616 ALLIS. [ior ll. 
surface of the hyomandibular, and separates almost immediately 
into its two main portions, the truncus mandibularis facialis (m/f) 
and the r. hyoideus facialis (Zf). The former lies anterior to 
the latter, turns downward, and soon separates into its two 
portions the r. mandibularis externus facialis (#ef) and the r. 
mandibularis internus facialis (sz/). 
The r. mandibularis externus facialis supplies only the sense 
organs of the operculo-mandibular canal and those of the pit 
lines of the cheek and mandible (No. 3, p. 518). The first 
branch of the nerve is given off before the main truncus has 
separated into its mandibular and hyoidean portions. It arises 
from the mandibular part of the truncus, while this truncus is 
still inside the canal of the hyomandibular, or immediately after 
it issues from that canal. It runs upward and backward, enters 
the preoperculum, and then separates into two parts, one going 
to organ 15 of the operculo-mandibular line, and the other to 
organ 16. This branch and the others also that go to organs in 
the preoperculum enter the bone along its inner, anterior edge. 
The second branch of the main nerve, in 35 mm. specimens, 
goes to the horizontal cheek-line of pit organs, and the third 
one to organ 14 operculo-mandibular. In the adult these two 
branches arise together, after the truncus has separated into 
its hyoidean and mandibular portion, but before the latter has 
separated into an externus and internus. The branch to the 
cheek-line (#ef:h/) passes outward through the adductor man- 
dibulae, near its origin from the preoperculum, and then runs 
forward along the outer surface of that muscle; the branch to 
organ 14 runs directly backward to that organ. In the adult 
specimen used for illustration a small communicating branch 
was sent from the branch to organs 15 and 16, to the one for 
organ 14; and in one 35 mm. specimen a similar branch was 
sent from the branch to the cheek-line to that for organ 14. 
Whether these communicating branches belonged to the exter- 
nus or were simply fibres of the internus, accompanying the 
branches of the externus, it was not possible to determine. In 
no other part of the lateral-line system were connecting branches 
of the kind found, and it is to be noted that in both dissections 
the connecting branch ran to organ 14 operculo-mandibular. 
