588 THE PHYSIOLOGY OF REPRODUCTION 



STRUCTURE OF THE MAMMARY GLANDS 



The mammary glands are composed typically of a number of 

 lobes, which are themselves divisible into lobules. Each lobule 

 consists of connective tissue in which the convoluted ducts of the 

 gland are bound together. If these ducts are traced backwards they 

 are seen to arise from groups of secretory alveoli. If they are traced 

 forwards they are found to unite together to form the lactiferous 

 ducts, which in the human subject are from fifteen to twenty in 

 number, and open to the exterior by minute apertures through the 

 teat. The lactiferous ducts at their point of origin from the lobular 

 ducts are provided with reservoirs in which the milk collects 

 during the periods of glandular activity (i.e. during lactation). 

 These reservoirs in some animals are of a very considerable size (e.g. 

 whales, as described above). The duct walls consist of areolar tissue 

 containing some unstriated muscle fibres. They are lined internally 

 by short columnar epithelial cells which become flattened in the 

 proximity of the nipple. A quantity of fat generally covers the 

 surface of the gland, excepting the nipple. This fat is connected 

 both with the skin in front and with the glandular tissue behind. 

 Like the latter it is tabulated by processes of areolar tissue. The 

 nipple also is formed of areolar tissue with unstriated muscle fibres. 

 It is richly supplied with vessels which give it an erectile structure. 

 The glandular tissue also is plentifully supplied with vessels, which 

 vary in size according to the condition of the gland. The glands 

 in man are innervated by branches from the anterior and lateral 

 intercostal cutaneous nerves. Sensitive papilla? are present on the 

 surface of the nipple, and around it there is a small area of skin, on 

 which the ducts of little secretory glands open to the exterior. 



In the sow the mammary glands are often pigmented, and the 

 bacon cut from this area is discoloured, the condition being kno\vn 

 among bacon-curers as " seedy-cut." It was formerly supposed that 

 the pigment was blood pigment derived from red corpuscles extra- 

 vasated during "heat," when the glands are apt to be congested, 

 and that it did not occur in pigs which had been subjected to 

 ovariotomy. It is now known, however, that the pigment is not 

 derived from blood (thus, it does not contain iron) and that it may 

 occur in spayed sows and even in males, but it is always in association 

 with mammary tissue. It has been identified by Hammond in foatal 

 pigs in the walls of the developing ducts as they dip down from the 

 skin. The pigment is presumably identical with that of the hair since it 

 only occurs in coloured pigs, and in Red Tarn worths is of a sandy colour. 1 



1 Mackenzie, Marshall, and Hammond, " On Ovariotomy in Sows, etc.," Jour. 

 Agric. Science, vols. iv., 1911-12 ; v., 1912-13 ; vi., 1914 ; and vii., 1915-16 ; and 

 "Physiology and Bacon Curing," Jour. Roy. Agric. >SV., vol. Ixxvi., 1915. 



