112 PROF. MARSHALL AND MR. SPENCER. 



the seventh nerve, given off immediately beyond the root of origin. It 

 is from the first connected with the ventral or secondary root (vn ft). 

 The condition at stage l is shown in fig. 10 (viii). At stage n (fig. 11) 

 its root, though still intimately connected with that of the facial, 

 shows a very evident line of separation from it; the ganglionic 

 character of the auditory nerve placing it in marked contrast with 

 the non-ganglionic root of the facial. This distinction between the 

 two roots becomes more marked in the later stages. 



General Considerations. — Several questions of a more general 

 character arise out of the facts we have recorded above, and we pro- 

 pose to conclude the present paper with a brief notice of the more 

 important of these. The problems in connection with the roots of 

 origin of the nerves have been already sufficiently discussed, so that 

 we at once turn to the consideration of their branches, concerning 

 which the most important points are the determinations of the equiva 

 lence of the branches of the different nerves to one another. 



We commence with the ophthalmic branches of the fifth and seventh 

 nerves, the branches named v a and vii a in our figures. These two 

 nerves, whose course and relations are well shown in figs. 11 and 12, 

 accompany one another very closely along their whole length ; they 

 appear to be both sensory nerves, their branches being distributed 

 exclusively to the skin of the top and front of the head, and more 

 especially to the mucous canals of these parts. Of the two nerves the 

 branch of the fifth nerve (v a) is the smaller and the more ventrally 

 placed of the two: though the smaller, its branches are, especially in 

 the earlier part of its course, more numerous than those of the seventh. 

 The two nerves in their course through the orbit lie dorsal to all 

 the other contents of the orbit. They are at first quite distinct from 

 one another (figs. 11 and 12) and lie close beneath the external 

 epiblast (fig. 5, vn a) ; the branch of the seventh being the more 

 superficial of the two. In the later stages of development, as in the 

 adult, the two nerves lie in very close contact with one another 

 (fig. 16, v a and vn a), the branch of the seventh lying immediately 

 dorsal to the branch of the fifth ; they also, as shown in fig. 1 6, lie at 

 a deeper level than at the earlier stages. 



The fourth nerve bears, as already noticed, a very close relation to 

 these branches. As shown in figs. 11 (rv) and 16 (iv) it crosses the 

 ophthalmic branches at right angles, lying at a slightly deeper level 



