8 9 4 METABOLISM. 



second place, the methods of synthesis in animal (and vegetable) 

 organisms are still an unsolved problem, in spite of the fact that 

 it is the rapid progress in our knowledge of the syntheses of organic 

 combinations which constitutes the greatest triumph of modern 

 chemistry. Chemists are already able artificially to build up atom for 

 atom out of their elements a series of organic compounds, some of a very 

 complicated nature. We no longer doubt that all the rest, even the 

 most complex, will be thus produced. Nevertheless the processes 

 employed in no way represent the synthetic processes of the living cell, 

 for all artificial syntheses can only be achieved by the application of 

 forces and agents which can never play a part in vital processes, such as 

 extreme pressure, high temperature, concentrated mineral acids, and free 

 chlorine agents which are immediately fatal to any living cell." 



It must nevertheless be admitted, in spite of the numerous instances 

 of syntheses of organic compounds which have accumulated of late years, 

 that, so far as the formation of bioplasm is concerned, the only material 

 from which the animal organism is capable of forming it is proteid, and 

 this proteid must be present as such in the food. No doubt the ultimate 

 change of the circulatory or blood proteids to the proteid of bioplasm must 

 depend upon a special synthesis, but we are necessarily completely ignor- 

 ant as to the manner in which such synthesis occurs, since we are ignorant 

 of the actual chemical constitution of both living tissue and dead proteid. 



With respect to the breaking-down of the bodystuffs in the process 

 of metabolism, there are reasons for believing that this consists of two 

 phases, namely, a splitting of the complex molecules into simpler mole- 

 cules, and an oxidation of some or all of the simpler substances thus 

 arising. It is probable that in the metabolism of proteid these two 

 phases usually, if not invariably, occur at different times, and even in 

 different places in the body ; for example, the materials derived from 

 the splitting up of the metabolised proteids of muscle do not all leave 

 the muscle in a fully oxidated condition, but are, in part at least, in 

 the form of oxidisable substances, such as lactic acid. Doubtless, in 

 the formation of the ultimate products, oxidation is the prominent 

 feature, for these products, in the form in which they leave the body, are, 

 as compared with the materials that enter the tissues, unquestionably 

 in a condition of oxidation, in some cases of complete oxidation. There 

 is, however, no distinct evidence that the process of splitting of the 

 complex molecules is necessarily immediately combined with that of 

 oxidation. On the other hand, there is reason to think that such 

 splitting may occur without immediate oxidation ; for example, the 

 splitting of proteids, which are taken in the food, into urea and non- 

 nitrogenous substances. For, in a dog fed with proteid, the urea was 

 found by Feder to make its appearance in the urine within fourteen 

 hours after feeding, whereas the removal of the remainder of the proteid 

 molecule in the form of carbon dioxide and water did not occur for 

 twenty-four hours after, so that the splitting of the proteid molecule 

 must have occurred at one time, and its complete oxidation at another. 1 



It is found that any conditions which tend to diminish the normal 

 oxidations of the body generally, or of the individual tissues (such as 

 the ingestion of prussic acid or the cutting off or diminution of the 

 arterial supply to an organ), cause such substances as lactic acid and 

 dextrose, which are probably products of proteid and carbohydrate 



1 C. Voit, Ztschr.f. SioL, Miinchen, 1891-2, Bd. xxviii. S. 292. 



