178 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



extends from one ganglion to the next, but in many teleosts and in 

 the higher groups (figs. 187, 188) the chain ganglia of a side are con- 

 nected by a longitudinal sympathetic trunk nerve. In these there is 

 also an extension of the system into the head. 



In the teleosts the cephalic sympathetic ganglia lie 

 beneath the floor of the cranium and are associated 

 with the ganglia of the fifth, seventh, ninth and tenth 

 cranial nerves; there is also an extension of the system 

 to the tip of the tail, the nerves lying in the haemal 

 arches of the vertebrae. In the perennibranchs the 

 cephalic part of the system extends as far forward as 

 the seventh nerve, its connexion with the more pos- 

 terior parts being in part by a separate trunk, Jacob- 

 son's commissure (fig. 189) extending forward from 

 the ninthtenth roots. In the anura (fig. 187) the 

 cephalic extension reaches only to the tenth nerve. 

 In the amniotes the cephalic part of the system is 

 well developed and is connected with several well-de- 

 veloped ganglia (cervical, sphenopalatine, otic, sub- 

 maxillary, etc.), which also receive nerves from the 

 regular cranial nerves. 



In the sympathetic system, besides the visceral 

 sensory and motor elements already referred to, there 

 are elements which may be grouped as purely sympa- 

 thetic. The ramus visceralis carries fibres which arise 

 either from ganglion cells in the dorsal ganglion or 

 from those in the lateral column (cornu) of the cord 

 (fig. 184). These develop their axons and dendrites 

 which interlace with motor and sensory ganglion cells 

 lying in the chain ganglia (fig. 188, eg). From these 

 some nerve fibres extend out to the viscera, while 

 others run backward to course in the dorsal and ventral 

 rami of the spinal nerves to reach the blood-vessels 

 and smooth muscle fibres in the peripheral parts of 

 the body. These purely sympathetic fibres are non- 

 meduUated and hence are gray in color, and in the 

 mammals the trunk carrying these from the chain 

 ganglia to the dorsal and ventral branches is called 

 the gray ramus (fig. 188, gr). 



In development some of the cells brought from 

 the spinal cord or ganglion of the dorsal root migrate from the sympathetic parts 

 and pass to various regions of the body, being usually closely associated with the 

 glands of so-called internal secretion — hypophysis, carotid gland, suprarenals, 

 etc. These cells possess a peculiar afl&nity for chromic acid salts and have 

 been called chromaffine cells. Little is known of their function. 



Fig. 187. — Sympa- 

 thetic system of right 

 side of a frog, after 

 Gaupp. Somatic nerves 

 dotted, sympathetic 

 black. a, atlas; at, 

 common intestinal 

 artery; ao, aorta; c, coc- 

 cyx; cr, crural nerve; j, 

 jugal ganglion; i, sciatic 

 nerve; r, radices aortae; 

 s, base of skull; sp, 

 splanchnic nerve; st, 

 sympathetic trunk; ih, 

 iliohypogastric nerve; 

 II-XI, second to 

 eleventh trunk nerves. 



