THE DUCTLESS GLANDS 1179 



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 secre- 

 tion). 



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 

 food- stuffs 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 there excites the act of 

 excretion. All tissues produce carbon dioxide, which passes to the lungs 

 to be excreted, 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 eliminate this waste 

 product. The liver receives ammonia from the alimentary canal and con- 

 verts 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 food-stuff 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 converting 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 oppaw, 

 6 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 



