2Q2 SECRETION 



tinct lobe ; so that the organ is really made up of a number of glands 

 identical in structure. 



The canals that discharge the milk at the nipple are called lactifer- 

 ous or galactophorous ducts. They are ten to fourteen in number. The 

 openings of the ducts at the nipple are small, measuring only -^ to ^ 

 of an inch (0.42 to 0.64 millimeter). As each duct passes downward, 

 it enlarges in the nipple to ^5 or yV of an incn (i or 2 millimeters) in 

 diameter, and beneath the areola it presents an elongated dilatation, 

 J to J of an inch (4.2 to 8.5 millimeters) in diameter, called the sinus of 

 the duct. During lactation a considerable quantity of milk collects in 

 these sinuses, which serve as reservoirs. Beyond the sinuses, the calibre 

 of the ducts measures % to J f an mc ^ ( 2<I to 4- 2 millimeters). The 

 ducts penetrate the lobes, branching and subdividing, to terminate in 

 the collections of alveoli which form the acini. There is no anasto- 

 mosis between the different lactiferous ducts, and each one is distributed 

 independently to one or more lobes. 



The lactiferous ducts have three distinct coats. The external coat 

 is composed of anastomosing fibres of elastic tissue with some inelastic 

 fibres. The middle coat is composed of non-striated muscular fibres, 

 arranged longitudinally and existing throughout the duct from its open- 

 ing at the nipple to the secreting alveoli. The internal coat is a thin 

 amorphous membrane, lined with flat polygonal cells during the inter- 

 vals of lactation and even during pregnancy, the cells being cylindrical 

 in form and frequently presenting multiple nuclei, when milk is secreted. 



The acini of the gland, which are very abundant, are visible to the 

 naked eye, in the form of small rounded granules of a reddish yellow 

 color. Between the acini, there exists a certain quantity of ordinary 

 white fibrous tissue with a number of adipose vesicles. The presence 

 of adipose tissue in considerable quantity in the substance of the glandu- 

 lar structure is peculiar to the mammary glands. Each acinus is made 

 up of twenty to forty secreting vesicles, or alveoli. These vesicles are 

 irregular in form, often varicose, and sometimes they are enlarged 

 and imperfectly bifurcated at their extremities. During lactation their 

 diameter is ^^ to 3^ of an inch (60 to 80 /-i). 



During the intervals of lactation, as the lactiferous ducts become re- 

 tracted, the glandular alveoli disappear ; and in pregnancy, as the gland 

 takes on its full development, the ducts branch and extend themselves, 

 and the vesicles are gradually developed around their extremities. 



Mechanism of the Secretion of Milk. With the exception of water 

 and inorganic matters, all the important and characteristic constituents 

 of the milk are formed in the substance of the mammary glands. The 

 secreting structures separate from the blood a great variety of inorganic 





