308 HANDBOOK OF PHYSIOLOGY. 



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, varying 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 very short columnar cells, with nuclei situated toward the 



Fig. 222. Section of mammary gland of biteh, showing acini, lined with epithelial cells of a 

 polyhedral or short columnar form. X 200. (V. D. Harris.) 



centre. The edges of the cells toward 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 toward the lumen, their nuclei divide, and 

 in the part of the cells toward the lumen a collection of oil globules and 

 probably of other materials takes place. 



The next stage is that the cells divide and the part of each toward the 

 lumen containing a nucleus and the materials of the secretion is, as it 

 were, broken off from the outer part and goes to form the solid part of 

 the milk. The cells also secrete, from the blood supplied to them, the 

 water, salts, and probably sugar. In addition to the actual casting off 

 parts of the cells containing fat and the other materials, oil globules 

 appear to pass out from the cells with the other materials into the lumen 

 of the alveoli. The cast-off parts of the cells disintegrate or break down, 

 undergoing a kind of solution in the more fluid part of the secretion. 



In the earlier days of lactation, epithelial cells partially transformed 

 are discharged in the secretion : these are termed colostrum corpuscles, 

 but later on the cells are completely transformed into fat before the 

 secretion is discharged. 



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 



