IO8 PHYSIOLOGICAL CHEMISTRY. 



reagents. While by prolonged treatment with superheated 

 steam or acids or alkalies they yield most of the cleavage prod- 

 ucts described as characteristic of the albumins, some are, 

 however, lacking. The tyrosine group, for example, is absent 

 from gelatin, or present in minute amount at most. 



In food value the albuminoids are quite distinct from the 

 other proteins. Most of these substances are so insoluble in 

 the digestive fluids that really no importance as foods could 

 be ascribed to them. Collagen, which yields gelatin, has a 

 limited food value of a peculiar kind which will be referred 

 to below. All these substances serve as supporting, connect- 

 ing or protective tissues in the body, and they are characterized 

 necessarily by a kind of permanence, which depends on insolu- 

 bility in the first degree. With increasing age of the body 

 the albuminoid tissues become harder, firmer and less elastic. 



COLLAGEN. 



The best known of all these albuminoids is the collagen, or 

 glue-forming substance, found as ossein in bone, in cartilage, 

 in the fibrils of connective tissue, in tendons, in fish scales and 

 elsewhere. This substance, wherever found, is insoluble in 

 cold water, but by prolonged heating with water it passes into 

 the soluble form known as gelatin, glutin or, in impure condi- 

 tion, as glue. The change seems to depend on the taking up 

 of a molecule, or more, of water. At the present time it is 

 made in enormous quantities from slaughter house by-products 

 and according to its purity is employed for different purposes. 

 When made by hot water extraction from clean bones or car- 

 tilage it is used as an adjunct to food and also in the prepara- 

 tion of emulsions for photographic plates or gelatin paper. 

 The product from common material is used as joiner's glue. 



Gelatin softens and dissolves in water at a temperature 

 above 30. But this solution point depends largely on the 

 treatment to which it has been previously subjected. By long 

 heating with water, and especially under the action of super- 

 heated steam gelatin gradually breaks down into the usual 



