THE NERVOUS SYSTEM 



383 



pers, however, calls attenlion to the fact that in arthropods the intestinal 

 plexuses, like the parasympathetic system of vertebrates, are limited to 

 the cerebral and caudal part of the intestine. 



So far as sympathetic ganglia are concerned, chordates appear to start 

 with a clean slate, since there is no evidence of 

 sympathetic ganglia in any of the protochordates. 

 The autonomic system is, however, represented in 

 protochordates, as in vertebrates, by the visceral 

 nerves, motor and sensory. In the myxinoids the 

 intestinal plexuses develop exclusively from the brain 

 region, mostly from the vagus nerve. Tretjakoff 

 has found in Petromyzon autonomic fibers in all 

 spinal nerves. 



Some sympathetic and parasympathetic ganglia 

 are found in elasmobranchs where, as in the higher 

 vertebrates, the ciliary ganglion is parasympathetic 

 and connected with the oculomotor and ophthalmic 

 nerves. Several sympathetic ganglia occur in the 

 trunk in connexion with a limited number of spinal 

 nerves. The metameric arrangement seen in the 

 embryos is modified in the adult through fusion. 

 No longitudinal connectives are found in elasmo- 

 branchs. But Allis has described, in the head region 

 of teleosts, segmental autonomic gangha chained 

 together by connectives. Nothing similar has been 

 found in other vertebrates. 



Fig. 338. — Sympa- 

 —,, . . , . ... thetic system of right 



i he autonomic system m tetrapods IS essentially side of a frog. Somatic 

 similar to that of man. A shift in the relations of "^rves dotted, sym- 



pathetic black, a, 

 atlas; at, common in- 

 testinal artery; ao, 

 aorta; c, coccyx; cr, 

 crural nerve; j, jugal 

 ganglion; i, sciatic 

 nerve; r, radices aortae; 

 s, base of skull; sp, 

 splanchnic nerve; st, 

 sympathetic trunk; ill, 

 ilio-hypogastric nerve; 

 II-XI, second to elev- 

 enth trunk nerves. 

 (From Kingsley's 

 "Comparative Anat- 

 . omy of Vertebrates," 



Ihe central nervous system of vertebrates arises after Gaupp.) 

 as a thickened placode of dorsal ectoderm anterior 

 to the blastopore. This placode is known as the neural plate. Next to 

 the notochord, the nervous system is the first organ to develop. By the 

 elevation of its edges, the neural plate is converted into a neural groove 



the autonomic fibers occurs in phylogenesis. In 

 Anamnia, visceral motor fibers have their cell bodies 

 in the lateral horn and have their exit from the tube 

 by way of the dorsal roots; those in the thoracico- 

 lumbar and sacral region of amniotes, on the other 

 hand, enter the ventral or somatic motor roots. 

 In the head region, the connexion with dorsal roots 

 is maintained throughout the vertebrate series. 



DEVELOPMENT OF THE BRAIN 



