722 PHYSIOLOGY 



which conies on slowly, often aftej the lapse of a day, and may last 

 two or three days. The process of protoplasmic disintegration 

 appears therefore to occur in a series of stages, which occupy a con- 

 siderable time and end in the production of substances qualitatively 

 distinct from that substance, urea, which is the almost exclusive 

 nitrogenous end-product of the energy metabolism of protein. 



THE FOOD-VALUE OF CERTAIN SUBSTANCES ALLIED 



TO PROTEINS 



PROTEOSES AND PEPTONES. In the digestion of the naturally 

 occurring proteins the first products of hydration consist of a mixture 

 of substances known as proteoses and peptones. In the further 

 processes of digestion, under the influence of the ferments of the 

 pancreas and small intestine, these substances are converted into 

 the amino-acids which we have learnt to regard as the proximate 

 constituents of the protein molecule. Many experiments have been 

 performed in order to determine the nutritive value of these digestive 

 products. In nearly all cases it has been found that the meat in 

 the diet of an animal can be replaced by a corresponding quantity 

 of the products of digestion of the same meat without interfering 

 with the nitrogenous equilibrium of the animal, and Loewi and others 

 have shown that the same result may be attained by feeding an 

 animal on the products of pancreatic digestion of protein, i.e. a 

 mixture consisting almost entirely of amino-acids. Since the proteins 

 of the body differ in their composition from the majority of the 

 proteins of the food, it is evident that each food-protein molecule 

 has to be taken to pieces and reconstructed before it can take its 

 place in the body fabric, and it is therefore only natural that, so far 

 as metabolism is concerned, the results should be identical whether 

 we feed the animal with the ordinary food-protein or with the products 

 of its metabolism. This mode of feeding cannot, however, be regarded 

 as presenting any advantages. Under normal circumstances the 

 food molecules are broken down by degrees. Their products of 

 hydrolysis are set free in small quantities at a time and can be 

 therefore absorbed and disposed of in proportion as they are set free. 

 On the other hand, a sudden flooding of the alimentary canal with 

 a large quantity of the products of digestion introduces an abnormal 

 factor which must tend to produce wastage of nitrogen in the body 

 and to disturb the normal chain of processes involved in the regular 

 course of digestion. 



COLLAGEN AND GELATIN. Among the various sclero-proteins 

 or albuminoids which occur as normal constituents of our foods 

 these two are the only substances which undergo digestion and solution 

 in the alimentary canal to any appreciable extent, other substances, 



