THE NERVOUS SYSTEM 



485 



The glossopharyngeal is the mixed nerve which supplies the third 

 visceral arch. It forks over the first gill-slit and a pretrematic branch is 

 distributed to the posterior wall of the hyoid arch. The post-trematic 

 branch contains sensory fibers from the floor of the pharynx and motor 

 fibers which innervate the muscles of the third arch. 



The vagus is a mixed nerve formed of fibers which are distributed 

 to the muscles and skin of the posterior visceral arches. This suggests 

 that a number of segmental nerves are united in the vagus. A lateralis 

 branch is the nerve of the posterior series of lateral line organs. A 

 visceral branch goes to heart, stomach, and intestine, and carries sym- 



I ST. SPINAL GANGLION 

 VAGUS 



SPINAL GANGLIA 



N . GLOSSOPHARYNGEUS 

 OTIC CAPSULE^^ 

 N. FACIAL 

 EYE 



^HYPOBRANCHIAL N. HYPOGLOSSUS 



MUSCLE. 



Fig. 399. — The head of Ammocoetes larva of Petromyzon in left lateral aspect, 

 showing that the hypobranchial muscle is innervated by the somatic motor fibers of 

 seven metaotic nerves which in higher vertebrates become the hypoglossal nerve. 



pathetic fibers connected with these organs. Each branchial branch 

 divides into pre- and post-trematic rami, which contain both visceral 

 sensory and visceral motor fibers. 



In the trunk region there is a general correspondence between the 

 number of myotomes and of spinal nerves, since for each myotome there 

 are usually a sensory and a motor nerve. In Petromyzon these are not 

 united; but in Myxinoids they join to form a mixed spinal nerve with two 

 roots, a dorsal ganglionated sensory root and a ventral motor root, each 

 neurite of which arises from a multipolar ganglion cell located in the gray 

 matter of the cord. Synaptic connexions between sensory and motor 

 neurones take place within the gray matter. Peripherally, each spinal 

 nerve divides into dorsal and ventral rami which supply skin and muscles. 



This simple one-to-one metameric correspondence of spinal nerves 

 and myotomes is, however, somewhat modified in the occipital region of 

 petromyzon where the first five post-otic myotomes are innervated by the 

 nerves of the fourth and fifth myotomes, and the nerves of the three 

 anterior myotomes have disappeared, at least as independent roots. 

 The nerves of post-otic myotomes 6 to 12 unite to form the hypoglossal 

 nerve which supplies the hypobranchial muscles. In this fusion of 



