HEAD CAVITIES AND NERVES OF ELASMOBRANCHS. 89 
mind that this branch in each case is the apparent direct ventral 
continuation of the main trunk, and in the early stages in close 
relation with the posterior wall of a head cavity, it will, I 
think, follow that the branch (111 4.) of the third, which supplies 
the rectus inferior and obliquus inferior, which is the apparent 
direct ventral continuation of the main trunk of the third, and 
which is in very intimate relation with the posterior wall of the 
first head cavity, is the corresponding branch of the third nerve, 
and is the strict homologue of the branches of the fifth and 
seventh nerves mentioned above. 
It would also appear probable, from the times of their appear- 
ance and their general relations, that the maxillary branch of 
the fifth and the mandibular branch of the seventh are equivalent 
nerves. 
Mr. Balfour, who was the first to describe the remarkabie 
ophthalmic branch of the seventh,’ has already shown that it is 
strictly comparable to the ophthalmic branch of the fifth. 
Schwalbe? describes and figures the samusophthalmicus superficialis 
in the adult Scy//iwm as arising by two roots, a posterior radix 
dorsalis and an anterior radix ventralis or profunda ; these enter 
the orbit by two separate foramina, run forward through the orbit 
as the portio major and portio minor of the ramus ophthalmicus 
superficalis, lying dorsad of all the eye muscles, and finally, on 
leaving the orbit anteriorly, end in branches to the anterior part of 
the head. As the two nerves (vii a. and v a.) in the embryo at 
any stage from L upwards exactly correspond to this description, 
there is, I think, no reason to doubt that these branches, which 
I have called the ophthalmic branches of the seventh and fifth, 
become respectively the portio major and portio minor of the 
ramus ophthalmicus superficialis of the adult. Schwalbe’s figures 
of the adult show that these branches acquire their final ar- 
rangement at a very early period, in fact, from their very first 
appearance ; his figure of these ophthalmic branches in the adult 
represents, with almost perfect accuracy, their arrangement in 
embryos of stage L. . 
Balfour leaves the fate of the ophthalmic branch of the 
seventh undecided,’ though he expresses himself as “ inclined 
to adopt” the view of which I have attempted to demonstrate 
the correctness. 
There appears to be no branch of the fifth corresponding to 
the palatine branch of the seventh ; this latter is a very singular 
nerve, lying, as it does, so extremely close to the maxillary 
branch of the fifth. I thought at one time that it might 
E\Op. cit... p-199: 
2 Loc. cit., p. 14. 
3 Op. cit., p. 200. 
