THE NERVOUS SYSTEM 



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become corpora quadrigemina by the division of each optic lobe into 

 anterior and posterior moieties. 



The cerebellum is slightly larger in reptiles than in amphibians. The 

 gray matter of the spinal cord as seen in cross section assumes the form of a 

 capital H with dorsal and ventral columns, as in mammals. 



In reptiles occipital vertebrae fuse with the cranium. Consequently, 

 two nerves, the spinal accessory which innervates shoulder and neck 

 muscles and the hypoglossus which supplies the tongue, both being spinal 

 in lower vetebrates, now become cranial. Each arises by a series of 

 segmentally arranged roots, and is therefore believed to be formed by 



Fig. 320. — Median section of brain of calf, a, aqueduct; ac, anterior commissure; 

 cc, corpus callosum;/, fornix; h, habenula; hy, hypophysis; i, infundibulum; itn, inter- 

 mediate mass ("soft commissure"); -mb, mammillary body; ob, olfactory bulb; oc, optic 

 chiasma; ol, optic lobes; p, pinealis; pc, posterior commissure; r, recessus suprapinealis; 

 s, septum pellucidum; III, IV, third and fourth ventricles. (From Kingsley's "Com- 

 parative Anatomy of Vertebrates," based on a figure by Biitschli.) 



the union of a number of spinal nerves. Other changes in the nervous 

 system in reptiles are relatively unimportant. 



Mammals. The brains of lower mammals differ little from those of 

 reptiles. Within the mammalian group, from monotremes to man, 

 there is an enormous enlargement of the cerebral hemispheres and of the 

 cerebellum. The expansion of the hemispheres affects chiefly the neopal- 

 lium, the beginnings of which were noted in reptiles. The archipallium 

 of reptiles, which serves chiefly as an olfactory center, becomes in mam- 

 mals the hippocampal lobe. As a result of the growth of the neopallium, 

 the hippocampus is crowded to the lower part of the brain. 



Increase in the size of mammalian brains is accompanied by com- 

 pHcation in form and structure. The cortex becomes mainly cellular, 

 and consequently gray. The amount of cortical material increases 

 many-fold, so that if the human cortex were spread out flat it would cover 



