376 H. W. NORRIS AND SALLY P. HUGHES 



tebrae, and just posterior to the origin of the third spinal nerve 

 fuses with the trunk formed by the union of the occipital and 

 first spinal nerves, forming what is known as the cervical plexus 

 (figs. 49 and 51). It will be seen that this account of the main 

 branches of the anterior spinal nerves in Squalus acanthias is 

 not wholly in agreement with the description given by Allen ('17) 

 of the corresponding parts of a posterior abdominal spinal nerve 

 in the same species. 



The form and branches of the third spinal nerve repeat with 

 little variation those of the two preceding nerves. The ventral 

 ramus passes out through a foramen in the second vertebra, and 

 after running posteriorly some distance comes into intimate re- 

 lations with the combined mass of the ventral rami of the occip- 

 ital and first and second spinal nerves. Only with extreme care 

 can it be followed in sections through the cervical plexus. It is 

 seen to receive a motor branch from the second spinal nerve, 

 but all its own motor fibers pass into the brachial plexus. It 

 sends anteroventrally one or more large sensory branches closely 

 paralleling the great hypobranchial nerve formed from the cer- 

 vical plexus (figs. 49, 51, and 53). This account is in close agree- 

 ment with that of Furbringer for Acanthias, but he states that 

 in some instances the third spinal nerve contributes to the cer- 

 vical plexus a sensory branch. 



Muller ('11) states that in Acanthias vulgaris there are three 

 occipital nerves, later in development two. In seventeen ex- 

 amples he finds that the third spinal nerve forms the first bra- 

 chial nerve; in six instances it is the second spinal, and in four 

 instances, the fourth spinal. 



THE HYPOBRANCHIAL NERVE 



The great nerve trunk formed by the union of the ventral rami 

 of the occipital and first two spinal nerves is composed of somatic 

 sensory and somatic motor fibers. Visceral sensory and motor 

 fibers, if present, are not in evidence. Careful examination of 

 cross-sections of the nerve shows that it is possible to distinguish 

 with considerable accuracy the parts contributed by the various 



