THE DUCTLESS GLANDS 1319 



stimulus, of a specific chemical substance, which passes into the cir- 

 culating blood to B, where in its turn it will excite the required state 

 of action. Such chemical messengers are designated hormones, from 

 opftaw, ' I excite.' We have already met with several examples 

 of such bodies. It may be interesting here to consider what must be 

 their general character if they are to fulfil the part of chemical 

 messengers. 



(1) In the first place, they must not be antigens, i.e. their injection 

 into the blood-stream must not evoke the production of an anti-body, 

 If this were the case, the hormone, on entering the blood-stream, 

 would meet its anti-body and would be unable to exert any effect 

 on the appropriate reacting organ. Practically all the complex colloid 

 bodies allied to the proteins, e.g. ferments, egg albumin, peptone, sera 

 of different animals, when injected into the blood-stream, cause the 

 production of the corresponding anti-body. The hormones must be 

 simpler in character than such substances and probably have a precise 

 and comparatively simple chemical or molecular constitution. 



(2) Since they must be carried by the blood-stream to the reacting 

 organ they must in most cases be susceptible to easy passage through 

 the walls of the blood-vessels if they are to excite a reaction within a 

 fairly short space of time. This consideration would also tend to 

 keep their molecular weight comparatively low. 



(3) As a rule the chemical messenger must excite a state of activity 

 in response to a change in some other part of the body. When the 

 primary change passes away, the action of the hormone should also 

 disappear. On this account it is necessary that the hormone should 

 either be susceptible of easy destruction, by oxidation or otherwise, 

 in the fluids of the body, or be readily excreted, so that its action may 

 not be continued indefinitely. 



In previous chapters we have already come across several examples 

 of correlated activities of different tissues effected by chemical means. 

 It is perhaps questionable whether we should regard carbon dioxide, or 

 the lactic acid produced by a contracting muscle, as a hormone in the 

 strict sense of the term, since both these products are produced in 

 large quantities as the final product of oxidation or disintegration of 

 the food-stuffs. Carbon dioxide is, however, rapidly eliminated from 

 the body, and lactic acid is equally rapidly oxidised in the body, and 

 there is no doubt that the activity of the respiratory centre is deter- 

 mined by the presence of this substance in the blood and is thereby 

 perfectly co-ordinated with the activities of the whole of the rest of 

 the organism. In the alimentary canal the secretion of pancreatic 

 juice at the precise moment when it is required in the duodenum for the 

 digestion of the food arriving there from the stomach is evoked by the 

 production in the cells of the intestinal mucous membrane under the 



