The Cranial Nerve Components of Petromyzon. 171 
to be considered as the representatives of the pharyngeal and pre- 
trematie rami of higher forms. 
After giving off the ramus visceralis the trunk descends rapidly 
to a point just over the cartilages surrounding the gill opening (463) 
where it divides into two branches. The smaller branch goes di- 
rectly caudad among the muscles, grows rapidly smaller and dis- 
Fig. 17. Fig. 18. 
680 
appears about 472. The larger branch, which may still be regarded 
as the trunk of the nerve, gives twigs to the muscles around the 
gill opening, bends backward and downward and gains the lower 
surface of the eartilage around the opening, sends a branch caudally 
which disappears in the muscles, goes downward and backward 
outside the branchial cartilage, and at 500 reaches the inner face 
of the ventro-lateral body musculature. Continuing downwards, me- 
sially- and slightly backwards it divides (509) into four branches, of 
