REPRODUCTION 31 



30. Structure of mammary glands. 1 The mammary 

 glands are made up of lobes which in turn are further 

 divided into lobules. The latter arise from secretory 

 alveoli. The lobule is chiefly connective tissue binding 

 together the milk ducts. The alveoli unite together 

 to form the lactiferous ducts which open externally. 

 These ducts are provided with reservoirs wherein the 

 milk is accumulated during the period of active lac- 

 tation. During lactation the alveoli secrete milk. This 

 secretion goes forward at all times, but is particularly 

 active during suckling. The milk drawn first is of 

 poorer composition in respect to solids than that drawn 

 near the end of the milking. This may be due to the 

 fact that the larger globules pass through the ducts 

 with greater difficulty and are thus retained longer in 

 the gland. 



The processes concerned in milk secretion are not 

 entirely understood, at least three views having been 

 held. One hypothesis is that the secretory cells them- 

 selves break down and thus set free their contents as is the 

 case with the sebaceous glands. Another view is that 

 the milk is simply excreted from the cell without causing 

 the breaking down of the cell, in a manner similar to 

 that which occurs in many secretory glands. 



" The third theory 2 was first suggested by Langer, 

 and has since been adopted, with various slight modifi- 

 cations, by Heidenhain, Steinhaus, Brouha and others. 

 According to their view the cells of the gland lengthen 

 out, so that their ends come to project freely into 

 the lumina of the alveoli. The projecting portions 

 then undergo a process of disintegration before or after 



1 Marshall, "The Physiology of Reproduction," p. 553. 



2 Ibid., p. 560. 



