116 PROF. MARSHALL AND MR. SPENCER. 



this nerve in Mustelus : should it prove to arise as in Scyllium, then 

 it must definitely be regarded as a branch of the third. 



As we have already pointed out, the nerves N.c and ]Sf. together 

 make up the ramus ophthalmicus profundus of zootomists, a nerve 

 which seems to have escaped Balfour's notice both in the adult and in 

 the embryo. Balfour does, indeed, in his description of the nerves of 

 the adult Scyllium, speak of a ramus ophthalmicus profundus, but 

 inasmuch as he says concerning it that " this latter nerve arises from 

 the anterior root of the fifth, separately pierces the wall of the orbit 

 and takes a course slightly ventral to the superior ophthalmic nerve 

 hut does not (as is usual in Elasmobranchs) run heloiv the superior rectus 

 and superior oblique muscles of the eye," 1 it is clear that he is describing 

 the ophthalmic branch of the fifth and not the true profundus, whose 

 existence he has overlooked. There appears to be considerable con- 

 fusion in the use of the terms ramus ophthalmicus superficialis and 

 ramus ophthalmicus profundus by different writers, a confusion 

 which our observations on Scyllium may help to remove. We find as 

 already stated, three perfectly distinct nerves to which the term 

 ophthalmic nerve can be, and is, applied • of these the two dorsal ones 

 (v a and vn a of our figures) are the rami dorsales of the fifth and 

 seventh nerves, and may be spoken of as the ophthalmic branches of 

 the fifth and seventh nerves respectively. Both these nerves are very 

 superficial along their whole course, and both lie dorsal to all the eye 

 muscles and other contents of the orbit. The two nerves are at first 

 perfectly distinct, but in the adult unite more or less closely together, 

 the extent of their union varying much in different Elasmobranchs. 

 The two together constitute the ramus ophthalmicus superficialis. 



The third of the ophthalmic nerves, the ramus ophthalmicus pro- 

 fundus, has a very different course, and is of a totally different nature ; 

 it is formed in Scyllium by the union of the connecting branch between 

 the fifth and third nerves {N.c) with the branch N of the third nerve. 

 It is very definitely characterised by its course ventral to the superior 

 rectus, superior oblique, and internal rectus muscles • by its close relation 

 with the inner wall of the eyeball ; by the fact that the ciliary ganglion 

 is either in its trunk or is connected with it directly ; by its having at 

 first no branches, and by its close connection with the olfactory nerve. 



We believe that the ophthalmicus superficialis and ophtlialmicus 

 profundus always maintain these relations ; that the profundus, which 

 1 Op. cit, p. 194 : the italica are our own. 



