196 DEVELOPMENT OF THE FIFTH NEKVE. 



sage a veiy small dorsal branch, which passes upwards and back- 

 wards 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 vagiis arises by a great number of distinct strands from the 

 sides of the medulla. In the example dissected there were twelve in 

 all. The anterior 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 sejDarated 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 im- 

 mediately 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 posterior 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 Stan- 

 nius, 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. 



The Fifth Kerve. 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 mentioned, 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 that they spring from the brain 

 about half way up its sides, and a little ventral to the border 

 of its thin roof. While this change has been taking place in 



