60 PROFESSOR J. C. EWART ON THE 



This plan necessitated the delay of any lengthened discussion of the cranial nerves, 

 and the breaking up of the work into a number of more or less independent investiga- 

 tions. Now that the work has advanced towards completion, I am satisfied that the 

 best plan was adopted. When I originally discovered four ganglia in connection with 

 the facial in the fish, I was inclined to believe that I ought to find their counterparts in 

 other vertebrates, and at once set about preparing schemes with a view to establishing 

 their relationships. By a roundabout process, however, I now know that there is no 

 necessity, in many cases, to look in the adult higher vertebrates for even the vestiges of 

 some of the nerves largely developed in fishes. 



As the optic and olfactory nerves dwindle or disappear when their related organs are 

 in a vestigial condition or absent, so some of the ancestral ganglia and nerves have 

 disappeared as their organs (only useful for an aquatic life) have degenerated. Prepared 

 for the complete absence of various nerves the work has been simplified, and time saved 

 which might have been wasted in making impossible comparisons. Now that the whole 

 ground has been roughly worked over, I propose, first, to describe the lateral sense 

 organs in several fishes, paying special regard to their innervation. This done, I shall 

 describe fully the cranial nerves in two or more members of the Elasmobranch group ; 

 and, finally, complete the work by making a comparative study of the cranial nerves of 

 the more important divisions of the vertebrata. 



THE LATERAL SENSE ORGANS. 

 I. Historical. 



In Elasmobranchs, the lateral sense organs consist of two distinct systems of canals, 

 and, in addition, of minute pit organs or follicles. Hitherto, the two kinds of canals 

 have usually been known as mucous canals ; but as they differ in structure and arrange- 

 ment, and, perhaps, also in function, distinctive names are obviously necessary. The 

 canals of the one system open on the surface of the skin by numerous usually short and 

 simple tubules ; and they further, in some cases, give off long branches, also provided 

 with tubules. The canals of the other system radiate from a given number of centres in 

 the head region ; and each canal presents an expansion (ampulla) at its proximal end, 

 and opens on the surface of the skin by its distal end. These radiating canals, however, 

 though often running a considerable distance side by side, never communicate with each 

 other ; nor do they give off tubules or branches. 



The canals with tubules, which include the canal of the lateral line and a number of 

 canals in the head region, I have, for various reasons, decided to speak of as the Sensory 

 Canals. They are not only characterised by their tubules and branches, but also, and very 

 specially, by the presence of numerous sense organs — structures which are present when 

 the tubules are absent, and when grooves or furrows take the place of the canals. The 

 canals without tubules and branches I shall invariably refer to as Ampullary Canals, 



