DEVELOPMENT OF THE FIFTH NERVE. 419 



not noticed a dorsal branch, similar to that described by Jackson and 

 Clarke. 



The auditory nerve arises immediately behind the seventh, but requires 

 no special notice here. A short way behind the auditory is situated the root 

 of the glossopharyngeal nerve. This nerve takes an oblique course back- 

 wards through the skull, and gives off in its passage a very small dorsal 

 branch, which passes upwards and backwards through the cartilage towards 

 the roof of the skull. At the point where the main stem leaves the cartilage 

 it divides into two branches, an anterior smaller branch to the hinder border 

 of the hyoid arch, and a posterior and larger one to anterior border of the 

 first branchial arch. It forks, in fact, over the first visceral cleft. 



The vagus arises by a great number of distinct strands from the sides of 

 the medulla. In the example dissected there were twelve in all. The an- 

 terior three of these were the largest ; the middle one having the most ventral 

 origin. The next four were very small and in pairs, and were separated by 

 a considerable interval from the next four, also very small, and these again 

 by a marked interval from the hindermost strand. 



The common stem formed by the junction of these gives off immediately 

 on leaving the skull a branch which forks on the second branchial cleft : a 

 second for the third cleft is next given off; the main stem then divides into a 

 dorsal branch the lateral nerve and a ventral one the branchio-intestinal 

 nerve which, after giving off the branches for the two last branchial clefts, 

 supplies the heart and intestinal tract. The lateral nerve passes back 

 towards the posterior end of the body, internal to the lateral line, and between 

 the dorso-lateral and ventro-lateral muscles. It gives off at its origin a fine 

 nerve, which has a course nearly parallel to its own. The main stem of the 

 vagus, at a short distance from its central end, receives a nerve which springs 

 from the ventral side of the medulla, on about a level with the most pos- 

 terior of the true roots of the vagus. This small nerve corresponds with the 

 ventral or anterior roots of the vagus described by Gegenbaur, Jackson, and 

 Clarke (though in the species investigated by the latter authors these roots 

 did not join the vagus, but the anterior spinal nerves). Similar roots are 

 also mentioned by Stannius, who found two of them in the Elasmobranchs 

 dissected by him; it is possible that a second may be present in Scyllium, 

 but have been overlooked by me, or perhaps may have been exceptionally 

 absent in the example dissected. 



I 



The Fifth Nerve. The thinning of the roof of the brain, in 

 the manner already described, produces a great change in the 

 apparent position of the roots of all the nerves. The central 

 ends of the rudiments of the two sides are, as has been men- 

 tioned, at first in contact dorsally ; but, when by the growth of 

 the roof of the brain its two lateral halves become pushed apart, 

 the nerves also shift their position and become widely separated. 

 The roots of the fifth nerve are so influenced by these changes 



