320 HUMAN PHYSIOLOGY 



teids. The amount of fat may vary from almost nil in white 

 fish to about 80 per cent, in fat bacon. 



Flesh is thus a source of proteids and albuminoids, and to 

 a smaller extent of fats. In animals specially fed the amount 

 of fat may be enormously increased, and ordinary butchers' 

 meat may have more fat than proteid. The extractives 

 include such bodies as creatin, xanthin, inosit, &c. (see p. 43), 

 which give the peculiar flavour to the flesh of various animals. 

 Flesh may be preserved in various ways e.g. by simply 

 drying, by salting, or by smoking. The result of each of 

 these procedures is to dimmish the amount of water, and 

 thus to increase the solids. 



3. Eggs. The egg of the domestic fowl need alone be 

 considered. The composition of the white and of the yolk 

 naturally differs considerably. The white of egg is nothing 

 more than a solution of proteids. 



In the yolk there is a very large amount of lecithin (p. 78) 

 along with ordinary fats, and a large amount of a phospho- 

 proteid, and the great value of eggs is thus that they con- 

 tain both proteids, ordinary fats, and this special fat. The 

 whole egg contains a little more than 10 per cent, each 

 of proteids and of fats. 



Speaking generally, we may say that the animal food- 

 stuff's are rich in proteids and fats, but are poor in carbo- 

 hydrates. 



Vegetable Food-stuffs. In the food of man vegetables 

 play as important a part as animal products. 



The peculiarity of special importance in vegetables is the 

 existence of a capsule to the cells, composed of cellulose a 

 substance allied to starch in its composition, but which is 

 very resistant to the action of the human digestive juices, 

 and thus hinders the utilisation of the cell contents. In 

 order that these may be digested and absorbed from the 

 stomach and intestine, this capsule must be broken down 

 either by some preparatory treatment, or by the teeth in the 

 act of chewing. Although of practically no value as a food- 

 stuff, it acts as a natural purgative by stimulating the intes- 

 tines, and is of great value in keeping up the regular action 

 of the bowels. 



