MILK. 



199 



anofher and about a tenth of an inch apart. They are 

 lined with a fine and highly sensitive membrane which is 

 continuous with the skin. The teat varies in form with 

 use and is subject to considerable alteration by manipu- 

 lation ; it is composed of longitudinal fibers which at the 

 [end are capable of a sort of erection under the influence 

 of stimulus, and thus act as a sphincter to close the ori- 

 fice and prevent the constant and passive flow of milk. 

 The udder (or udders, there being really four of them 

 in the cow) is made up, in addition to the organs de- 

 scribed above, of connective tissue, arteries, veins, ca- 

 pillary vessels, nerves and absoi'beots. It is supplied 

 with blood by the external 

 pudic artery and requires 

 two sets of veins to com- 

 plete the circulation, one 

 deep, which follows the ar- 

 teries, and one superficial, 

 which converges into the 

 great abdominal vein which 

 passes from the udder near 

 the skin and enters the ab- 

 domen behind the umbilli- 

 cal region. This large vein 

 is commonly called the milk 



vein.and is rightly supposed i 



to indicate by its prominence the larger milking capacity 

 of the cow. 



When gestation is not going on the above described 

 glandular culs-des-sacs and tissue connected with them 

 are shrunken and contracted (figure 23, a), the lining 

 membrane is shriveled and folded upon itself and cov- 

 ered only by contracted epithelium. When gestation has 

 progressed to a certain stage the vesicles are enlarged 

 and new ones are developed, the epithelium expands, be- 

 comes globular in shape, and is charged with fat granules, 



