GJnntrll ICaro Btfynal ICibrarg QL 933.B32 ne " UnlVers ">' Library „Xf!,? bra 'n as t Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924017493978 THE BRAIN AN ORGAN OF MIND t „ ,, under Surface. (Owen after Cuvier) .... 116 52. „ „ Carp. (Ferrier) 117 53. ,, „ Ray or Skate, upper aspect. (Mivart) 117 64. ,, „ Gar-Pike. (Owen) 118 55. „ „ Whiting. (Solly) n 8 56. Brain and Spinal Cord of the Frog. (Solly) 119 67. Brain of the Cod, under surface. (Owen) 122 58. Brain and Cranial Nerves of Boa Constrictor. (Rymer Jones after Swan) . ]26 69. Brain of Lizard. (Owen) 127 60. ,, the Perch, vertical longitudinal section of. (Mivart) . . . 128 61. ,, Turtle, side view. (Solly) 129 62. ,, Pigeon. (Ferrier) 131 63. Brain and part of Spinal Cord of Chick sixteen days old. (Owen after Anderson) 131 64. „ ,, „ „ twenty days old. (Owen after Anderson) ^ 131 65. Brain of Common Fowl. (Spurzheim) . . 133 66. „ Pigeon, side view. (Mivart) .133 67. „ Sea Gull. (Owen after Anderson) .... 134 68. Brain and Spinal Cord of Kangaroo. (Owen) .... " 2,15 69. Brain of the Horse, outer surface. (S jlly after Leuret) ... ! 256 70. „ Agouti. (Owen) * 95- 71. „ Beaver. (Owen) * 257 72. „ Horse, inner surface. (Solly after Leuret) . * 2'8 7u. „ Rabbit, under surface. (Solly after Leuret) . * 261 74. „ Dolphin, under surface. (Owen after Tiedemann) . 262 75. ,, Borse, upper aspect. (Owen) .... 76. „ Hare, upper aspect (Spurzheim) . 77. „ Porpoise. (Solly) [ ' * il 78. „ Bat, side view. (Solly) .... * 79. Cerebellum of the Cat. (Ferrier) . - . 265 y 0- » n Dog. (Ferrier) ..."."." 'lit 81. Brain of the Squirrel, dissected. (Solly) .... ' ' LIST OF WOODCUTS. (Solly) 100. 101. 102. 103. 104. 105. 1(16. 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 29. Head and Brain of a Squirrel, aide view. Brain of a Chelonian. (Gegenbauer) „ „ Foetal Calf. (Gegenbauer) ,, ,, Cat. (Gegenbauer) ... „ the Dolphin, dissected. (Owen after Tiedemann) . ,, „ Dog, dissected. (Ferrier) . ... ,, ,, Rock Coney. (Owen) .... Left Cerebral Hemisphere of the Horse. (Owen) .... »» „ „ Rhinoceros. (Owen) » ,, ,, Stag. (Owen) » m ,, Giraffe. (Owen) .... Brain of the Rock Coney, side view. (Owen) .... ,, ,, Giraffe, side view. (Owen) Right Cerebral Hemisphere of the Elephant. (Owen) ... Brain of the Cat. (Tiedemann) Dog. (Tiedemann) Coati. (Owen) Cat, side view. (Owen) Fox. (Owen) Dolphin, upper aspect. (Owen after Tiedemann) . Brown Macaque Aye-aye. (Owen) Marmoset. (Owen) Squirrel-Monkey. (Owen) Macaque. (Owen) Gibbon. (Owen) a fifth month Human Fostus. (Owen) the Howler Monkey. (Duncan) Mangabey, upper aspect. (Vogt) ,, „ side view. (Vogt) „ Wanderoo, upper aspect. (Vogt) „ „ ,, side view. (Vogt) „ „ Baboon, upper aspect. (Vrolik after Leuret) . . . . „ „ Chimpanzee. (Vogt after Marshall) . . . . . „ „ a Human Idiot. (Vogt after«»heile) . . . ,, „ Gorilla, side view. (After Bolau and Pansch) . „ „ Orang, base of. (Owen after Tiedemann) .... ,, ,, „ upper aspect. (Duncan) ,, „ Gorilla, inner aspect. (Bolau and Pansch) ,, „ Orang, side view. (Vogt after Gratiolet) Diagrams illustrating the Progressive Changes in Fostal Brain. (Mivart) . Sketches of the Early Form of the Cerebro-Spinal Axis of the Human Embryo. (Sharpey after Tiedemann) Vertical Section of the Brain of a Human Embryo of fourteen weeks. (Sharpey after Reichert) Brain and Spinal Cord of a Fostus of four months. (Sharpey after K51- liker) ... Brain of Human Fcetus of fourth month. (Owen) .... ,, Turtle. (Owen) The outer aspect of the Fcetal Brain at six months. (Sharpey after R. Wagner) The upper aspect of the Fcetal Brain at six months. (Sharpey after R. Wagner) PAGE . 267 268 2 LIU 270 280 2S0 2S1 2S1 282 2S3 283 2S3 2S3 283 284 2S7 289 239 289 289 289 290 291 202 293 293 294 296 297 208 299 300 301 302 334 340 341 342 X LIST OF WOODCUTS. PAGE 130.' The inner aspect of the right half of the Foetal Brain at six months. ^ (Sharpey after Rcicbert) ' " ' ' „.„ 131. Dura Mater with its vessels enveloping the Brain. (After Hirschlelcy . o o 132. Human Cerebrum and Cerebellum, side view. (After Hirsehfeld) . • «» 133. Brain of the Hottentot Venus, side view. (Vogt after Gratiolet) . . ■ ill 134. _ ( „ „ upper aspect. (Vogt after Gratiolet) . . 378 13s! " a Bushwoman, upper aspect. (Heath after Marshall) . . .380 136. " „ lateral aspect. (Heath after Marshall) . . .381 ■137! Eight Cerebral Hemisphere of a Scotchman, outer aspect. (Turner) . . 382 138. Upper aspect of the Brain of a Scotchman. (Turner) . . . 139. Inner Face and Tentorial Surface of the Left Cerebral Hemisphere. (Tur- 140. View of the Orbital Lobule and of the Island of Hell. (Turner) . . 385 141. Brain of Gauss, the celebrated Mathematician and Astronomer, upper aspect. (Sharpey after B. Wagner) ™7 142. Brain of Gauss, side view. (Vogt after E. Wagner) 3S9 143. Brain of a Journalist, front view of the Frontal Lobes 391 141. Under surface of the Human Brain. (Allen Thomson) 397 145. Under surface of Cerebral Peduncles, Pons, and Medulla. (Sappey after Hirsehfeld) 39S 146. Section through the left Occipital Lobe of a Human Brain . . . .399 147. View of Occipital Lobes and of Cerebellum from behind, showing the Occi- pital Groove .... . . 402 148. Posterior diagrammatic view of Dura Mater with great Venous Sinuses. . (Todd) 403 149. Upper surface of the Cerebellum. (Sappey after Hirsehfeld) . . .405 150. Inferior surface of the Cerebellum. (Sappey after Hirsehfeld) . . .406 151. The Lateral Ventricles and their Cornua.with Contiguous Structures. (After Sharpey) 43 ° 182. Third and Fourth Ventricles of the Brain'exposed. (After Sharpey) . . 431 103. Longitudinal Vertical Section through the Left Hemisphere, showing the Lateral Ventricle and its three Cornua. (Sappey after Hirsehfeld) . 432 154. Decussation of the Motor Fibres in the Medulla and Pons. (Broadbont) . 4)4 155. Under surface of Brain, dissected. (Broadbent) 435 156. Central Ganglia of the Brain, together with the Cerebellum and its Supe- rior Peduncles. (Sappey after Hirsehfeld) 437 157. Transverse Section of the Cerebrum, showing course of certain Fibres. (Broadbent) 439 158. Transverse Section through the anterior part of the left Frontal Lobe . . 445 159. Section through the third Frontal Convolution of Man. (Ferrier after Meynert) 446 160. Large Pyramidal Cell with its processes, a so-called 'Giant-Cell.' (Charcot) 447 161. Section of the Involuted Layer of the Hippocampus. (Meynert) . . 450 162. Longitudinal Section through the centre of the Brain, showing the inner face of Left Cerebral Hemisphere. (Sappey after Hirsehfeld) . . . 452 163. Horizontal Section through the Cranium and the Cerebral Hemispheres, showing the ' Centrum ovale.' (Sappey after Vicq d'Azyr) . . . 454 164. Horizontal Section through the Cerebrum at a deeper level, showing the Third Ventricle and its Commissures. (Sappey) 455 165. The Upper Peduncles of the Cerebellum, with the Fourth Ventricle and contiguous parts. (Sappey after Hirsehfeld) 461 166. The Middle Cerebellar Peduncles and Pons, with contiguous parts. (Sappey after Hirsehfeld) 463 LIST OF WOODCUTS. xi "°- PAGE 367. Section of the Grey Matter of the Cerebellum. (Sharpey after Sankey) . 466 168. Part of the Base of the Brain to which the Cranial Nerves are attached. (Ferrier after Allen Thomson) 471 169. Left Pneumogastric Nerre, with Cervical and Thoracic Portions of the Great Sympathetic. (.Jamin after Hirschfeld) 473 170. One of the Sympathetic Ganglia from the right Lateral Cord of the Rabbit. (Owen after KOUiker) 475 171. Transverse section through the Cerebrum of a Dog, showing the posterior portion of the : internal capsule.' (Charcot after Duret) . . 489 172. Left Hemisphere of the Brain of a Monkey. (Ferrier) 5*0 173. Internal aspect of the right Cerebral Hemisphere of a Monkey. (Ferrier) . 532 174. Brain of Monkey, showing a shaded wrea corresponding with so-called Visual Centre in the Cortex of the left Cerebral Hemisphere. (Ferrier) . 533 175. Brain of Monkey, showing a shaded area corresponding with the so-called ' Auditory Centre ' in the Cortex of the right Cerebral Hemisphere. (Ferrier) 534' 176. Brain of Monkey, showing shaded area in Temporal Lobe, the destruction of which is said to cause loss of Smell and loss of Taste. (Ferrier) . 537 177. Internal aspect of the right Hemisphere of the Brain of a Monkey, show- ing a darkly shaded area corresponding with the so-called ' Tactile Centre. ' (Ferrier) 530 178. Groups of Cells in connection with the Anterior Roots of the Spinal Nerves in Spinal Cord of a Sheep. (Flint after Dean) 550 170. Nerve Cell with many branches, from one of Anterior Cornua of a Human Spinal Cord. (Max Schultze) 565 180. Transverse Section of the Brain of a Dog, showing the anterior part of the 'internal capsule.' (Carville and Duret) 566 181. Lateral view of the Brain of a Monkey, showing the boundaries of the so- called ' motor area ' of the right Cerebral Hemisphere. (Ferrier) . . 571 182. Lateral aspect of a Monkey's Brain, showing the relative positions of the so-called 'Motor Centres ' in the left Cerebral Hemisphere. (Ferrier) . 573 183. Upper aspect of Monkey's Brain, showing the relative positions of some of the so-called 'Motor Centres ' in the left Cerebral Hemisphere. (Ferrier) 574 184. Brain of a Woman who suffered from Aphasia, showing the traces of a lesion in the posterior part of the 'third Frontal Convolution.' (Prevost) 676 %* The Writer desires to acknowledge his obligation to various publishers and authors for the right that has been courteously granted him to introduce a number of the illustrations which appear in this volume. Messrs. LoDgmans and Co. have been good enough to supply him with electros from Quain's "Elements of Anatomy," Solly on "The Human Brain," Owen's "Anatomy of Vertebrates," and "The Cyclopaedia of Anatomy and Physiology." Messrs. Macmillan and Co., Messrs. Smith, Elder and Co., the Council of the Anthropological Institute, and various authors, both at home and abroad, have also placed him under similar obligation. THE BRAIN AS AN ORGAN OF MIND. CHAPTEE I. THE USES AND ORIGIN OP A NERVOUS SYSTEM. A. lifeless object makes no appreciable response to ex- ternal impressions. If we touch a rock or a stone, no answering movements follow. Day and night, summer and winter succeed one another, and yet, though inaDi- mate objects undergo imperceptible molecular changes, they yield no active and visible response either to diurnal or to seasonal vicissitudes. It is wholly different, as we know, with the members of the vegetable kingdom existing around and amongst these inanimate things. The seasonal changes shown by them are familiar to all. The putting forth of the leaf, the period of active growth, the bloom of flowers, the shedding of seed, the fading and fall of leaves, are so many manifestations of an internal activity which dis- play themselves with never-failing regularity. ... /' 2 THE USES AND ORIGIN Plants respond, however, to more definite external changes than those dependent upon seasonal mutations. Their flowers open and shut at particular hours of the day, in accordance with the varying amounts of heat and sunlight falling upon them. They grow more rapidly by night than by day, though as a general rule the activity of their internal changes is closely related to the degree of heat to which they are subjected. Again, whilst they generally grow best in directions where they meet with most air and light (not because of the latter agency, but rather on account of the heat which goes with it), many of them will, in the course of a few days or within shorter periods, bend very perceptibly, so as to bring them- selves more under the influence of this latter agent. Amongst some representatives of plant life, the corre- spondence between internal and external changes is undoubtedly less obvious than in many of the instances just referred to. Thus is it with the black or grey film of Lichen which marks as with a patch of paint the damp surface of some weather-beaten rock. Yet, watch it care- fully from time to time, and, even in this lowly form of life, responsive though sluggish changes may be detected, sufficient to remove it from the category of inanimate things to which the rock itself belongs. The comparative complexity of life exhibited by mem- bers of the vegetable kingdom is, however, small; and for this two principal causes may be cited. (1.) As a rule — to which there are only few though interesting exceptions, to be mentioned further on — they subsist on inorganic materials, deriving their food from the gaseous or dissolved mineral elements existing in the air or water with which their surfaces are bathed. In their natural or healthy state plants decompose carbonic acid, fixing its carbon and setting free its oxygen. They Chap. I.] OF A NERVOUS SYSTEM. 3 decompose water, so as to retain its hydrogen; whilst they also abstract nitrogen either directly from the atmosphere, or indirectly from the nitrate of ammonia formed therein and brought to the soil in refreshing showers. This work of decomposition, under the in- fluence of light and heat, goes hand in hand with one of an opposite kind, resulting in the elaboration of those organic and living compounds which enter into the com- position of vegetal tissues. (2.) Then again, as a rule, plants exhibit no inherent powers, of movement other than those connected with their growth. The movements of the Sunflower and its allies are exceptional ; and there are very few plants which more or less immediately respond to a touch by a movement, in the way that the Sensitive-plant or the Venus fly-trap is known to do. To this subject, however, and to the causes of such motions in plants, it will be necessary to return. For the present it is of importance to recollect that plants do not move at all in search of food. The comparative simplicity of the life-processes of plants is in the main due to these two peculiarities. They are also, perhaps, the most fundamental attributes of plants as distinguished from animals. This subject is well worthy of our brief attention, since if its considera- tion should lead us to anything like a correct apprecia- tion of the mode in which some of the simplest vegetal organisms differ from some of the simplest animal organ- isms, this insight may — apart from its own intrinsic interest — prove of the highest importance in regard to our present inquiry. It may enable us, in a measure, to comprehend why a Nervous System is absent from Plants, and why it comes into existence in Animals. It may help us further to comprehend why this nerve tissue gradually 4 THE USES AND ORIGIN increases in complexity in ascending to more and more highly organized types of animal life. ***** In the present day it is commonly admitted that many of the lowest forms of life cannot positively be assigned either to the Vegetal or to the Animal Kingdom. Their characters as living things are not sufficiently specific or constant to enable us to say that they belong to one king- dom rather than to the other. In some of their life-phases such organisms seem to display the attributes of vegetal life, whilst in others those of animal life are bo less pronounced. They constitute, in fact, an underlying indeterminate plexus of changeable and more or less related forms, appearing now as animals, now as plants — and they "may give rise to descendants, or to a series of them, totally unlike themselves and their own immediate • ancestors. Amongst such forms variability reigns supreme. These creatures of circumstance, which become metamor- phosed in a most striking and apparently irregular manner, the writer has proposed* to include under the general designation, of ' ephemeromorphs.' True ' species,' in the strict acceptation of the term, are not to be found amongst them. Starting from this neutral and changeable ground^ however, forms of life appear "that habitually reproduce' their like, either directly or indirectly ; some of which are unmistakably members of the vegetal kingdom, whilst 1 others are no less characteristic representatives of the animal world. Owing to the frequency and rapidity with which transi- tions from vegetal to animal, or from animal to vegetal,' modes of growth have been observed to occur amongst* * " Beginnings of Life," 1872, vol. ii. pp. 559, 571. Chap. I.] OF A NERVOUS SYSTEM. 5 ' ephemeromorphs,' we are compelled to believe that such passages from the one mode of molecular composition and activity to the other, may be determined without any great difficulty by internal chemico-nutritive changes, whether these latter have or have not been in part induced by external influences. Such transitions from vegetal to animal modes of life, or the reverse, are regarded by the writer as comparable with some well-known metamor- phoses of form and nature amongst simpler kinds of matter.* It is certain, as Prof. Graham showed, that one and the same saline substance may exist with its molecules now in the crystalloid and now in the colloidal mode of aggre- gation, according to the different influences under which it has been produced, or to which it has been afterwards subjected. This, for instance, is the case with silica, with the sesquioxides of chromium and iron, and with other mineral substances. ■ On the contrary, it is also known that certain typical colloids may, under some conditions, be converted into crystalloids. Again, transformations of a similar order, though of different degrees of complexity, are met with amongst saline and elementary substances, when these assume different ' allotropic' conditions. Well known illustrations of this kind of metamorphosis are met with in the dif- ferent interchangeable states of carbon, of phosphorus, and of sulphur. The passage from one to the other allotropic state amongst these elementary substances may take place either with difficulty or with comparative readi- ness, though the ease and celerity with which analogous transformations are effected in the case of certain saline substances is still more interesting in its bearing upon the transformations of simple living units. No better instance # "Beginnings of Life," vol. ii. pp. 38, 55, 82. 6 THE USES AND ORIGIN can be selected than the case of mercuric iodide, a sub- stance well known to exist in two totally distinct crystalline forms which differ also in colour. Watts says—" The red crystals turn yellow when heated, and resume their red tint on cooling. The yellow crystals obtained by subli- mation retain their colour when cooled ; but, on the slightest rubbing or stirring with a pointed instrument, the part which is touched turns scarlet, and this change of colour extends with a slight motion, as if the mass were alive, throughout the whole group of crystals as far as they adhere together." Thus, it would appear that the phenomena of allo- tropism and dimorphism, and the fluxes from the crys- talloid to the colloid state and the reverse, are strictly comparable with the transformations from the vegetal to the animal, and from the animal to the vegetal, modes of growth so common amongst ' ephemeromorphs.' The members of the animal and the vegetal worlds may be regarded as self- multiplying and progressively varying products, resulting from developments which are con- tinually taking origin from what may be regarded as different allotropic states of Living Matter. ***** Of the organisms appearing as constituents of the ephemeromorphic assemblage of vital forms, Amoebae may perhaps be cited as the simplest types of unquestionably animal life; just as some of the smallest Confervae or Moulds are amongst the simplest known forms of the vegetal type or mode of growth. Confervas or Moulds, after the fashion of plants generally, feed upon the inorganic elements existing around them either in water or in air ; Amoebae, after the manner of animals generally, feed upon matter which is either living or which has once lived. This difference between plants Chap. I.] OF A NERVOUS SYSTEM. 7 and animals in their mode of nutrition is so fundamental, so much depends upon it, that we shall find it worth our while to inquire a little more particularly how the depar- ture from the more primordial mode of nutrition, met with amongst animals, can be accounted for. If we examine some simple vegetal unit through a microscope — the germ from which a Conferva grows, for instance — we find it exhibiting no distinct changes of form; and, if unprovided with one or more vibratile filaments, it also shows no movements from place to place. It manifests no tendency to seize, nor has it any means of taking, solid food. As soon, therefore, as the changes incident upon the active growth of such a unit have ceased, the outer portion of its substance remains constantly in contact with the medium in which it lives, and shortly becomes modified. It condenses and is otherwise changed into an investing envelope, which commonly goes by the name of a ' cell- wall.' In the Amoeba, on the other hand, we have an organism which, like the fabled Proteus, is for ever changing its form. It is composed of a clear jelly-like material, endowed with a superabundance of that intrinsic activity character- istic of animal life generally. Those internal molecular movements, indeed, which are inferred to occur to a marked extent in all living matter, seem to take place in it in a pre-eminent degree. Its whole substance shows a mobility of the most striking kind. It continually moves through the water or over surfaces, by alternate projec- tions and retractions of its active body-substanoe. Two consequences flow from this high inherent activity of the Amoeba. In the first place, owing to the creature's rapid alterations in shape, no one portion of its substance is continuously exposed to contact with its medium, and, as a consequence, that first step in organ- 8 THE USES AND ORIGIN ization, above referred to in connection with the Conferva unit, does not take place. So long as the Amoeba remains in full vigour and constantly changes its shape, a cell- wall cannot be formed. Secondly, during the movements of the organism from place to place, portions of its projected body- substance,; come into contact with others-more minute organisms/' such as unicellular algse and'diatoms, or with small portions of organic refuse, and these are oftentimes drawn into its interior when the projections with which they are in contact are retracted. The activity of the Amoeba and its allies is excited by contact with matter of this and of other kinds, though inorganic fragments are subsequently rejected. The surplus inherent activity of the Amoeba being,: therefore, one of the immediately determining causes of its absorbing solid food, may also be regarded as one of the causes of its departure from the more elementary mode of nutrition met with amongst the simpler or less vitalized organisms from which it has been derived. A word, however, is required as to the ' selective '. power which the Amoeba seems to manifest. A magnet ' selects ' minute fragments of iron or steel from any heap of heterogeneous particles containing such matter with which it may be brought into contact. Certain plants, also, such as the Sun-dew and the Venus fly-trap, ' select,' and seem capable of discriminating, nitrogenous from other substances with which they come into contact. The leaves of these plants, however, possess no nervous tissues of any kind ; so that the fact that they seem to ' select ' nitrogenous substances merely implies the existence of some relation between the molecular com- position and activities of the leaves and those of suck substances— by virtue of which mutual contact keeps- Chip. IJ OF A NER.VOTJS SYSTEM. 9 up a state of excitation in the tissues of the plant. Simi- larly, there must be some definite molecular relation between a magnet and pieces of iron or steel, leading to their ' selection ' whenever they come within certain degrees of proximity. In the latter case we have, unquestionably, to do with problems of molecular physics ; and in the case of the affinity which seems to exist between the nerveless Amoeba ami the organic fragments or minute living things which it absords as food, we probably have to do with an allied problem. There may be differences of degree, but none of kind ; all must be included as problems of mole- cular physics. At any rate, be the cause what it may, the coming into contact of a fragment of organic matter with projected portions of the substance of an Amoeba is followed by the closure of this mobile substance round it. The organic mass is gradually drawn into the interior of our Proteus, where, after being thus appropriated, it slowly disappears by a rudimentary process of ' digestion.' After feeding, in this way, and assimilating the organic matter taken into its interior, the Amoeba rapidly increases in size, and per- haps still continues its active movements. Or, as happens at other times, its movements may cease : the creature grows sluggish from over-feeding, and then, as a consequence of its motionless condition, its outer layer soon becomes differentiated into a cyst-wall. Simple as this mode of nutrition may appear to those who are familiar with it, its initiation in the Amoeba is "followed by consequences of the most profound importance. The assimilation, after such a fashion, of already elaborated organic matter is strongly calculated to increase that high degree of vitality which originally led the organism to take in solid food. This mode of nutrition, in fact, entails a liberation within the organism of much of the molecular 10 THE USES AND ORIGIN motion which was potential in its food ; and molecular motion thus liberated becomes a cause of further active movements in the organism — provided its constitution is, at the time, able to accommodate itself to such powerful internal causes of change. Where it is not in such a condition the assimilation of much solid food is followed by an interval of apparent rest, during which a thorough re- adjustment of the molecular constitution of the organisa occurs. In the latter case the encysted mass of living matter may after a time divide into a swarm of smaller though most active Monads. Or else traces of higher organization may reveal themselves in the encysted mass as a whole — so that the previous Amoeba shortly emerges from its cyst as an active creature of larger size and higher . type. Ciliated Infusoria, Rotifers, and other forms of animal life of different degrees of complexity, may take origin in such encysted masses of protoplasm, forming the resting I stages of previously active Amoebae.* The extent to which this occurs, however, and the real significance of the pro- cesses, are subjects upon which all naturalists are far from being of the same opinion. Be the interpretation, however, what it may, the fact remains that Ciliated Infusoria, Rotifers, and other organisms may be seen to develop directly from encysted matrices of vegetal or of Amoeboid origin. Nay more, any forms of the animal series thus initiated exhibit, in an even more marked degree, the fundamental properties of the Amoeba — the power, that is, of executing well-' marked independent movements and of feeding upon solid food. And as channels for the reception of such food become more and more formed, we may find the or<*an- * " Beginnings of Life,'' vol. ii., chaps, xxi. and xxii. Chap. I.] OF A NERVOUS SYSTEM. 11 ism's increasing powers of movement more definitely ministering to this capacity. Its motions, instead of being wholly at random, show more and more signs of purposiveness — they become, to an increasing degree, subservient to the capture of food. Look, then, at the differences already indicated both in grade of organization and mode of life, by virtue of which even the simpler kinds of animals become strikingly un- like vegetal organisms. The unit of vegetal life before it has attained any great size exhibits, by reason of its lower degree of inherent activity, a tendency to undergo the first stage of organiza- tion, that is, to develop a cell-wall which imprisons the more active living matter within and causes it to under- go certain secondary modifications. Before this occurs, however, the vegetal unit, if it does not divide, may seg- ment or bud ; the bud grows into a unit similar to its parent, and this in its turn may also segment or bud. By repetition of such a process motionless cellular organisms are produced, which, though presenting almost endless differences in form and in the ultimate arrangement of their units, are in the main composed of mere aggrega- tions of similar parts — these being not solid units of pro- toplasm, but mostly vesicular elements, in which a cavity filled with fluid contents is bounded by a layer of pro- toplasm and outside this by an inert cell-wall. "We may have, in the more simple combinations, long strings of such elements forming cellular filaments, as in the Con- ferva and other thread-like algffi ; or we may have flat cellular expansions, such as exist and brighten many a rock pool, in the rich green fronds of Ulva. Organisms like this present us with life changes of extreme simplicity. If they move it is because they are swayed to and fro by 12 THE USES AND ORIGIN the elements. They require not to seek their food, siucej the inorganic materials and simple compounds sufficing for their nutrition habitually exist around and in contact with them. On the other hand, in animal organisms next above the Amoeba — such as the various forms of Ciliated Infusoria and Eotifers — well-marked powers of locomotion are dis- played, and we have to do with creatures which; if they do not ' seek,' at all events seize and swallow solid food. We find in the latter of these forms of pond life, distinct channels through which food is taken in and absorbed; we have glandular structures of various kinds ; we have organs of locomotion, internal and external. Thus, though we have not yet been able to detect with any certainty even the rudiments of a nervous system, the grade of vitality of these animal organisms must be at once ad- mitted to be notably higher than that of plants. The degree of correspondence existing between such creatures and their surroundings is already much more varied than that existing between vegetal organisms and their medium ; and this kind of complexity of relation steadily increases in animal organisms only a little higher than those to which we have already referred. Their responses, moreover, to the varied external influences to which they have become amenable are effected by movements direct, rapid, and comparatively complex — the motions them- selves being brought about by muscular contractions, partly simultaneous and partly successive,"' and mostly occurring in groups which are definitely related to different external impressions. Eefei'ence to a few of their common muscular actions will illustrate this. Conjoined movements of the head and its appendages are needed for the seizure of fragments serving as food ; and these motions must be followed by certain others in Chap. I.] OF A NERVOUS SYSTEM. 13 the upper parts of the alimentary canal before the morsel that has been captured can be swallowed. A series of movements of this kind may occur in response to some touch upon the external surface of such an organism; and, after a rudimentary sense of sight has once been established, impressions produced by an object not in con- tact may lead to complex locomotions in pursuit, followed by others for capture, and others again for the swallowing of food or prey. The sight of a different object may, how- ever, lead to movements of flight rather than to those of pursuit. The organism may hasten away, to avoid a pos- sible attack — since in the past this kind of experience may often have followed the appearance of a similar object. Again, the process of digestion in such animal organ- isms is aided by certain accessory glandular organs, whose activity is stimulated by the contact of food with different portions of the alimentary canal. Absorption of the pro- ducts of digestion is either simple and direct from the alimentary canal into some general body-cavity whose fluid comes into contact with most of the organs ; or it takes place through definite channels, and empties itself into a circulatory system proper in which blood is pro- pelled throughout the body by means of a contractile heart containing one or more chambers. Glands also exist. whose office it is to modify the constitution of the blood. There may be either gills or lungs to renovate it by contact with oxygen and to get rid of effete products though in this latter function the organs of respiration are powerfully aided by renal and other emunctories. All these are functions having to do with the preserva- tion of the life of the individual, though another set of activities also come into play in animals that have attained a grade of organization of the kind to which we are refer- ring. These new activities pertain to the sexual function — 2 14 THE USES AND ORIGTN leading to the union of male and female, the begetting of young, and the consequent perpetuation of the species. Thus it may be dimly gathered how complex the relation of the animal organism to its environment soon becomes, and also what an amount of interdependence is established between the actions of the several parts or organs of the animal economy. The contrast between the animal and the vegetal organism in both these respects becomes most marked. It is during the establishment of the complex relations above indicated between an animal and its environment, and between the several parts or organs of an animal, that nervous tissues first take origin, develop, and subsequently increase in complexity. How and why this should be may become a little more plain after a brief consideration of the nature of simple nervous functions and structures, and after some reference to the manner in which these increase in complexity, not only in the individual but (by virtue of the principles of heredity and ' natural selection ') during the life of that succession of individuals consti- tuting the race or ' species ' to which the organism belongs. From what has been already said it will be seen that the preliminary conditions necessary for the initiation of a Nervous System are, first, the existence of a living sub- stance whose excitability is high ; and, secondly, the pos- session by such substance of a well-marked contractile power. This statement carries with it the implication that the living matter in which a nervous tissue is to develop must not, in the first place, subdivide itself very minutely into separate units ; or, at all events, that it must not become differentiated into cells with fully de- veloped cell-walls. Much of the substance of the organism, Chap. L] 01' A NERVOUS SYSTEM. 15 if not comparatively structureless, must be composed of plastic units of living matter, not marked off from one another by definite and lowly vitalized cell-walls. The vegetal mode of growth is, therefore, as already indicated, precisely of such a kind as to unfit it in an eminent degree for developing any notable power of appre- ciating varied external impressions and yielding immediate and discriminative responses thereto. The nearest approach to such powers and actions in the vegetal world is met with amongst the so-called "Insec- tivorous Plants," upon whose peculiarities Mr. Darwin has lately given us much information. If we dwell for a few moments upon these highest manifestions of the kind known to occur amongst plants, the reader may the better comprehend the great gulf which separates the vegetal from the animal world in regard to their respective powers of discrimination and motor response. When the three hair-like projections on the upper surface of the leaf of the Venus fly-trap are touched, they almost instantly communicate a stimulus to the cells on each side of the mid-rib, whereby some change is induced in them, and the two halves of the leaf are made to approach one another. The nature of the change has not yet been fully ascertained, though the evidence adduced by Darwin seems to show that it is, at least in part, due to the contractility of the cells above mentioned. A simi- lar influence appears to be transmitted from the glands that tip the hair-like projections fringing the leaves of the Sun-dew, to certain' cells near the base of these bodies, whereby motion is produced. In this latter plant, a very appreciable interval occurs between the time of irritation and the answering movement. Mr. Darwin has never known the interval to be less than ten seconds, though even in the one case in which it took place so rapidly as 16 THE USES AND ORIGIN this, two and a half ^minutes were needed for the hair, or 'tentacle' as it has been termed, to more through an angle of 45°. As a rule, the rate of movement is even much slower. The stimulus which provokes movement may come to the base of a marginal tentacle either from its own sensitive tip, or by radiation from some of the shorter hair-like projections near the centre of the leaf whenever their terminal glands have been excited by con- tact with a foreign body. The transmission of a stimulus from one of the glands tipping a marginal tentacle in the Sun-dew, to certain cells near its base, though consisting only of molecular move- ments, beeomes in a manner visible, owing to the fact that during its passage the protoplasm within the cells of the tentacle undergoes certain obvious changes. Protoplasm previously in a state of uniform diffusion throughout each cell, is caused to aggregate into masses of different size and shape as the invisible wave of molecular movement passes through it. This ' aggregation ' is therefore a visible sign marking the passage of the invisible stimulus. And as Darwin points out, the phenomenon is analogous in certain respects to that which occurs when, after stimulus, an invisible molecular change traverses a nerve in an animal organism.* The same observer has discovered that the chief delay in the transmission of the stimulus along the tentacle of the Sun-dew is caused by its having to traverse tho successive cell-walls which lie across its path. At each barrier of this kind an appreciable retardation occurs, as is evidenced by the interval that elapses between the com- pleted aggregation in one cell and the commencement of the process in the protoplasm of that which stands next "Insectivorous Plants." 1875, p. 63. Chap. I.] OF A NERVOUS SYSTEM. 17 along the line traversed by the stimulus. It has been found that a stimulus radiated from the centre traverses the leaf in a longitudinal more rapidly than it does in a transverse direction — a circumstance apparently to be explained by the fact that, in the longitudinal direction, owing to the elongated shape and disposition of the cells, the stimulus has to pass through a smaller number of obstructive cell-walls. The irritability and answering movements just described are, however, altogether exceptional events in plant life ; more especially if we refer, as at present, only to cases where there is reason to suppose it possible that the move- ments are in part due to contractility, rather than to mere disturbance of tension in some of the cells — movements of the latter order being not unfrequent in stamens, seed- pods, or other parts of plants. Yet even in these plants, where contractility appears to exist to a more marked extent than in any other known members of the vegetal kingdom, there is no development of a specialized con- tractile tissue, and still less is there an appearance of any nerve fibres along which the molecular disturbance consti- tuting the stimulus may be transmitted. The obstacles opposing the passage of the stimulus, to which reference has been made, would indeed also tend to impede the formation of a special tissue along the line of discharge. In Animal Organisms, however, we have a highly im- pressible and very active variety of protoplasm, the units of which, particularly as met with in the lowest forms of animal life, do not go on to the formation of a distinct cell-wall, and are for the most part aggregated into mere semi-fluid or gelatinous tissues capable of transmitting vibrations in different directions with the greatest ease. This is .the case, for instance, in Medusae, which are perhaps the lowest animals in whom a nervous system is 18 THE USES AND ORIGIN met with. The recent investigations of Gr. J. Bomanes* in regard to this subject are particularly interesting^ because they seem to show such a system actually in pro- cess of evolution. The contractions of the bell-shaped swimming disc of common Medusae must be familiar to most dwellers by the seaside, and we now learn that this part is lined internally by a very thin layer of highly contractile protoplasm, not yet presenting the definite characters of muscle. We learn also that this contractile layer is permeated by a network of incipient nerve fibres, in connection with rudimentary ganglia, near its free margin. The degree of irritability of these altogether elementary animal tissues, and the rate at which stimuli traverse them, is alike remarkable, and far ahead of what may be met with in the plants in which analogous changes are most marked, — such as the Yenus fly-trap or the Sun-dew. According to Bomanes the molecular discharges issuing from a single rudimentary ganglion, in the swimming bell of a large Aurelia weighing thirty pounds, were sufficient to incite vigorous contractions throughout the whole mass — though this mass weighed 30,000,000 times as much as the ganglion itself. When all the ganglia have been removed, he has found that a wave of contraction, starting from any part of the disc which is touched, will travel equally in all directions at the rate of a foot and a half per second, so that the contraction of the whole bell is practically simultaneous — and therefore, in marked con- trast with the very slow bending of the irritated tentacle of a Sun-dew. Thus the preliminary conditions already asserted to be necessary for the initiation of a nervous system are here present to a well-marked degree, and in notable contrast * " Phil. Trans.," Part I., 1876. Chap. I.] OF A NERVOUS SYSTEM. 19 to what obtains amongst the members of the vegetable world. As to the mode by which, in Medusae or other low types of animal life, the first rudiments of a nervous system are evolved, only a few brief statements can be made. On this subject inferences have only too often to take the place of positive knowledge. Fortunately, however, the data on which such inferences may be based are now fairly well established, thanks more especially to the writings of Herbert Spencer* — whose speculations on this subject have been to some extent confirmed by the recent investi- gations of Romanes and Eimer. In the lower forms of animal life, we have to do with a body substance composed, as already stated, almost wholly of undifferentiated protoplasm. This substance, if not ' sensitive ' in the strict sense of the term, is highly impressible— or capable of receiving a stimulus — and is also highly contractile. But neither the impressi- bility nor the contractility of the protoplasm in lower forms of animal life is localized — both properties are, so far as they exist, uniformly possessed by all parts of the organism. In some of the larger Ciliated Infusoria, in Gregarinse, and in the hydroid Polyps, distinct rudimen- tary ' muscles ' become differentiated, and such tissues are, moreover, now known to exist in many other organisms in which no traces of a nervous system are to be found. Muscular tissue, therefore, makes its appearance before nervous tissue, and it becomes developed in those situa- tions where the protoplasm is stimulated to undergo frequent contractions. It is, in fact, one of the most fundamental truths in biology that the performance of functions, or, in other * " Principles of Psychology," vol. ii. p. 69. 20 THE USES AND ORIGIN ■words, the occurrence of actions of any kind in living matter, tends to occasion structural changes therein*) Such a fact is implied in the common statement that living matter is an organizable matter. "We suppose nothing unusual, therefore, when we imagine that frequently recurring contractions in any one portion of living protoplasm will almost certainly lead to a structural" change therein. And, further, we are warranted in supposing that such structural change will be of a kind to favour the occurrence of the actions by which it has itself been produced — that is, that the modified protoplasm will be more highly contractile than the original protoplasm from which it has been produced. But what, it may be asked, is the cause of these locally recurring contractions, the occurrence of which is supposed^ eventually to lead to the production of muscular tissue ? Contraction so invariably follows upon stimulation, that we may safely say the cause in question can be no other than the incidence of certain stimulations — and we pro- bably shall not be very far wrong if we suppose that these result from, or take their origin in, shocks or other physical-; impressions upon definite though related parts of the external surface of the organism. Its form or its mode of progression by cilia may lead it to come into contact with external objects most frequently by some particular.; part of its surface, and such local shocks produce waves of molecular movement, which pass more especially in some one or more directions and act as stimuli. It is pretty certain that impressions or shocks made upon protoplasm, or even the incidence of physical agents such as light or heat, liberate molecular movements, therein, and that these molecular movements may be transmitted from their point of origin through it in all directions. Yet it occasionally happens, owin« to the Chap. I.] OP A NERVOUS SYSTEM. 21 shape of the part struck, or owing to the fact that an impression made upon one region — say a tentacle— is usually followed pretty quickly by a second impression made by the same moving object upon another surface region, that an impression or stimulus comes, as Herbert Spencer points out, habitually to traverse a certain path. Much of the molecular motion consequent upon the ' stim- ulus ' is drafted along this path. This being so, the stim- ulus necessarily tends to excite contractions in particular parts, and thus leads to the differentiation of the pro- toplasm of such parts into the more or less definite Muscular Tissue found in some of the lowest animal organisms. This, however, is not all. The localization of the path of the stimulus leads to structural results of another kind. Whenever external impressions produce molecular movements which traverse with frequency some definite path, the transference of such movements is made easier by each repetition, and there is a tendency to the initia- tion of a structural change along this path. Just as the frequent repetition of contractions in certain parts of the protoplasm leads to the production of distinct muscular tissues, so the frequent passage of a wave of molecular movement along a definite track through protoplasm or through juxtaposed plastides, leads to the differentiation of the protoplasm thus acted upon. At first the actual structural change may be unrecognizable, although a ' line of discharge ' may have become established along which impressions are habitually transmitted with ease, as seems to be the case with the majority of Medusas. Ultimately, however, by the constant repetition of such a process, we should have the gradual formation of an actual ' Nerve Fibre' — this being a tissue element whose special use and duty is to transmit molecular movement, and which may 22 THE USES AND ORIGIN be seen in its earliest form as a barely recognizable structure in Sarsia.* From all this it would appear that the primitive ' nerve fibre ' is a structure serving to connect impressions made upon the exterior of the organism with certain responsive muscular contractions quickly following thereupon. This is perfectly true, though only part of the truth. The path taken by stimuli from impressible surfaces to muscles is not generally the shortest and most direct route. In the great majority of organ- isms these paths are more or less bent upon themselves. Those for ingoing impressions may run nearly parallel with one another towards some central situation ; and thence they may be distributed to muscles in various parts of the body — some of these being perhaps not very distant from the surface stimulated. In the latter case the track of the stimulus wave is found to be bent at an acute angle, or ' reflected.' At the turning point or ' nerve centre,' whence impres- sions are distributed outwards in various directions to muscles, what are called ' Nerve Cells ' become developed. * Since the above was written and in type the observations of Schafer (Proceed, of Roy. Soc, January, 1878), and of O. and R. Hertwig, have revealed the existence of distinct nerve tissues in several species of Medusas. Fig. 1. Different kinds of Nerve Cells, fied about 350 diameters.) (Magni- Cn»p. I.] OF A NERVOUS SYSTEM. 23 These bodies are interposed so as to constitute part of the actual path of the stimulus wave, and accordingly, they may be, in effect, junctions for ingoing impressions or dividing stations for out-going impressions. The matter composing them seems to be endowed with extreme mole- cular mobility. It is owing to the multitudinous com- binations of these bodies with one another, and with ingoing and outgoing fibres, in modes which will be sketched in the next chapter, that the complex work of the nervous system is enabled to be carried on. Nerve tissue, in the lower forms of animal life, is essentially subservient to the bringing about of move- ments in more or less immediate response to external shocks or other localized impressions, or of movements and glandular activity as a result of impressions upon internal surfaces. These various movements gradually become more definitely related and appropriate as responses, in proportion as the organism becomes better able to discriminate the differences between the several kinds of impressions made upon different parts of its surface. Even amongst Medusae definite responses to stimuli are occasionally met with. Thus in the hemispherical Tiaropsis, from the inside of which hangs a long funnel- like body or polypite, this structuie, as Eomanes says, is found to be capable of " localizing with the utmost pre- cision any point of stimulation situated in the bell. For instance, if the bell be pricked with a needle at any point, the polypite immediately moves over and touches that point. ... If immediately afterwards any other part of the bell be pricked, the polypite moves over to that part, and so on." From this it may be concluded " that all parts of the bell must be pervaded by lines of discharge, every one of which is capable of conveying a 24 THE OSES AND OEtGIN separate stimulus to the polypite, and so of enabling the polypite always to determine which of the whole multi- tude is being stimulated. . . . It is no doubt a benefit to this Medusa that its polypite is able to localize a seat of stimulation in the bell; for the end of the polypite is provided with a stinging apparatus, and is, besides, the mouth of the animal. Consequently, when any living object touches the bell — whether it be an enemy or a creature serving as prey — it must alike he an advantage to the Medusa that its polypite is able to move over quickly to the right spot, in the one case to sting away the enemy, and in the other to capture the prey."* It is, in all probability, the delicate impressions pro- duced by contact of the sea- water with the surface of the organism, acting through the intermediation of the rudi- mentary ganglia near the edge of the swimming-bell, which tend to incite its apparently * spontaneous ' movements. At all events, when these little bodies are removed the habitual rhythmical contractions of the swimming-bell cease, and a single stimulation of any portion of the bell is then followed by a single contraction. The contrast between the behaviour of such an animal and one which is uninjured, is very striking.! Multiply the kind of correlation above typified, and it may be seen that as organisms, or their descendants, increase in their ability to discriminate different impres- sions made upon them from without, so will there grow up muscular responses suitable to each. And the struc- tural modifications, or ' tissues,' through the intervention of which any of these impressions, discriminations, and responses are rendered possible, are no more isolated from others which the creature is capable of receiving or making, * " Nature," vol. xvi. p. 290. t Loc. Qut 800 diameters. '(Max Schultze.) Chap. II.] A NERVOUS SYSTEM. 37 cesses,' which are either much branched (fig. 12) (ramifying processes) or simple. It is by means of these different kinds of processes that nerve cells are united to the central extremities of the nerve fibres and to one another. It is worthy of note that the substance, both of the nerve cell and of its processes, when examined under high magnifying powers can often be seen to be dis- tinctly fibrillated in the same manner as the ' axis band ' of a nerve fibre, with which, or with the ramifications of which, some o" these processes are continuous. If the fibrillations of the axis band, and of the nerve process into which it may be continued, correspond with functionally if not structurally, distinct fibrils — that is, with separate paths for stimulus waves — so, in all probability, the fibrillations of the nerve cells will indicate as many distinct paths of stimulus waves through them in different directions. The appearances presented by the cells are quite consistent with this view (fig. 8). Fibrillations, for instance, can be seen passing from one nerve process in a carved direction through the body of the cell and into another process ; whilst others in the same process can be followed through the cell in quite different directions. There is no difficulty in supposing that many nerve currents may pass through one of these compound nerve fibres, just as many electric currents might pass simultaneously through a single telegraphic or telephonic wire. These fibrillations of the nerve cell are probably sequen- tial to, and gradually differentiated in the course of, its functional activity. It is not unreasonable to expect that there would be a gradual marking out of the paths of habitual nerve currents, through the previously structure- less though slightly granular substance of the nerve cell, during their passage from fibre to cell and from one of these bodies to another. 3 38 THE STRUCTURE OF In accordance with this view, we should not expect to find in the majority of ganglion cells terminations or origins of such fibrils — whether in the nucleus or free in the body of the cell. If the fibrillations are the structural correla- tives of nerve currents, they should be generally as con- tinuous and unbroken as the latter, and just as devious, winding and irregular in their path. We should scarcely look for free ends or beginnings to such fibrils elsewhere than at the periphery. And if the semblance of free ends are ever recognizable within the body of the cell, it will probably be in young cells in which the functional (and therefore the structural) current lines have not yet been sufficiently developed by constant repetitions. Much obscurity, however, still reigns in regard to all these matters. We do not, indeed, know definitely how far this kind of fibrillation of the nerve cells is general, and whether there may not be whole groups of them in which no such arrangement exists. It is quite conceivable that in some nerve centres, where ' spon- taneity ' of action appears to prevail (or, in other words, whence widespread and sudden irradiations of motor stimuli may emanate on slight provocation), we might have a different kind of action altogether. The nerve cells of such centres may approach nearer to H. Spencer's ideal, and be true ' libero-motor ' elements. The Neuroglia, or intermediate substance, exists most abundantly in the larger nerve centres, such as the Brain and Spinal Cord. It has been most commonly regarded as a comparatively insignificant connective tissue, though some few physiologists have always - been willing, and even anxious, that it should be credited with higher developmental and functional capacities. It is composed in part of minute corpuscles or cells, Chip. II.] A NERVOUS SYSTEM. 39 united to one another by means of a network of slender ramifying fibrils (fig. 9), and in part of an interspersed homo- geneous or simply granular basis substance. It bas been long known to contain some small branched corpuscles, almost indis- tinguishable from young nerve cells ; and of late the much branched processes of many fully developed nerve cells have also been thought to have a structural continuity with this minute net- work of the neuroglia. If these i , • . . ,. Fig. 9. — Portion of Neuroglia ooservations are correct, portions from the Spinal cord, open of the 'intermediate substance' meshea < aa seen with 8ma11 . nuclei or cells at intervals, WOuld Often Constitute part Of the but at two places close lamelli- circuits traversed by nerve currents ^TSTsSX in their passage through the meters - centres. This intermediate tissue is, in short, the probable matrix wherein and from which new nerve fibres and new nerve cells are evolved in animals, of whatsoever kind or degree of organization, during their advance in reflex, in instinctive, or in intellectual acquirements. Some such process must take place, pari passu with the acquisition of new know- ledge and powers, of all kinds and howsoever acquired : whether it comes, as in lower animals, from mere intercourse with natural phenomena ; or, as amongst ourselves, from similar means, supplemented by individual application in the mastery of educational or professional pursuits and of all kinds of handiwork ; or whether the new knowledge and powers come to us as a result of that more general edu- cation or ' experience ' which is gained by daily intercourse with the pleasures, troubles, turmoils, and exertions in- separable from social life. The acquirement of new powers 40 THE STRUCTURE OP or accomplishments must correspond either with more or less alteration of old, or with the development of new structures, in one or more of the various nerve centres. The Structural Relations of Nerve Cells with Nerve Fibres, and with one another. Nerve cells are supposed to communicate with nerve fibres and with one another in the following modes : — 1. The nerve cell occurs as a round or elongated swell- ing in the course of a nerve fibre, as may be seen in figs. 10 and 11. Here an undivided nerve fibre swells more or less abruptly into the nerve cell, and similarly emerges therefrom, so that the cell in this case is only a nucleated expansion of the fibre. The fibrils of the axis band may be seen pass- ing through the cell in a divergent and re-convergent fashion, having the finely granular basis substance of the cell between them. The sheath of the fibre, though usually not the medullary substance (fig. 10), also passes over the cell. A point which will be found more doubtful in other cases is most distinctly illustrated here : viz., that a struc- tural continuity exists between the substance of the cell and that of the nerve fibre. There is no distinct line of demarcation between the two. But, so far as we know at present, this particular relation of fibre and cell exists principally in ganglia peculiar to the ingoing ner ves and situated near the great centres to which these are attached. There is, it is true, some reason for believing that a similar relationship may exist in some of the ganglia on Chap. II.] A NERVOUS SYSTEM. 41 .Fig 10.— Three bipolar Ganglion Cells from the fifth nerve of the Tike (Strieker after Bidder). Fig. 11. — Three "bipolar Ganglion Cells from the auditory nerve of the Pike : a, entirely enclosed within the medullary sheath ; 6, entirely, and c, partially, exposed, to show that those ganglion cells arc only expansions of the axis band. the visceral nerves, and tha*t something like it also exists, 42 THE STRUCTURE OF Chap. II.] A NERVOUS SYSTEM. 43 though on a much smaller scale, in the course of ultimate peripheral nerve fibres (Beale). 2. The ingoing nerve fibre, on subsequently reaching its centre, divides into its elementary fibrils, and these become structurally continuous with a fine network of fibrils (Ger- lach) forming the rootlets of ramifying nerve pro- cesses belonging to one or more contiguous nerve cells (fig. 12). This kind of connec- tion is thought to exist not only in the spinal cord, but also in the superficial grey matter of the brain (both cere- brum and cerebellum), though it is by no means certain whether the fibres which unite with the cells in this fashion in the latter or- gans constitute ingoing or outgoing. channels. It is into such a union as this that the fibrils and corpuscles of the ' neuroglia ' (fig. 9) seem to enter. Cer- tainly its network can- not be distinguished or clearly separated, in many nerve centres, from that formed by the ultimate nerve fibrils and the branchlets of ramify- ing cell processes. Fig. 13.— Multipolar Ganglion Cell from an- terior grey matter of Spinal Cord of Ox. o, Axis cylinder process ; b, branched processes. Magnified 150 diameters. (Deiters. ) 44 THE STRUCTURE OF 3. In other nerve cells, furnished with many ramifying processes, one long simple process may be seen (figs. 13, a, 14), which is occasionally traceable into direct continuity with the entire axis cylinder of a nerve fibre (Deiters)« This mode of union is now generally admitted to exist, and it is not improbable that nerves so arising are, usually at least, outgoing fibres. Whilst this view cannot be defi- nitely verified, it is a fact that such processes have been found principally in the spinal cord in connection with the nerve cells of the anterior, or motor, regions of its grey matter. There is thus some ground for believing that ingoing fibres, in the majority of cases, swell in the posterior spinal ganglia and their analogues into nerve cells (fig. 10) ; that within the larger nerve centres these fibres, which convey ingoing currents, break up into a pencil of ultimate fibrils, and that these ultimate fibrils may be partly in structural continuity with the neuroglia, and partly with the radicles of a much branched nerve process (fig. 12), the divisions of which unite (like the radicles of a vein), till they are gathered into one or more branches directly continuous with the substance of the nerve cell. Such arrangements may suffice to break the force of Ingoing Cur- rents as they impinge upon highly excitable centres ; or their diffusion therein may thus be facilitated, and as a con- sequence they may be enabled to come into relation with the ultimate ramifications of processes pertaining to several cells. On the other hand, there is ground for believing that Outgoing Currents leave the cells of the spinal motor centres by undivided processes, which are directly con- tinuous with the axis-bands of dark bordered nerve fibres. Should these suppositions be correct as to the mode in which currents impinge upon the sensory side, and subsequently issue from the motor side of a nerve centre, Chap. II ] A NERVOUS SYSTEM. 45 then, in order to complete our mental survey of the path of a stimulus wave through such a nervous arc as is called into play in one of the higher animals during the perform- ance of a 'reflex ac- tion,' it only remains to consider the modes of connection exist- ing between the several cells of sensory groups and of motor groups, together with the kinds of communica- tion existing between these two orders of nerve units. 4. Between the contiguous cells of a motor and perhaps also of a sensory group, union is brought about in some cases by means . , . . Fig. 14.— Motor Nerve Cells connected by inter- 01 a SIlOrt Simple cellular processes (6, 6), and giving origin to outgo- intercellular process, iug fibre f (c : '• c - * nd a) - *■ MuIti P° lar cel1 contain - r ' vag much pigment around nucleus. Diagrammatic. such as we see repre- (Vogt.) sented in figs. 1 and 14. But whether this is the most frequent means of union, or whether, in the majority of cases, especially amongst sensory groups, it is not rather by the inosculation of the rootlets of ramifying processes (with the possible interme- diation of the neuroglia) we cannot at present say. There is reason to believe that both modes of union may exist. 5. The cells of a sensory group are united with the 46 THE STRUCTURE OF cells of a motor group by one or other of these modes — though in regard to this point we have even less certain knowledge than concerning the last. Of the existence of such connecting or ' commissural ' fibres— which are either short or long according to the proximity or re- moteness of the two groups of cells — there can be no doubt. Uncertainty exists, however, with regard to the precise mode of their connection with the sensory nerve cells on the one side and the motor on the other — whether at either extremity they are continu- ous with undivided cell-processes, or break up and inosculate with ramifying cell-processes. More room for doubt, therefore, exists in regard to the precise modes in which stimulus waves traverse nerve centres, than con- cerning the manner in which they impinge upon or depart there- from. 6. In the ' sympathetic' or vis- ceral ganglia of the Frog and other animals another kind of relation between fibres and cells has been shown to exist (Lionel Beale). J£J? «?£?%;££ The cells are pear-shaped and the magnified ; according to Beale. naiTOW extremity of each of them Reduced and adapted from one of • ■• i • , t • i his figures, a a , straight fibre; 6», ls continued into a process which coiled fibre ; c, smaller one joining i n turn becomes COntinUOUS with it. (Quain.) a dark-bordered fibre, whilst one or, it may be, two or more smaller fibres seem to arise from Chap. II ] A NERVOUS SYSTEM. 47 »•» the surface substance of the same extremity of the cell, whence, after twisting round it and the straight process several times, they pass away in different directions. Occasionally L. Beale has seen the spiral process con- tinuous with a dark-bordered fibre, though in such cases it is not certain whether the straight process is or is not continuous with a fibre of the same kind. J. Arnold has also described cells of this type, and believes that the pro- cesses are in connection with the nucleus of the cell, an arrangement which has not been confirmed by other observers. The fig- ures given by Axel Key and Eetzius agree closely with those of Beale. 7. But in the ' sympathetic ' or visceral ganglia of man and other higher vertebrates it is most common to find many simple processes issuing from large and very granular ganglion cells. Whether each is directly con- tinuous with a single nerve fibre, after the FlG irgan displays among vertebrate animals, the organ of hearing throughout the Invertebrata is remarkable for its sim- plicity, and remains in all of them notably inferior to the very high type attained by this sensorial apparatus in many Mammals and in Man. Like the sense of Sight and the sense of Smell, that of Hearing, even in its simplest grades, serves to bring the organism into relation with more or less distant bodies. It is only necessary that these latter should be capable of transmitting sonorous vibrations through water or air to the auditory organs which become attuned to receive them. It seems just possible, however, that the so-called ' auditory saccules ' of the Invertebrata, may have more to do with the ' sense of Direction,' or of the organism's relations with space, than with the sense of Hearing.* In Vertebrate Animals, it would appear, that both these functions are associated with the auditory apparatus, and it is by no means certain that the ' sense of Direction,' or of the organism's space-relations, may not be an endow- ment more primordial than that of Hearing. No auditory perception seems to be present at all — certainly none has as yet been detected or inferred to exist — in many of the lower forms of life ; while in other animals, though possibly existing, its organs remain as yet unrecognized. The latter condition obtains, for instance, with the majority of Crustacea, Spiders, and Insects. * See p. 238. G6 THE USE AND NATURE Judging from the instances in which ' auditory sac- cules ' have been detected in Mollusks, and in some few representatives of the classes above named, it seems (and the information may be novel to many readers) that the endowment in question is not habitually, or even usually, found in the head, or in direct relation with one of the gangljft composing the brain of Invertebrates. In some Heteropida, and their allies, however, the ' saccules,' what- ever may be the function to which they are subservient, seem to be in immediate relation with the brain ganglia.* Further remarks on this subject must, however, be de- ferred until a brief description has been given in future chapters, of the nature and distribution of the nervous system in some of the principal groups of the Invertebrata. The foregoing are the commonly received modes by which organisms are impressed from without, and by which they attune themselves to the conditions and actions occurring in their medium. It was recognized by Democritus and other ancient writers, that they are all of them derivatives, or more specialized modes of a primordial common sensi- bility, such as is possessed by the entire outer surface of the organism. Touch, taste, smell, vision, and probably hearing, are sense endowments, having their origin in organs formed by a gradual differentiation of certain por- tions of the external or surface layer of the body — that is, of the part in which common sensibility is most frequently called into play. And just as this common sensibility is a crude or general sense of touch, so are the several special senses to be regarded as more or less highly refined modes of the same sense endowment. The distribution and arrangement of nerves in the various impressible surfaces have certain characteristics * Siebold, "Manuel d'Anat. Oomp.," p. 309, Note 1. Ch,p. III.] Ol' SENSE ORGANS. 67 which have been clearly pointed out by Herbert Spencer. " At the surface of the body," he says,* " where the ex- tremities of nerve fibres are so placed as to be most easily disturbed, we generally find what may be called multipliers of disturbances. Sundry appliances, which appearing to have nothing in common, have the common function of concentrating, on the ends of nerves, the actions of ex- ternal agents." This effect is produced by lenses in the eyes, otoliths and other bodies in the organs of hearing, vibrissae and corpuscula tactus in the skin ; all of which serve to exaggerate the effects of incident forces upon especially sensitive peripheral expansions of the nervous system. " The ultimate nerve fibriUse, ramifying where they are most exposed to disturbances, consist of nerve protoplasm, unprotected by medullary sheaths, and not even covered by membranous sheaths. In fact they appear to consist of matter like that contained in nerve vesicles, .... and may be regarded as, like it, more unstable than the matter composing the central fibres of the fully differentiated nerve tubes. . . . This peri- pheral expansion of the nerve on which visual images fall contains numerous small portions of the highly unstable nerve matter, ready to change, and ready to give out molecular motion in changing. It is thus, too [in higher animals], with those terminal ramifications of the auditory nerve on which sonorous vibrations are concentrated. And there is an analogous peculiarity in the immensely expanded extremity of the olfactory nerve. Here, over a large tract covered by mucous membrane, is a thick plexus of the grey unsheathed fibres ; and among them are distributed both nerve vesicles and granular grey sub- stance, such as that out of which the vesicles arise in the nervous centres." * " Principles of Psychology," vol. i. p. 35. 68 THE USE AND NATURE The movements of locomotion, or of limited parts of the body, which become established in correspondence with various kinds of external impressions, tend with time to in- crease in number, definiteness, and complexity. They are, for the most part, to be classified as actions subservient to the pursuit and capture of prey, to the avoidance of enemies, to the union of the sexes, or to the care of young. All such movements are found, as a general rule, to have the effect of prolonging the action of any influences which previous individual or race experiences have proved to be favourable to the life and well-being of the organism ; and, on the other hand, of cutting short or avoiding influ- ences which past individual or race experiences have proved to be contrary to its general well-being. The capture and swallowing of food are ends to which a very large proportion indeed of the definite motions of most of the lower organisms are directed ; and this direction of their energies is only a special case to be included under the rule above indicated — just as efforts to escape from predatory neighbours, are other opposite instances of the same rule. Visceral Sensations and the ' Muscular Sense.' — In addition to the various modes of impressibility by external influence which we have hitherto been considering, there are also certain other modes due to changes in the condition of internal parts of the organism. These are divisible into two categories : (1) impressions emanating from one or other of the various sets of viscera — such as the alimentary canal and its appendages, the respiratory organs, the genital organs, or other internal parts ; and (2) impressions derivable from, or in some way attendant upon, the contractions of muscles. The first category of internal impressions — those eman- Chap. III.] OF SENSE ORGANS. 69 ating from the viscera — are undoubtedly very important in relation to animal life generally. In part, they have the effect of causing contractions of related muscular por- tions of the viscera — as when the presence and pressure of food in certain portions of the alimentary canal excites — it may be through local ganglia — contractions by which _ the food is propelled farther on. In part, however, they act upon the principal nerve ganglia — those constituting the brain — in such a way as to excite the external sense- organs with which they are connected to a higher order of activity. Visceral impressions of one kind may cause an animal more eagerly to pursue prey, whilst those of another sort may tend to an increased alacrity in dis- covering a mate. In these, and in many other instances, internal impressions, reaching the cerebral ganglia, would seem to excite a higher receptivity for certain kinds of external impressions and a corresponding increased readi- ness to respond on the part of the moving organs whose activity is related to such conjoined impressions and promptings. With the second set of impressions, those of the so- called 'muscular sense,' we have at present nothing to do. They differ altogether from others, whether of ex- ternal or of internal origin, by the fact that they follow or accompany movements whose intensity they are supposed to measure, and do not of themselves incite movements. Granting that such impressions have a real existence, it is obvious we can know nothing about them among Invertebrate Animals, since they have only a subjective existence and do not of themselves alone lead to move- ments. Our only knowledge of such impressions, as subjective states, must be derived from our own sensations together with what other fellow-men are able to describe. CHAPTER IV. THE NERVOUS SYSTEM OF MOLLUSKS. Foe several reasons it will be advantageous to depart from the usual zoological order, and consider first the disposition of the nervous system in some of the principal types of the sub-kingdom Mollusca. These are animals mostly aquatic and wholly devoid of hollow, articulated, locomotor appendages. Their organs of vegetative life attain a disproportionate development, as may be imagined from the fact that some of the simplest representatives of the class consist of mere motionless sacs or bags, containing organs of digestion, respiration, circulation, and generation. The most complex Mollusks, however, are active predatory creatures, endowed with remarkable and varied powers of locomotion, and with sense organs which are both keen and highly developed. The simpler forms are represented by the motionless Ascidian, and the higher by the active and highly endowed Cuttle-fish. It should be mentioned, however, that the tendency^ several recent investigations has been to separate the class to which the Ascidians belong altogether from the Mollusca, and to place them as an independent group, having affinities to the lowest Vertebrates. The solitary Ascidians may be taken as the type of Chap. IV.] THE NERVOUS SYSTEM OF MOLLUSKS. 71 the Tunicata. Their life of relation with the external world is of the simplest description. They are sta- tionary creatures, having even no prehensile organs — their food being brought to the commencement of the alimentary canal by ciliary action. In correspondence with such a simple mode of life, we might expect to find a very rudimentary nervous system, and this expectation is fully realized. The Tunicata possess a single small nervous ganglion lying between the bases of the two funnels through which water is taken in and discharged (fig. 21, c). This ganglion receives branches from the tentacula guarding the orifice of the oral funnel, and possibly from the branchial chamber; whilst it gives off outgoing fila- ments to the various parts of the muscular A ^° Ai ^ 1 ' "~ w -^ sac, and perhaps to the alimentary canal rough diagiam- , « .-i .. . , -, t matic sketch of and some of the other internal organs. In its Nervous Sys . some of the solitary Tunicata a rudimentary tem - ( Sol, y aftcr * " Ouvier.) a, Bran- visual function is presumed to exist. At chiai orifice : &, all events, pigment-spots are situated on, or 1™^XX* in very close relation with, the- solitary with its afferent and efferent ganglion. -nerves. The recent investigations of Kupffer tend to show that this extremely simple nervous system, never- theless, represents a decidedly higher type of organization than had been previously supposed. Further details cannot, however, here be given.* The Brachiopods are among the oldest and most wide- spread of the forms of life in the fossil state, and the geographical distribution of their living representatives at * See Gegenbauer's " Corop. Anatomy," English Translation, p. 395. 72 THE NERVOUS SYSTEM OF MOLLUSKS. the present day is also very wide. Like the Tunicata, they are also headless organisms, and lead a sedentary existence, attached to rock or stone either hy a pedicle or by one division of their bivalve shells. The mouth is unprovided with any appendages for grasping food — nutritive par- ticles being again brought to it by means of ciliary currents. Numerous muscles exist which connect the valves of the shell to one another, and. with the enclosed animal. Though the visceral organization of the Brachiopods is somewhat complex, no definite Sense Organs have yet been detected in any of them. The nervous system of these sedentary animals, moreover, comprises nothing an- swering to a ' brain ' as it is ordinarily constituted — though ganglia exist around the oesophagus which must receive afferent impressions of some kind, and from which branches proceed to the various muscles and viscera of the body. Such low sensory endowments would be wholly incompatible with that degree of visceral complexity of organization which the Brachiopods possess, had it not been for the fact that these animals lead a passive exist- ence in respect to quest of food. The absence of sense- organs and of a brain is, indeed, only compatible with such a semi-vegetative existence. The Lamellibranchs, or ordinary headless bivalve Mollusks, also include some representatives — such as the Oyster and its allies- — which lead a sedentary life. The valves of the shell in Lamellilranchs generally are lateral, instead of being dorsal and ventral as amongst the curious Brachiopods above referred to. The mouth of the Oyster is surrounded by four labial processes whose functions are not very definitely known. It presents no other appendages of any kind in the neighbourhood of the mouth, and, as in the two types of Chap. IV.] THE NERVOUS SYSTEM t)F MOLLUSKS. 73 Molhisca already described, the food which it swallows is brought to the entrance of its oesophagus by means of ciliary currents. It has two small anterior or 'labial' ganglia (fig. 22, a, a,) one being situated on each side of the mouth. They are connected by a commissure arching over it, and also by a more slender thread beneath the mouth. From this lower commissure, filaments (e) are given off to the stomach. The anterior ganglia receive nerves (/) from the labial processes which are probably for the most part afferent in function — at all events, these processes have no distinct muscular struc- ture. Two long parallel commis- sures (d, d) connect the anterior ganglia with a single large com- pound 'branchial' ganglion (b), o^^X"*^*™ Situated posteriorly, and close tO -Anterior or labial ganglia ; b, ,1 ■ j j . i t, posterior or branchial ganglion the great adductor muscle. It (double ) ; /, ^ iA nerV e 8 * Ci gives Off branches tO this mUSCle, branolua l nerves ; d, d, commis- ■ sures between labial and branchial to each halt ot the mantle, and gaDgiia. to the gills (c, c). Other more active Lamellibranchs possess a muscular appendage known as the 'foot', which is in relation with an additional single or double nervous ganglion (' pedal'), and is used in various ways as an organ of locomotion. Speak- ing of the diverse uses of the foot among bivalves, Prof. Owen says :* "To some which rise to the surface of the water it acts, by its expansion, as a float; to others it serves by its bent form as an instrument to drag them along the sands ; to a third family it is a * " Lect. on Comp. Anat. of Invert. Animals," p. 505. 74 THE NERVOTjS SYSTEM OF MOLLUSKS. burrowing organ; to many it aids in the execution of short leaps." The bivalves possessing a foot, therefore, present three pairs of ganglia instead of two — the anterior or ' labial', Pig. 23.— Nervous System of the Common Mussel. (After Owen.) I, Labial ganglia connected by a short commissure above or in front of the mouth; 6, b, branchial ganglia similarly connected, and also united by very long cords (d, d) with the labial ganglia; p, bilobed pedal ganglion sending branches to the muscular foot (r), and closely connected with the ' auditory saccules ' (s) ; h, k', circum-palHal plexus ; y, byssus, by which the animal attaches itself to external substances. the posterior or 'branchial', and the inferior or 'pedal'. It occasionally happens that the ganglia of the posterior or even of the inferior pair may become approximated and fused into one. Cbat. IV] THE NERVOUS SYSTEM OF MOLLUSKS. 75 The fusion of the posterior ganglia takes place, as in the Oyster (6g. 22, b), when the branchiae from which they receive nerves (c, c) come close together posteriorly. On the other hand, in those Mollusks in which the branchiae are farther apart, the two ganglia remain separate and are connected by a short commissure, as in the Common Mussel (fig. 23, b). The separateness or fusion of the inferior or ' pedal ' gan- glia depends upon the size and shape of the foot, since the nerves in relation with them are distributed almost wholly to this organ and its retractor muscles. Where the foot is broad the ganglia remain separate, and are merely connected by a commissure. But where the foot is small and narrow, as in the Mussel, the two ganglia become fused into one (fig. 23, p). Some of the special senses are unquestionably repre- sented amongst these headless Mollusks, though the distribution of the different organs is very peculiar. Thus in Pecten, Pinna, Spondylus, Ostrsea, and many other genera, very distinct and often pedunculated ocelli are distributed over both margins of the 'pallium' or mantle. These vary in number from forty to two hundred or more, and are in connection with distinct branches of the circumpallial nerves. In the Kazor-fish, Cockle, Venus, and other bivalves possessing prolongations of the mantle known as ' siphon-tubes', the ocelli are situated either at the base or on the tips of the numerous small tentacles arranged round the orifices of these organs. And these parts, in such bivalves as live in the sand, are often the only portions of the body which appear above the surface. The margins of the mantle are also garnished by a number of short though, apparently, very sensitive tentacles, in which the creature's most specialized sense of touch seems to reside. 76 THE NERVOUS SYSTEM OF MOLLUSKS. Some of these tactile appendages, as well as some of the ocelli, send their nerves to the branchial ganglia, while others, situated on the anterior borders of the mantle, send filaments to the labial ganglia. The latter also receive filaments from the so-called labial appen- dages, whose function is uncertain, though it has been suggested that they may be organs of taste or smell. Lastly, in close relation with the pedal ganglia or ganglion, there are two minute saccules (fig. 23, s), to which an auditory function is usually ascribed. Thus we find amongst these headless Mollusks a distri- bution of specially impressible parts or sensory organs, such as cannot be paralleled among any other animals. The functions which we shall find pertaining to the ' brain ' in other creatures are in them distributed in a very remarkable manner — so that such organisms may be said to be brainless as well as headless. The Pteropods constitute another interesting class of Mollusks, which lead us on from the comparatively sluggish Lamellibranchs to the Gasteropods and the Cephalopods — organisms which possess definite and wide- reaching powers of locomotion, as well as a distinct head carrying sense-organs and a more or less developed brain. The possession, by many members of this class, of two fin-like muscular expansions attached to the side of the head induced Cuvier to give them the above class name. According to Owe'n, " All the species of Pteropoda are of small size; they float in the open sea, often at great distances from any shore, and serve, with the Acalephm, to people the remote tracts of the ocean. In the latitudes suitable to their well-being, the little Pteropoda swarm in incredible numbers, so as to discolour the' surfaoe of the sea for leagues ; and the Clio and the Limacina con- Chap. IV.] THE NERVOUS SYSTEM OF MOLLUSKS. 77 stitute, in the northern seas, the principal article of food of the great whales." Some of the least highly organized members of this class, such as the Hyalleidse, possess a bivalve shell, and no distinct head ; but in other Pteropods devoid of a shell, we meet with a higher organization. Thus in Clio there is a distinct head hearing sensory appendages, in the form of two tentacula and two eyes, and containing ' a brain ' within. The brain is represented by two connected ganglia above the oesophagus, which are in relation, by means of ingoing nerves, with the above mentioned sensory organs. In connection with another commissure uniting these two cerebral ganglia and which passes under the first part of the alimentary canal, are two 'pedal' and two 'branchial' ganglia pretty close together. These two pairs of ganglia exist separately in Cli© and its allies, though they are combined into one quadrate mass in Hyalea. In Clio two ' auditory saccules ' are in connection with the anterior sub-cesopha- geal ganglia — that is, with the pair which corresponds with the ' pedal ' ganglia of the common bivalve Mollusks. Gasteropods constitute a class of organisms which, in point of numbers, can only be compared with the still more Tiumerously represented class of Insects. Their name is derived from the fact that they crawl by means of a large muscular expansion or ' foot ' stretched out beneath the viscera. The locomotion of members of this class may be said to be, in the main, dependent upon their own individual efforts, so that, in this respect, they differ widely from Pteropods, whose movements from place to place are brought about chiefly by winds driving them along the surface of the water on which they float. Some Gasteropods are terrestrial, air-breathing animals, though by far the greater number are aquatic and breathe 78 THE NERVOUS SYSTEM OF MOLLUSKS. by means of gills. Bat being all of them, as Prof. Owen says, " endowed with power to attain, subdue, and devour organic matter, dead and living," we find their Nervous System not only better developed, more complex and con- centrated, but also in relation with more highly evolved organs of special sense and exploration. This system offers considerable variations in general arrangement, and as regards the relative positions of its ganglia, though these modifications are, to a great extent, referable to differences in the outward configuration of the body. The wide differences in external form which are to be met with among Gasteropods may be well illustrated by comparing the Limpet or the Chiton with the Snail. Here differences in habit are also marked, so that we almost necessarily meet with very notable variations in the disposition of the principal parts of the nervous system. In the Limpet two small cerebral ganglia (fig. 24 a) exist, which are widely separated from one another, and lie at the side of the oesophagus. Each receives a rather large nerve from one of the tentacles, and a smaller optic nerve. A commissure above the oesophagus con- nects, these cerebral ganglia with one another, while each of them is also in relation by means of two descending fig. 24. — Nervous commissures with a series of four con- ilm^S. °^od C d° m aXr nected ganglia forming a transversely Gamer.) a, cerebral arranged row beneath the oesophagus. ganglia; o, branchial ,-.»,■, ,, . ,. , ,. , . and b, pedai' ganglia: (Ji these tne two median ganglia (b) 2m£3?.^ i, corres P° nd with the pedal, while the two commissures ; g, tenta- external (c) correspond with the bran- eular nerve; i, optic ^^ ^^ thougQ ^ ^ ^ separated from one another by an un- usually wide interval. Ohap. IV.] THE NERVOUS SYSTEM OF MOLLUSKS. 79 However small and undeveloped the duplex brain of the Limpet may be, this organ exists in an even more rudimentary state in its close ally, the Chiton, which is about the most simply organized of all the Gasteropods. It has neither tentacles nor eyes, and, as a consequence, no distinct supra- cesophageal ganglia are found (fig. 25). There is, in fact, nothing to which the term ' brain ' can be appropriately ap- plied. If we turn, however, to the very actiye Snail, we find the nervous sys- tem existing in a much more deve- loped and concentrated form. There is a large ganglionic mass (fig. 26, I) situated over the oesophagus, each half of which receives a considerable bundle of nerve-fibres (J) from the fig. 25. — Nervous sys- eye (b) of the same side, which is situ- Xtf't m™ ated at the tip of the larger tentacle, ganglion (left) ; b, pedal Ti. 1 • ,1 1 Ti j. ganglion (right) ; c, bran- it also receives another bundle of C hi a i ganglion; », upper nerves (k) from the small tentacle on portion of ^ophagoai ring . t t devoid of any distinct each side, which has in all probability cerebral ganglia. a tactile function. The ' auditory sac- cules ' are here in their exceptional position — that is in immediate relation with the posterior aspect of the ganglia constituting the brain, though in most other Gasteropods they are, as in bivalve Mollusks, found in connection with the pedal ganglia. There is one group, however — the Heteropoda — in which the ' auditory saccules ' seem to be always in direct relation with the cerebral ganglia, as in Carinaria and Pterotrachea.* * See Fig. 187, p. 354, Gegenbauer's " Comp. Anat." (Engl. Transl.) 80 THE NERVOUS SYSTEM OF MOLLUSKS. Naturalists now generally admit that Snails and their allies are endowed with a rudimentary sense of smell, though hitherto they have heen unable to locate the endow- ment in any particular organ or surface-region. The brain of the Snail is connected, by means of a thick cord or commissure on each side of the oeso- phagus, with a long and curved double ganglionic mass (m). This latter body, situated beneath the oesophagus, represents the pair of pedal and the pair of branchial ganglia of the bivalve Mollusks. Here nerves are received from the integument and given off to the muscles of the foot ; while they are also, received and given off from the respiratory and other The nervous system of of the Nudibranoh Fig. 26.— Head and Nervous System of the Common Garden Snail. (Owen.) (.Cere- bral ganglia receiving nerves from smaller oi'ganS. (a) and from larger tentacles bearing ocelli (6); to, sub-03sophageal ganglionic mass, representing a pair of pedal and a pair of jjg branchial ganglia. Two of the tentacles are represented in different states of retraction. Mollusks has been repre- sented in fig. 17. It is also highly developed and concentrated, whilst its sensory and motor ganglia are unusually distinct and separate from one another. A somewhat analogous arrangement of the prin' cipal nerve centres exists in the Common Slug (fig. 27), only here the motor ganglia of the two sides are fused together, as in the Snail, instead of being widely separated as in Eolis and its allies. They consequently occupy an inferior rather than a superior and lateral position in regard to Chap. IV.] THE NERVOUS SYSTEM OF MOLLUSKS. 81 the oesophagus. The branchial ganglia are, moreover, fused with them, instead of with the cerebral as they are in Eolis. The nervous system in the Cephalopoda presents many peculiarities, which can, however, be only very briefly referred to here. Owing to an extreme amount of shortening of their commissures, the principal ganglia are closely aggregated in the head. The nervous system is, in- deed, more concentrated and com- plex than in other Mollusks, and the animals themselves are notable for the high degree of development of some of their sensory organs as well as for their great powers of locomotion. The body of the Pearly Nautilus, contained within the last chamber of its coiled and loculated shell, is en- veloped by a muscular mantle open anteriorly, round the head and its numerous sensory appendages. Ac- cording to Owen,* " the number of tentacles with which the Pearly Nautilus is provided amounts to no less than ninety, of which thirty- eight may be termed digital, four ophthalmic, and forty- ei^ht labial." The eyes, not so well developed as in the Cuttle-fish, are also in relation with smaller optic ganglia (fig. 28, o o). Near them are two hollow bodies, regarded by Valenciennes as olfactory organs, the nerves from which join the same ganglia. The situation and * " Lectures on Comp. Anat. and Physiol of Invert.," p. 581. Fig. 27.— Nervous System of the Common Slug (Solly after Baly.) a a, Cerebral ganglia ; b b, branchial ganglia and c, pedal ganglia fused into one >, pharyngeal ganglia. 82 THE NERVOUS SYSTEM OP MOLLUSKS. relations of auditor}' organs in this animal have not been definitely settled. In regard to organs of taste and touch, Owen writes as follows, — " The complex and well developed tongue of the Pearly Nautilus ex- hibits in the papillte of its anterior lobes and in the soft ridges of its root the requisite structure for the exercise of some degree of taste : . . . the sense of touch must be specially exercised by the numer- ous cephalic tentacles, which, from their softness of texture, and especially their laminated inner surface, are to be regarded as organs of exploration not less than as organs of prehension." The nerves of these tentacles, must be both sensory and motor ; they are in connection with a large double Fig. 28.— Nervous System of v ° Pearly Nautilus. (Gegenbauer ganglionic maSS (b 0) Situated after Owen.) a a i Cerebral gall- beneath the CBSOphagUS but in glia, constituting th.e brain ; o o, x o optic ganglia in communication front of the Other Sub- (Esophageal ilTn^rtflwe^g! ganglion (c c), which is thought lionicmass(6 6), receiving nerves by Owen to represent "the homO- (t *') from the tentacles and other , „ , , . . p*rts about the mouth, partly logues oi both the branchial and sensory and partly motor The pe ^ al rr ang li on s in the inferior cerebral ganglia are in addition ■*- ° ° united to a posterior sub-oeso- MoliUSCa." The latter pairs of ^XtTpi^'pSZV: g^glia are clearly combined in pair of branchial ganglia, m m, function, since the locomotions of Motor nerves; d d, braDchial ,-, XT .., ,., ,. n nerves and ganglia. the JNautiius, like the much more rapid locomotions of other Cepha- lopods, seem to be principally effected "in a succession Chap. IV.] THIS NERVOUS SYSTEM OF MOLLUSKS. 83 of jerks, occasioned by the reaction of the respiratory currents upon the surrounding water "— these currents Fig. 29. — Nervous System of the Common Cuttle-Fish {Sepia officinalis). (Owen.) 1, Double supra-cesophageal ganglion developed from upper commissure ; p p, cut surfaces of the cartilaginous cranium ; 2 2, optic ganglia ; 4 4, posterior sub-oeso- phageal ganglia (anterior sub-cesophageal ganglia in connection with nerves of feet and tentacles, 6 6, not seen in this view) ; 7 and 8, ganglia in connection with the pharynx and mouth, connected by nerves (5 5) with the cerebral lobes ; 13 13, great motor nerves, of the mantle and other parts, with (d) their ganglia ; 14, c c, respiratory nerves ; k k, small tubercles in connection with optic ganglia. being produced by the expulsive contractions of a powerful muscular funnel continuous -with a portion of the mantle. In the Cuttle-fish one of the most striking character- 84 THE NERVOUS SYSTEM OF MOLLUSKS. istics of the principal nerve-centres is the fact of the exist- ence of a very large optic ganglion (fig. 29, 2) on each side, in connection with an extremely well- developed eye. Each optic lobe, according to Lockhart Clarke, is "as large as the rest of the cephalic ganglia on both sides taken together." From each of these lobes an optic peduncle passes inward to join a supra- oesophageal gan- glionic mass, which bears on its surface a large bilobed ganglion (1), thought by Clarke to be homologous with the cerebral lobes of fishes. It is connected, by means of two short cords, with a much smaller bilobed ganglion, known as the pharyngeal (r). This latter ganglion re- ceives nerves from what are presumed to be the organs of taste and smell, and gives off nerves to the tongue and powerful parrot-like jaws with which the creature is pro- vided. The supra-cesophageal mass is connected by cords, at the sides of the oesophagus, with a very large ganglion lying beneath it (4), which is partially divided into an anterior and a posterior division. The anterior division — regarded by Huxley as in part homologous with the pedal ganglia of lower Mollusks — is in relation by means of large nerves (e) with the feet and tentacles. A com- missure also unites it with the pharyngeal ganglion, so that the tentacles and arms are thus able to be brought into correlated action with the jaws. The posterior" portion of the sub-cesophageal mass receives nerves from, and also gives off nerves (u) to, the branchiae and other viscera, as well as to the muscular mantle (13, n). The ' auditory saccules ' and their nerves are connected with this great branchio-pallial ganglion. These organs are lodged in the substance of the cartilaginous framework (p p) investing the nerve-ganglia — a structure which seems to answer to a rudimentary skull or cranium. Chap. IV.] THE NERVOUS SYSTEM OP MOLLUSKS. 85 The locomotions of Cattle-fishes are largely brought about by contractions of the pallial chamber, though these same contractions of the pallium are also subservient, as in the Nautilus, to the respiratory function. The large share, therefore, which the branchio- pallial ganglia take in bringing about and regulating the move- ments of these animals, would seem in part to explain the connection of the ' auditory saccules ' with them, since in the great majority of other Mollusks in which these organs are known to occur, they are found to be in primary relation with the principal motor centres. Whatever may be the full explanation of these remarkable relations, the fact remains that, even in the Cattle-fish tribe, the super- ficial connections of the so-called ' auditory saccules,' are still away from the brain. 5 CHAPTER V. THE NEEVOUS SYSTEM OF VERMES. Nothing distinctly answering to a Brain is to be found in some other of the lowest animals in which a nervou3 system exists. It is thus, for instance, with Star-fishe3 and the larger Nematoid Entozoa. What most nearly resembles such an organ in Star-fishes, consists of a mere band of nerve fibres, surrounding the commencement of the oesophagus, and containing a few nerve-cells partly between its fibres and partly in groups slightly removed therefrom. The absence of any distinct ganglia in the neighbourhood of the mouth is doubtless due, in the main, to the form of these animals, and their low type of organi- zation. Each arm or ray contains its own nervous system, so that the ring or band round the mouth seems to be little more than a commissure connecting such otherwise distinct parts of the common system. These Echinoderms are, however, here only incidentally re- ferred to. In the larger parasitic Nematoids the nervous system is more concentrated. The oesophageal ring and imme- diately adjacent parts constitute almost all that is as yet known of their nervous system, but it contains, or is in relation with, a larger number of ganglion-cells than the similar part in Star-fishes. Thus, in addition to the cells intermixed with the fibres of the ring itself, there are five or six groups adjacent to and in connection with it, Chap. V.] THE NERVOUS SYSTEM OF VERMES. 87 which receive fibres from certain large papillae surrounding the mouth. These papillae would seem to be the principal sensory organs of the Nematoid. By means of the connect- ing nerve-fibres and ganglion- cells they are brought into relation with the nervous ring, and from this latter out- going fibres are, doubtless, given off to the four great longitudinal muscular bands by which the movements of the organism are effected. The distribution of such motor nerve-fibres, however, has not been distinctly traced. The absence of ganglionic swellings on, or in connec- tion with, the oesophageal ring of Nematoids is probably dependent upon the comparative simplicity and limited number of the impressions capable of being received through their cephalic papillae. Among other representatives of the sub - kingdom Veemes, the nervous system varies a good deal in minor details, in accordance with the degree of organization, and with the diversity of the sensory and locomotor endow- ments of the several organisms. The broad features of the nervous system, however, are comparatively similar in all — especially in the most typical representatives of this sub-kingdom, which contains so many aberrant types. Only a very few forms will be here referred to. The Nemerteans, a class of marine worms, possess a very simple nervous system. They have soft, un- segmented, and highly contractile bodies, covered with cilia, but are otherwise wholly devoid of all external appendages. On the anterior extremity of the body, a little posterior to the mouth, two, four, or more specks of pig- ment are met with (fig. 30, e, e), which are conjectured to serve the purpose of rudimentary ocelli ; and whilst the animal is moving from place to place this anterior part of its body doubtless acts also as its principal tactile surface. Nerve-fibres proceed from these regions, and converge so 88 THE NERVOUS SYSTEM OF VERMES. as to form three or four nerve-trunks on each side, which enter a comparatively large compound ganglionic mass (a, a) lying on the lateral aspects of the sheath of the proboscis. Each of these masses is pyriform in shape, and composed of a sensory and a motor ganglion fused into cne. It is connected with its fellow by means of two commissures, one of which passes over, and the other beneath, the proboscis. It is difficult to trace the ultimate distribution of the nerve-fibres in these crea- tures ; so that, ■ although fibres can be followed nearly up to the pigment-spots, none have been detected in immediate continuity there- with. The inferior com- FiG. 30.— HeadandBrainofaNemertean. „.*„„„,.„ /„\ i,„j. j.i,„ i (Tetrast„ m na wlanocepkala.) a, .,, Com- mSSOre (c) between the two pound lateral ganglia ; 6, narrow upper ganglionic maSSeS is shorter commissure between which and the much -. ■. , ■, . -. ,-, , . thicker inferior commissure, c, the oasopha- &UQ. milCIl tfllCKer tnan tllC giw passes ; d, d, the great lateral nerve U p pe r. The two great late- cords ; e, e, pigment spots, or rudimentary ° ocelli. (After Mcintosh.) ral nerve-trunks (d, d) start from the ganglia, and, pro- ceeding along the sides of the body, give off numerous branches to the longitudinal and circular muscles between which they are situated. Tactile and possibly gustatory impressions, together with impressions produced by light or darkness, doubtless come from the anterior extremity of the organism to the anterior part of the pyriform ganglia on either side ; and are thence reflected from the posterior parts of these bodies Chap. V.] THK NERVOUS SYSTEM OF VERMES. 89 along related channels in the great efferent bundles, the fibres of which proceed to the contractile proboscis and also to the muscles on one or both sides of the body. Other departments of the nervous system may exist in these animals, though as yet none have been detected. In the common Medicinal Leech the nervous system is somewhat differently developed. The lateral ganglia of the Nemertidse are replaced by two small upper ganglia (fig. 31, a), connected by lateral commissures with a single lower ganglion (c) ; and, as a consequence of this coalescence of the two sub-oesophageal gan- glia, we have, instead of the two lateral (tf^ cords of the Nemertidse, a double ventral |5£?, nervous cord traversing the whole length ^^f of the body. The two cords approximate so closely as to be almost fused into one, and ^53 they bear a series of ganglia — one for every three or four of the segments into which the body of the animal is obscurely divided. The bilobed ganglion above the oesopha- gus, which is mainly sensory, receives fibres from the tactile lips, together with ten dis- tinct filaments from as many pigment- spots \-^> or ocelli (b b), situated round the margin of the upper lip. From this bilobed ganglion, \0*s corresponding with the brain proper of V^ higher animals, a cord descends on each side of the oesophagus, and the two join the heart-shaped sub-oesophageal ganglion (c), from which efferent nerves are given off to Fjg. 31. Nervous System of the Medicinal Leech. (Owen.) a, Double supra- ossophageal ganglion connected by nerves with b, b, rudimentary ocelli ; c, the double infra-osaophageal ganglionic mass, which is continuous with the double ventral cord, bearing distinct compound ganglia at intervals. 90 THE NERVOUS SYSTEM OF VERMES. the muscles whose business it is to move its three saw- like jaws, as well as to the muscles of the oral sucker. This lower ganglion is in part analogous to the ' me- dulla oblongata ' of vertebrate animals. It is continuous with the double ventral cord, on which twenty equidistant rhomboidal ganglia are developed. Each of these ganglia gives off two nerves on either side, whose branches are distributed to the parietes and the muscles of adjacent segments. In this animal a simple filament is also given off from the posterior part of the supra-cesophageal ganglion, which is distributed along the dorsal aspect of the alimentary canal. It foreshadows an important system of fibres in higher animals, corresponding partly with the pneumo- gastric nerves, and partly with the ' sympathetic system.' As it exists amongst the Invertebrates it is known as the ' stomato-gastric system ' of nerves. In other members of the invertebrate series it frequently takes origin from the commissures connecting the upper and lower oeso- phageal ganglia, rather than from the upper ganglia them- selves. In some of the worms, in which such an ar- rangement exists, the stomato-gastric system is also more complicated. In the Earthworm the body is composed of a multitude of ring-like segments, provided with lateral setae which the animal calls into play during its subterranean loco- motions. It possesses no distinct ocelli, and, having regard to its mode of life, this is not surprising. The supra-cesophageal ganglia, which together represent the brain of the Earthworm, receive a nerve trunk on each: side, composed of fibres coming from the tactile upper lip; and, as no sensory filaments of a different order are known to be immediately connected therewith, the functions of the brain in this animal must be comparatively simple. Chap. V.] THE NERVOUS SYSTEM OF VERMES. 91 The lip is regarded as an organ of touch, but it is equally probable that it is capable of receiving more special impressions representing rudimentary tastes. The separation between these modes of sensibility in such low organisms is probably somewhat indefinite. The double ventral cord has a fibrous structure along its upper surface, whilst below there is an irregular stratum of ganglion cells. These cells are more abundant about the centre of each body-segment, so that their aggregation gives rise to a series of rudimentary ganglia in these situations. From every one of the ganglionic swellings two nerves are given off on each side.; whilst a third pair issues from the cord itself, just anterior to the swelling, and is dis- tributed along the anterior boundaries of the segment. In Serpula, one of the small tube-dwelling marine worms, the ventral ganglia are also very minute, and those of the two sides, together with the ventral cords, lie some distance apart, and are connected by a series of • r- nn t ii ■ t FlG - 32,-Nervoua Sya- COmmiSSUreS (hg. 32, 0). in thlS dlS- tern of Serpida mntartu- position of the great nervous cords we ~ ^™^ have something intermediate between pbageai ganglia; &, sub- ... . . . • ,i xt i oesophageal ganglia; V one their lateral position in tneJNemerteans, of gangiiouated cords; », and their contiguous mid- ventral posi- motor bucoal nerves ' ( ' tac - ° x tile nerves. tion in the Leech and the Earthworm. As in the latter, so in Serpula, the afferent nerves entering the brain (t) seem to be in the main tactile. The oesophageal ganglia in the Earthworm are, propor- tionately to the rest of the nervous system, much smaller 92 THE NERVOUS SYSTEM OF VERMES. than in the Nemerteans ; and this is perhaps due in great part to the existence in it of the numerous segmental ganglia, — structures which are absent in the above-men- tioned marine worms. The movements of the Nemer- teans, like those of the Nematoids, are probably much more exclusively under the control of the oesophageal ganglia than are those of the segmented Earthworm — in which each of the body ganglia, doubtless, has much to do with bringing about the contraction of its contiguous muscles in the same segment. The Earthworm has a more complex visceral structure than is to be met with among the Nemerteans ; and it presents distinct evidences of a nervous interconnection between its internal organs and some of the principal nerve-eentres. Lockhart Clarke has described a complicated ganglionic network on each side of the oesophagus, start- ing from the lateral commissures and sending prolonga- tions to the intestine and other parts. By means of this principal visceral system of nerves, the internal organs are brought into relation with one another, and with the nervous system of animal life — that is, with those parts of it having to do more especially with the relation of the organism to its medium. CHAPTER VI. TIIE NERVOUS SYSTEM OP ARTHROPODS. The next sub-kingdom, Arthropoda, comprises the Myriapods, Crustacea, Spiders, and Insects. They are all characterized by the possession of hollow and jointed organs of locomotion provided with distinct muscles, instead of the mere lateral setae or bristles often met with amongst Vermes. The lowest types of these various classes possess a nervous system closely analogous to that of the various kinds of Worms ; but in the higher kinds of Crabs, Spiders, and Insects, we meet with a great increase in the complexity of animal organization, and this further complexity, as might have been expected, extends to the nervous system. Among Insects, for example, the respiratory organs assume a marvellous degree of elaboration, and the develop- ment of this system, together with a correlated organiza- tion of their nervous and muscular systems, contributes greatly to confer upon these denizens of the air those enor- mous powers of locomotion for which they are remarkable. But the acuteness, discriminative power, and structural elaboration of sense-organs, is almost sure to be greatly increased in creatures endowed with such activity ; and, looking to the constitution of the Brain as well as to the nature of the ' intelligence ' of these lower animals, it may easily be conceived that increased sensorial activity is 94 THE NERVOUS SYSTEM OF ARTHROPODS. likely to be associated with greater brain development and with higher or more complex brain functions. Among the lower Myriapods, such as lulus and Geophi- lus, in which the limbs, though very numerous, are feeble and ill-developed, the nervous system exhibits only a slight advance over the forms which it presents among the higher Annelida. In lulus (fig. 33) the single abdominal cord shows almost no traces of ganglionic swellings, owing to the great number of the small nerves given off on each side, along its entire length, -which are distributed to the hundreds of small segments entering into the composition of the body. Fio 33. —Anterior part of the Nervous System of lulus (Owen), a, a, Cerebral ganglia ; c, c, optic nerves ; d, d, antenna! nerves ; 6, nerves of the palpless mandibles ; g, oesophageal cords ; e, f, stomato-gastric nerves ; h, motor nerves to the maxillae, proceeding from the part which corresponds with the sub-cesophageal ganglia, here fused with i, i, the ventral cord. The brain (a, a), elongated transversely, is divided by a slight median furrow, and is continuous with the short and thick optic nerves (c, c). Two separate nerves are received from the antennae on each side (d, d), below CnAp. VI.] THE NERVOUS SYSTEM OF ARTHROPODS. 95 and in front of the optic nerves ; whilst nearer the middle line two other nerves on each side (b) are in relation with the palpless mandibles. The thick oesophageal cords (g) are continued from the posterior and inferior angles of the brain ; and, as they descend to enter the me- dullary or sub-cesophageal ganglion at the commencement of the abdominal cord (i, i), they are united by a cross branch, as in many Crustacea (fig. 36). From this sub- cesophageal ganglion large nerves are given off on each side (h) to supply the maxillse and other parts about the mouth. " The stomato-gastric nerves, which arise from the posterior part of the brain immediately, form a third slender ring (e) about the oesophagus, from the middle of the upper part of which the trunk of the stomato-gastric system (/) is continued a short way back upon the stomach," when it divides into two branches which " bend abruptly backwards, and run parallel with each other along the dorso-lateral parts of the wide and straight alimentary canal." (Owen.) In the more powerful predatory Myriapods, of which the common Centipede may be taken as a type, a distinct advance is met with. This carnivorous creature has a smaller number of better- developed limbs, and its nervous system closely resembles that found amongst the larvae or Caterpillars of higher Insects (fig. 39). The supra-oesopha- geal ganglia, or brain, receive nerves from the two pairs of antennse, and from the groups of ocelli on each side of the head. They are connected by oesophageal cords with a bilobed infra- oesophageal ganglion, which distributes nerves to the jaws and other parts about the mouth. This bilobed infra-cesophageal ganglion is the first and largest of a series of ventral ganglia, numbering about twenty, which are connected together by a double ventral cord. Every 96 THE NERVOUS SYSTEM OF ARTHROPODS. ganglion sends off lateral nerves to a pair of limbs. The stomato-gastric nerves are connected with the posterior part of the brain or with the cesopha- fig.31. fig. 35. g ea j C0Y & S> an d they distribute them- •\i ./, > selves over the alimentary canal in the usual manner. Among Crustacea great differ- ences are met with in the degree of concentration of the nervous system, the variations being, in the main, de- pendent upon differences of external form and in the arrangement of locomotor appendages, in the different representatives of the class. In some of the lower terms of the series, such as the Sandhopper and its allies, in which the body is elongated and com- posed of many almost similar seg- ments, the nervous system is not very different from that of many "Worms. In the Sandhopper, indeed, the ven- o f F,G Co™%°L S C m r tral C ° rdS and S aD g Ha ( fi S- 34 ) 0f ttG (Taiurus lomsia). (Grant.) two sides of the body are separate Showing separate cerebral « . , . ganglia, each about the same "'om one another as they are in Ser- size as other ganglia, situated pu l a (fig. 32) although the ganglia on the separate ventral cords. o . 00 Fig. 85.— Nervous System are here fewer in number and much of Cymotlioa. (Grant.) Cere- distinct. bral ganglia almost wholly absent from oesophageal In slightly higher forms of Crus- ring. Oesophageal cords dis- , . , . . tinct, and uniting below into tacea, however, the two divisions of a sh.gie ventral- cord, with the originally double ventral cord compound ganglia at inter- i vais. always become fused together, whilst, at the same time, the equality of the several ganglia diminishes. Thus, in such forms as the Chap. VI.] NERVOUS SYSTEM OF ARTHROPOES. 97 Lobster and the Crayfish, the ganglia of the thorax, whiah supply nerves to the limbs, are distinctly larger than those of the abdominal segments, though these are also of good size, since the tail-segments are actively called into play during locomotion. - In the Prawn a further development and concentration of the nervous system is seen. The thoracic ganglia are fused into a single elliptical mass, whilst those of the abdominal segments still re- main separate. But in the ordinary edible Crab (fig. 36) and its allies, an even more remarkable concentration of the nervous system is met with. All the thoracic and all the abdo- minal ganglia are here fused into one large perforated mass of nervous matter (c, c), situated near the middle of the ventral region of the body.* From this large and compound ganglionic mass nerves are received from, and given off to, the limbs, to the abortive tail, and to other adjacent parts. The brain (a) of the Crab is repre- sented by a rather small bilobed ganglion. It receives nerves from the pedunculate i r Fig. 36. — ^ervous compound eyes, from the two pairs of system of a crab . ip .i i ■ i • (Palinurus vulgaris). antennas, and from the palpi-bearing man- ] i? Fused cerebral dibles. The posterior antennas (or anten- s^gUa receiving ,, optic, tactile, and nules, as they are sometimes termed) con- olfactory (?) nerves ; tain in their basal joint a body which is ^ 1 s ong c oes ° pb ^ supposed to represent an olfactory organ, ventral ganglionic though others have regarded it (on very ZITds.) insufficient grounds) as an organ of hear- ing. This small bilobed brain is, indeed, thought by * A large artery passes through the aperture in this ganglion. 98 NERVOUS SYSTEM OP ARTHROPODS. many naturalists really to embody three pairs of ganglia, in relation with three pairs of sensory organs, viz., eyes, tactile antennae, and the supposed olfactory antennules. The brain is connected, by means of a long cord on each side (6, b) of the oesophagus, with, the anterior ex- tremity of the great ventral ganglion. Nerves in relation with the organs of mastication join the cords about mid- _ way between the brain and the great abdominal ganglion, and small ganglia are to be found in this situation. Just behind these small ganglia a transverse commissure con- nects the cords with one another. The unusual length of the oesophageal cords is one of the most notable character- istics of the nervous system of the higher Crustacea, and this seems due in part to the fact that the sub- oesophageal ganglia remain separate instead of uniting with one another, as they do in fig. 18. The ' stomato-gastric ' system of Crustacea is very similar to that which exists in Centipedes. One part of it is given off from the oesophageal cord on each side, while another median branch proceeds from the posterior part of the united cephalic ganglia, as in lulus (fig. 33,/). Where the main nerve lies on the upper surface of the stomach, in the higher Crustacea, it is connected with one or two ganglia from which branches pass to the walls of this organ. They send filaments also to the right and left, into the liver. This principal visceral nerve is brought into communication with the above-mentioned nerves, going to the organs of mastication, by means of two filaments which join the ganglionic swellings on the oesophageal cord at the part whence they issue. Among Arachnida forms of the nervous system exist which agree in many respects with those belonging to members of the class last described — especially where Chap. VI.] NERVOUS SYSTEM OF ARTHROPODS. 99 there are general similarities in the external configuration of the hody. Thus in Scorpions the arrangement of the nervous system is not very dissimilar from that met with in the Prawn and .its allies. The thoracic ganglia have coalesced with one another and with the anterior abdominal ganglia ; thereby forming a large stellate ner- vous mass which supplies the limbs and the anterior part of the abdomen. The ventral cord throughout the remainder of the abdomen, and its caudal prolongation, is marked at intervals by a series of small ganglionic swellings. In Spiders proper, the nervous system attains its maxi- mum amount of concentration. The bilobed brain (fig. 37, c) receives nerves on each side (o), corresponding in number with the ocelli which the animal may happen to possess. Pig. 37.— Head and Nervous System of a Spider {MygaU). (Owen after Duges.) c, Cerebral ganglia (side view), receiving (o) optic nerves, and (m) nerves (sensory and motor) from the powerful mandibles, m'. The cerebral ganglia are connected by very short oesophageal cords with a large stellate ventral ganglion (s), from which five large nerves issue on each side (p, I, I); a, mouth ; &, oesophagus ; d, stomach. It also receives two large nerves (ra), which probably con- tain outgoing as well as ingoing fibres, from the so-called mandibles (»«•'). Owing to the suctorial habits of these fierce and predatory creatures, the oesophagus is very narrow ; and as a consequence, the oesophageal cords are very short, so that the brain is — unlike the arrangement which obtains 100 NERVOUS SYSTEM OF ARTHROPODS. in the common Crab and its allies (fig. 36)— quite close to the great stellate systemic ganglion (s), into which are fused the representatives of the sub-cesophageal, the thoracic, and the abdominal ganglia. From this ganglion (fig. 38, i) five principal nerves are sent off on each side, " the first to the pediform maxillary palpi; the second to the more pediform labial palpi, which are usually longer than the rest of the legs, and used by many Spiders rather as instruments of exploration than of locomotion ; the three posterior nerves supply the re- maining legs, which answer to the thoracic legs of hexapod Insects." (Owen.) Since the sub-cesophageal ganglia are in part analogous, as already stated, with the ' me- dulla oblongata' of vertebrate animals, their fusion with the thoracic ganglia in Arachnida, as well as in Myriapoda, tends, Fig. 38. — Nervous System of a . „ ,. great scorpion-iike Spider (Thtiy- in a measure, to confirm the plwmxcaKdatus). (Gegenbauer, after T j ew ^eld fo y gome ana t mistS, Blanchard.) s, Cerebral ganglia ; i, . . ' great ventral ganglion, communicat- that it is better to regard the i&rT.&^n^;^ ' meduUa ' as a Prolongation of c taii-iike prolongation. the spinal cord, than as an in- tegral part of the brain. The artificial line, that is, which for convenience is drawn between the brain and the cord in Vertebrates, should be placed at the upper rather than the lower or posterior Chap. VI.] NERVOUS SYSTEM OF ARTHROPODS. 101 boundary of the ' medulla,' so that the latter part may be regarded as the more highly developed portion of the spinal cord by which fusion with the brain is effected. The visceral nerves are well developed in the higher Arachnida. They consist of one or two filaments, on which a ganglion may exist, in connection with the posterior part of the brain, and thence proceeding to the stomach and other internal organs. There are, moreover, two or three branches given off from the great ventral ganglion which, after passing through smaller ganglia, distribute numerous filaments to the intestines, the respiratory and genital organs, as well as other viscera. The former set may be in the main afferent, and the latter perhaps principally efferent visceral nerves. Organs of vision are much more elaborate in Crustacea, Spiders and Insects, than among Worms or Centipedes. And, whilst organs of touch and taste are further perfected, two sensory endowments, found ' among higher Mol- lusks, seem also to manifest themselves. These higher Ar- thropods are capable of being impressed by, and of discri- minating, the different odours of some substances anterior to their contact with the mouth. This power must mate- rially aid them in their ' search ' for or recognition of food. Some Arthropods seem to be also capable of appreciating those vibrations of the medium they inhabit which induce impressions recognizable by us as sounds or noises. Still, in some of the most highly organized forms of Insects a sense of hearing appears to have no existence. Much uncertainty, in fact, exists in regard to this sense-endow- ment.* Extreme sensibility of the tactile order may cause the organism to display an apparent sensitiveness to sounds. A delicate general ability to appreciate aerial vibrations, therefore, must not be confounded with the * See pp. 65 and 205. 102 NKRVOUS SYSTEM OF ARTHROPODS. more special auditory perception. On the other hand, it is quite possible that sounds not appreciable by our Fig. 40. '%' P\ Fig. 39.— Nervous System of full-grown Caterpillar of Privet Hawk-Moth (Sphinx igustri), about two days previous to its change to the chrysalis state. Fig. 40. — Nervous System of the Privet Hawk-Moth thirty days after changing to the chrysalis state. The abdominal cords are now seen to be much shortened, and bearing five instead of seven ganglia. Fig. 41. — Nervous System of the perfect Insect. A, Greatly enlarged cerebral, and B, optic ganglia. The numerals refer to the numbers of the ganglia. i>, o, o, o, respiratory nerves, 'nervi transversa' (Solly after Newport.) organization may be perceptible by the sensory organs and centres of some of the lower organisms. Additional sensory endowments like Smell and Hearing Chap. VI.] NERVOUS SYSTEM OF ARTHROPODS. 103 would, of course, be of importance to any organisms, but more especially to those possessing active powers of loco- motion. They would serve, on the one hand, to assist in bringing their possessors into relation with food, or with sexual mates, and, on the other, to warn them of the approach of enemies. The nervous system of Insects varies not only among different classes and orders, but even in the same indivi- dual in different stages of its development. The cater- pillar of a Moth (fig. 39) or Butterfly presents a nervous system not very different from that met with in the Centipede; while in the imago or perfected Insect, the same system has undergone some re- markable changes — there is, for instance, an increased size of the cerebral ganglia, and also a notable development of some of the ganglia pertaining to the ventral cord, while concentra- tion or even suppression of others is met with. . Fig. 42.— Brain and Adjacent Parts In SUch insects as Butter- f Nervous System of a rather flies, Bees, Dragon-flies, and slu sf sh ' a P t % 0US Be t e f e - T ™ arc,ia ' -"-"-^"J (3 ^ > tenebricosa. (Nevrport.) A, BTam re- Othei'S where the Visual Ol'ganS ceiving the antennal nerves, and also . -, ■, -, j e, the optic nerves ; c, origin of the are enormously developed, ana sympatheti0 from and rear t he com- in which the power Of Vigorous mencement of the oesophageal cords; t <=> Dj the sub-cesophageal ganglia; b, the and Sustained flight is COrreS- vagus, or visceral nerve before reach- pondingly increased, the nervous ££*»**»; c lateral visceral system as a whole attains its maximum of development among the Arthropoda. The brain of these creatures differs from that existing in all other members of the class by reason of the great develop- 104 NERVOUS SYSTEM OF ARTHROPODS. ment of those portions of it in relation with the visual organs, as may be seen by fig. 45, representing the nervous system of the Common Fly, and by fig. 42, representing the brain of a Beetle. A ganglionic swelling is fre- quently found where the optic nerve joins the brain, and in some Insects there are also small ganglionic swellings at the corresponding parts of the antennal nerves. It is in Ants, Bees, and Flies, however, that the brain of Insects seems to attain its greatest development. Speak- ing of the brain of the Blow- fly, B. T. Lowne says* : — " Next to bees and ants that of the blow-fly is the largest known in any insect proportionally to its size, being about thirty times larger than the cephalic ganglia of the larger beetles." The same writer adds : — " But a more positive indication of a higher type of organization than even the relative bulk of the sensory ganglia is found in the fact that two very remarkable convoluted nerve centres, con- nected by a commissure, each about l-30th of an inch in diameter, surmount the Gephalic ganglion, and are con- nected to it by a pair of distinct peduncles ; t these are extremely like the pedunculated convoluted nerve centres which occupy the same position in bees and ants, first described by M. Felix Dujardin (" Ann. des Sc. Nat." (Ser. iii.), t. xiv. p. 195), and considered by him as analogous to the cerebral lobes of the higher animals. That naturalist failed to distinguish these organs in the fly, probably owing to their being imbedded in the substance of the cephalic ganglion." In the Bee, according to Dujardin, these peculiar bodies are attached to the sensory ganglia by a single peduncle, and their united bulk is said by him to equal -Jth of the whole brain. Further details concerning these interesting structures are much needed. The double cerebral ganglion is connected in nearly * " Anat. of the Blowfly," p. 14. f Loc cit, PI. vii. fig. 4. Chap. VI.] NERVOUS SYSTEM OF ARTHROPODS. 105 all Insects with a separate sub-cesophageal ganglion, from which nerves are given off to the mandibles, the maxilla?, and the labium. But, as in Spiders, the oesophageal ring is often very narrow, owing to the greatly diminished size of the oesophagus in the imago forms of higher Insects. In Spiders and Myriapods, as before stated, the sub-oeso- Fig. 43 Pig. 44. Fig 45 Fig. 43.— Nervous System of a White Ant (Termes). (Gegenbauer after Lespes.) Fig. 44. — Nervous System of a Water Beetle [Dytiscus). (Gegenbauer.) Fig. 45. — Nervous System of a Fly (Musca). (Gegenbauer after Blanchard.) o, Eyes ; gs, supra-cesophageal ganglia (brain) ; gi, sub-ossophageal ganglion ; gr, ft, g 3 fused ganglia of the thorax. phageal ganglion has no separate existence apart from the thoracic ganglia. In many Insects the three thoracic ganglia preserve a separate existence (fig. 43), though in others of the higher types above referred to these ganglia are more frequently fused into a single lobed mass (fig. 45). The 106 NERVOUS SYSTEM OF ARTHROPODS. eight abdominal ganglia, which are always much smaller than the thoracic, also continue to have a separate exist- ence among some of the less developed types of Insects (fig. 43) though it is more frequent for some, or even all, of them to disappear (figs. 44, 45). The ' stomato-gastric ' system of nerves attains a con- siderable degree of complexity in these animals. In front there is a median ganglion (fig. 42) lying below and often anterior to the brain. This oral ganglion is a swelling situated on the great median (afferent) visceral nerve, at the spot where it bifurcates in order to proceed to each half of the brain. It receives branches from the mouth and adjacent parts. The main nerve, or else the ganglion, is also connected with other branches (c), proceeding from one or two pairs of lateral ganglia situated close to the oesophageal cords, and often' in structural relation with them. This visceral system of nerves receives branches from the stomach, the intestines, and other internal organs. tn Insects, moreover, we meet with another semi- independent set of visceral nerves, connected with a chain of minute ganglia lying upon the great ventral ganglionated cord, and united thereto by means of minute nerve fila- ments. The nerves (fig. 41, o, o, o) in connection with this chain of minute ganglia are received from and distributed to the all-pervading respiratory organs (air tubes) of the Insect. They are known to anatomists, on account of the disposition of their main branches, as ' nervi transversi,' and are much more highly developed in these animals than are anything corresponding to them amongst other Arthropods. CHAPTER Vn. DATA CONCERNING THE BEAIN DERIVED FROM THE STUDY OF THE NERVOUS SYSTEM OF INVERTEBRATES. This survey of some of the principal varieties of the Nervous System among the Invertebrata, brief though it has been, should have sufficed to call attention to many important facts and to show the warrant for certain related inferences, many of which are embodied in the following propositions : 1. Sedentary animals, though they may possess a Nervous System, are often headless, and they then have no distinct morphological section of this system answering to what is known as a Brain. 2. Where a Brain exists, it is invariably a double organ. Its two halves may be separated from one another ; though at other times they are fused into what appears to be a single mass. 3. The component or elementary parts of the Brain in these lower animals are Ganglia in connection with nerves proceeding from special impressible parts or Sense Organs ; and it is through the intervention of these united Sensory Ganglia that the animal's actions are brought into harmony with its environment or medium. 4. That the Sensory Ganglia, which in the aggregate constitute the Brain of invertebrate animals, are connected 108 THE BRAIN OF INVERTEBRATES. with one another on the same side and also with their fellows on opposite sides of the body. They are related to one another either by what appears to be continuous growth or by means of * commissures.' 5. The size of the Brain as a whole, or of its several parts, is therefore always fairly proportionate to the develop- ment of the animal's special Sense Organs. The more any one of these impressible surfaces or organs becomes elaborated and attuned to taks part in discriminating between varied external impressions, the greater will be the proportionate size of the ganglionic mass concerned. 6. Of the several sense-organs and Sensory Ganglia whose activity lies at the root of the Instinctive and Intelligent life (such as it is) of Invertebrate Animals, some are much more important than others. Two of them especially are notable for their greater proportional development : viz., those concerned with Touch and Vision. The organs of the former sense are, however, soon outstripped in importance by the latter. The visual sense, and its related nerve-ganglia, attain an altogether exceptional development in the higher Insects and in the highest Mollusks. 7. The sense of Taste and that of Smell seem, as a rule, to be developed to a much lower extent. In the great majority of Invertebrate Animals it is even difficult to point to distinct organs or impressible surfaces as certainly devoted to the reception of either of such impressions. Nevertheless, as we shall subsequently find, there is reason to believe that in some Insects the sense of Smell is mar- vellously keen, and so much called into play as to make it for such creatures quite the dominant sense endowment. It is pretty acute also in some Crustacea. 8. The sense of Hearing seems to be developed to a very slight extent. Organs supposed to represent it have been Chap. VII.] THE BRAIN OF INVERTEBRATES. 109 discovered, principally in Mollusks and in a few Insects. It is, however, of no small interest to find that where these organs exist, the nerves issuing from them are most fre- quently not in direct relation with the Brain, but imme- diately connected with one of the principal motor nerve- centres of the body. It is conjectured that these so-called ' auditory saccules ' may, in reality, have more to do with what Cyon terms the sense of Space than with that of Hearing (p. 218). The nature of the organs met with supports this view, and their close relations with the motor ganglia also become a trifle more explicable in accord- ance with such a notion. 9. Thus the associated ganglia representing the double Brain are, in animals possessing a head, tbe centres in which all impressions from sense-organs, save those last referred to, are directly received, and whence they are reflected on to different groups of muscles — the reflection occurring not at once but after the stimulus has passed through certain ' motor ' ganglia. It may be easily under- stood, therefore, that in all Invertebrate Animals perfection of sense-organs, size of brain, and power of executing manifold muscular movements, are variables intimately related to one another. 10. But a fairly parallel correlation also becomes estab- lished between these various developments and that of the Internal Organs. An increasing visceral complexity is gradually attained ; and this carries with it the necessity for a further development of nervous communications. The several internal organs with their varying states are gradually brought into more perfect relation with the principal nerve centres as well as with one another. 11. These relations are brought about by important visceral nerves in Vermes and Arthropods — those of the ' Stomato-Gastric System ' — conveying their impressions 6 110 THE BRAIN OF INVERTEBRATKS. either direct to the posterior part of the Brain or to its peduncles. They thus contribute internal impressions which impinge upon the Brain side by side with those coming through external sense organs. 12. This Visceral System of Nerves in invertebrate ani- mals has, when compared with the rest of the Nervous System, a greater proportional development than among vertebrate animals. Its importance among the former is not dwarfed, in fact, by that enormous development of the Brain and Spinal Cord which gradually declares itself in the latter. 13. Thus impressions emanating from the Viscera and stimulating the organism to movements of various kinds, whether in pursuit of food or of a mate, would seem to have a proportionally greater importance as constituting part of the ordinary mental life of Invertebrate Animals. The combination of such impressions with the sense- guided movements by which they are followed, in complex groups, will be found to afford a basis for the development of many of the Instinctive Acts which animals so fre- quently display. CHAPTER VIII. THE BUAIN OF FISHES AND OF AMPHIBIA. In all Vertebrates the relation of the principal nervous ganglia to the commencement of the alimentary canal is different from that existing among the Invertebrates. We no longer find, as in the Mollusk, the Worm, or the Insect a ring of nerve matter encircling the oesophagus. The parts which in Fishes answer to the supra- and sub- cesophageal ganglia lie altogether above the oesophagus, and they are, moreover, directly continuous with one another, instead of being connected by long or short com- missures. In Fishes, as well as in other Vertebrates, all the parts constituting the Brain, as well as the Medulla Oblongata, are enclosed within a distinct ' skull' or ' cranium,' while within this they are again surrounded by two membranes — one of which, and the thicker of the two, lines the inner surface of the cranium ; while the other, which is delicate and transparent, immediately envelops the great nerve centres. The Spinal Cord, which is directly con- tinuous with the Medulla, is also lodged in a bony case known as the ' spinal canal' ; and this is formed by the contiguous posterior arches of the several vertebra? com- posing the spine or vertebral column. Among the Invertebrata, it is the nervous system 112 THE BRAIN OF FISHES of Insects and other Arthropods which approaches most closely to that of Fishes, inasmuch as they possess a single or double ganglionated nervous cord running through the body, which is fairly comparable with the spinal cord. In Insects and their allies, however, this cord is situated in the ventral region; while the spinal cord of Verte- brates lies above the alimentary canal in the dorsal region of the body. No such structure exists or is needed among Mollusks, because these organisms have no articulated locomotor appendages, and are otherwise notably different in form and organization ; yet it is true that among the highest representatives of this latter class (viz., the Cephalopods), we get the first approach to the formation of a distinct brain case or ' cranium.' All the nerve-centres situated within the cranium have been regarded as parts of the Brain in Vertebrates, whilst those lying beyond it, and within the spinal canal, con- stitute the Spinal Cord : the two together are sometimes spoken of as the ' Cerebro- Spinal Axis.' But in addition to the Sensory Ganglia, and the Medulla Oblongata, there are certain highly important supple- mentary parts entering into the composition of the Brain of the Fish. There is, for instance, a pair of bodies known as the Cerebral Lobes ; whilst further back, in connec- tion with the Medulla, we have another new nervous ganglion, single, but having equal parts on each side of the middle line, which is known as the Cerebellum. That representatives of these parts (seemingly superadded to the brain of Fishes and other Vertebrates) are really non-existent in the highest Mollusks and Insects it would not be safe to affirm ; especially as ganglia, which have been compared to Cerebral Lobes, exist in the Cuttlefish, and even more distinctly in Ants, Bees and some Flies. On the other hand, both the Cerebral Lobes and the Chap. VIII.] AND OF AMPHIBIA. 113 Cerebellum tend to increase in size and become more and more complex as we pass from Fishes to Eeptiles, from Eeptiles to Birds, and from Birds to Mammals. The relative size of these parts, however, as well as of other divisions of the Brain, will be found to vary greatly in different kinds of Fishes. Fig. 46.— Brain and Cranial Nerves of the Perch, side view. (Gegenbauer, after Cuvier.) A, Cerebral lobe with olfactory ganglion in front ; B, optic lobe ■. C, cere- bellum ; D, medulla oblongata ; /, olfactory nerve coming from a, the nasal sac ; //, optic nerve cut across ; ///, oculo-motor ; IV, trochlear nerve ; V, trigeminal ; VII, auditory ; VIII, vagus, with its ganglion ; k, lateral branch of the vagus ; I, upper twig of the same ; m, dorsal branch of the trigeminus, which is joined by n, the dorsal branch of the vagus ; a, j3, y, three branches of the trigeminus ; fie, f icial nerve •_ X, Uronchial branches of tho vagus. The Spinal Cord of Fishes is more or less cylindrical in shape (fig. 47, h) and almost uniform in thickness through- out, except that it tapers to a point posteriorly. It occurs only rarely that there is, as in the Bay, a slight swelling in the region where the nerves from the great pectoral fins 114 THE BRAIN OF FISHES are received, and sent forth. From the whole length of the spinal cord a series of nerves is given off on each side, and each of them is connected therewith hy an ante- rior (or motor) and a posterior (or sensory) root, the latter swelling into a more or less distinct ganglion just where its fibres begin to mingle with those of the anterior root. This mode of connection of the spinal nerves with the spinal cord exists throughout the class of Fishes and also in all other Vertebrates. Anteriorly the cord is continuous with a slightly more swollen prolongation — the before-mentioned Medulla Oblongata (fig. 47, d). Many very important nerves, to which reference will subsequently be made, are attached to this part. Growing from the back of the anterior extremity of the medulla is a semi-ovoid or tongue-like projection, which has been already referred to as the Cerebellum. Though single in appearance, it is really double and composed of two symmetrical halves. No distinct connection of nerves with this body can be detected by the naked eye. The cerebellum exists in its simplest form in the parasitic Cyclostomes, in the Sturgeon, and also in Polypterus and Le- vidmg and penetrat- pidosiren, where it appears merely as a ^o^rXi- si ^Ple bridge or commissure, crossing the !y) anterior and upper part of the medulla. In most osseous Fishes it is larger, and projects backwards over the medulla in the form of a Fig. 47.— Brain of the Pike, a, Olfac- tory ganglia; B, ce- rebral lobes ; c, optic lobes; E,cerebellum ; h, spinal cord ; x, olfactory nerve, di- Chap. Till.] AND OF ViMPIUBIA. 115 smooth, convex, semi-ovoid, or tongue-like body (fig. 49, d). According to Professor Owen, the cerebellum is " very small iu the lazy Lump-fish, and extremely large in the active and warm-blooded Tunny." It attains its highest develop- ment, however, in Sharks (fig. 48, c). In these most active and predaceous fishes the cerebellum not only covers much of the medulla, but advances forwards over the optic lobes, and the extent of its surface is further increased by the existence of numerous superficial folds or indentations. In front of the cerebellum are two rounded ganglia known Fig. 48. Pig. 48. —Brain of the Shark (Cardiavia^, side view: (Owen.) p, Cerebral hemi- sphere ; o, optic lobe ; c, cerebellum with surface folds (m) ; R, olfactory ganglion, giving off (1) olfactory nerves ; z, junction of olfactory peduncle with cerebral lobe ; x Crus cerebri ; w, pineal body ; n, hypoaria ; p, pituitary body ; 2, optic nerve ; 3, oculo-motor nerve ; 5, trigeminus ; 7, auditory ; 8, vagus. F, G . 49.— Brain of Roach, a, Olfactory peduncles ; b, cerebral lobes ; c. optic lobes ; d. cerebellum ; e, medulla ; /, optic nerves. (After Spurzheiru.) as the Optic Lobes (fig. 49, c), which correspond with the principal part of the Insect's brain. The optic nerves are connected with their under surface ; and they decussate (figs. 51, 57), so that the one proceeding from the right eye passes to the left optic lobe, and that from the left eye to the right optic lobe. This new kind of cross arrangement will, in a later chapter, be referred to in detail, since, with slight differences, it also exists in other Vertebrates, and, moreover, seems gradually to extend to other parts of the nervous system. 116 THE BRAIN OP FISHES In many of the lower Fishes the eyes are very rudimen- tary. In the young Lamprey two pigment spots replace the single ' eye spot ' of the Lancelot. In the genus Myxine the eyes are represented by small bodies, which, though in connection with slender optic nerves, are covered over by muscle as well as by skin. The ocular muscles for moving the eyeball are absent in many Fishes ; this is the case even in the Gar-Pike, in which, though Fig. 51. Fig. 50.— Brain of Perch, upper surface. (Owen after Cuvier.) a, Cerebellum ; 6, optic lobes ; c, cerebral lobes ; i, olfactoiy ganglia ; ff, medulla ; p, n, r, s, t, cra- nial nerves. Pig. 51.— Brain of Perch, under surface. (Owen after Cuvier.) a, Medulla; e. hypoaria ; /, pituitary body ; n, optic nerves, decussating ; c, cerebral lobes ; i, olfac- tory ganglia ; p, q, r, *, t , cranial nerves. small, the eyes are at the surface. In the great majority of Fishes, however, these organs are large and attain a remarkable development. The optic lobes are usually the largest divisions of the brain in osseous fishes, as in the Perch (fig. 50), and they are commonly united by one or more transverse com- missures. Each of them generally contains a distinct cavity or ' ventricle,' and they often bear on their under surface two smaller ganglionic projections, known as Chap. VIII. ] AND OF AMPHIBIA. 117 'hypoaria.' These bodies are well developed in the Perch, and in the Cod (figs. 51, 57). Their use is un- known, and it is remarkable that they are structures peculiar to the brain of Fishes. In connection with the optic lobes there are also two peculiar structures, one above and the other below, known as the ' Pineal ' and ' Pituitary ' Bodies (figs. 53, 3; 60, 3, e ). In front of the optic lobes are the already men- Fig. 52. Fia. 53. Fig. 52.— Brain of Carp. (Ferrier.) a, Cerebral lobes ; b, optic lobes ; c, cerebel- lum and medulla. Fig. 53.— Upper aspect of the Brain of a Hay, or Ekate (Raia balie). 1, Olfactory lobes ; 2, the conjoined cerebral lobes ; 3, the pineal gland ; 4, optic lobes ; 5, cere- bellum ; 6, medulla, with ganglionic projections. (Mivart.) tioned Cerebral Lobes. They, like the cerebellum, have no obvious connection with nerves, and vary much in size in different Fishes, though they are mostly, as in the Carp (fig. 52) and the Perch (fig. 50), smaller than the optic lobes. The Cerebral Lobes are smallest in the Lamprey and its allies, in the Herring, and in the Cod ; while they are most developed in the Skate, the Shark, Polypterus, and Lepidosiren. In the Skate (fig. 53), they coalesce 118 THE BRAIN OF FISHES into a somewhat flattened, transversely elongated mass, showing only slight indications of a median fissure. In the Shark (fig. 48) they also unite to form a large almost globular mass with little trace of a median furrow. A similar fusion of the two lobes occurs in some other Fishes, though in the majority they exist as spheroids united only by a transverse commissure. In Lepidosiren the cerebral hemispheres are larger than all the rest of the brain; each of them also contains a cavity or ventricle, which is Fig. 55. Pig. 54.— Brain of Lepidostcus or Gar-Pike\ (Owen.) n, Olfactory ganglia ; p, cerebral lobea ; o, optic lobes ; c, cerebellum ; 7t, medulla ; /, fourth ventricle ; rf, lower boundary of medulla. Fig. 55.— Brain of the Whiting. (Solly.) a, Olfactory ganglia ; B, cerebral lobes; c, optic lobes ; e, cerebellum and medulla. prolonged into the olfactory lobe. In these respects they closely agree with the cerebral lobes of Reptiles. In the Gar-Pike (fig. 54), the Perch, the Mackerel, and many other Fishes, two additional ganglia known as the Olfactory Lobes lie immediately in front of the cerebral lobes, and each of them receives a long olfactory nerve.* * The Lancelot has a single olfactory sac and a single nerve; in all other fishes, except in the Lamprey and its allies, there are two nerves (see Huxley, " Journ. of Linn. Soc." (Zool.), vol. xii. p. 224). Chap. VIII.] AND OF AMPHIBIA. 119 But in such Fishes as the "Whiting (fig. 55), the Carp (fig. 52), the Skate (fig. 53), the Shark (fig. 48), and others, the olfactory ganglia are situated at a distance from the cerebral lobes, with which they are connected only by means of two long and narrow outgrowths or peduncles. In these latter Fish the ganglia are to be found close to the olfactory organs, from which they receive numerous short nerves. Such are the essential parts in the brain of the Fish. Their relative size or development is, however, subject to almost countless diversities in different genera. From the foregoing description, it will be seen that ona of the principal characteristics of the Brain of Fishes is to be found in the serial arrangement of its parts, in a line with one another and with the spinal cord ; whilst another is the small mass of the Brain as compared with that of the Spinal Cord, and still more in comparison with the mass and weight of the entire body. In the former respect, at least, the Brain of Amphibia (fig. 56) agrees closely with that of Fishes. The principal • Pig. 56.— Brain and Spinal Cord of the Frog a, Olfactory lobes ; B, cerebral lobes; it, pineal body ; c and D, optic lobes ; E, cerebellum ; H, spinal cord. divisions of the brain also in these animals are identically the same. The Brain of the Frog is notable principally for the smaller size of its Cerebellum, and also for the diminished bulk of its Optic Lobes and Olfactory Ganglia. lxi - Cerebral Lobes, are, therefore, proportionately large. The Serial Cord is shorter than usual, and does not occupy the -vhole length of the ' spinal canal.' 120 THE BRAIN OF FISHES Though the Cerebellum itself does not appear to be immediately connected with any nerves, the Medulla Oblongata, from which this part is an outgrowth, is remarkable in Fishes, as well as in other vertebrates, for the number and importance of the nerves with which it is connected. Indeed, if the limits of the Medulla are taken to be those originally defined by Willis and most anatomists anterior to Haller (1762), they will include the ' crura cerebri ' ; and in that case all the Cranial Nerves (that is, the nerves which pass inwards or outwards through holes in the cranium), except the olfactory and the optic, would have to be described as in direct con- nection with the medulla oblongata. The Cranial Nerves of Fishes and of Amphibia are, with few exceptions, similar in number and nature to those existing throughout the vertebrate series, so that they may with advantage be here enumerated. According to the classification of Willis (1664), which is generally followed, they are said to consist of nine pairs, counting from before backwards. (See figs. 46, 57, 58.) Olfactory. Optic. Motor oculi communis; supplying all but two of the muscles of the eyeball and the circular fibres of the iris. Trochlearis ; supplying the superior oblique muscle of the eye. fLarge root : the nerve of general sen- sibility for the side of the head, face, &c. Small root : supplying muscles con- nected with the jaw (muscles <>* mastication). 6th „ Motor oculi externus ; supplying th<> external rectus muscle of the eyeball. ' 1st Pair. 2nd » 3rd " «j M 5 4th 97 ^l *\ < £ < o 5th » Trigeminus to 5 < s o 8th Chap. VIII.] AND OF AMPHIBIA. 121 .Auditory. 7th „ . Facial; supplying the superficial muscles of the face, &c. /■Glosso-pharyngeal (gustatory nerve and nerve of common sensibility for the pharynx). Vagus, or Pneumogastrio (seusory nerve of respi- ratory organs, heart, alimentary canal, liver, I kidneys, &o. I Spinal accessory ; supplying the muscles of the * larynx, &c. 9th „ Sublingual, or Hypoglossal; motor nerve of tongue and of muscles which move it. From this table it will bo seen that three of the ' pairs ' of cranial nerves (5th, 7th, and 8th) are compound in their nature. Their parts have, moreover, little in common, except for the fact that the components of each so-called ' pair ' in man and many of the lowe* animals pass side by side through the same hole in the skull. This, indeed, seems to have been the principal reason actuating the earlier anatomists when they grouped them together.* No cranial nerves answering to the 9th pair exist in Fishes : their functions being discharged by branches from the first spinal nerve. The motor root of the 8th, the ' spinal accessory,' is also less distinct as a separate nerve in Fishes and some Reptiles, than it is in higher vertebrates. Looked at from the point of view of the functions which * Except in the case of the two divisions of the 5th nerve, this grouping was not respected in the classification of Soemmering (1778). According to him, the cranial nerves were to be regarded as twelve pairs, the first six agreeing with those of Willis, whilst the facial is called the 7th, the auditory the 8th, the glosso-pharyngeal the 9th, the vagus the 10th, the spinal accessory the 11th, and the sublingual the 12th. 122 THE BRAIN OF FISHES I. Nerves of Special Sense. II. Nerves of General Sensibility. III. Motor Nerves. they subserve, these Cranial Nerves fall into the following groups : I Olfactory. Optic. Auditory. Gustatory. I Large root of 5th. Part of Glosso-pharyngeal. Vagus (the visceral nerve). ( Motores oculi (3rd, 4th, and 6th pairs). I Small root of 5th. "* Facial nerve. Spinal accessory. v Sublingual or Hypoglossal. Taking the larger view held by Willis and others, as to the limits of the Medulla Oblongata, and including under this name all those parts of the Brain, with the exception of the cerebellum, posterior to the optic lobes, we find the several pairs of true cranial nerves (from 3rd to 9th inclusive) attached to it on each side, and for the most part in the order of their numeration (the 3rd issuing from it close to the optic lobes, and the 9th close to the junction of the medulla with the spinal cord), with the reservation that in Fishes the nerves of the 8th pair are the last which pertain to the medulla. The ' sensory ' nerves attached to the Medulla, are, like those of the spinal cord marked by ganglionic swellings near or at the points of attachment of such nerves (p. 44). Thus the roots of the Vagus or Pneumogastric in a large Fig. 57.— Brain of tbe Cod, uncU r surface. (Owen.)p,Cerebrallobes ; c, optic lobes ; n, bypoa- ria ; p, pituitary body ; a, anterior pyramids ; 2, optic nerves, crossing ; 3, oculo-mocor ; 5, trige- minus ; 6, external ocu- lar ; 7, auditory ; S, vagus and glosso-pbaryngeal. Chap. VIII. ] AND 0F AMPHIBIA. 123 number of fishes become swollen into distinct ganglia at their point of junction with the Medulla, and in some- such as the Carp, the Torpedo, the Electric Eel, and the Skate — these lateral ganglia, situated at the side of the cerebellum, are exceptionally large. The Glosso-pharyn- geal is in reality only a large separate branch of the vagus. In some fishes it joins one of the roots of the vagus ; and, even where this external junction does not exist, an internal union is effected by the smaller nerve entering the nucleus of the larger one. A little anterior to the ganglia of the Vagi, large swellings are also frequently met with in connection with the roots of the Trigeminal nerves (fig. 10), which in fishes are mostly very large, and have an extensive dis- tribution even beyond the region of the head. The remaining sensory nerves of the medulla — the Auditory — are attached to it by two or three roots, between the vagi and the 5th nerves. These nerves are large, though it is only rarely that a distinct ganglionic swelling is found at their point of junction with the medulla (fig. 11). The ganglia are usually embedded in the Medulla itself, and some of its roots soon join another large ganglion : viz., the Cerebellum. This apparent connection of the auditory nerves with the great motor ganglion in Verte- brates, whatever its explanation may be, is quite in har- mony with the close relation of the ' auditory saccules ' and nerves to the pedal ganglia in Mollusks, and with their relation to the most active motorial centres of the ventral cord in those Insects (such as Locusts and Grass- hoppers) in which the so-called ' auditory saccules ' have been positively detected.* * The Organs of Hearing in Fishes are always double, as in invertebrate animals. They are, moreover, situated within the body, and mostly have no connection with its surface. Sometimes 124 THE BRAIN OF FISHES AND OF AMPHIBIA. The ganglia at the roots of the Olfactory and Optic nerves are sufficiently obvious and remarkable, so that no further reference need here be made to them, except to point out that they, together with the ganglia at the roots of the Trigeminus and Vagus, undergo a propor- tionate diminution in size as the Cerebral Lobes become better developed, among Reptiles and Birds — changes which seem to imply that functions previously discharged by lower sensory ganglia are gradually passed on and merged as products of a higher order of cerebral activity, when such higher co-ordinating centres arise and come into fuller action. The ganglia at the roots of the Auditory nerves, how- ever, do not seem to attain their maximum size till we come to Reptiles, a fact which may be accounted for by the probably rudimentary state of this sense endowment among Fishes. It will be found, therefore, to be a peculiarity of all Sensory Nerves in vertebrate animals that their fibres pass through such Ganglia before they impinge upon the great nerve centres — a fact originally noticed by Sir Charles Bell. No corresponding ganglia exist in connection with motor nerves, outside the anterior cornua of the spinal cord. they are lodged outside the cranial cavity, sometimes in the walls of the cranium, and sometimes half within and half ontside this cavity. Their structure is extremely simple, and in some fishes they are only a very little more complex than the 'auditory saccules ' met with in the Cuttle-fish. In the fact that in Fishes, as in other vertebrates, the auditory organs are always situated in the head, we have a departure from the rule so commonly obtaining among Invertebrates. Perhaps, in its simplest forms, this apparatus may have as much to do with the organism's Space relations as with Hearing (see p. 218). CHAPTER IX. THE BRAIN OF REPTILES AND OF BIRDS. The nervous system of Reptiles generally exists in a slightly more developed form than that which is common amongst Fishes. The Spinal Cord occupies the whole length of the spinal canal. It is slender and almost uniform in thick- ness in Serpents, though it is relatively stouter in Croco- diles and their allies. In the latter it also presents decided swellings in those regions whence the nerves are given off, on each side, for the fore and hind limbs. The principal divisions of the Brain are the same in all kinds of Reptiles, though, as might have been expected from the varied form and nature of the different representatives of this great class, the respective development of the several divisions of the organ varies much in different orders. The Medulla Oblongata, directly continuous with the spinal cord, slightly widens at its upper part, where it is surmounted by the Cerebellum. This latter structure, in the Lizard (fig. 59) and its allies, is very small, consisting only of a thin lamella. The cerebellum is larger, how- ever, among Serpents (fig. 58), and it becomes still more developed in Turtles (fig. 61) and Crocodiles. The Optic Lobes are relatively smaller in most Reptiles than they are among Fishes ; and in the Boa Constrictor 126 THE BRAIN OF REPTILES they show a transverse fissure which divides the two bodies into four parts, corresponding to the ' corpora quadrigemina' Fig. 58.— Brain and Cranial Nerves of Boa Constrictor. (Rymer Jones, after Swan.) a, Cerebral lobes ; 6, optic lobes with transverse depression ; c, cerebellum ; d, mem- brane of the ncse ; 1, olfactory nerve ; 2, optic nerve ; 3, third, or common oculo- muscular nerve ; 4, fourth, or trochlear nerve to the superior oblique muscle of the eye ; 5, first trunk of the fifth ; 6, se :ond trunk of the fifth ; 7, third trunk of the fifth ; 8, hard portion of the seventh nerve ; 9, auditory nerve ; 10, glosso-pharyngeal nerve ; 11, trunk of the vagus nerve; 12, ninth nerve. The last three nerves are intimately connected with, one another, and -with 13, a sympathetic ganglion Chai\ IX.] AND OF BIRDS. 127 of higher Vertebrates (fig. 58, b). Between the optic lobes and the next great division of the brain, the cerebral lobes, we find the so-called ' pineal body ' (fig. 61, j), projecting upwards, and in a more developed form than that which is met with in Fishes. The nature and uses of this body are wholly unknown. It is chiefly notorious from the fact that Descartes pointed to the corresponding structure in the human brain as the " seat of the Soul." The Cerebral Lobes in the Lizard (fig. 59) and its allies, as well as in Amphibia, are, in comparison with other parts of the brain, much larger than in Fishes. This is due only in part to an absolute increase in their development, as there seems to be some diminution in the size of the olfactory and optic lobes and the cerebellum. In Serpents, Crocodiles, Tur- tles (fig. 61), and their allies, however, we meet with a decided absolute increase in the size of the cerebral lobes. In Crocodiles, for instance, they are much larger and broader than other parts of the brain, though their surface is still quite smooth. Each lobe contains a cavity or ' ventricle ' in its interior, as in some of the higher Fishes. But in Reptiles the ventricle is larger, and, projecting from its anterior and inner surface there is a rounded emi- nence, supposed by some anatomists to represent a body of considerable importance — which is known amongst higher vertebrates as the ' Corpus Striatum ' or striate body. Each Cerebral Lobe is connected with its corresponding optic lobe and with the same half of the medulla oblongata, by means of a thick and composite prolongation called the Fig. 59.— Brain of Lizard (Lacerbx viri- dls). a, Cerebral hemispheres ; 6, op- tic lobes ; c, cerebel- lum ; d, spinal cord ; e, fourth ventricle ; /, pineal body; g, olfactory ganglia. (Owen.) 128 THE BRAIN OP REPTILES ' cerebral peduncle.' On the upper and inner part of each of these composite peduncles, just anterior to the optic lobes, there is a small projection, supposed to answer to another very important ganglionic body; which, in higher vertebrates, is known as the ' Thalamus.' As to the identity of these bodies, however, some difference of opinion exists. They, together with the inner faces of the peduncles on which they are situated, constitute the lateral boundaries of another brain cavity, known as the ' third ventricle,' which is mostly covered over above, by the backward extension of the cerebral lobes. A band of fibres, termed the ' anterior commissure,' which connects certain regions of the two cerebral lobes — hereafter to be specified — arches across the anterier part Fig. 60.— Vertical Longitudinal Section o{ the Brain of Perch. (Mivart.) 1, Olfac- tory lobe ; 2, cerebral lobe ; 3, pineal body : 4, optic lobe, with large cavity within ; 5, cerebellum : 6, pituitary body ; 7, hypoarium. of this Third Ventricle ; whilst the upper strata of the two cerebral peduncles are connected by means of a smaller 'posterior commissure,' crossing the posterior boundary of this ventricle, just in front of the optic lobes. The peduncles or attachments of the before-mentioned ' pineal body ' Eire structural relation with the posterior com- missure. The Third Ventricle is continuous below with a funnel- like prolongation, at the extremity of which is a structure named the 'pituitary body,' not altogether unlike the 'pineal body,' and whose use is similarly unknown. Though present in Fishes and higher Vertebrata, the pituitary body is especially large in many Reptiles. Chap. IX.] AND OF BIRDS. 129 The Olfactory Lobes have, throughout the class of Reptiles, a smaller proportionate size than in Fishes. In Serpents (fig. 58) and Crocodiles they are situated, as in "some of the last-named creatures, at a distance from the cerebral lobes — being connected with them by long peduncles. In Lizards and their allies the olfactory lobes are more or less continuous with the cerebral lobes (fig. 59) ; while in the Turtle and other Chelonians, they are marked off from the anterior extremities of the cerebral hemispheres only by a slight constriction (fig. 61, a), and each olfactory lobe is penetrated by a prolongation from the corresponding cerebral ' ventricle.' With regard to the Cranial Nerves of Reptiles, it may be remarked that the Trigeminus and the Vagus (or visceral nerve) are still very large, but neither of them swell at their roots into such dis- tinct ganglia as in Fishes. The Glosso- pharyngeal, or nerve of taste, joins the internal nucleus of the Vagus in Amphi- bia, though in Serpents and higher Rep- tiles it has a nucleus of its own, distinct from that of the latter. The Auditory nerves are large, and in Turtles, Croco- diles, and their allies, they swell into distinct ganglionic enlargements at the back of the medulla, on each side of the floor of the ' fourth ventricle.' The brain of Reptiles, like that of P iMalt0 ^- Fishes, is still characterized by the arrangement of its Fio. 61.— Brain of Tur- tle, side view. (Solly.) Olfactory ganglion ; B, cerebral hemisphere ; , optic ganglion ; e, ce- rebellum ; o, ganglion at root of vagus nerve ; j, 130 THE BRAIN OF REPTILES several parts and the spinal cord in the same horizontal plane, and by the small size of the Brain as compared with the latter structure. Still, the brain is more nearly equal in weight to the cord than it is in Fishes, and it also bears, in the majority of Eeptiles, a greater proportion to the total body-weight. But in Bieds we find the Brain attaining a notably greater size in proportion to the bulk of the Spinal Cord than it has among Eeptiles, and also presenting other signs of increased development. According to Leuret, the average proportional weight of the brain to the body in the four undermentioned classes, as deduced from numerous observations on different representatives of each, may be stated to be as follows : In Fishes as 1 to 5,668 In Birds as 1 to 212 In Reptiles as 1 to 1,321 In Mammalia... as 1 to 186 These figures must, of course, be regarded merely as approximate averages. No peculiarity worthy of note exists in the Spinal Cord of Birds, except that in the situation of its posterior enlargement, corresponding with the attachment of the great nerves of the legs, the posterior columns of the cord diverge from one another, and shortly again approxi- mate so as to form a space, known as the ' rhomboidal sinus.' This, however, is an anatomical peculiarity to which no physiological significance is attached. The Medulla Oblongata, from the back of which the cerebellum is developed, is, in Birds, decidedly broader than the spinal cord. As in lower vertebrates, the diver- gence of the upper or posterior columns of the cord leaves at the corresponding surface of the medulla the space known as the ' fourth ventricle,' which becomes much Chap. IX.] AND OF BIRDS. 131 more completely roofed over than it is in Fishes or Sep- tiles, by the under surface of the now larger cerebellum (fig. 64). The Auditory nerves arise from about the middle of the floor of the fourth ventricle, where, as in some Reptiles, they are connected with a distinct ganglionic emi- nence on each side of the middle line. The Trigeminus is always large, and exceeds all the other cranial nerves in size, with the exception of the Optic. The Cerebellum is much larger than we have hitherto met with it — with the single exception of that of the Fig. 02. Fig. 62. — Brain of Pigeon. (Ferrier.) a, Cerebral hemispheres ; B, optic lobe ; o, cerebellum with transverse furrows and very small lateral lobes. Fig. 63. — Brain and part of Spinal Cord of Chick 16 days old, showing the optic lobes (6) still in contact — at their inner borders. (Owen, after Anderson.) Flo. 64. — Brain and part of Spinal Cord of Chick, 20 days old, showing the optic lobes (6) now widely separated, and cerebellum (c) greatly developed. (Owen, after Anderson.) Shark. It now consists of a more or less ovoid median lobe (deeply scored by transverse furrows), and of two much smaller lateral portions, which project slightly be- hind the optic lobes (fig. 62, c). These Optic Lobes are pushed aside and depressed so that they are partly covered by the large cerebral hemi- spheres (figs. 63, 64). In form they are rounded bodies, •showing no trace of a transverse division. Each contains a cavity, opening below and internally iuto a subjacent ■passage or canal, which serves to connect the fourth with 132 THE BRAIN OF REPTILES the third ventricle. The two optic lobes are connected with one another by a wide commissure, which constitutes the roof of the above-mentioned passage. The optic nerves arise from the under surface of these lobes. They are lamellated structures ; and at the place where the two nerves cross one another, their lamellae interlock ; instead of the one nerve, as a whole, passing over the other, as is the case in Fishes. In front of the optic lobes are the cerebral peduncles or ' Crura Cerebri/ between which the ' third ventricle ' is situated. Stretching across this space, immediately in front of the optic lobes, is the ' posterior commissure ' of the brain, with which (as in Reptiles) the peduncles of the ' pineal body ' are connected — a structure sometimes seen to project in the brain of Birds between the cerebral hemispheres and the cerebellum. A little in front of this ' posterior commissure ' a rounded prominence may be seen on the upper and inner aspect of each cerebral peduncle — that is, on the portion which constitutes part of the lateral boundary of the third ventricle. A similar projection has been previously alluded to as occurring in some Eeptiles, and it is supposed to correspond with the important structures termed the 'Thalamus' of a Mammal's brain". The anterior part of the floor of the third ventricle still communicates, by a short hollow peduncle, with the peculiar ' pituitary body ' — a structure which, in Birds (fig. 66, e) is proportionately less de- veloped than in Eeptiles and Fishes (fig. 60,