THE NERVOUS SYSTEM 



377 



the dorsal and the ventral nerve of each trunk segment unite to form a 

 mixed nerve. In this way every spinal nerve possesses two "roots," a 

 dorsal sensory and a ventral motor root. 



Another phylogenetic change is the loss of motor fibers in the dorsal 

 roots. This takes place in the vertebrate series. Anamnia retain in 

 their dorsal spinal roots the visceral motor fibers characteristic of the 

 dorsal nerves of Amphioxus. In amniotes, however, these visceral motor 

 fibers have their exit from the spinal cord by way of the ventral roots. 

 The meaning of this shift is not clear. 



ixbcd. 



FiG- 334- — -4, diagram of collector nerve; B, of a nerve plexus. (After Braus.) 

 C, brachial plexus of Salamandra maculata. (From Kingsley's "Comparative Anatomy 

 of Vertebrates," after Fiirbringer.) 



Nerve plexuses are not found in Amphioxus. A cervical plexus appears 

 in cyclostomes. In higher vertebrates four plexuses arise — cervical, 

 brachial, lumbar and sacral. To form a plexus, the fibers of a number 

 motor nerves unite in an interlaced network. In this way any muscle 

 may be innervated by more than a single nerve and thus a summation of 

 stimulation is effected. 



THE AUTONOMIC NERVOUS SYSTEM 



In the higher chordates a special system of nerves, the autonomic, is 

 distributed to the smooth muscles of the digestive and circulatory systems 



