VENTRAL SPINAL NERVES IN AMPHIOXUS 157 
large bundle 3, but which does not enter the myotome, passing 
along its inner face to join the nerve plexus of the aortic trunk. 
It is a splanchnic nerve. All the other branches of the ventral 
nerve, so far as I have observed, are distributed to the myotome. 
The four branches to the myotome are, counting from before 
backward: nerve 1 leaves the anterior part of the root and runs 
dorsolateral and enters the dorsal fork of the preceding myotome; 
nerve 2 is the main supply of the dorsal fork of the myotome 
belonging to the root, it runs between the two sections of the 
muscle and extends the entire length of the muscle; nerve 3, 
the largest and longest given off by the ventral root, enters the 
posterior edge of the ventral fork of the myotome belonging to 
the root, between the closely applied surfaces of the longus dorsi 
and the rectus divisions of the muscle ventrad of the junction 
of the two forks and runs the entire length of the ventral fork; 
nerve 4 runs caudad and enters the tip of the ventral fork of the 
next succeeding myotome; while nerve 5, as stated above, runs 
into the body to join the nerve plexus associated with the cen- 
tral vascular mechanism. ‘The ventral spinal nerves are there- 
fore composite nerves supplying not only the trunk muscles, but 
also visceral organs. 
In life, the muscle plates are semifluid and are applied close 
to the connective structures forming the walls of the base of the 
dorsal fin, the spinal cord, the notochord and the sheet that 
forms the supporting wall of the body cavity. The trunk muscle 
is attached to this connective-tissue skeleton and some of the 
fibers in the vicinity of the motor nerve interdigitate among the 
fibers of the issuing ventral root. 
The fact that muscle fibers are attached to the dural sheath 
and penetrate among the bundles of nerve fibers of the ventral 
root was largely responsible for the controversy as to whether 
the motor fibers are striated like the muscle fibers. None of the 
nerve fibers show striation. Rhode’s figure 34 shows two bun- 
dles of muscle fibers which he calls striated nerve fibers. These 
belong to the group of muscle fibers indicated in figure 4, M’’, 
some of which attach direct to the dura. 
