658- TRANSACTIONS OF SECTION 1. 
remains, I find, pretty widely spread, the feeling—due to earlier biological teaching 
—that, apart from substances which are obviously excreta, all the simpler products 
which can be found in cells or tissues are as a class mere dejecta, already too remote 
from the fundamental biochemical events to have much significance. So far 
from this being the case, recent progress points in the clearest way to the fact 
that the molecules with which a most important and significant part of the 
chemical dynamics of living tissues is concerned are of a comparatively simple 
character. The synthetic reactions which we have already considered surely 
prepare us for this view; but it may be felt that, however important, they 
represent abnormal events, while the study of them has been largely confined 
to determining the end-products of change. Let me now turn to normal 
metabolic processes and to intermediary reactions. 
We know first of all that the raw material of metabolism is so prepared as 
to secure that it shall be in the form of substances of small molecular weight; 
that the chief significance of digestion, indeed, lies in the fact that it protects 
the body from complexes foreign to itself. Abderhalden has ably summarised 
the evidence for this and has shown us also that, so far as the known con- 
stituents of our dietaries are concerned, the body is able to maintain itself when 
these are supplied to it wholly broken down into simple Bausteine, any one of 
which could be artificially synthesised with the aid of our present knowledge. 
Dealing especially with the proteins, we have good reason to believe that the 
individual constituent amino-acids, and not elaborate complexes of these, leave 
the digestive tract, while Folin, Van Slyke, and Abel have recently supplied 
us with suggestive evidence for the fact that the individual amino-acids reach 
the tissues as such and there undergo change. But still more important, when 
things are viewed from my present standpoint, is the fact that recent work 
gives clear promise that we shall ultimately be able to follow, on definite 
chemical Jines, the fate in metabolism of each amino-acid individually; to trace 
each phase in the series of reactions which are concerned in the gradual break- 
down and oxidation of its molecule. Apart from the success to which it has 
already attained, the mere fact that the effort to do this has been made is 
significant. To those at least who are familiar with the average physiological 
thought of thirty years ago, it will appear significant enough. So long as there 
were any remains of the instinctive belief that the carbonic acid and urea 
which leave the body originate from oxidations occurring wholly in the 
vague complex of protoplasm, or at least that any intermediate products 
between the complex and the final excreta could only be looked for in the 
few substances that accumulate in considerable amount in the _ tissues 
(for instance, the creatin of muscle), the idea of seriously trying to trace within 
the body a series of processes which begin with such simple substances 
as tryosin or leucin was as foreign to thought as was any conception that 
such processes could be of fundamental importance in metabolism. How- 
ever vaguely held, such beliefs lasted long after there was justification for 
them; their belated survival was due, it seems to me, to a _ certain 
laziness exhibited by physiological thought when it trenched on matters 
chemical; they disappeared only when those accustomed to think in terms of 
molecular structure turned their attention to the subject. But it should be 
clearly understood that the progress made in these matters could only have come 
through the work and thought of those who combined with chemical knowledge 
trained instinct and feeling for biological possibilities. Our present knowledge 
of the fate of amino-acids, as of that of other substances in the body, has only 
been arrived at by the combination of many ingenious methods of study. It is 
easy in the animal, as in the laboratory, to determine the end-products of 
change; but, when the end result is reached in stages, it is by no means easy to 
determine what are the stages, since the intermediate products may elude us. 
And yet the whole significance of the processes concerned is to be sought in the 
succession of these stages. In animal experiments directed to the end under 
consideration, investigators have relied first of all upon the fact that the body, 
though the seat of a myriad reactions and capable perhaps of learning, to a 
limited extent and under stress of circumstances, new chemical accomplishments, 
is in general able to deal only with what is customary to it. This circumstance 
has yielded two methods of determining the nature of intermediate products in 
metabolism. Considerations of molecular structure will, for instance, suggest 
