THE NERVOUS SYSTEM 



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ramus ophthalmicus superficialis innervates the supraorbital series of 

 lateral-line organs. The buccalis supplies the infraorbital series. The 

 deep palatme branch is distributed to the skin of the roof of the mouth. 

 The hyomandibular innervates the muscles of the hyoid arch and the skin 

 of the mandibular region. 



The auditory supplies the otic vesicle which in cyclostomes is chiefly 

 an organ of equilibration. As might be expected from its origin as a 

 branch of the facial nerve, its roots are closely associated with those of 

 the facial. Since the otic vesicle is a modified lateral-line organ, the fibers 

 of the auditory nerve belong to the group of lateralis or special somatic 

 sensory components. 



The glossopharyngeal is the mixed nerve which supplies the third 

 visceral arch. It forks over the first gill-sht 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 sympathetic 

 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 

 neurons 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 1 2 unite to form the hypoglossal nerve 

 which supplies the hypobranchial muscles. In this fusion of occipital 

 nerves may be seen the beginnings of the cervical plexus which persists 

 throughout the vertebrate series. Since paired appendages are wanting 

 in cyclostomes, no thoracic or lumbar plexuses are formed. 



