CHAPTER XIII 

 LACTATION 



" Nunc femina queeque, 



Cum peperit, dulci repletur lacte, quod omnis 

 Impetus in mammas convertitur ille alimenti." ' 



LUCRETIUS. 



THE possession of mammary glands is an essentially mammalian 

 character. Their function is to provide nourishment for the newly- 

 born young. They are present in both sexes, but are usually 

 functional in the female only. Their number and position vary 

 considerably in different species. There may be only a single pair 

 (man), or as many as eleven pairs (Centetes). The number in any 

 particular species usually bears a relation to the normal size of the 

 litter, or to the requirements of the newly-born offspring. Thus in 

 the guinea-pig, in which the young are born in an advanced state 

 of development, and can feed without being suckled, there are only 

 two mammary glands, while in the rabbit, in which the newly-born 

 young are naked and helpless and the gestation period is far shorter, 

 there are seven or eight mamnue. In animals which possess a 

 number of mammary glands, these are usually arranged in two 

 nearly parallel rows along the ventral side of the thorax and 

 abdomen. In other cases they are restricted to the thorax (Primates, 

 excepting some lemurs, Cheiroptera, Sirenia, elephants, sloths) ; while 

 in others again they are confined to the inguinal region (most 

 Ungulates, Cetaceans). 



In the cow and most other Ungulates the mammae are contained 

 within a definite milk-bag or udder, which is surrounded by a fibrous 

 envelope and is suspended below the abdomen. The udder is 

 provided with milk cisterns or galactophorous sinuses into which 

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

 acini. Each sinus communicates with the exterior by a teat, there 

 being four teats in a cow, corresponding to the four mammary glands 

 (and sinuses) commonly called the four "quarters." One quarter 

 may run dry without the others. 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. 

 There are very frequently extra teats (sometimes as many as three) 



