330 WILLIAM PATTEN. 



Hence the term vagus is applicable to these nerves, for, 

 owing, as we shall see, to the almost complete disappearance 

 of their proper field of distribution, they have not only wan- 

 dered into other segments, but to organs which they do not 

 normally supply. 



Little is certainly known about the vagus nerves of Verte- 

 brates, but at present I see no serious objection to supposing 

 they are derived from the vagus of Scorpions. The most im- 

 portant resemblance between these remarkable groups of 

 nerves are the following : — (1) The vagus nerves in both Scor- 

 pions and Vertebrates extend backward (although the neuro- 

 meres to which they belong have been pushed forward), and 

 supply muscles and internal organs to which the corre- 

 sponding nerves of the other segments are not normally 

 distributed. (2) This wandering of the nerves in both Verte- 

 brates and Scorpio is probably due to the same cause, i. e. 

 the great concentration of their neuromeres and the absence of 

 their mesomeres; the result is that the nerves must also 

 disappear or wander to other tissues. This point is an im- 

 portant one, because these conditions are not found in any other 

 animals besides Vertebrates and Arthropods. (3) In the Scor- 

 pion the main vagus nerve is formed by the early and remark- 

 ably complete fusion of four neural nerves of an accessory brain. 

 In Vertebrates the vagus is formed in the same way; but 

 there is nothing to show whether these fused nerves, either in 

 Scorpio or Vertebrates, represent neural nerves or only their 

 sensory branches, or both. (4) The hsemal vagus nerves of 

 Scorpio form a compact and isolated group of nerves evidently 

 undergoing profound secondary changes ; already they are 

 partly fused with one another, and their roots have moved 

 backward at the same time that the neural roots have moved 

 forward. Since in Vertebrates there has probably been a similar 

 movement in the vagus region (Gegenbaur), it is possible that 

 in Petromyzon the four posterior vagus roots of the eight de- 

 scribed by Ahlborn represent haemal vagus roots which have 

 moved backward along the medulla oblongata only a little more 

 markedly than the hajmal vagus roots in Scorpio. A part 



