the mammary glands of agricultural animals. In ruminants and mares the udder is located in the inguinal region, between the thighs. In swine udders are symmetrically placed right and left of the white line on the belly. The udder of the cow, camel, and reindeer consists of two fore, or belly, parts and two rear, or hip, parts. Milk is synthesized in the secreting epithelium of tiny sacs called alveoli. Each cell synthesizes milk with each of its constituent parts. Alveoli, the largest of which include up to 100 epithelium cells each, are placed radially around the milk ducts. These ducts unite to form larger ones and open into milk cisterns. Milk is retained in the udder because of capillary action, as well as the presence of circular closing muscles (sphincters) in the teats. The udder is well supplied with blood, because 500 liters of blood must pass through the udder in order to make 1 kg of milk.
In heifers the glandular tissues of the udder begin to grow with the onset of sexual maturity and develop very intensely not long before calving. (In pregnant cows this is in the second half of the dry period, a month before calving.) In dairy cows the udder is goblet-shaped and set forward. It is firmly attached to the body (not hanging), and its parts are even and symmetrically placed. This kind of udder is soft, pliant, and elastic to the touch; after milking it becomes smaller and has long, twisted, clearly visible veins. The udders of sheep, goats, and mares each consist of two complexes of glands and two teats.