554 THE PHYSIOLOGY OF REPRODUCTION 



to the four mammary glands (and sinuses) commonly called the 

 four " quarters." One quarter may run dry without the others. 

 There is a fibrous di ision consisting of yellow elastic tissue 

 between the two lateral halves of the cow's udder, but not 

 between the anterior and posterior parts. 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 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 prob- 

 ably 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 sub- 

 aqueous 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 



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 mam- 

 mary 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 wicktiges Stadium in der Entwickelung der mensch- 

 lichen Milchdriise," Anat. Anz., vol. xxiv., 1904). 



2 Wiedersheim, Comparative Anatomy of Vertebrates, Parker's transla- 

 tion, 2nd Edition, London, 1897. 



3 Flower and Lydekker, An Introduction <o the Study of Mammals. 

 London, 1891. 



