104 PROF. MARSHALL AND MR. SPENCER, 



tinguishes between the roots of the fifth and the seventh. He describes 

 the fifth as arising by the union of two trunks of about equal size, an 

 anterior and a posterior ; of these the anterior (a) arises from the 

 ventral surface of the medulla by two roots situated very close together ; 

 the posterior (5) has also two distinct roots, a dorsal one (a) arising 

 from the side of the medulla by a large swelling projecting into the 

 fourth ventricle, and a ventral one (j3) situated immediately above the 

 root of the facial, and in front of, and above that of the auditory. 



The seventh nerve in Hexanchus is described as arising by two 

 roots, a larger one immediately in front of the auditory, and a smaller 

 one passing to it from the fifth. 



Jackson and Clarke 1 describe the combined fifth and seventh nerves 

 in Echinorhinus as arising by three roots ; an anterior inferior root 

 (v a), itself with two well-marked rootlets, a second root (v j3) arising 

 by a well-marked superior rootlet from the lobus trigeminus and by a 

 smaller inferior one, and a third root (v y and vn) closely connected 

 with the second one, 



Balfour 2 describes the fifth nerve in Scyllium stellare as arising by 

 three roots : — (1) an anterior more or less ventral root ; (2) a root 

 rather behind this arising by two strands, a dorsal and a ventral, and 

 closely connected behind with the root of the seventh ; and (3) a quite 

 distinct dorsal and posterior root situated slightly behind the dorsal 

 strand of the second root. The seventh nerve is described as arising 

 by a single root close to, and behind, the second root of the fifth. 



On comparing these descriptions of adult Elasmobranchs with our 

 own observations on embryos and adults we are led to the following 

 conclusions. 



The fifth nerve in the adult arises by two roots : 



a. An anterior one arising from the ventral surface of the medulla 

 by two non-ganglionic rootlets, whose distinctness varies much in 

 different adult Elasmobranchs. These rootlets are the tertiary or 

 anterior roots of our embryos (figs. 10, 11, and 14, v y). This root 

 corresponds to the first root of Stannius ; the anterior root (a) of 

 Gegenbaur ; the anterior root (v a) of Jackson and Clarke ; and the 

 anterior root (1) of Balfour. 



b. A posterior larger ganglionic root, the ventral or secondary root 



of our embryos (figs. 10, 11, and 14, v /3). This is at first quite 



1 "The Brain and Cranial Nerves of Echinorhinus spinosus.'" 'Journal of Anat. and 

 Phys.,' vol. x, p. 81, 



* Op, cit., pp. 194 and 195. 



