486 



FOOD 



[CH. XXX. 



The nipple is composed of areolar tissue, and contains unstriped muscular fibres. 

 Blood-vessels are also freely supplied to it, so as to give it an erectile structure. On 

 its surface are very sensitive papillae ; and around it is a small area or areola of 

 pink or dark-tinted skin, on which are to be seen small projections formed by 

 minute secreting glands. 



Blood-vessels, nerves, and lymphatics are plentifully supplied to the mammary 

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

 with short columnar cells (see fig. 340). 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 divide and the part of each towards the lumen 

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

 form the constituents of the milk. 



In the earlier days of lactation, epithelial 

 cells only partially transformed are discharged in 

 the secretion ; these are termed colostrum cor- 



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 pro- 

 ceeds, 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. 

 No secretory nerves of the mammary gland have yet been discovered. It is 

 possible they do not exist, but the normal stimulus to mammary activity is a 

 chemical one formed by the ovary. 



FIG. 340. Section of mammary gland 

 of bitch, showing acini, lined 

 with epithelial cells of a short 

 columnar form, x 200. (V. D. 

 Harris.) 



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 protein 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 



