486 PROF. MILNES MARSHALL AND W, B. SPENCER, 
It has by previous observers been almost invariably described as 
a root of the fifth, and never as a true root of the seventh; ow 
description and figures here given leave no room for doubt that it 
belongs to the seventh. As already noticed, the retention of this 
root marks the seventh as being more primitive than the spinal, 
and possibly more so than any of the other cranial nerves, all 
the other nerves apparently retaining their secondary roots only. 
This root is the third or dorsal root of Stannius; the dorsal 
rootlet (a) of the posterior trunk (4) of the fifth of Gegenbaur ; 
the superior rootlet of the second root (v (3) of Jackson and 
Clarke, and the dorsal and posterior root (8) of the fifth of 
Balfour. 
6. A ventral root arising from the side of the medulla at a 
rather lower level than the posterior root of the fifth. This is 
the secondary or ventral root of the seventh of our embryos 
(figs. 8, 6, 9, 10, 11, and 15, vir (3). The auditory nerve is at 
first derived from this root, but in the adult appears to be more 
distinct from the facial than is the case in the embryo. This 
root is, at first, some little distance behind the secondary one 
(v 8) of the fifth nerve (figs. 10 and 11), from which it is 
perfectly distinct; in the later stages the two roots gradually 
approach one another (fig. 14), and in the adult are usually in 
close contact. 
This root is the posterior part of the second root of Stannius, 
the root of the seventh of Gegenbaur; part, or possibly the 
whole of the third root (v y and vit) of Jackson and Clarke; 
and the single root of the seventh of Balfour. 
It would appear, therefore, that the fifth nerve loses its pri- 
mary root, retains its secondary, and acquires tertiary roots, 
while the seventh retains both primary and secondary. Con- 
cerning the fourth nerve we would suggest the possibility that 
it may prove to be the primary root of a nerve of which the 
third nerve is the secondary root, which has, in this case, 
acquired complete independence. 
c. Lhe branches of the seventh nerve.—At stage k the seventh 
nerve, which, as we have already seen, has acquired its secondary 
as well as its primary roots of origin (fig. 3), expands below the 
secondary root into a large ganglionic swelling lymg immediately 
behind the third head cavity. From this enlargement three 
branches arise: (1) from the anterior and superior angle of the 
ganglion a large nerve with a ganglionic base arises, and runs 
forward along the dorsal surface of the head, lying just beneath 
the superficial epiblast; this is the ophthalmic branch of the 
seventh, and is referred to in our figures as VII a. 
(2) The second branch, which is also large, and has a 
