STRUCTURE OF MAMMARY GLANDS. 281 



breast, are close to the pelvis that is, to the cavity with which 

 the special organs of reproduction are connected. This is 

 their situation in the mare, the cow, the ewe, the sow, and the 

 bitch. In what may be regarded as the lower tribes of mam- 

 mals, as in the duck-billed animals, and even in the whale 

 tribe, the structure of these glands has a much more simple 

 conformation than in such mammals as specially belong to 

 this treatise. There is, however, a similarity of structure 

 throughout the entire order of mammalians. Each gland con- 

 sists of innumerable minute secreting cells grouped together in 

 lobules and lobes. Delicate excretory ducts, taking their rise 

 from the ultimate cells, join again and again together, until 

 capacious ducts or rather reservoirs for the milk are produced. 

 In the female of the human race the lactiferous canals open by 

 numerous orifices upon the extremity of the nipple ; but in 

 animals in which the nipples or teats are of large size, as in 

 the cow, there is generally a wide cavity where the milk ac- 

 cumulates in considerable quantity, and is discharged through 

 one or two orifices only. In the cow the reservoirs are so 

 capacious as to be capable of containing at least a quart of the 

 secretion. In the cow a large reservoir is situated just above 

 each teat. The teats are four in number, or occasionally even 

 six, by the superaddition of extra mammary glands in some 

 cows ; these, however, for the most part, are imperfectly de- 

 veloped, and -therefore hardly yield any supply of milk.* Each 

 teat is perforated by a duct which, communicating with the 

 milk reservoir above, affords a copious stream of the secretion. 

 This duct is lined by a mucous membrane, with a thin covering 

 of tesselated epithelium for defence. The same membrane is 

 prolonged into the reservoirs and lactiferous tubes, so as to 

 line them throughout ; but in the smallest ramifications, and 



* Professor Simmonds " On the Mammary Gland of the Cow," 'Agricul- 

 tural Journal of England,' vol. xix. p. 81. 



