94 PROFESSOR A. MILNES MARSHALL, 
arising from the brain in the adult, as shown in fig. 22, in the 
position held by normal anterior roots (cf. fig. 30). The same 
explanation may possibly apply to the fifth also. 
The Development of the Eye Muscles.—I have made certain 
observations on the development of the eye muscles, which, 
though as yet imperfect, I desire to record here, because they 
seem to throw some light on the morphology of this very 
interesting and perplexing group of muscles. 
In a previous section I have described the way in which the 
first head cavity becomes partially doubled up on itself so as to 
form a cup, closely embracing the inner and posterior surfaces of 
the eyeball, and have further mentioned that the outer wall of 
this cup becomes converted in part into certain of the eye muscles. 
I propose now to examine these changes more in detail. 
Previous observers, with one very important exception, have 
maintained an almost complete silence concerning the develop- 
ment of these muscles. Balfour, in his great memoir on Elas- 
mobranch Fishes, to which I have referred so often, states 
concerning the first head cavity, ‘‘1 have not yet succeeded in 
tracing the subsequent fate of its walls, but think it probable that 
they develope into the muscles of the eye.”' These three lines 
which have furnished the clue I have attempted to follow up, 
contain, I believe, all that has been hitherto published concerning 
the embryology of these muscles. 
Plate V, figs. 19 and 20, represent, as already described, 
longitudinal and vertical sections through an embryo at stage M. 
The first head cavity (1) is doubled up in the manner described 
above, and its outer wall is converted in part into muscles. 
Figs. 32 to 40 show the same parts in a more advanced state of 
development at stage o. In figs. 34 and 35 the muscle fibres 
derived from this outer wall of the first head cavity are clearly 
divided into three sets, wpper, middle, and lower. Of these the 
middle set, figs. 34 and 35 (7.7.) run horizontally forwards in 
close connection with the inner surface of the eyeball; with this 
muscle the anterior branch of the third nerve (111 a.) is seen to 
be in very close relation. This muscle, which appears to be 
developed partly from the outer and partly from the inner wall 
of the first head cavity, has exactly the position and relations of 
the rectus internus of the adult. The upper portion (7.s., fig. 
34.) is quite distinct from the middle portion first described ; it is 
situated entirely above the ramus ophthalmicus profundus (tu 
a.) it rans upwards and outwards, and terminates in the upper 
wall of the eyeball (fig. 32, 7.s.) This muscle, from its course 
and relations, can be none other than the rectus superior. The 
1 Loc. cit., p. 208. 
