THE NERVOUS SYSTEM 



489 



In the spinal cord, dorsal and ventral columns of gray matter are 

 differentiated. The dorsal column, however, remains unpaired. 



The cranial nerves are identical with those of cyclostomes but rela- 

 tively enlarged both in correlation with the increased size of sense organs 

 and muscles, and by the addition to the nerve fibers of medullary and 

 neurolemma sheaths. A cranial sympathetic ganglion, the ciliary, has 

 developed in association with the oculomotor and profundus nerves. 

 The profundus has now become a branch of the trigeminal. Some of 

 the supraorbital series of lateral line organs are innervated by fibers of the 

 superficial ophthalmic branch of the fifth nerve, while the remainder are 

 supplied by the superficial ophthalmic of the facialis. 



The somatic motor nerves of five post-otic myotomes unite to form 

 the h>'poglossal, which supplies hypobranchial muscles. A thoracic 



Fig. 405. — Brain of Prolopterus, a Dipnoan fish, ch, cerebellum; e, epiphysial 

 structures; h, hypophysis; i, inf undibulum ; m, mid-brain; se, saccus endolymphaticus; 

 sp, spinal nerves; t, cerebral hemisphere; 1-12, cranial nerves. (From Kingsley's 

 "Comparative Anatomy of Vertebrates," after Burckhardt.) 



plexus is formed by the union of the nerves immediately posterior to those 

 of the cervical plexus. But the number of nerves which participate 

 varies greatly in different elasmobranchs. In many species the fibers 

 of the cervical and thoracic plexuses unite as a cervico-thoracic plexus. 

 In the region of the pelvic fin is a similar but smaller lumbo-sacral plexus. 



Well developed sympathetic ganglia appear in the trunk region, 

 in the vicinity of the dorsal aorta. Their arrangement is metameric, 

 but the anterior and largest is formed by the union of primarily separate 

 ganglia. Each is connected by a ramus communicans with a spinal 

 nerve. A longitudinal sympathetic cord or connective is only imper- 

 fectly developed. An intestinal plexus occurs, as in all vertebrates. 



Amphibia. Amphibia have a relatively simple brain like that of 

 cyclostomes and dipnoi. Olfactory lobes are relatively large, and merge 

 without constriction into the cerebral hemispheres. The pallium is 

 thick, and cells have migrated from the gray matter into the external 

 marginal zone of white matter. The lumen of each hemisphere is reduced 

 by the thickening of its median and lateral walls. An inner longitudinal 

 sulcus or groove divides these walls into dorsal and ventral halves. The 

 dorsal half of the lateral wall is the paleocortex, the ventral half is the epis- 

 triatum, and below the epistriatum is the paleostriatum. In the median 



