THE NERVOUS SYSTEM 



359 



terminalis. Habenular and posterior commissures persist in the roof of 

 the diencephalon. The epiphysis forms a pineal gland. A chorioid plexus 

 invaginates into the third ventricle. The saccus vasculosus of fishes has 

 disappeared. The thickened walls of the mid-brain reduce the lumen 

 to a narrow passage, the aqueduct. The cerebellum is rudimentary like 

 that of cvclostomes. 



Fig. 318. — Side and dorsal views of brain of young alligator, c, cerebrum; cl, 

 cerebellum; e, epiphysial structures; /;, hypophysis; i, infundibulum; ol, optic lobes; 

 II-XII, cranial nerves. (From Kingsley's "Comparative Anatomy of Vertebrates," 

 after Herrick.) 



The ten cranial nerves of fishes persist in Amphibia. Urodeles have 

 lateralis nerves, but these disappear in the Anura in correlation with 

 the loss of lateral-line organs. With the loss of gills, the number of 

 branches of the vagus is reduced. 



Cervical and lumbar enlargements of the spinal cord appear in cor- 

 relation with the enlargement of the appendages. 



Sympathetic nerve cords or connectives unite the series of sympathetic 

 ganglia. 



Reptiles. The cerebral hemispheres of reptiles are larger than those 

 of Amphibia and by extension caudad have partially overgrown the 



