

NUTRITION ^_ 297 



In muscular work, so long as it is not excessive, no evidence of increased 

 nitrogen excretion, due to wear and tear, or otherwise, is to be obtained. It 

 appears that a complex substance, containing nitrogen and of a high energy 

 content, is broken down to give the energy of the contraction process. The 

 nitrogenous constituent is normally used again for resynthesis of the " inogen," 

 while the carbon and hydrogen are burnt up. 



The chief function of carbohydrate food, as also of fat, is to afford energy; 

 but, in the process of its oxidation, a number of intermediate products are 

 produced, given in the form of a diagram in the text (page 273). These 

 substances are of importance in that they give opportunity for the occurrence 

 of reactions of importance to the organism in other ways. Pyruvic aldehyde, 

 lactic, and pyruvic acids may be especially mentioned. All of these reactions, 

 with the exception of the last stages of oxidation, have been shown to be 

 reversible under conditions obtaining in the living' organism. 



Fat is of additional importance as being readily stored in considerable quantity. 

 It can be formed from the carbohydrates of the food, and the manner in which 

 there is every reason to suppose that the process takes place is, in general terms, 

 as follows. By condensation of an aldehyde with a ketonic acid, we obtain 

 another aldehyde with two more carbon atoms than the original one and, by 

 repetition of the process, with final reduction, fatty acids with straight chains of 

 carbon atoms, increasing by two at a time, are produced. 



The frequent occurrence of pyruvic acid in the processes of metabolism, of 

 carbohydrate, fat and protein, is pointed out. This substance forms, as it were, 

 a meeting place of the three different classes of food-stuffs. 



The value of perfusion experiments and experiments in vitro as extended to 

 processes in the whole organism is discussed in the text. 



Carbohydrates and fats are readily stored in the tissues as glycogen and 

 neutral fats, respectively. There does not seem to be any particular form in which 

 protein is stored, except as tissue or protoplasmic substance, although amino-acids 

 can be adsorbed by tissue colloids. 



The effect of ammonia and of urea in diminishing the nitrogen loss is probably 

 due to a diminution by mass action of the de^amination of amino-acids and of the 

 formation of urea from ammonia. 



The question of optical activity is discussed in the text and the way in which 

 compounds of this kind may have first arisen is described. The preferential use 

 of one optical isomer, at all events for energy purposes, is shown to be merely one 

 of degree, although the cell constituents are finally composed of one set of isomers. 



Results obtained by the growth of tissues in vitro show that proteins can be 

 utilised, or dealt with in some way, by cells themselves, a fact also evident from 

 the using up of cell substance in starvation. The process is to be explained by the 

 presence of autolytic enzymes. Under normal conditions, the proteins of the blood 

 do not serve as nitrogen food to the cells of the tissues. 



The fact that an organ of one animal has not been satisfactorily transplanted 

 into another one, apart from exceptional cases, argues an extraordinary complexity 

 of some kind or other on the part of the protoplasmic systems of the cell. 



There is no evidence that the processes of nutrition in cells are directly 

 influenced by the nervous system ; although the existence of nerve fibres supplying 

 cells forbids a categorical denial of the possibility of such influence. 



Certain processes of growth and metabolism obey definite known mathematical 

 laws. 



It is pointed out that the investigation of functions of the lower organisms is 

 less likely to lead to valuable knowledge than that of the higher organisms. 

 The methods of comparative physiology are of value in enabling us to exclude 

 unessential factors and, in certain cases, allow experiments to be made under 

 conditions in which it would be impossible to preserve the organs of warm-blooded 



