OBSERVATIONS ON THE CRANIAL NERVES OF SCYLLIUM. 473 
transverse section, it is still the same. In the adult its course, 
like that of the other nerves, becomes altered owing to the 
varying rates of growth of the brain, the skull, and the face ; 
the rapid growth of the skull relatively to that of the brain causing 
the fourth nerve to run some distance forward within the brain 
case before passing out, when, as before stated, it runs imme- 
diately beneath and in very close contact with the ophthalmic 
branches of the fifth and seventh nerve. 
In attempting to trace the fourth nerve in stages earlier than 
nw we have met with considerable difficulties, and have hitherto 
obtained only a moderate amount of success. At m the relations 
are the same as at N, the sole difference being that the nerve is 
more slender than at the later stage. At stage L, though we have 
examined a very considerable number of specimens in excellent 
histological preservation, we have as yet recognised the fourth 
nerve in one specimen only, and even in that one not with abso- 
lute certainty. In the specimen in question the nerve has the 
same position and relations as at N, but is very much more 
slender, so thin, in fact, as to be almost unrecognisable. arlier 
than 1 we have failed, after the most careful search, to find any 
trace of the nerve. © 
Though our observations are imperfect they yet seem to point 
to certain conclusions of importance touching the morphology of 
this important nerve. In the first place the fourth nerve is the 
only one in the body which, in the adult, arises from the dorsal 
surface of the brain ; it is, therefore, of great importance to notice 
that from the very earliest stage at which we have seen it the 
point of origin is that of the adult nerve.! Inasmuch as the ma- 
jority of the cranial nerves, as well as the dorsal roots of the 
spinal nerves, arise af first from the dorsal surface of the brain or 
spinal cord, it seems natural to suggest that the fourth nerve 
differs from all the rest, not in its mode of origin, but in the fact 
that, whilst all the other nerves shift their attachment to a greater 
or less extent, it alone preserves the primitive position of its roots 
of origin, This shifting of the roots is, in part,’ due to the 
rapid growth of the dorsal part of the brain forcing the roots of 
the two sides from each other; and it becomes of interest to 
notice that the fourth nerve arises from a portion of the brain 
where this rapid growth of the roof does not occur, and where; 
consequently, one cause of the change in the other nerves is 
absent. 
The fact that the direction of the fourth nerve is at first at 
right angles, or nearly so, to the axis of the part of the head 
1 Balfour, ‘ Elasmobranch Fishes,’ pp. 156 and 191. 
° We say “in part,” because it will be shown further on in this paper 
that another process contributes greatly to this shifting. 
