494 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



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 move- 

 ments of the body, after it has been cut off from all connexion with the 

 brain, is very considerable. We find that, in Reptiles, as in Fishes, the 

 Spinal Cord may have a nearly uniform size from one extremity to the 

 other, like the ventral cord of the lower Articulata ; or it may present 

 considerable enlargements at particular spots, like the ganglionic cord 

 in the thoracic regions of Insects. This difference depends upon the 

 degree of development of the special locomotive organs. Thus in the 

 Eel and Serpent, whose movements are accomplished by the undulations 

 of the entire trunk, and which are destitute of members, we find a uni- 

 form development of ganglionic matter in the spinal cord. On the other 

 hand, in the Flying-fish, in which the pectoral fins or anterior extremi- 

 ties effect the greater part of the propulsion of the body, we find a great 

 ganglionic enlargement of the Spinal cord, at the part with which the 

 nerves of those members are connected : in the Frog, whose movements 

 are chiefly effected by the posterior extremities, we find a similar enlarge- 

 ment at the roots of the crural nerves : and in the Turtles and Lizards, 

 the two pairs of whose members are nearly equal in function, and serve 

 to effect the principal movements of the body, we find an anterior and 

 posterior enlargement of the Spinal Cord, corresponding to the parts 

 with which the nerves of these members are connected. 



872. We find in Birds a considerable advance in the character of the 

 Encephalon, towards that which it presents in Mammalia. The Cere- 

 bral Hemispheres (Plate II., Figs. 11, 12, 13, 5) are greatly increased 

 in size ; and they cover-in, not merely the olfactory ganglia, but in 

 great part also the optic ganglia. The former are of comparatively 

 small size ; the organ of smell in Birds not being much developed. The 

 latter are very large, in conformity with the acuteness of sight which is 

 highly characteristic of the class. The cerebellum is of large size, as 

 we should expect from the number and complexity of the muscular 

 movements performed by animals of this class ; but it is still undivided 

 into hemispheres. The Spinal Cord is still of considerable size in com- 

 parison with the Encephalon ; and it is much enlarged at the points 

 whence the legs and wings originate. In the species which have the 

 most energetic flight, such as the Swallow, the enlargement is the 

 greatest where the nerves of the wings come off; but in those which, 

 like the Ostrich, move principally by running on the ground, the poste- 

 rior enlargement, from which the legs are supplied with nerves, is much 

 the more considerable. 



873. In the Mammalia we find the size and general development of 

 the Encephalon presenting a gradual increase, as we ascend the series, 

 from the non-placental Monotremes and Marsupials, towards Man. In 

 the former, the Hemispheres exhibit no convolutions ; and the great 

 transverse commissure, or connecting band of fibrous structure, termed 

 the corpus callosum, is deficient. As we rise through the true viviparous 

 division of the class, we notice a gradually-increasing prolongation of 

 the Cerebral Hemispheres backwards ; so that first the optic ganglia, 

 and then the cerebellum, are covered-in by them. The latter partly 

 shows itself, however, in all but Man and the Quadrumana, when we 



