VENTRAL SPINAL NERVES IN AMPHIOXUS 159 
The neuromuscular mechanism thus arranged insures coérdi- 
nation of the contraction of the myotomes of one side as a physi- 
ological unit. Besides the innervation by the motor root there 
is a further innervation by the branches of the dorsal spinal 
nerve through the ramus ventralis. These branches pass into 
the myotome where the visceral ramus of the dorsal nerve crosses 
the interspaces between the myotomes (fig. 7) and enters the 
muscle. These nerves probably terminate in the connective 
tissue, but I have-not seen their endings. 
Another point of interest in connection with the dorsal roots 
is the group of branches given off between the so-called dorsal 
branch and the point where they pass lateral of the myotomes 
(fig. 7). These branches I have not followed to their endings; 
they are mainly distributed to the connective-tissue structures 
between and about the mesial surfaces of the myotomes. 
Here is material out of which the dorsal nerves of the higher 
vertebrates could be made up. By a reduction of the branches 
outside the myotomes accompanied by an increase in these 
mesial branches, the characteristic relation of the dorsal root in 
Fig. 1 Dissection showing spinal cord of Amphioxus with dorsal and ventral 
roots and the branching of the motor nerve. The cord is represented as trans- 
parent to show the sweep of the motor fibers as they assemble to leave the ventral 
angle of the cord. The horns of the myotome are placed so as to show the motor 
nerve to advantage. 
Fig. 2 Dissection of myotome of Amphioxus seen from mesial face to show 
motor nerve and its branches1to5. The rectus (mesial) portion of the myotome 
is removed. The drawings fail to reproduce the translucent, silver-gray color 
of the nerves and the delicate outlines of the thin plates before they break up 
into the smaller nerve bundles. The branches shown in the figure are only the 
main bundles which lie between the lateral and mesial halves of the myotome. 
The innumerable branches given off from these to pass immediately among the 
muscle plates to their motor endings are not shown. 
Fig.3 Dissection of motor roots of Amphioxus, showing three adjacent nerve 
roots and their course in the muscles. The myotomes and ventral roots are sep- 
arated longitudinally, hence the figure shows the nerves connecting adjacent 
myotomes as stretched to the extent of the separation of the myotomes. The 
double line A-B, indicates the position of the aorta in this region, to which 
branches of the motor roots make their way. The arrow points cephalad. The 
heavy lines represent the motor roots as they issue from the dura. They are 
displaced to the right, their normal position is along the line of junction of the 
ventral and dorsal forks of the myotome. 
