336 HANDBOOK OF PHYSIOLOGY. 



lobes and lobules are bound together by areolar tissue; penetrating be- 

 tween the lobes, and covering the general surface of the gland, with the 

 exception of the nipple, is a considerable quantity of yellow fat, itself 

 lobulated by sheaths and processes of tough areolar tissue (Fig. 228) 

 connected both with the skin in front and the gland behind; the same 

 bond of connection extending also from the under surface of the gland 

 to the sheathing connective tissue of the great pectoral muscle on which 

 it lies. The main ducts of the gland, fifteen to twenty in number, 

 called the lactiferous or galactophorous ducts, are formed by the union 

 of the smaller (lobular) ducts, and open by small separate orifices 

 through the nipple. At the points of junction of lobular ducts to form 

 lactiferous ducts, and just before these enter the base of the nipple, the 

 ducts are dilated (Fig. 228); and, during lactation, the period of active 

 secretion by the gland, the dilatations form reservoirs for the milk, 

 which collects in, and distends them. The walls of the gland-ducts are 

 formed of areolar with some unstriped muscular tissue, and are lined 

 internally by short columnar, and near the nipple by squamous epithe- 

 lium. The alveoli consist of a membrana propria of flattened endothe- 

 lial cells lined by low columnar epithelium, and are filled with fat 

 globules. 



The nipple, which contains the terminations of the lactiferous ducts, 

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

 Blood-vessels are also freely supplied to it, so as to give it a species of 

 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, varying very greatly under certain conditions, especially 

 those of pregnancy and lactation. 



Changes in the Glands at certain Periods. The minute changes 

 which occur in the mammary gland during its periods of evolution (preg- 

 nancy), and involution (when lactation has ceased), are the following: 



The most favorable period for observing the epithelium of the mam- 

 mary gland fully developed is shortly before the end of pregnancy. At 

 this period the acini which form the lobules of the gland, are found to 

 be lined with a mosaic of polyhedral epithelial cells (Fig. 229), and sup- 

 ported by a connective-tissue stroma. 



The rapid formation of milk during lactation results from a fatty 

 metamorphosis of the epithelial cells. 



In the earlier days of lactation, epithelial cells partially transformed 

 are discharged in the secretion: these are termed " colostrum cor- 

 puscles," but later on the cells are completely transformed into fat be- 

 fore the secretion is discharged. 



