Johnston, Nerves of Petromyzonts. 579 



nerve of the tongue and these are the only fibers entering the velum 

 besides those of the velar nerve. 



The fibers of the lateral line system have been traced only in 

 part and nothing is to be added to my former descriptions (1902 

 and 1905). 



THE SOMATIC MOTOR COMPONENTS. 



The results to be reported here concern the distribution and 

 mode of ending of the motor fibers in the spinal nerves of the 

 branchial region. The postotic myotomes persist throughout life, 

 going to form the great parietal muscle. The ventral spinal 

 nerves innervating this show certain features of peculiar interest. 



As has been shown by the embryological studies of Neal and 

 KoLTZOFF, the first two postotic myotomes reach forward dorsal 

 and ventral to the orbit so that they lie far forward from the roots 

 of the first spinal nerves. In the number and segmental relations 

 of the ventral spinal nerves Lampetra seems to agree closely with 

 P. dorsatus. The first ventral spinal nerve appears to belong to 

 the second postotic myotome and the first three myotomes are 

 innervated by the first two spinal nerves, which are much larger 

 than the following ones. In my previous study I was unable, 

 with the method used, to trace these nerves very far forward with 

 the anterior prolongation of the first myotomes. In the Golgi 

 sections, however, several fibers of the combined trunk of the 

 first and second nerves are impregnated and their general course 

 is indicated in the figures above described. The endings of the 

 fibers in the first and second myotomes are beautifully impregnated 

 and one or two fibers are traced to fhe extreme anterior end of the 

 first myotome. 



' The ventral nerve roots consist of a small number of very coarse 

 fibers. Both these features have strongly impressed me since 

 my first study of the lamprey. It is not easy to make a count of 

 the number of fibers in a ventral nerve from sections in any of 

 the conventional planes because, owing to the direction taken by 

 the root, the section is never transverse to the root. In the 

 ammocoetes of P. dorsatus a reasonable estimate of the number 

 of fibers in the ventral roots in the branchial region seems to me 

 to be about twenty. In Lampetra the conditions are a little more 

 favorable for counting, because the fibers arising from the motor 

 cells run caudad just within the ventral surface of the spinal cord 



