MAMMARY GLAND
The udder of the cow comprises four individual glands, referred to as quarters. The skin of the udder is covered with fine hair; however, the teat is completely hairless. The right and left halves of the udder each consist of a cranial (front) quarter and a caudal (hind) quarter.
Each side of the udder operates with a degree of autonomy due to its distinct blood supply, nerve supply, and suspensory apparatus. Ventrally, the two halves of the udder are demarcated by a longitudinal furrow, the intermammary groove, which corresponds to a median septum of connective tissue dividing left and right halves. Because of the relative isolation of each side, half of the udder can be removed surgically without damaging the other half, as might be done to treat a tumor. The two quarters in each half are separate from one another as far as the gland tissue and duct system are concerned. Thus, all the milk from one teat is produced by the glandular tissue of that quarter. The vasculature, nerve supply, and lymphatic drainage, however, are common to both quarters of a given half. The parenchyma of the lactating mammary gland consists of secretory tissue and the ducts of the gland. The secretory units, the alveoli, are lined by a simple epithelium that varies from columnar to cuboidal in height. The alveoli are the chief structures for actual milk production, although the initial portion of the associated duct is also lined with secretory epithelium.The various small initial ducts converge to form larger ducts, and these converge to form yet larger ones, all of which eventually terminate in a large single basin, the lactiferous sinus. The lactiferous sinus is sometimes described as being divided into a large cavity within the quarter itself, the pars glandularis (gland cistern), and a smaller cavity within the associated teat called the pars papillaris (teat cistern). The demarcation between the gland cistern and teat cistern is frequently marked by a circular ridge (annulus) that contains a vein and some smooth muscle fibers. The wall of the empty cistern contains numerous overlapping longitudinal and circular folds that are obliterated through expansion of the wall when it is full of milk. There may also be diverticula (pockets) in the wall of the gland cistern. The teat cistern is continuous with the exterior of the teat through a narrow opening at the end of the teat, the papillary duct (commonly called streak canal or teat canal), which opens at the ostium papillae. The bovine streak canal is about 8.5 mm long, and its lumen is normally closed by epithelial folds that project inward from the wall of the streak canal, leaving only a star-shaped potential opening. A sphincter of smooth muscle fibers surrounds the streak canal at the distal end of the teat.
22.3