Green, Chorda Tympani in Selachians. 413 



has disappeared and the pre-spiracular or mandibular half gill 

 has in most cases been reduced to a vestigeal condition, the 

 pseudobranch. 



Even in man these relations are not wholly obscured ; for 

 here we can recognize the main facial trunk as homologous with 

 the post-spiracular ramus, the great superficial petrosal as the 

 palatine branch and the chorda tympani as a probable pre-spi- 

 racular ramus. These homologies are fairly well established, 

 in the various groups of lower vertebrates ; except in the case 

 of the chorda, but on the latter point there has been the greatest 

 diversity of opinion. 



Now, first, what are the criteria of the chorda tympani? 

 As to its composition, it is in part, at least, a gustatory nerve 

 and therefore belongs to the communis system of nerves as 

 defined b)^ Strong. As to its course, it forms a part of the 

 sensory facialis root, its fibers are related to cells to the facialis 

 (geniculate) ganglion and they pass out by a circuitous course 

 above and in front (cephalad) of the middle ear and Eustachean 

 tube, then downward along the inner side of the mandible to 

 enter the tongue from the side. Remembering that the Eusta- 

 chean tube is the phylogenetic derivative of the spiracular canal 

 and that the tongue is built upon the hyoid arch, it is obvious 

 that the chorda is one of the pre-spiracuL-r branches. It has 

 been assumed by some authors who hold this view (e.g.. Cole) 

 that it represents the pre-trematic ramus of the fishes in the 

 strict sense, viz., the branch for the gill on the anterior face of 

 the spiracular cleft, or mandibular hemibranch. Stannius, how- 

 ever, has given an account which would seem to put the matter 

 in a slightly different light. In his great monograph on the 

 peripheral nervous system of fishes, published in 1849, he de- 

 scribes for Raja and Spinax three pre-spiracular twigs, as 

 follows : 



"Der N. palatinus ist bei Raja, wie bei Spinax, compli- 

 cirter, als bei den Knochenfischen. Er wird durch drei Zweige 

 reprasentirt : einen zarten hinteren und zwei starkere vordere, 

 welche bei Raja clavata und R. batis ein zehr zierliches Geflecht 

 bilden. 



