XITl] FEMALE GENERATIVE ORGANS 160 



which are visible externally are known collectively as the vulva. 

 The clitoris is a small rod-like organ and represents the penis of 

 the male, but unlike the latter is solid. 



The Mammary Glands, the function of which is to provide a 

 nutritive secretion for the newly born young, though rudimentary 

 in the male are normally active onl}' in the female. Their numbers 

 and position vary in different species. In animals which possess 

 a number of mammary glands, as with the pig, these are usually 

 arranged in rows approximately jjarallel on the ventral side of the 

 thorax and abdomen. In the cow the mammae are contained 

 within a definite milk-bag or udder which is surroiuided b}^ a 

 fibrous envelope and suspended below the abdomen. This udder 

 is provided with milk cisterns or galactoj^horous sinuses into which 

 the ducts of the glands open and convey milk from the secretory 

 alveoli. Each sinus communicates with the exterior by a teat, 

 there being t;y'pically four teats in a cow, corresponding to the 

 four mammarj^ glands (and sinuses), commonly called the four 

 'quarters.' One quarter may run dry without the others but this 

 does not happen normally. There is a fibrous division consisting 

 of yellow elastic tissue between the two lateral halves of the cow's 

 udder, but not between the anterior and posterior parts. Very 

 frequently there are one, two, or even three extra teats which are 

 placed posteriorly to or between the other teats, but are usually 

 smaller than the normal ; these supernumerary teats are associated 

 with extra glands from which the milk constituents may be re- 

 absorbed into the blood (the milk sugar sometimes appearing in 

 the urine) since the secretion is not usually drawn off through the 

 teats. In the sheep there are normally only two glands, sinuses 

 and teats (occasionally four), and the mare is similar excepting 

 that there may be two or even four sinuses opening into one teat. 



The actual glandular tissue is divided into lobes, which are 

 sub-di\aded into lobules, containing secretory alveoli. These are 

 bound together by connective tissue. The ducts through which 

 the milk passes have walls containing alveolar tissue and some 

 unstriated muscle fibres, and are lined internally by columnar 

 epithelial cells. In the secretor}' cells of the alveoli an active and 

 a resting stage can be distinguished. In the latter the lumina are 

 wide, and the cells of the lining epithelium form a single flat 

 layer with centrally situated nuclei. In the active condition the 

 epithelial cells are long and columnar and j)roject into the cavity. 



