PRESIDENTIAL ADDRESS. 765 
of a new food, are quite good enough to demonstrate its general properties, 
and its price will then gradually settle itself as the food gets known. : 
Turning to the more strictly scientific aspects of animal nutrition, entirely 
new ideas have arisen during the last twerty years. I propose to discuss these 
shortly, beginning with the proteins. Twenty years ago the generally 
accepted view of the role of proteins in nutrition was that the proteins ingested 
were transformed in the stomach and gut into peptones, and absorbed as such 
without further change. Splitting into crystalline products, such as leucin 
and tyrosin, was thought only to take place when the supply of ingested 
protein exceeded the demand, and peptones remained in the gut for some time 
unabsorbed. It is now generally agreed that ingested protein is normally 
split into crystalline products which are separately absorbed from the gut, 
and built up again into the various proteins required by the animal. If the 
ingested protein does not yield a mixture of crystalline products in the right 
proportions to build up the proteins required, those crystalline products which 
are in excess are further changed and excreted. If the mixture contains none 
of one of the products required by the animal, then life cannot be maintained. 
This has been actually demonstrated in the case of zein, one of the proteins 
of maize, which contains no tryptophane. The addition of a trace of trypto- 
phane to a diet, in which zein was the only protein, markedly increased the. 
survival period of mice. 
The adoption of this view emphasises the importance of a knowledge of 
the composition of the proteins, and especially of a quantitative knowledge of 
their splitting products, and much work is being directed to this subject 
in Germany, in America, and more recently in Cambridge as a result of the 
creation there of an Institute for Research in Animal Nutrition by the Board 
of Agriculture and the Development Commission. This work ‘is expected 
ultimately to provide a scientific basis for the compounding of rations, the 
idea being to combine foods whose proteins are, so to speak, complementary 
to each other, one giving on digestion much of the products of which the 
other gives little. Meantime, it is desirable that information should be collected 
as to mixtures of foods which are particularly successful or the reverse. 
Here the question arises, for what purpose does the animal require a pecu- 
liarly complicated substance like tryptophane? The natural suggestion seems to 
be that the tryptophane grouping is required for the building up of animal 
proteins. It has also been suggested that such substances are required for 
the formation of hormones, the active principles of the internal secretions 
whose importance in the animal economy has received such ample demonstra- 
tion in recent years. The importance of even mere traces of various sub- 
stances in the animal economy is another quite recent conception. Thus it 
has been shown, both in Cambridge and in America, that young animals fail 
to grow on a diet of carefully purified casein, starch, fat, and ash, although 
they will remain alive for long periods. In animals on such a diet, however, 
normal growth is at once started by the addition of a few drops of milk or 
meat juice, or a trace of yeast, or other fresh animal or vegetable matter. 
The amount added is far too small to affect the actual nutritive value of the 
diet. Its effect can only be due to the presence of a trace of some substance 
which acts, so to speak, as the hormone of growth. The search for such a 
substance is now being actively prosecuted. Its discovery will be of the 
greatest scientific and practical interest. 
Evidently new ideas are not lacking amongst those who are engaged in 
investigating the réle of the proteins and their splitting products in the animal 
economy. But of more immediate practical interest is the question of the 
amount of protein required by animals under various conditions. It is obviously 
impossible to fix this amount with any great accuracy, since proteins differ so 
widely in composition, but from many experiments. in which a nitrogen balance 
between the ingesta and the excreta was made, it appears that oxen remain 
in nitrogenous equilibrium on a ration containing about one pound of protein 
per 1,000 pounds live-weight per day. All the British exveriments of a more 
practical nature have been recalculated on a systematic basis by Ingle. and 
tabulated in the ‘Journal of the Highland and Agricultural Society.” From 
them it appears that increase of protein in the ration, beyond somewhere 
between one and a half and two’ pounds per 1,000 pounds live-weight per day 
