96 PROFESSOR A. MILNES MARSHALL, 
the muscle (0.s.) being as I have named it, the obliquus 
superior. 
The last of the muscles of the eye, rectus externus, is the 
easiest of all to recognise and determine. It can be clearly 
recognised in stage M (vide 18 to 20, 7.¢.) as arising behind 
the main stem of the fifth nerve (between the fifth and seventh 
nerves in fact), and running forwards and outwards to the eye- 
ball, passing to the imner side of the mandibular branch of the 
fifth nerve (figs. 19 and 18) and ending by being inserted into 
the outer and posterior wall of the eyeball. The course of the 
muscle and of the nerve supplying it (the sixth) is well 
shown at stage o in figures 32 to 38. Fig. 38 shows, as already 
noticed, the origin and trunk of the sixth nerve; fig. 37 shows at 
r.e. the nerve terminating in the posterior part or origin of 
the muscle; the muscle itself can be traced running forwards 
and outwards in figures 36 and 35 (r.e.). In fig. 34, it is seen 
crossing the mandibular branch of the fifth, lymg to its inner 
side, while figs. 33 and 82 show it running forwards in front 
of the fifth nerve towards the eyeball. 
The rectus externus is also shown in transverse section in 
figs. 28 and 29 as already described, the former figure showing 
its close relation to the fifth nerve. 
The most important fact in connection with this muscle, and 
a fact proved in the most conclusive manner by the figures 
referred to above, is that 2¢ has nothing whatever to do with the 
jirst head cavity. Though it ultimately reaches the eyeball, yet, 
as shown in figures 18 and 32, it lies superficially to this cavity 
and never comes in contact with it at all. Thisis a point whose 
importance can hardly be overrated, as it furnishes us with 
an explanation of the fact that the rectus externus is supplied, 
not by the third nerve, but by a totally distinct nerve—the sixth. 
Of the four muscles supplied by the third, three are certainly, 
and the fourth possibly, developed from the walls of the first 
head cavity. The rectus externus has nothing to do with this 
cavity; it is in fact a muscle belonging to a segment further back, 
and consequently has a distinct and separate nerve supply. 
We have seen above that the dorsal ends of the second and third 
cavities disappear by stage m; it will be seen from the figures that 
the rectus externus, which appears at stage M, occupies exactly the 
position previously held by these cavities. I have not yet suc- 
ceeded in determining whether the musele is directly developed 
out of the wall of one or other or both of these cavities, but 
when we consider that the walls of the other cavities become 
muscles, that this muscle appear immediately after the disap- 
pearance of the cavities, that it occupies the position previously 
held by the cavities, and that, finally, it is, for a time at least, 
