THE CRANIAL NERVES OF SCYLLIUM. 105 



distinct from the root of the seventh, but during the later stages of 

 development gradually approaches this latter, and in the adult cannot 

 be clearly distinguished from it. 



This root is the anterior part of the second root of Stannius ; the 

 ventral division (/3) of the posterior root (b) of the fifth of Gegenbaur ; 

 apparently the inferior rootlet of the second root (v j3) and possibly 

 part of the third root (v y and vu) as well, of Jackson and Clarke ; 

 and the second root (2) of the fifth of Balfour. 



The seventh nerve in the adult arises by two roots : 



a. A dorsal root arising far up the side of the medulla, at the 

 junction of the thickened side and thin roof of the fourth ventricle. 

 This root is the primary or dorsal root of the seventh nerve of our 

 embryos (figs. 2, 3, 6, 9, 12, 14, and 15, vu a). It has by previous 

 observers been almost invariably described as a root of the fifth, and 

 never as a true root of the seventh ; our 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 (b) of the fifth of Gegenbaur ; the superior 

 rootlet of the second root (v /3) of Jackson and Clarke ; and the dorsal 

 and posterior root (3) of the fifth of Balfour. 



b. 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. 3, 6, 9, 10, 11, and 

 15, vu /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 /3) 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 vu) of Jackson and Clarke ; and the single root of 

 the seventh of Balfour. 



It would appear, therefore, that the fifth nerve loses its primary 



