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Basic Structure of Vertebrates 



via the autonomic ganglions, with the smooth or involuntary visceral 

 muscles and with glands. 



The two spinal roots unite near the ganglion to form a mixed nerve 

 which then divides into three trunks (Fig. 152). Two of them pass out- 

 ward into the body-wall. One, the dorsal ramus, goes to the epaxial 

 division of the body-muscle and to the corresponding region of the skin. 

 The other, the ventral ramus, innervates the hypaxial muscle and the 

 corresponding ventral region of skin. The third and smaller trunk, the 

 ramus communicans, is a visceral branch which passes into a neigh- 

 boring ganglion of the autonomic system. 



Plexuses. In general, the distribution of a spinal nerve is fairly 

 closely restricted to its particular segment of the body, but, in the re- 

 gion of the paired appendages, this relation seems to be modified by 

 union of the ventral rami of several nerves to form an interlacing net- 

 work or plexus (Fig. 153) from which nerves emerge distally and pass 

 into the appendage. Four such plexuses occur in vertebrates, cervical, 

 brachial, lumbar, and sacral. Commonly, the anterior two join to 

 form a cervicothoracic plexus, and the posterior two form a lumbo- 

 sacral plexus. The number of spinal nerves involved in the plexus of 



Fig. 153. (A) Diagram of a collector nerve. (B) Diagram of a nerve plexus. 

 (After Braus.) (C) Diagram of a brachial plexus of Salamandra maculata. (After 

 Furbringer.) (Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," 

 Philadelphia, The Blakiston Company.) 



