CHAPTER XIII 



LACTATION 



" Nunc femina quseque, 



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 help- 

 less and the gestation period is far shorter, there are seven or 

 eight mammae. 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 (Un- 

 gulates, Cetaceans). 



In the cow and 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 



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