SENSORY CANALS OF L^EMARGUS. 75 



a physiological sense) sensory fibres for the lateral sense organs. These fibres may be 

 known as special somatic sensory fibres. 



In order to distinguish between these two kinds of fibres, those innervating the sense 

 organs might be known as the supra-branchial fibres of the dorsal branch. This would 

 admit of the term dorsal, so long in use, being retained in fishes and also in the higher 

 vertebrates in which the lateral sense organs, with some possible exceptions, and the 

 nerve fibres (supra-branchial) supplying them, have completely disappeared. Seeing 

 that not one of the many hundreds of sense organs in the skin of Selachians has been 

 found in the skin of adult higher vertebrates, we must expect in studying the dorsal 

 branches of the cranial nerves, even in fishes, some of the supra-branchial fibres, either 

 in process of degenerating, or entirely absent. 



In the case of the ophthalmicus profundus in Lsemargus, there are no lateral sense 

 organs (neither nerve hillocks, ampullae, nor pit organs) supplied by its dorsal branch, 

 i.e., what I have designated the supra-branchial fibres of the dorsal branch are either 

 absent altogether or have changed their function, and are now playing the part of 

 ordinary sensory somatic fibres. That they have degenerated (supposing they once 

 existed) and not changed their connections, may be inferred from the fact that when 

 the supra-orbital sensory canal disappears, its nerve, the superficial ophthalmic of the 

 facial, also disappears. That there is a supra-branchial sense organ in connection with 

 the ganglion of the profundus in the embryo, seems to have been placed beyond doubt, 

 but it has not yet been shown that the embryonic sense organ in connection with the 

 profundus either develops into canals or pit organs. 



What then is the distribution of the dorsal branch of the profundus ? The dorsal branch 

 (o.n., fig. 2), on leaving the large profundus ganglion (o.n.g., fig. 2) passes over the 

 eyeball under the rectus suj)erior, rectus internus, and obliquus superior muscles, and 

 eventually reaches the anterior part of the snout, where it terminates in the skin. The 

 profundus, which has a similar distribution in Eaia and in Amia, also gives off several 

 branches which enter the eyeball. Some of these branches (long ciliary) arise from the 

 dorsal branch as it passes through the orbit (lc, fig. 2), others spring from the ganglion, 

 and either pass directly to the eyeball, or unite with a branch of the deep division of 

 the oculo-motor, having a similar destination. The ciliary ganglion (e.g., fig. 2) lies at 

 the junction of the profundus and oculo-motor fibres. Large ganglionic cells sometimes 

 extend a considerable distance along the dorsal branch, and ganglionic cells extend 

 from the profundus ganglion to form the ciliary ganglion at the junction of the deep 

 branch of the profundus (lr., fig. 2) with a branch (sr., fig. 2) from the oculo-motor (3). 



The second nerve having a dorsal branch is the trigeminal (Tr., fig. 2) ; the dorsal 

 branch is the ophthalmicus superficialis trigemini. In sharks and rays this nerve sup- 

 plies the eyelids, and the skin over the anterior part of the cranium, but it also sends 

 fibres into the snout. More or less distinct in sharks, the superficial ophthalmic of the 

 trigeminal in rays consists of very few fibres which, on leaving the trigeminal, at once 

 more or less completely unite with the superficial ophthalmic of the facial. 



VOL. XXXVII. PART I. (NO. 5). N 



