THE TASTE OF ACIDS 455 



by the reactions of ^olosoma, a freshwater oligochaet (Kribs, 

 '10). 



It adds somewhat to the clearness of the discussion if, as I 

 have previously proposed (Crozier, '14, p. 16), we restrict the 

 word 'stimulus' to mean the change induced in a receptor by 

 the action of a stimulating agent. The explanation of the acid 

 taste is made easier by the fact that one is not called upon to 

 account for a sour taste resulting from heterologous stimula- 

 tion; there is no good evidence for the existence of a sour taste 

 not directly produced by acid. The extreme specialization 

 of the acid taste makes it a very favorable case for analysis. 



III. It is sufficiently obvious that only the surface of the 

 receptor is immediately concerned in stimulation. This view 

 is reasonable upon purely morphological grounds, such as the 

 modifications of the exposed ends of sensory cells- and the rela- 

 tions of nerve fibrils to the surface of secondary sense cells. 

 The way in which this conception of the cell surface as the organ 

 of irritability has been elaborated by R, S. Lillie and others 

 need not be discussed here. But it may be pointed out that the 

 method of interpreting mechanical excitation has been indicated 

 by Osterhout ('15) and by such observations as that of Evans 

 and Winternitz (Evans and Schulemann, '14, p. 453). Un- 

 equivocal evidence in this direction is afforded by Harvey's 

 work on the penetration of cells by alkalies, (Harvey, '14 c), 

 by his experiments with acids (Harvey, '14 c), and by my own 

 stud}' of this latter subject (Crozier, '15, '16); the evidence 

 referred to is, in brief, to the effect that penetration of the body 

 of the cell substance is altogether too slow a process. to account 

 for the rapidity of taste stinuilation. The slowness of acid 

 penetration is tlue to the resistance offered by the cell surface, 

 a resistance which practically disappears with the death of the 

 cell. An examination of cell penetration by various acids, 

 employing a wide range of dilutions, should therefore yield 

 valuable information regarding the composition and behavior 

 of the resistant cell surface. In several cases a study of this 



2 The propable chemoreceptors of the earthworm have been described by 

 Miss Langdon ('95) and Bovard ('04). 



THE JOURNAL OF COMPARATIVE NEIROLOGY, VOL. 26, NO. 4 



