374 H. W. NORRIS AND SALLY P. HUGHES 



Occipital nerve z originates by four or five rootlets from the 

 region where the spinal cord merges into the medulla. It passes 

 at once through a small foramen into the vagal canal, almost im- 

 mediately on entering giving off the small dorsal branch that 

 passes out dorsally with a corresponding branch of the nerve y. 

 As stated above, it accompanies the preceding occipital nerve 

 posteriorly and finally unites with it and a branch of the first 

 spinal nerve. It gives off small branches to the first interbasal 

 muscle. 



The occipital nerve x, when present, is very small. In the 

 single instance in which it was traced from origin to innervation, 

 it was found to leave the brain wall some distance anterior to 

 the exit of y. Passing back ventral to the vagus roots it enters 

 a minute canal in the base of the cranium, merging into the vagal 

 canal, where it runs posteriorly at the ventral border of the bran- 

 chial ganglia (fig. 42). On leaving the vagal canal, it takes a 

 position at the lateral border of the basal plate, soon joining the 

 nerve y, with which it passes back some distance, finally turning 

 mesially into the mesial portion of the first interbasal muscle, 

 which it innervates. Figure 44 is a horizontal section through 

 the cranial basis of a specimen with five occipital nerves. The 

 destinations of nerves v and w were not traced. 



According to Ftirbringer, there are usually three nerve branches 

 supplying the interbasal muscles in selachians. One or two of 

 these, derived from y or y and z, innervate the first interbasal 

 (subspinalis) . One or two branches derived from y and z supply 

 interbasals 2-3. The writers find in Squalus acanthias five nerve 

 branches supplying the first interbasal muscle : one derived from 

 nerve x, two from y, one from z, and one from the first spinal 

 nerve. That the other interbasal muscles are innervated as 

 Furbringer describes is probable, but the writers are unable to 

 confirm his statements. 



The small foramen of exit of the nerve z into the vagal canal 

 is designated by Wells ('17) as a secondary foramen of the vagus 

 nerve. 



