THE NERVOUS SYSTEM 455 



the title of Vagus (wandering), since it becomes distributed 

 to the oesophagus and stomach, the heart, and, in higher forms, 

 the lungs. In spite of its great length and extensive distribu- 

 tion, however, it is not to be considered,, like the ramus lateralis, 

 a compound nerve, but its length is due rather to the extension 

 posteriorly of parts once placed far forward and thus within 

 the legitimate province of the original nerve. Thus, the heart 

 has primarily a very anterior position ; the oesophagus and 

 stomach were probably once very far forward, and the lungs 

 are diverticula of the primary oesophagus. The wide distribu- 

 tion of this branch is thus a striking illustration of the prin- 

 ciple of conservatism of nerve distribution enunciated above, 

 and belongs in the same category as the case of the stapedial 

 nerve or that of the innervation of the mimetic musculature. 



In the transition to terrestrial life the Vagus group suffers 

 naturally the loss of the branchial elements in which metamer- 

 ism zvas so clearly displayed, but has gained by the greater de- 

 velopment of the tongue and the sense of taste, and has differ- 

 entiated the Accessorius element. The intestinal branch also f 

 corresponding to the higher development of its field of distribu- 

 tion, is still more extensive and complex. 



The extreme differentiation of this group may be learned 

 from Plate VII, which here especially, in the separation of the 

 Accessorius and in other points, suggests the mammalian con- 

 ditions. The communicating branch, the tympanic nerve, be- 

 comes somewhat more complicated and forms connections be- 

 tween four ganglia ; starting from the ganglion petrosum of the 

 Ninth nerve it runs forward and sends branches to the gan- 

 glion geniculare of the Seventh, and to the otic and sphenopala- 

 tine ganglia of the sympathetic system. It is also involved in a 

 small plexus of sympathetic nerves which surround the carotid 

 artery. The relationships of this nerve, which are so complex 

 in mammals, become especially so in Man, owing to the short- 

 ening of the longitudinal axis of the skull and the formation 

 of the cervical flexure, both of which tend to the shortening 

 of the distance between the nerve roots. The relations 

 found in Man are shown in the accompanying figure (Plate 



