462 FOOD. [CH. xxvin. 



In the earlier days of lactation, epithelial cells only partially transformed 

 are discharged in the secretion : these are termed colostrum corpuscles. It 

 is stated that colostrum possesses a purgative action. 



During pregnancy the mammary glands undergo changes (evolution) which 

 are readily observable. They enlarge, become harder, and more distinctly 

 lobulated : the veins on the surface become more prominent. The areola 

 becomes -enlarged and dusky, with projecting papillae ; the nipple too be- 

 comes more prominent, and milk can be squeezed from the orifices of the 

 ducts. This is a very gradual process, which commences about the time of 

 conception, and progresses steadily during the whole period of gestation. In 

 the gland itself solid columns of cells bud off from the old alveoli to form 

 new alveoli. But these solid columns after a while are converted into tubes 

 by the central cells becoming fatty and being discharged as the colostrum 

 corpuscles above mentioned. 



After the end of lactation, the mamma gradually returns to its original 

 size (involution). The acini, in the early stages of involution, are lined with 

 cells in all degrees of vacuolation. As involution proceeds the acini diminish 

 considerably in size, and at length, instead of a mosaic of lining epithelial 

 cells (twenty to thirty in each acinus), we have five or six nuclei (some with 

 no surrounding protoplasm) lying in an irregular heap within the acinus. 

 During the later stages of involution, large yellow granular cells are to be seen. 

 As the acini diminish in size, the connective-tissue and fatty matter between 

 them increase, and in some animals, when the gland is completely inactive 

 it is found to consist of a thin film of glandular tissue overlying a thick 

 cushion of fat. Many of the products of waste are carried off by the 

 lymphatics. 



Eggs. 



In this country the eggs of hens and ducks are those particu- 

 larly selected as foods. The chief constituent of the shell is 

 calcium carbonate. The white is composed of a richly albuminous 

 fluid enclosed in a network of firmer and more fibrous material. 

 The amount of solids is 13-3 percent.; of this 12-2 is proteid 

 in nature (egg-albumin, with smaller quantities of egg-globulin, 

 and of a mucinoid substance called ovo-mucoid), and the re- 

 mainder is made up of sugar (0-5 per cent.), traces of fats, 

 lecithin and cholesterin, and 0*6 per cent, of inorganic salts. 

 The yolk is rich in food materials for the development 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 vitdlin. Small quantities of sugar, 

 lecithin, cholesterin and inorganic salts are also present. 



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

 digestible ; 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 



