( 710 ) 
PR fig. IV). When they have quitted the canal each root separates 
into two filaments, one of these filaments is ascending (Asc. f.) and 
one is descending (Dese. f. fig. VI). Both ascending -root filaments 
unite to form a nerve, the internal branch of the posterior division 
(fig. VD, the filaments of which pass over in the intermyotomal 
septum at the places indicated by 3 D—5 D fig. IV, and leave the 
septum to go over in the skin at the places indicated in the same 
way in fig. V. Before these filaments go over into the corium 
they each give off a small twig innervating the distended part of the 
intermyotomal septum, which is adjacent to the skin. 
Before the two ascending rootfilaments join, they each give off a 
small branch, which also unite to form a small nerve, the first external 
branch of the posterior divisions (fig. VI) entering the septum at 
2D fig. IV and leaving the septum to pass over in the skin at the 
corresponding place of fig. V. 
Both descending rootfilaments before joining each give off a 
small branch, which form together a small nerve, the second 
external branch of the posterior division (fig. VI), which enters and 
leaves the intermyotomal septum at the places indicated by 1D in 
fig. IV and V. 
The nerves described, all together, innervate the dorsal part of the 
myotome and the intermyotomal septum, and form the posterior 
primary division of the spinal nerve. 
The descending rootfilaments also unite to form a nerve which pretty 
soon divides into two branches, one of these innervating the lateral 
part of the myotome and the intermyotomal septum ; the other is, the 
continuation of the maintrunk, crosses the lateral part of the myotome 
and innervates the ventral part of the myotome and the intermyotomal 
septum. The branch innervating the lateral part of the myotome 
divides into two branches, an external and internal braneh of the 
lateral division (fig. VI). The external branch splits up into two filaments 
one of which is recurrent (recurrent br. fig, VI) and innervates the 
top of the lateral part of the myotome. The external branch enters 
the septum at £ 1.2. fig. IV and leaves the septum at 1, 24 
fig. V. The internal branch gives off several branches passing over 
in the skin at 3 L—6 LZ fig. V. 
The branch innervating the ventral part of the myotome and the 
intermyotomal septum shows the same arrangement as the branch 
for the lateral part of the myotome. It divides into two branches 
one being the external branch of the anterior division, the other the 
internal branch (fig. VI). The external branch passes over in the 
septum at V 1.2. fig. IV, splits up into two smaller branches of 
