PRESIDENTIAL ADDRESS; 86o 



hundred and twenty years have enabled us to assign ultimate physical and chemical 

 sources to the energy and material leaving the body in various forms. We can 

 assign to such sources the energy of animal heat, muscular work, glandular, nervous, 

 and other activity : also the carbon dioxide, urea, salts, and many other substances 

 which leave the body or are formed within it. All of this new knowledge may 

 be regarded as progress towards a physico-chemical explanation of life. 



But there is another aspect to be considered ; for side by side with what I have 

 just referred to there has been a different kind of increase of knowledge with 

 regard to animal metabolism. This growth of knowledge relates to the manner 

 in which the passage of energy and material through the body is regulated in 

 accordance with what is required for the maintenance of the normal structure 

 and activities of the body. In Liebig's time, for instance, it was believed that 

 the rate of respiratory exchange was regulated simply by the supply to the body 

 of oxygen and food-material. If one breathed faster, or if the barometric pressure 

 or percentage of oxygen in the air increased, the respiratory exchange was 

 assumed to be also increased, just as ordinary combustion outside the body would 

 be increased by an increased supply of oxygen. If, again, one took in more food 

 it was supposed that the excess went to increase the rate of combustion in the 

 blood {lu.vtis co)isinnptio7i), just as a fire is increased when more fuel is supplied. 

 We now know that these assumptions were wholly mistaken, and that the re- 

 spiratory movement?, respiratory exchange, and corresponding consumption of 

 food material in the body are regulated with astounding exactitude in accordance 

 with bodily requirements. If, for instance, the body consumes more proteid, it 

 economises a quantity of fat or carbohydrate equivalent in energy value to the 

 proteid ; and from day to day the amount of energy liberated in the body is very 

 steady. With regard to the excretion of material by the kidneys a similar growth 

 in knowledge can be traced. It is scarcely a century since the urine was re- 

 garded as equivalent more or less to the liquid part of the blood separated from 

 the corpuscles, which were unable to pass through the very tine capillary tubules 

 supposed to exist in the kidney substance. Gradually, however, we have learnt 

 how extraordinarily delicate is the selective action which occurs in the kidney 

 substance, and how efficiently this selective action maintains the normal com- 

 position of the blood. Scarcely a remnant is now left of the old filtration 

 theories. Our ideas of tissue nutrition and growth have undergone a similar 

 change ; and it is hard to realise that only about seventy years ago b'chwann 

 could put forward the theory that cell formation and growth is a process of 

 cry.stallisation. 



One can multiply instances like these almost indefinitely : but I have, per- 

 haps, said enough to show that if in some ways the advance of Physiology seems 

 to have taken us nearer to a physico-chemical explanation of lile,"in other wa^s 

 it seems to have taken us further away. On the one hand we have accumulating 

 knowledge as to the physical and chemical sources and the ultimate destiny of 

 the material and energy passing through the body : on the other hand an equally 

 rapidly accumulating knowledge of an apparent teleological ordering of this 

 material and energy; and for this ttlcological ordering we are at a loss for 

 physico-chemical explanations. There was a time, about fifty years ago, when 

 the rising generation of physiologists in their enthusiasm for the first kind of 

 knowledge closed their eyes to the second. That time is past, and we must once 

 more face the old problem of life. 



Let us first look at the answer given to this problem by many of the older 

 physiologists. Koughly speaking they carried physical and chemical explanation 

 of physiological processes as far as they could, and for the rest assumed that at 

 some point or other the physical and chemical factors are interfered with and 

 ordered in a teleological direction by something peculiar to living organisms — the 

 ' vital principle' or ' vital force.' This theory, if one can call it a theory, had 

 the negative merit that it did not lead physiologists to ignore facts which they 

 could not explain. But in practice the 'vital force' became simply a convenient 

 resting-place for these facts. It was assumed that the vital force' could do anv- 

 thing and everything, and that it acts ' from the blue 'on physical and chemical 

 1908. 3 K 



