362 



CLASSIFICATION OF VERTEBRATES. 



the aplacental mammals the anterior commissure is especially 

 well developed, and forms the chief connection between the 

 two sides of the brain, while the corpus callosum remains more 

 rudimentary, as in sauropsida. In the placentalia, on the other 

 hand, the corpus callosum or commissure between the two 

 hemispheres becomes the most important connection between 

 the right and left sides, the anterior commissure remaining 



behind. The longitudinal 

 commissures, the fornix, and 

 the cornua ammonii, are also 

 well developed. The lateral 

 ventricles are large, and in 

 them several sub-regions are 

 distinguished, — anterior and 

 descending, and in the 

 higher mammals posterior 

 cornua. The olfactory lobes 

 are comparatively small, and 

 in the whales are absent. 

 In development a diverticu- 

 lum of the lateral ventricle 

 extends into each olfactory 

 lobe, but except in a few 

 forms, like the horse, this 

 disappears in the adult. 



The twixt brain and op- 

 tic lobes (corpora bi- or 

 quadrigemini) are poorly 

 developed, and are covered 

 in by the hinder lobe of 

 the cerebrum. The epiphysis is small and lacks any sensory 

 structures. In the cerebellum the vermis is large in the apla- 

 centals, but in the placentalia the lateral lobes of the cere- 

 bellum are in the ascendancy. A corresponding increase from 

 lower to higher is seen in the pons varolii. The spinal cord 

 extends back only to the sacral region, the posterior part of 

 the spinal canal being occupied by a cauda equina formed of 

 the more posterior nerves before their exit. 



Fig. 349. Brain of rabbit from above, 

 irom Wiedersheim. Bol, olfactorj- bulb; 

 Ftp, pallial fissure ; Gp, pinealis ; ////, cere- 

 bellar hemispheres; XH^ medulla; VH, 

 •cerebral hemispheres ; /Fw, vermis. 



