164 PHYSIOLOGY OF NERVE [CH. XIV. 



to increase in its activity ; it is much more difficult to understand 

 how it can possibly produce a lessening of action such as occurs in 

 inhibition. .Nevertheless the "discharge hypothesis," as it used to 

 be called, has been revived of late in modified, form, and electrolytic 

 changes with liberation of ions occurring between the fibrils and the 

 interfibrillar material are supposed to constitute the main feature of 

 the impulse. Macdonald considers that the potassium salts in organic 

 combination within the axis cylinder are the principal materials that 

 undergo the change which is propagated along the nerve; he thus 

 reduces the phenomenon of nervous conduction to electrolytic dis- 

 sociation and association of inorganic ions. The comparatively slow 

 rate at which the change is propagated must, if this is so, be due to 

 admixture or combination of the salt with the less mobile colloid 

 substances of the conducting core. It is interesting to state, if only 

 in outline, the kind of theories which are in the air at present. We 

 must await with patience to see whether they or any of them contain 

 a germ of truth, or whether, like so many theories in the past, they 

 will be forgotten in the future. 



Receptive Substances. 



Langley, as a result of the study of certain poisons on various 

 tissues and organs, has made the interesting suggestion that in all 

 cell-protoplasm two classes of constituents at least are present : (1) a 

 chief substance or substances concerned with the main function of 

 the cell ; and (2) receptive substances which may be acted upon by 

 chemical materials, or in certain cases by nervous stimuli. The 

 receptive substance affects, or can affect, the metabolism of the chief 

 substance. A cell, for instance, can contain a motor receptive sub- 

 stance, or an inhibitory receptive substance, or both, and the effect of 

 a nerve impulse will then depend on the proportion of the jbwo 

 kinds of receptive substance which is affected by the impulse. 



Eeceptive substances are at present entirely hypothetical, and we 

 have no knowledge of their chemical composition. The assumption 

 that they exist does, however, explain certain difficulties, particularly 

 in the action of such drugs as nicotine and curare, which are agents 

 that act on nerve-endings in muscle. If the receptive substances 

 really exist, the drugs mentioned probably act on them and not on 

 the nerve-endings proper. 



In support of the new theory, Dixon has shown that chemical 

 substances are produced in the heart during inhibition which can be 

 dissolved out by alcohol, and then used to produce inhibition in 

 another heart. 



The theory is an attractive one, but is not much more than a 

 theory at present. If, however, a muscle is rendered active by the 



