1232 PHYSIOLOGY 



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 mole- 

 cular 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 substances are produced in large quantities as the 

 final product of oxidation or disintegration of the foodstuffs. 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 determined 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 

 agency of the acid of the gastric juice (the specific stimulus) of a substance, 

 secretin. This substance, which is heat stable and diffusible but is easily 

 destroyed by oxidation, is absorbed into the blood and carried to the pan- 

 creas, where it acts as a specific stimulus for the secretory cells. The same 

 substance excites also the secretion of bile by the liver cells and the secretion 

 of intestinal juice by the glands of the small intestine. These are perhaps 

 the two best examples of chemical reflexes, i. e. adaptations effected by 

 chemical means rather than by impulses passing along the nerve channels. 

 There are however many other examples of a chemical influence exerted 

 by one organ on another, in which the interaction probably depends on the 

 production of minute quantities of some substance acting in virtue of its 

 excitatory qualities rather than of its value as a source of energy. For 

 many of the organs of the body we know, in fact, no other function than the 

 production of some substance, the presence of which in the blood is a necessary 



