AUTHOR'S ABSTRACT OF THIS PAPER ISSUED 
BY THE BIBLIOGRAPHIC SERVICE, MAY 9 
VENTRAL SPINAL NERVES IN AMPHIOXUS 
HOWARD AYERS 
SEVEN FIGURES 
The spinal cord of Amphioxus presents a sinuous outline, 
when viewed either from the dorsal or ventral face. This 
sinuosity is due to the curved projections of the cord (fig. 6) 
where the motor roots issue, and since they emerge, as a rule, 
alternately on the right and left sides, the superficial curvatures 
result. The main structures of the cord are not affected thereby. 
The motor roots are composed of fibers which lack protective 
covering of any kind. The number of fibers varies from about 
100 to 250, in keeping with the variation in the size of the myo- 
tomes. The larger the myotome the greater the number of 
muscular elements requiring nerve control. The terminal myo- 
tomes in head and tail are the smallest of the lot and the size 
increases in both directions toward the middle of the body. 
The motor nerves increase in size in the same way—the increase 
in size being due solely to the addition of nerve fibers. 
It has been stated that no motor nerve has been found for 
the first myotome. I have dissected the first myotome and 
find the nerve supplying it small, and could locate with cer- 
tainty only two branches, one for each fork of the myotome. 
Rhode’s description of the motor nerve is more complete and 
accurate than his figures, but is far from correct either as to the 
size and the course of the nerve or its method of branching. It 
is, however, the best I have seen. 
He describes three sections of the motor nerve, an anterior 
which spreads like a fan on the dorsal part of the myotome, a 
median section composed of ventral fibers which run to the 
rectus abdominis and the ventral portion of the outer part of 
the myotome, and a posterior section or bundle of fibers which 
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