200 ME. F. J. COLE ON THE STRUCTUKE AND MORPHOLOGY OF 



Whilst on the subject of the facial nerve it is necessary that AUis's views on tliis nerve 

 in Amia should be discussed. He says (1897, 6, pp. 615, 616) : " After issuing from that 

 [=facial] canal the truncus facialis continues dowiiward, outward, and backward, lying 

 in the depression on the outer surface of the hyoniandibular, and separates almost 

 immediately into its two main j)ortious, the truncus mandibularis facialis and the 

 11. hyoideus facialis. The former lies anterior to the latter, turns downward, and soon 

 separates into its two portions, the 11. mandibularis exteruus facialis and the R. mandibu- 

 laris internus facialis." On p. 618 the "internal mandibular" is described as entering 

 into relations with the lower jaw. Before going further and discussing the homology of 

 the latter nerve with the chorda tynipani, it is necessary to point out that Allis has 

 misnamed his nerves. As I have already pointed out (18S)6, 46, p. 657 et neq.), the terms 

 internal mandibular and Jiijoidetis as first used by Stannius are not only synonymous hut 

 ap2)ly to a motor post-spiracular nerve related to the anterior face of the liyoid arch, just 

 as the pre-spiracvilar nerve should be related to the posterior face of the mandibular arch. 

 On this ground alone (and there are others) Allis's "iuternal mandibular" must be a 

 morphological pre-spiracular nerve, and as sucli is wrongly iiamed by him. As the 

 whole question, however, involves also the question of the homology of the chorda 

 tympani, it is first necessary to show what Allis's views are on the latter question. He 

 says (6, pp. 638, 639): "As the nerve [i. e. the "iuternal mandibular"!, in Amia, lies 

 behind the spiracular canal, it is a post-trematic branch of the facitilis, and cannot, 

 therefore, be the chorda tympani, for the course of that nerve in man through the upper 

 portion of the tympanic cavity and then downward anterior to that cavity certainly 

 indicates that it is a pre-spiracular nerve. That this nerve, in Amia, is the homologue 

 of the nerve of the same name described by Ewart, Pollard, and Strong, in other 

 Ichthyopsida, and considered by them as the homologue of the chorda tympani, is hardly 

 open to qitestion. The nerve in Amia is probably to be compared to the branch which, 

 on each side of the branchial arches, runs downward over the anterior face of the arch 

 on to the inner surface of its ventral portion. Its position in Amia, along the inner 

 surface of the mandible, could be easily derived from that in Selachians as given by 

 Vetter. In Heptanchus what seems to be the nerve is shown lying along the posterior 

 edge of the mandible ; tVom this position, as tlie hyoideo-mandibular fold of Amia was 

 formed, the nerve could as naturally come to lie along the inner sui-face of the mandible 

 as along the lateral surface of the hyoid." An examination of this passage reveals 

 several flaws. It is true that the chorda tympani is a pre-spiracular nerve,'as I have 

 previously shown (1896, 46), but is Allis's " iuternal mandibular " a pos^-spiracular 

 nerve ? We have seen that Ramsay Wright failed to identify the homologue of the 

 pre-spiracular in Lepidostens and Amia, and Allis himself (18S9, 4, \)\). 501 et seq.) says 

 nothing about it. The fact is, nothing is known of the relation of the "internal 

 mandibular" nerve of Amia to the spiracular cleft, since the embryonic condition hns 

 not yet been worked out, and the cleft itself degenerates in the adult. We are, therefore, 

 not in a position to say whether this nerve is pre- or post-spiracular. Further, what is 

 the definition of a pre-spiracular nerve ? It should fulfil three conditions : (1) it should 

 rim in frcmt of the spiracle : (2) it should pass akaig the posterior face of the mandibular 

 arch ; and (3) it should consist of visceral sensory fibres. With regard to the first 



