558 SCIENCE PROGRESS 



to all the other cells of the body. It becomes possible, there- 

 fore, for different parts of the body to carry out a common 

 action, the regulating nexus being provided by chemical 

 substances produced in the metabolism of one of the co-operating 

 parties, and carried by the circulating fluid all over the body. 

 The conception that among the constituents of the internal 

 nutrient fluid of organisms are certain substances which 

 function, not as foods in the ordinary sense of the term, but as 

 excitatory substances {Reizstoffe), is one that has long been 

 familiar to botanists, though even here we find an ambiguity 

 of defining line between those bodies which are necessary, even 

 in minimal quantities, for the building up of the framework of 

 the cells, and those whose part it is to modify the functions 

 of the already formed protoplasm. Foodstuffs are valuable in 

 proportion as they furnish energy to the organism or material 

 for its construction and growth. These " Reizstoffe" are, so far 

 as we can tell, non-assimilable, and yield no appreciable amount 

 of energy. It is their dynamic effects on the living cell which 

 are of importance. In this respect they present a close analogy 

 to the substances which form the ordinary drugs of our 

 pharmacopoeias. Since in the normal functioning of the body 

 they have to be discharged at frequent intervals into the blood- 

 stream, and carried onward by this to the organ on which they 

 exercise their specific effect, they cannot belong to that class of 

 complex bodies, which include the toxins, of animal or vegetable 

 origin. These, which are supposed by Ehrlich to ape the part 

 of a foodstuff and so be built up into the living framework of 

 the cell itself, give rise, probably in consequence of this self-same 

 property, when injected into the blood-stream, to the formation 

 of antibodies. The formation of such bodies, in the case of the 

 chemical agents of correlation, would annul their physiological 

 effect. We must therefore conceive the latter as substances, 

 produced often in the normal metabolism of certain cells, of 

 definite chemical composition, and comparable in their chemical 

 nature and mode of action to drugs of specific action, such as 

 the alkaloids. This conclusion is borne out by the few in- 

 vestigations which have been made as to the nature of the 

 chemical messengers in the case of certain well-marked corre- 

 lations of function in the higher animals. In consequence of the 

 distinctive features of this class of bodies, and the important 

 functions played by them in the higher organisms, I have 



