128 THE THIliD DAY. [CHAP. 



brief statement of some of the main results arrived at. The 

 outgrowth for the vagus nerve supplies in the embryo the fourth 

 and succeeding visceral arches, and from what we know of it 

 in the lower vertebrate types, we may conclude that it is a 

 compound nerve, composed of as many primitively distinct 

 nerves as there are branches to the visceral arches. 



The glossopharyngeal nerve is the nerve supplying the third 

 visceral arch, the homologue of the first branchial arch of Fishes. 

 The development of the hypoglossal nerve is not known, but it is 

 perhaps the anterior root of a spinal nerve. The spinal accessory 

 nerve has still smaller claims than the hypoglossal to be regarded 

 as a true cranial nerve. The primitively single root of the 

 seventh auditory nerves divides almost at once into two branches. 

 The anterior of these pursues a straight course to the hyoid arch 

 and forms the rudiment of the facial nerve. Fig. 67, vii ; the second 

 of the two, which is the rudiment of the auditory nerve, develops 

 a ganglionic enlargement, and, turning backwards, closely hugs 

 the ventral wall of the auditory involution. The sixth nerve 

 appears to arise later than the seventh nerve from the ventral 

 part of the hind-brain, and has no ganglion near its root. 



Shortly after its development the root of the fifth nerve shifts 

 so as to be attached about half-way down the side of the brain. 

 A large ganglion is developed close to the root, which becomes 

 the Gasserian gangUon. The main branch of the nerve grows 

 into the mandibular arch (Fig. 67), maintaining towards it similar 

 relations to those of the nerves behind it to their respective 

 arches. 



An important branch becomes early developed which is 

 directed straight towards the eye (Fig. 67), near which it meets 

 and unites with the third nerve, where the ciliary ganghon 

 is developed. This branch is usually called the ophthalmic 

 branch of the fifth nerve, and may perhaps represent an inde- 

 pendent nerve. 



Later than these two branches there is developed a third 

 branch, passing the upper process of the first visceral arch. 

 It forms the superior maxillary branch of the adult. 



Nothing is known with reference to the development of the 

 fourth nerve. 



