314 OUTLINES OF CHORDATE DEVELOPMENT 



four mesodermal segments. It arises as two pairs of roots, 

 similar to the ventral spinal nerve roots, in the region of the 

 last two segments of the head. The two roots join and the 

 nerve then passes around, posteriorly to the last gill-pouch, to 

 the floor of the pharynx, where it is distributed to the muscles 

 of the tongue; it is thus visceral efferent. 



The XI Cranial Nerve (Spinal Accessory}. The development 

 of this nerve in the chick is unknown. The XI nerve is to be 

 regarded as a separated caudal portion of the X nerve and 

 since it is visceral efferent (although supplying voluntary mus- 

 cles in the higher vertebrates), its development is probably not 

 unlike that of the visceral efferent elements of the X nerve. 



The X Cranial Nerve (Vagus or Pneumogastric}. This large 

 nerve is really a complex, consisting of the nerves associated 

 with the third and fourth visceral pouches. It is chiefly 

 visceral, both afferent and efferent, and m addition to its 

 visceral-pouch branches, it sends branches back to certain of 

 the viscera. 



The formation of the primary vagus ganglion has been de- 

 scribed. This divides into two parts, one remaining in its 

 original position (ganglion nodosum), the other moving down 

 between the fourth and fifth visceral arches (ganglion jugulare). 

 Its ventral root is multiple, a large number of outgrowths from 

 the cells of the medulla converging to the ganglion nodosum, 

 from which connections are made with the sympathetic system. 

 From the ganglion jugulare branches grow out to the gill-pouch 

 region, and a large branch passes posteriorly to supply the 

 thoracic and abdominal organs. Neuroblasts accompany these 

 branches and later form the sympathetic ganglia of the organs 

 innervated. 



The IX Cranial Nerve (Glossopharyngeal) . This is to be re- 

 garded as a separate anterior portion of the vagus group. It 

 is the nerve of the second visceral pouch (first branchial pouch). 

 Its development is essentially similar to that of the branchial 

 portions of the X nerve, with which it forms a connection. 



The VIII Cranial Nerve (Auditory). As already noted the 

 VIII nerve arises hi common with the VII. A posterior part 



