SECRETION. 421 



The nipple is a conical eminence that is capable of erection from 

 mechanical excitement. This is mainly produced by the contraction 

 of its unstriped, muscular tissue, aided by its numerous blood-vessels. 

 All tend to give it an erectile structure. The nipple is perforated by 

 numerous orifices: the apertures of the lactiferous ducts. On its 

 surface are very sensitive papillae. Near the base of the nipple and 

 upon the surface of the areola are numerous sebaceous glands. These 

 become enlarged during lactation, their fatty secretion serving as a 

 means of protection during the act of sucking. 



Fig. 156. Mammary Gland of Human Female. (After LIEGEOIS.) 

 (From Mills's "Animal Physiology," copyright, 1889, by D. Appleton 

 and Company.) 



1, Sinus, or dilatation of one of lactiferous ducts. 2, Extremities of the 

 ducts. 3, Lobules of gland. 4, Nipple, retracted in center. 5, Areola. 



The nipple is made up of areolar tissue interspersed with num- 

 erous blood-vessels and plain muscular fibers. The fibers are ar- 

 ranged chiefly in a circular manner around the base, some fibers, how- 

 ever, radiating from the base to the apex. 



Structure of the Mammae. The mammae consist of gland-tissue. 

 Like other glands, they are composed of large divisions, or lobes, 



