HEAD CAVITIES AND NERVES OF ELASMOBRANCHS, 81 
The branches of the third nerve beyond the ganglion (c. g.) 
are shown at stage m in the series of longitudinal and vertical 
sections drawn in figs. 15 to 20; of which series fig. 15 is the 
most superficial, and figs. 19 and 20 (which belong to opposite 
sides of the body) the deepest. In fig. 20, the branches already 
described are well seen. The ganglion (ec. g.) receives the com- 
municating branch (v.d.) from the Gasserian ganglion on the 
fifth, while from it proceed the branches 1a. and u14é. already 
described at an earlier stage. Of these the anterior (111 @) runs 
directly forward through the walls of the first head cavity (1), 
then along or rather through the substance of the inner wall 
of the eye, in front of which it continues its course straight 
forward, as shown in figs. 19,18, 17, and 16 (im a), until it 
ends, as seen in fig. 15 (11 @.), at the extreme anterior end of 
the head, just dorsad of the olfactory pit (o/f). 
The second branch (1m 4) from the ganglion (c.g.) is 
practically the direct continuation of the main stem of the nerve; 
it runs down, as seen in figs. 19 and 20 (a1 4), in very close 
contact with the posterior wall of the first head cavity (the 
second cavity having already disappeared). This posterior wall, 
as will be fully described later on, 1s by this time converted in part 
into certain of the eye muscles; and this branch of the third 
nerve, which lies in close contact with these muscles (vide 
fig. 19) and supplies them, ends in the most ventrally situated of 
these muscles (0. 7., figs. 19 and 20). 
At stage o the third nerve has still the same appearance, which 
is indeed almost identical with that of the adult, as is shown 
in the series of figures 33 to 40. Fig. 40, the deepest sec- 
tion of the series, shows the large ganglionic root and the 
smaller anterior non-ganglionic roots very clearly. From its 
root the nerve can be traced running backwards and outwards 
in figs. 38, 37, and 36, until it reaches, in fig. 36, the 
posterior wall of the first head cavity. Fig. 35 shows the 
branches (111 a) and (1m 4), also the communicating branch 
from the fifth nerve; and in figs. 34 and 33, the terminal 
branch (111 4) is clearly seen ending in the muscle (0. 7.). 
On comparing the condition of the third nerve here described 
in embryos from stage K upwards with that occurring in the 
adult, there can, I think, be no doubt whatever that the ganglion 
(c.g.) which lies wedged in between the first and second head 
cavities is the ciliary ganglion. Professor Schwalbe has recently 
pointed out, in a very important memoir on the morphology of 
the ciliary ganglion, that it is really a ganglion belonging to the 
main stem of the third nerve! He has shown that in Hlasmo- 
' Schwalbe, ‘‘Ueber die morphologische Bedeutung des Ganglion 
Ciliare,” ‘Sitzungsberichte der Jenaischen Gesellschaft ftir Medicin und 
VOL, XXI.—NEW SER. F 
