Johnston, Nej-zrs of Petromyzouts. 581 



before they enter the ventral roots. When the ventral roots are 

 traced in sections stained in haematoxylin, they pass to the inner 

 surface of the myotomes, and divide into dorsal and ventral rami 

 which spread over this surface of the myotome so that the individ- 

 ual fibers are soon lost. It is readily seen, however, that the fibers 

 increase still more in thickness before they reach the myotome. 

 The great thickness of the motor fibers before they enter the 

 myotome is well brought out in Golgi preparations. In all cases, 

 before the fibers begin to give off branches to their endings in 

 relation with the muscle fibers, they have acquired an enormous 

 thickness which can best be appreciated by examining figs. 14 

 and 15. In fig. 15, which represents a horizontal section of the 

 right half of the body, the size of these fibers relative to that of 

 the myotomes and of the whole body can be seen. The fibers 

 are a Httle longer in proportion to their thickness than appears 

 here, because six sections of 75// are projected on one plane. 

 Some of these fibers have a thickness of 24/^, which is about one- 

 fourth of the thickness of the largest muscle bands in the myotomes. 

 How much the fibers increase in thickness in their course between 

 the spinal cord and the myotome can be seen from fig. 14, in 

 which aty^ is drawn the right half of a transverse section of the 

 spinal cord at the same magnification as the rest of the figure. 

 Near the ventral surface of the cord are seen the fibers of the 

 ventral root in cross section. The colossal size of these fibers can 

 be indicated again by calhng attention to the fact that although 

 naked fibers are measured here, they equal in diameter the coarsest 

 medullated fibers in man and mammals. The explanation of 

 the thickness of the fibers is found in the fact already pointed out 

 that each motor fiber must supply a large number of muscle fibers. 



Figs. 14 and 15 show two forms of nerve endings in the myo- 

 tomes. One of these is so difi^erent from typical motor endings 

 that it was at first thought that it might serve the muscle sense. 

 Both kinds of fibers, however, are traced with certainty to the 

 ventral roots and furthermore the larger part of the myotome is 

 free from the typical endings and must be innervated by the simple 

 endings to be described below. 



Typical motor endings. — Those fibers which spread over the 

 inner surface of the myotomes present endings which, although 

 simple in form, resemble typical motor end plates. In figs. 14 

 and 15 are shown the spreading of such fibers and their ending 



