SENSORY CANALS OF THE COMMON SKATE. 97 



prominence. At the anterior margin of the supra-scapula, it gives off a long (scapular) 

 branch (sc, fig. 6), which runs backwards and outwards towards the margin of the fin, 

 communicating on the way with the dorsal extension of the hyomandibular canal. A 

 second (post-scapular) branch (p.sc, fig. 6) springs from the canal opposite the posterior 

 margin of the scapula, and runs backwards in a similar direction to the first (scapular) 

 branch. The main canal having given off the posterior scapular, curves inwards, and 

 runs along first the trunk, and then the side of the tail, terminating at or near its tip. 

 The lateral canal and its scapular branches are flattened, and have a small lumen — the 

 diameter diminishing as the tip of the tail is reached. 



From the part of the canal in front of the scapular branch there spring eleven tubules 

 (t.-t. n , fig. 6) ; ten of these, which are of about the same length, curve inwards, and 

 one, longer than the others, bends forwards and outwards. The part of the canal lying 

 on the supra-scapula gives off no tubules, but from the origin of the post-scapular branch 

 to the tip of the tail tubules are regularly given off. These tubules, which first run 

 backwards and outwards, and afterwards backwards and inwards across the canal, 

 gradually diminish in length from before backwards. The scapular branch gives off twelve 

 tubules before, and fourteen after, communicating with the dorsal part of the hyomandi- 

 bular canal. The inner tubules, which are of considerable length, run outwards and 

 forwards, while those beyond the junction run almost directly outwards. The latter 

 gradually diminish in length, and lie nearer each other than the former. 



The post-scapular branch gives off twenty tubules, which arch outwards ; the last eight 

 rapidly diminish in length as the end of the canal is reached. 



Innervation of the Lateral Canal. — The precommissural and commissural portions, 

 and the part of the trunk portion of the canal up to near the origin of the scapular branch, 

 are supplied by a special nerve which springs from the lateralis division of the vagus at 

 or near its ganglion. This branch passes upwards behind the auditory capsule, and as it 

 approaches the surface breaks up into three bundles. One runs forwards to innervate 

 the precommissural, another bends inwards to the commissural part of the canal, the 

 third passes backwards at some distance below and to the outer side of the first part of 

 the trunk portion of the canal, which it supplies as far as its eighth tubule (fig. 6). 

 For the part of the canal immediately in front of the scapular branch, the two scapular 

 branches, and the short part of the main canal between them, the lateralis gives off two 

 special branches. The first, a fairly large nerve, springs from the lateralis in front of the 

 shoulder-girdle, and, after approaching the surface, runs backwards and outwards immedi- 

 ately behind the scapular branch of the canal. It sends three twigs to the main canal in 

 front of, and a similar number to the main canal behind, the scapular branch ; and, as it 

 accompanies the scapular branch outwards, it gives off thirty-two twigs, the majority 

 of which enter the canal opposite the tubules (fig. 6). The second, or post-scapular 

 nerve, springs from the lateralis as it reaches the posterior border of the shoulder-girdle. 

 Having sent a twig to the main canal, it proceeds outwards behind the post-scapular off- 

 shoot, to which it contributes twenty-four twigs, the greater number of which enter 



