1906.] NERVES OF CHLAMYDOSELAOHUS AXGUIXEUS. 975 



tliose belonging to the Trigeminal ; bnt, even there, a. large 

 cutaneous component could not be expected, for Johnston writes 

 (15. p. 185): "That even in Fetrom9/son the dorf^ixl and lateral 

 portions of this area " (the cutaneous region of the hyoid segments 

 in non-operculate forms) " are being encroached upon bj^ the 

 branches of the Trigeminus." 



After a course of about 4 cm. the truncus hyomandibularis 

 divides into three branches, which vary slightly on the two sides. 

 The most cephalad, the r. externus niandibularis A (fig. 1, 

 E.M. VII. A) passes forwards to supply that closed portion of the 

 hyomandibular canal (H.L.) which lies above the mouth, and 

 which is the nucleus of the whole complicated hyomandibular 

 canal of Chlamydoselachus. This branch innervates neuromasts 

 only. The second branch (E.M. YII. B) passes downwards and 

 forwards to supply the open lateral-line canal (H.L. A.) which lies 

 along the side of the lower jaw. The nerve is continued beyond 

 the canal to break up into a number of small branches, part of 

 which supply a very small group of ampulla? (H.A.), the remainder 

 being distributed to the skin. Two fine twigs (E.M. VII. D & E) 

 are given oiF on the caudad side near the origin of this branch. 

 The first (E.M. VII. D) curves upwards to supply the dorsal 

 extension (L to M) of the hyomandibular canal ; the second 

 (E.M. VII. E) passes caudad to supply a few neuromasts in the 

 first part of the hyomandibular canal B (J to H). These two 

 twigs are not constant in their oi-igin. Another branch of the 

 t. hyomandibularis, the largest and most caudad (E.M. VII. 0), 

 passes downward and then cephalad to supply the large open 

 lateral line canal (H.L. B) which lies towards the middle line of 

 the lower jaw. During the fii'st part of its course this branch 

 lies at some distance from the canal, to which it sends numerous 

 fine nerves. The latter anastomose frequently, recalling the 

 condition of the long fine branches of the r. buccalis. It 

 supplies neuromasts principally, but also sends a number of fine 

 twigs to the skin (C.F.). Johnston finds that in Petromyzon (15) 

 " The hyomandibularis certainly innervates the ventral cutaneous 

 area of this segment." He further adds in the same paper : " It 

 is to be expected that the same will be found in other Cyclostomes 

 and in Selachians, and it is not surprising to find in a ganoid 

 [Amia) this component persisting in the Vllth root." It still 

 remains to be found whether the sense-organs supplied are general 

 cutaneous or communis, and whether, if they are the foi'mer — 

 which is probable, — that component is present in the facialis root 

 itself or derived from the trigeminal. The former seems more 

 likely, as there is no anatomical reason why the trigeminal 

 cutaneous components should pass to the lower jaw by the truncus 

 hyomandibularis when they could more easily pass directly by the 

 ramus mandibularis V., which is typically both motor and sensory 

 (general cutaneous). About halfway between its origin and 

 termination r. externus mandibularis gives off ventralty a nerve 

 nearly as large as itself (H + M), which after a course of 4 cm. 



