580 'Journal of Comparative Neurology and Psychology. 



and finally emerge as a compact root. On this account a trans- 

 verse section of the head passing just cephalad from the root 

 gives a transverse section of nearly or quite all of its fibers. The 

 first two nerves are larger than the others, the second containing 

 about 70 fibers. In six following roots counted, part on the 

 right and part on the left side, the count varied from 31 to 40. 

 Even a cursory examination shows that the nerves are much 

 larger in Lampetra than in the ammocoetes of P, dorsatus, but 

 whether this is a difference in species or a difference between the 

 ammocoetes and the adult is an interesting question which I have 

 no means of deciding. If we accept 40 as a reasonable estimate 

 of the number of fibers in the ventral root, it will be seen at once 

 that an interesting relation exists between this and the number 

 of fibers to be innervated in the myotome. 



The parietal muscle compared with other organs of the body is 

 large in the lamprey, as in the fishes generally. Each myotome 

 is made up of bands or plates of muscle fibers. Within each 

 muscle band are distinguished parietal and central fibers. The 

 parietal fibers are smaller and are provided with sarcolemma. 

 The central fibers are without sarcolemma ( ?) and have the 

 muscle fibrillae more closely packed than in the parietal fibers. 

 The central fibers also vary greatly in size. The number of fibers 

 in each band varies from 7 to 8 in the small bands at the lower 

 border of the myotome to 55 to 60 in the large bands. As the 

 majority of the bands are large, having above 35 to 40 fibers, a 

 fair average for the muscle bands would be about 40 muscle fibers. 

 The total number of muscle bands in a myotome and the corre- 

 sponding segment of the ventral muscle in the branchial region 

 varied in the counts made between 107 and 120. From these 

 figures the round number 4000 might be taken as fairly indi- 

 cating the number of muscle fibers to be innervated by each 

 ventral spinal nerve in the branchial region. In other words, each 

 nerve fiber of the ventral spinal nerve must innervate about 

 100 muscle fibers. These figures are to be regarded as only a 

 rough approximation but they express a general relation between 

 the nerve and its muscle of which there can be no doubt. The 

 mode of distribution and ending of the motor fibers is evidently 

 influenced by this relation. 



As the axones leave the motor cells in the spinal cord they are 

 fibers of moderate thickness and increase somewhat in diameter 



