THE CHORDA TYMPANI. 163 



course fails because it contravenes the first criterion. 

 The first and second criteria are perfectly fulfilled by the 

 mandibularis internus VII of the Amphibia (Strong, '95) 

 and by that nerve of the fishes in general, for it supplies 

 the mucosa between the hyoid and mandibular arches and 

 along the inner side of the latter (see also Ruge, '97, 

 p. 209). 



Several recent writers in emphasizing the third criterion 

 above have called attention to the fact that the r, man- 

 dibularis internus VII of several of the fishes is a post- 

 spiracular nerve and therefore cannot be homologous 

 with the chorda tympani. This point was made by Allis 

 in his preliminary paper in 1895 (p. 488), but his discus- 

 sion of this nerve in his later paper ('97, p. 638) is not 

 altogether clear. He distinctly homologizes this nerve in 

 Amia with the nerve so named by Ewart, Pollard and 

 Strong in the other Ichthyopsida, for it certainly has the 

 corresponding course for its whole length. Since it 

 appears to take no part in the innervation either of ter- 

 minal buds externally or of taste buds in the mouth, he 

 seems inclined, however, to conclude that it is not a com- 

 munis nerve at all, but perhaps general cutaneous. This 

 he bases on the false assumption elsewhere expressed 

 (p. 642), that all, or nearly all, communis fibres are for 

 specialized sense organs. The fact is that they may go to 

 the general mucous surfaces, and I have no doubt that the 

 r. mandibularis internus VII of Amia is structurally, as 

 well as topographically, the same as that nerve in Menidia 

 and the other forms mentioned. As this nerve in Amia 

 and in selachians lies behind the spiracular canal, the cor- 

 responding nerve in these other types must also be a real 

 post-trematic ramus and cannot be regarded as a pre- 

 trematic ramus which has coalesced with the post- 



