4.72 PROF. MILNES MARSHALL AND W. B. SPENCER, 
proximity of the third to the fifth nerve it is only with extreme 
difficulty that the nerve JV. c. can be distinguished at all. 
We find, therefore, that the main stem and the branches 111d 
and ie of stage n become directly the neryes which have the 
same course and relations in the adult. The ganglion e.g. be- 
comes the ciliary ganglion of the adult.1 The nerves WV. and 
N.c. become directly continuous with one another, and together 
form the nerve known as the Ramus ophthalmicus profundus. 
The discussion of the morphological import of these two very 
remarkable nerves we postpone till after the description of the 
fifth and seventh nerves. . 
The Fourth (Pathetic) Nerve.—Concerning the development 
of the fourth nerve no description has yet appeared, and though 
our observations do not yet enable us to give a complete account, 
still, so far as they go, they are of so definite a character that we 
think it» well to record-them here rather than wait for the possi- 
bility of completing them at some future time. 
The condition of the fourth nerve at stage N is well shown in 
figures 1] and 13, of which the former shows the greater part 
of its course, and the latter its root of origin. The nerve arises 
(fig. 13, tv) from the: dorsal surface of the extreme hinder 
border of the mid-brain, so far back indeed that very careful 
examination is necessary to determine. whether its origin is 
really from mid and not from hind-brain. | The roots of the two 
nerves are in close contact on the dorsal surface of the brain. 
From its root the nerve runs at first almost directly outwards, 
following the curvature of the brain, until it comes to lie a very 
short distance below the surface; it then runs backwards and 
downwards as a long, straight, and very slender nerve 
(fig. 11, 1v), which very commonly branches early in its 
course, and: terminates in the odliquus superior muscle. Just 
before reaching the muscle (fig. 11) the fourth nerve crosses 
the ophthalmic branches of the fifth and seventh nerves at right 
angles, lying at a slightly deeper level than these nerves. It 
also divides into a number of branches, of which one or more 
appear to become connected with similar branches from the 
ophthalmic branch of the fifth, the remainder entering the obliquus 
superior, whilst in the later stages (fig. 16) the main nerve is 
seen to come into very close contact with the two ophthalmic 
branches themselves of the fifth and seventh nerves. 
It will be seen from the above description that the fourth 
nerve has by stage N acquired its adult relations and distribution, 
and at stages o-p (fig. 15) and Q (fig. 16), where it is shown in 
1 Marshall, loc. cit., p. 87, and Schwalbe, “ Das Ganglion Oculomotorii,” 
‘Jen. Zeit.,’ Bd. xiii. 
