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Show 1906.] NERVES OF CHLAMYDOSELACHUS ANGUINEUS. 9 7 5 tliose belonging to the Trigeminal; but, even there, a large cutaneous component could not be expected, for Johnston writes (15. p. 185): " That even in Petromyzon the dorsal and lateral portions of this area. " (the cutaneous region of the hyoid segments in non-operculate forms) " are being encroached upon by the branches of the Trigeminus." After a course of about 4 cm. the truncus hyomairlibularis divides into three branches, which vary slightly on the two sides. The most cephalad, the i*. externus mandibularis 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 w'liole complicated hyomandibular canal of Chlamydoselachus. This branch innervates neuromasts only. The second branch (E.M. VII. 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 ampullae (H.A.), the remainder being distributed to the skin. Two fine twigs (E.M. VII. D E) are given off 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 twTo twigs are not constant in their origin. Another branch of the t. hyomandibularis, the largest and most caudad (E.M. VII. C), 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 first 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 former- 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 0 gives off ventrally a nerve nearly as large as itself (H + M), which after a course of 4 cm. |