THE CRANIAL NERVES OP SCYLLIUM. . 91 



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, iv), which very commonly branches early in its course, 

 and terminates in the obliquus 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 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 immediately beneath and in very close contact with 

 the ophthalmic branches of the fifth and seventh nerves. 



In attempting to trace the fourth nerve in stages earlier than n 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 absolute 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. 

 Earlier than l 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 



