466 



FOOD 



[CH. XXVIII. 



cent.), traces of fats, lecithin and cholesterin, and 0'6 per cent, of 

 inorganic salts. The yolk is rich in food materials for the develop- 

 ment of the future embryo. In it there are two varieties of yolk- 

 spherules, one kind yellow and opaque (due to admixture with fat 

 and a yellow lipochrome), and the other smaller, transparent and 

 almost colourless ; these are proteid in nature, consisting of the 

 nucleo-proteid called wtellin. Small quantities of sugar, lecithin, 

 cholesterin and inorganic salts are also present. 



The nutritive value of eggs is high, as they are so readily digest- 

 ible ; but the more an egg is cooked the more insoluble do its proteid 

 constituents become. 



Meat. 



This is composed of the muscular and connective (including adipose) 

 tissues of certain animals. The flesh of some animals is not eaten ; 

 in some cases this is a matter of fashion, in others, owing to an 

 unpleasant taste, such as the flesh of carnivora is said to have ; and 

 in other cases (e.g. the horse) because it is more lucrative to use the 

 animal as a beast of burden. 



Meat is the most concentrated and most easily assimilable of 

 nitrogenous foods. It is our chief source of nitrogen. Its chief solid 

 constituent is proteid, and the principal proteid is myosin. In addition 

 to the extractives and salts contained in muscle, there is always a 

 certain percentage of fat, even though all visible adipose tissue is 

 dissected off. The fat-cells are placed between the muscular fibres, 

 and the amount of fat so situated varies in different animals ; it is 

 particularly abundant in pork ; hence the indigestibility of this form 

 of flesh : the fat prevents the gastric juice from obtaining ready access 

 to the muscular fibres. 



The following table gives the chief substances in some of the 

 principal meats used as food : 



The large percentage of water in meat should be particularly 

 noted; if a man wished to take his daily supply of 100 grammes of 



* The flesh of young animals is richer in gelatin than that of old ; thus 1000 

 parts of beef yield 6, of veal 50, parts of gelatin. 



