THE SYMPHATHETIC NERVOUS SYSTEM 367 



linated and form separately the gray rami. Nerve fibers appear in the paired 

 longitudinal cords, which were at first purely cellular, in such a manner that seg- 

 mental masses of cells {sympathetic ganglia) become linked by fibrous, commissural 

 cords. 



In the head region the sympathetic gangha are not segmentally arranged, but 

 are derived from cells of the cerebrospinal ganglia which migrate to a ventral 



Fig. 366. — The sympathetic system in a 16 mm. human embr3'0 (Streeter in Lewis and Stohr). 

 X 7. The ganghonated trunk is heavily shaded. The first and last cervical, thoracic, lumbar, sacral 

 and coccygeal spinal ganglia are numbered, a., Aorta; ace, accessory nerve; car., carotid artery; cil., 

 ciUary ganglion; coe., coeliac artery; Ht., heart; nod., nodose ganglion; ot., otic gangUon; pet., petrosal 

 ganglion; s-m., submaxillary ganglion; s.mes., superior mesenteric artery; sph.-p., sphenopalatine 

 ganghon; spl., splanchnic nerve; St., stomach. 



position (Fig. 365) . These cells Kkewise give rise to nerve fibers which constitute 

 longitudinal commissures connecting the various ganglia of the head with the 

 ganglionated cord of the trunk region. The small cranial sympathetic ganglia 

 are probably all derived from the anlage of the semilunar ganglion (Fig. 366). 



