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



opposite the tubules (fig. 6). No other large, branches proceed from the lateralis nerve. 

 As it proceeds backwards, first at some distance from the canal, but afterwards in close 

 contact with it, the lateralis gives off slender branches which either directly, or after 

 dividing, enter the canal to terminate in the sense organs. 



The Histology of the Sensory Canals. 



Many investigators have studied the minute structure of the lateral sense organs of 

 fishes, but to Leydig (8), Merkel (4), Solger (9), and Fritsch (6) we are most indebted 

 for information on this subject. In this paper it will only be necessary to give a short 

 account of the minute structure of the canals of the skate. 



Hitherto it seems to have been taken for granted that the cranial canals differed in 

 structure from the lateral canal. This, however, is not the case ; for in the skate, for 

 example, some parts of the cranial canals exactly agree in structure with the canal of the 

 trunk. It would be more accurate to say that the canals of the dorsal surface differ in 

 structure from the canals of the ventral surface ; but even to this statement there are 

 exceptions, for the anterior (rostral) part of the supra-orbital canal and the anterior dorsal 

 part of the infra-orbital canal, as well as the dilated dorsal portion of the hyomandibular 

 canal, all differ from the canal of the lateral line, and agree with the canals of the ventral 

 aspect. Apparently the difference in structure depends on the relation of the canals to 

 the skin ; for the canals which are imbedded in, or lie immediately in contact with, the 

 skin, differ from those that lie more or less deep in the subcutaneous tissue. The canals 

 intimately related to the skin, i.e., the lateral canal, and, with the exceptions just 

 mentioned, all the dorsal cranial canals, are flattened, have a small lumen, and are to a 

 large extent composed of fibro-cartilage. 



The ventral canals and the portions of the supra-orbital, infra-orbital, and hyomandi- 

 bular already specified, have, on the other hand, a rounded form ; the lumen is often five 

 to six times greater than that of the lateral canal, and there is little or no fibro-cartilage 

 in the wall. Further, while the wall of the ventral canals is of nearly the same thickness 

 all round, the wall of the lateral canal, and of the cranial canals which resemble it, while 

 presenting thick fibro-cartilaginous sides, have only a thin roof and floor. It may here be 

 mentioned that Fritsch (6) gives a figure of a " trunk canal" of the torpedo, and states 

 that while the trunk canals are fibrous, the head canals bear a fibro-cartilaginous character. 

 This may be true of the torpedo, but it does not hold for the skate. 



The Dorsal Canals. — The lateral canal may be taken as an example of what might be 

 known as the dorsal or cutaneous type, while the ventral part of the infra-orbital may 

 serve as an example of the wide, thin-walled ventral or subcutaneous type. The lateral 

 canal, which lies either in or immediately beneath the skin, is flattened, and somewhat 

 resembles a long narrow slightly tapering ribbon, having one of its surfaces parallel with 

 the surface of the skin. The small lumen occupies less than a third of the width of the 

 ribbon ; and, while it is bounded at each side by a thick fibro-cartilaginous wall, the floor 



