THE PHYSIOLOGY OF LACTATION 



47 



placed in the aperture of the teat, thus opening it slightly and 

 facilitating the release of milk. In other cases a surgical opera- 

 tion becomes necessary, by which the muscle is cut and weakened 

 sufficiently to render milking less difficult. The skin of the teat 

 has neither hair nor sweat-glands, and the mucous membrane 

 which lines the teat duct expands in the cistern. The cistern, 

 therefore, may be regarded as the expansion of the teat duct 

 (Fig. 13). The cistern (sinus lactiferus) is a pouch which is sepa- 

 rated from the teat by a circular muscle, over which the cow has 

 little control. This cistern is of variable size and may contain 

 from less than \ pint to 1 pint of milk. It is connected with other 

 parts of the udder by a network of milk-ducts whose lumen is 

 largest at the point where it opens into the cistern. These ducts 



Fig. 13. a, Upper end of teat duct; 6, cistern. (Ernst.) 



anastomose and end in the ultimate follicles or gland lobules. 

 Anastomosis is more pronounced in the upper regions of the udder 

 than near the cistern. The upper parts of the two quarters of 

 the udder that belong to the same gland are connected by these 

 milk-ducts, so that communication is established. At the junc- 

 ture of the milk-ducts there are small cisterns and a system of 

 sphincter muscles protects all junctures of the ducts. These 

 sphincter muscles are under control of the cow and enable her to 

 withhold milk at will, or perhaps by an involuntary reflex, so that, 

 when she suffers from nervousness caused by disease, maltreat- 

 ment, or dislike of the milker, she is able to hold up the greater 

 part of the milk. Such conditions frequently interfere with the 

 productiveness of a cow and are difficult to control. 



