634 VETERINARY PHYSIOLOGY 



tion of the organ from the central nervous system. The peri- 

 pheral mechanism, like that of the intestine and bladder, is 

 capable of independent action. But normally a centre in the 

 lumbar enlargement of the spinal cord appears to be excited 

 reflexly. This centre is further acted upon by the brain, and 

 various disturbances, accompanied by emotional changes, may, 

 for a time, arrest uterine contraction. 



IX. Lactation. 



1. The Mammary Gland. — The mammary glands consist 

 essentially of a collection of specially developed sebaceous 

 glands, the function of which has been modified to jdeld a 

 nutritive secretion for the young. In some of the lower 

 mammals (Monotremes) the secretion is provided, by the 

 enlarged skin glands which open direct on to the surface of 

 the abdominal wall, and the young are nourished by licking 

 the wall where aggregations of these occur. In the higher 

 mammals the glands are mure highly developed and more 

 definitely collected into groups forming the mammar}- glands, 

 with sinuses to act as reservoirs for the secreted fluid, and a 

 teat for convenience of the young in sucking. 



The number of glands present is roughly in proportion 

 to the usual number of young born at a time. The mare, 

 the sheep, and the goat have two glands. The pig has ten 

 to fourteen, the dog eight to twelve. The udder of the cow- 

 is usually spoken of as having four "quarters." A fibrous 

 septum in the median line divides the udder into two 

 halves. There is no dividing line between the two quarters 

 of the same side, though the sinuses of the same side do not 

 communicate. 



2. Physiology — (1) Development. — Rudimentary glands 

 are present in both male and female animals. In the male, 

 normally they remain undeveloped. In the female, as 

 sexual maturity is reached, the gland increases in size, the 

 increase consisting chiefly of fibrous tissue with a large 

 amount of fat. In pregnancy, proliferation of the glandular 

 tissue occurs. The tubules, which were solid blocks of cells, 

 grow outward and alveoli develop. As the cells of the 

 tubules and the alveoli divide, some remain attached to the 



