RESPIRATION 387 



The mechanistic interpretation of life fails to take account of 

 the mutual dependence throughout a living organism of these 

 reactions. When we remove any part of the organism from its 

 physiological connection with its environment including the other 

 parts, we at the same time necessarily alter its reactions and the 

 stability of its living structure. Hence we cannot investigate an 

 organism as we investigate the parts of a machine by taking them 

 apart and ascertaining the properties and structure of each sepa- 

 rate part. The same criticism applies to what may be called the 

 ''hormone" theory of the interconnection between the parts of an 

 organism. On this theory the interconnection is brought about 

 through the existence of special chemical messengers, or "hor- 

 mones," produced in minute quantities by each organ, and bring- 

 ing about specific excitatory effects, resulting in coordinated 

 action. The hormone theory, like the mechanistic theory, tacitly 

 assumes that, apart from the influence of hormones, and of the 

 central nervous system, each part of an organism leads an inde- 

 pendent existence. The truth is that every substance which enters 

 into the life processes of any part of an organism is as much a 

 hormone as any other such substance. Water, for instance, is the 

 most abundant constituent of the body, and a very minute excess 

 in the diffusion pressure of water in the blood excites very striking 

 reaction in the kidneys. This minute excess seems, therefore, to 

 act as a hormone, just as a minute deficiency in alkalinity or in 

 oxygen pressure acts as a hormone to the respiratory center. 

 Since, however, water, hydrogen and hydroxyl ions, and oxygen 

 are influencing the body continuously, the conception of them as 

 hormones, acting only occasionally, is quite misleading. The 

 physiological interconnection between different parts of the body 

 is continuously in existence and far more intimate than is assumed 

 by either the ordinary mechanistic theory or the hormone theory. 



In the case of chemical compounds which we ordinarily regard 

 as being stable in their existing environment, and not in a constant 

 state of association and dissociation, it is well known that the 

 particular nature of one of the atomic linkings may make a great 

 difference to the others. Thus the general properties of an or- 

 ganic compound may be greatly changed when a hydrogen atom 

 is replaced by a chlorine atom or a methyl radicle. We have also 

 seen in Chapter IV how in oxyhaemoglobin the affinity of the 

 haemochromogen part of the molecule for oxygen is affected by 

 changes in environment affecting primarily another part of the 

 molecule. From the point of view of our present chemical knowl- 



