186 Basic Structure of Vertebrates 



they must find their exciters in the gangliated plexuses. The exciter- 

 fibers which proceed from the posterior mesenteric ganglion enter this 

 same intricate region and, presumably, pass directly to effectors in 

 the walls of the organs. Apparently, posterior-mesenteric exciters and 

 terminal exciters related to hypogastric connectors may innervate the 

 same organ. The chief organs which receive innervation from one or 

 both sources are the large intestine (except, perhaps, its anterior 

 region), urinary bladder and urethra, uterus and other urinogenital 

 ducts, and external genital organs. 



Origin of Autonomic Neurons 



That all cells of the autonomic system are of ectodermal origin 

 has been well established. The cells of the lateral ganglions have com- 

 mon origin with those of the spinal ganglions from a ridge (ganglionic 

 or neural crest) of ectodermal cells lying dorsolateral^ along each 

 side of the neural tube (Fig. 161). From the developing lateral gan- 

 glions (Fig. 134), some cells move into deeper positions and establish 

 the collateral ganglions. In short, all neurons of the terminal ganglions 

 and plexuses, however deeply situated, are derived from the ganglionic 

 crest by successive migrations of cells from one ganglionic site to a 

 deeper one. In general, the nerves connecting the more remote gan- 



Fig. 161. Drawing showing closure of the neural tube and formation of the 

 neural crest. From pig embryos of (A) 8 somites; (B) 10 somites; (C) 11 somites; 

 (D) 13 somites. (X 135.) (Courtesy, Patten: "Embryology of the Pig," Phila- 

 delphia, The Blakiston Company.) 



