540 



COMPARATIVE ANATOMY 



As a system, therefore, the sympathetic of vertebrates is a new addi- 

 tion which arises late both ontogenetically and phylogenetically. Kap- 

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

 plexuses, Uke the parasympathetic system of 

 vertebrates, are limited to the cerebral and caudal 

 part of the intestine. 



So far as sympathetic ganglia are concerned, 

 chorda tes 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 ganglia chained 

 together by connectives. Nothing similar has been 

 found in other vertebrates. 



The autonomic system in tetrapods is essentially 

 similar to that of man. A shift in the relations of 

 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. The meaning of this shift is not clear. 

 In the head region, the connexion with dorsal roots is maintained through- 

 out the vertebrate series. 



Pig. 



445. — Sym- 



pathetic system of right 

 side of a frog. Somatic 

 nerves dotted, sym- 

 pathetic black, a, 

 atlas; ai, 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; ih, 

 ilio-hypogastric nerve; 

 II— XI, second to elev- 

 enth trunk nerves. 

 (From Kingsley's 

 "Comparative Anat- 

 omy of Vertebrates," 

 after Gaupp.) 



DEVELOPMENT OF THE BRAIN 



The central nervous system of vertebrates arises as a thickened placode 

 of dorsal ectoderm anterior to the blastopore. This placode is known as 



