xm 



PHYLUM CHORDATA 



157 



exterior, passes to the first branchial cleft, where it bifurcates, one 

 branch going to the anterior, and the other to the posterior 

 wall of the cleft. The last nerve of the series the pneumogastric or 

 vagus (vag., X) is a large nerve which emerges from the skull by 

 an aperture 

 situated be- 

 tween the audi- 

 tory region and 

 the foramen 

 magnum. It 

 first gives off a 

 series of four 

 branchial 

 branches, each 

 of which bifur- 

 cates to supply 

 the anterior and 

 posterior bor- 

 ders of the last 

 four branchial 

 clefts. The 

 lateralis nerve 

 (kit. vag., X.I) 

 is frequently re- 

 ferred to as a 

 branch of the 

 vagus since it 

 runs in intimate 

 connection with 

 the trunk of 

 that nerve for 

 some distance, 

 but it has a 

 distinct origin 

 in the medulla : 

 after becoming 

 separated from 

 the vagus trunk 

 it runs along be- 

 neath the peri- 

 toneum opposite the lateral line, which it supplies, to the 

 posterior end of the body. The rest of the vagus runs backwards 

 to divide into cardiac branches for the heart and gastric branches 

 for the stomach. 



It will be observed that the system of neuromast organs (lateral 

 line and ampullary organs) are supplied by nerve-fibres which pass 

 out in various branches of the facial and in the lateralis : 



