182 Types and Maricet Classes of Live Stock 



a duct called the "teat-canal," having a capacity, when distended, of 

 from 1 to 11 2 ounces in moderate-sized teats. There are both circular 

 and longitudinal muscle fibers, and at the lower end the circular fibers 

 form an involuntary sphincter muscle, the function of which is to keep 

 the teat-canal closed under ordinary pressure. At the upper end of 

 the teat is another involuntary sphincter muscle, but it does not com- 

 pletely close the canal, and sometimes a third is found farther above. 

 The sphincters are sometimes so relaxed that very slight pressure is 

 sufficient to open them, and the cow leaks her milk; of other cows the 

 reverse is true, making them hard milkers. 



The milk -cistern, ducts, and alveoli. — Above the teat is a reservoir, 

 called the "milk-cistern," seldom holding more than half a pint. 

 This cistern is divided into pockets of various sizes into which the large 

 milk-ducts empty. At the point of entrance of these ducts are sphincter 

 muscles, and while they cannot entirely close the openings, they may 

 very nearly do so, and in this way the cow is enabled to "hold up her 

 milk." These large ducts ramify to all parts of the gland and anasto- 

 mose (intercommunicate) freely; and at the intersections are still other 

 voluntary sphincter muscles. A strong effort on the part of the cow 

 is required to close the larger ducts in the lower part of the udder, but 

 a slight effort will close the smaller canals further up in the glands. 

 Cows differ greatly in their control over these muscles and in their 

 disposition to exert it. The most common causes of holding up milk 

 are fright, the presence of strangers in the stable, lack of familiarity 

 with surroundings, irregularity in the time or manner of feeding or 

 milking, and sexual heat. With some cows it becomes a habit, much 

 injuring the usefulness and milking capacity of the animal. The large 

 ducts subdivide into smaller ducts, and these again into smaller ones, 

 until they terminate in groups of small sac-like bodies known as the 

 "ultimate follicles," "acini," or "alveoli." The teat-canal, milk-cis- 

 tern, and ducts are lined with columnar epithelium. 



The alveolus (plural, alveoli) is the sacculated distention found 

 in groups of three to five on the end of the minute milk-ducts. It is 

 the essential part of the gland. It is lined by a single layer of epithelial 

 cells which are especially concerned in milk production. The alveolus 

 is only 1-30 of an inch in diameter, and its cavity is from 1-250 to 1-100 

 of an inch in length and from 1-1,300 to 1-800 of an inch in diameter. 

 New ducts and alveoli may be formed up to about the fifth or sixth 

 year, thereby increasing the producing capacity of the cow. 



The entire gland may be compared to a large bunch of grapes; 

 the main duct of the gland branches very much as the stem of the 

 bunch of grapes branches; and just as the branches and sub-branches 

 of the stem lead to the grapes, so the branches of the duct lead to the 



