CARBOHYDRATES, FATS AND PROTEINS 105 



The experimental procedures are as follows : 



(a) Various chemical substances, which form compounds 

 with some of the possible intermediate products, are ad- 

 ministered. The recovery of compounds of the intermediate 

 products with the substance administered does not prove 

 that the former is a normal stage in metabolism, but if the 

 compound is not formed the evidence fs plainly against there 

 being any such intermediate product. 



In order to favour the excretion of the compound of the 

 intermediate product large doses of the original substance 

 may be given along with the substance that forms the 

 compound with the intermediate product. 



(b) The supposed intermediate product may be administered 

 to an animal and the resulting products isolated. If the same 

 end products are produced that are yielded by the original 

 substance the substance administered may be an intermediate 

 stage, but if the same end products are not produced the 

 substance administered is probably not an intermediate stage 

 in metabolism. The only reservation is that substances 

 administered in large doses cannot be so conveniently dealt 

 with as when they are slowly produced in the cells. For 

 instance, the intermediate substance may be poisonous in 

 large doses or its metabolism may lead to some other inter- 

 mediate substance which cannot be further altered with 

 sufficient rapidity. 



In other cases the supposed intermediate substance may 

 be combined with some substance resistant to oxidation and 

 the fate of this compound investigated. 



(c) Surviving organs may be perfused with blood or saline 

 solution containing certain substances, and the liquid, after 

 perfusion, examined for changes in composition. Allied to 

 this method is the preparation of extracts and testing these 

 extracts on various substances to find out if any enzymes 

 are present. This method is especially useful in tracing the 

 details of metabolism and for allocating the various steps in 

 metabolism to the different organs. 



(d) Pathological conditions sometimes lead to the excretion 

 of unusual substances, and these conditions may be useful in 

 furnishing evidence as to the intermediate stages in metabolism. 



When we turn to the results obtained we find that much 

 more is known about the fate of fatty acids than about the 

 fate of carbohydrates. The reason for this difference is that 

 the products are easier to separate and identify in the former 

 than in the latter case. 



