MAMMARY GLANDS 73 



teat. IMilk escapes from the cistern through a single large 

 canal. 



The milk duct is closed and controlled at the end of the teat 

 by a circular involuntary muscle and is therefore not under vol- 

 untary control. The milk ducts draining each lobe are sur- 

 rounded by circular voluntary muscle fibers and are therefore 

 under voluntary control. By these 

 she is able to "hold up" her milk. 



The gland structure is undevel- 

 oped and rudimentary until matur- 

 ity is reached, and does not become 

 active until the close of the first 

 pregnancy. 



The growth of the mammary fig. 34.-Alveoli and Outlet 

 gland is apparently stimulated by Ducts. Magnified. 



and dependent upon some substance 



given to the blood by the body of the fetus or the "yellow body" 

 of the ovary. This same substance appears to check milk secre- 

 tion. Removal of the body of the fetus by birth, or absorption of 

 the yellow body after birth, permits the glands to produce milk. 



Blood supply. — The mammary glands receive their supply 

 through the mammary artery, which distributes branches 

 through the two glands in each half of the udder, one mammary 

 artery on each side. This artery divides and branches until the 

 finest arterioles terminate in capillaries distributing blood to 

 every portion of the udder. The nutrition for one half of the 

 udder thus comes through the external iliac artery, then through 

 the prepubic, a branch of the external iliac, then through a 

 branch of the prepubic, the external pudic. The mammary 

 artery is one of the terminal branches of the external pudic. 

 From this the distribution is through arterioles, capillaries, and 

 lymph spaces to the cells. 



The large vein which may be felt in front of the udder on 

 each side, and called by dairymen the "milk vein," is properly 

 the subcutaneous abdominal vein. Three veins, the external 

 pudic, the perineal vein, and the abdominal subcutaneous vein, 

 drain each lateral half of the udder. 



Nerve supply. — The mammary glands have their nerve supply 

 through the first lumbar pair of spinal nerves. 



Products. — Colostrum is the fluid which accumulates in the 

 udder during the latter part of pregnancy. It differs from milk 



