THE DUCTLESS GLANDS 1231 



correlation of activities did not however do away with, the necessity for the 

 more primitive method. Even in the higher animals, where rapidity of 

 reaction is not required, we find adaptations carried out in response to some 

 change in distant parts of the body, the message having been chemical 

 and not nervous in character (e.g. the secretin mechanism for pancreatic 

 secretion). 



When we speak of the chemical correlation of the activities of the different 

 parts of the body, it is important not to confuse processes which have little 

 or nothing in common. In one sense we may say that every cell in the body 

 is chemically connected with and dependent on all the other cells in the body. 

 This interdependence is a necessary consequence of the differentiation of 

 function associated with increased complexity of the organism. Thus the 

 foodstuffs are digested and absorbed by the cells lining the alimentary 

 canal and are then transmitted, more or less changed by these cells, to all the 

 other tissues of the body. The liver stores up glycogen and is ready to give 

 of its store to any tissue in need of carbohydrate. All the tissues probably 

 produce urea, which passes to the kidneys and is there excreted. All tissues 

 produce carbon dioxide, which passes to the lungs to be eliminated, but as it 

 traverses the respiratory centre it arouses respiratory movements, which are 

 exactly proportioned to the tension of the carbon dioxide and therefore to 

 the need of the whole body to get rid of this waste product. The liver 

 receives ammonia from the alimentary canal and converts it into urea, thus 

 shielding all the other tissues from the poisonous effects which would be 

 produced by the entrance of the ammonia into the general circulation. Thus 

 one organ may receive and modify any substance or foodstuff so as to prepare 

 it for more ready assimilation by other tissues. It may shield these other 

 tissues from the poisonous effects of certain waste products, either by con- 

 verting these into harmless substances or by excreting them from the body. 

 In all these cases the tissues are dealing with some substance which is utilised 

 in bulk or which, by its accumulation, could exert a toxic influence on other 

 tissues. We are probably justified in treating apart a group of phenomena 

 in which the substance transmitted from one part of the organism to another 

 is significant almost entirely as an excitatory agent, and has little or no 

 value as a source of energy. When the adaptation to a change of A consists 

 in the activity of an organ B, the activity of B can be evoked either by a nerve 

 impulse passing from A to the central nervous system and from this to B, 

 or by the production at A, as a direct consequence of the stimulus, of a specific 

 chemical substance, which passes into the circulating blood to B, where in its 

 turn it will excite the required state of action. Such chemical messengers 

 are designated hormones, from d^aco, ' 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 evolve the production of an anti-body. 

 If this were the case, the hormone, on entering the blood stream, would meet 



