448 DISEASE IN WILD MAMMALS AND BIRDS 
contain several proteins or groups of proteins, whose 
biological adequacy depends upon their yield of amino- 
acid. Experiment has shown that many proteins are 
entirely lacking in one or more of these essential radicles ; 
and no food can be adequate unless it contains at least 
all the aminoacids that the individual animal is unable to 
manufacture for itself. So far as is known, no animal can 
produce in itself either lysin or tryptophane. Gliadin, the 
principal protein of wheat and lacking in lysin, is unable 
to support growth even when given in amounts sufficient 
to insure the storage of nitrogen, and is associated with a 
diet adequate in all other factors. Absence of tryptophane 
prevents not only growth but maintenance. Any of the 
aminoacids, whose radicles are contained in tissue pro- 
teins, may contribute to the maintenance of adult 
equilibrium ; but no growth occurs unless all the necessary 
groups are present. Except in laboratories, diets are 
never made up of isolated proteins, but they are often 
composed of proteins derived from one plant and are 
often deficient. McCollum and his associates in their 
studies showed that while there were pronounced differ- 
ences in the composition of many foods used by men and 
animals not only in their protein content but in water, 
fats, carbohydrates, etc., yet in the combinations found 
even in rather restricted diets, the errors, as a rule, cor- 
rected each other. 
During digestion the protein molecule is broken down 
into the component aminoacids which are absorbed and 
sjTLthesized in the intestinal walls, and appear on the 
other side as the specific blood protein, which serves as 
the substrate for the anabolism of all the special tissue 
proteins. Excessive protein is stored to a slight extent as 
aminoacid for the future maintenance of the blood pro- 
tein, the integrity of which is tenaciously protected during 
hibernations, sexual migrations, and even during starva- 
tion. The animal body tends to adjust its nitrogen 
