The Cranial Nerve Components of Petromyzon. 175 
plying the museles about the gill opening and the skin of the ventro- 
lateral surface. The innervation of the pit organs above the gill 
opening was not made out. 
As the vagus trunk continues caudad it receives the ventral 
ramus of the St" dorsal spinal at 780 and that of the 9% ventral 
spinal at 795. In this region the trunk contains a ganglion. At 
820 the ventral ramus of the 9" dorsal spinal and at S70 that of 
the 10% ventral spinal join the trunk. 
At 865 and following, the fourth branchial nerve goes ventrally 
and contains a ganglion. At 884 it reaches the mucosa and at 900 
gives off the ramus visceralis. This has the same disposition as 
the previous visceral rami and supplies sense organs in 885 and 
adjacent sections. Many ganglion cells are found in this fourth 
branchial nerve both above and below the point of origin of the 
ramus visceralis. 'T'he meaning of this is not elear. It probably 
indicates the presence of a third component besides the motor and 
general eutaneous in the trunk beyond the visceral ramus. This may 
be the neuromast component for the pit organs over the gill slit. 
It is to be remembered that the neuromast component in the vagus 
has its ganglion cells somewhere in the trunk or rami, since root 7b 
(that portion of the lateral line root which joins the vagus) is not 
supplied with a ganglion. After supplying the gill muscles and 
descending below the gill slit the ramus museulo-ceutaneous instead 
of piereing the ventro-lateral museles as the previous rami do, bends 
dorsally along their inner face toward the gill slit and gradually 
diminishes. It probably pierces the muscle in small strands and 
supplies the. skin. 
At 890 the ventral ramus of the 10% dorsal spinal joins the 
vagus and at 912 the ventral ramus of the 11! ventral spinal, 
which is larger than any of the preceding, enters it. A part of this 
ramus passes lateral to the vagus and innervates the lateral body 
muscle. Caudal to 901 the vagus trunk differentiates into two areas, 
one containing the fine fibers, the other the coarse fibers. The fine 
fibered area comes to lie mesially and to form a mesial trunk, while 
the coarse fibered area forms a lateral trunk. The separation is 
complete by 972, and the two continue eaudad, the coarse fibered 
trunk becoming more and more ventral as compared with the fine 
fibered. In 940 and adjacent sections the fine fibered trunk is 
ganglionated. At 972 the fifth branchial nerve separates from the 
fine fibered trunk, and at 998 it goes ventrally, gives off a visceral 
