SECRETION. 329 



and, during lactation, the period of active secretion by the gland, the 

 dilatations form reservoirs for the milk, which collects in them and dis- 

 tends them. The walls of the gland-ducts are formed of areolar and elas- 

 tic with some muscular tissue, and are lined internally by short columnar 

 and near the nipple by squamous epithelium. The alveoli consist of a 

 membrana propria of flattened endothelial 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 evolu- 



FIG. 222. Section of mammary gland of rabbit near the end of pregnancy, showing six acini, e, 

 epithelial cells of a polyhedral or short columnar form, with which the acini are packed. ' X 200. 



tion (pregnancy), and involution (when lactation has ceased), are the fol- 

 lowing: 



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. 222), and sup- 

 ported by a connective tissue stroma. 



The rapid formation of milk during lactation results from a fatty 

 metamorphosis of the epithelial cells: "The secretion may be said to be 

 produced by a transformation of the substance of successive generations 



