ii4 HISTORY OF THE HUMAN BODY 



from two to five sacs which open into the rectum immediately 

 within the anus, from which in some cases they may be pro- 

 truded by being turned inside out. The best known of these 

 are the two lateral ones of the skunk, weasel, and allied forms, 

 which are covered with a muscular layer derived from the 

 levator ani muscle, and secrete an ill-smelling fluid as defense. 

 Other similar glands secrete odoriferous fluids employed as a 

 mode of sexual attraction, some of which are agreeable to man 

 and are used in the manufacture of perfumes (musk, civet). 



In their simplest form such glands open separately, but near 

 together, the surface covered by the opening being designated 

 as a glandular area, usually free from hair or nearly so ; but in 

 many cases this glandular area becomes depressed and forms a 

 sac or bursa sunken beneath the surface and serving at times 

 as a reservoir for the secretion. It is as such a structure that 

 the mammary or milk glands, so characteristic of the Class 

 of Mammalia, have arisen, and their appearance in the momo- 

 treme, Echidna, exhibits nearly their original condition. The 

 female of this animal possesses upon the ventral side an integu- 

 mental pouch, the marsupium, in which the eggs are placed, 

 and in which the young are nurtured when hatched. Opening 

 into the sides of this pouch is a pair of glandular sacs or 

 pouches, supplied with glands which are probably of the tubu- 

 lar type, although long supposed to be acinous. These sacs 

 are the mammary pockets, at the bottom of which lies the 

 glandular area with its numerous openings. The secretion, 

 which is a form of milk, pours out into the pockets, where it 

 is taken up by the young. There are no traces of nipples, but 

 the lips of the sac, the corium wall, fit around the nose of the 

 young and prevent loss (Fig. 31, a). A slight advance is 

 seen in the young Halmaturus, a marsupial, where the 

 mammary pocket is deeper, and a rudimentary nipple is 

 formed by the elevation of the middle of the glandular 

 area at the bottom (d). This structure is still further 

 developed in the young opossum (e), and in this latter animal 

 the functional activity of these organs causes the complete 

 extrusion of the nipples, and the mammary pocket is lost (f). 



