90 PROFESSOR J. C. EWART AND MR J. C. MITCHELL ON THE 



ness. As it approaches the tip of the snout it is reduced to 1*4 mm., and this diameter 

 it retains for some distance as it extends backwards under the rostrum. The rostral part 

 of the canal receives twenty branches from the superficial ophthalmic nerve as it proceeds 

 to the tip of the snout. 



The ventral part of the supra-orbital canal (subrostral canal of Garman ; S.O. 3 , fig. 7) 

 may be said to consist of three segments — (1) a part lying nearly parallel with the middle 

 line ; (2) the loop (S.O. 4 , fig. 7) already mentioned ; and (3) a slightly curved part that runs 

 outwards to the infra-orbital (I.O. 4 , fig. 7). The first part, as it runs backwards, increases 

 slightly in diameter, and then expands considerably as it turns sharply forwards and out- 

 wards to form the loop. As the top of the loop is reached, the canal contracts and again 

 expands slightly as it returns to bend backwards and outwards in front of the nasal 

 capsule. It lies embedded in the gelatinous tissue of the snout, from 2 to 3 mm. from 

 the surface. The first part opens to the exterior by ten extremely short tubules (t.-t. ss , 

 fig. 7), the first two being under 2 mm. in length. There are no tubules from the looped 

 portion ; there are, however, twelve regularly arranged but short tubules from the terminal 

 portion (t.~t. 45 , fig. 7). Passing to the sense organs in the ventral part of the supra-orbital 

 canal, forty-five nerves were counted — sixteen to the first part, seventeen to the loop, and 

 twelve to the terminal portion. All the nerves to the sense organs of the supra-orbital 

 canal spring from the ophthalmicus superficialis of the facial — a nerve which has almost 

 universally been described as a branch of the trigeminal, and which is still apt to be con- 

 fused with the superficial ophthalmic branch of the trigeminal or with the ophthalmicus 

 profundus. 



Innervation. — According to Bfard's scheme, referred to in the Lsemargus paper (1), 

 the three ophthalmic nerves — viz., the ophthalmicus profundus, the ophthalmic of the 

 trigeminal, and the ophthalmic of the facial — are supra-branchial branches, and each 

 should have sense organs in connection with it ; in other words, all three nerves should 

 take part in supplying sense organs in the supra-orbital region or in the snout. As to the 

 ophthalmicus profundus, which lies in front of the trigeminus proper, we are satisfied that, 

 as in Lsemargus and Amia (7), it takes no part in supplying lateral sense organs. The 

 superficial ophthalmic branch of the trigeminal so completely blends with the ophthal- 

 micus superficialis division of the facial that it is all but impossible to trace its fibres. On 

 the other hand, in the Elasmobranchs in which the ophthalmic division of the trigeminus 

 is a separate nerve, it has not been found supplying any of the sense organs ; while, even 

 in the skate, the numerous branches for the sensory canals have been found springing 

 directly or indirectly from the main trunk of the superficial ophthalmic division of the 

 facial. This nerve (s.o.f., fig. 6), on leaving the buccal division of the facial, escapes from 

 the cranium along with the ophthalmicus profundus, leaving which it runs forwards super- 

 ficial to the orbital muscles, and reaches the snout by traversing a canal at the junction 

 of the nasal capsule and the cranium. On the way it gives off sixteen branches, which 

 penetrate the roof of the orbit. These, after dividing, enter the first (cranial) portion of 

 the supra-orbital canal, and terminate in sense organs, probably twenty-three in number. 



