28o rxixcirLEs OF GENERAL PHYSIOLOGY 



ANALYSIS OF METABOLIC PROl'KssKS 



The ivailt-r will have noticed that many of the reactions described as stages 

 in the metabolism of proteins, fats, or curbc (hydrates rest upon evidence derived 

 either from perfusion of isolated organs or from experiments with extracts of 

 tissues in rifro. Some investigators appear to doubt whether legitimate con- 

 clusions, as regards the processes taking place in the organism as a whole, 

 can be drawn from experiments of this kind. 



It does not seem to me that sucli criticism is justified. Investigation of the 

 excreta compared with the ingesta, valuable as the information is for certain 

 purposes, gives us very little knowledge of the chemical reactions by which the 

 latter are converted into the former. The comparison of the organism to a town, 

 where various occupations are carried on, is often made. If we notice that a large 

 quantity of milk goes into the town and that a corresponding amount of cheese 

 comes out, we conclude that the milk has been used to make the cheese, but we 

 learn nothing about the method employed. Still less is learned by such methods 

 with regard to the more intellectual occupations, such as that of the poet or 

 musician. Hopkins has made use of the simile of a conjuror, who puts a loaf 

 into a hat and takes out a rabbit. What we want to know about are the 

 intermediate stages between the loaf and the rabbit. 



Looking at the question from another point of view and taking, for example, 

 the oxidation of glucose, it is surely permissible to consider what are the possible 

 chemical changes that might take place. Suppose then that we find certain of the 

 possible reactions to be brought about by extracts of tissues, while others, also 

 chemically possible, are not; we are, it seems to me, justified in stating that the 

 former is the way in which the organism works, at all events until the contrary 

 has been actually proved. 



Again, as to perfusiou experiments : when pyruvic acid is added to blood 

 passing through the liver and alanine is found in the issuing blood, it cannot be 

 denied that, even in the whole organism, if pyruvic acid be present in the portal 

 blood, alanine will be found in the blood of the hepatic vein. If it be objected 

 that alanine might come from the substance of the cells themselves, it may be 

 pointed out that, when the related a-ketonic acid of butyric acid is perfused, u- 

 amino-butyric acid is formed. It seems extremely improbable that the permeability 

 of the cells should be affected by closely related ketonic acids in such a way that 

 the corresponding amino-acid, and this only, is washed out of them. 



Since the reactions under discussion are reversible, take for example that 

 between lactic acid and glucose, it is scarcely credible that lactic acid should wash 

 out or cause the cells to give up glucose, while glucose causes them to lose 

 lactic acid. 



STORAGE 



The organism is capable of storing, in some form or other, the chief classes of 

 food-stuffs, although in different degrees. 



Carbohydrates. These are stored, mainly in the form of glycogen, in the liver 

 and muscular tissues for the most part. Glycogen, being an insoluble substance, 

 is kept out of the risk of undesirable participation in chemical reactions ; on the 

 other hand, while soluble carbohydrate is required, the action of the enzyme, 

 amylase, converts it into maltose or glucose. In the plant, starch is the iimst 

 common form of stored carbohydrate, but saccharose is very frequently met with. 

 Soluble carbohydrates do not appear to be stored in the animal, at all events in 

 more than very small amounts. 



Fat is stored in practically all cells in larger or smaller quantity, frequently 

 in the form of lecithin, or related substances. The main store of neutral fat is 

 in the subcutaneous connective tissue. In the plant, neutral fats are found in 

 some fruits and practically only in fruits. 



Proteins. It is impossible to name any particular form in which nitrogen 

 is stored. Protoplasm in general is capable of increase and, in starvation, as we 

 have seen, the less important tissues give up amino-acids for the benefit of the 

 more vital organs, thanks to the presence of autolytic enzymes. Whether there 



