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 
brain, is very considerable. 
45-5. 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, b. 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 
the neighbourhood of 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 
Fig. 194.— Brain of 
Ostrich. 
Fiig. 193— Brain 
or Reptile. 
a, cerebral hemi¬ 
spheres b, optic 
ganglia; c, cere¬ 
bellum ; d, spinal 
cord. 
