HEAD CAVITIES AND NERVES OF ELASMOBRANCHS. 95 
third or dower portion of the muscular mass derived from the 
first head cavity is situated below the rectus internus, and has 
the main trunk (111 4.) of the third nerve running along its 
posterior border ( wide fig. 33, 7. mmf.) ; from its position and 
relations I believe it to be the rectus inferior. 
Immediately below the lowest portion of the first head cavity 
is a very conspicuous muscle (0.7., figs. 19 and 20, and 32 to 
35). This muscle runs forward (figs. 19, 32, 34) at a lower 
level than the optic nerve, but does not actually pass below it, 
stopping just as it reaches it. The upper and posterior end of 
this muscle is in close contact with the posterior wall of the 
first head cavity, 7.e. with the rectus inferior wuscle (figs. 19 
and 33) ; it also receives the terminal branch of the third nerve 
as shown in figs. 19 and 83 (i114.). From the position and 
relations of the muscle, and especially from the fact of the third, 
nerve ending in it, I have no doubt that it is the obliquus 
inferior muscle; it differs from the adult mainly in its origin not 
being in front of the optic nerve. 
Concerning the relation of this muscle to the first head cavity, 
my observations leave me in doubt; its dorsal end is undoubt- 
edly from its first appearance part of the outer wall of the first 
head cavity ; but its ventral end is situated considerably further 
forward than the end of the head cavity. 
My observations thus enables me to state with considerable 
confidence that of the four eye muscles supplied by the third 
nerve the rectus superior, rectus internus, and rectus inferior are 
developed out of the walls of the first head cavity ; and that the 
obliquus wferior is, very possibly, also developed from the same 
source. 
Concerning the obliquus superior, my observations are much 
less definite. In embryos from stage m upwards, I have 
observed a muscle situated above the eyeball, below the oph- 
thalmic branch of the fifth, and in front of the rectus superior. 
This muscle (0.s., figs. 19 and 20, and 32 to 35) has the posi- 
tion and relations of the obliquus superior, and I believe it to be 
this muscle. Though it lies not far from the top of the first 
head cavity (figs. 19 and 20), especially in the earlier stages, I 
have not observed that it has any genetic relation to that cavity. 
The only nerve I have succeeded in tracing to this muscle is a 
small branch (ve.) shown in fig. 20, as arising from the com- 
municating branch (vd.) between the fifth and third nerves, 
and running upwards to the under surface of the muscle in 
which it ends. As a communicating branch between this branch 
of the fifth nerve and the fourth nerve (the special nerve of 
the obliquus superior) is described by Schwalbe as existing in 
the adult Scyldium, this may be taken as evidence in favour of 
