314 THE AMERICAN NATURALIST. [Vol. XXXVII. 



the anatomical findings which have laid the foundation historic- 

 ally for the physiological differentiation of these sense organs. 



In the skin of fishes there are three types of sensory nerve 

 endings belonging to the cerebro-spinal system, aside from 

 sympathetic nerves of uncertain physiological significance. ( i ) 

 there are free endings everywhere in the skin, but especially on 

 the lips, barblets and exposed surfaces generally. (2) Sense 

 organs in canals or pits obviously belonging to the lateral line 

 system and termed canal organs and pit organs, or collectively 

 nerve hillocks or neuromasts. (3) Sense organs resembling 

 taste buds of the mouth, lying freely exposed on the surface of 

 the skin, never sunk below the surface, and variously termed 

 flask-shaped organs, end-buds, terminal buds, etc. 



The nerves which end free, those of the first type, usually 

 lose their medullary sheaths some distance below the skin and 

 have sometimes been overlooked. Merkel, for instance, in his 

 great monograph denies the presence of these endings in the 

 skin of fishes. Recent students of nerve components have 

 shown that the nerves of this type are anatomically distinct from 

 those for both the other types of sense organs for their entire 

 extent, in all cases being provided with separate ganglia and 

 entering the brain by distinct roots. In the trunk region these 

 " general cutaneous" nerves make up the greater part of the 

 dorsal spinal roots and terminate in the dorsal horns; in the 

 head they enter by the V, X and sometimes by the IX pairs of 

 cranial nerves and all terminate in the funicular nucleus or the 

 gray matter associated with the spinal V tract, all morphological 

 equivalents of the dorsal horns of the spinal cord. Morpholog- 

 ically the general cutaneous system of nerves is a well defined 

 unit. Physiologically its function is unquestionably in the main 

 tactile. Practically all parts of the body are sensitive to touch 

 and are reached by these nerves, whether they are supplied by 

 other types of sensory nerves or not. The more acute the tac- 

 tile sensibility, the more rich the innervation by fibers of this 



The differentiation of the sense organs of the second and third 

 types has proven very difficult, for the reason that rows of 

 undoubted lateral line organs which are certainly homologous 



