366 NERVOUS CENTRES OF REPTILES, BIRDS, AND MAMMALS. 



save that the Cerebral hemispheres are usually larger, extend- 

 ing 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 

 krain, is very considerable. 



455. In BIRDS, however, we find a consi- 

 derable 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, 6. The Cere- 

 bellum, 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 ener- 

 getic, the enlargement is the greatest in 

 ^ e ne ig nDournoo( i f the wings ; but in 

 those which, like the Ostrich, move 

 principally by running on the ground, the posterior en- 

 largement, from which the legs are supplied with nerves, is 

 the more considerable. 



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 pro- 

 portionally 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 



