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 viliaie (fig. 193), 
The Cerebellum is generally smaller, as we should expect from 
the inertness of these animals, and the want oi 
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 
keeping-up the movements of the body, after 
it has been cut-off from connexion with _ 
Fig. 193—Braix rain, is very considerable. 
PTILE. 
SS eeeetical’ baint- 455. In Birps, however, we find a co 
te op b,optle erable advance in the character of the bral 
beliam ‘ispinal_ towards that which it presents in 
met The Cerebral hemispheres (a, fig. 194) an 
ents increased in size, and cover-in, not only the olfactor 
ganglia, but also in great part the optic ganglia, b. The Cere 
bellum, ¢, also, is much more developed 
as we should expect from the number anc 
complexity of the movements performec 
by the animals of this class ; but it is sti 
undivided into hemispheres. The Spina 
Cord, d, is still of considerable size, and i 
much enlarged at the points from whiel 
the nerves of the wings and legs originate 
in the species whose flight is most enel 
getic, the enlargement is the greatest i 
Fig. 191."Braix or the neighbourhood of the wings; but i 
those which, like the Ostrich, mo 
principally by running on the ground, the posterior & 
largement, from which the legs are supplied with nerves, 18 
the more considerable. | 
456. In Mammats, we find the size of the Cerebral 
hemispheres very greatly increased, especially as we Tis 
towards Man ; whilst the olfactive and optic ganglia are pr 
portionally diminished, and are completely covered-in 
them. The surface of the cerebral hemispheres is no longe 
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 wi 
the nervous matter is very greatly increased ; and we find the 
