28 CHEMICAL CONSTITUENTS OF BODY AND FOOD. 
usual method which a chemist follows in attempting to unravel the 
constitution of any substance, is first to discover the way in which it decom- 
poses (analysis), and then to build up the original material once more from 
the simple compounds soobtained (synthesis). In the case of the proteids 
there have been many observations on the analytical side, but synthesis 
lias not yet been successful. We will first consider the results of 
analysis, next the attempts at synthesis, and finally state some of the 
theories founded on these observations. 
The decomposition products of proteids. — The experiments which 
have been performed fall into two 
main groups : the first, designed with 
a view to determine the series of 
changes a proteid undergoes in its 
passage through the body ; the second, 
with the object of investigating the 
chemical substances obtained as 
cleavage products by artificial means 
in the laboratory. In the first group 
the progress which has been made 
is slight, great and obvious difficulties 
being encountered at nearly every 
step ; the end products, carbonic 
anhydride, water, urea, uric acid, 
ammonia, etc., are known, but the 
intermediate substances, resulting 
from metabolic changes within the 
cells and tissues, are still in the 
region of conjecture. 
In the alimentary canal itself there are, however, changes which are 
within the grasp of the investi- 
gator, and the proteoses, al- 
buminates, and peptones there 
formed will be treated under 
the head of " Digestion." Here, 
too, under the prolonged action 
of the pancreatic juice, simpler 
nitrogenous substances, such 
as leucine, tyrosine, aspartic 
acid, and ammonia, arc formed 
in small quantities. Leucine 
(C 6 H 13 N 2 0) is empirically 
amido-caproic acid, but of the 
numerous possible isomerides 
which could be included under 
that name, leucine has 
been shown to be a-amido- 
isobutylacetic acid, (CH 3 ).,CH. 
CH 2 CH 2 ( NH 2 ) COOK "The 
leucine obtained on pancreatic 
digestion is dextrorotatory. 
Levorotatory and optically in- 
active varieties of leucine exist, 
and some of them have been 
Fig. 7.— Leucine crystals. — After Ktiline. 
Fig. S. — Tyrosine crystals. — After Frey. 
