THE CRANIAL NERVES OF SCYLLIUM. 113 



but turning outwards immediately beneath them, to end in the 

 superior oblique muscle (o.s.). At the point of crossing the branches 

 of the two nerves are in very close relation with one another, and we 

 are inclined to believe that a communication exists between the fourth 

 nerve and the ophthalmic branch of the fifth at this point, though we 

 have failed to determine this with certainty. 



In determining the morphological value of these ophthalmic branches 

 of the fifth and seventh nerves, very valuable evidence, by which we have 

 been much influenced, is afforded by the condition of the glossopharyn- 

 geal nerve. This nerve, at stage l, gives off, just beyond its root of 

 origin, a slender dorsal branch (fig. 10, is a), which, at first passing 

 upwards and backwards, soon curves round the hinder end of the 

 auditory vesicle (caccl), and reaching the dorsal surface of the head, runs 

 forward a short distance, lying immediately beneath the superficial 

 epiblast. It gives off branches along its whole course, which are 

 distributed, as shown in the figure, to the integument of the top of the 

 head. At stage n this nerve (fig 12, ix a) has the same course and 

 appearance, the only differences being that it extends rather further 

 forwards than at the earlier stage, so as to reach some distance 

 in front of the middle of the auditory vesicle ; and, secondly, 

 that its branches are now seen to be in connection with the 

 commencing mucous canals of this region. This branch of the 

 ninth nerve is clearly the ramies dorsalis, and an examination of 

 the figures 11 and 12 will, we think, leave no doubt that the nerves 

 vn a and v a, which have a similarly superficial course and a like 

 distribution to mucous canals, must be viewed as the equivalent 

 branches of the seventh and fifth nerves. We are, therefore, led 

 to adopt the view put forward by Balfoiu, that the ophthalmic 

 branches v a and vn a of the fifth and seventh nerves are the rami 

 dor sales of these nerves} Stannius and Gegenbaur speak of the ophthal- 

 mias as rami dorsales, but refer them entirely to the fifth. 



What the causes are which have led to the very marked extension 

 forwards of the rami dorsales of these nerves is not very evident; we 



1 In my paper on the head cavities of Elasmobranchs I abandoned the view previously put 

 forward ('Quait. Journ. Micr. Sc ,' Jan., 1878, p. 30), that the ophthalmics were persistent 

 remains of the commissure connecting together the roots of the nerves at their first appearance, 

 but did not expressly adopt the view that they were rami dorsales. I have been led to adopt 

 this view mainly because it now appears that, instead of being perfectly exceptional in 

 their course, as I had previously supposed them to be, the ophthalmics merely express an 

 exaggerated condition of a state of things shown in a less extreme form by the ramus dorsalis 

 of the glossopharyngeal.— A. M. M. 



