THE GLANDULAR SYSTEM 247 



Changes in the Mammary Glands. During preg- 

 nancy the mammary glands become larger, firmer, and more 

 lobulated; the areola darkens, and the veins become more 

 prominent. During the period of lactation the gland is 

 the seat of active histologic and physiologic changes cor- 

 related with the production of milk. At the close of 

 lactation the glands diminish in size, undergo involution, 

 and gradually return to their original, non-secreting con- 

 dition. 



Structure of the Mammary Glands. Each gland con- 

 sists of an aggregation of some fifteen to twenty lobes, 

 each of which is surrounded by a framework of fibrous 

 tissue. The lobe is provided with an excretory duct, 

 which, as it approaches the base of the nipple, expands 

 to form a sinus or reservoir, beyond which it opens by 

 a narrowed orifice on the surface of the nipple. On 

 tracing the duct into the lobe it is found to divide and 

 subdivide, and finally to terminate in lobules or acini. 

 Each acinus consists of a basement membrane, lined by 

 cells,* and supplied with blood-vessels, lymphatics, and 

 nerves. 



THE SEBACEOUS GLANDS 



The sebaceous glands are embedded in the true skin, 

 and consist of grape-like masses opening by a duct upon 

 the surface of the epidermis or into a hair-follicle. They 

 are found in almost every part of the body, but most 

 abundantly upon the face. They secrete an oily sub- 

 stance, known as sebum. It is obstruction of these ducts 

 that produces what are commonly known as "black- 

 heads," which are seen so often upon the face. 



The deposit, so abundant upon some children at birth, 

 and which consists of a white, glue-like substance, is the 

 residue of the sebaceous matter, and is known as the 

 vernix caseosa. 



