SENSORY CANALS OF L^MARGUS. 77 



It seems at first sight remarkable that the sense organs of the snout in Selachians — 

 the sensory canals and ampullae — are not supplied as has hitherto been supposed by the 

 trigeminal, but by the facial, a nerve which, without doubt, belongs to a posterior 

 segment. It is, however, no more remarkable than the innervation of the entire length 

 of the lateral canal by the lateralis division of the vagus, and may be accounted for by 

 the two portions of the branchial sense organ, that in the embryo lies above the hyoid 

 cleft, growing forwards over the head (carrying their nerves with them) in very much 

 the same way as the lateral canal grows backwards along the trunk. 



Although I have not thought it necessary to discuss the question as to the segmental 

 value of the ophthalmic and buccal divisions of the facial, I may state that, with Van 

 Wijhe (35), I regard them as together representing a single dorsal branch, i.e., that 

 they have both been developed in connection with a single branchial sense organ. 



The next dorsal nerve, the hyomandibular (hm., fig. 2), also belongs to the facial. 

 It is an extremely large nerve in many Selachians, more especially in the skates, and, 

 unlike the dorsal nerves already considered, it runs outwards almost at right angles to 

 the long axis of the head. While the superficial and buccal branches of the facial have 

 in all probability been formed by the splitting of a single dorsal branch, much might be 

 said in favour of considering the large bundle of sensory fibres that proceeds outwards 

 behind the spiracle, as representing an independent dorsal branch. If the sensory fibres 

 of the hyomandibular nerve represent one dorsal branch, and the ophthalmic and buccal 

 branches of the facial together represent a second dorsal branch, there is no escape from 

 the conclusion that there are two nerves between the trigeminal and auditory — a con- 

 clusion which supports strongly Dohrn's contention that there is a hyomandibular as 

 well as a hyoid segment. On the other hand, it is possible that all the three dorsal 

 nerves, viz., ophthalmic, buccal, and hyomandibular, and their respective canals and 

 ganglia, have been formed in connection with a single branchial sense organ which grew 

 outwards behind the spiracle, as well as forwards above and below the orbit. 



The large nerve which supplies the hyoid and mandibular* groups of ampullae, 

 and the hyomandibular sensory canal, has until recently been looked upon as a branch 

 of the trigeminal, and strangely enough has not found a place in the various schemes 

 prepared with a view to illustrating the branchial, sense organs, and the segmental 

 arrangement of the cranial nerves. The hyomandibular nerve, made up entirely of 

 supra-branchial fibres, extends outwards behind the spiracle towards the great hyoid 

 group of ampullae (H.A., fig. 2). A large number of the fibres abruptly end in the 

 ampullae, others pass in front of or through the ampullary eapsule and end in the sense 

 organs of the hyomandibular canal (HM., fig. 2), and the mandibular group of ampullae 

 (M.A., fig. 2). A nerve (f.a., fig. 2) corresponding to the facial of the higher vertebrates 

 lies in contact with the hyomandibular. It contains no supra-branchial fibres, but a branch 

 which runs backwards to the first branchial cleft may include some fibres for the skin. 



* The mandibular group of ampullae, which is easily found in the skate, has not hitherto been referred to by any 

 writers on the lateral sense organs. 



