ii8 BIOLOGICAL CHEMISTRY 



reducing power of the solution and the rotation of polarised 

 light are, however, more accurate.* 



Process of Secretion. The production of saliva by the 

 salivary glands is an example of secretion which is well worth 

 studying. There has been much discussion as to whether 

 secretion can be explained on purely physical grounds. In 

 one sense it is quite true that all processes can be explained on 

 chemical and physical grounds. The Law of Conservation of 

 Energy holds in biological processes, hence there must be a 

 balance between the income and output of energy. On the 

 other hand, a mechanical explanation of the phenomena is 

 not forthcoming. 



The production of saliva cannot be due to filtration because 

 if the duct of a salivary gland is attached to a canula and the 

 canula connected with a manometer, on stimulation of the 

 nerve to the gland the pressure in the duct rises higher than 

 the blood pressure. Nor can the secretion be due to osmosis 

 because the concentration of the saliva is less than that of the 

 blood. Hence the liquid should pass back from the saliva 

 to the blood. Even if the concentration of the secretion, as 

 happens in the case of other glands, were greater than the con- 

 centration of the blood we must still enquire how the more 

 concentrated solution was separated from the blood. Further 

 differences in surface tension do not give any plausible 

 explanation of the formation of saliva. 



In any case work is done and at the same time increased 

 oxidation occurs. It is obviously futile to discuss whether 

 this is a " vital " phenomenon or a physico-chemical process. 

 What we have to find out is the manner in which the energy 

 derived from oxidation is converted into the work of secretion. 

 When we know this the problem is settled without further 

 discussion. At the present time, the best way to deal with 

 the process of secretion is to look upon it as a sort of pumping 

 action by the cell ; the energy for the performance of work is 

 produced by the oxidation that occurs in the cells. 



In this discussion we have not touched upon the difference 

 in the chemical composition of the secretion from that of the 

 blood. In some cases the gland separates some substance 

 which is present in the blood (urea by the kidneys), and in 

 others the gland manufactures the material of the secretion 

 (lactose in mammary gland). Even in the former case the 

 concentration is increased so work must be done against the 

 osmotic pressure. Similar considerations apply to the secre- 



* C. L. Evans, Journ. PhysioL, 1912, vol. 44, p. 220. 



