EXTERNAL SECRETIONS 



483 



physiologic actions of their secretions have in large part been considered 

 in the foregoing chapter on Digestion. There remains, however, to be con- 

 sidered the mammary glands, and the liver. 



MAMMARY GLANDS 



The mammary glands, which secrete the milk, are two more or less 

 hemispheric organs situated in the human female on the anterior surface of 



the thorax. Though rudimentary in 

 childhood, they gradually increase in 

 size as puberty approaches. The gland 

 presents at its convexity a small conical 

 eminence termed the mammilla or nipple, 

 surrounded by a circular area of pig- 

 mented skin, the areola. The gland 

 proper is covered by a layer of adipose 



FIG. 212. MAMMARY G L A N D . i. 

 Lactiferous ducts. 2. Lobuli of the 

 mammary gland. , 



FIG. 213. ACINI or THE MAMMARY 

 GLAND or A SHEEP DURING LACTATION. 

 a. Membrana propria. b. Secretory 

 epithelium. 



tissue anteriorly and is attached posteriorly to the pectoral muscles by a net- 

 work of fibrous tissue. 



During utero-gestation the mammary glands become larger, firmer, 

 and more tabulated; the areola darkens and the blood-vessels, especially 

 the veins, become more prominent. At the period of lactation the 

 gland is the seat of active histologic and physiologic changes correlated 

 with the production of milk. At the close of lactation these activities cease, 

 the glands diminish in size, undergo involution, and gradually return to their 

 former non-secreting condition. 



Structure of the Mammary Gland. Each mammary gland consists 

 of an aggregation of some 1 5 or 20 irregular pyramidal lobes, each of which 

 is surrounded by a framework of fibrous tissue. This tissue affords support 

 for blood-vessels, lymph-vessels, and nerves. Each lobe is provided with a 

 single excretory duct, the lactiferous duct, which as it approaches the areola 

 expands into a fusiform ampulla or reservoir. At the base of the nipple 

 the ampullae contract to form some 15 or 20 narrow ducts, which, ascending 

 the nipple, open by constricted orifices 0.5 mm. in diameter on its apex 

 (Fig. 212). 



On tracing the lactiferous duct into a lobe, it is found to divide and 

 subdivide into a number of branches, which pass into smaller masses the 

 lobules. The lobule in turn is composed of a large number of tubular acini 

 or alveoli, the final terminations of the lobular ducts. Each acinus consists 



