File:The chordates (1950) (19990222983).jpg

Title: The chordates
Identifier: chordates00rand (find matches)
Year: 1950 (1950s)
Authors: Rand, Herbert W. (Herbert Wilbur), 1872-1960
Subjects: Chordata
Publisher: Philadelphia : Blakiston
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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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-
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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.)

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