DEVELOPMENT OF CRANIAL NERVES IN THE CHICK. 3l 



neural canal, but that they arise as outgrotoths from a continuous 

 longitudinal neural ridge. What becomes of the intervening 

 parts of the neural ridge between the successive pairs of nerves ? 

 The neural ridge itself is such an exceedingly definite structure 

 that it would be a most remarkable circumstance if these inter- 

 vening parts, which early attain a very considerable size {vide 

 fig. 7), were destined to disappear without undergoing further 

 development. We shall see shortly that there is direct evidence 

 of a very strong nature, tending to prove that the intervening 

 parts of the neural ridge form commissures connecting together 

 the posterior roots of the spinal nerves in the body, and in the 

 hinder part of the head connecting the roots of the vagus with 

 the posterior root of the first spinal nerve behind, and with the 

 root of the glosso-pharyngeal in front. 



Seeing, therefore, that in all cases in which their subsequent 

 development has been traced these intervening parts of the neural 

 ridge develope into commissural nerves, I would suggest — 



1. That the intervening parts persist in the anterior part as in 

 the posterior part of the brain and the body ; and that, as in the 

 latter, they persist in the form of a longitudinal commissure 

 connecting the several segmental nerves together. 



3. That the ophthalmic nerve is the persistent commissure 

 connecting the fifth with the third and olfactory nerves. 



On this view we should get a ready explanation of several 

 otherwise perplexing points, such as the course of the ophthalmic 

 nerve and the bend it makes near its root ; the connection of the 

 ophthalmic nerve with the third nerve at the ophthalmic ganglion ; 

 and the remarkably close relation between the ophthalmic and 

 olfactory nerves in the olfactory organ of most vertebrates. 



Further, if we assume that the commissure between the fifth 

 and seventh nerves persists in like manner, we shall at once get 

 a clue to the otherwise perplexing variations presented by the 

 ramus ophthalmicus superficialis and ramus ophthalmicus pro- 

 fundus, which might be explained as due to the commissure 

 splitting along its length into two or more branches, which may 

 unite together again. Finally, it is just possible that the fourth 

 nerve may be a part of this commissure, whose relations have 

 been altered by the growth of the brain ; the origin of the fourth 

 nerve from one of the intervening parts of the brain supports this 

 suggestion. 



As to the other branches of the fifth nerve there can, I think, 

 be no doubt that Y 3 is the segmental nerve belonging to the 

 anterior border of the mandibular arch. Y 2 arises from Y 3 in 

 exactly the same manner that the anterior branch arises from the 

 posterior in each of the other segmental nerves. If this be so, 

 then the cleft between the maxillary and mandibular arches must be 



