NERVOUS CENTRES OF FISHES. 365 proceeding from it, are largest in those animals in which the brain is smallest. B ce ор 453. It is in FISHES that we find the brain least developed, and the cerebral hemispheres bearing the smallest proportion to the other parts. On opening the skull, we usually observe four nervous masses (three of them in pairs) lying, one in front of the other, nearly in the same line with the spinal cord. Those of the first pair are olfactory ganglia, or the ganglia of the nerves of smell (fig. 192 a, ol). In the Shark, and some other Fishes, these are separated from the rest by peduncles or foot-stalks (B, ol); a fact of much interest, as explaining the arrangement which we find in Man (§ 458). Behind these is a pair of ganglionic masses (c h), of which the relative size varies considerably in different fishes ch (thus in the Cod they are much smaller than those op which succeed them, whilst ce in the Shark they are much larger); these are the cerebral sp hemispheres. Behind these, again, are two large masses (op), the optic ganglia, in which the optic nerves terminate. And at the back of these, overlying the top of the spinal cord, is a single mass, the cerebellum (ce); this is seen to be much larger in the active rapacious Shark, the variety of whose movements is very great, than in the less energetic Cod. The spinal cord (sp) is seen to be divided at the top by a fissure, which is most wide and deep beneath the cerebellum, where there is a complete opening between its two halves. This opening corresponds to that through which the œsophagus passes in the Invertebrata; but, as the whole nervous mass of Vertebrated animals is above the alimentary canal (§ 74), it does not serve the same purpose in them; and in the higher classes the fissure is almost entirely closed by the union of the two halves of the cord on the central line. Fig. 192.-BRAINS OF FISHES. 454. In REPTILES we do not observe any considerable advance in the character of the brain, beyond that of Fishes; 366 NERVOUS CENTRES OF REPTILES, BIRDS, AND MAMMALS. save that the Cerebral hemispheres are usually larger, extending forwards so as to cover-in the Olfactive ganglia (fig. 193). The Cerebellum is generally smaller, as we should expect from the inertness of these animals, and the want of variety in their movements (§ 480). The Spinal Cord is still very large, in proportion to the nervous masses contained in the skull; and, as we shall hereafter see, its power of keeping-up the movements of the body, after it has been cut-off from connexion with the brain, is very considerable. 27 Fig. 193-BRAIN OF REPTILE. a, cerebral hemispheres b, optic ganglia; c, cere bellum; d, spinal cord. 455. In BIRDS, however, we find a considerable advance in the character of the brain, towards that which it presents in Mammalia. The Cerebral hemispheres (a, fig. 194) are greatly increased in size, and cover-in, not only the olfactory ganglia, but also in great part the optic ganglia, b. The Cerebellum, c, also, is much more developed, as we should expect from the number and complexity of the movements performed by the animals of this class; but it is still undivided into hemispheres. The Spinal Cord, d, is still of considerable size, and is much enlarged at the points from which the nerves of the wings and legs originate; in the species whose flight is most energetic, the enlargement is the greatest in the neighbourhood of the wings; but in those which, like the Ostrich, move principally by running on the ground, the posterior enlargement, from which the legs are supplied with nerves, is the more considerable. b с Fig. 194.-BRAIN OF 456. In MAMMALS, we find the size of the Cerebral hemispheres very greatly increased, especially as we rise towards Man; whilst the olfactive and optic ganglia are proportionally diminished, and are completely covered-in by them. The surface of the cerebral hemispheres is no longer smooth, as in most of the lower classes, but is divided by furrows into a series of convolutions (fig. 196); by these, the surface over which the blood-vessels come into relation with the nervous matter is very greatly increased; and we find the 368 NERVOUS CENTRES OF MAMMALS. convolutions more marked as we rise from the lowest Mammalia, in which they scarcely exist, towards Man, in whom the furrows are deepest. The two hemispheres are much more closely connected with each other, by means of fibres running across from either side, than they are in the lower tribes; and in fact, a considerable part of their mass is made up of fibres that pass among their different portions, uniting them with each other. The Cerebellum, also, is divided into two hemispheres (b, fig. 195); and the grey matter in its interior has a very complex and beautiful arrangement, which causes it to present a tree-like aspect when it is cut across (d, fig. 196). The Spinal Cord is much reduced in size, when compared with the other parts of the nervous centres; the motions of the animal now depending more upon its will and being more guided by its sensations, and the simply reflex actions bearing a much smaller proportion to the rest. 457. The general arrangement of the nervous centres, and distribution of the nervous trunks, of Man, are shown in fig. 195. At a are seen the hemispheres of the Cerebrum; at b those of the Cerebellum; and at c, the Spinal Cord. The principal motor nerve of the face (the facial) is seen at d; and and at e is seen the brachial plexus, a sort of net-work of nerves, originating by several roots from the spinal cord, and going to supply the arm. From this plexus there proceed the median nerve, f; the ulnar nerve, g; the internal cutaneous nerve, h; and the radial and musculo-cutaneous nerves, i. From the Spinal Cord are given off the intercostal nerves, j, passing between the ribs; the nerves forming the lumbar plexus, k, from which the front of the leg is supplied; and those forming the sacral plexus, 7, from which the back of the leg is supplied. The latter gives origin to the great sciatic nerve; which afterwards divides into the tibial nerve, m; the peroneal or fibular nerve, n; the external saphenous nerve, o; and other branches. 458. We shall now examine the structure of the Brain itself, and the arrangement of the nerves which proceed from it; confining ourselves to the points of most physiological importance, and neglecting those which are interesting only to the professed anatomist. In fig. 196 is represented a perpendicular section of the Human Brain down its middle; the two hemispheres forming the Cerebrum having been separated from each other by the division of the broad fibrous band f, termed the corpus callosum, which unites them. Each hemisphere is considered as made up of three lobes or divisions, the anterior a, the middle b, and the posterior c; but these are not by any means distinctly marked-out, either on the external surface or in the internal structure of the organ. The vesicular or ganglionic nerve-substance is disposed for the most part upon the exterior, forming a continuous layer, whose extent is greatly increased by the convoluted folds in which it lies; and it is very copiously supplied with blood from the pia mater, a membrane which consists almost entirely of bloodvessels and of the areolar tissue that holds them together, and which so closely enfolds the hemispheres as to dip down into all the furrows of their surface. The principal part of the internal substance of each hemisphere is composed of nerve-fibres, of which some pass between its convolutions and the chain of ganglionic masses on which the cerebrum is superposed, others BB |