CH. XXVIII.] 



EGGS 



465 



glands ; the calibre of the blood-vessels, as well as the size of the glands, varies 

 very greatly under certain conditions, especially those of pregnancy and lactation. 

 The secretory nerves of the mammary glands have not yet been discovered. 



The alveoli of the glands during the secreting periods are found to be lined with 

 very short columnar cells, with nuclei situated towards the centre. The edges of 

 the cells towards the lumen may be irregular and jagged, and the remainder of the 

 alveolus is filled up with the materials of the milk. During the intervals between 

 the acts of discharge, the cells of the alveoli elongate towards the lumen, their 

 nuclei divide, and in the part of the cells towards the lumen a collection of oil 

 globules and of other materials takes place. 



The next stage is that the cells fiivide and the part of each towards the lumen 

 containing a nucleus and the materials of the secretion, disintegrates and goes to 

 form the solid part of the milk. The cells also secrete water, salts, and milk sugar. 

 The fat, etc. , of milk are not simply picked out from the blood by the secreting 

 cells, but these materials are formed by metabolic 

 processes within the protoplasm of the cells. 



In the earlier days of lactation, epithelial 

 cells only partially transformed are discharged in 

 the secretion ; these are termed colostrum cor- 

 puscles. 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 sur- 

 face become more prominent. The areola becomes 

 enlarged and dusky, with projecting papillae ; the 

 nipple, too, becomes 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 particularly 

 selected as foods. The chief constituent of the shell is calcium car- 

 bonate. 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 per cent. ; 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 remainder is made up of sugar (0*5 per 



2 G 



FIG. 390. Section of mammary gland 

 of bitch, showing acini, lined 

 with epithelial cells of a short 

 columnar form, x 200. (V. D. 

 Harris.) 



