MAMMARY ORGANS IN MARSUFIALIA. 769 



alba at the back of the marsupial depressions, or of the pouch ; 

 they are not fewer than two on each side (Macropus, Hypsi- 

 prymnus, Phalangista, Petaurus, Phascolarctos. Phascolomys) ; nor 

 more than thirteen, six on each side and one midway (Didelphys 

 virginiana). The follicles, from the inner surface of which the 

 milk-cells are detached, are cylindrical in shape, -£^t\i in. in 

 diameter ; grouped in clusters of from ten to twenty on short, 

 slender ducts, which enter the sides of larger canals, these 

 uniting to form four or six conical dilatations, from the apices of 

 which as many slender ducts pass to the apex of the nipple. 

 This is peculiar for its length and slenderness when in use ; but 

 in the young and virgin Marsupial it is much shorter, and lies at 

 the bottom of an inverted part of the skin of the back of the 

 pouch, which becomes thin and is reflected over the end of the 

 nipple, like the prepuce over the glans penis. The mammary 

 glands enlarge after impregnation, and rapidly a day or two 

 before uterine birth ; when, partly from development of the 

 nipple, partly from pressure of the enlarging gland, aided perhaps 

 by the action of its compressor muscle, the sheath is everted and 

 the nipple protruded. The preliminary infolding of the integu- 

 ment provides for the covering of the long nipple, which now is 

 pendant at the back of the pouch. The compressor muscle arises 

 from the ilium between or near to the lower attachment of the 

 internal oblique and ( transversalis abdominis : ' it passes out of 

 the abdominal ring, bends round the marsupial bone, expands as 

 it turns upward and inward behind the pouch to surround partly 

 by carneous, partly by sclerous fibres, the mammary glands, 

 dividing into as many insertions as there are glands of its own 

 side. This muscle (' ileo-marsupialis' of Cuvier) is the homotype 

 of the ( cremaster ' in the male (p. 10) ; and the chief function of 

 the ossification of the internal pillar of the abdominal ring (mar- 

 supial bone) is to add the power of the pulley to the compressor 

 of the mammary gland, and effect the requisite change in the 

 course of the contractile fibres. In the pouch of a young Mar- 

 supial the nipples are indicated by the inconspicuous orifices 

 of the teat-sheaths. Once naturally protruded and the sheath 

 everted, the nipples continue external. In the Kangaroo, after 

 being some weeks in use, they present a slight terminal expan- 

 sion, fig. 604, d. This part lies in a deep longitudinal fossa on 

 the dorsum of the tongue, ib. a ; and the originally wide mouth 

 of the uterine foetus is changed to a long tubular cavity, with a 

 terminal sub-circular or triangular aperture, just large enough to 

 admit the nipple, to which the young Marsupial thus very firmly 

 VOL. III. 3 D 



