf, Mesial Fillet; mob. Mesial Geniculate Body; noc, Oculomotor Nucleus; nru. Nucleus Ruber; N3, Oculomotor Nerve; pd, Predorsal Bundle; pl, Posterior Longitudinal Fasciculus; ref, Reticular Formation; sbn. Substantia Nigra; sc, Superior Colliculus; spt. Spinothalamic Tract. [Accession No. 146. Section 167. Actual Size 10X6 mm.] In this particular, the case is similar to that of the inferior colliculus which has apparently retained much of its primordial auditory function. The neces- sity for quick, decisive motor responses as a reaction to visual stimuh on the part of the marmoset must be obvious. Most of the animal's reactions to visual stimulation resuh in very rapid movements of the head, eyes, neck and limbs. CALLITHRIX JACCHUS, THE MARMOSET 179 These movements have much of the quality of immediate reflex responses. They resemble but little the motor reactions characterized by any marked degree of deliberate rcllection in selecting alternative courses of action which may arise in connection with any given situation. For this reason apparently the superior colliculus retains much of its primitive conspicuity as well as much of its original structural organization. The evidence afforded by marmoset indicates that vision has attained only partial telencephalization in this form. Also of importance is the appearance of the large substantia nigra (Sbn) which again points to the possibility of the animal's large endowment in automatic associative control of movement. In the most ventral portion of the basis are the fibers constituting the cerebral peduncle (CP). A lateral protrusion in juxtaposition to the substantia nigra is the mesial geniculate body (Mgb) along whose periphery is situated the brachium conjunctivum posticum containing auditory fibers on their way from the inferior colliculus. Dorsal to the mesial fillet ( M f ) is a collection of large motor cells, the nucleus ruber (NRu), which gives origin to the rubrospinal tract, the connecting link between the cerebellum and the final common pathway of the cerebro- spinal axis. The central gray matter ( C e n ) is now greatly increased in size and surrounds a small aqueduct of Sylvius. At its ventromesial angle it contains the nucleus oculomotorius (Noc) which gives rise to the third cranial nerve supplying all the muscles of the eyeball with the exception of the external rectus and the superior oblique. An important feature of this nucleus is the fact that it is very poorly supplied with internuclear or commissural fibers. The two nuclei maintain a certain degree of independence. This perhaps is no more marked than in lemur, but it is distinctly more pronounced than in the higher apes and man. It is probable that marmoset possesses but a small degree of power to converge the visual axes and in consequence has a limited amount of stereoscopic vision. Passing forward from the nucleus of the third nerve are its emergent fibers. i8o THE LOWER PRIMATES LEVEL OF THE OPTIC CHLA.SM (FIG. 89) At the level of the optic chiasm the identifying feature is the decussation in the optic pathway (Opt). The optic thalamus is also represented. This is FIG. 89. MARMOSET. LE\EL OF THE OPTIC CHIASM. CIN, Internal Capsule; cph, Corpus Hypotlialamicum; fdp, Descending Pillars of the Fornix; glb, Globus Pallidus; hab. Nucleus Habenulae; mf. Mesial Fillet; nmt, Medial Nucleus of the Thalamus; nli, Internal Lateral Nucleus of the Thalamus; nlt, External Lateral Nucleus of the Thalamus; opt. Optic Tract; OPX, Optic Chiasm. [Accession No. 146. Section 219. Actual Size 19 X 12 mm.] an important structure inasmuch as it represents the last great relay station for the conduction of all types of sensibility with the exception of the olfac- tory sense. Situated ventral to the main nuclei of the optic thalamus is a diffuse collection of fibers and gray matter, the corpus hypothalamicum CALLITHRIX JACCHUS, THE MARMOSET i8i (Cph). The nucleus which forms the central core of this body is surrounded by two fields of white matter constituting field Hi and field Hi of Forel. Lateral to the optic thalamus and the nucleus hj-pothalamicus is a dense FIG. 90. MARMOSET. LEVEL OF THE ANTERIOR COMMISSURE. AC, Anterior Commissure; cin, Internal Capsule; cph, Corpus Hypothalamicum; for. Fornix; len. Lenticu- lar Nucleus; nca. Caudate Nucleus; nmt. Medial Nucleus of the Thalamus; inli. Internal Lateral Nucleus of the Thalamus; tt, Taenia Thalami. [Accession No. 146. Section 241. Actual Size 15 X 6 mm.] band of medullary substance comprising a portion of the internal capsule (Cin); while ventral to this massive bundle of myelinized axons is a large nuclear mass, the globus pallidus (Gib). This structure takes its importance from the fact that it is one of the main basal ganglia regulating automatic associated movements. Immediately mesial to the globus pallidus is a bundle of faintly staining fibers, the descending pillars of the fornix (Fdp). i82 THE LOWER PRIMATES LEVEL OF THE ANTERIOR COALMISSURE (FIG. 90) At the level of the anterior eommissure (AC) is ilhistrated the anterior extremity of the optie thalamus and a portion of the tubercuhmi anticum thalami (NMt). Lateral to the thalamus is a dense mass of iiJDers repre- senting the internal capsule ( Cin ), hiteral to which is a portion of the corpus striatum, the k'nticuiar nucleus ( Len). Chapter VI RECONSTRUCTION OF THE GRAY MATTER IN THE BRAIN STEM OF CALLITHRIX JACCHUS Dorsal Sensory Nuclei THE nucleus of Goll first becomes apparent in the reconstruction as a small sessile projection connected with the dorsal surface of the cen- tral gray column. It rapidly extends dorsally as a narrow prolonga- tion of the central gray column and presents at its extremity a laterally flaring termination. It is situated close to the midline and separated from the dorsal median septum by a thin lamina of libers. At a somewhat higher level a small collection of gray matter appears in the dorsal portion of the reticular forma- tion. This consists of the nucleus of Burdach which, like the sensory nucleus of Goll, rapidly expands as it extends cephalad. Attached laterally to the main nuclear mass and presenting the characteristic arboreal form seen in the lemur is the external nucleus of Burdach (Blumenau). The effect of the opening of the fourth ventricle is already apparent in the divergent arrangement of the overhanging dorsal masses of the nuclei of Goll and Bur- dach. As these nuclei take their origin from the central gray column and the reticular formation mesial to the point of confluence between the dorsal gray column of the spinal cord and the central gray column, the former is slowly and gradually shifted laterally, and its expanded cap, the substantia gelat- inosa Rolandi, under the influence of the opening ventricle, shows a similar lateral inclination. The transition from the Rolandic to the trigeminal gelat- inosa occurs without sharp line of demarcation except that the trigeminal gelatinosa is constantly increasing in size as it ascends. In outline this struc- ture presents a serrated surface, the indentations being occupied by the 183 1 84 THE LOWER PRIMATES descending trigeminal tract irbers. The nuclear mass itself is triangular in form, its apex being mesially directed, while the base is disposed laterally in contact with the descending trigeminal fibers. The nucleus continues upward in its fixed location so constant as to afford an orienting point for other internal structures of the bulb, to the midpontine level where, after expand- ing, it rapidly undergoes contraction and disappears. The Inferior Oli\^\ry Nucleus Reconstruction of this mass of gray matter resembles very closely the nucleus as described in lemur. The inferior ohve forms a flat, double ribbon with its single plication situated ventrolaterally and close to the surface of the brain stem. Its free extremities are directed dorsomcsially and turn slightly dorsally at their terminations. There is httle indication of the devel- opment of the secondary plications characteristic of this nuclear mass in the higher primates. The approach of the nucleus to the surface occasions a slight elevation which corresponds to the relief of the olive so conspicuous in the human brain stem. The chief olivary nucleus, as in the lemur, is poorly developed. The plicated turn of the nucleus appears somewhat thicker and heavier than the two limbs of the nuclear mass formed by the accessory olivary nuclei which stretch dorsomcsially toward the median raphe of the oblongata. The inferior olivary nucleus in the marmoset, as in the lemur, consists mainly of the two accessory nuclei. The main inferior olivary nucleus is only suggested. The ventral accessory nucleus appears at the upper decus- sational level of the pyramidal tract as a flattened band directed mesially and slightly dorsally. It ascends, diminishing in width and approaching the midline. The dorsal accessory nucleus is a mass of nuclear material which rapidly flattens out and is embedded in the ventral surface of the reticular formation. It also tends to approach the midline and attenuates as it ascends. RECONSTRUCTION OF CALLITHRIX JACCHUS 185 The main olivary nucleus appears as a small mass of nuclear material between the actual extremities of the accessory olives with which it fuses. It presents no plications. FIG. 91. VENTRAL SURFACE OF THE GRAY MATTER OF THE BRAIN STEM, CALLITHRIX JACCHUS. Key to Diagram, inf. olive. Inferior Olive; lat. gen. body. Lateral Geniculate Body; pontile. Pontile Nuclei; ret. form.. Reticular Formation; subst. nigra, Substantia Nigra; vent, cochl., Ventral Cochlear Nucleus; v. G. C. and vent. cr. col., Ventral Gray Column. The Reticular Formation Reconstruction of the reticular formation reveals in general the same characteristics found in the lemur. It provides a matrix through which numerous fasciculi make their way. It is surrounded, except dorsally, by fiber masses passing upward or downward through the stem. Dorsally it comes into contact and merges with the subependymal gray matter which appears as the continuation of the central gray column. The reticular forma- tion originates as small masses of nuclear material appearing between the decussating bundles of the pyramidal tract, which, as these fibers are i86 THE LOWER PRIMATES gradually drawn together into a compact bundle, coalesce to form a recog- nizable structure. In the pontile levels, the reticular formation is separated from the deep layer of the pontile nuclei by the trapezoid body. This structure at higher levels forms a part of the lateral boundary of the reticular formation. The disposition of the reticular formation in marmoset is similar to that in lemur. It supplies the main gray mass of the brain stem, affording a matri.x* in which condensations of nuclear material appear as discrete nuclei. The lateral nuclei of the reticular formation and the superior olivary nuclei are but little better developed than in the lemur. In the region of the midbrain the nucleus ruber appears as a special condensation of the reticular forma- tion, surrounded by its fiber capsule derived from the superior cerebellar peduncle and the striato-rubral tract. In its dorsomesial aspect there are embedded successively the nuclei of the mesial somatic motor cell column, the hypoglossal, the abducens, the trochlear and the oculomotor nuclei. The vestibular nuclei appear, as in the lemur, in the dorsolateral angle of the tegmentum. The nucleus of Deiters first appears as a wedge between the reticular formation and the central gray matter. The cephalic extremity of the reticular formation comes into rather intimate relation with the sub- stantia ii'gra but does not fuse with it. Actual fusion takes place between the reticular formation and the zona incerta of the diencephalon and with the caudoventral pcM-tion of the central thalamic gray matter. The Pontile Nuclei In reconstruction these nuclear masses appear rather abruptly at the level of the trapezoid body. There is no substantial indication of the arciform nuclei in the oblongata which appear in higher primates as caudal prolonga- tions of the pontile nuclei along the ventral surface of the pyramidal tracts. The pontile nuclei appear as bilateral nuclear masses mainly disposed RECONSTRUCTION OF CALLITHRIX JACCHUS 187 mesially and laterally to the descending pallio-pontile and pallio-spinal sys- tems of fibers. These mesial and lateral nuclear masses are connected by relatively light bridges of nuclear material. There is but little evidence FIG. 92. DORSAL SURFACE OF THE GRAY MATTER OF THE BRAIN STEM, CALLITHRIX JACCHUS. Key to Diagram, inf. coll., Inferior Colliculus; lat. gen. body. Lateral Geniculate Body; nucl. of BURDACH, Nucleus of Burdach; nucl. of deiters. Nucleus of Deiters; nucl. of coll. Nucleus of GoII; ret. FRM. and RET. form., Reticular Formation; subst. gel. rolando, Substantia Gelatinosa of Rolando; vent, cochl., Ventral Cochlear Nucleus. pointing toward the development of the invading strips of nuclear material so richly present in the higher forms. Cephalically the pontile nuclei become continuous with the relatively massive substantia nigra chiefly by means of the mesial and lateral condensations of nuclear material already mentioned in the studv of the brain stem of lemur as the lateral and mesial buttresses. The Vestibular Nuclei Reconstruction of this mass of nuclear substance discloses a small, wedge-shaped area lying between the nuclei of the columns of Goll and Bur- i88 THE LOWER PRIMATES dach, the central gray matter and the reticular formation at the dorsolateral angle of the latter. The large nucleus of Deiters first appears at the lower decussational level of the pyramidal tract and, rapidly expandmg, reaches its greatest diameter at the midventricular level of the brain stem. From this region it diminishes as higher levels are approached. As it diminishes in bulk the vestibular nuclei increase to their maximum and then recede to give place to the cochlear nuclei. At the midventricular level the nucleus of Deiters and the nucleus of Schwalbe both begin to diminish. The vestibular nuclei are carried upward about the lateral walls of the ventricle by the small nucleus of von Bechterew which is so poorly diflerentiated as to make adequate representation in this model impossible. The Cochlear Nuclei As shown in the reconstruction, the cochlear complex consists of both the ventral cochlear nucleus and the dorsal cochlear nucleus which are situ- ated directly ventral to the subependymal gray matter of the floor of the lateral recess of the fourth ventricle. The dorsal cochlear nucleus is situated in the lateral ventricular recess and is more or less oval in outline. It extends for a short distance above and below the limits of the lateral ventricular recess. It also extends mesially for a short distance under the ventricular gray matter. The Substantia Nigra The reconstruction of this mass of gray matter resembles the nucleus found in lemur. It appears to be continuous with the deep layer of the pontile nuclei, supported throughout its entire transverse extent by this lamina, while at its extremities it rests upon the lateral and mesial buttresses of the pontile nuclei. Mesially it is connected with its fellow of the opposite side by the undifferentiated ventral interpeduncular gray matter. In its lateral por- RECONSTRUCTION OF CALLITHRIX JACCHUS 189 tion is lound the discrete nucleus described in connection with lemur, from which nerve fibers pass dorsomesially into the mesencephalic tegmentum. Its cephalic termination is uncertain as it seems to attenuate and disappear FIG. 93. LATERAL SURFACE OF THE GRAY MATTER OF THE BRAIN STEM, CALLITHRIX JACCHUS. Key to Diagram, dors, cochl.. Dorsal Cochlear Nucleus; inf. olive, Inferior Olive; nucl. of burdach, Nucleus of Burdach; nucl. of deiters, Nucleus of Deiters; pontile, Pontile Nuclei; ret. form.. Reticular Formation; subst. gel. rolando. Substantia Gelatinosa of Rolando; sup. coll., Superior Colliculus; VENT. COCHL., Ventral Cochlear Nucleus. with no continuity with the thahimic structures, although it appears to be rephiced by the zona incerta of the diencephalon. The Colliculi In the reconstruction, the inferior collicuhis arises at the lower mes- encephahc level from the reticular formation laterally and the tectal gray matter surrounding the upper extremity of the fourth ventricle mesially. It is separated from the superior cerebeHar peduncle by a narrow lateral prolonga- tion from the reticular formation of the mesencephalic tegmentum. Mesially ipo THE LOWER PRIMATES it is in contact with the dorsal undillcrciitiatcd ^ray matter surrounding the ventricle and the Sylvian aqueduct, while its lateral extremity appears to rest upon the lateral and dorsal extensions of the reticular formation. The inferior colliculus is c|uite massiAC and presents a marked expansion on the dorsal surface of the mesencephalon. It is separated from the superior collic- ulus by a narrow groove in which the reticular formation underlying the inferior colliculus comes to the surface. The superior colliculus is a massive structure and presents a well-marked surface elevation on the dorsal aspect of the mesencephalon. It also is continuous mesially with the unditlerentiated dorsal gray matter. Laterally it comes into close relation with the inferior brachium and ventrally with the dorsal extensions of the reticular formation. Cephalically the superior colliculus does not make contact with the thalamic nuclei, being separated from them by a dorsal extension of the reticular formation. The Central Gray Matter In the reconstruction, the sheet of gray matter in the floor of the fourth ventricle seems featureless and is disposed as a ilat, smooth stratum. The underlying structures produce no marked prominences or depressions in its surface. As the midbrain is approached, the lloor of the ventricle decreases in size, the side walls and roof approach each other and thicken materially so that when the aqueduct of Sylvius is formed, the lateral walls and roof present subependymal gray matter fully as bulky as the gray matter of the floor of the aqueduct. Cephalically the central gray matter is continuous with the gray matter underlying the ependymal lining of the third ventricle, forming the mesial group of the thalamic nuclei. The dorsal gray matter is continuous with the epithalamic structures, the ventral gray matter with the hypencephalic region. Chapter VII MVCETES SENICULUS, ITS BRAIN AND BEHAVIOR //,s' Position amonii the Primates; Measurements and Brain Indices; Surface Aj)])earance oj the Brain; Internal Structure of the Brain Stem in Cross Section jT' II ^HE Cebidac arc in general much larger than the Hapahdae and I more apc-hkc in appearance than the lemurs. The tail, although 11 , short in a few instances, is usually long and definitely prehensile. It is not covered with hair at its extremity and in this part especially manifests its prehensile characters. These animals vary considerably in size; some species are but little larger than the marmoset, while the larger members of the family are only a little smaller than some of the old-world monkeys. In general they are about as large as a fox-terrier. The development of the tail attains its highest degree of specialization in the spider monkey (Ateles) and the woolly monkey (Lagothrix). In these Cebidae the tail is so much differen- tiated in its prehensile function as to justify speaking of it as a Jijtb band. By means of this fifth hand the monkey swings its way along among the branches, procuring many advantageous positions of the body which enable it to use both fore- and hindlimbs in the manner of hands. It can reach, grasp and even hurl small objects by means of its tail. When not in use, the tail is carried erect over the head. This highly specialized sensory organ adds an important element to the behavioral organization of the animal, some evidence of which must be reflected in the central nervous system. The howling monkeys constitute an interesting group of the Cebidae. While they arc not possessed of a tail so highly differentiated as the spider monkeys, this organ none the less has great efficiency in its prehensile Courtesy, American Museum oj Natural History FIG. 94. HABITAT GROUP OF MYCETES SENICULUS, THE RED HOWLING MONKEY OF SOUTH AMERICA. [192] MYCETES SENICULUS 193 capacities. In addition to this specialization, the howHng monkeys have a sinus-like diverticulum in the larynx larger than that of any of the other American monkeys. The hyoid bone is much enlarged and contains a cavern- CouTlesy, American Museum oj Natural History FIGS. 95 AND 96. TWO VIEWS OF MYCETES SENICULUS, THE RED HOWLING MONKEY OF SOUTH AMERICA, SHOWING ITS PREHENSILE TAIL. ous dilatation, while the mandible, in order to protect the underlying struc- tures, is likewise unusually large and heavy (Fig. 95). The howling monkeys also possess an opposable thumb and well- developed toes. They are described as being the most ferocious of South American monkeys, at the same time having the lowest degree of intelligence. Their mournful howlings are often audible for miles around. Their cries are supposed to be employed as a means of defense in order to intimidate their enemies. Their vocal powers are so awe-inspiring that the 194 THE LOWER PRIMATES natives have associated certain almost supernatural attributes with these animals. For example, it is claimed that these monkeys are capable of form- ing a bridge to span a river by means of their tails. But while the prehensile Courtesy, New York Zoological Garde; FIGS. 97 AND 98. TWO VIEWS OF MYCETES SENICULUS, THE RED HOWLING MONKEY. tail makes possible many leats of locomotion little short ot miraculous, it seems clear that these bridge-building propensities arc quite legendary. Not only is the thumb of mycetes well developed and opposable, but the hand as a whole is highly specialized. The face is naked with the exception of a heavy beard which hangs beneath the chin. The movements of the howling monkey are relatively slow when compared to those of the lemur or even of the spider monkey. The animals appear to be sullen in temper, and are practi- cally untamable, soon dying in captivity. Whether or not their intelligence is of a low order and their adaptability to conditions outside of their natural environment small, it is certain that they are among the least attractive of all the primates in disposition. Their fur is usually black, but in some cases it is brown or reddish-brown. In most species the sexes are alike in color. Courusy. American A/uMum ../ Wiliual History FIG. 99. HABITAT GROUP OF ATELES ATER, THE SPIDER MONKEY. [195I 196 THE LOWER PRIMATES The species here described is Mycetes seniculus, also known as Alouatta senicukis, the red howling monkey. The red howler has its habitat in Car- tagena and Colombia. It lives in the forests near the Rio Negro but has also Courtesy^ American Museum oj Natural History FIG. 100. ATELES ATER, THE SPIDER MONKEY, SHOWING PREHENSILE TAIL. been described in Brazil. The animal's fore- and hindlimbs are about of equal length, that is, between ten and twelve inches long. The length of the body, MYCETES SENICULUS 197 exclusive of the tail, is from eighteen to twenty inches, while the tail is about thirty inches in length. When these howlers are seen in the forests, three or four of them are usually together on the topmost branches of the trees. They live largely on fruit, although, like other South American monkeys, they are said to feed also on caterpillars and msects. General Behavioral Tendencies — Thorndyke's Observations In connection with the general behavioral tendencies of Mycetes seniculus, observations made by Professor Edwin L. Thorndyke, of Columbia University, upon several South American monkeys of the Cebus type (species not stated) are of particular importance. They represent the pioneer effort in the attempt to investigate by accurate psychological method certain aspects of simian behavior. Professor Thorndyke was chiefly interested in the manner in which monkeys may vary from other mammals in the general mental functions revealed by their methods of learning, as well as how they may vary from adult ci\'ilizcd human beings. He recognized three diflerent modes by means of which knowledge may be acquired, namely, learning by trial and accidental success, learning by imitation, and learning by ideation. In the latter case the situation calls up some idea which then arouses the act or may in some way modify it. This, in fact, is the method of learning obviously employed in all advances of civilization. Professor Thorndyke devised certain more or less complicated experiments by means of which he tested the monkeys. These tests consisted chiefly of boxes with pegs, bolts, single bars, double bars, hooks, strings and wooden plugs. He arranged the plugs or loops in various combinations, the mastery of which was essential to release the animal from confinement or to admit it to a goal containing food. The tests made with such apparatus yielded negative results and made it clear that monkeys do not learn by reasoning. They do, however, form more associations and associations of 198 THE LOWER PRIMATES greater variety than other mammals. Their combination of such associations is remarkably slow and ineflective in providing any new behavioral acquisi- tion by this means. Nor were experiments involving the discrimination of Courtesy, American Museum of Nalurat History FIGS. 101 AND 102. HAND AND FOOT OF MYCETES SENICULUS. Left. Palmar surface of hand, showing digitation, well-developed balls of fingers and long, opposable thumb. Right. Plantar surface of foot showing fairly well-developed heel and narrow sole, long finger-like toes, with long, prehensile hallux. certain signals designed to set in motion definite lines of action any more pro- ductive of evidence. In experiments under the inlluence of human tuition the problem was — can the monkey learn and does he commonly learn and do things, not by mere selection of the act from amongst the acts done by him, but by getting some idea and then himself providing the act because it is associated in his mind with that idea? As the result of these tests, the conclusion was a negative one. The records which were carefully made show no signs of any influence of tuition to A\hich the animal was subjected. The systematic experiments designed to detect the presence of the ability to learn from MYCETES SENICULUS 199 human beings were almost unanmious against this possibihty. Theoretically, it is likely that monkeys learn more from watching each other than from watching human beings. Professor Thorndyke's observations in this Courusy, American Museum oj Natural History FIGS. 103 AND 104. HAND AND FOOT OF MYCETES SENICULUS. Left. Dorsum of hand showing well-marked fingers and finger-nails. Right. Dorsum of foot showing well-developed hallux, long finger-like toes and prominent toe-nails. connection were somewhat limited. They do not seem to favor the hypothesis that these monkeys have any general abihty to learn to do things by see- ing them done by others, even of their own kind. This question is still to some extent an open one, requiring much more extensive study than it has yet received. Concerning the general mental development of monkeys, Thorndyke believes that they represent a certain advance from the generalized mam- malian type toward man. This is particularly true of their sensory equipment and their localized vision. Their motor equipment provides for the coor- 200 THE LOWER PRIMATES dinated movements of the eye and the hand. In their method of learning, associative processes are quicker in formation of associations and there is a greater number of such associations as well as greater delicacy, complexity Courtesy, American Museum oj Natural History FIGS. 105 AND 106. HAND AND FOOT OF SPIDER MONKEY. Left. Palmar surface of hand showing well-developed palm, marked digitation with complete absence of tlie thumb. Right. PLintar surface of the foot showing long sole, small heel, short opposable hallux and long toes. and permanency in their representation. Yet, in spite of this increase as compared with lower mammals, these associations fail in their full signif- icance as utilizable behavioral components, probably because they lack close interassociation. Thorndyke feels that there is nothing surprising in the comparative absence of free ideas in these monkeys. The only demon- strable intellectual advance of the monkeys over the mammals in general is the change from a few, narrowly confined, practical associations to a far greater assortment of them. This fact mav turn out to be at the bottom of the MYCETES SENICULUS 201 only demonstrable advance in man. It is in reality an advance due to the brain acting with increased delicacy, bringing in its train the functions which distinguish human mental faculty from that of all other animals. CoUTte&y, American Museum oj Natural History FIGS. 107 AND 108. HAND AND FOOT OF SPIDER MONKEY. Left. Dorsum of hand, showing well-developed fingers and finger-nails, absence of the thumb. Right. Dorsum of the foot showing long metatarsus, long toes, opposable hallux, well marked toe-nails. Brain Measurements and Indices in Mycetes Seniculus Diameters of the skull Occipito-nasal -8 mm. Bitemporal 42 mm. Length of the brain case 56 mm. Brain, includmg cerebellum and brain stem Longitudinal 52 mm. Transverse 40 mm. 202 THE LOWER PRIMATES Total weight of the brain 24 . 5 gms. Water displacement of the brain 24. 5 c.c. Weight of the forebrain 20 gms. Weight of the midbrain , i gm. Weight of the hindl^rain 3.5 gms. On the basis of these figures, the following indices were computed for the several divisions of the brain: Forebrain index 81.6 per cent Midbrain index 4.8 per cent Hindbrain index 13.6 per cent These indices definitely place the animal in the manual group. Surface Appearance of the Brain in Mycetes Seniculus the fissural pattern The surface of the hemispheres of the brain is gyrenceJDhahc with the most marked fissuring in the parietal, temporal and occipital regions. The frontal lobe has but few sulci. The fissure of Sylvius is a prominent landmark upon the hitcral surface. Below and parallel to it is the superior temporal fissure, separating an inferior from a superior temporal convolution. Several interrupted sulci in the parietal region occupy the position of the interparietal fissure, while a small sulcus in the usual position of the fissura semilunaris separates the occipital frt)m the parietal lobe. A short preccntral sulcus extends horizontally from the sagittal line toward the fissure of Sylvius. The ventral extremity of this fissure falls considerably short of the latter sulcus. While this fissure may be easily discerned as the fissure of Rolando, its perpendicular relation to the great longitudinal fissure is reminiscent of the fissura cruciata of lower mammals. One deep fissure is present in the rostral Courtesy, American Museum uj Nalural History MYCETES SENICULUS, HOWLING MONKEY ATELES ATER, SPIDER MONKEY FIGS. 109 AND 110. DISTAL EXTREMITIES OF PREHENSILE TAILS. The rugae in the tails indicate the high degree of sensory specialization in consequence of which these monkeys are able to employ the tail as a "fifth hand." [203] 204 THE LOWER PRIMATES extremity of the frontal lobe separating a small inferior frontal convolution from a large superior frontal convolution. Several indefinite, interrupted sulci are seen on the orbital surface of the frontal lobe. Upon the mesial surface FIG. III. DORSAL SURFACE OF BRAIN, MYCETES SENICULUS. [Actual Length, 46 mm.] Key to Diagram, fis. p. o. lat., Fissura Parietooccipitalis Lateralis; sulc. prec. inf., Sulcus Precentralis Inferior; sulc. retrocent. inf.. Sulcus Retro-centralis Inferior; sulc. temp, sup.. Sulcus Temporalis Superior. the indication ot the general line and direction of the marginal sulcus is indicated by a scries of interrupted fissures. The calcarine and collateral fissures are well defined, as is also the supracallosal fissure. The corpus callosum is larger than in marmoset and lemur. The splenium in particular is somewhat thicker than in the lower forms already considered. The lateral appearance of the hemisphere in myeetes gives the impres- sion of a marked advance as compared with lemur and marmoset. The brain is definitely gyrencephalic and its fissural patterns are rendered conspicuous by the appearance of the well-marked Sylvian sulcus which is no longer dominated by any suggestion of a circumsylvian arrangement of convolu- tions. Although there is no such marked development in the sulcus simiarum Nn'CETES SENICULUS 205 as appears in the more advanced primates, the inception of this fissure is seen in the occipital lobe. The central fissure extends from the vertex of the hemisphere downward and forward toward the main Sylvian fissure in the K "^ ' \ FIG. 112. BASE OF BRAIN, MYCETES SENICULUS. [Actual Length, 46 mm.] Key to Diagram, obln.. Oblongata. region between the parietal and frontal areas. Its angulation with the superior longitudinal sinus is something less than go°. Lilcewise, the angle of the Sylvian sulcus with the base line of the brain is a little less than 45°, showing a general disposition on the part of this fissure to depart from its primitive vertical position and incline itself more toward the horizontal. LOBATION OF MYCETES BRAIN The lobation in the mycetes' brain is much more distinct than in the primates below it. This is due to the fact that the major sulci are all well defined. A short fissure indicates the position of the precentral sulcus in the frontal area, and deep fissures situated well forward toward the orbital sur- face indicate the positions of the sulcus prccentralis inferior and the sulcus 206 THE LOWER PRIMATES frontomarginalis. The parietal lobe is well demarcated, its caudal iaoundary being established by the inception of the sulcus simiarum. The occipital lobe as a whole shows considerably more expansion than in the case of lemur or FIG. 113. LEIT LATERAL SURFACE OF BRAIN, MYCETES SENICULUS. (Actual Length, 52 mm.] Key to Diagram, obl.. Oblongata; ram. post., Ramus Posterior of Superior Temporal Sulcus; sulc. PRECT. INF., Sulcus Precentralis Inferior; sulc. ret. inf.. Sulcus Retrocentralis Inferior. marmoset. The temporal lobe, perhaps more than any other portion of the lateral surface, shows a tendency toward that progressive advance which eventuates in the fully developed characters of this region in the primate brain. THE BASAL SURFACE OF THE HEMISPHERE AND THE OCCIPITAL CONCAVITY On the basal surface of the hemisphere, the two orbital concavities are well defined, as are also the interorbital keels. The olfactory bulb and tract show a considerable decline in prominence and are detachable as far back as the trigonum olfaetorium. The lateral root of the olfactory tract is much less prominent than in either the marmoset or the lemur, indicating in a general way the tendency toward deflorescence in the development of the olfactory central mechanism. The occipital concavity is pronounced in mycetcs due both to the expan- sion of the lateral lobes of the cerebellum and the further expansion of the MYCETES SENICULUS 207 occipital lobe of the hemisphere itself. This concavity is deepest in the midline where it appears as the postsplenial fossa for the accommodation of the protuberant superior vermis of the cerebellum. FIG. 114. RIGHT LATERAL SURFACE OF BRAIN, MVCETES SENICULUS. [Actual Length, 52 mm.] Key to Diagram, ram. post., Ramus Posterior of .Superior Temporal Sulcus; SULC. occip. lat., Sulcus Occipitalis Lateralis; SULC. precnt. inf.. Sulcus Precentralis Inferior; sulc. retrc. inf.. Sulcus Retro- centralis Inferior. THE CEREBELLUM In the cerebellum, certain advances are prominent, consequent primarily upon the pronounced expansion of the lateral lobes. The tentorial surface is entirely overhung by the occipital lobes. It is gabled from its lateral extrem- ity toward the median ridge-pole formed by the vermis cerebelh. The inter- foHal sulci pass without interruption from the vermis to the lateral lobes. On the occipital surface the expansion of the hemispheres of the cerebelhim is an even more conspicuous feature. The inferior vermis still occupies a prominent position on this surface but its proportions are reduced to about a sixth of the entire expanse of this region. In the lower forms the vermis constitutes a third of this area. Two deep paramedian sulci interrupt the passage of the interfolial fissures from the vermis to the lateral lobes. A feature of much importance in connection with the petroso-ventricular sur- face of the cerebellum in mycetes is the extreme development of the flocculus. 208 THE LOWER PRIMATES The flocculus in these monkeys, as well as in Atelcs and Lagothrix, all species in which the prehensile tail has i)ecome highly specialized, reaches a higher point of development than in any other primates. Whether this prominence FIG. I 15. VENTRAL SURFACE OF BRAIN STEM, MYCETES SENICULUS. [Actual Length, 34 mm.| Key to Diagram, cereb. peduncle, Cerebral Peduncle; opt. ped. space, Opticopeduncular Space; pvr. DEC, Pyramidal Decussation; trap, body. Trapezoid Body; ventro. med. sulc, Ventromcdian Sulcus. of the flocculus is due, as has been thought to be the case by several author- ities, to the additional responsibilities imposed upon the coordinative mechanism in response to the prehensile tail is a question needing further investigation. THE BRAIN STEM The surface markings of the brain stem in mycetes are more pronounced than in lemur or marmoset, although in only a few instances do they attain the prominence obser\ed in the higher apes. The oblongata upon its ventral MYCETES SENICULUS 209 surface presents a ventromesial sulcus which is interrupted at its caudal extremity by the decussating pyramidal fibers. Upon either side of this sulcus are two well-defined pyramids indicating an animal having a wider FIG. I 16. DORSAL SURFACE OF BRAIN STEM, MYCETES SENICULUS. [Actual Length, 34 mm.] Key to Diagram, d. m. fis., Dorsomedian Fissure; d.m.s., Dorsomedian Sulcus; inf. colliculus. Inferior Colliculus; sup. cerebl. peduncle, Superior Cerebellar Peduncle; sup. collicul., Superior Colliculus; TUB. tr., Tuberculum Trigemini. range of volitional control than is true of either lemur or marmoset. Lateral to each pyramid is a small ohvary eminence. The dorsal surface is characterized by the presence of the two enlarge- ments representing the cohmms of GoII and Burdach. While both of these structures are prominent, the column of Goll is slightly larger than the column of Burdach. The increment in the column of Goll reflects the appear- ance of a highly developed prehensile tail which adds what some authorities 210 THE LOWER PRIMATES speak of as a fifth hand to the animal's motor and sensory equipment. Although the prehensile adaptabihty of the tail in mycctes is hardly to be compared with that of the woolly monkey (Lagothrix) or the spider monkey (Ateles), the caudal appendage of the howhng monkey is an important organ which the animal uses with great deftness and skill. The increment in the column of Burdach is also significant. It rellects a further difl'erentiation of the hand with the consequent development of new motor capacities as well as acquisitions in manual dexterity and precision. In proportion as the fore- limb has emancipated itself from its limitations as a locomotor organ, it has expanded its potentialities in mastery of the environment and has added immeasurably to the upbuilding of new behavioral reactions. One feature of the brain stem which dilferentiates mycctes from lemur and marmoset is the size of the cerebellum. The pons Varolii is also considerably larger than in the lower forms. The inference from the size of the cerebellum in conjunction with that of the pons seems to be that mycctes possesses a fairly wide range of acquired skilled movements and employs the forelimbs for other purposes than those of locomotion. DORSAL ASPECT OF THE MIDBRAIN The dorsal aspect of the midbrain presents two well-defined sets of collicular eminences whose prominence suggests the persistence of certain visual and auditory functions primordially vested in the mesencephalon. Internal Structure of the Brain Stem in Mycetes Seniculus The internal structure of the brain stem in mycetes gives the impression that all of the structures thus far recognized as criteria in estimating behav- ioral reactions stand out with a clearness of definition not observed in either the marmoset or the lemur. MYCETES SENICULUS 211 LEVEL OF THE PYRAMIDAL DECUSSATION (FIG. II7) At the level of the pyramidal decussation the chief feature is the crossing of the pyramidal bundles and the effect which these crossing fibers have upon FIG. 117. MYCETES SENICULUS. LEVEL OF THE PYRAMIDAL DECUSSATION. CB, Column of Burdach; cen, Central Gray Matter; CG, Column of Goll; dt, Deiterso-spinal Tract; fle. Dorsal Spinocerebellar Tract; cow. Ventral Spinocerebellar Tract; hel, Spino-olivary Tract of Helweg; NR, Nucleus of Rolando; pv. Pyramid; p\-x. Pyramidal Decussation; rst. Rubrospinal Tract; spt, Spino- thalamic Tract; trd. Descending Trigeminal Tract; ven, Ventral Gray Column; xpv, Crossed Pyramidal Tract. [Accession No. 148. Section 15. Actual size 8X7 mm.] the arrangement of the gray matter. The decussation is shown at Pyx. Its final accompHshment and the formation of a crossed pyramidal tract about 212 THE LOWER PRIMATES to pass into the spinal cord is indicated at XPy, wJiilc tlie as yet uncrossed portion of the pyramid is shown at Py. Compared \\ith the lower forms, both the pyramid and the bundles constituting its decussation are larger in rela- tion to the rest of the cross section. This increase in size of the pyramidal system is significant of accessions to vohtional control over the muscles, more particularly the muscles of the upper extremities. The nnportance of this accession is noteworthy since this animal, when compared with those ahxady discussed, has developed a highly speciahzed hand. Even in this particular, however, it lias some of the defects character- istic of the higher anthropoid apes, especially in the relative shortness of the thumb when compared with the other digits. Nevertheless, because the thumb is opposable, it gives a dehnitely new capacity to the upper extremity as a grasping organ and thus induces a train of consequences with far-reach- ing Influences. The hand now becomes an instrument for analyzing elements in the environment. It assumes new activities in grasping and manipulating objects, and linally in submitting them to closer visual scrutiny. The manual development, because it adds to the equipment of these animals by providing a more efficient means for exploring their surroundings, inaugurates the proc- esses necessary to the eventual psychological differentiation between what is intrinsically part of the animal and what is definitely external to it. Thus an important step in constructing the psychic elements which distinguish between self and extra-self is established. The large size of the pyramidal decussation indicates an increment in volitional control especially demanded to meet the new motor possibilities of a well-developed hand. Quite as much does the expansion of a dorsal sensory field denote func- tional increments in the sphere of sensibility in consequence of manual and caudal development. The additions in the column of Goll (CG) represent sensor}^ expansions mainly clue to the prehensile tail which, if it has not attained in mycetcs the degree of functional differentiation warranting the MYCETES SENICULUS 213 designation of a fifth hand, as in Ateles, none the less has become an organ possessed of delicate sensory discrimination. It is probable that the acquisition of such a prehensile tail is chiefly accountable for expansion in the column of GoII since no further provocative specialization has occurred m the hind leg or foot. One particular development in connection with the nucleus of Goli is the median unpaired nucleus of Bischoff which is situated at the caudal extremity of the dorsal sensory nuclei in juxtaposition to the dorsal median septum. This nucleus was first described by Bischoff in 1899. It develops according to Zeehandelaar in animals possessed of tails used pre- hensilely, as in the spider monkeys, or as supporting organs in the kangaroos, or as in the Cetacea acting as propelling organs. Its presence in mycetes indicates the high degree of development of the prehensile tail in this animal. Even more striknig, however, is the expansion of the column of Burdach (CB) at this level, for which but one mterpretation seems reasonable, namely, the appearance of a highly diff'erentiated hand. The degree of dis- criminative sensibility now vested in the cutaneous and subcutaneous struc- tures of this hand provides the ability to estimate the consistency of objects, to detect differences in their size and shape, texture and temperature, mois- ture and dryness, as well as other physical qualities which may be appreci- ated by manual contact. To these sensory discriminations are added others equally important because they take their significance from the sensory impressions created by ne\\' ranges of motion. These new possibilities suggest that there has evolved from the simple and but little differentiated forelimb originally specialized for locomoticjn, an organ so highly modified that it may almost be considered new. New also are the avenues of contact with life which the hand has created, adding immeasurably to the stream of behavioral reactions of which the animal is capable. 214 THE LOWER PRIMATES It is difficult to estimate all of the far-reaching consequences of this pro- gressive differentiation of the upper extremity. The advent of the hand not only brings into existence an acquisitive explorer in the environment, but, by releasing the forehmbs from the responsibihty of locomotion, it inlluences profoundly both the posture and the method of locomotion itself. In such a capacity it dictates new tendencies in the selection of habitat. It provides new means of defense and offense and thus may aflect the matter of food supply and metabohsm. It becomes an instrument of investigation and con- trivance, the creator of a wide range of gestures and hence of symbols, and finally through its agency as a means of communication, leads on to the vocal accompaniments which ultimately eventuate in verbal speech. Having such importance in the synthesis of reactions \\hich characterize the complex output of behavioral performances in the highest form, the significance of this dorsal sensory field in the oblongata cannot be overestimated. There may be certain difficulties, perhaps, in maintaining that the progressive differen- tiation of the hand has been one of the chief factors in the later expansions of consciousness. Yet it must be clear that the animal possessed of such a discriminating organ requires more extensive sensory areas in the brain than the animal not similarly equipped. The increase of these sensory areas which represent the acquisitions of manual discrimination cannot fail to have a widespread influence by amplifying the sensory syntheses which enter into consciousness. The growing importance of the hand as a sensory organ is witnessed by another interesting fact. The portion of the dorsal sensory field in mycetes allotted to the transmission of sensory impulses from the head and face is not proportionally large when compared with the areas for receiving impressions from the extremities. Both the substantia gelatinosa and the descending trigeminal tract (Trd) are relatively smaller than in lemur or marmoset. The apparent reason for this relative decrease in prominence may be sought in the fact that the face and head have lost some of their MYCETES SENICULUS 215 primitive responsibility 111 the direction of locomotion. This iunction, in part at least, is now delegated to the more eflectively organized receptive organ, the hand. FIG. no. MYCETES SENICULUS. LEVEL OF THE CAUDAL EXTREMITY OF THE INFERIOR OLIVE. CB, Column of Burdach; cen, Central Gray Matter; dt, Deiterso-spinal Tract; fle. Dorsal Spinocerebellar Tract; cow. Ventral Spinocerebellar Tract; hel, Spino-olivary Tract of Helweg; lo, Inferior Olive; MP, Mesial Fillet; nb. Nucleus of Burdach; nbs, Nucleus of Bischoff; nbl. Nucleus of Blumenau; ng, Nucleus of Goll; NR, Nucleus of Rolando; pl, Posterior Longitudinal Fasciculus; pd, Predorsal Bundle; pv, Pyramid; REF, Reticular Formation; rst, Rubrospinal Tract; spt. Spinothalamic Tract; trd, Descending Trigeminal Tract. [Accession No. 148. Section 85. Actual Size 11 X 7 mm.| LE\EL OF THE CAUDAL E.XTREMITY OF THE INFERIOR OLIVE (FIG. I 1 8) At the level ol the caudal extremity of the inferior olive, a clearer impres- sion is obtained of the rehitive size of the pyramidal system (Py). There 2i6 THE LOWER PRIMATES is evidently a defrnite accession in motor control, particularly in the regula- tion of the movements in the upper extremity. The structures in the dorsal sensory field have occasioned a general broadening in the diameters in this region and also show an increase both in fiber richness and extensiveness of the nuclear collections. The nucleus of Goll (NG) appears to be of about the same size as the nucleus of Burdach (NB). The increase in the latter element indicates the addition of a highly developed hand. A corresponding increase in the mesial fillet (Mf) indicates increments in the function of discriminative sensibility already made apparent by expansions in the dorsal columns of Coil and Burdach. A feature of much importance is the inferior olivary body (10) whose caudal extremity is seen in this section. Here, as elsewhere in the brain stem of mycetes, the olivary body fails to show that clearcut delimitation characteristic of those forms in which the structure attains its highest differentiation. Since the inferior olive is function- ally active in the coordination of simultaneous movements in head, eyes and forearm, and since it facilitates the coordination of all skilled learned perform- ances, it should be more highly specialized in this species than in the lemur or marmoset. Differentiation of the olive depends upon the extent to which cooperative movements of the eyes, head and hand are coordinated. Such movements manifest an increase of effectiveness in proportion as the animal is able, by means of the hand, to bring objects into closer scrutiny by the eyes. This ocular, cephalic and brachial adjustment is also proportional to the degree of accuracy with which head and eye movements are capable of following manipulations of the hand. The substantia gelatinosa of Rolando (NR) and its accompanying descending tract of the fifth nerve (Trd) are prominent features at this level although comparatively smaller than in lemur or marmoset. The signifi- cance of this apparent decrease in the sensory representation of the head MYCETES SENICULUS 217 and face has already been identified as a concomitant development depend- ent upon the increase in functicMial capacity of the hand lor directing locomotion. FIG. 119. MYCETES SENICULUS. LEVEL THROUGH THE MIDDLE OF THE INFERIOR OLIVE. DO, Dorsal Olive; dt, Deiterso-spinal Tract; cow. Ventral Spinocerebellar Tract; hel, Spino-olivary Tract of Helweg; icp. Inferior Cerebellar Peduncle; 10, Inferior Olive; mf. Mesial Fillet; nb, Nucleus of Burdach; NFS, Facial Nucleus; nhv. Nucleus Hypoglossus; nr. Nucleus of Rolando; nvd. Dorsal Vagal Nucleus; N12, Hypoglossal Nerve; pd, Predorsal Bundle; PL, Posterior Longitudinal Fasciculus; py, Pyramid; ref. Reticu- lar Formation; rst. Rubrospinal Tract; spt, Spinothalamic Tract; trd, Descending Trigeminal Tract; vo. Ventral Accessory Olive; iv. Fourth Ventricle. [Accession No. 148. Section 145. Actual Size 13X6 mm.] LEVEL THROUGH THE MIDDLE OF THE INFERIOR OLIVE (pIG. I 1 9) At the level through the middle of the inferior olive (10) it is evident that the unfolding which has occurred in the olivary body is considerable. Not only is there an increase in size, but there is also a tendency for this struc- ture to assume the characteristic outline which identifies it in the higher species. It has, however, little of the convoluted appearance so prominent in the larger simians. It is now possible to recognize the main olive and several 2l8 THE LOWER PRIMATES accessory portions, i.e., the ventral accessory olive (VO) and the dorsal accessory olive (DO). The significance of this increase I^oth in definition and size of the olivary body may be understood in relation to the animal's FIG. I 20. MYCETES SENICULUS. LEVEL OF THE NUCLEUS AMBIGUUS. AMB, Nucleus Ambiguus; dt, Deiters' Nucleus; do, Dorsal Accessory Olive; cow. Dorsal Spinocerebellar Tract; hel, Spino-olivary Tract of Helweg; ICP, Inferior Cerebellar Peduncle; 10, Inferior Olive; mf, Mesial Fillet; nhv. Hypoglossal Nucleus; nfs. Fasciculus Solitarius and Nucleus; nsc, Nucleus of Schwalbe; nr, Nucleus of Rolando; N12, Hypoglossal Nerve; pl, Posterior Longitudinal Fasciculus; pd, Predorsal Fas- ciculus; PY, Pyramid; ref. Reticular Formation; rst, Rubrospinal Tract; spt, Spinothalamic Tract; trd, Descending Trigeminal Tract; vo, Ventral Accessory Olive. [Accession No. 148. Section 155. Actual Size 13X6 mm.] need of more accurate coordination in the simultaneous movements of eyes, head and hands for the new motor attainments acc[uired in consequence of manual development. LEVEL OF THE NUCLEUS SUPPLYING MOTOR FIBERS TO THE LARYNX (fIG. I20) At this level, a noteworthy specialization appears in the more fixed por- tion of the brain stem which throughout the primate series, generally speak- MYCETES SENICULUS 219 ing, shows little or no adaptive modification. In the case of the howling monkey, however, there is a striking degree of prominence in the develop- ment of those nuclei which have control of the larynx and hence regulate voice production. It is perhaps not surprising to find the nucleus amhiguus ( Amb) as well as the dorsal vagal nucleus, which are related to motor and sensory control of the larynx, highly developed in these animals. One of the characteristic physiological features of mycetes is the terrifying sound which they produce by their vocal organs and from which they have earned their distmguishing cognomen, howling monkeys. LEVEL OF THE \'ESTIBULAR AND CEREBELLAR NUCLEI (FIGS. 121 AND 122) Here at the level of the vestibular and cerebellar nuclei there are indi- cations of the extent to which the balancing and coordinating mechanisms have developed. The vestibular nuclei, comprising the triangular nucleus of Schwalbe (NSc) and the nucleus of Deiters (ND), occupy a position in relation with the floor of the fourth ventricle; while the cerebellar nuclei, which consist of the nucleus dcntatus (Ndt) and the nucleus fastigii (Nfg), are situated in the medullary substance of the vermal portion of the cerebellum. The vestibular nuclei are important receiving centers of the balancing mechanism and this, to a certain extent, is also true of the nucleus fastigii, which is connected with the vestibular mechanism by means of the juxtarestiform body. The dentate nucleus, on the other hand, has quite a different significance, being an index of the degree of etTerent conduction provided for the cerebellar impulses. The major outflow from the cerebellum is conveyed by axons which take their origin in the dentate nucleus and constitute the superior cerebellar peduncle. Thus, the nucleus dentatus offers a means of estimating the relative amount of coordinative influence which the cerebellum is capable of contributing to the muscles. 220 THE LOWER PRIMATES In regard to this nucleus in mycetcs, it is important to note that the struc- ture is not only more extensive in its general dimensions, but also begins to manifest, even if in a somewhat ill-defined manner, that tendency toward FIG. 121. MYCETES SENICULUS. LEVEL OF THE CEREBELLAR NUCLEI. CBL, Lateral Lobe of Cerebellum; dt, Deitersal Tract; Gow, Ventral Spinocerebellar Tract; icp, Inferior Cerebellar Peduncle; lo. Inferior Olive; mf, Mesial Fillet; nd, Deiters' Nucleus; ndt. Dentate Nucleus; NFG, Nucleus Fastigii; nr, Nucleus of Rolando; NSC, Nucleus of Schwalbe; pd, Predorsal Bundle; pl, Pos- terior Longitudinal Fasciculus; pv. Pyramid; rst, Rubrospinal Tract; spt. Spinothalamic Tract; trd. Descending Trigeminal Tract; ver, Vermis of Cerebellum. [Accession No. 148. Section 160. Actual Size 13X9 mm.] convolution of its surfaces typical of the higher primates. It may be inferred, therefore, that the degree of coordination which the cerebellum furnishes to the somatic muscles is greater in mycetes than in lemur or marmoset. The necessity of such coordination becomes clear in the light of the increased MYCETES SENICULUS 221 complexity in motion resulting from a greater volitional control of the upper extremities. Not alone is this increment in coordination essential to the newer movements of the hand, but now there is the further need of coordina- %i«.- FIG. 122. MYCETES SENICULUS. LEVEL OF THE VESTIBULAR NUCLEI. CTT, Central Tegmental Tract; dt, Deiterso-spinal Tract; do, Dorsal Accessory Olive; cow, Ventral Spino- cerebellar Tract; icp, Inferior Cerebellar Peduncle; lo, Inferior Olive; mf, Mesial Fillet; nd, Deiters' Nucleus; NFS, Facial Nucleus; nsc. Nucleus of Sclnvalbe; n8. Auditory Nerve; nr. Nucleus of Rolando; nhy. Nucleus Hypoglossus; pd, Predorsal Bundle; PL, Posterior Longitudinal Fasciculus; py. Pyramid; ref. Reticular Formation; rst. Rubrospinal Tract; spt, Spinotfialamic Tract; trd. Descending Trigeminal Tract; tub, Tuberculum Acusticum; vo. Ventral Accessory Olive. [Accession No. 148. Section 165. Actual Size, 13X5 mm.] tion arising from the fact that the animal depends more upon the hindhmbs in locomotion than do the lower forms. This requirement appHes hkewise to the partial attainment of the upright posture as well as the animal's tendency to sit upon its haunches and to balance itself in this position. All of these factors demand a more extensive development of coordina- tive as well as equihbratory control, and this control in its more remote secondary effects has aided in the development of the hand. 222 THE LOWER PRIMATES LEVEL OF THE EMERGENCE OF THE SIXTH NERVE (FIG. 1 23) A remarkable feature at this level of the brain stem in mycetes is the appearance in its entire course of the emergent root of the abducens nerve. FIG. 123. MYCETES SENICULUS. LEVEL OF EMERGENCE OF SIXTH NERVE. CEN, Central Gray Matter; cow, Ventral Spinocerebellar Tract; mf, Mesial Fillet; nab, Abducens Nucleus; NBE, Nucleus of Bechterew; nr. Nucleus of Rolando; n6, Abducens Nerve; N7, Facial Nerve; n8, Auditory Nerve; pd, Predorsal Bundle; pl. Posterior Longitudinal Fasciculus; pv, Pyramid; ref, Reticular Formation; scp, Superior Cerebellar Peduncle; so, Superior Olive; trd, Descending Trigeminal Tract; trp, Trapezoid Body; TUR, Tractus Uncinatus of Russel (Hook Bundle). [Accession No. 148. Section 230. Actual Size 18X7 mm.) The origin of this nerve is seen in its nucleus (Nab), and its course directly ventrad through the trapezoid body (Trp ), the lateral portion of the mesial fillet (Mf), and its emergence lateral to the pyramid (Py), are exceptional in cross sections of the primate stem. No fibers of the pons are as yet appar- ent at this stage. The central gray matter (Ccn ) is a narrow zone, which in its mesial angle contains the second portion of the facial nerve (N7). Its lateral angle is continuous with the nucleus of Bechterew (NBe). MYCETES SENICULUS 223 Situated above this latter nucleus are three important bundles of libers superimposed one above the other: the superior cerebellar peduncle (Sep), the fasciculus uncinatus of Russel (Tur), and the ventral spinocerebellar tract (Gow). The trapezoid body (Trp) and mesial fillet (Mf) lie dorsal to the pyramid (Py ). At the lateral apex of the corpus trapezoideum is the superior olive (SO). The emergence from the axis of the seventh nerve in its relation to the libers of the eighth nerve is clearly seen (N7 and N8). LEVEL THROUGH THE AHDDLE OF THE PONS VAROLII (fIG. 1 24) At the level through the middle of the pons Varolii a clearer conception may be gained regarding the size and complexity of the pons. The stratum superficiale is a dense and relatively broad bundle of fibers making its way transversely into the middle cerebellar peduncle ( Mcp). Dorsal to this is the stratum complexum containing the scattered bundles of the pyramidal system (Py), some transverse fibers and the pontile nuclei (PN). In the most dor- sal position is a fairly wide zone constituting the stratum profundum, the transverse pontile libers of which also enter the middle cerebellar peduncle. Perhaps at no other level is it possible to obtain so comprehensive a view of the animal's motor capacity from structural indices. The size of the pons, including its transverse fibers and nuclear masses, indicates a degree of cortical expansion in the cerebral hemisphere much above that attained by lemur or marmoset. It is significant of an animal capable of considerable coordinative control, one which has developed the lateral lobes of the cere- bellum more than is to be observed in lower mammals. Because of this development it has acquired control over the upper extremity such as is commensurate only with animals possessing a more or less highly developed hand. Nor is all of the prominence attained by the pons Varolii directly ascribable to the increased demands in coordinative regulation of the upper extrcmity.Thesedemandsmay be a primary incident or even an incentive to the 224 THE LOWER PRIMATES increase in coordinative control of the body as a whole. Such additional con- trol in coordination would arise from the profound readjustments occasioned by the assumption of the semi-erect posture and partial biped locomotion FIG. 124. MYCETES SENICULUS. LEVEL THROUGH MIDDLE OF PONS VAROLII. CEN, Central Gray Matter; ctt. Central Tegmental Tract; cow. Ventral Spinocerebellar Tract; lf, Lateral Fillet; mf. Mesial Fillet; mcp. Middle Cerebellar Peduncle; N4, Trochlear Nerve; N5, Trigeminal Nerve; PD, Predorsal Bundle; pl. Posterior Longitudinal Fasciculus; pn, Pontile Nuclei; pns, Pons; pv, Pyramid; REF, Reticular Formation; rst, Rubrospinal Tract; scp, Superior Cerebellar Peduncle; tur, Tractus Uncina- tus of Russel (Hook Bundle). [Accession No. 148. Section 305. Actual Size 15 X 10 mm.) as well as from the fact that the animal is now able to sit upon its haunches and in this position use its hands for various new purposes. Similarly the scattered bundles of the pyramidal system, although disseminated amidst the stratum complexum, present a greater degree of prominence in mycetes than they do in lemur or marmoset. They afford the basis for a structural MYCETES SENICULUS 225 index to contrast the probable dirterence in volitional control inherent in these several species. That the quantitative diOerencc favors mycetes, there can be no doubt. Its Ijchavioral reactions both in its learned and automatic movements are consonant with this estimation. On the [boundary between the stratum profundum and the remainder of the brain stem is the mesial fillet (Mf) whose size is considerably above that observed in lemur and marmoset. Here it is possible to form an idea of the relative vohime of this ascending sensory pathway and so estimate the degree to which the animal has developed its discriminative sensibihty. The increments to this ascending fasciculus of fibers appear to be due to additional sensory contributions arising in the upper extremity and particularly in the hand. In the tegmentum hiteral to the mesial fdlet is the lateral fillet (Lf), now approaching the inferior colliculus for another relay in the pathway of hearing. A dense bundle situated mesial to the lateral fdlet constitutes the superior cerebellar peduncle (Sep) which affords an opportunity of esti- mating to what extent the animal is equipped with a conduction system for coordinative control of the muscles. The superior cerebellar peduncle is larger than in lemur or marmoset, from which it may be inferred that mycetes is possessed of a more highly elaborated coordinating control. The central gray matter ( Cen ) surrounds the much reduced ventricular canal, the aqueduct of Sylvius. The roof of the aqueduct is formed by the medullary velum in which are the decussating fibers of the trochlear nerve (N4) on their way to the superior oblique muscle of the eyeball. Ventral to these fibers is the ascending or mesencephalic root of the fifth nerve. LEVEL OF THE INFERIOR COLLICULUS (fIG. I 25) At the level of the inferior colliculus se\'eral features of importance make their appearance. Among these is the tectal specialization of the inferior FIG. 125. MYCETES SENICULUS. LEVEL OF THE INFERIOR COLLICULUS. CEN, Central Gray Matter; Ctt, Central Tegmental Tract; ic. Inferior CoIIiculus; lf, Lateral Fillet; MF, Mesial Fillet; ntr. Nucleus Trochlearis; pd, Predorsal Bundle; pl, Posterior Longitudinal Fasciculus; PN, Pontile Nuclei; pns, Pons; pv, Pyramid; ref, Reticular Formation; scp, Superior Cerebellar Peduncle; xscp, Crossing of the Superior Cerebellar Peduncle. [Accession No. 148. Section 370. Actual Size 13 X 13 mm.] [226] MYCETES SENICULUS 227 colliculus (IC). The histological organization of the tectal region discloses a stratification almost as complex as in lemur and marmoset and thus suggests a functional capacity in the primitive correlating center of hearing of similar importance. The relation of such a correlating center to direct automatic acts of defense and offense in response to auditory stimuli appears clear. The necessity for such a relation is as important in this arboreal animal as in those already considered. A prominent feature at this level is the extensive substantia nigra, an index suggesting the probable persistence of many highly complex automatic associated movements. Ventral to the mesial fillet is the pons Varolii (Pns). It contains the pallio-ponto-cerebellar system of fibers as well as the fibers of the pyramidal system which latter, as in the oblon- gata, are not aggregated in a single bundle. From the size of these two fiber systems it is apparent to what extent the animal is endowed with volitional control from the cerebral cortex, as well as what concurrent cerebellar impulses must accompany the volitional stream which designs, initiates, directs and finally inhibits all voluntary movements. LEVEL OF THE SUPERIOR COLLICULUS (FIG. I 26) At this level, the superior colliculus (SO serves as an important relay in the pathway of vision. Since there is still a considerable histological organi- zation in this tectal region, it seems probable that the superior colliculus (SC) may retain some of its primitive visual function. That much of this function, however, has now been delegated to the occipital lobe is evi- dent by the pronounced development in the calcarine or visual area of the cerebral cortex. The ventral portion of the central gray matter contains the nucleus oculomotorius (Noc) whose fibers pass forward and inward to the oculomotor sulcus from which they emerge to supply all of the extrinsic muscles of the eyeball w ith the exception of the superior oblique and external 228 THE LOWER PRIMATES rectus muscles. This nerve also supplies all of the intrinsic ocular muscles and the levator muscle of the upper eyelid. It is especially signiiicant that the commissural connections of the nucleus oculomotorius are much more prom- FIG. 126. MYCETES ^il.\ UA LL S. LEVEL OF THE SUPERIOR COLLICULUS. CEN, Central Gray Matter; ctt, Central Tegmental Tract; cp. Cerebral Peduncle; mgb. Mesial Geniculate Body; MF, Mesial Fillet; noc. Oculomotor Nucleus; nru, Nucleus Ruber; N3, Oculomotor Nerve; pd, Predorsal Bundle; ref, Reticular Formation; rst, Rubrospinal Tract; sc, Superior Colliculus; sbn, Sub- stantia Nigra; spt, Spinothalamic Tract. [Accession No. 148. Section 401. Actual Size 18X9 mm.] inent than m lemur or marmoset, thus implying a closer association in interocular movements of the eyes and thereby securing a nearer approach to binocular vision and stereoscopic fusion. Ventrolateral to the nucleus oculomotorius is the red nucleus (NRu) which is iairly well dellned. It constitutes a rehiy station in the course of the cerebello-spinal pathway. From it arise the fibers of the rubrospinal tract (Rst) over which pass impulses necessary to muscular coordination. In MYCETES SENICULUS 229 this sense the size of the nucleus nil)er furnishes some chie of the coor- dinating capacity of the animal. Although more prominent than in marmoset or lemur, the nucleus ruber is not so large as in some of the higher apes, which FIG. 127. MYCETES SENICULUS. LEVEL OF THE OPTIC CHIASM. ciN, Internal Capsule; for. Fornix; fpd. Descending Pillar of Fornix; glb. Globus Pallidus; opx, Optic Chiasm; put, Putamen; th, Thalamus. [Accession No. 148. Section 501. Actual Size 28 X 17 mm.] seems to justify the supposition that the animal's eoordinative powers lie somewhere intermediate between the lowest and highest differentiation in the primate scries. LEVEL OF THE OPTIC CHIASM (FIG. 1 27) At this level the section shows the changes incident to the appearance of the optic thalamus (Th) and the optic decussation (Opx). The massive 230 THE LOWER PRIMATES structure of the internal capsule (Cin) is indicative, as are the cerebral peduncle and the pons Varolii, of the high degree of neokinetic expansion exhibited by these monkeys. The behavioral patterns have become greatly FIG. 128. MYCETES SENICULUS. LEVEL OF THE ANTERIOR COMMISSURE. AC, Anterior Commissure; cin, Internal Capsule; len. Lenticular Nucleus; NC, Caudate Nucleus; th, Thalamus; vl. Lateral Ventricle. [Accession No. 148. Section 575. Actual Size 28 X 13 mm.] amplified in consequence of those new capacities made available by quad- rumanous differentiation. To this new range of capacity should also be added those facilities of motion made possible through the prehensile tail, also spoken of as the fifth hand. Bordering on the outer surface of the internal capsule are the two major divisions of the lenticular nucleus, namely, the globus pallidus (Gib) and the putamen (Put). MYCETES SENICULUS 231 LEVEL OF THE ANTERIOR COMMISSURE (FIG. 1^8) At the level <>r the anterior commissure the brain stem has terminated in its cephalic extremity. The upper hmit of the optic thahimus (Th) is seen as it forms the anterior thalamic tubercle. The anterior commissure (AC) is approaching the midline from either side, about to establish com- munication between the two halves of the axis. The other important struc- tures in this level are indicated by corresponding letters in the captions. Chapter VIII RECONSTRUCTION OF THE GRAY MATTER IN THE BRAIN STEM OF MYCETES SENICULUS MYCETES scniculus has been selected for reconstruction as repre- senting the Ceiiidae. This animal shows a deiinite advance over the lemur and the marmoset. In size it exceeds considerably the animals already mentioned and the brain reconstruction is correspondingly larger than that found in either of the two preceding forms. The Dorsal Sensory Nuclei The nucleus of Goll first appears in the reconstruction as a dorsal extension of the central gray matter between the point of attachment of the dorsal gray column to the central gray column and the dorsal median septum. The nuclear mass is a narrow prolongation from the central gray column. It rapidly extends dorsally until it forms a core in the column of Goll. The nucleus is narrow, laterally compressed and presents a somewhat bulbous enlargement at its dorsal tip. At a somewhat higher level than the origin of the nucleus of Goll there appears on the dorsal margin of the central gray column a flat, sessile condensation in the central gray column which is the beginning of the nucleus of Burdach. At about the same level in the dorsal portion of the mass of white fibers forming the column of Burdach appear isolated masses of gray matter which coalesce and become attached to the dorsal extension of the central gray column to form the external nucleus of Burdach. The arboreal character of this nucleus, already mentioned in lemur, is further developed in mj'cetes. The appearance of the nuclei of Goll and Burdach between the dorsome- dian septum and the point of confluence of the dorsal gray column with the 234 THE LOW ER PRIMATES central gray column tends further to separate this latter structure from the midhne. As the mass of the dorso-medullarv nuclei reaches its maximum, the sufjstantia gelatinosa Rohindi which at this point is passing over into the sulxstantia gelatinosa trigemini is shifted into its fixed lateral position. The heavy dorsal mass of the nucleus of Burdach overhangs laterally the sub- stantia gelatinosa trigemini. The central gray matter at this point has merged to a considerable extent with the reticular formation. The transition between the substantia gelatinosa Rolandi and the substantia gelatinosa trigemini is essentially without a definite line of demarcation. As the latter structure is traced upward, however, it constantly increases in size except for the constriction found at the midolivary level, termed the waist of the trigeminal nucleus. The Inferior Olivary Nucleus In the reconstruction, the inferior olivary nucleus shows a moderate advance over that found in the Lemur mongoz and the Callithrix jacchus. A slight indication of secondary plication now appears in the dorsal and ventral branches of the u-shaped nucleus. The ventral accessory olivary nucleus appears at about the point at which the dorsal sensory nuclei begin to assume proportions of any size. It first manifests itself as a round collection of gray material which flattens out obliquely from before backward and inward. At a somewhat higher level, isolated gray islands appear which coalesce and form a similar parallel flattened band, the dorsal accessory olivary nucleus. The chief nucleus appears between the lateral extremities of this accessory nucleus as a rounded mass which rapidly assumes the form of a loop, the ventral branch of which fuses with the lateral extremity of the ventral accessory nucleus, while the dorsal branch is continued mesially parallel to the dorsal accessory nucleus. The mesial extremities of the accessory nuclei then fuse together. The accessory nuclei attenuate and dis- RECONSTRUCTION OF MYCETES SENICULUS 235 appear leaving the loop of the chief nucleus to continue upward for a short distance, after which it also comes to an end. The location of the inferior olivary nucleus corresponds to that already described in lemur and marmoset. FIG. 129. VENTRAL SURFACE OF THE GRAY MATTER OF THE BRAIN STEM, MYCETES SENICULUS. Key to Diagram, inf. olive, Inferior Olive; lat. gen. body. Lateral Geniculate Body; meso-gen. body. Mesial Geniculate Body; pontile. Pontile Nuclei; ret. form.. Reticular Formation; subst. gel. rolando, Substantia Gelatinosa of Rolando; subst. nigra.. Substantia Nigra; vent. coch. and vent, cochl.. Ventral Cochlear Nucleus; vent, gray col., Ventral Gray Column. The base of the nucleus approaches the ventrolateral angle of the brain stem, producing a slight elevation on the surface which corresponds to the olivary body found on the surface of the brain stem. The Reticular Formation The reconstruction of this mass of gray and white matter appears as a discrete body at about the middle of the pyramidal decussation. It is sepa- rated from and lateral to the ventral gray column. It rapidly increases in size 236 THE LOWER PRIMATES as the ventral gray columns diminish and linally the latter disappear by merging into the former at the level of the inferior ohvary nucleus. The reticular formation furnishes the same matrix for the ascending and descending fiber tracts found in the two preceding forms. It is surrounded on all sides, except the dorsal, by the ascending and descending fiber bundles of the stem. It is connected laterally with the mesial surface of the substantia gelatinosa trigemini and dorsaliy with the bases of the nuclei of Goll and Burdach. Embedded in it ventromesially is the dorsal lamina of the inferior ohvary nucleus together with the dorsal accessory olivary nucleus. As the reticular formation proceeds upward in the stem it gradually increases in size until it assumes considerable proportions. The nucleus lateralis of the reticular formation and the superior olive are developed to a somewhat greater extent than in lemur and marmoset. In the region of the midbrain the pronounced nuclear condensation of the reticular formation appears in the form of the nucleus ruber which is developed to a somewhat greater extent in this form than in either of the preceding types. In the dorsal aspect of the reticular formation are successively embedded the nuclei of the mesial somatic motor cell column, namely, the hypoglossus nucleus in the region of the medulla, the nucleus abducentis in the midpontile region, the nucleus trochlearis and the nucleus oculomotorius in the mesencephalic segment. In the dorsolateral angle, between the subependymal gray matter and the cephalic extremity of the nucleus of Burdach, appears a condensation in the reticular formation which gives rise to the vestibular complex. The nucleus of Deiters separates the subependymal gray matter and the nucleus of Burdach, reaching its maximum diameter at a level somewhat above the midventricular level of the stem. At this point the triangular nucleus of Schwalbe appears, and continues upward in its characteristic position. The nucleus of von Bechterew lies lateral and dorsal to the rest of the vestibular complex in the lateral wall of the fourth ventricle. The reticular RECONSTRUCTION OF xMYCETES SENICULUS =37 formation seems to be continuous with the matrix of gray matter from which develop the zona incerta of the diencephalon and also that of the less differ- entiated hypencephalic gray matter. FIG. 130. DORSAL SURFACE OF THE GRAY MATTER OF THE BRAIN STEM, MYCETES SENICULUS. Key to Diagr-\m. inf. coll., Inferior Colliculus; nucl. ofburdach, Nucleus of Burdach; nucl. of deiters. Nucleus of Deiters; nucl. of coll, Nucleus of GoII; ret. form., Reticular Formation; subst. gel. rolando. Substantia Gelatinosa of Rolando; sup. colliculus, Superior Colliculus; vent, coch.. Ventral Cochlear Nucleus. The Pontile Nuclei In reconstruction the pontile nuclei appear rather abruptly at the level of the trapezoid body. The nucleus is considerably more massive than in the Lemur mongoz and the Callithrix jacchus, and is tunnelled on both sides by the descending pyramidal and pallio-pontile tracts. This again produces the typical arrangeiiient of a lateral and mesial buttress, connected ventrally and dorsally by the superficial and the deep layer of the pontile nucleus. The simple arrangement of the nucleus as found in the Lemur mongoz and the marmoset now becomes somewhat more complicated w ith masses of nuclear 238 THE LOWER PRIMATES material beginning to infiltrate intu the bundles of the pyramidal and the pallio-pontile system of fibers, producing a lacework of nerve cells and fibers between the mesial aspects of the two lateral and mesial buttresses. As is the case in the lemur and marmoset, the deep layer of the pontile nuclei, together with the dorsal portions of the lateral and mesial buttresses, becomes continuous with and serves, so to speak, as a support for the mesencephalic substantia nigra. The Vestibular Complex The reconstruction of this nucleus has been described as a condensation in the reticular formation arising at about the middle of the ventricular por- tion of the medulla oblongata. It appears first as a small, wedge-shaped mass of gray matter between the subependymal gray matter of the ventricular iloor and the dorsal mass of the nucleus of Burdach. The nucleus of Deiters expands rapidly, becoming roughly triangular in shape, with its base upon the subependymal gray matter, its lateral boundary facing toward the nucleus of Burdach, and its mesial boundary toward the subependymal ventricular gray matter and the reticular formation. The triangular nucleus of Schwalbe continues somewhat cephalad to the lateral ventricular recess and then merges w ith the reticular formation. The nucleus of von Bechterew is situated in the lateral wall of the fourth ventricle cephalad to the lateral recess. This last nucleus is so feebly developed as to be scarcely demonstrable in the model. The Cochlear Complex As reconstructed, this nuclear collection conforms with the type found in the lemur and the marmoset. It represents a trough which covers the caudal, lateral and cephalic aspects of the entering cochlear nerve root. The RECONSTRUCTION OF MVCETES SENICULUS 23Q cochlear nerve, therefore, is uncovered by this nuclear material mesially where it comes into contact with the brain stem. The nuclear material extends along the course of the nerve, interspersed between its fibers. As FIG. 131. LATERAL SURFACE OF THE GRAY >L\TTER OF THE BRAIN STEM, MYCETES SENICULUS. Key to Diagram, lat. gen. bodv. Lateral Geniculate Body; m.g.b.. Mesial Geniculate Body; nucl. of BURDACH, Nucleus of Burdach; N. 01- deiters and nucl. of deiters, Nucleus of Deiters; pontile, Pon- tile Nuclei; ret. form., Reticular Formation; subst. gel. rolando, Substantia Gelatinosa of Rolando; SUBST. nigra. Substantia Nigra; ventral cochlear. Ventral Cochlear Nucleus. the cochlear nerve is traced into the dorsolateral angle of the brain stem another nuclear complex connected with the cochlear apparatus makes its appearance — the dorsal cochlear nucleus which hes in the lateral recess of the fourth ventricle, dorsal to the vestibular complex. This nucleus is tri- angular in shape, its base resting upon the subependymal gray matter, its lateral border directed toward the inferior and middle cerebellar peduncles and its mesial border Ij'ing against the vestibular complex. 240 THE LOWER PRIMATES The Substantia Nigra In the reconstruction, the substantia nigra appears in the reticular forma- tion of the mesencephalon, apparently as a cephahc specialization in the deep layer of the pontile nucleus. Mesially it is continuous with the undifTeren- tiated interpeduncular gray matter. Vcntraliy its surface is somewhat cor- rugated by the passage of the libers of the pes peduncuH. Dorsally it is in contact with the reticular formation of the mesencephalon which is demar- cated from it by the circumferential fiber bundles of this nuclear mass. Ventrolaterally in its upper portion it is hollowed out to a marked extent by a special nuclear accumulation which takes place in this part of the substantia nigra. This nuclear mass presents a tangle of myelinated nerve fibers which seem to rise in the nucleus and pass into the reticular formation of the mesen- cephalon. Cephalically it is continuous with the subthalamic gray structures of the diencephalon and laterally it merges with the reticular formation in which are embedded the mesial geniculate bodies. The Colliculi The reconstruction of the inferior colliculus appears as a rapidly increas- ing mass of gray matter supported laterally by the dorsal extension of the reticular formation. Dorsally it is continuous with the dorsal gray matter of the tectum which separates it from its fellow of the opposite side and also provides a pathway for the inferior collicular commissure. At the mid- mescncephalic level the inferior colliculus diminishes in thickness and gradually disappears. In this region the dorsal extension of the tegmental reticular formation comes to the surface of the tectum of the midbrain. Immediately succeeding this, the superior colliculus appears, situated in an essentially similar position to that occupied by the inferior colliculi. Its dorsal and ventral extremities receive the same type of support as that of the RECONSTRUCTION OF MYCETES SENICULUS 241 inferior colliculus and it is connected across tlie midline with its fellow of the opposite side by the superior collicular commissure. The Nucleus Ruber In the reconstruction, the nucleus ruber in Mycetes senicukis appears as a fairly well-diflerentiated and encapsulated mass of gray matter in the mesial portion of the reticular formation. It receives at its caudal extremity the decussating fillers of the superior cerebellar peduncle. These libers are con- tinued about the nucleus, forming a capsule for it and many of them are continued upward beyond the nucleus ruber into the subthalamic region. The upper extremity of the nucleus ruber projects into the diencephalic reticular formation and the nucleus in general seems to be located at a more cephalic level than is the case in the human brain stem. The Central Gray Matter The reconstruction of the central gray matter as lirst observed in the higher cervical level is somewhat cordiform in outline and is separated from the ventral gray column. To its dorsolateral corners are attached the bases of the dorsal gray columns. As the ventricular level is approached a narrow prolongation of gray matter is seen to arise from its dorsal aspect and passing along the dorsal median septum it divides into two tongue-shaped processes which flare out on either side. This is the first indication of the opening of the fourth ventricle. The central gray matter passes outward and backward, gradually flattening into a narrow ribbon of gray matter lying under the ependyma of the fourth ventricle. There the narrow strip of gray matter is continued around the lateral boundaries of the ventricle, both in the inferior and superior medullary velum so that the entire ventricular cavity is sur- rounded by material derived from or continuous with the original central gray matter. The gray matter of the floor of the fourth ventricle is relatively 242 THE LOWER PRIMATES smooth and presents little modelling of its surface in the lower half of the fourth ventricle. The walls are formed successively by the nuclei of Goll and Burdach and the nucleus of Deiters. In the upper half of the ventricle the door shows a single well-marked and rounded medial emmence produced b\- the mass of gray matter forming the nucleus alxlucentis, close to the mid- line and just above a line joining the two lateral recesses of the fourth ventricle. As the upper portions of the ventricle are approached the walls rapidly contract to form the narrow aqueduct of Sylvius which traverses the mesencephalon. The central gray matter of the mesencephalon is consider- ably thicker than that found in the lemur or the marmoset. It contains the dorsal extension of the trochlear and oculomotor nuclei which lie embedded in the dorsal region of the mesencephalic tegmentum. At the upper extremity of the mesencephalon the central gray matter is directly continuous with the subependymal gray matter of the third ventricle and the mesial thalamic nuclei. The most dorsal portion of the central gray matter in the mesencephalon is continuous with the epithalamic group of structures, while the most ventral portion is continuous with the hypenccphalic structures. Chapter IX COMPARATIVE SUMMARY OF STRUCTURES HAVING EVOLUTIONAL SIGNIFICANCE IN THE BRAIN STEMS OF THE LOWER PRIMATES A Critical Comparisoii oj the Pyramidal Tract, Olivary Body, Dorsal Sensory Nuclei, Vestibular, Cerebellar and Pontile Nuclei, the Midbrain Colliculi and Oculoinotor Decussation. Their EvolutKmal Significance in Relation to Behavior CoMPARAXn E Re\TE\V OF StKUCTURAL AND BeHA\ lORAL ADAPTATIONS CERTAIN homologous constituents in the brain stem of the lemur, tarsier, marmoset and howling monkey manifest a measurable vari- ability in structural clelinition and relative size. Such variations seem to harmonize with equally well-defined modifications in the sensory and motor equipment utilized by these animals in their highly differentiated behavioral adjustments. That these variables should particularly involve the neokinetic elements of behavior might be presupposed from the fact that the progressive addition of new motor complexes paved the way to the highest evolutional development. I. The Pyramidal Tract in Relation to the Vollintary Control of THE Extremities with Especial Reference to the Hand Especially striking are the variations indicative of modification in volitional inlluence over the somatic muscles. The pyramidal system has quite as much significance with regard to the degree of neokinetic activity as the cortex of the cerebral hemispheres. In direct proportion as the phyletic expansion of the neopallium has made possible new accessions of highly 244 THE LOWER PRIMATES skilled performances, the pyramidal tract has progressively enlarged. Although this neopalhal expansion adds ne\\- cortical fields for more extensive sensory correlation, its ultimate object is the behavioral expression made manifest in the total motor output from the ceref^ral cortex. The possibilities of increasing the richness of sensory associations within a single sphere of sensibility such, for example, as vision, are obviously dependent upon increase in its structural substratum. But when combinations of sensi- bility such as those within the reahii of vision are incorporated in compound association of sight and hearing, taste and smell and all qualities of somatic sensibiHty, then even vaster possibihties for sensing the world are open to the animal. In its turn, this gradually advancing conquest of the environment through avenues of the senses must find expression in new currents of behavior. It might be expected that the afferent convergence of this sensory influx would require a correspondingly expanded channel in the efferent pyramidal system. Indeed, it seems surprising that the pyramidal tract is no larger than it actually is. The pyramid, however, is a newcomer in the central axis. It is essentially a mammalian character in the brain, particularly implicated in the difTerentiation of the appendicular musculature. Its infTuence over the axial muscles, although potential, seldom reaches a high degree of specialization. The impulses which it conducts are preeminently concerned in the execution of such skilled performances as belong to the group of complex learned reactions. Undoubtedly the most highly organized of these skilled acts are dependent upon the operation of the distal portions of the upper and lower extremities, namely, the feet and toes, the hands and fingers. It is in relation to the progressive adaptation apparent in the upper extremity that the pyramidal system is most intimately connected. Sub- stantially little change occurs in the specialization of the lower extremity in the lower primates. The addition of the prehensile tail in certain of the South American monkeys undoubtedly requires an accession in volitional control SUMMARY OF STRUCTURES 245 of the caudal musculature. It is by no means so insistent in its demands as the rapidly differentiatinp; structures of the upper extremity and particularly of the hand. In lemur and tarsius, the pyramidal tract appears to be less prominent than in Mycetes seneculus. Such is the case also in marmoset. The hand ot this animal has made a hesitating advance toward manual differentiation, and its pyramidal tract is hence less conspicuous than in the howling monkey. PLANIMETRIC COEFFICIENT These facts are clearly illustrated by certain coefficients which show that the pyramidal area in proportion to the remainder of the cross section of the oblongata is greater in the howling monkey than in either marmoset, tarsius or lemur. The numerical expression of this proportion may be termed a plani- metric coefficient. It is obtained by means of projection drawings of the cross section in which the structure to be measured appears, the projection being produced at a fixed magnification. The area occupied by the structure under consideration is then determined by means of the planimeter, and in a similar manner the area of the hemisection in which the structure lies. The ratio of the structure whose coefficient is sought to the total hemisection of the axis is then computed and the figure taken to represent the planimetric coeffi- cient. The planimetric coefficients of the pyramidal tract in lemur, tarsius, marmoset and mycetes are: Planimetric Coefficients of Pyramidal Tract in Lower Primates Species CoefFicient Lemur . no Tarsier .032 Marmoset .064 Mycetes ■13" 246 THE LOW ER PRIMATES While these figures may not in any sense be accepted as dclinitive, they afford a close approximation to an actual estimate of the fact. From them it seems permissible to conclude that the increment in pyramidal volume is proportional to that increasing demand for control of motor performances made possible through the progressive development ol the hand. MORPHOLOGICAL CONSIDERATIONS IN THE COMPARISON OF UPPER EXTREMITIES IN MAN AND LOWER PRIMATES If the human upper extremity be accepted as the structural standard in the process of manual differentiation, certain morphological conditions must be taken into account when comparing these parts of the human body with similar structures in the lower primates. Among these conditions are the proportional length of the arm to the body, of the forearm to the arm, ot the hand to the forearm, as well as the proportions of the metacarpals and phalanges, particularly the proportion of the metacarpal bones and phalanges of the thumb. There should likewise be included the degree of opposability of the thumb and the differentiation of the fingers, especially the linger nails and the cutaneous pads connected with the distal phalanges. Estimated in the light of such criteria, the upper extremity in lemur and tarsius falls consider- ably short of complete manual differentiation. In the case of marmoset, the proportions in the major segments in the limb are at variance with the accepted human standard. The differentiation of the fingers is particularly primitive. The thumb is short, the cutaneous pads of the distal phalanges correspond more with the conditions presented by animals possessed of claws, and finally, the nails upon the lingers are much more claw-like than in any other form of primate. In mycetes, however, the behavioral reactions made possible through differentiation of the hands place the animal much closer to man than lemur, tarsier or marmoset. The performances of the howling monkey are SUMMARY OF STRUCTURES 247 much more humanoid than those ol the still lower primates. Although it is distinctly subhuman in its manual achievements, none the less mycetes must be assigned a place well up in the class of delinite manual ditlerentiation. II. The Inferior Olivary Nucleus in Relation to the Regulation OF Movements in the Eves, Head and Hands FUNCTION of the INFERIOR OLIVARY NUCLEUS There is considerable doubt concerning the function of the inferior olivary nucleus. Certain facts regarding it, however, are fairly obvious. The olive must be closely related in function to the cerebellum since its major connections are with that organ. The axons arising in the olivary substance pass as olivo-cerebellar fibers to the inferior cerebellar peduncle and thus reach the vermis and lateral cerebellar lobes. Whatever the precise function of this structure may be, its intimate association in the cerebellar reflex arc implies an activity related to the coordinative control of somatic muscula- ture. The inferior olive is intercalated as a relay nucleus in some important pathway whose impulses are destined to the cerebellum. It apparently serves to diffuse these impulses more extensively and thus bring into operation larger fields of cerebellar tissue in the interest of a highly specialized coordinat- ing activity. Von Bechterew and other neurophysiologists are largely agreed that the inferior olive is functionally concerned with static coordination. Experiments upon dogs in which the olive of one side has been injured cause a peculiar paralysis of the eye muscles with irregular nystagmic movements and simultaneous torsion of the body about its long axis. This torsion immedi- ately follows the injury and attains its highest degree directly after the operation. In the course of time the torsion becomes less pronounced and the paralysis of the eye muscles together with the nystagmus diminishes. 248 THE LOW ER PRIMATES Recent experimental work upon cats (Pike), in which the inferior olive on one side was destroyed, produced ahnost identical results with those reported Idv Bcchterew. The paralysis of the eye muscles, the nystagmus and the torsion of the body were all prominent symptoms as a result of discrete olivary lesions. The most striking feature of the disorder, however, was the torsion which took place in the trunk and neck. This torsion determined such a position of the body that the head and face of the animal together with its forehmbs were pointed one way, while the hind extremities pointed in the opposite direction. Repetitions of this experiment produced similar effects in all cases. These experiments were controlled in such a way as to exclude the results of injury to neighboring structures. The conclusion that the inferior olive is involved in the coordinative control of the eye, neck and arm musculature seemed unavoidable. Clinico-pathological observation sheds little light upon this problem. Cases in which lesions of the olive have been observed are usually masked by encroachment ol the pathological process upon some important atlerent or eflerent tracts in the oblongata. They have in no instance been discrete enough to permit of valid deductions regarding the function of this structure on the basis of pathological alteration in man. CONNECTIONS OF THE INFERIOR OLIVE These connections are of much signiiicance in this cjuestion. The out- standing fibers related to the olivary body are those already mentioned as constituting the olivo-cerebellar pathway. These fibers undoubtedly establish the ultimate connection between the olive and various portions of the cerebellar cortex including both the vermis and the lateral lobes. Another important connection is the central tegmental tract which lies along the ventrolateral aspect of the olive and may be traced upward in the tegmentum of the pons into the midbrain in the region of the nucleus oculomotorius and SUMMARY OF STRUCTURES 249 mesencephalic nucleus of the trigeminal nerve. By some authorities this latter nucleus is accredited with proprioceptive functions, receiving sensory stimuli from the eye muscles. Some fibers of the central tegmental tract may pass further cephalad in the direction of the basal ganglia of the endbrain and also into the posterior commissure. The main bulk of this bundle seems to terminate in the region of the nucleus oculomotorius. By means of it connection is established between the chief nuclei concerned in regulation of ocular movements and the inferior olive. A small and poorly myelinized tract of fibers may be traced to the olive from origins in the cervical and upper thoracic segments of the spinal cord. This is the spino-olivary tract of Helweg. In the absence of other ascending or descending connections with this important nuclear structure it seems probable that the olivary nucleus is an intermediary station for impulses received from the muscles of the eye, neck, upper extremity and perhaps the upper portion of the trunk. It ultimately delivers these impulses to the cerebellum. Such connections might well serve the purposes of simultaneous coordination in the ocular, neck, arm and upper trunk muscles. The fact that experimental lesions of the inferior olive disturb the coordmation of the eye muscles and cause pathological torsion in the neck lends support to the theory that one of the functions of the olive, if not its chief function, is simultaneous coordination of eye, head and hand movements and also the performance of all highly skilled acts. This presump- tion is borne out by the progressive increase in the conspicuity of the olive while passing from the lowest of the primates to the highest members of this order. OLIVARY SUBDIVISIONS Morphologically several olivary subdivisions have been recognized. These include a portion which is old, often referred to as the paleo-olire, and 250 THE LOWER PRIMATES a portion which is more recent, the neo-ohve. Each of these subdivisions has been accredited with special connections in the ccreijcilum. This question, however, does not necessarily concern the phyletic problem at present under consideration. Discrete differentiation of ohvary segments and the significance of their connections are matters needing further investigation before their real value as to the function of this nuclear structure is determined. Although there is undoubted significance underlying the distinction between the phyletically old and new portions of the inferior olivary body, attention is here directed to the evolutional unfolding which has involved the structure as a whole rather than any changes affecting its individual parts. EVE. HEAD AND HAND MOVEMENTS, AND THEIR CONTROL As the hand gains in its capacity to utilize motor patterns, the wider becomes its range of purposive reactions. The process of acquiring manual performances, as well as the actual execution of them once they have been acquired, necessitates certain directive influences. Vision especially becomes an important supplementary and even dominating factor. Without the aid of sight many complex movements of the hands could not be learned, and quite as certainly many acts would be deprived of their full efTectiveness if the eyes did not supply the proper idea of distance and perspective. Accepting the cooperation of vision as essential to the organization and performance of many highly skilled acts acquired by the animal, especially those resulting in the movements of the hands and fingers, it becomes clear that a close functional inter-relation between the movements of the eyeball and of the hand must exist. It is necessary that the visual axes hold in focus the movements of the hand during the performance of acts which have been acquired with the supplementary cooperation of the visual function. In this sense the eye and the hand become essentially one organ, since the move- ments in the one must follow and harmonize with the movements SUMMARY OF STRUCTURES 251 in the other. The nuiscular striietures producinjj; these movements must therefore be integrated in a manner to produce this harmony of motor effect. Indeed, certain other effectors are also implicated in this intricate activity which manifests itself in these simultaneous movements of eye and hand. To this group belong the muscles of the trunk which sustain the body in delinite positions to support the movements of the head upon the neck. The muscles of the hindlimbs should also be included in this category. The animal, espe- cially in the erect posture, reqiiires the cooperation of the hindlimbs in support of the body thus to provide a stable basis for the eye and head move- ments which accompany the movements of the hand. Synergic Units. One of the requisites for the precise control of such an extensive grouping of muscles in the l^ody is the muscular coordination in each of the groups cooperating to produce the result. Such coordination is dependent upon the proper intermuscular relation of all thespecialized groups forming the agonist and antagonist muscles. Within each group there exists, under normal conditions, a definite relationship in regard to muscular tension. When one muscle, it may be a flexor, contracts to produce a flexor movement, its antagonistic extensor likewise contracts to such an extent as to impose a check or guiding effect upon the muscle producing the movement. Each group of muscles because it acts in this manner to maintain this specialized inter- muscular relation, has been called a synergic miit. The entire body muscula- ture is composed of synergic units which maintain this definite intermuscular relation when in action. To secure such coordmate movement, one entire division of the cen- tral nervous system is set apart, namely, the cerebellum. Under the control of this organ the synergic units of the body are maintained in their proper relations to each other. By the same means proper intermuscular relation between the synergic units is established. The function which regulates coordinated action in the muscles of the body is known as syiiergia. In the 252 THE LOWER PRIMATES operation of this function, the cerebellum must be in touch from instant to instant with the varying degrees of muscle tension existing in each muscle group of the body. Cerebellopetal fd^ers furnish a physical means by which such communication is established. A continuous stream of atl'erent impulses is thus passing to the cerebeUum during all phases of muscular activity. It is interesting in this connection to note that most of the afferent pathways to the cerebellum from the musculature of the body pass upward in the spino- cerebellar tracts whose destination is the vermis cerebelli. This portion of cerebellar organization is chiefly concerned with the axial and paraxial muscu- lature, while the lateral lobes are much more engaged with the coordinative control of the appendicular muscles in the limbs. All of the ascending cere- bellar fibers are believed to receive intermediate relay in precerebellar nuclei. The column of Clark in the spinal cord is an example of such a precere- bellar nucleus. It receives peripheral afterent fibers and in turn gives rise to fibers which constitute the dorsal spinocerebellar tract. A similar longi- tudinal nucleus, perhaps not so discretely limited as the column of Clark, provides a relay for the libers constituting the ventral spinocerebellar tract. Both of these precerebellar nuclei have the form of long cell columns extend- ing through many segments of the spinal cord. In this respect they differ from the inferior olive which is also a conspicuous precerebellar nucleus. This nucleus is saccular in form and limited to the segments of the oblongata. EVOLUTIONAL EXPANSION IN THE INFERIOR OLIVE Such expansion as occurs in the spinal precerebellar nuclei must in the main be longitudinal. It is for this reason less conspicuous than the expan- sion in the inferior olive which is largely in the transverse diameters. The progressive enlargement of the inferior olivary nucleus in passing from the lowest of the primate order to man gives the impression of a much more striking evolutional process than is the case with other precerebellar nuclei. The olive is also more impressive from the fact that in its expansion there is SUMMARY OF STRUCTURES 253 a tendency for it to become more definitely convoluted. This feature goes hand in hand with the progressive development of the lateral cerebellar lobes. Those forms having the largest cerebellar hemispheres have also the most highly convoluted inferior olivary nuclei. The parallelism in development of olive and lateral lobes of the cere- bellum depends upon the fact that the inferior olivary nucleus, in its capacity of a precerebellar relay station, sends its fibers not only to the vermis, but in very large measure to the lateral lobes as well. Such is not the case with the precerebellar nuclei situated in the spinal cord. Most of the fibers arising in Clark's column, as well as those which give origin to the ventral spinocere- bellar tract, have their destination in the vermis of the cerebellum. The contrasts drawn by these morphological facts are not without evolutional significance. The spinocerebellar tracts represent a portion of the musculature with striking phyletic constancy, namely, the axial muscles of the body. The representation of these muscles when projected upon the cerebellar cortex requires for its elaboration the limited areas of the vermis only. This muscu- lature is restricted to the trunk, the neck and such axial structures as those innervated by certain of the cranial nerves. No such functional limitation prescribes the representation of the inferior olivary body as a precerebellar nucleus. It expands as the cerebellar lobes expand, and having a widespread connection with them, exists in response to portions of the muscular system whose representation when projected upon the cerebellar cortex reaches all areas both in the vermis and in the lateral cerebellar lobes. Factors Underlying Progressive Expansion of Inferior Olivary Nucleus. Apparently the same dynamic influences which have produced expansion in the cerebellar hemispheres are operative in the progressive expansion of the inferior olivary nucleus. Were it possible to select the one most compelling factor underlying such influence, it would doubtless be the progressive differentiation of the forelimb. This differentiation does not alone 254 THE LOWER PRIMATES participate in the intimate specializations occurring in the upper extremity. It involves those widespread adaptations concurrent with and consequent upon the hberation of the forehmb from the responsibilities of locomotion. It enters into the complete assumption ol upright posture, and modilication of the lower extremities essential to this purpose, together with acquisitions of muscular coordination incident both to bipedal locomotion and the per- fection of manual dexterity. Planimetric and Longitudinal Coefficients of Olixarv Body. Evi- dence in support of this view is tound even in the lower primates where the comparison of lemur, tarsier, marmoset and mycetes reveals a progressive ex- pansion in the inferior olive. The accompanying tabulation of the planimetric coefficients of the nucleus indicates a volumetric increment of lOO per cent or more in the olive of mycetes as compared with the other three species. Longitudinal coefficients of the olivary body, however, show even more extensive change. Together these two mensurations confirm the supposition that the inferior olive almost entirely expresses its expansion transversely within the limits of the oblongatal segments. It differs from the spinal pre- cerebellar nuclei whose expansion is chielly longitudinal. From a functional comparison of these four species it is clear that manual performances in mycetes are more complex and have a greater range of adaptability. As a structural indicator of this phyletic progress the inferior olive is of especial significance. It denotes the proficiency attained in the simultaneous move- ments of the eyes, head and hands. Coefficients of the Inferior Oli\ e in the Lower Primates Species Planimetric Longitudinal Lemur .060 .290 Tarsier .042 .180 Marmoset .038 .230 Mycetes . 120 .260 SUMMARY OF STRUCTURES 255 III. The Dorsal Sensory Nuclei in Their Relation to Discriminative Sensibility in the Extremities the nuclei of goll, burdach and blumenau The nuclear structures in the dorsal columns, intercalated in the path- way of discriminate sensibility, assume importance in relation to motor per- formances which depend upon kinesthetic organization. The more perfectly an animal senses the movements and postures in the several parts of its body, the more completely is it able to ad just these parts to complex motor patterns. A low degree of kinesthetic organization is indicative ofa limited range of reac- tion patterns. No specialized area of the nervous system affords a more illumi- nating index concerning the discriminative sensory inilux than the dorsal sensory field. Experiments and clinical pathology have demonstrated a discrete division in the oblongata! nuclei of these dorsal columns. Discriminative impulses fi'om the muscles, tendons, joints and bones in the leg and tail find relay stations in their advance toward the cerebral cortex, in the nucleus of Goll. Similarly, impulses from the upper extremity are relayed in the nucleus of Burdach. The ancillary nucleus of Blumenau, known also as the lateral nucleus of Monakow, is connected with the nucleus cuneatus. It presents certain features in which it differs from the dorsal nuclei. Its staining reaction is more intense and its cells are somewhat larger. Various opinions have been held concerning its relay function, both for and against the belief that it is intermediary in advancing impulses from the upper extremity to the cere- bellum. It seems clear, particularly in lemur and mycetes, that many fibers do make their way to the inferior cerebellar peduncle from this nucleus of Blumenau. It is equally certain that the pathway itself gains in prominence with the increasing degree of differentiation in the hand. While, therefore, no final opinion may be expressed with reference to the function of the nucleus of Blumenau, there is evidence to show that it serves as a relav for certain 256 THE LOWER PRIMATES impulses which arise in the proprioceptive organs of the upper extremity and are destined for the cerebelkim. THE NUCLEUS OF ROLANDO The remaining nuclear structure in the dorsal sensory field, namely, the nucleus of Rolando (substantia gelatinosa trigemini) accounts for the sensory innervation of the face and of the head rostral to the interparietal line. The exact course followed by the hbers conveying discriminative sensibihty from the trunk, from the neck and from the back of the head is not so well under- stood as the tracts which constitute the main conduction pathway from the extremities. It is possible that many of the fibers from the trunk, back of the head and neck make their way to the oblongata after relay in the reticular formation. They may incorporate themscKes in the great sensory pathways of the dorsal columns to receive their relay in the nuclei of Goll, Burdach and Rolando. In any event, these axial fibers, representing the distribution of the dermatomes in the trunk, in the neck and in the back of the head, constitute more or less constant factors which in all probability vary but little from species to species. It is unlikely that any great variation, either in the capac- ity of sensibility or in the demands for conduction, arises in connection with the trunk. Such modifications as do occur appear to be induced by conspicu- ous variables such as the tail, the forelimb and the hindlimb. Thus it might be expected that in an animal possessed of a highly efficient prehensile tail, the nucleus of Goll would appear in larger dimensions than in an animal either possessed of no tail or having one of much more generalized functional significance. The chief nuclear expression connected with the development of the tail appears in the nucleus of Bischoff. The development of the hand affects the column of Burdach which is much larger in animals having a high degree of manual differentiation than in those manifesting but slight differ- entiation of this kind. SUMMARY OF STRUCTURES 257 III the case of the Rolandie nueleus, although the area of innervation which it represents (the face and ventral portion of the head) is exquisitely axial in its distribution, this sensory territory of the body manifests consider- able iunetional variabiHty according as the animal depends more or less upon the head and face tor the direction of its locomotion. A COMPARISON OF THE NUCLEI OF GOLL AND OF BLRDACH IN THE LOWER PRIMATES A comparison of the nucleus of Goll in lemur, tarsier, marmoset and mycetes shows an actual increase in size in the howling monkey, accounted for largely by the presence of a well-defined nucleus of Bischotf. In lemur the planimetric coetlicient of the nucleus gracilis (Golf) is 4.1 per cent, in tarsius 2.6 per cent, in marmoset, 6.8 per cent, while in mycetes it reaches 13.1 per cent. All of these forms are essentially quadrumanal. Such increase as occurs in the size of the nucleus of Goll in these species cannot be attributed to essential modifications in the hindlimbs which, specialized as they are for arboreal life, become more or less standardized by this fact. They are subject to but little variation. The motive of this nuclear expansion must in conse- quence be sought elsewhere than in the specialization of the feet. The most obvious possibility to suggest itself is the specialization of the tail. In lemur, tarsier and marmoset the planimetric coefficient of the nucleus gracilis shows no striking diflerences. In these animals the tail serves as a balancing and steering organ. It has developed no prehensile qualities and thus has become but little specialized in its sensory capacity-. As might be presumed, such an organ would not exert much influence tending to efl'ect expansion in the sensory receiving nuclei. In the case of mycetes, however, the nucleus of Goll shows a marked expansion. Its coefficient exceeds that of marmoset by nearly ~ per cent and that of lemur by 9 per cent. Such a striking advance as this must have more 258 THE LO\\ER PRIMATES than passing significance. The skillfull manner in which mycetcs employs its tail supplies the reason for calling this organ a hfth hand, and hence the need of sensory specialization in this caudal appendage is evident. The tail at its tip is free of hair and presents upon its ventral surface parallel rugae not unlike the rugous markings upon the balls of the fingers and thumb. The ani- mal utilizes its tail not merelj- for purposes of suspension during locomotion, but in many selective acts in which sensory discrimination is necessary. A tail manifesting such a degree of deftness at once implies the accjuisition of a large series of motor patterns. It must therefore be the case that the kines- thetic development in connection with the movements of the tail is largely expanded in mycetes as compared with lemur, tarsier and marmoset. This expansion, however, is not confined to the nucleus of Goll. The planimetric coeflicicnts of the nucleus of Burdach show a similar increase in the size. In lemur this nucleus has a planimetric coeflicient of 4.9 per cent, in tarsius, of 2.9 per cent, in marmoset, of 4.3 per cent. In mycetes, the coeflicient is 1 1.3 per cent. Since the nucleus of Burdach, in the main, repre- sents the discriminative sensory influx from the upper extremitjs the hand would prove a dynamic factor most likely to exert an influence favoring expan- sion. In lemur, manual specialization is not far advanced. The thumb is short and lacks much of the opposability characteristic of the higher species. Both the forearm and arm are short and do not approximate the ideal proportions of human conditions. In fact, the forelimb is still so largely implicated in locomo- tion that it possesses many characters .inherent in a locomotor organ. Even more submanual is the forelimb in tarsier and marmoset. In both of these species, differentiation of the hand has made ineflectual attempts in the direc- tion of human conditions. In the case of mycetes, however, the forelimb has advanced decisively toward ultimate manual difTerentiation. Its locomotor oflices have been supplemented by the use of the prehensile tail which, per- mitting suspension of the body, aflords opportunities for the freer use ot the SUMMARY OF STRUCTURES 259 hand in acts of discrimination and selection. The planimctric coefficient of the nucleus of Burdach in mycetes is sHghtly in excess ot that of any of the other primates including the anthropoids and man. This observation will be more fully discussed in connection with humanoid and human manual ditler- entiation. In general, it seems to indicate a strong adaptive iniluence operat- ing in direct response to the specialization of the prehensile tail which has largely freed the forelimbs from functions of locomotion. It has thus made them available for more complex motor patterns utilized in exploring and further dominating the environment. The tabulation of coefficients follows: Planimetric Coefficients of Dorsal Sensory Nuclei in Lower Primates Species Goll Burdach Lemur .041 .049 Tarsier 1 . 026 .029 Marmoset . 068 • 043 Mycetes .131 .113 Longitudinal Coefficients of Dorsal Sensory Nuclei in Lower Primates Species Goll Burdach Lemur . 200 .290 Tarsier .190 .240 Marmoset .190 .210 Mycetes .210 .280 Advances in the nucleus of Goll and more especially in the median nucleus of Bischoff are measurably demonstrable in mycetes as an apparent response to the adaptive modifications incident to the development of the prehensile tail. This species, compared with lemur, tarsier and marmoset, shows a similar advance in the nucleus of Burdach as a consequence of further manual diiferentiation. 26o THE LOWER PRIMATES IV. The Vestibular Nuclei in Their Relation to the Balancing Mechanism extreme sensitiveness of the balancing mechanism This group of nuclei is fundamentally associated with the function of balancing, that is, the maintaining of the body in the optimum physiological posture, or of righting the body in the event that it may for any reason be forced out of this posture. This posture itself, although subject to numerous modifications, presents a fairly well-generalized pattern in all vertebrates. In accordance with this pattern the animal's best posture appears that in which the ventral surface of the body is nearest to the surface of the earth. This is true of fish, amphibia, reptiles and birds. From such a posture these animals may most readily initiate locomotion or remain in a resting phase preparatory to locomotion. This posture in mammals, with a few notable exceptions, such as the sloth and the bat, is similar to that in the lower vertelsrates. Any sHght deflection in this posture tends to cause extensive dis- organization in the animal's behavior. Such disturbances may be artificially induced by pathological lesions affecting the nervous system, more par- ticularly that part of the nervous system connected with the proprioceptive organization upon whicli balancing function depends. Tlius the destruction of one semicircular canal will so thoroughly disorganize the animal's capacity for assuming and maintaining the optimum physiological posture as to make locomotion impossible. It also makes tlie animal incapable of maintaining itself in any position. The mechanism upon which this important activity depends must needs be highly organized. This applies to its receptor organs as well as its central representation for receiving, converting and transmitting postural impulses to the musculature. Furthermore, this mechanism is extremely sensitive to many conditions of habitat and behavioral adjustments. Aquatic, aerial and SUMMARY OF STRUCTURES 261 arboreal adaptations create the most marked variations in this neural apparatus. The responsibility of the balancing mechanism in animals whose life is spent largely in flight, soaring to great heights, or dropping swiftly from the air to alight upon the branches of trees is most exacting. It necessitates a mechanism capable of adjusting the body in postures most advantageous for the use of wings. Adaptation to arboreal life imposes similar requirements upon the balancing apparatus. Such of the apes, for example, as are able to live almost exclusively in the trees and move from place to place by leaping, climbing and swinging, require a most delicately adjusted mechanism in the interest of equilibration. FUNCTION OF THE VESTIBULAR NUCLEI The primary centers associated with balancing are the vestibular nuclei. These nuclei serve as the chief receiving stations for the impulses flowing inward from the semicircular canals, the utricle and saccule. It is probable that even from the earliest stages of life these nuclear groups in the oblongata exert a pronounced influence upon the behavior of the young animal. With rare exceptions among the mammals, the power to assume and maintain the optimum physiological posture develops shortly after birth. The structural substratum for the early maturing of this function is found m the lact that the nerve fibers permitting communication between the proprioceptive organs have consummated their connections with the vestibular nuclei. These nuclei are in turn provided with efferent fibers necessary to the conduction of impulses destined to the somatic muscles. In this manner the reflex arcs essential to maintain the physiological optimum posture are completed. The vestibular nuclei in the oblongata are, of themselves, sufficient to mediate the reflex impulses necessary to the proper balance of the body. On the other hand, there is evidence that many myelinized fibers, even at an early period of development, pass to the region of the inferior colliculus. These axons 262 THE LOWER PRIMATES have connections with the lower portions of the neuraxis. They provide a series of reflex ares which participate in the balancmg function. That this function is primarily the concern of the lower segments of the brain below the cliencephalon and endbrain seems clear on the basis of ontogenesis. It may be held with a fair degree of certainty that the reflex arcs made possible through the vestibular nuclei of the oblongata, perhaps supplemented by influences from the midbrain, are sufiicient for a considerable period of tmie m early life to carry on the reflex activities involved in the balancing reaction. CONNECTIONS OF THE VESTIBULAR NUCLEI The several connections of the vestibular nuclei become ol considerable importance in this relation. These nuclei, as already indicated, are primarily connected by afferent fibers with the proprioceptors of the internal ear, including the saccule, the utricle and the semicircular canals. The nuclei have connection with the vermis of the cerebellum through the corpus juxtaresti- forme, also with the midbrain, particularly the oculomotor nuclei. COMPARATIVE DIMENSIONS OF DEITERs' AND SCHWALBe's AREA IN THE LOWER PRIMATES Conclusions based on comparative dimensions of Deiters' and Schwalbe's nuclei respectively indicate that the function of balancing and maintaining the optimum physiological posture in all of the primates is essentially the same in efleetiveness. Deiters' nucleus throughout the primate series manifests strikingly constant dimensions. It presents a maximum variation in its planimetric coefficients of less than 7 per cent, while its variation in its longitudinal coefficients does not exceed 4 per cent. In the case of Schwalbe's nucleus apparently the same constancy obtains in so far as the planimetric coefficient is concerned, although in the comparative longitudinal measurements of the nucleus a variational range of 6 per cent SUMMARY OF STRUCTURES 263 has been noted. The following tables give the planinietrie and longitudinal coefficients of Deiters' and Schwalbe's nuclei in lemur, tarsier, marmoset and mycetes : Pla.nimetric Coefficients of the Vestibular Nuclei in Lemur, Tarsier, Marmoset and Mycetes Species . Deiters' Nucleus Schwalbe's Nucleus Lemur .082 .045 Tarsier .180 .062 Marmoset .077 .060 Mycetes .114 .090 Longitudinal Coefficients of the Vestibular Nuclei in Lemur, Tarsier, Marmoset and Mycetes Species Deiters' Nucleus Schwalbe's Nucleus Lemur .21 .21 Tarsier .22 • 17 Marmoset .21 .20 Mycetes .18 ■15 Such ditlerences as do exist in the vc^stibular nuclei of these specit^s appear to favor tarsius, a fact which denotes how much more this animal depends upon its balancing function than lemur, mycetes or marmoset. This added functional responsibility may be inherent in tarsius due to its locomotion. In mycetes the problem in balancing includes not only the ordinary locomotion of climbing, leaping and swinging by hands and feet, but also embraces the many postures oi the body incident to suspension by means of the tail. Mycetes is capable of assuming positions in which the body is suspended while the hands and feet are freed for other activities than those of locomotion. In such positions as these the requirements of balance must be considerably greater than in other primates. The entire process of motor 264 THE LOWER PRIMATES adjustment, operating upon a less stable base in mycetes ealls into play the functional activity of additional balancing reflexes. While the problems of balancing change from species to species according as the locomotor adapta- tion is made to arboreal, terrestrial or intermediate modes of life, the demands for equilibration remain essentially the same throughout the primate order. This undoubtedly is due to the fact that although the equili- bratory requirements of arboreal life may recede as the animal approaches nearer to a strictly terrestrial habit of living, these are replaced by new requisites for balancing induced by the gradual assumption of the erect posture, by bipedal and ultimate plantigrade locomotion. That tarsius and mycetes should exceed other primates in the size of their vestibular areas is due to the fact that they have added to their balanc- ing equipment certain ancillary mechanisms essential to peculiarities in their locomotor specializations. Such specializations are conditioned by the pre- hensile tail or saltatory locomotion. In their general evolutional signilicance, the vestibular nuclei do not contribute so striking an example of progressive unfolding as is the case with many other structures. Nevertheless, their high specialization in mycetes is worthy of note. They reveal to what extent the development of a plastic charac- ter like the prehensile tail may influence so fundamental a function as that regulating the equilibrium of the body. The absence of such a tail in lemur, tarsier and marmoset seems to make this differential element all the more significant in the evolutional sense. v. The Cerebellar Nuclei and the Nucleus Ruber in Relation TO Coordination Several relatively prominent cell groups are recognized and distinguished as cerebellar nuclei. They include the nucleus dentatus, nucleus emboliformis, nucleus globosus and nucleus fastigii. The globosal and fastigial nuclei belong SUMMARY OF STRUCTURES 265 to what may be called the medial cerebellar division. The nucleus dentatus and nucleus emboliformis, occupying a more lateral position, are regarded as constituents of the lateral cerebellar lobes. In the preceding descriptions attention has been directed particularly to the nucleus globosus and the nucleus dentatus. Reconstructions of these structures have indicated how difficult It IS to make intrinsic distinctions between the medial and the lateral nuclei. For this reason the dentate nucleus has been taken to include both the nucleus dentatus and the nucleus emboliformis, while the nucleus globosus includes the globosal and fastigial nuclei. The division of these nuclei into a medial and lateral group has much functional significance. The lateral group is in large measure representative of the appendicular muscles of the body, that is, the upper and lower extremi- ties. That this group represents the appendicular muscles exclusively is not the case; but the main expansions in the nucleus dentatus occur in response to the progressive differentiation of motor capacity in the arms and legs. THE DENTATE NUCLEUS AND ITS CONCURRENT EXPANSION WITH THE CEREBELLUM AND CEREBRAL HEMISPHERES Inasmuch as the dentate nucleus is the principal relay station for efferent impulses from the cerebellum, its significance in connection with this organ as a whole becomes correspondingly important. From the physiologi- cal point of view, it is fairly well established that conditions demanding greater coordinative control of the musculature have their structural response in definite expansions of the cerebellum. A comparison of different species of mammals discloses the fact that the central or vermal portions of the cerebel- lum have participated less in such development than the lateral lobes. This is especially notable in the primates, in which the proportional size of the cerebellar hemispheres in relation to the vermis progressively increases in 266 THE LOWER PRIMATES passing from the lowest apes to man. Concomitant with this increase of the lateral cerebellar lobes, there is an expansion in the dentate nucleus. The reason for this growth in the cerebellar hemispheres and the corresponding development of the dentate nucleus is found in the connections of the coor- dinating organ. The vermis of the cerebelhim has a much more restricted connection than the lateral lobes. It is in communication by means of atTerent spinocerebellar tracts \\ith the spinal cord and by olivo-cerebellar libers as well as vestibulo-cerebellar libers, with the oblongata. The lateral lobes, on the other hand, although connected with the axial portions ol the brain by olivo-cerebellar fibers, receive their main tributaries from the pallio-ponto- cerebellar fibers. These axons connect the lateral lobes of the cerebellum with the hemispheres of the cerebrum. In other words, the vermis ol the cerebel- lum responds to such inllux ol impulses as may arise Irom cerebellar representation in the spinal cord and oblongata; whereas the cerebellar hemispheres are responsive to more complex neural syntheses created within the cerebral cortex as well as in the oblongata. Impulses trom these latter sources appear to require more expansive cerebellar receiving areas tor their elaboration than is afforded by the vermal cortex. It is apparent that the phyletic growth of the cerebral hemispheres has determined cerebellar expansion. In proportion as the cerebral cortex becomes more highly con- voluted, the connections between the cerebral and cerebellar hemispheres enlarge and the lateral lobes of the cerebellum expand correspondingly. The case of the vermis is dillerent. Its connections are exclusively seg- mental, that is to say, with the definitely segmented portions of the central axis. The lateral cerebellar lobes, although possessing certain segmental connections, establish their preeminent communications by means of supra- segmental fibers. These latter fibers arise in portions of the central nervous system which have developed as superstructures over and above the primor- dial segmented neuraxis. SUMMARY OF STRUCTURES 267 The dynamic significance of the suprasegmental divisions of the nervous system has ah-eady been mentioned in rehition to the actual and potential expansions of the brain. Such expansions provide further opportunity for increasing the range and number of neural syntheses in the progressive adaptations of animal behavior. To find the dentate nucleus expanding con- currently with the lateral hemispheres of the cerebellum and the cerebral hemispheres reflects the tendency of this nucleus to participate in the progres- sive development of behavioral adaptation. For this reason, the nucleus may be accepted as a reliable index of the coordinative adaptability of the animal. Its size, proportions and definition may, to an extent at least, be taken to indicate the range and intricacy of behavioral adjustments of which an ani- mal is capable. Connections of the Dentate Nucleus. As the gateway of impulses passing out of the cerebellum, the nucleus dentatus has its major connection, by way of the superior cerebellar peduncle, with the red nucleus m the mid- brain. This latter structure acts as an intermediate relay for the conduction of impulses through the axis to their various levels of distribution in the brain stem and spinal cord. The course of the fibers constituting the superior cere- bellar peduncle need not be considered in detail further than to observe that by means of two major decussations, both of which occur in the midbrain, the final connection between the dentate nucleus and the muscles is ipsilateral. In this way, the dentate nucleus on one side distributes the impulses from the cerebellum to the musculature of the corresponding side of the body. Acting thus in the capacity of the chief efferent distributing station in the cerebellum, the dentate nucleus is connected by means of cortico-dentate fibers with areas in the lateral lobes of the cerebellum as well as in the vermis. Syntheses of coordinating impulses arising in the cortex of the vermis and the lateral lobes thus make their way to the dentate nucleus and here find an outlet for their stabilizing influence over the muscles of the body. 268 THE LOWER PRIMATES THE NUCLEUS GLOBOSUS The nucleus globosus chiefly represents the central or vermal portions of the cerebelhim. Functionally the vermis is more rigidly fixed and responds but little to the progressive expansions of behavior. It supphes coordinative control to the axial muscles which show considerably less adaptive variation than the appendicular muscles. These axial muscles determine primordial postural patterns of the body. From the phyletic standpoint their coordina- tive control is palcostatic. It has been present from the beginning of verte- brate organization when trunkal movements were the preeminent requisites of locomotion. The expansion of such trunkal movements, as behavior becomes progressively more complicated, is relatively small in comparison with that of limb movements. Hence the nucleus globosus shows no marked degree of development concomitant with the cerebellar lobes or the cerebral hemi- spheres. In this respect it differs from the dentate nucleus. COMPARISON OF NUCLEUS DENTATUS AND NUCLEUS GLOBOSUS IN LOWER PRIMATES In general, the dentate nucleus appears most prominent in such animals as are possessed of the most highly diflerentiated forelimbs. These nuclear relations when applied to the lemur, tarsier, marmoset and mycetes are made clear in the planimetric and longitudinal coefficients as shown in the tables: Planimetric Coefficients of the Nucleus Dentatus and Nucleus Globosus IN Lemur, Tarsier, Marmoset and Mycetes Species Nucleus Dentatus Nucleus Globosus Lemur . no .030 Tarsier .059 • 037 Marmoset .077 .050 Mycetes .130 .032 SUMMARY OF STRUCTURES 269 Longitudinal Coefficients of the Nucleus Dentatus and Nucleus Globosus IN Lemur, Tarsier, Marmoset and Mycetes Species Nucleus Dentatus Nucleus Globosus Lemur .230 .10 Tarsier .180 . 10 Marmoset .150 . 10 Mycetes 1 .150 . 10 Although the diftcrcnccs in these species arc not so stril:ing as in the higher primates, they are sufficient to indicate that an evohitional process has impressed its influence upon the chief cerebellar nuclei. THE NUCLEUS RUBER The nucleus ruber of the midbrain, which is the principal relay station between the cerebelhim and the spinal cord, is particularly sensitive to cere- bellar expansion. It thus oflers a reliable index of augmentation in coordina- tive control over the muscles. Since the red nucleus receives impulses directly from the nucleus dentatus, the increments in the former should be propor- tional to those in the latter. In other words, if there does occur an actual increase in the coordinative control arising from the cerebellum both the dentate and the red nuclei should show increments of expansion in their coefficients pari passu. That such is not the fact, either in the lower primates or in the primate series as a whole, calls for explanation. In comparing the red nucleus in the lower primates, there is a progressive rise from the lemur to mycetes. This increase is so pronounced as to leave little cjuestion of a progressive expansion in this nuclear structure of the midbrain. The longi- tudinal coefficients likewise show a corresponding increment in the nucleus. 270 THE LOWER PRIMATES These coefficients are shown in the accompanying table: Coefficients of the Red Niclels in the Lower Primates Species Planimetric Longitudinal Lemur .012 .090 Tarsier .025 .160 Marmoset .044 .150 Mycetes .081 . 140 LACK OF STRICT FUNCTIONAL PARALLELISM BETWEEN THE DENTATE NUCLEUS AND THE NUCLEUS RUBER One fact, however, is not satisfactorily explained by these data; namely, that the increment of the dentate nucleus in these species does not vary pari passu with the increment of the red nucleus. This disparity probably indi- cates a lack ot strict tunctional parallelism between the dentate nucleus on the one hand and the nucleus ruber on the other. Such disparity may be explained by certain fibers arising in the red nucleus and passing forward through higher levels to the suprasegmental portions of the axis. There is also some evidence to indicate that certain descending fibers from the region of the corpus striatum connect the latter body to subadjacent aggregations of gray matter in the nucleus ruber. This nucleus may, therefore, represent a composite relay station into which enter impulses not only from the cerebel- lum, but certain others from the corpus striatum as well. Thus, insofar as the functional significance of the red nucleus is concerned, variations in the increment of its expansion are perhaps less important as bearing upon the function of coordination than those of the dentate nucleus. It may be regarded, however, as collateral evidence in relation to this function, and is not without value in estimating the progressive changes going forward in the cerebellum. SUMMARY OF STRUCTURES 271 \i. The Pontile Nuclei in Their Relation to the Regulation of Skilled Mo\ements, Particularly in the Upper Extremity THE pons \'AR0LII AS AN INDEX OF INTELLIGENCE A Statement to the effect that the pons Varolii may be held as an index of the inteUigence possessed by an animal has found considerable acceptance. Allowing for a degree of exaggeration inherent in ail such axiomatic formu- lations, it may be safe to say that the pons Varolii does to a large extent indi- cate the proficiency attained in the power, range and complexity of skilled performances. A structure, therefore, providing direct indications of such specializations would c/t- factii represent the extent to which the intelligence has been developed. The size of the pons Varolii varies conspicuously in the different orders of mammals. Being primarily a mammalian structure, it has its lowest repre- sentation in those animals which have least developed the use of the fore or hind legs. Its highest development occurs in those animals in which the forelimbs have become freed from the function of carrying the body, and are employed for purposes other than those of locomotion. Thus, while in monotremes and edentates the pons Varolii is small to the point of being almost negligible, in primates and most particularly in man, it reaches its greatest dimensions. The physiological substratum underlying this variation in size is definite and significant. In the execution of any learned skilled performance, it has been shown that there is need for the concurrence of two simultaneous streams of innervation. In the first place, the design, pattern, extent and duration of the act must be devised and directed; its incentive must be constructed and its limitations prescribed. While the stream of nerve impulses constituting this synthesis is being built up and distributed to the ertector structures, a second stream must run parallel with it in order to maintain the proper coordination of the muscles. That the incentive syn- 272 THE LOWER PRIMATES thesis to any such skilled or learned performance has its origin in many areas of the cerebral cortex is the generally accepted opinion of the present day. According to this view each specialized area of the neopallium may, and probably does, participate in the formulation of this incentive synthesis. Kinesthetic sense, general body sense, vision, hearing, the sense of smell and even of taste, together with certain higher discriminative faculties imparting the elements of judgment, all contribute to this composite body of impulses which finally combine to form an incentive synthesis. The genesis of the second of the two concurrent streams is of equal importance. Each major functional region of the cerebral cortex — such, for example, as the frontal, the parietal, the temporal and the occipital lobe — gives rise to a group of fibers which, becoming collected, enter the corona radiata, thence pass through the internal capsule, into the cerebral peduncle, and ultimately end in the pons Varolii. These are known as the pallio-pontile fibers. Their several subdivi- sions are specifically indicated as the fronto-pontile, parieto-pontile, tcm- poro-pontile and occipito-pontile contingents. Each contingent by means of synapses in a large nuclear mass of the pons VaroUi, the pontile nuclei, gains ultimate connection with the lateral lobes of the cerebellum. Those animals with small-sized lobes of cerebral cortex contribute correspondingly small contingents to this pallio-ponto-cerebellar system. In consequence, not only the number of fibers entering into the pons from this source, but the size of the pontile nucleus necessary to relay them therein, is correspondingly small. Hence, the animal equipped with a small cerebral cortex must of necessity have a small pons Varolii. From the functional standpoint, the smallness of the pons is indicativeof an animal poorly equipped in the more complex varieties of skilled learned performances. For these reasons the size of the pontile nuclei may also be employed as a reliable index in estimating the degree of expansion in the cerebral cortex, and, from such estimation, in arriving at an opinion as to the degree to which that animal SUMMARY OF STRUCTURES 273 has developed its skilled performances. No more cogent indicator of these specialized functional capacities may be obtained in the brain stem. It is, perhaps, even safe to say that of all the structures in this part of the central nervous system, none is more reliable or important than these nuclei lodged in the pons Varolii. COMPARISON OF PONTILE NUCLEI IN LOWER PRIMATES In looking to this structure, therefore, for evidence of evolutional unfolding, a comparison of the lower primates gives the impression of a pro- gressive development in this nuclear group of the pons. Such a structural expansion is also well borne out physiologically by the increment manifested by these animals in their accjuircd learned reactions, and more particularly those performances executed by means of the upper extremities. A tabulation showing the planimetric coefTicients of the pontile nuclei in lemur, tarsier, marmoset and mycetes is appended. From it, a progressive expansion is readily apparent, mycetes having nuclei nearly twice the size of lemur. There may be some question concerning the relatively high pontile differentiation manifested by the marmoset, an animal known to possess a manual differentiation quite inferior to that of lemur. In spite of this appar- ent discrepancy, there is a strong probability that the entire family of Hapalidae employ the upper extremities in a much more hand-like manner than is true of the lemurs. Hence, while structurally the hands in the marmo- Coefficients OF THE Pontile Nuclei in the Lower Primates Species i Planimetric j Longitudinal Lemur .055 { .270 Tarsier .057 1 •330 Marmoset ■095 1 .230 Mycetes .103 •350 274 THE LOWER PRIMATES set have not attained the degree of speeialization that eharacterizcs lenuir or mycetes, they still have developed sufficient capacity for complex skilled performances to necessitate relatively extensive pontile nuclei. THE PALLIO-PONTO-CEREBELLAR SYSTEM By means of the relay already referred to in the pons, the pallio-pontile system is continued into the cerebellum. This entire collection of fibers con- stitutes what is termed the pallio-ponto-cerebellar system. As these libers take their origin in the cerebral cortex, they are spread out much m the manner of a fan, becoming convergent as they approach the internal capsule and cerebral peduncle. In their termination, also, they again spread out in relation with the lateral lobes ol the cerebellum. Thus, the pallio-ponto- cerebellar system has in relation with it two conspicuous fan-like radiations: one at its origin in the cerebral cortex, another in its termination in the lateral lobe of the cerebellum. This is a morphological fact of much significance, clearly indicating as it does the many regions ot the cerebral cortex which are connected with all areas of the lateral lobe of the cerebellum. Further- more, as the cerebral hemisphere increases in size and prominence during the course of evolutionary expansion, this pallio-ponto-cerebellar system likewise gains in volume. It contains a greater number of fibers in those animals possessed of large hemispheres with well-delined lobation than in those species in which lobation is more inconspicuous. It follows that in proportion as the number of fibers arising in the cerebral cortex is large, so also are the pontile nuclei. Functional Significanceof Pallio-ponto-cerebellar Fibers. Clin- ical and pathological conditions have not as yet yielded satisfactory evidence concerning the functional significance of these pallio-ponto-cerebellar fibers. The reason for this deficiency is not far to seek. Lesions in the cerebral hemisphere near the surface of the cortex where these fibers take origin, in SUMMARY OF STRUCTURES 275 the corona radiata, or in the internal capsule where they become convergent toward the cerebral peduncle, are not discrete enough to produce exclusive involvement of the pallio-pontile axons. Thus it is that no pure disturbance involving this system has yet been observed. Either the motor system or some part of the sensory system is simultanecnisly implicated by the lesion, and so adds elements which raise doubts as to the specific function of the pallio-ponto-cerebcllar fibers. Experimental lesions are complicated by similar difficulties. Neither of these sources of infVirmation has, therefore, contributed as much as might be desired in revealing the functional activities of this system of axons. On the other hand, the increase of this system in animals capable of highly complex performances bespeaks a function which has a capacity for expansion directly proportional to the evolutional develop- ment of the cerebral hemispheres. The origin of these fibers in the cerebral cortex, their relay in the pons Varolii, their termination in the lateral cerebellar lobes, declare them to be of utmost importance in some phase of neural activity whose mechanism ultimately depends upon the cerebellum. This mechanism at the same time has as its essential coadjutor the controlling influence of the cerebral cortex. Whether it be finally decided that the functional capacity of the pallio- ponto-cerebellar system is primarily in the interest of coordination for the performance of complex skilled acts, or whether it is shown that there exists some other reciprocal relation between the cerebellum on the one hand and the cerebral cortex on the other, is a question for further investigation to decide. In either case the pontile nuclei serve as a relay for a great system of fibers arising in the neopallium. They show progressive expansion in propor- tion to the evolutional development of the endbrain. Such expansion goes hand in hand with the acquisition of more complex, more numerous, more varied \'oluntary performances, capable of producing a greater continuity in action. That motor activities characterized by such qualities as these are 276 THE LOWER PRIMATES most commonly seen in those animals which have gamed comparative free- dom of the upper extremities from the oflices of locomotion, is obviously true without further proof. As a corollary to the supposition that the pontile nuclei increase in proportion as the animal becomes more capable in its skilled performances, it is true that these nuclei also expand as greater manual differentiation is attained. It may therefore be held that the progres- sive increment in this important group of nuclei bears definite relation to the increasing capacities acquired as the forelimbs become more precisely cap- able of manual function. VII. The Colliculi of the Midbrain in Their Relation to Sight and Hearing their primordial eminence in auditory and visual functions The history of the mesencephalon indicates that this portion of the brain has presided over many important functions during the development of the phylum. It quite as clearly demonstrates the waning predominance of this part of the brain as evolution proceeded. Not the least important among the earlier functions of the midbrain are those connected with the special senses of sight and hearing. One portion of the midbrain has become so highly specialized for visual function that it has earned for itself in many of the lower forms of vertebrates the title of optic lobe. Thus in the fish, in the amphibia, in the reptiles and in the birds the optic lobe is one of the most conspicuous parts of the entire brain. Similarly, a more caudal portion ot the roofplate of the midbrain became highly differentiated in connection with the function of hearing. But one of the most striking marks of progress as the brain found opportunity to provide for more ample syntheses in the ditTerent sensory spheres is the manner in which the midbrain has lost its ancient prestige. Those extensive areas developed in relation to sight and hearing SUMMARY OF STRUCTURES 277 have gradually been deprived of their morphological prominence. In the primates they appear as inconspicuous elements covered by the greatly expanded endbrain. In viewing the mesencephalon and its two sets of colliculi, the tact ot their primordial eminence in auditory and visual functions must not be overlooked. Nor must it be considered a sudden departure from an old architectural design that these two areas of the brain have passed into rela- tive insignificance. It was by the gradual delegation of visual and auditory functions to other regions that the midbrain became progressively reduced. Apparently the object of this delegation of former power to other areas was to enlist the activities of a more promising region. There was need of suffi- cient expansion to accommodate the rapidly growing demand for more complex associations in the realms of sight and hearing. The portion of the nervous system which has shown the chief propensity in this direction is the telencephalon. It is the last of all the brain constituents to make its appear- ance in the process of individual as well as phyletic development. In the end- brain there are special regions known respectively as the auditory cjr temporal area and the visual or occipital area, which have largely taken over the functions of hearing and of vision. COMPARISON OF THE INFERIOR COLLICULI OF LOWER PRIMATES Yet in spite of this assumption of functional activity on the part of the cerebral hemispheres, the midbrain has not altogether given over its original autonomy. A comparison of the lower primates, such as lemur, with man clearly reveals this fact. Even a comparison of closely allied lower primates indicates that to some extent at least the progressive delegation of function from the colliculi to the endbrain is in process. The inferior colliculus, con- nected with hearing, shows in its planimetric coefficients a definite decline almost as striking in its longitudinal coefficients. 278 THE LOWER PRIMATES Coefficients of the Inferior Colliculi IN THE Lower Primates Species Planimetric Longitudinal Lemur .223 .210 Tarsier .230 .140 Marmoset 1 .210 .200 Mycetes .182 . 190 Coefficients of the Superior Colliculi IN THE Lower Primates Species , Planimetric 1 Longitudinal Lemur . 140 .200 Tarsier •337 .290 Marmoset .154 .130 Mycetes ifii .130 This would scum to sir; c.li. Column ..I Bunlaclii ..., C.ilunin ..I G. .1! ; I>l, I Xitci s..-.|>in.il IiaLt; I Lr. DoismI S|.iii,.crirlHll,ii liact; .,(ju. \rnt.;il S|)in. .ic i . brILn Tr.ict, nil. Spin..-, ,liv:n \ Tr;n. t .,1 I klucc;; NK, N,Rkus..ri-!..l;inci..: i-. , P\r;iinid; |.^.\, P\ i.niil. [Xsoiulin- Ii i-iniln.il Ti:Kt; \i \. \',nlr:il Gi:iv C.liinin. |Accrsvi..n N... I4<). S<-i.li..n i... ActiKil Si/a' \n • - niiii,| the sLiljstant la ^clatinosa tn^ciiiini (NR). The narrow, isthnnis-liko hand ol ,i:;ra\ matter (.■oniu\-t mii this latter with tlu' eeaitral .^rax matter lorms the eer\ i\ ol the dorsal horn. Dorsal to thi' eentral ,^ra\ matter (Cen), the eer\ i\ o| the dorsal horn and the substantia irelatmosa tnuemmi is the PITHECUS RHESUS, MACACUS RHESUS 367 expansive dorsal field in which, however, there is as yet no sign of those great sensory nuclei which serve as relays for the sensory pathways trom the extremities. The broad expanse of this dorsal sensory field indicates the FIG. 172. MACACUS. LEVEL OF CAUDAL LIMIT OF DORSAL SENSORY NUCLEI. CEN, Central Gray Matter; CB, Column of Burdach; cc, Column of Goll; dt, Deiterso-spinal Tract; fle. Dorsal Spinocerebellar Tract; cow, Ventral Spinocerebellar Tract; hel, Spino-olivary Tract of Helweg; NB, Nucleus of Burdach; ng, Nucleus of Goll; nr. Nucleus of Rolando; py. Pyramid; pyx. Pyramidal Decus- sation; REF, Reticular Formation; rst, Rubrospinal Tract; spt. Spinothalamic Tract; trd. Descending Trigeminal Tract; ven. Ventral Gray Column. [Accession No. 149. Section 35. Actual Size 12X8 mm.] capacity of the central axis for the conduction of sensory impulses concerned in discriminative sensibility. The faint indenture on the dorsal aspect of the section indicates the position of the dorsal paramedian sulcus which sepa- rates the column of Goll (CG) fVom the column of Burdach (CB), thus furnishing some idea as to the relative dimensions of these two tracts. 368 THE INTERMEDIATE PRIMATES Judgment based upon this eomparison would favor the pathway from the upper extremity as being more capacious for the purpose of sensory con- duction. Should this impression be borne out by other facts, the inference that there has been an advance in the upper extremity, and especially the hand as a sensory organ, seems justified. This fact, taken in conjunction with the relatively small size of the substantia gelatinosa, as compared with the similar structure in other species, implies that the facial region of the animal has lost some of its original importance as a sensory directing organ. In all probability much of this function has been delegated to more capable sensory structures of the forclimb, particularly the hand. Occupying the most ventral position in the cross section is the pyramidal system (Pyx), most of whose libers are actively engaged in the process of decussation. On the ventrolateral aspect of the section, lying external to the ventral gray matter, is the medullary substance constituting the circum- ferential zone, and mesial to this is the intermediate zone. LEVEL OF THE CAUDAL LIMIT OF THE INFERIOR OLIVARY NUCLEUS (fIG. I 73) At this level certain striking changes have appeared. Most notable among these is a dense mass of gray matter situated in the ventral portion of the section, the caudal extremity of the inferior olivary nucleus (10). In the dorsal sensory Held there appear the huge masses oi nuclear sub- stance constituting respectively the nucleus of Goll (NG) and the nucleus of Burdach (NB). A feature of the inferior olivary nucleus is the sharper definition of its borders, giving it the appearance of a more discrete structure than in any of the lower species. This nucleus, as is the case with other structures, conveys the impression of an emergence from a more primitive indefiniteness. The central gray matter (Cen) surrounding the central canal occupies a position somewhat more dorsal than in the previous level, indicatmg a general tendency of this portion ot the gray substance to move PITHECUS RHESUS, MACACUS RHESUS 369 into the position ultimately oeeupieci by tlie lloor of the fourth ventricle. Doi'sal to it, and adjacent to the dorsomedian septum, is a large nuclear mass surrounded by a small capsule of medullary substance, the nucleus FIG. 173. MACACUS. LEVEL OF CAUDAL LIMIT OF INFERIOR OLIVARY NUCLEUS. CEN, Central Gray Matter; CB, Column of Burdach; cc, Column of Goll; dt, Deitcrso-spinal Tract; fle. Dorsal Spinocerebellar Tract; cow, Ventral Spinocerebellar Tract; hel, Spino-olivary Tract of Helweg; 10, Inferior Olive; mf. Mesial Fillet; nb, Nucleus of Burdach; nbl, Nucleus of Blumenau; ng, Nucleus of Goll; NFL, Facial Nucleus; nr. Nucleus of Rolando; py. Pyramid; ref, Reticular Formation; rst. Rubrospinal Tract; spt. Spinothalamic Tract; trd. Descending Trigeminal Tract. [Accession No. 149. Section 90. Actual Size 11X8 mm.] of Goll (NG), lateral to which, and separated from it by the dorsal paramedian sulcus, is the much larger nucleus of Burdach (NB). De- 370 THE INTERMEDIATE PRIMATES tached from this nucleus and lying near the periphery is an equally large sensory element, the nucleus oi Blumenau (NBI). In a position ventral to the nucleus oi" Blumenau is the substantia gelatinosa trigemini (NR), in contact externally with the fibers of the descending trigeminal tract (Trd). It is now possible to compare with more critical sense the relative dimensions of the three major elements entering into the dorsal sensory field, and representing the dermatomic areas of the body from the soles of the feet to the crown oi the head. These elements serve all modalities of dis- criminative sensibility within the same limits. It is apparent that the column and nucleus of Burdach i'ar exceed in size either of their neighboring structures, the nucleus of Goll (NG) situated mesial to them, or the substantia gelatinosa trigemini (N R) situated ventral to them. On the strength of this comparison, the conclusion seems tenable that the hand and upper extremity have gained much in prominence as sensory organs over the head and face, w hich latter are only mcagerly represented bj'' the relatively small receiving station of the trigeminal nerve. Quite as much do the lower extremity and tail suifer by the comparison as sensory organs. In Macacus rhesus the tail is short and has developed none of that prehensile speciali- zation which characterizes the new-world monkeys. Passing through the substantia gelatinosa trigemini (NR) are a few emergent fibers of the eleventh nerve, making their way toward the periph- ery of the axis. The fibers of the twelfth nerve are also seen taking origin in the ventral portion of the central gray matter (Cen ) and passing forward in the direction of the inierior olivary nucleus ( 10 ). The reticular formation ( Ref ) occupies a central position, and through it pass a number of internal arcuate fibers from the nucleus of Burdach, while large masses of decussating fibers extend inward iVom origins in the nucleus of Goll and pass mesially to enter the decussation of the mesial iillet ( Mf ). The collected bundle of the pyramid (Py) occupies its usual ventromesial position and afiords a clear PITHECUS RHESUS, iMACACUS RHESUS 371 idea as to the relative size of the pyramidal system in maeaeus. Lying upon the ventrolateral periphery is the circumferential zone, mesial to which is the intermediate zone. Some external arcuate fibers, apparently arising in the nucleus of Burdach, pass forward and inward to their decussation in the median raphe. LEVEL THROUGH THE NHDDLE OF THE INFERIOR OLI\ ARV NUCLEUS (PIG. I 74) At this level, the addition to the inferior olive (10) ot the mesial and dorsal accessory olives (DO, VO) is clearly shown. This level, however, is more significant as again indicating the relative sizes of the three major nuclear relay stations for discriminative sensibility, occupying the dorsal sen- sory field. Thus the nucleus of Goll (NG) appears much smaller as com- pared with the nucleus of Burdach (,NB), and the accessory nucleus of Blumenau (NBI) (now showing some of its points of confluence with the more mesial cell mass of Burdach). It offers an almost convincing argument that the leg, foot and tail have been superseded in their sensory importance by the development of the forclimb and particularly the hand. The relative functional significance of the forclimb and hand, as represented by the nuclei of Burdach and Blumenau ( N B, NBI ), when compared with the sensory significance of the face and head, is clearly illustrated in the relatively small size of the substantia gelatinosa trigemini (NR). The section also indicates the manner in which the internal arcuate fibers, arising in the nucleus of Goll, sweep forward and inward as dense col- lected bundles, skirting the ventral gray matter, while those arcuate fibers arising from the nucleus of Burdach make their way by longer, more graceful arcs, in less dense compact bundles, penetrating in their passage the main mass of the reticular formation. Fibers of the twelfth nerve (N 12 ) (nervus hypoglossus) make their way from their nucleus (Nhy) in the ventral portion of the central gray matter to the inferior olivary nucleus 372 THE INTERMEDIATE PRIMATES (10). Dorsal to the pyramid ( Py ), and containing at this level many of the compact bundles of decussating libers, is the mesial fdlct ( Mf ) as its libers are beginning to turn cephalad after crossing the median raphe. FIG. 174. MACACUS. LE\EL THROUGH MIDDLE OF INFERIOR OLIVARY NUCLEUS. DO, Dorsal Accessory Olive; dt, Deiterso-spinal Tract; fle, Dorsal Spinocerebellar Tract; cow, Ventral Spinocerebellar Tract ; hel, Spino-olivary Tract of Helweg; 10, Inferior Olive; mf, Mesial Fillet; nb, Nucleus ofBurdach; nbl, Nucleus of Blumenau; NFS, Facial Nucleus; ng. Nucleus of Coll; nhy, Hypoglossal Nucleus; r>rvD, Dorsal Vagal Nucleus; nr. Nucleus of Rolando; N12, Hypoglossal Nerve; pd, Predorsal Bundle; PL, Posterior Longitudinal Fasciculus; py. Pyramid; ref. Reticular Formation; rst. Rubrospinal Tract; SPT, Spinothalamic Tract; trd. Descending Trigeminal Tract; ven 4, Fourth Ventricle; vo. Ventral Acces- sory Olive. [Accession No. I4g. Section 141. Actual Size 13X8 mm.] LEVEL OF THE N'ESTIBULAR NUCLEI (FIG. 1 75) Here the cross section shows considerable alteration in its general configuration. Its diameters in the dorsal portion of the field are more exten- sive than in the lower sections. This is primarily due to the widening of the PITHECUS RHESUS, MACACUS RHESUS 3-3 fourth ventricle and the appearance of the hiteral recesses. Among the intrinsic modifications of the axis at this level is a group of nuclei connected with the internal ear, serving as relays for impulses concerned with the special FIG. 175. MACACUS. Il\ll i)i THE \EST1BULAR NUCLEI. DT, Deiterso-spinal Tract; GOw, Ventral Spinucerebellar Tract; icp, Inferior Cerebellar Peduncle; lo, Inferior Olive; mf. Mesial Fillet; nd, Deiters' Nucleus; nr, Nucleus of Rolando; nsc. Nucleus of Schwalbe; n8, Auditory Nerve; pd, Predorsal Bundle; pl, Posterior Longitudinal Fasciculus; pv. Pyramid; ref. Reticular Formation; rst, Rubrospinal Tract; spt. Spinothalamic Tract; trd. Descending Trigeminal Tract; TLB, Tuberculum Acusticum. (Accession No. 149. Section 185. Actual Size 17X7 mm.| sense of hearing as well as with the balancing mechanism. Situated immedi- ately beneath the floor of the fourth ventricle, underlying the ependymal gray matter, is a large triangular nucleus of small cells, the nucleus of Schwalbe (NSc), and lateral to this a nucleus containing many large cells, the nucleus magnocellularis of Deiters (ND). Dispersed among the cells of the latter nucleus are many small bundles of heavily mye- linated axons. These two nuclei constitute the vestibular complex. They have assumed, in a general way, the positions occupied in the lower levels by the nuclei of Goll and Burdach. In the topographical relation of these 374 THE INTERMEDIATE PRIMATES nuclear masses there is complete correspondence in all species thus far considered, from which it must be apparent that a definite principle of structural design regulates the manner in which this series of nuclei is laid down in the brain stem. Beginning at the caudal extremity of the oblongata are the nuclei for discriminative sensibility in the tail, leg and arm; immediately succeeding them in the next higher lc\els are the nuclear relay stations receiving impulses from the proprioceptors of the internal ear in the interest of the balancing function. Thus a superposed series of nuclei in the dorsal lield of the oblongata in the form of a long column makes provision for the primary relay of all sensory impulses arising in the proprioceptors of the body. This is not merely of generic significance, for its long antecedent history in the vertebrates makes it a matter of phyletic moment. Lateral to Deiters' nucleus, forming the extreme lateral structure of the cross section, is an elevation on the surface of the axis, the tubcrculum acusticum (Tub), into which enter some of the fibers of the cochlear divi- sion oi the eighth nerve (N8). This elevation forms a relay station in the pathway of hearing, so that in the tuberculum acusticum and the two vestibu- lar nuclei at this level, the chiet receiving stations for the eighth nerve appear. Interposed between the tuberculum acusticum and Deiters' nucleus is a dense oval bundle of fibers constituting the restiform body ( ICP). A4ention should be made of the large size of the vestibular area, inas- much as this element of the brain stem causes a pronounced elevation in the lloor of the fourth ventricle. Furthermore, the large dimensions of the vestibular complex in macacus indicate to what extent the balancing func- tion is necessary in this animal. Another feature at this level is the inferior olivary nucleus (10) con- nected with which are the two accessory olivary nuclei. Attention is especially called to the clear-cut boundaries of the inferior olivary nucleus PITHECUS RHESUS, iMACACUS RHESUS 375 in macacus, together with the tenclcnev toward clehnite convolution ol its structure, which appears m this species in such conspicuous development for the lirst time. This tenclcnev to convolution is one which will be seen to become more marked as the higher apes are approached. It reaches its greatest degree ot complexity m the brain stem ol man. The pyramid ( Py ) occupies its typical ventromesial position and affords an opportunity for estimating its size in relation to the rest of the cross sec- tion. Dorsal to It are the bundles constituting the mesial lillet (i\H ). Ventral to Deiters' nucleus, laintly outlined at this level, is the substantia gelatinosa trigemini (N R), upon the lateral border ol which are the collected fibers of the descending trigeminal tract (Trd). The reticLilar formation (Ref) is quite extensive and is penetrated by many arcuate fibers apparently arising in the nucleus of Deiters, to form the Deiterso-spinal tracts. If one feature may be signalized at this level, it is the great prominence of the vestibular com- plex with the implication which it bears concerning the balancing function of the macacus. LE\EL OF THE CEREBELLAR NUCLEI (FIG. 1 76) At the level of the cerebellar nuclei the organization of the cerebellar connection is indicated. The two groups of cerebellar nuclei, namely, the mesial and the lateral groups, have all of that indefinite development which characterizes the lower primates. The lateral group (Ndt), whose largest constituent part is the nucleus dentatus, is a more or less diffuse mass of nuclear substance surrounded by medullary tissue. It has none of the convoluted appearance or discrete outline prominent in the higher anthro- poids. In this respect it allies itself closely with all the lower forms pre- viously described. Situated in the roof of the fourth ventricle is the mesial nuclear group ( N f g ), consisting of the nucleus fastigius and the nucleus globosus. Between 376 THE INTERMEDIATE PRIMATES the mesial and lateral groups of the cerebellar nuclei are the massive bundles of the juxtarestiform body. The roof of the fourth ventricle is formed by the nuclear substance of the cerebellar nuclei. Its floor still contains a large FIG. 176. MACACUS. LE\'EL OF THE CEREBELLAR NUCLEI. CEBL, Cerebellum; GOW, Ventral Spinocerebellar Tract; icp, Inferior Cerebellar Peduncle; 10, Inferior Olive; MF, Mesial Fillet; nd. Nucleus of Deiters; ndt. Cerebellar Nuclei, Lateral Group; nfg, Cerebellar Nuclei, Mesial Group; nr. Nucleus of Rolando; NSC, Schwalbe's Nucleus; n8. Auditory Nerve; pd, Predorsal Bundle; pl. Posterior Longitudinal Fasciculus; py. Pyramid; ref. Reticular Formation; trd. Descending Trigeminal Tract; ver, Cerebellar Vermis; ven 4, Fourth Ventricle. (Accession No. 149. Section 190. Actual Size 18 X 15 mm.] representation of Schwalbe's and Deiters' nuclei (NSc, ND). The other elements at this level are indicated by letters in the caption. PITHECUS RHESUS, MACACUS RHESUS 377 The principal Icatuif illustrated by this region is the ditluse character of the cerebellar nuclei. This condition at once indicates an animal poorly provided with coordinative control over its skilled acts. This view of the behavior of macacus is further supported by the relatively small size of the lateral cerebellar lobes. The animal is noted, and even notorious, for its prankishness and its giTat agility as a climber. It is possible to teach these monkeys many amusing tricks, so that the conception of them as limited in their neokinetic performances perhaps does them injustice. On the other hand, the fact should not be overlooked that howe\'er much they may improve by training, these animals use their new accjuisitions but little in the ordinary routine of their lives, and escaping from captivity, relapse to the simpler motor patterns essential to their primitive arboreal adaptations. It must, therefore, be accepted from such evidence as is elicited from the cerebellar nuclei and their connections, that the macacus species are, in fact, capable of a very limited range of skilled learned performances which they habitually employ in the pursuits of their daily lives. LEVEL OF EMERGENCE OF SLXTH CRANIAL NERVE, NER\'US ABDUCENS (FIG. 1 77) At this level the section has undergone considerable change due to the appearance of the caudal portion of the pons Varolii (PN). This structure now adds a basal element to the neuraxis and makes distinguishable in it the basis and the tegmentum pontis. The pons here consists of but a few transverse fibers constituting the stratum superficiale pontis, and the caudal extremity of the pontile nuclei contained within which latter are the somewhat scattered libers of the pyramidal system (Py). The lateral continuation of the superhcial stratum of the pons enters into and forms part of the massive middle cerebellar peduncle (Mcp). The tegmentum at this level is separated from the basis by a number of transverse decussating fibers constituting the trapezoid body (Trp) in whose lateral extremity is the 378 THE INTERMEDIATE PRIMATES superior olivary bocly( SO ). Lateral to this is a small portion of the nueleus facialis ( N f ) which gives rise to the first portion of the se\eiith nerve, a series ol line, myelinated fibers arranged more or less as a spray. It extends inward FIG. 177. MACACUS. I I \ II Ol IliH EMERGENCE OF THE SIXTH NERVE. LIN, Llnguhi; MCP, Middle Cerebellar Peduncle; mf, Mesial Fillet; nab, Nucleus Abducentis; nf. Facial Nucleus; nr, Nucleus of Rolando; n6, Abducens Nerve; Ny, Facial Nerve; I'D, Predorsal Bundle; PL, Posterior Longitudinal Fasciculus; pn, Pontile Nuclei; pv, Pyramid; ref, Reticular Formation; trd, Descending Trigeminal Tract; scp, Superior Cerebellar Peduncle; so, Superior Olive; trd. Descending Trigeminal Tract; trp, Trapezoid Body; vex 4. Fourth Ventricle. [Accession No. I4<). Section 230. Actual Size 20 X I'l iiini.l and backward toward the lloor of the fourth ventricle. From the superior oli- vary nueleus a similar, but somewhat larger spray of myelinized libers PITHECUS RHESUS, MACACUS RHESUS 379 extends backward and inward, apparently terminating in the large nuclear mass situated on the lloor of the fourth ventricle. This hitter is the nucleus abducentis (Nab) from which arise the libers of the sixth nerve to supply the external rectus muscle of the eyeball. They pass successively through the reticular formation ( Ref), the trapezoid body (Trp) and the stratum superficiale pontis. The fiber connections between the superior olive and nucleus of the sixth nerve constitute the superior olivary peduncle. Axons from the superior olive to the sixth nerve nucleus serve the purpose of turn- ing the eyes rellexly toward the side from which the animal may hear a sudden or unexpected sound. The immediate dellection of the gaze toward the source of possible danger would thus be of service in the quick detection of an approaching enemy or other unfavorable factor. Dorsomesial to the nucleus of the sixth nerve, lying between it and the subependymal gray matter on the lloor of the ventricle, is a dense bundle of fibers constituting the second portion of the facial nerve (N7) in its intramedullary course. The beginning of the third portion of this nerve, the genu facialis, lies immediately dorsal to the abducens nucleus, while the fourth division of the facial nerve forms a long bundle of heavily myelinized fibers extending obliquely forward and outward between the nucleus of the seventh nerve and the substantia gelatinosa trigemini (NR). Extending dorsolaterally from the substantia gelatinosa of Rolando is a radiating fasciculus which makes its way backward and inward toward the angle of the fourth ventricle where it apparently terminates in a heavily myelinized bundle of libers, the fasciculus mesencephalici trigemini. The fourth ventricle is much reduced preparatory to transition into the Sylvian aqueduct. Above it lies the superior medullary velum upon which rests the lingular lobule of the cerebellum (Lin). Dorsal to the mesencephalic root of the fifth nerve is a dense bundle of fibers constituting the superior cerebellar peduncle (Sep). 38o THE INTERMEDIATE PRIMATES LE\ EL OF THE MIDDLE OF THE PONS \AROLII (FIG. 1 78) Here this structure attains its lull dimensions. It presents the three characteristic pontile layers, the stratum superficiale pontis, the stratum complexum pontis and the stratum profundum pontis. In the complex layer of the pons Varolii opportunity is atlorded to estimate the size and extent of the pontile nuclei (PN) which appear to be considerably larger than in baboon. The pyramidal libers (Py) occupy their usual position in the center of this layer, their bundles being much dispersed by the interposition of nuclear substance and many decussating fibers of this middle layer of the pons. The impression conveyed by the pons and its large nuclear mass is that it belongs to an animal possessed of relatively high coordinative control in its skilled movements. Situated between the basis and the tegmentum, as the boundary line between these two, is the mesial fillet (Mf); lateral to the mesial fillet is the lateral lillet (Lf), in the dorsal extremity of which is the nucleus of the lateral fillet. A dense and large bundle of myelinized fibers in the extreme lateral position of the cross section is the middle cerebellar pe- duncle (Mcp), while situated in the dorsolateral angle of the section is a smaller dense bundle of iibcrs which represents the superior cerebellar pe- duncle (Sep). On the periphery over this bundle of fibers is a thin band of myelinized axons, the ventral spinocerebellar tract (Gow) on its way to the vermis of the cerebellum. Interposed between this latter tract and the superior cerebellar peduncle is the tractus uncinatus of Russel (Tur). Ventral to the dense fasciculus forming the middle peduncle, many fibers enter the axis. These axons are the dorsal root fibers of the trigeminal nerve (N5). Their course may be followed through the peduncle into the lateral aspect of the tegmentum where some of them end in relation with the cephalic extremity of the substantia gelatinosa trigemini ( N R). PITHECUS RHESUS, MACACUS RHESUS 381 The central gray matter (Cen) surrounds the much reduced ven- tricular space which is here approaching the caudal orifice of the Sylvian aqueduct. The major portion of the tegmentum is occupied by the reticular FIG. 178. MACACUS. LEVEL THROUGH THE AUDDLE OF THE PONS VAROLIL CEN, Central Gray Matter; ctt, Central Tegmental Tract; GOW, Ventral Spinocerebellar Tract; lf. Lateral Fillet; mcp. Middle Cerebellar Peduncle; mf, Mesial Fillet; nbe, Nucleus of Bechterew; nr, Nucleus of Rolando; N5, Trigeminal Nerve; N7, Facial Nerve; pd, Predorsal Bundle; pl, Posterior Longitudinal Fascicu- lus; PN, Pontile Nuclei; pns. Pons; py. Pyramid; ref. Reticular Formation; rst. Rubrospinal Tract; scp, Superior Cerebellar Peduncle; spt. Spinocerebellar Tract; trd. Descending Trigeminal Tract; tlr, Tractus Uncinatus of Russel (Hook Bundle). (Accession No. 149. Section 280. Actual Size 21X10 nim.| formation (Ret) ni which are many transverse decussating iibers, some of which at least appear to take origin in the substantia gelatinosa trigemini (NR), passing inward, it may be, as a secondary pathway for the con- duction of impressions from the face and head. These fibers appear to decus- sate, as is the case of all sensory axons, in the median raphe. LEVEL OF THE EMERGENCE OF THE FOURTH OR TROCHLEAR NERVE (fIG. I 79) At this level, the neuraxis passes through its isthmial portion at or near the junction between the hindbrain and the midbrain. In its roofplate, which 382 THE INTERMEDIATE PRIMATES forms the superior medullary velum above the beghmino; of the Sylvian aqueduct (IT), are the decussathig fibers of the fourth or trochlear nerve (N4) as they emerge from the brain stem. This ner\e supphes the FIG. 179. MACACUS. LE\EL OF THE EMERGENCE OF THE TROCHLEAR NERVE. CEN, Central Gray Matter; ctt, Central Tegmental Tract; it. Iter; lf. Lateral Fillet; mcp, Middle Cerebellar Peduncle; mf, Mesial Fillet; N4, Trochlear Nerve; pd, Predorsal Bundle; pl, Posterior Longitudinal Fascicu- lus; PN, Pontile Nuclei; pns. Pons; py. Pyramid; ref. Reticular Formation; rst, Rubrospinal Tract; scp, Superior Cerebellar Peduncle; spt, Spinothalamic Tract. [Accession No. 149. Section 305. Actual Size 18 X 12 mm.] superior obhque muscle of the eyeball and is the only one of the cranial nerve series which undergoes complete decussation before its emergence. The three layers of the pons Varolii arc still clearly defined and further opportunity is afforded to estimate the extensiveness of the pontile nuclei (PN) whose bearing upcMi the de\elopment of skilled mo\'ements has PITHECUS RHESUS, MACACUS RHESUS 383 already been discussed. The fibers constituting the pyramidal system are still further separated by the interposition of transverse fibers and portions of the pontile nuclei. The tegmentum is separated from the basis here by the mesial fillet (Mf) at whose lateral extremity, and extending dorsally, is the lateral fillet (Lf). Mesial to the lateral fillet is a dense bundle, now somewhat scattered, the superior cerebellar peduncle (Sep), about to make its characteristic swing forward and inward preparatory to decussa- tion m the midbram. The reticular formation (Ref) occupies the major portion of the tegmentum. It contains at this level no specialized aggregations of nuclear diflerentiation. LEVEL OF THE INFERIOR COLLICULUS (FIG. l8o) Here the contour of the axis is considerably altered by the appearance of two elevations on its dorsal surface. These elevations, the inferior colliculi (IC), still show a marked degree of stratification. Their prominence, together with this specialization, may be taken to indicate that some of the primitive function over which they preside is still retained. Fibers of the lateral fillet (Lf) ha\e already entered into the inferior colliculus; still others are approaching it. The prominence of these tectal structures, both in surface relief and histological differentiation, doubtless indicates that some portion, at least, of the function of hearing still has representation in these primordial centers of that special sense. It is noteworthy, however, that the coefficients of this structure, as compared with the lower primates, show an actual decrease in size. This fact uncjuestionably implies that the telen- cephalizing process which is gradually transferring the major functional activi- ties of the auditory sense to the temporal lobe of the cerebral hemisphere, has made considerable advance in such animals as the macaque, as a consequence of which the tectum of the midbrain has lost much of its primitive auditory dominance. The central gray matter (Cen) surrounds the Sylvian aque- 384 THE INTERMEDIATE PRIMATES duct. In its lateral portion are several dense bundles of myelinized axons representing the descending fibers of the trochlear nerve (N4) as they pass downward to the level of their linal decussation before emergence from FIG. 100. MACACUS. LEVEL OI UlL IMLKIOK COLLICULUS. CEN, Central Gray Matter; ctt. Central Tegmental Tract; ic, Inferior Colliculus; lf, Lateral Fillet; mf. Mesial Fillet; N4, Trochlear Nerve; pd, Predorsal Bundle; pl. Posterior Longitudinal Fasciculus; pn, Pontile Nuclei; pns. Pons; pv, Pyramid; ref. Reticular Formation; rst. Rubrospinal Tract;scp, Superior Cerebellar Peduncle; spt. Spinothalamic Tract; xscp, Crossing of the Superior Cerebellar Peduncle. [Accession No. 149. Section 320. Actual Size 16 X 14 mm.] PITHECUS RHESUS, MACACUS RHESUS 385 the superior medullary velum. Lateral to the fibers ol the trochlear nerve, on the outer border of the eentral gray matter, are the scattered fibers from the tractus meseneephalieus trigemini. The territory occupied by the reticular formation (Ref) has at this level become somewhat obscured by the libers of the superior cerebellar peduncle (Sep) which are sweeping inward and forward, about to undergo their decussation before coming into relation w ith the red nucleus. This decussation is shown as fully developed in the next section. The boundary between the basis and tegmentum, as at other levels, is indicated b\- the presence of the mesial fillet (Mf) from whose lateral angle extend dorsally many of the fibers of the lateral fillet ( Lf ), now making their way to the inferior colliculus. The basis of the section still contains some of the typical characteristics of the pons Varolii in its three layers and also the disseminated condition of the pyramidal fibers (Py). There is, however, a tendency for the ven- tral groove of the pons to deepen and thus to foreshadow the beginning divergence of the basal portioii of the axis, which ultimately occurs in the cerebral peduncles where an assemblage of the cortico-pontile fibers takes place and the pyramidal axons are reassembled into a single compact bundle. LEVEL OF THE SUPERIOR COLLICULUS (FIG. l8l) Here the contour of the brain stem has again undergone modification due to the appearance of two fairly prominent elevations on its dorsal aspect and the development of a wide cleft in the middle portion of its base. These elevations are the superior colliculi (SO which represent remnants ol the optic lobes of the lower vertebrates. That they are reduced in size in all mammals, particularly the primates, is significant of the fact that the process of telencephalization of the visual function has been steadily progressing since the advent of mammalian forms. Visual function has for the most part been taken over by the occipital lobe in the cerebral hemisphere, a 386 THE INTERMEDIATE PRIMATES portion of the rndbrain which in the primates has gained particular promi- nence. In all probability, httle of the primordial responsibility for the function ot vision is still vested in the superior colhcuii. This portion of the brain, FIG. lOI. MACACUS. LE\EL OF THE SUPERIOR COI LICULUS. CEN, Central Gray Matter; cp. Cerebral Peduncle; ctt, Centra! Tcgemental Tract; mf. Mesial Fillet; MGB, Mesial Geniculate Body; noc, Oculomotor Nucleus; XRU, Nucleus Ruber; N3, Oculomotor Nerve; pd, Predorsal Bundle; pl, Posterior Longitudinal Fasciculus; rep, Reticular Formation; sc, Superior Colliculus; SBN, Substantia Nigra; spt, Spinothalamic Tract. (Accession No. 149. Section 381S. Actual Size 19 X 10 mm.] which tor such a long period in vertebrate e\'ohition held dominance in the animal's visual functions, has now become much reduced in this respect and appears as a relatively insignificant structural tcature on the dorsal surface of the midbrain. As a matter of fact, the superior colliculus is somewhat larger than the inferior colliculus, and it still retains some degree of stratihca- PITHECUS RHESUS, MACACUS RHESUS 387 tion rcMiiiniscL'nt ofits rornicr high speciaHzatiun when, as in the reptiles and birds, it presented fourteen distinet layers of alternating nerve cells and fibers. FIG. 182. MACACUS. LEVEL OF THE OPTIC CHLA.S.\L CIN, Internal Capsule; fdp, Descending Pillar of the Fornix; for. Fornix; glp, Globus Pallidus; nli, Nucleus Lateralis Internis Thalami; nlt, Nucleus Lateralis Thalami; nmt, Nucleus Mesialis Thalami; OPX, Optic Decussation; put, Putamen; \y. Fasciculus of Vicq d'Azyr. (Accession No. 149. Section 505. Actual Size 30 X 16 mm.] The central gray matter (Cen) surrounds the Sylvian aqueduct as it is approaching its cephalic orifice to communicate with the thud ventricle. Its ventral portion is much prolonged by the appearance in it ot an extremely important nuclear body, the nucleus oculomotorious (Noc) of the third cranial nerve from which emerge libers destined for the oculomotor groove. Here they leave the stem en route to supply all of the muscles of the eyeball, both extrinsic and intrinsic, with the exception of the external rectus and the superior oblique. In the midline and connected with this nucleus are a number of fibers crossing from one side to the other. These are the commissu- ral and decussating fibers associating the oculomotor nuclei. Their nnpor- 388 THE INTERMEDIATE PRIMATES tance has already been mentioned in connection with the development of binocular vision. Special reference is made to them in this phice in order that their significance in the animal's behavior may be subsequently discussed. FIG. 183. MACACUS. LE\EL OF THE ANTERIOR COMMISSURE. AC, Anterior Commissure; cin. Internal Capsule; fdp, Descending Pillar of the Fornix; for, Fornix; len. Lenticular Nucleus; NAT, Nucleus Anterior Thalanii. [Accession No. 149. Section 565. Actual Size 30X8 mm.] Ventral to the nucleus oculomotorius, although as yet poorly differentiated, is the ULicIeus ruber (NRu) through which some of the emergent fibers of the oculomotor nerve pass. Lateral to this nucleus is the reticular forma- tion ( Ref ), whose general appearance suggests the lack of highly special- ized differentiation in this species. The lateral portion of this formation is in continuation with a small protuberance on the lateral surface ot the midbrain, the mesial geniculate body ( Mgb) which serves as one of the relay stations in the secondary pathway of hearing. The \'entral aspect of the section shows a cleft in and about the mid- line. This is the beginning of the optico-peduncular space which sepa- rates the basis into the right and left cerebral peduncles (CP). These PITHECUS RHESUS, MACACUS RHESUS 389 peduncles now contain the compact bundles of the pallio-pontile system which connect the cerebral cortex with the lateral lobes of the cerebellum and the compact bundles of the cortico-spinal tract for the transmission of voHtional impulses to the somatic muscuhiture. Immediately dorsal to the cerebral peduncle is a large mass of gray matter, the substantia nigra (Sbn) whose relation to the control of certain automatic associated movements has been mentioned. LEVEL OF THE OPTIC CHLA.SM (FIG. 1 82) At this level, one of the most cephalic sections of the brain stem in this series is shown. A prominent feature is the optic thalamus which is sepa- rated by the internal capsule (Cin) from the lenticular nucleus, whose major portions are well illustrated, the globus pallidus (GIp) and the putamen (Put). As this specialization in the diencephalic portion of the brainstem is highly complex and deserves separate treatment, the structures at this level are merely referred to as indicative of the marked changes in transition in the level at which the brain stem forms its final continuity with the endbrain. It is here that contact is ultimately estab- lished with those great expansions of the brain, the cerebral hemispheres. They, in later stages of evoIutitMi, have dominated all of the far-reaching modifications in behavior characteristic of mammals, reaching their con- summate expression in the achievements of man. Chapter XIII RECONSTRUCTION OF THE GRAY MATTER IN THE BRAIN STEM OF PITHECUS RHESUS Y^ I ^HE reconstruction of this species follows the general lines already I laid down in the preceding reconstructions. It is particularly valu- H ^ able as showing the structures in their interrelation and contin- uity, and thus furnishes a more realistic idea of the actual disposition of the more important masses of gray matter in the brain stem. So far as possible, the descriptions have been made almost exckisively objective, reserving the space for comment on the signilicance of the structures for the chapters on summaries and conclusions. The High Cervical Level of the Spinal Cord In the high cervical level of the spinal cord at which the reconstruction of the gray matter in macacus begins, the ventral gray cohnnn is already detached from the central gray cohimn by the decussating libers of the pyramidal tract. In outline the ventral gray column is somewhat oval, directed ventrodorsally and laterally. It occupies the normal position of the ventral gray column in the spinal cord but is reduced in size. The central gray matter is somewhat cordiform in shape, presenting a ventral apex and a dorsal base from the lateral angles of which proceed the laterally compressed cervices of the dorsal gray column. The substantia gelatinosa trigemini is seated like a cap upon the dorsal extremity of the dorsal gray column and is already beginning to show the lateral swing occasioned by the appearance of the dorsal medullary nuclei and the opening of the fourth ventricle. As the higher cervical segments pass into the lowermost levels of the oblongata, the ventral gray column becomes increasingly interspersed with 392 THE INTERMEDIATE PRIMATES longitudinal fiber bundles and it gradually assumes the characteristic appearance of the reticular formation into which it merges. The Dorsal Sensory Nuclei The nucleus of the cohinin of GoII makes its first appearance in the reconstruction as a prolongation from the central gray column, in close proximity to the dorsal median septum. It rapidly assumes its maximum ventrodorsal extent and proceeds upward as a gradually thickening mass of gray matter maintaining its median position and separated from its fellow of the opposite side by only a narrow encapsulation of white matter. Somewhat above the level of the appearance of the inferior olivary nucleus, there appear scattered masses of gray matter in the column of Goll, lateral to the main mass of the nucleus of the column of Goll. They establish connection with the stem of the nucleus, which becomes somewhat rounded in shape and then rapidly expands laterally. This expansion con- tinues laterally until the dorsal extremity of the nucleus assumes a massive appearance, occupying the major portion of the dorsal aspect of the oblon- gata. It is somewhat arborescent in character, presenting masses of gray matter broken up and surrounded by penetrating, longitudinally coursing bundles of white libers. Cephalically, the nucleus of the column of Goll comes to rather an abrupt termination and its most cephalic portion mesially forms part of the caudolateral boundary of the fourth ventricle. Laterally it becomes sub- merged from view by the appearance in this region of the nucleus of Deiters. The nucleus of the ct)Iumn of Burdach appears first as a rounded mass of nuclear material in the dorsal portion of the reticular formation. It is at this point encapsulated by fibers arising from the nucleus of Goll and, passing ventrally in an arcade as the internal arcuate libers, gives rise to the mesial fillet. Increasing in size and extending dorsallv at about RECONSTRUCTION OF PITHECUS RHESUS 393 the mid-level of the nucleus of the eohmin of Burdaeh, there appears a secondary portion of the nucleus situated laterally in the collection of the ascendino; fibers of the column of Burdaeh, hrst as isolated groups of gray FIG. 184. VENTRAL SURFACE OF GRAY MATTER OF BRAIN STEM, PITHECUS RHESUS. Key to Diagram, inf. olive., Inferior Olive; lat. gen. body. Lateral Geniculate Body; meso-gen. body. Mesial Geniculate Body; nuc. of burdach. Nucleus of Burdaeh; pontile. Pontile Nuclei; ret. form.. Reticular Formation; subst. gel. rolando. Substantia Gelatinosa of Rolando; subst. nigra. Substantia Nigra; vent, cochlear, Ventral Cochlear Nucleus; vent, gray col.. Ventral Gray Column. matter which coalesce to form a large nuclear mass lying lateral to the stem of the nucleus of the column of Burdach; this is the nucleus of Blumenau. It rapidly establishes connection with the mesial portion of the nucleus, its nuclear masses presenting the typical leafy arrangement characteristic of it. It extends laterally for a considerable distance and overhangs the prolongation of the substantia gelatinosa trigemini. Gradually encroached upon by the vestibular complex, it disappears from view, being entirely replaced by the vestibular nuclei. 394 THE INTERMEDIATE PRIMATES The substantia gelatinosa Rolandi has become relatively slender, and placed as a cap over the dorsal extremity of the dorsal gray cohimn. As the highest cervical levels are approached, it is transformed, without any change whatsoever in its morphological characteristics, mto the substantia gela- tinosa trigemini which thereafter serves as a reception nucleus for the descending fibers from the Gasserian ganglion. It passes gradually into the lateral position which is characteristic of this mass of gray matter in the primate brain, and at the level of the inferior olivary nucleus it assumes its permanent unchanging position. The nucleus continues upward, embedded more or less irregularly in the lateral surface of the reticular formation, appearing at times on the surface and at other times being submerged from view by the reticular formation. The usual constriction which has previously been noticed is present at about the middle of the inferior olivary nucleus. Here the substantia gelatinosa trigemini reaches its minimum diameters. From this point it continues gradually to increase in size until it reaches its maximum at about the mid-ventricular region of the stem, above which it continues and develops its expanded cephalic extremity called the caput. Just prior to its disappearance it presents at its mesial and dorsal aspects the motor nucleus of the trigeminal nerve. The Inferior Olivary Nucleus The inferior olivary nucleus first appears in the reconstruction at the level of the cephalic extremity of the pyramidal decussation. Here it is a flattened lamina of gray matter directed mesially and slightly dorsally. This is the ventral accessory olivary nucleus (paleo-olive). It extends mesially and somewhat dorsally forming a flat ribbon with its mesial extrem- ity twisted so as to lie in a more direct ventrodorsal diameter of the stem. It ends cephalically by merging with the dorsal extremity of the dorsal lamina of the inferior olivary nucleus. The main mass of the inferior olivary RECONSTRUCTION OF PITHECUS RHESUS 395 nucleus appears somewhat above this level, dorsolateral to the ventral accessory olive where it is a pHcated fold of gray matter, its fundus directed toward the vcntrohueral angle of the brain stem. It presents a FIG. 105 DORSAL SURFACE OF GRAY MATTER OF BRAIN STEM, PITHECUS RHESUS. Key to Diagram, dors, cochl., Dorsal Cochlear Nucleus; inf. coll., Inferior Colliculus; nucl. of burd. and NUCL. of burdach, Nucleus of Burdach; nucl. of deiters. Nucleus of Deiters; nucl. of goll. Nucleus of Goll; RET. form., Reticular Formation; subst. gel. rolando. Substantia Gelatinosa of Rolando; sup. coll., Superior Colliculus; vent. coch. and vent, cochlear. Ventral Cochlear Nucleus. tendency toward plication, an undulating strand of gray matter showing a number of bends and folds. The ventral lamina is almost devoid of redupli- cations, whereas the dorsal lamina shows an increased irregularity produced by the appearance of numerous redupHcations. At a level somewhat above the appearance of the main mass of the inferior olivary nucleus, the dorsal accessory olivary nucleus appears as a flat, narrow strand of gray matter placed almost in the lateral meridian of the brain stem and overlying the hihis of the inferior oHvary nucleus. It extends upward as a ilat band in this 396 THE INTERMEDIATE PRIMATES same location and, gradually diminishing, it appears cephalically as a plug in the hilus of the nucleus. Before it completely disappears it seems to fuse with the dorsal extremity of the inferior olivary nucleus. The entire inferior olivary complex extends upward to a level somewhat below that of the middle of the ventricular cavity, where it ends rather abruptly, merging with the reticular formation in whose ventral surface it is embedded. The Reticular Formation The reticular formation, as shown in the reconstruction, begins in the upper cervical levels by the appearance of isolated groups of nerve cells lying dispersed between the fibers of the lateral portion of the decussating pyramidal tract. It here receives the termination of the ventral gray column and then rapidly becomes the outstanding feature in the tegmen- tum of the rhombencephalon. It is quite irregular in outline and presents embedded in its substance and along its surfaces the various condensations of gray matter forming the nuclei of the brain stem and the collections of nerve fibers which form the ascending and descending tracts. At its ventro- lateral angle it presents the constant condensations of nuclear material which form the lateral nucleus of the reticular formation and the superior olive, the two latter structures being somewhat more prominent and extensive than those found in the preceding forms. On its ventral surface, extending from the cephalic limit of the pyramidal decussation to the mid-ventricular level of the stem, it forms a bed for the dorsal accessory olivary nucleus of the olivary complex which at its beginning emerges from, and at its termina- tion disappears in the matrix of the reticular formation. Laterally the reticular formation receives the substantia gelatinosa trigemini, while dorsomesially it affords attachment to the stem of the nucleus of the column of Goll and, somewhat more cephalically, to the base of the nucleus of the column of Burdach. It continues upward, RECONSTRUCTION OF PITHECUS RHESUS 397 increasing ni size, into the tegmentum of the pons, where it is massive in form and suppHes the main bulk of the gray matter of the meteneephalic tegmentum. FIG. 1 86. LATERAL SURFACE OF GRAY MATTER OF BRAIN STEM, PITHECUS RHESUS. Key to Diagram, dors, cochl., Dorsal Cochlear Nucleus; inf. coll.. Inferior Colliculus; inf. olive, Inferior Olive; lat. gen. body. Lateral Geniculate Body; nuc. of burdach. Nucleus of Burdach; nucl. of DEiTERs, Nucleus of Deiters; nucl. of goll. Nucleus of GoII; pontine. Pontile Nucleus; subst. gel. ROLANDO, Substantia Gelatinosa of Rolando; subst. nigra, Substantia Nigra; vent, cochl.. Ventral Cochlear Nucleus; vent, gray col., Ventral Gray Column. Above the level of the olivary complex the ventral surface of the reticular formation recedes rather rapidly from the surface, providing along its ventral surface a deeply excavated bay for the lodgment of the trapezoid body, formed by the secondary fibers from the cochlear system. Continuing upward in the metencephalon the reticular formation gradually diminishes in bulk, separated at first from the deep layer of the pontile nuclei by the trapezoid body. Above the level of the trapezoid body, the reticular formation comes into fairly close contact with the deep layer of the pontile nuclei and presents 398 THE INTERMEDIATE PRIMATES in this region the beginning mesencephalic appearance of the reticular formation with a large mesial mass and a lateral prolongation which turns outward and backward to encircle the superior cerebellar peduncle as it enters into the mesencephahc tegmentum from the superior medullary velum. Below the appearance of the inferior collicuh a prolongation of the reticular formation passes around dorsally toward the midline. Into the space between the superior colliculus and inferior colliculus a similar prolongation of the mesencephalic reticular formation passes laterally from the lateral portion of the reticular formation almost to the midline, thus separating the colliculi from each other. Similarly, above the superior colliculus a rather narrow extension from the most cephalic region of the reticular formation again comes to the surface close to the junction between the mesencephalon and the diencephalon. In the mesencephalon itself, the major portion of the mesial mass of the reticular formation gives place to that most characteristic nucleus of the mesencephalon, the nucleus ruber; while the lateral portion extends upward and outward and at the junction of the mesencephalon with the diencephalon it forms a bed for the mesial surface of the mesial geniculate body and then fuses cephalically with the indifferent reticular formation of the diencephalon. In the upper medullary levels of the brain steni the dorsal surface of the reticular formation is in fairly close contact with the subependymal gray matter of the fourth ventricle and presents at its dorsolateral angle the specialization of the reticular forma- tion, producing the vestibular complex. The Pontile Nuclei In the pontile nuclei of Macacus rhesus is found for the first time an extension indicating the arciform nuclei which appear as a flattened layer of gray matter applied to the ventral surface of the pyramid, conforming to its convex surface. This arciform nuclear formation is relatively poorly developed RECONSTRUCTION OF PITHECUS RHESUS 399 and almost immediately after its first identification the typical appearance of the pontile nuclei develops. In its most caudal portions the pontile nucleus appears as an incomplete ring of nuclear material deficient on the lateral aspect and surrounding the relatively undivided pyramidal tract fibers. As this structure is traced upward, the ring soon becomes complete, surrounding the pyramidal tract and the lowest fibers of the pallio-pontile system of fibers and, at the same time, there are distinctly developed the two layers of the pontile nucleus (the superficial and deep layers), which meet mesially and laterally the two buttresses of the pontile nuclei. There is some indication of migration of masses of nuclear material in between the fibers of the two longitudinally coursing systems, producing a poorly developed, lace-like appearance. The pontile nuclei are materially heavier in formation and more extensive in distribution than in the preceding forms. As the mesencephalon is approached the mass of the pontile fibers gradually diminishes and the superficial layer contracts and extends further upward for only a short distance ventral to the developing cerebral peduncle. The deep layer becomes increasingly closer in proximity to the mesencephalic reticular formation and gradually is transformed into the substantia nigra. The Vestibular Complex The vestibular complex makes its appearance first as a wedge-shaped mass of gray matter inserted between the subependymal gray matter forming the floor of the fourth ventricle above the cephalic extremity of the nucleus of the column of Coll, and the mesial surface of the nucleus of the column of Burdach. Rapidly increasing in size, it replaces the nucleus of the column of Goll and displaces laterally the nucleus of the column of Burdach, thus bringing about through the agency of the opening of the fourth ventricle that lateral displacement which is characteristic of all of the dorsal medullar}- nuclei of the brain stem. 400 THE INTERMEDIATE PRIMATES The micleus of Deiters' area rapidly increases in size and maintains its position of contact with the subependymal gray matter of the iloor of the fourth ventricle. As the upper hmit of the nucleus of the cohimn of Burdacli is approached, the nucleus of Deiters extends ventrally and laterally, assum- ing the position previously occupied by the nucleus of the cokmm of Burdach and presenting two prolongations — one mesial, closely applied to the lateral surface of the reticular formation, the other lateral and in close proximity to the entering root of the cochlear nerve. In this recess formed by the mesial and lateral prolongations is located the ascending nuclear mass of the sub- stantia gelatinosa trigemini (Rolando). Somewhat above the mid-ventricu- lar level of the stem it rapidly diminishes in size and is superseded by the triangular nucleus, the uLicleus of Schwalbe, belonging to the vestibular complex which has fn"st appeared just below the lateral recess of the fourth ventricle. The triangular nucleus rapidly becomes a comparatively bulky nucleus extending mesially to the base of the lateral wall of the ventricle, and laterally to the dorsolateral angle of the reticular formation. It extends upward almost to the upper limit of the metencephalon and then disappears gradually. The ascending nucleus of the vestibular complex, the nucleus of Bechterew, is contained in the lateral wall of the fourth ventricle and is not of sufiicient bulk to be represented satisfactorily in the reconstruction. The Cochlear Complex The cochlear complex of nuclei makes its appearance at the upper level of the inferior olivary nucleus as two masses of gray matter form- ing the ventral and dorsal cochlear nuclei. The large ventral cochlear nucleus is located at the extreme ventrolateral angle of the brain stem at the point of entrance of the cochlear nerve into the brain stem. It forms a fairly well-developed trough in which the cochlear nerve lies, covering this nerve on its cephalic, lateral and caudal surfaces. RECONSTRUCTION OF PITHECUS RHESUS 401 The dorsal cochlear nucleus, on the other hand, appears below the lateral recess of the fourth ventricle as a more or less triangular mass of gray matter in the extreme dorsolateral angle of the brain stem. At this point it is located dorsal to the nucleus. A certain amount of nuclear material lies interspersed between the fibers of the cochlear nerve extending between and connecting the dorsal and ventral cochlear nuclei. The dorsal and ventral cochlear nuclei appear below the level of the cochlear nerve and extend to a level higher than that of the nerve itself. The Substantia Nigra The substantia nigra appears in the reconstruction as a well de- veloped and extensive mass of gray matter situated dorsal to the cerebral peduncle and supported by the deep layer of the pontile nucleus and the mesial and lateral buttresses. Between these two structures it rests upon the deep layer of the pontile nucleus from which it seems to develop. It extends upward throughout the extent of the mesencephalon, diverging laterally and somewhat dorsally to terminate in a rounded, blunt extremity which is separated from the mesial geniculate body by a relatively small intervening mass of the reticular formation and the fiber capsule of the mesial geniculate body itself. At the junction of the mesencephalon and the diencephalon it comes to an end, separated from the corpus of Luys by a thin lamina of white matter. The Colliculi The inferior colliculus constitutes a poorly developed structure lying as a crescentic mass of gray matter in the tectum of the mesencephalon. It is supported laterally by the lateral extension of the mesencephalic reticular formation, while mesially it lies against the undifferentiated dorsal gray matter. It is separated from the superior colliculus by a deep narrow^ 402 THE INTER-MEDIATE PRIMATES extension of the reticular formation which passes between it and the superior colliculus. The inferior colliculi are closely connected across the midline by the commissure of the inferior colliculi. The superior coIHcuIar elevation is much more massive than the inferior colhculus and is similarly situated, being separated from the central gray matter surrounding the aqueduct of Sylvius by a thin lamina derived from the dorsal extension of the reticular formation. It is supported laterally by the same lateral extension of the mesencephahc reticuhir iormation. Cephahcally it is limited by a thin exten- sion of the lateral mesencephalic reticular formation. Mesially it rests against the undiflerentiated dorsal gray matter and is connected with its fellow of the opposite side by the commissure of the superior colliculus. The Red Nucleus The nucleus ruber is contained within the mesial mass of the mesence- phalic reticular formation and receives at its caudal extremity the decussat- ing superior cerebellar peduncle which forms an encapsulating mantle of white libers about it, thus deiinitely outlining it from the surround- ing matrix of the reticular formation. Although it is the outstanding feature of the mesencephalon, it is relatively poorly developed in this form, extend- ing upward even beyond the junction of the mesencephalon and diencephalon into the substance of the latter division of the bram stem. The Central Gray Matter The central gray matter, as has been previously described, appears in the higher levels of the cervical cord as a roughly cordiform structure already separated from the ventral gray columns and providing a connection with the laterally compressed dorsal horns. Continuing upward in this same position, it gives origin to the dorsal extensions which form the nucleus of the column of Goll, and at a somewhat higher level it presents the RECONSTRUCTION OF PITHECUS RHESUS 403 condensations situated more laterally than the preceding, which extension forms the nucleus of the cohmm of Burdach. At the point where the nucleus of the column of Goll begins to increase in size, there appears a narrow, tongue-shaped prolongation of the central gray matter, which gradually approaches the dorsal surface and then, extending laterally, the entire mass of the central gray matter is drawn dorsally and laterally flattening out to form the floor of the fourth ventricle. In its ventral surface lie embedded the dorsal nuclei of the ninth and tenth cranial nerves, and close to the midline appears the nucleus of the twelfth cranial nerve. Separat- ing the gray matter of the ventricular floor and the reticular formation, lie the longitudinally coursing fiber bundles of the posterior fasciculus mesially, the fasciculus solitarius laterally, together with the peripheral condensation of fibers which circumferentially limits the reticular formation. In its lateral portion are embedded first the nucleus of the column of Goll, arid then the nucleus of the column of Burdach, which become separated from contact with the central gray matter by the appearance of the nucleus of Deiters. In the cephalic portions of the myelencephalon and the metencephalon appear small masses of gray matter connected with the ventral aspect of the central gray matter, extending along the tegmental raphe. These form a connected series of nuclei lying between the posterior longitudinal fasciculus, the predorsal bundle and the mesial fillet. The central gray matter is in close contact with the vestibular and cochlear complexes throughout their extent in the metencephalon. At the upper limit of the triangular nucleus (Schwalbe) of the vestibular com- plex, the lateral walls of the ventricle begin to approach each other and as the upper extremity of the ventricle is reached, the floor, roof and lateral walls rapidly draw together and the aqueduct of Sylvius appears. The gray matter surrounding the aqueduct of Sylvius is relatively massive on all sides. Ventrally the central gray matter is continued forward as a long, tongue- 404 THE INTERMEDIATE PRIMATES shaped process which contains embedded within it the nuclei of the fourth and third cranial nerves. As the diencephalon is approached the ventral extension of the central gray matter rapidly increases until the third ven- tricle appears where the central gray matter becomes continuous with the diencephalic subependymal gray matter and the mesial mass of the thalamic nuclei. The floor of the fourth ventricle shows practically no modeling in its lower half. Above the level of the lateral recess, however, there appear a relatively well-marked eminences on either side of the midline, separated from each other by a rather deep sulcus, representing the underlying con- densation of the gray matter to form the nucleus of the sixth cranial nerve. Chapter XIV HVLOBATES HOOLOCK, THE GIBBON, ITS BRAIN AND BEHAVIOR Its Position amons: the Primates; The Proantbropoid Stage; Measureirienis and Brain Indices; Surjace Appearance 0/ the Brain; Internal Structure oj the Brain Stem in Cross Section T "^HE gibbons are usually assigned to the group of the higher anthro- poids, with the quahlication, however, that in this group they are the most remote from man. In the general appearance of their brain they ally themselves much more closely with the intermediate primates. This, in combination with the fact that by their adaptive radiation they have become equipped preeminently if not exchisively as tree dwellers, seems to justify their inclusion with those intermediate forms already described. The designation of them as "Hylobates" is intended to indicate how exquisitely these "tree walkers" are specialized for arboreal habitat. Their lives are passed in the trees, through whose branches they move with a swiftness and ease similar to those of a bird. When on terra firma they pass over its surface a little awkwardly, yet with much facility although compelled to balance themselves by holding their long arms over their heads and hastening their footsteps lest they fall. Once, however, their slender hands come in contact with the branches of a tree, a really marvelous change appears in their pro- gression. Their uncertain gait is immediately replaced by a locomotion which has all the grace, the accuracy, the speed of a bird in flight. Appearance and Habits of the Gibbon The body and head of the gibbon are relatively small, little larger indeed, than some of the smaller macaques. The animal's legs are relatively short and it has no tail. The characteristic feature of the gibbon is the Courtesy. New York Zoolomcal Garden FIG. 187. HYLOBATES SYNDACTYLUS (gibbon). [406] H^LOBATES HOOLOCK, THE GIBBON 407 exceptional length of the forearm which so increases the entire length of the arm that the tips of the lingers touch the ground when the animal stands erect. The slender hand is longer than the foot and the thumb is also long in FIG. 188. HYLOBATES SYNDACTYLUS (GIBBON). Gibbon's brachiating locomotion has favored erect posture and thus initiated the Proanthropoid Stage. proportion. These animals are possessed of remarkable vocal power and can be heard for great distances. They go in troupes and usually early in the morning give voice to their peculiar chorus of calls which seems to cease, as if regulated by some prearranged schedule, in the early forenoon. 4o8 THE INTERMEDIATE PRIMATES One young is produced at a time and the mother carries it under her body, the young one chnging to her fur with hands and feet. This appendage to the mother does not seem in the slightest to embari-ass her progress as she Courtesy, Ameriian Museum oj Natural History FIGS. 189 AND 190. HAND AND FOOT OF HYLOBATES SYNDACTYLUS. Left. Palmar surface of hand, showing the extremely long fingers developed in consequence of brachiation. The swinging type of locomotion of the gibbon has produced this specialization in the hand. Fingers show some syndactylism. Right. Plantar surface of the foot showing short heel, long, powerful great toe, marked syndactylism ol second and third toes. The foot is adapted to prehensile purposes involved in the brachiating locomotion. makes her way through the forest, executing her great swings from tree to tree which are just as prodigious in their length as those made by the unen- cumbered male. The gibbon is a delicate animal and rarely survives long in captivity. By disposition it is gentle, often affectionate. It is not averse to handling by strangers and will come close to the sides of its cage with arms extended through the bars so that its hands may be grasped and stroked. The gibbons H^LOBATES HOOLOCK, THE GIBBON 409 never leave the forest and are distributed for the most part throughout southern Asia and the adjaeent islands. A lew species venture from the inland jungles to the vicinity of the coast. Courtesy, American Museum of Natural History FIGS. 191 AND 192. HAND AND FOOT OF HYLOBATES SYNDACTVLUS. Left. Dorsum of hand showing long, slender fingers, well-developed finger-nails, a hand adapted to grasping the branches in the gibbon's brachiating type of locomotion. Right. Dorsum of foot showing marked hand-like characters in adaptation to brachiating locomotion. The long, powerful, opposable great toe is specialized for grasping purposes, cooperating in this function especially with the syndactylic second and third toes. THE HOOLOCK GIBBON In general, the typical hue of the hoolock is black, although many varieties are found, as far as coloring is concerned. The hoolocks, however, are most consistent in this regard. Such variations in color as do occur affect the female more than the male. The hoolock gibbon is confined in its range to a limited district bounded by the Brahmaputra and Irawadi Rivers. It has a distinct aversion to water and cannot swim, a fact which 410 THE INTERMEDIATE PRIMATES probably circumscribes its habitat to the region in which it is found. Like all members of this family, the hoolock is exquisitely arboreal. While it makes some successful etTorts to progress over the surface of the ground, balancing its Ix)dy in a rather awkward manner by holding its arms over its head, its life is passed essentially in the trees. The effects of brachiation are seen in the greater erectness of body which marks the beginning of the Proanthropoid Stage. Mr. Candler, who has studied the habits of this proanthropoid, gives such an interesting account of its actions, that it may be here quoted in part: "He [Hoolock] swings along to the thinnest part of the bough or to the slender end of a bamboo until it bends to his weight, then with a swing and a sort of a kick-off, he flies through the air seizing another bough and swinging along it with the unerring accuracy of a finished trapeze performer. I fancy he does very little walking in the wild state, for I have never seen a wild Hoolock on the ground. Moreover, they are only found in the dense jungle where the ground is everywhere covered with tangled vegetation. It is puzzling to me why these anthropoids, being so entirely arboreal in habit, should be lacking in such a useful appendage as the tail. The Hoolocks are extremely shy and it is most difficult to watch them as they are concealed by leaves high up on the tops of the bamboo clumps or forest trees. You may hear their cries all around as you ride quickly along the jungle track; but the moment you leave the path or look up at them there is a dead silence and scarcely a leaf stirs until, tired of waiting, you move on again. The cry of the Hoolock is a characteristic sound in the Cachar jungle. It is a very pleasing note, rising and falling in intensity and reminding one somewhat in its rhythm of a pack of beagles giving tongue on a scent, which is waxing and waning in strength as a larger or smaller number of the band join in the chorus. It is heard chiefly in the early morning, then all through the heat of the day there is silence but towards evening, as the sun sets, you may hear it again. 'Hoo'loo! Hoo'Ioo! Hoo'loo!' HYLOBATES HOOLOCK, THE GIBBON 411 is supposed to describe the soLind, but it is really quite indescribable in writing." As in other species of apes, there is a special modification of the larynx which acts as a sort of resounding box and helps to make the sound carry, as it does, long distances. There is also a peculiar arrangement of the upper aperture of the larynx with its small and inadequate-looking epiglottis, which resembles more the arrangement in birds than the leaf-like epiglottis in man. Mr. Candler believes that the hoolocks work their ground systemati- cally in their search for food just as the planter plucks one section of his tea to-day and another section in a different part of the garden to-morrow. For he has found them fdling the air with their cries along a particular stretch of jungle road one day, while the next day not one was to be heard; and then, perhaps a week later, they were back again in the same place as at first. Living as they do in fairly large communities, they are constantly on the move, and from what is known of their relatively good intelligence, it seems highly probable that their movements are guided by definite plans, and that they have some sort of governmental system. In Cachar, the tea planters often keep hoolocks for years at a time, permitting them to run about the compound quite freely. Several such tame hoolocks were seen by Mr. Candler and were under his observation for several months at a time. They often would disappear and be away in the tree tops for days together, and at such times nothing tempts them to come down. But at length one of them seems to prefer to be sociable and he will then come and sit on the arm of a chair at breakfast, although he will never reach and snatch things off the table. His manners, in fact, are quite irreproachable, and he keeps himself remarkably clean. At sunset it is his custom to settle down to sleep, jammed tightly into the fork of a tree in a squatting position, usually with his arms over his head. In this semi-domesticated state, the hoolock seldom uses his voice, for he apparently finds no necessity for chattering or callmg to his 412 THE INTERMEDIATE PRIMATES fellows. A long list of foods eaten by the hooloek has been given by the numerous observers. This includes iruits, leaves, young shoots, spiders, insects, birds' eggs and young birds. But much of this may be considered as conjecture, although there is good reason to believe that the diet is extremely varied. If captured young, the hooloek is readily tamed. It is gentle, well disposed and good tempered. It shows a marked intelligence, particularly in the acquisition of tricks and in adjusting itself to household regulation. Measurements and Indices of Hylobates Hoolock The measurements of the hoolock show on the average: Body and head about 520 mm. Foot 150 mm. Skull 114 mm. Occipito-nasal length 99. Q mm. Interparietal width 49 mm. Breadth of brain case 61.5 mm. The dimensions of the brain including the cerebellum and brain stem are: Longitudinal diameter 65 .5 mm. Transverse diamc^ter jj mm. The brain is distinctly gyrencephalic in type, presenting, as do the brains of all the other intermediate primates, a Sylvian, a Rolandic and a simian fissure, which distinctly set the boundaries of the frontal, parietal, temporal and occipital lobes upon the lateral convexity of the cerebral hemisphere. The weights of the several divisions of the brain are: Forebrain 54 gms. Midbrain 2 gms. Hindbrain 13 gms. Total weight of brain 69 gms. HYLOBATES HOOLOCK, THE GIBBON 413 The water displacement ol the brain is: Forebrain 53 c.c. Midbrain 2 c.c. Hindbrain 10 c.c. Total displacement 65 c.c. The lorebram indices, computed on the basis ot weight and vohime, give the gibbon a forebrain index of 81 per cent by weight and 82 per cent by volume, thus placing it in the group characterized by manual differentia- tion, although it occupies a rather low position in this group. Surface Appe.ar.ance of the Brain in Hylobates Hoolock From some of the more important superficial features on the external surface of the cerebral hemisphere, it is evident that the brain of the gibbon in many respects is little more advanced in its development than that of the baboon or macacus. The two great concavities on the basilar surface, i.e., the orbital concavity and the cerebellar concavity, are equally as pronounced as in the other intermediate primates. This fact in itself calls attention to the degree of limitation imposed upon the frontal and occipital lobes in their expansion. For it is almost in direct proportion as these basal concavities become less pronounced, that the two important territories of the cortex, the frontal and the occipital lobes, show progressive expansion. The olfac- tory bulb and tract are detachable as far back as the trigonum olfactorium. A rudimentary olfactory sulcus is present and a correspondingly rudimen- tary gyrus rectus. The interorbital keel is prominent. As in the case of the baboon and the macacus, the boundaries of the superior longitudinal fissure tend to diverge as they approach the occipital pole. This divergence permits of a marked widening of the fissure which is in the interest of accommodating the superior vermis of the cerebellum whose appearance on the tentorial 414 THE INTERMEDIATE PRIMATES surface of this organ is sharply convex from side to side and thus forms a decisive ridge in the midline of the cerebellum. This ridge becomes progres- sively less distinct in the higher anthropoids and man. The divergence in FIG. ig3 DORSAL SURFACE OF BRAIN, HYLOBATES HOOLOCK. [Actual Length, 70 mm.] Key to Diagram, sulc. inter-par.. Sulcus Interparietalis, s. prec. inf., Sulcus Precentralis Inferior; sulc. PRECNT. SUP., Sulcus Precentralis Superior; sulc. ret. inf., Sulcus Retrocentralis Inferior; s. ret. s., Sulcus Retrocentralis Superior. the occipital portion of the superior longitudinal fissure of the higher forms gradually lessens, with the general effect that the occipital region eventually conceals from view the entire tentorial surface of the cerebellum. FISSURES AND LOBES In its fissures and lobations, also, the cerebral hemisphere corresponds closely with the brain of the macaque and baboon. The fissure of Rolando separates the frontal and parietal lobes, the Sylvian fissure intervenes between the parietal and temporal lobes, the simian fissure separates the parietal and temporal lobes from the occipital lobe. The convolutional pattern in all H^'LOBATES HOOLOCK, THE GIBBON 415 of these cortical areas is relatively simple. It is most complex in the parietal region and least conspicuous in the frontal lobes. In respect to fissural devel- opment, the gibbon shows less advance than either the baboon or macacos. FIG. 194. BASE OF BRAIN, HYLOBATES HOOLOCK. [Actual Length, 70 mm.] Key to Diagram, o.c, Optic Chiasm. On the Other hand, there is a richness of convolutional pattern in the occipi- tal lobe, which does not exist in cither of the other two forms. This fact, taken in conjunction with the much reduced appearance of the superior colliculus of the midbrain, indicates that in the gibbon most of the actual supervision of vision has been transferred to the occipital lobe. The fissures in the temporal lobe, especially the superior temporal fissure, are well devel- oped and this latter has connected \\ith its extremity a well-defined angular gyrus. The fissures of the basal surface of the frontal and temporal lobes give these regions an extremely simple appearance. In fact, the impression obtained from a survey of the cerebral hemispheres in gibbon allies this form much 4i6 THE INTERMEDIATE PRIMATES more closely with the group here identified as intermediate primates. Cer- tainly, the superlicial appearance of the gibbon's cerebral hemisphere places between it and the more highly complex endbrain of the great anthropoid FIG. 195. LEFT LATERAL SURFACE OF BRAIN, HYLOBATES HOOLOCK. [Actual Length, 73 mm.| Kev to Diagram, cerebl.. Cerebellum; obl., Oblongata; sulc. occip., Sulcus Occipitalis; SULC. occip. LAT., Sulcus Occipitalis Lateralis; sulc. precnt. inf., Sulcus Precontralis Inferior; sulc. ret. sup.. Sulcus Rctroccntralis Superior; sulc. simiarum. Sulcus Simla rum. apes a wide interval, so wide as to justify the opinion that in descent, the hneal relation between the gibbon and the anthropoid, however direct, must be quite remote. THE CEREBELLUM The cerebcHum I^ears out this impression in conclusive manner. Its tentorial surlace shows that marked convexity or gabling which culminates in a well-defined vermal ridge-pole. This feature is characteristic of intermedi- ate and lower primates, but gradually disappears in the higher anthropoids. On the tentorial surlace of the cerebellum the interfolial fissures pass from the vermis to the lateral lobe without sulcal interruption, while on the occip- ital surface, a paramedian sulcus upon either side of the vermis interrupts the folial sulci in their passage from the median to the lateral expansions of HYLOBATES HOOLOCK, THE GIBBON 41- the cerebellum. The petroso-ventricular surface of the cerelx'lhim presents a fairly large flocculus occupying the cerebello-pontile angle. The mesial por- tion of this surface is in relation with the roof of the fourth ventricle. FIG. ig6. RIGHT LATERAL SURFACE OF BRAIN, HYLOBATES HOOLOCK. [Actual Length 73 mm.] Key to Diagram, obl., Oblongata; sulc. occip., Sulcus Occipitalis; sulc. occip. lat., Sulcus Occipitalis Lateralis; cerebl., Cerebellum; sulc. precnt. inf., Sulcus Precentralis Inferior; sulc. ret. sup., Sulcus Retrocentralis Superior; sulc. simiarum, Sulcus Siraiarum; s. ret. i.. Sulcus Retrocentralis Inferior. THE BRAIN STEM In the brain stem, the markings on the several surfaces in gibbon are more precise than in the lower primates, and about equally as distinct as they arc in the baboon and macacus. The Oblongata. The oblongata upon its ventral surface presents a well-defined ventromedian sulcus, which is flanktxl upon either side by t\\o fairly prominent pyramidal elevations. Lateral to the pyramid and separated from it by a pre-olivary sulcus is a well-defined olivary eminence whose contour and proportions are somewhat more marked than in the other intermediate primates. The lateral surface shows the gradual gathering of the ascending spinocerebellar fibers until this collection forms the restiform body at the lower border of the pons Varolii. In the lower portion of the lateral surface there is a slight protuberance which marks the surface relief 4i8 THE INTERMEDIATE PRIMATES of the substantia gelatinosa trigemini. This trigeminal protuberance at the more ccphahc levels of the oblongata is concealed from view by the superposition of the fibers entering the restiform body. In the most cephalic FIG. 197. VENTRAL SURFACE OF BRAIN STEM, HYLOBATES HOOLOCK. [Actual Length 45 mm.] Key to Diagram, cer. ped. and cerbl. peduncle, Cerebral Peduncle; opt. ch.. Optic Chiasm; ventro MED. SULCUS, Ventromedian Sulcus; 3RD N., Third Nerve; 8th nr.. Eighth Nerve. portion of the lateral surface appears a well-defined protuberance, the tuberculum acusticum. The dorsal surface of the oblongata presents its characteristic divisions, the ventricular and infraventricular portions. The infraventricular portion is characterized by the tj^pical dorsomedian septum, upon either side of which appears a well-defined clava, separated by a dorsal paramedian sulcus from an equally well-defined cuneus. These elevations on the dorsal surface indi- cate respectively the presence of the nucleus of Goll and the nucleus of Burdach. HYLOBATES HOOLOCK, THE GIBBON 419 In the ventricular portion of the dorsal surface appears the caudal angle of the fourth ventricle bounded laterally by the elevations of the clava and cuneus. At the angle of this ventricle is the trigonum hypoglossi which FIG. 198. DORSAL SURFACE OF BRAIN STEM, HYLOBATES HOOLOCK. [Actual Length 45 mm.] Key to Diagram, d. m. septum, Dorsomedian Septum; dorso. med. fissure, Dorsomedian Fissure; inf. COLL., Inferior Colliculus; sup. cerebr. ped., Superior Cerebellar Peduncle; sup. coll., Superior Colliculus; TUB. tricem. ,TubercuIum Trigeniini; 4TH ven.. Fourth Ventricle. marks the position of the nucleus of the twelfth nerve; lateral to it, and separated from it by the sulcus limitans, is situated the fovea vagi above the dorsal nucleus of the pneumogastric nerve. The lateral walls constituted by the cuneus and clava become progressively reduced in altitude as they proceed cephalad, and finally at the level of the lateral recess have reached the plane of the ventricular floor. At this level, fibers of the eighth nerve make their entrance, as the striae acusticae; most of these fibers pass directly transversely inward to the dorsomedian sulcus. Below the level of the striae 420 THE INTERMEDIATE PRIMATES acusticae, a large elevation appears in the floor marking the vestibular area. This fact accords well with the exquisitely arboreal locomotion of the gibbon and reveals one of the most important regulating mechanisms which makes possible its almost bird-like passage through the forest. The median fissure becomes somewhat deeper as the caudal (jrihce of the Sylvian aqueduct is approached, while the superior cerebellar peduncles, formmg the lateral boundaries of the ventricle, converge toward the acjueduct. Immediately above this orifice of the aqueduct the trochlear nerve emerges from the brain stem after decussation in the superior medullary velum. At this level appear the characteristic developments in the roof of the midbrain, the collicular eminences. Of these, as in other primates, the inierior colliculi are the smaller. However, the disparity m size between these two sets of colliculi in gibbon is not so marked as in the macacus or baboon. This modification appears to be clue to the fact that the superior colliculus has lost somewhat in promi- nence as compared with the other two intermediate primates. The correctness of this observation gains support from the more complex development of the occipital lobe in gibbon when compared with the baboon or macacus, thus implying a still further delegation of ^•isual function to the occipital lobe than in either of the other primates mentioned. The Pons Varolii. Upon the ventral surface of the axis the pons Varolii appears as a fairly prominent transverse structure, separated from the oblongata by a well-defined bulbopontile sulcus. It is also separated at its cephalic extremity from the cerebral peduncles by the pedunculo-pontile sulcus. The pons in the gibbon gives the impression of a greater degree of prominence than in either the baboon or macacus. The inference based upon the pontile appearance accredits the gibbon with the possession of skilled movements more complex than either the baboon or the macacus. The Ventral Surface of the Brain Stem. The brain steni upon its ventral surface terminates cephalically by the typical divergence of the two H^'LOBATES HOOLOCK, THE GIBBON 421 cerebral peduncles bounding the optico-peduncular space. From this space emerge the two oculc^motor nerves and in it are contained the mammillary bodies, the attachment of the infunchbular stalk, and the tuber cinereum. Internal Structure of the Brain Stem in Hylobates Hoolock In some particulars the cross sections through the brain stem of gibbon show a slight advance, particularly in the definition ot the structures when compared with the baboon and macacus. This applies more especially to the inferior olive and pontile nuclei. In other structural details the definition in gibbon does not show any marked degree of progress over the other inter- mediate primates. LE\EL OF THE PYRAMIDAL DECUSSATION (FIG. I Q9 ) Here the crossing libers of the pyramid { Pyx) determine a conspicu- ous rearrangement in the axis. The ellects of this decussation are seen in several notable features. In the lirst place, the usual separation ot the ventral gray column ( Ven) from the central gray matter (Cen) is clearly shown. This behavior on the part of the pyramidal fibers is ot particular interest. It is primarily a mammalian characteristic. Furthermore, the pallio-spinal connection represented by the pyramid is peculiar to mammals. Decussation in conduction pathways of all ^'arIetIes, whether intersegmental or projec- tional, is the almost invariable rule in the vertebrate neuraxis. Whether, as Cajal believes, such crossing was determined by the universal decussation ot the optic libers throughout this phylum, or whether some other factors have been operative, the irresistible tendency for long fiber tracts to pass from one side of the brain to the other has exerted its influence on the pyramidal system. The nerve fibers in this system are all newcomers among the conduct- ing axons of the neuraxis. They were introduced by the mammals and repre- sent the last accessional elements in the control of motor reactions. Thev are 422 THE INTERMEDIATE PRIMATES what may be called the modern agents of volitional behavior. According as their volume is large or small, they indicate the directing capacity of the cerebral cortex and as such transmit the orders of the supreme motor author- FIG. 199. GIBBON. LEVEL OF THE PYRAMIDAL DECUSSATION. CEN, Central Gray Matter; cb, Column of Burdach; CG, Column of Goll; dt, Deiterso-spinal Tract; FLE, Dorsal Spmocerebellar Tract; Gow, Ventral Spinocerebellar Tract; hel, Spino-olivary Tract of Helweg; MR, Nucleus of Rolando; pyx, Pyramidal Decussation; rst, Rubrospinal Tract; spt, Spinothalamic Tract; TRD, Descending Trigeminal Tract; ven. Ventral Gray Matter; xpv. Crossed Pyramidal Tract. [Accession No. 141. Section 5. Actual size 8X7 mm.] HYLOBATES HOOLOCK, THE GIBBON 423 ity. Regardless of their number, these pyramidal libers have submitted them- selves to the inlluences which determine decussation. They do not follow- exactly the same manner of crossing in all mammals, nor is their decussation equally decisive in all species. In some instances the pyramid is small and the crossing fibers produce relatively inconspicuous changes in the oblongata. Where, as in the primates, the decussation is large, it occasions a most out- spoken feature in cross section. In certain mammals, the decussating fibers of the pyramid make their way almost furtively into the dorsal columns for their further descent into the spinal cord. In others, they cross with more conspicuity directly into the lateral columns. The primates, from one end of their order to the other, have established a fashion of pyramidal decussation which is distinctively their own. With the possible exception of the transitional forms represented by lemur or of such anthropoid incipiency as is seen in the tarsiers, the pyram- idal system of primates is larger than in all other mammals. The secondary effects of pyramidal decussation are witnessed in the wide detachment of the ventral gray column (Ven) from the central gray matter (Cen) and also in the large field eventually occupied by the decussated fibers. This structural disposition and rearrangement is so decisive that any primate may be identified as such by the appearance of its pyramidal decussation. Were these facts of structural importance alone, they might carry but little weight in the presence of so many other satisfactory identifying char- acters. It is their dynamic significance, however, that gives them their true value. In this sense, they indicate the degree of neopallial development in the brain and the extensions in the cortical control of behavior. Through the gradual expansion of this mammalian contribution to the nervous sys- tem, the primate has progressively advanced to the highest stages of differentiation. 424 THE INTERMEDIATE PRIMATES The dorsal sensory field is represented by the eokinins of Goll and of Burdach (CG,CB), together \\ith the substantia gelatinosa trigemini (NR) and its aeeompanying deseending trigeminal traet (Trd). A com- parison of these elements shows that the eohunn of Burdach has increased in size relatively to the cokmm of Goll. It is evident froni this relation that the sensory territories of the upper extremity of gibbon have gained in promi- nence. This advance is no doubt consecjuent upon the high degree of brachial and manual specialization essential to the animal's arboreal life. The com- paratively small size of the luicleus of Rolando ( N R ) indicates that much of the tactile direction of locomotion must have been delegated to a more effec- tive and highly specialized hand. In the circumferential zone are the ascending libers of the spinocere- bellar tracts ( Fie, Gow). The medullary area surrounding the ventral gray column contains the Deiterso-spinal tracts (DT), while in the intermediate zone are the rubrospinal and spinothalamic tracts ( Rst, Spt). LEVEL OF THE CAUDAL EXTREMITV OF THE INFERIOR OLIVE (FIG. 200) Here the transverse diameters of the axis have increased considerably. This change especially aflects the dorsal sensory field, due to the appearance in it of the nuclei of Goll and Burdach ( NG, NB). Both nuclei are large but by actual measurement the nucleus of Burdach has a volume about twice that of Goll. The latter nucleus shows no median specialization characteristic of the nucleus of Bischofl. Since the gibbon does not develop a tail, the absence of this unpaired median nucleus might be expected. Connected with Burdach's nucleus there is a large isolated mass which constitutes the nucleus of Blumenau (NBl). The substantia gelatinosa presents marked prom- inence and is in relation with a descending trigeminal tract (Trd) of considerable size. Estimated as a whole, the dorsal sensory fietd denotes the apportionment of the afferent influx. The territory of the upper extrem- H\LOBATES HOOLOCK, THE GIBBON 425 ity has occasioned a notable mcrenicnt in consequence ot the annual's high specialization for arboreal life. The ultimate formula of the elements in the dorsal field of the great apes and man is here more nearly approached FIG. 200. GIBBON. LEVEL OF THE CAUDAL EXTREMITY OF THE INFERIOR OLI\ E. CB, Column of Burdach; cen, Central Gray Matter; dt, Deiterso-spinal Tract; fle, Dorsal Spinocerebellar Tract; cow, Ventral Spinocerebellar Tract; 10, Inferior Olive; mf. Mesial Fillet; nb, Nucleus of Burdach; NBL, Nucleus of Blumenau; ng. Nucleus of Goll; nr, Nucleus of Rolando; pv, Pyramid; ref, Reticular Formation; rst. Rubrospinal Tract; spt. Spinothalamic Tract; trd, Descending Trigeminal Tract. [Acces- sion No. 141. Section 16. Actual Size 9X8 mm.] than in any ot the primates below this stage. Numerous coarse bundles of arcuate fibers arising in the nucleus of Goll pass forward toward 426 THE INTERMEDIATE PRIMATES the median raphe. In their passage they ahiiost completely separate the central gray matter (Cen) from connection with the substantia gela- tinosa of Rolando (NR). The central gray matter has manifested decisive alteration. It is now almost quadrilateral in outhne, with a narrow dorsal extension reaching backward into the dorsomedian septum. This tongue- lil:e process indicates the prehminary rearrangement incident to the open- ing of the fourth ventricle. In the lateral field the circumferential zone is characterized by the usual dorsal migration of the spinocerebellar tracts. The dorsal spinocerebellar fasciculus (Fie) has changed its course to an obhque passage along the outer side of the descending trigeminal tract where its heavily myelinized fibers may be readily seen. The intermediate zone contains the rubrospinal ( Rst), the spinothalamic (Spt) and the Deiterso- spinal (DT) tracts. The ventral held contains the fibers which constitute the pyramid (Py), dorsal to which are the bundles of the posterior longitudinal and predorsal fasciculi. The internal arcuate libers, as they cross the raphe to form the mesial fillet (Mf), somewhat obscure the outlines of the predorsal and posterior longitudinal bundles. Dorsolateral to the pyramid is a slender body of gray matter, the caudal extremity of the inferior olive (10). An extensive, diffuse reticular formation (Ref), lies between the olivary body and the nucleus of Rolando. It is traversed by numerous internal arcuate fibers and contains several indefinite nuclear aggregations, one constituting the lateral reticular nucleus and a second, the dorsal reticular nucleus. A comparative survey of the six species of primates thus far considered must make impressive the striking similarities which characterize the cor- responding cross sections of the brain stem. Level for level in lemuroid and anthropoid, the component structures are remarkably identical. So alike are they that the repeated descriptions of the several levels in the different species creates a degree of tedium felt by the reader no doubt as much as by HYLOBATES HOOLOCK, THE GIBBON 427 the author. The repetition would be wholly objectionable and out of place were it not for one compelling argument which seems to make it justifiable. To recognize the morphological consistency, the unity of structural design, in the relation of so many intricate parts should suffice to dispel all doubts concerning the close generic association of these animals. This unity is like- wise convincing as to that iniluence which, working through the primate stock, has employed the same fundamental pattern to establish such condi- tions as result in progressively more efficient neural mechanisms and in better behavioral adjustments. LEVEL THROUGH THE MIDDLE OF THE INFERIOR OLIVE (FIGS. 201, 202) Here the most conspicuous feature is the appearance of the inferior olivary nucleus (10). This structure here presents its full degree of devel- opment in the gibbon. Certain features of it are to be contrasted with the homologous structure in the lower primates. It has gained considerably in its general dimensions, both transversely and longitudinally. But its real increase in prominence is due to its more convoluted configuration character- istic of the human and humanoid structure. In connection wTth it are its two accessory olives (VO, DO). The central gray matter occupies a position immediately beneath the floor of the fourth ventricle. In its most mesial portion it contains the nucleus of the twelfth nerve (Nhy), whose emergent fibers (N12) pass from the nucleus and make their way ventrolaterally toward the inferior olive. Lateral to the nucleus hypoglossus, and separated from it by the sulcus limitans in the central gray matter, is the dorsal nucleus of the pneumogastric nerve (Nvd), some entering fibers of which may be discerned approaching this nuclear collection. Immediately adjacent and lateral to the dorsal nucleus of the tenth nerve is a dense collection of medullated fibers constituting the fasciculus solitarius surrounded by the nucleus of this fasciculus (Nfs). Ventral to the fasciculus solitarius is the 428 THE INTERMEDIATE PRIMATES substantia gelatinosa trigemini (NR), while lateral to it and contiguous with its external surface is the descending trigeminal tract ( Trd ). Occupy- ing the most dorsal position in the lateral field is the cephalic extremity of FIG. 201. GIBBON. LEVEL THROUGH THE MIDDLE OF THE INFERIOR OLIVE. DO, Dorsal Accessory Olive; dt, Deiterso-spinal Tract; fle, Dorsal Spinocerebellar Tract; cow, Ventral Spinocerebellar Tract; lO, Inferior Olive; iv, Fourth Ventricle; mf. Mesial Fillet; nb, Nucleus of Burdach; NFS, Nucleus Fasciculus Solitarius; nhv. Hypoglossal Nucleus; nr. Nucleus of Rolando; nvd. Dorsal Vagal Nucleus; N12, Twelfth or Hypoglossal Nerve; pd, Predorsal Bundle; PL, Posterior Longitudinal Fasciculus; pv, Pyramid; ref. Reticular Formation; rst. Rubrospinal Tract; spt. Spinothalamic Tract; trd, Descending Trigeminal Tract; vo. Ventral Accessory Olive. (Accession No. 141. Section loi. Actual Size 14 X 10 mm.] the nucleus of Blumenau (NBl), fn^n which numerous internal arcuate fibers make their way forward and inward through the reticular formation toward the median raphe, where they undergo decussation and enter the mesial iillet ( Mi ) on their way to higher levels m the brain. HYLOBATES HOOLOCK, THE GIBBON 429 The general size of the nucleus of Blumenau ( N B I ) is suggestive, as in other levels, of the degree to which the forelimb and hand have developed. Emphasis has been laid on the fact that the relatively poor development of FIG. 202. GIBBON. LIALl IHKOLGH THE MIDDLE OF THE INFERIOR OLI\ E. AMB, Nucleus Ambiguus; do. Dorsal Accessory Olive; dt, Deiterso-spinal Tract; gow. Ventral Spinocerebellar Tract; icp. Inferior Cerebellar Peduncle; lO, Inferior Olive; mf. Mesial Fillet; nbl. Nucleus of Blumenau; NFS, Nucleus Fasciculus Solitarius; nhv. Hypoglossal Nucleus; nr, Nucleus of Rolando; nvd. Dorsal Vagal Nucleus; nig. Tenth Cranial Nerve; ni2, Twelfth or Hypoglossal Nerve; pd, Predorsal Bundle; pl, Posterior Longitudinal Fasciculus; pv. Pyramid; ref, Reticular Formation; rst. Rubrospinal Tract; spt. Spinothala- mic Tract; trd. Descending Trigeminal Tract; \o. Ventral Accessory Olive. [Accession No. 141. Section 131. Actual Size 14 X 8 mm.] the hindlimb in gibbon and the high degree of proficiency attained by the upper extremity and hand, especially for the purposes of its fleet locomotion, give marked structural preponderance to the nucleus of Blumenau. The prominence of the inferior oIi\ary nucleus compared with the other forms may also be in direct relation with the Ilight-Iike locomotion ol the animal 430 THE INTERMEDIATE PRIMATES which has been likened to the most skillful attainment of expert trapeze per- formers. The execution of such acts would require most active and precise visual supervision, both in the recognition of distance as well as in the selec- tion of adequate supports to sustain the weight of the body as the animal makes its prodigious swinging flights through space from one limb to another. Inaccuracy in visual judgment and particularly in the simultaneous adjust- ment of head, eye and hand in the execution of such acts could not fail to be attended with catastrophe. Hence it is that the simultaneous movements of head, eye and hand become of greater importance to the gibbon than to the ground-living baboon or in the more conservative aerial feats of the macacus and allied monkeys. The pyramid (Py) occupies its characteristic position in the ventral area of the cross section, but from its general dimensions, it appears to show no marked increase over this structure in the macaque and baboon. The cross section bears out the general impression conveyed by the appear- ance on the external surface of the oblongata. It seems probable that, despite the high development of the hand and especially of the forearm in the adjust- ment to its arboreal locomotion, the gibbon has shown no great specialization in its skilled movements, generally speaking. The proficiency which the animal exhibits in the hindlimb would indicate less need for the volitional control in these parts of the body than that possessed by many of the other primates. Dorsolateral to the olive is the reticular formation (Rcf) through which pass many of the internal arcuate fibers. Its definition is less well marked than in the other intermediate primates. At the lateral extremity of the section is a bundle of fibers constituting the restiform body (ICP) which is carrying spinocerebellar and other ascending cerebellar fibers upward toward the cerebellum. HYLOBATES HOOLOCK. THE GIBBON 431 LE\EL OF THE VESTIBULAR NUCLEI (FIG. 203) At this level the eonliguration of the cross section is altered by the further marked enlarfj;enient of the fourth ventricle. Intricate changes depend FIG. 203. GIBBON. LEVEL OF THE VESTIBULAR NUCLEI. CTT, Central Tegmental Tract; cow, Ventral Spinocerebellar Tract; hel, Spino-olivary Tract of Helweg; ICP, Inferior Cerebellar Peduncle; 10, Inferior Olive; mf, Mesial Fillet; nd, Deiters' Nucleus; nr, Nucleus of Rolando; NSC, Nucleus of Schwalbe; n8, Auditory Nerve; pd, Predorsal Bundle; pl. Posterior Longitudinal Fasciculus; py, Pyramid; ref. Reticular Formation; rst, Rubrospinal Tract; spt. Spinothalamic Tract; TRD, Descending Trigeminal Tract; tub, Tuberculum Acusticum. [Accession No. 141. Section 141. Actual Size 14 X 7 mm.] upon the replacement of the dorsal columns of Goll and Burdacli by the large elements of the vestibular area. The existence of a long proprioceptive column in the alar plate of the oblongata has already been noted. In its more caudal portion this column makes provision for the transmission of proprioceptive stimuli arising in the muscles and joints of the extremities and trunk. The cephalic division of this column serves in a similar capacity for the proprioceptors in the vestibular portion of the internal ear. This 432 THE INTERMEDIATE PRIMATES system is here represented by the nucleus triangularis of Schwalbe (NSc) and the nucleus magnocellularis of Deiters (ND). Both of these nuclear structures are of relatively larger size than in most other primates and indi- cate a balancing mechanism commensurate with the locomotor specializa- tion of the gibbon's arboreal life. The fibers of the vestibular division oi the acoustic nerve penetrate the substantia gelatinosa trigemini (NR) to enter Deiters' nucleus. Lateral to Deiters' nucleus is the corpus restiforme (ICP) superposed upon which is the tuberculum acusticum (Tub ). The circumferential area is now occupied largely by the ventral spinocerebellar tract (Gow ), mesial to which is the heavy bundle of the descending trigeminal tract (Trd). The inferior olivary nucleus with its accessory bodies (10) occupies its characteristic position and has its usual form. There is evident in it a slightly greater tend- ency toward convolution than observed in lower primates. Its definition is, however, somewhat more hazy than is true of higher members of this order. Dorsolateral to the olive is the central tegmental tract (Ctt), important because of its probable connections with the oculomotor nuclei and the mesencephalic root of the trigeminal nerve. This mesencephalic root, accord- ing to good authority, represents the proprioceptive conduction from the eye muscles. In this light the central tegmental tract may be regarded as the intersegmental link between the primary midbrain nuclei which receive afferent stimuli from ocular muscles, and the inferior olivary nucleus. Immediately dorsolateral to the olive and mesial to the central tegmental tract is the spinothalamic tract ( Spt ), while ventral to the substantia gela- tinosa trigemini (,NR) is the rubrospinal tract ( Rst). The reticular forma- tion (Ref) is relatively large. Ventromesial to the olive is the pyramid ( Py) and dorsal to it in succession toward the lloor of the ventricle are the mesial fillet (Mf), the predorsal fasciculus ( PD) and the posterior longi- tudinal fasciculus (PL). H^LOBATES HOOL.OCK, THE GIBBON 433 LE\'EL OF THE CEREBELLAR NUCLEI (FIG. 20^) At this level the cerebelhim eontains evidenee of a deeisive advance. This advaiiee atlects the eerei^eHar nuelei and partieularly the nucleus den- FIG. 104. GIBBON LE\'EL OF THE CEREBELLAR NUCLEI. icp, Inferior Cerebellar Peduncle; mf, Mesial Fillet; ndt, Cerebellar Nuclei, Lateral Group; nfg, Cerebellar Nuclei, Mesial Group; n8. Auditory Nerve; pv. Pyramid; ref. Reticular Formation; scp, Superior Cere- bellar Peduncle. (Accession No 141. Section 151. Actual Size 19 X 12 mm.) tatus (Ndt ). For the first time this nucleus shows some of the tendencies in its development which foreshadow its ultimate configuration in the higher primates. In the species lower than gibbon, the dentate nucleus has had a more ditfuse and somewhat amorphous appearance — amorphous at least in that it does not present any of those distinctive features which mark it in the great apes and man. A suspicion of the tendency to become convoluted may be detected in the baboon and macacus. In general, however, this tendency in both of the latter species is confined to ca^cumscribed and quite 434 THE INTERMEDIATE PRIMATES limited portions of the nuclear substance. The nucleus in the gibbon shows a number of well-defined plications throughout its entire extent. Its hilus is dorsomesial in position and so disposed as to place its fundus in a ventro- mesial position. The general saccular arrangement of the nuclear substance is most pronounced along the lateral wall. Here also the folia or convolutions are best defined. The mesial wall is much less developed in this respect. It gives the impression of a structure about to emerge from a diffuse matrix, but as yet not possessed of the definition characteristic of ultimate develop- ment. In this sense the dentate nucleus in gibbon represents a transitional stage in which only a portion of it has attained its full evolutional differen- tiation. It affords an illustration of one of the most important movements in the process of unfolding which appears both in ontogenetic and phylogenetic development. The gradual emergence of recognizable features out of a diffuse, more or less indefinite anlage is a rule in the genesis of all organs. The spinal cord, for example, in early fetal stages, shows but little differentiation of its principal histological features. In the early period of human development the dorsal and ventral gray columns are distinguishable but have many points of similarity. Only in infancy do these two columns of gray matter assume dis- tinctive characters. During the first year of life it is often difficult to dis- tinguish histologically between the configuration of the several levels of the spinal cord, even when the comparison involves such widely removed portions as the cervical and sacral segments. Throughout adolescence and into adult life these differential features progressively assume their ultimate character. A similar process passing through gradient stages appears in the phyletic evolution of many structures in the nervous system. Such structures have their inception in a diffuse matrix and gradually acquire sharpness of defini- tion together with specific characteristics. The dentate nucleus in gibbon, therefore, is of great interest morphologically if for no other reason than representing a decisive stage of evolutional transition. Its physiological signif- m'LOBATES HOOLOCK, THE GIBBON 435 icance is equally important. It indicates the pronounced accessions in coordi- native control ^vhich tliis more advanced primate has acquired in response to its specialized arboreal locomotion. This detail of its neural organization approaches much nearer to the higher anthropoids than any of the inter- mediate primates. Dorsomesial to the dentate nucleus is the nucleus fastigii (Nfg), while the inferior vermis projects into and fills most of the fourth ventricle. LEVEL OF THE EMERGENCE OF THE SLXTH CRANLA.L NERVE (fIG. 20j) At this level the most important features in the section are the abducens nucleus (Nab) situated in the iloor of the fourth ventricle, the superior olive (SO) connected with the secondary cochlear pathway, and the appearance of the transverse fibers constituting the trapezoid body. The pyramid (Py) occupies its usual position in the ventromedian portion of the section, while dorsal to it, stretching across the section in a trans- verse manner, are many fibers connected with the secondary cochlear path- way of the trapezoid body. At the lateral extremity of this is a nuclear collection which forms the superior olive (SO), from which many fibers make their way to the nucleus abducentis. These latter constitute the peduncle of the superior olive, the fibers of which participate in reilex acts necessary to adjusting lateral gaze to the direction from which sudden sounds may arise. The importance of such a direct reflex mechanism is apparent when the needs of instantaneous visual detection are considered in relation to acts of self-protection. Mesial to the nucleus abducentis (Nab) are the fibers which form the second part in the emergent course of the facial nerve. Other fibers constituting the fourth part of this nerve sweep forward and outward, mesial to the substantia gelatinosa ( N R). The central gray matter occupies the floor of the fourth ventricle whose roof is formed by the vermis of the cerebellum. 436 THE INTERMEDIATE PRIMATES LE\ EL THKOUGH THE MIDDLE OF THE PONS X'AROLH (FIG. 2o6) At this level all three layers ol this structure may be discerned, namely, the stratum superticiale pontis, the stratuni complexum pontis, containing FIG. 205. GIBBON. LE\EL Ol I III I MERGENCE OF THE SIXTH CRANIAL NERVE. MCP, Middle Cerebellar Peduncle; mf, Mesial Fillet; nab, Abducens Nucleus; nbe, Nucleus of Bechterew; NDT, Cerebellar Nuclei, Lateral Group; nfg. Cerebellar Nuclei, Mesial Group; nr. Nucleus of Rolando; n6, Abducens Nerve; N7, Facial Nerve; n8. Auditory Nerve; i>v. Pyramid; so, Superior Olive; tkd. Descending Trigeminal Tract. [Accession No. 141. Section 172. Actual Size 20 X 16 mm.| some scattered fasciculi ol the pyramid ( Py ), the pontile nuclear mass ( PN ) and some decussating pontile libers, and the stratum |)rolundum pontis largely made up of transverse decussating libers. At the lateral extremity of the section are the collected bundles constituting the middle cerebellar peduncle ( Mcp ). Entering the brain stem at this \v\c\ near the mesial sur- face arc some dorsal root axons of the fifth nerve ( N5 ) on their way to the H^'LOBATES HOOLOCK, THE GIBBON 437 substantia gclatinosa. From this nucleus many fibers pass dorsally and mesi- ally toward the angle of the fourth ventricle to form the tractus mesence- phalicus trigemini. FIG. 206. GIBBON. LEVEL THROUGH THE MIDDLE OF THE PONS. Mcp, Middle Cerebellar Peduncle; mf. Mesial Fillet; N5, Trigeminal Nerve; pn, Pontile Nuclei; pns. Pons; FY, Pyramid; ref, Reticular Formation; scp, Superior Cerebellar Peduncle; tur, Tractus Uncinatus of Russel (Hook Bundle). [Accession No. 141. Section 186. Actual Size 21 X 16 mm.] The central gray matter occupies a position mimediately beneath the floor of the fourth ventricle and serves to form the lateral ventricular bound- ary. At the lateral extremity of the section is a dense mass of fibers, the superior cerebellar peduncle (Scp), an index concerning the degree of coordinative control which the animal possesses. Compared with the other intermediate primates, the gibbon shows no marked increase in this par- 438 THE INTERMEDIATE PRIMATES ticular. The boundary between the tegmentum and basis pontis is provided by the mesial iillct (Mf), while the reticular formation ( Ref) shows no specialization of nuclear collections at this level. FIG. 207. GIBBON. LEVEL OF THE EMERGENCE OF THE TROCHLEAR NERVE. CEN, Central Gray Matter; ctt. Central Tegmental Tract; lf, Lateral Fillet; mf. Mesial Fillet; mcp, Middle Cerebellar Peduncle; N4, Trochlear Nerve; pd, Predorsal Bundle; PL, Posterior Longitudinal Fasciculus; PN, Pontile Nuclei; pv. Pyramid; ref, Reticular Formation; rst. Rubrospinal Tract; scp, Superior Cerebellar Peduncle; spt. Spinothalamic Tract. [Accession No. 141. Section 250. Actual Size 16X12 mm.] level of the emergence of the trochlear or fourth cranial nerve (fig. 207) Here the appearance of the section has undergone considerable modifi- cation due to the fact that the axis is approaching its mesencephalic portion H^'LOBATES HOOLOCK, THE GIBBON 439 where the large space of the foLirth \entricle is reduced as it approaches the aqueduct of Sylvius. The central gray matter (Ccn) surrounds the ventricle whose roof is formed by the superior medullary velum in which the fibers of the trochlear nerve (N4 ) undergo complete decussation. These libers subse- quently emerge m relation with the dorsal aspect of the isthmus. In the central portion of the central gray matter (Cen) are the dense bundles constituting the fasciculus longitudinalis posterior (PL) and the fasciculus predorsalis (PD). Lateral to the central gray matter are the fibers con- stituting the superior cerebellar peduncle (Sep) preparatory to their inward dellection toward their decussation in the midbrain. Ventral to these libers on the circumference of the axis is a mass of medullated axons forming the lateral fillet (Lf). At the boundary line between the tegmentum and basis are the transversely disposed bundles of the mesial lillet ( Mf), while the general arrangement of the pons Varolii in its three characteristic layers appears ventral to this boundary. The pons here contains the stratum superficiale, the stratum complexum and the stratum profundum lying immediately ventral to the tegmental portion of the axis. LEVEL OF THE INFERIOR COLLICULUS (fIG. 2o8) Here the cross section shows further modification by the appearance of the two dorsal elevations constituting the primordial stations for the auditory pathway. These colliculi (IC) form a prominent elevation in the quadri- geminal plate of the midbrain and show a degree of stratification which harks back to then" highest specialization in the lower vertebrates. Entering into the inferior colliculus are the collected bundles of the lateral fillet (Lf) which serves to convey impulses from the lower relay stations in the pathway of hearing. The central gray matter ( Cen) completely surrounds the aque- duct (jf Sylvius. In its ventromesial pcjrtion is the caudal extremity of the 440 THE INTERMEDIATE PRIMATES nucleus trochlearis (Ntr) from which the fourth cranial nerve arises (N4). In the lateral border of the central gray matter are the scattered bundles forming the tractus mesencephalicus trigemini (Tmt), while in a ventral FIG. 208. GIBBON. LEVEL OF THE INFERIOR COLLICULUS. CEN, Central Gray Matter; ctt. Central Tegmental Tract; ic. Inferior CoIIiculus; lf. Lateral Fillet; mf> Mesial Fillet; ntr, Trapezoid Nucleus; N4, Trochlear Nerve; pl, Posterior Longitudinal Fasciculus; pns, Pons; pv. Pyramid; rst. Rubrospinal Tract; scp, Superior Cerebellar Peduncle; spt, Spinothalamic Tract; TMT, Tractus Mesencephalic! Trigemini. [Accession No. 141. Section 278. Actual Size 16 X 15 mm.] HYLOBATES HOOLOCK, THE GIBBON 441 position are fibers forming tlie posterior longitudinal fasciculus ( PL) and the fasciculus predorsalis. Tiie tegmentum at this level is separated from the basis by the libers of the mesial fillet ( Mf ). The superior cerebellar peduncle (Sep) is about to make its decussation prior to entering the red nucleus. LE\EL OF THE SUPERIOR COLLICULUS (PIGS. 20g, 2 1 o) At this level the configuration of the brain stem again shows some conspicuous alteration. This is chiefly due to the appearance of the two elevations in the dorsal region of the midbrain which form the superior coll liculi (SC). In a ventral position appears a divergence of the two cerebra- peduncles which forms the caudal boundary of the optico-peduncular space. The superior colliculus in the gibbon, while somewhat less prominent than in the lower primates, even than in baboon or macacus, still retains a degree of its previous specialization. Traces of stratification may yet be detected in it. The central gray matter (Cen) surrounds the small ventric- ular space of the aqueduct of Sylvius and contains the nuclear specializa- tion forming the nucleus oculomotorius (Noc). Near the median raphe many fibers from this nucleus cross to the opposite side, forming the oculo- motor decussation. The nucleus ruber (NRu), whose large size and clear definition are in contrast with all other forms heretofore discussed, occupies its characteristic position in the tegmentum. Because of its much greater defini- tion and its more apparent emergence from the surrounding reticular forma- tion, it seems to show some further advance in the development of that system which has control over the coordinative regulation of movement. On the other hand, the functional duality of this nucleus as a relay station should be borne in mind. It is possible that its increments, both in definition and in size, are due to augmentation from the striatal structures of the endbrain rather than to an increase of fibers arising in the cerebellum. In all probabilit}-, 442 THE INTERMEDIATE PRIMATES both factors have contributed some additional prominence to the red nucleus in gibbon. That its striorubral portion has undergone expansion might be expected in an animal presenting such complex arboreal locomotion. That FIG. 209. GIBBON. LEN'EL OF THE SUPERIOR COLLICULUS. CEN, Central Gray Matter; cp. Cerebral Peduncle; ctt. Central Tegmental Tract; mf, Mesial Fillet; mgb, Mesial Geniculate Body; noc. Nucleus Oculomotorius; nru. Nucleus Ruber; n'3. Oculomotor Nerve; pd, Predorsal Bundle; pl, Posterior Longitudinal Fasciculus; ref, Reticular Formation; sbn, Substantia Nigra; sc, Superior Colliculus; spt. Spinothalamic Tract. [Accession No. 141. Section 310. Actual Size 30 X 16 mm.] the cerebello-rubral portion of this nucleus has also undergone some expan- sion may be inferred from the increased size of the dentate nucleus. Surround- ing the nucleus on all sides, and lying lateral to the central gray matter H^'LOBATES HOOLOCK, THE GIBBON 443 (Cen), is the reticular formation ( Ref), in this species somewhat reduced in size and prominence as compared with the lower forms. This reduction is due largely to the emergence from it of the large red nuclear mass. In the FIG. 210. GIBBON. LE\EL OF THE SUPERIOR COLLICULUS. CEN', Central Gray Matter; cp, Cerebral Peduncle; ctt, Central Tegmental Tract; mf. Mesial Fillet; mgb. Mesial Geniculate Body; noc. Nucleus Oculomotorius; nru, Nucleus Ruber; N3, Oculomotor Nerve; pd, Predorsal Bundle; pl, Posterior Longitudinal Fasciculus; ref. Reticular Formation; sbn. Substantia Nigra; sc, Superior Colliculus. [Accession No. 141. Section 350. Actual Size 30 X 16 mm.] lateral extremity of the section is seen a small protul^erance, the mesial geniculate body (Mgb). The gray matter of this nucleus seems to be confluent with another large nuclear aggregation situated ventromesial to it, the substantia nigra (Sbn). This mass of gray matter extends obliquely inward and forward from the region of the mesial geniculate body toward the optico-peduneular space and separates the basal portion of the mesen- 444 THE INTERMEDIATE PRIMATES cephalon from the tegmentum. The specific functions of the substantia nigra arc yet surrounded by doubt, although many writers attribute to it some regulating control of the automatic associated movements of the body. FIG. 211. GIBBON. LEVEL OF THE OPTIC CHIASM. CIN, Internal Capsule; cph, Corpus Hypotlialamiciim; fdp, Descending Pillars of the Fornix; for, Fornix; GLP, Globus Pallidus; nl, Lateral Nucleus of the Thalamus; nli, Internal Lateral Nucleus of the Thalamus; opx. Optic Chiasm; put, Putamen. (Accession No. 141. Section 440. Actual Size 34 X 19 mm.] This opinion is in part conjectural, although the structure as a whole has the appearance of a very important element in the brain stem. LEVEL OF THE OPTIC CHIASM (fIG. 21 I ) At this level the configuration of the section again shows those marked alterations due to the fact that the brain stem is about to reach its cephalic termination. Some of the massive lateral additions of the endbrain are ah-eady HYLOBATES HOOLOCK, THE GIBBON 445 apparent. The most ventral structure in the cross section is the optic chiasm (Opx) which talces the form observed in all primates, of a lateral elonga- tion, together with the obtuse angulation in relation to the optic nerves and FIG. 212. GIBBON. LEVEL OF THE ANTERIOR COMMISSURE. AC, Anterior Commissure; ciN, Internal Capsule; fdp, Descending Pillars of the Fornix; for, Fornix; glp. Globus Pallidus; nca. Nucleus Caudatus; nl. Lateral Nucleus of the Thalamus; nm. Nucleus Medialis Thaianii; put, Putamen. [Accession No. 141. Section 486. Actual Size 34 X 10 mm.] optic tracts. The supra-optic portion of the third ventricle appears immedi- ately dorsal to the optic chiasm. Flanking either side of the ventricle are the dense masses of the optic thalami, which in turn are separated by the internal capsule (Gin) from the lenticular portion of the stratum. This nucleus presents its two characteristic portions, the mesial and darker area constituting the globus pallidus (Glp) and the outer and lighter portion, the putamen (Put). The evolutional significance of such com- plex elements as the thalamus and the lenticular nucleus could scarcely be approached within the limits of this work. 446 THE INTERMEDIATE PRIMATES Lli\ EL ()I- THE ANTEKIOK COMMISSURE (FIG. 2 12) Here tlu' brain striii, as eonsulcTcd in this wnrk, cdiiu-s ti> its ccphalu' termination. In the nii(l-\ eiitral hue is seen the naimw elell n| the thiicl xc-ntriele, aiul l)iiiinclin;4 this an elevated ixirtmn nl the thahinuis, the tnUer- eiiluin antieuni, external tn whieh is the eei:)hahe purtion ol the lateral thal- aniie mieleus ( N I ). This nueleus is st'parated l)\ a tleiiso mass nl hea\ il\ mxelinizecl lihers re'presiMitinj^ the antermr Imil) (it the internal eapsnle (Cinl IrDin an e\[)ansiw luielear jjurtidn nl the enrpns striatum (Cilp. Put). Passin^ \entrall\ inward toward the midline, thron^li this pnrtinn ul the corpus striatum, are the collected bundles lormin;^ the anterior com- missure ( AC ). Chapter XV RECONSTRUCTION OF THE GRA\' MATTER IN THE BRAIN STEM OF HYLOBATES HOOLOCK T "^HE reconstruction of the gray matter of the brain stem in Hylo- bates hoolock begins at the caudal extremity of the inferior olivary nucleus. The higher levels of the spinal cord are not represented in the model and the ventral gray cokimns have ah-eady merged with the reticular formation. The Dorsal Medullary Nuclei The nucleus of GoII, as first represented in the reconstruction, is already of considerable size. It is roughly quadrilateral in shape with a broad base which is confluent with the central gray matter. A large dorsal portion shows some tendency towards that lateral swing which characterizes these nuclei in the various brain stems ah-cady studied. The nucleus of Burdach arises slightly more cephalad than the nucleus of GoH, in fact, at about the same level as that in which the inferior ohvary nucleus appears. Its origin is represented by a thickening at the point of junction between the nucleus of Goli and the central gray matter. The nucleus rapidly increases in size, expanding laterally and, at its periphery, somewhat ventrally, thus showing the same tendency towards the lateral swing evidenced by all of these dorsal nuclear structures. It soon reaches its maximum size and continues upward with httle change. At the level of the lower thud of the fourth ventricle it begins to diminish and rapidly comes to an end. Directly ventral to the nucleus of Burdach is the substantia gelatinosa trigemini. This nucleus, as in other primates, has ah-eady reached a position widely removed from the midline. It is oval in outHne with its long axis 448 THE INTERMEDIATE PRIMATES directed somewhat obliquely from before backward and inward. It rests in a space hollowed out of the reticular formation which surrounds it upon its dorsal, ventral and mesial surfaces. Dorsomesially the substantia gelatinosa trigemini is in relation first with the nucleus of Goll and, more cephahcally, with the nucleus of Burdach. In the gibbon, the constriction in the substantia gelatinosa trigemini described in connection with the other primate brain stems is well defmed, occurring at about the level of the junction of the upper and middle thirds of the inferior ohvary nucleus. Above this point the substantia gelatinosa again enlarges and passes upward in the lateral portion of the tegmentum to the upper metencephalic levels. At this point it expands to form its caput and is associated with the motor nucleus of the trigeminal nerve on its mesial aspect. The Inferior Olivary Nucleus In Hylobates hoolock the inferior olivary nucleus has I^ecome a relatively massive and prominent structure. It presents a great number of secondary plications in its surface. The fundus of the inferior olive is deep and wide, with a number of secondary loops. The accessory olivary nuclei are well developed. The ventral accessory nucleus begins below as a flat, elongated band applied to the inner half of the ventral branch of the main olivary nucleus. It extends upward in this position as a compressed lamina of gray matter lying between the inferior olivary nucleus and the pyramidal tract. As the ventral accessory nucleus is traced upward it gradually diminishes in size until it comes to an end apparently by fusing with the mesial extremity of the ventral branch of the principal nucleus. The dorsal accessory olive begins at about the middle of the main olivary nucleus and appears as a narrow layer of gray matter applied to the inner portion of the dorsal branch of the olivary nucleus. It rapidly reaches its maximum transverse diameter and then, gradually diminishing, ends below RECONSTRUCTION OF HYLOBATES HOOLOCK 449 the uppermost level of the mahi nucleus by fusing with the dorsal branch at its mesial extremity. The arrangement of the nucleus is similar to that found in other primates. Its deep surface lies embedded in the reticular for- FIG. 213. \ ENTRAL SURFACE OF GRAY MATTER OF BRAIX STEM, HYLOBATES HOOLOCK. Key to Diagram, lat. gen. body. Lateral Geniculate Body; pontile, Pontile Nuclei; ret. form.. Reticular Formation; sup. olive, Superior Olive; vent. coch. and ventral cochlear, Ventral Cochlear Nucleus. mation and the dorsal extremities ot its two laminae are turned dorsally at their mesial extremities. The hihis opens toward the contrahiteral inferior cerebellar peduncle. The Reticular Formation The ventral surface of the reticular formation in its oblongatal and lower pontile portions is, to a great extent, in contact with the inferior oli- vary nucleus. Lateral to the fundus of the olive the reticular formation approaches the surface of the stem and is luicovered except by peripheral fiber bundles. Dorsal to this uncovered area is the substantia gelatinosa 450 THE INTERMEDIATE PRIMATES trigcmini which is situated in the dorsolateral angle of the reticular formation. Dorsal to the substantia gelatinosa this formation is covered by the fibers of the columns of GoII and Burdach and gives origin to the basal portions of the dorsal sensory nuclei. Mesially the reticular formation is continuous with the central gray matter. Its mesial surface is separated from its fellow of the opposite side by the longitudinal bundles situated adjacent to the raphe. The pontile portion of the reticular formation on its ventral surface presents a deep excavation produced by the trapezoid body. Lateral to this excavation are located two nuclear masses of moderate size, forming the supe- rior olive and the lateral reticular nucleus. As the superior cerebellar peduncle sinks deeper into the reticular formation, a lateral reticular prolongation envelops the peduncular bundle over its lateral surface. In the mesencephalic portion of the neuraxis the reticular formation comes to the surface laterally, dorsal to the pontile nucleus and ventral to the colliculi. Somewhat ventral to the point at which the superior peduncle extends into the tegmentum is the pathway of the lateral fdlet. The reticular formation, as it is followed further upward in the stem, becomes more disseminated by the superior cerebellar peduncle. It is finally separated into a mesial and a lateral portion by the appearance within it of the red nucleus. At the level of this nucleus and above this level the reticular formation becomes irregular and forms a matrix in which the mesial genicu- late body develops. In the diencephalon the reticular formation seems to merge with the zona inccrta and other less w ell-defined nuclear masses in the caudal portions of the diencephalon. The Pontile Nuclei The pontile nuclei begin in the more cephalic levels of the medulla oblongata by the appearance of the arciform nuclei which partially surround the pyramidal tract on its ventral, mesial and dorsal aspects. This arciform RECONSTRUCTION OF H\ LOBATES HOOLOCK 451 development becomes more extensive until it completely invests the pyram- idal tract, thus forming the superficial and deep layers of the pontile nucleus. The superficial layer is convex in contour, corresponding to the FIG. 214. DORSAL SURFACE OF GRAY MATTER OF BRAIN STEM, HYLOBATES HOOLOCK. Key to Diagram, inferior collicul., Inferior Colliculus; nucl. of deiters, Nucleus of Deiters; nucl. of GOLL, Nucleus of GoII; ret. form.. Reticular Formation; tub. accousticum, Tuberculum Acusticum. surface configuration of the pons itself, whereas the deep layer is more or less straight or somewhat concave. Dorsally the ^mesial buttress comes into approximate contact with the ventral surface of the reticular formation. The same arrangement is found in the lateral buttress which, although irregular in outline, comes into intimate relation with the ventrolateral angle of the reticular formation. Between these two points of fusion, however, the ventral surface of the reticular formation and the deep layer of the pontile nuclei are hollowed out by the trapezoid body. The scattered collections of gray matter which pass across the tunnel formed within the pontile nuclei are considerably increased. Continuing 452 THE INTERMEDIATE PRIMATES upward, the superficial layer of the pontile nucleus remains in contact later- ally with the ventral surface of the reticular formation of the mesencephalon. As the pontile nucleus approaches the isthmus mesencephah it rapidly decreases in size, leaving only the deep nuclear layer which continues upward to merge directly with the substantia nigra. The Vestibular Complex The vestibuhu- complex hrst comes to view on the surface of the recon- struction at the lower level of the fourth ventricle where it appears as a small, triangular, wedge-shaped mass, the nucleus of Deiters. It is intercalated between the lateral surface of the gray matter of the lloor of the fourth ventricle and the cephalic portion of the nucleus of Burdach. The nucleus is continued forward, presenting a prolongation which overhangs the sub- stantia gelatinosa trigemini. It reaches its maximum at about the mid- ventricular level. At this point its dorsal surface is covered by the beginning development of the triangular nucleus complex which gradually increases in size as the Deitersal nucleus diminishes. It passes upward iii this position between the gray matter of the lloor of the fourth ventricle and the reticular formation but diminishes in size as the upper pontile levels are approached. The nucleus of von Bechterew is continued from the upper portion of the triangular nucleus into the lateral wall of the fourth ventricle. The Cochlear Complex The cochlear complex is relatively large in contrast to that of Cyno- cephalus babuin, which is relatively small. The ventral nucleus of hylobates is well developed, having the same trough-like formation present in the other members of the primate group. The roots of the nerves passing into the nucleus are partially surrounded by the gray matter, leaving the mesial surface free. RECONSTRUCTION OF H\TOBATES HOOLOCK 453 The dorsal cochlear nucleus is situated over the trianguhir nucleus of the vestibular complex and is connected with the ventral cochlear nucleus bv numerous strands of gray matter. The size of the cochlear nucleus is FIG. 215. LATERAL SURFACE OF GRAY ^LATTER OF BRAIN STEM, HYLOBATES HOOLOCK. Key to Diagr.-\m. dorsal cochlear, Dorsal Cochlear Nucleus; inf. coll., Inferior Colliculus; lat. gen. BODY, Lateral Geniculate Body; nucl. of burdach. Nucleus of Burdach; nucl. of deiters, Nucleus of Deiters; nucl. of coll. Nucleus of Goll; pontile. Pontile Nuclei; ret. form.. Reticular Formation; subst. GEL. ROLANDO, Substantia Gelatinosa of Rolando; sup. coll., Superior Colliculus; ventral cochlear. Ventral Cochlear Nucleus. commensurate with the relatively large size of the trapezoid body which produces a marked excavation in the ventral surface of the metencephahc reticular formation, separating this structure from the deep layer of the pontile nucleus. The Sl'bstantla Nigra The substantia nigra is developed as a massive nuclear collection by a specialization from the deep layer of the pontile nucleus. This deep layer 4,-4 THE INTERMEDIATE PRIMATES .supports tin- cntiri' tianswrsi' (.■xtnit ol thr substantia nii^ra, tliickt'iiiufi; at Ijoth the nu'sial and lateral (.■xtreniitics, w lure- thr deep ponlili' la\rr fuses with the diirsal extremities of the mesial and lateral buttresses. It presents a roui^hened xcntral surlaei.' in e()nlai.'t with the desc't'ndin;j; pallio-spmal and palliii-pontile tracts. \U'siall\ it is eiuitinuous with the indillercait intca- peduneular }.::ra\ matter, while laterall\ it presents a hooked extrcMiiitx whleh conu's into elose relation w ith tlu' two 28 by Paul B. Hotbtr, Inc., N. Y. PHE-PAL/EOLITHIC OR, E.OL1THIC ? PLEISTOCENE PLIOCENE ">