. Animal physiology. Physiology, Comparative; Physiology, Comparative. 34S NERVOUS SYSTEM OF BIRDS AND Fig. 183.—Brain of Ostrich. 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 («, Fig. 183) 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
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 34S NERVOUS SYSTEM OF BIRDS AND Fig. 183.—Brain of Ostrich. 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 («, Fig. 183) 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 hemi- spheres. The Spinal Cord, d, is still of con- siderable 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 neigh- bourhood of the wings; but in those which, like the Ostrich, move principally by running on the ground, the posterior enlarge- ment from which the legs are supplied with nerves, is the more considerable. 456. In the Mammalia, 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 sur- face of the cerebral hemispheres is no longer smooth, as in most of the lower classes, but is divided by furrows into a series of con- volutions (Fig. 185) ; by these, the surface over which the blood- vessels come into relation with the nervous matter is very greatly increased; and we find the 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 t
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