156 HOWARD AYERS 
passes in between the rectus abdominis and the longus dorsi 
and supplies both these muscles. 
The current opinion seems to be that ‘‘the motor nerves 
spread out like a fan and terminate upon the muscle fibers of 
the myotomes,” to use Willey’s phrasing. This is not correct, 
for the ventral spinal nerves of Amphioxus are relatively long 
nerves and the muscle fibrils most distant from the cord are 
controlled by means of nerve fibers which span the interval 
between the spinal cord and muscle. The longest of these 
fibers are, consequently, as least as long as the vertical diameter 
of the animal, since the fibers run ventrocaudad in the ventral 
fork of the myotome. 
Another point should be kept in mind—the motor root divides 
into nerve branches, distinct and compact bundles which, while 
without noticeable sheaths, nevertheless hold their fibers firmly 
together as nerves, to give them off as they pass along the terri- 
tory of their distribution by the familiar method of nerve branch- 
ing until the ultimate nerve filaments are set free singly or in 
small groups of two to eight fibrils. These single fibers or small 
groups are the elements which pass to the muscle for contact or 
fusion and also for the formation of nerve nets upon the surface 
of the muscle plates. . 
It is common experience of those who have tried to lay bare 
the connection of the ventral spinal nerves with the cord, that 
the root too easily tears away from the cord at its junction 
therewith. A dissection of a motor nerve with the parts in 
place is shown in figure 1. The fibers issue from the ventro- 
lateral angle of the cord as a wide, thin plate of fibers, which, as 
Rhode has shown, is composed of groups of bundles of fibers. 
This differentiation in the structure of the root I have followed 
back into the spinal cord. It indicates a specialization of the — 
nerve supply assembled from several parts of the cord for the in- 
nervation of the structures under the control of the ventral roots. 
The motor roots have been thought to belong exclusively to 
the striated muscles of the myotomes but, as indicated in 
figures 1, 2, and 3, there is a small bundle of fibers (relatively 
too large in the figures) marked 5, which runs out with the 
