v. 9 VAGUS NERVE 155 



and becomes more and more nearly a simple pathway between the 

 brain and the periphery. 



However this may be, the vagus is certainly a nerve compounded 

 of the dorsal roots of several segments and it is a mixed nerve, con- 

 taining both receptor and motor fibres. Some of the more posterior 

 rootlets of this series are separated off in higher animals (not the dog- 

 fish) to form the eleventh cranial nerve, the accessorius or spinal 

 accessory, which in mammals sends motor-fibres to certain muscles 

 of the neck, the sternomastoid, and part of the trapezius. Its motor 

 nature has led some to suppose that this nerve is a ventral root, but 

 these muscles are derived from lateral plate musculature and the 

 accessorius represents the motor portion of the hinder dorsal roots 

 of the vagus series. 



The ventral roots of this post-otic region have become much 

 reduced. Several myotomes are always missing completely, so that 

 there are no ventral roots corresponding to the glossopharyngeal and 

 first three or four vagal segments. The more anterior of the surviving 

 post-otic somites are to be found not in the dorsal region but ventrally, 

 as the hypoglossal musculature of the tongue. The muscle-buds have 

 grown round into this portion behind the gill-slits, and the nerve 

 (hypoglossal) that innervates them represents the ventral roots of the 

 more posterior segments of the vagus-accessorius series (Fig. 104). 

 The origin of this nerve from the floor of the medulla is a clear sign 

 that it is a ventral root. 



Thus the entire series of cranial nerves is : 



Two cranial nerves have not yet been considered, the first, olfactory, 

 and second, optic. Our thesis is that all connexions between centre 

 and periphery are made by means of a segmental series of dorsal and 

 ventral roots and therefore these nerves, too, should be fitted into the 

 series. No embryological or other studies have enabled this to be done 



