SENSORY CANALS OF THE COMMON SKATE. 95 



(fig. 7). The main branch runs backwards nearly parallel with the inner limb of the 

 loop. From this branch (1) slender twigs reach and penetrate the middle third of the 

 inner limb, and (2) longer delicate filaments proceed to the posterior third of the inner 

 and posterior half of the outer limbs. The rest of the outer limb, as far forward as 

 the point where it begins to bend inwards, is innervated by four branches, each of which 

 has a separate exit from the ampullary capsule. The bend formed by the outer loop 

 receives its nerves from the precapsular branches, which reach it by passing under the 

 anterior portion of the canal. The straight, most anterior, ventral portion, and the 

 dilated anterior portion on the dorsal aspect, are supplied from a large superficial pre- 

 capsular branch which runs directly forwards and divides into two main branches (hm. 2 , 

 fig. 6). The outer dips downwards and sends six twigs to the ventral part ; the inner 

 sends a long slender branch forwards which gives six twigs to the longitudinal part of 

 the dilatation, and a short branch inwards, which sends several twigs to the transverse 

 part of the dilatation. From the same precapsular branch three filaments proceed to 

 supply the part of the canal immediately beyond its connection with the suborbital 

 tubules. The next segment of the canal, the part from which the second to the four- 

 teenth tubules are given off, is supplied by twelve twigs which radiate from a second 

 precapsular branch. The rest of the dorsal portion of the hyomandibular canal is supplied 

 by four branches, which escape from the capsule and break up into long, delicate fila- 

 ments as they run outwards and backwards. This part of the canal gives off twenty-five 

 tubules, and is penetrated by twenty-seven nerves, most of which enter near the origin of 

 the tubules ; the two last nerves enter the terminal part of the canal which runs inwards 

 to communicate with the anterior (scapular) offshoot from the lateral canal. 



The mandibular canal is supplied by a branch (hm. 1 , fig. 7) of the hyomandibular 

 nerve, which leaves it and passes downwards between the first branchial chamber and the 

 jaw muscles. This branch, after parting with the majority of its fibres to the mandibular 

 ampullae (M. A., fig. 7), runs along the'posterior border of the mandible, and sends thirteen 

 filaments to the sense organs of the canal. 



The hyomandibular canal, with its remarkable ventral loop (subpleural of Garman) 

 and its long dorsal extension (pleural of Garman), has many points of interest. In the 

 first place, these pleural or pectoral extensions are characteristic of the Batoidei. In all 

 of them the dorsal extension, which bends backwards to join the scapular branch of the 

 lateral canal, is invariably present ; and, except in the torpedoes, the ventral extension is 

 also present, though in some of them, e.g. Raja ocellata, the outer limb of the loop is 

 absent. Sappey's description of what may be called the pectoral portions of the hyo- 

 mandibular canal is in many respects remarkable. Having failed, apparently, to establish 

 a connection between the ventral and dorsal portions of the canal, he describes the 

 ventral loop as forming the external and internal branches of the ventral mucous canal, 

 which he describes as beginning near the front of the snout and extending backwards 

 and then forwards to terminate near its origin. The inner limb is thus made to include 

 the ventral part of the infra-orbital, which he evidently failed to discover was continuous 



VOL. XXXVII. PART I. (NO 6). Q 



