THE MAMMARY GLANDS. 265 



sexual maturity, when they increase in size and rotundity in consequence 

 chiefly of the deposition of fat. The full development of the true gland is 

 deferred until the occurrence of pregnancy, when active proliferation and 

 increase of the gland-tissue takes place in preparation for its activity as a 

 milk-producing organ. After lactation has ended, the mammae undergo 

 involution, the glandular tissue being reduced and returning to a condition 

 resembling that before pregnancy. With the recurrence of the latter, the 

 gland again enters upon a period of renewed growth and preparation, to be 

 followed in time by return to the resting condition, in which the amount of 

 glandular tissue is inconspicuous. After cessation of menstruation, the 

 mammary gland gradually decreases in size, and in advanced age the corpus 

 mammae may be reduced to a fibrous disk in which gland-tissue is almost, if 

 indeed not entirely, wanting. 



The blood-vessels supplying the mammary gland, in addition to their 

 distribution to the skin and more superficial parts of the breast, send deeper 

 twigs to the glandular tissue which break up into capillary networks surround- 

 ing the alveoli. During lactation the vascular supply is materially increased. 

 The veins from the corpus mammae join the superficial vessels and, in the main, 

 follow the arteries. Within the areola, the subcutaneous veins form a plexus 

 that encircles the nipple and receives its blood. The lymphatics are 

 exceptionally numerous and important. The deeper ones lie within the 

 interlobular connective tissue and pass towards the surface, where they join 

 the rich subareolar network. With the exception of a few trunks that follow 

 the perforating arteries and become efferents of the internal mammary lymph- 

 nodes, the lymphatics of the breast form two or three large trunks that pass 

 to the axillary nodes. The nerves supplying the glandular tissue are chiefly 

 sympathetic fibres, some ending in the blood-vessels and others forming 

 plexuses upon the membrana propria of the alveoli, a few fibrils terminating 

 between the secreting cells. 



