PROTEINS OR ALBUMINOUS SUBSTANCES 437 



There is evidently much work yet to be undertaken by the 

 chemist and physiologist. 



The study of nutrition, at any rate from the chemical side, has 

 already made some progress, but the proteins of the animal 

 body are very numerous and very diverse, and it is at present 

 uncertain how many even of those which are known are necessary 

 for the accomplishment of healthy normal changes in digestion, 

 and in the building up of new tissue. It is interesting to learn 

 from the researches of Abderhalden that a mixture of ammo- 

 acids alone without polypeptide is capable of maintaining life, 

 that is to say that the complex proteins of food are not indis- 

 pensable, and an animal can be kept in health when supplied 

 with the products of their hydrolysis, combined only with suit- 

 able amounts of pure fat, starch, cane sugar, and the necessary 

 inorganic salts. One reservation only is necessary here, and 

 that is that a minute amount of the mysterious substance or 

 substances belonging to the class of agents called hormones must 

 be added to the food. What these compounds are is unknown, 

 except that they are not proteins and are usually soluble in 

 alcohol and ether. Normally these substances are secreted in 

 the body by special glands, such as the thyroid, or in digestion 

 from the salivary glands. 



In some of the experiments which have been made on animals 

 an alcoholic extract of fresh milk is the form in which this non- 

 nitrogenous material has been supplied. In any case the quantity 

 required is not more than perhaps 1 per cent of the food given. 



Among the amino-acids which are produced by hydrolysis of 

 proteins, tryptophane seems to play a special and peculiar part. 

 A supply of this substance in the food was shown to be necessary 

 some years ago, and Dr. Gowland Hopkins has described ex- 

 periments in which the absence of this compound from the diet 

 was immediately followed by a falling off of the body weight of 

 the animal under observation. Tryptophane has the formula : 



-C-CH 2 -CH(NH 2 )-COOH 



>CH 

 NH 



