1318 PHYSIOLOGY 



which the consensus partium could be maintained by the rapid propa- 

 gation of molecular changes along differentiated paths in the proto- 

 plasm. The development of this second mode of 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 inter- 

 dependence is a necessary consequence of the differentiation of func- 

 tion 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 move- 

 ments 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 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 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 sub- 

 stance 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 trans- 

 mitted 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 at 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 



