894 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 syn theses 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 bodystufls 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. Z/schr.f. Biol., Mimelien, 1891-2, Bd. xxviii. S. 292. 
