LACTATION 587 



situated posteriorly (or one may be between two of the chief teats) 

 and with glands associated with them. During lactation these extra 

 glands may secrete, but the milk or other fluid secreted by them, 

 since it is not usually drawn off, is absorbed into the circulation (see 

 below, p. 603). In the sheep there are only two glands (lateral 

 halves), sinuses, and teats (occasionally four), and the mare is similar 

 excepting that there may be two or even four sinuses opening into 

 one teat. In the sow the teats extend forward to the thorax in two 

 rows. In many animals there is a sphincter at the end of each teat. 



In Monotremes the mammary glands are stated to be modified 

 sweat glands, whereas in other mammalian orders they are commonly 

 regarded as representing sebaceous glands. 1 In Monotremes alone 

 there are no teats, the orifices of the glands being mere scattered 

 pores in the skin, the exuding milk probably passing along the hairs, 

 which in this region are arranged in bunches. In Echidna the 

 orifices open into two depressions which have been called the 

 mammary pockets. 2 Teats, which are present in all other orders 

 of Mammals, are of two kinds. In one kind the skin in the region 

 of the gland becomes raised up in a circular rim, and in this. way 

 gives rise to a tubular teat or nipple, into the base of which the 

 ducts of the gland open. This form of teat occurs in Carnivora and 

 Ungulata. In the other kind of teat the gland itself is raised into a 

 papilla, as in man and other Primates, in Rodents and in Marsupials. 

 The use of the teats is to facilitate the process of sucking. In the 

 Cetacea, however, where the action of sucking is incompatible with 

 the subaqueous life of these animals, the ducts of the mammary 

 glands are enlarged into reservoirs (somewhat similar to the galacto- 

 phorous sinuses of Ungulates), from which the milk is ejected into 

 the mouths of the young by means of a compressor muscle. 3 



In the male mammal, as just mentioned, the mamm?e do not 

 usually function, though milk is occasionally produced in man at 

 birth and at puberty, and more rarely at other times. Male goats 

 and sheep have been known to yield milk exceptionally, and the 

 same is also said to be the case with male rats 4 (see p. 614). 



1 Brouha and certain other authorities regard the mammary glands in all 

 the Mammalia as modified sweat glands (" Recherches sur les Diverses Phases 

 du Developpement et de 1'Activite de la Mamelle," Arch, de Biol., vol. xxi., 

 1905. This paper contains many references). Eggelung regards the mammary 

 glands either as homologous with sweat glands, or else as organs which are 

 sui generis, being derived independently from the primitive merocrine skin- 

 gland ("Ueber den wichtigen Stadium in der Entwickehtng der menschlichen 

 Milchdriise," Anat. Am., vol. xxiv., 1904). 



2 Wiedersheim, Comparative Anatomy of Vertebrates, Parker's translation, 

 2nd Edition, London, 1897. 



3 Flower and Lydekker, An Introduction to the Study of Mammals, London, 

 1891. 



* Wiedersheim, loc. cit. 



