SEXUAL ORGANS. 55 



be viewed as accessories of the sexual organs. The number, posi- 

 tion, and external form of the mammae are very different in the sev- 

 eral orders ; it is the special province of Zoology to describe more 

 minutely these diversities. Frequently, as in the Hedgehog, Dog, 

 Hare, and other rodent and carnivorous animals, there occur from 10 

 to 12 mammas ; the number, however, varies between 2 and 12. It 

 is only in the higher orders, as in the Apes and Cheiroptera, though 

 also in the herbivorous Cetacea, that the mammae are present two 

 in number, as in Man, and situated upon the breast. In the rest of 

 the Cetacea, indeed, and in the Solipedia, there are found only two 

 mammae, but they lie far back, near to the anus or the sexual organs. 

 The Pachydermata and Ruminants have mostly from 2 to 4 upon the 

 belly. In the Carnivora and Rodentia, the number varies from 4 to 

 12, and they then lie in two adjacent rows, upon the belly, extend- 

 ing from the breast to the perineum. The number of the lactic 

 glands, which are frequently blended together, is indicated externally 

 by that of the nipples ; these have a soft cuticular covering, are perfo- 

 rated by the excretory ducts of the glands, and differ in number and 

 arrangement. In the Cow, the ducts pass into a large simple sinus, 

 which has only a single papillary opening ; the structure is similar 

 in the Whale and Dolphin. In the Rabbit and Cat we find 5 small 

 openings, and ten in the Dog, while in Man from 15 to 20 occur. 

 The nipples are seldom completely wanting, as they are in the Mono- 

 tremata, where the young can only suck, by making a kind of fold 

 of the skin upon the breast by means of their snout. In this order, 

 however, as in the Cetacea, a peculiar tegumentary muscle occurs, 

 which can compress the gland, and so spurt, as it were, the milk into 

 the young one's mouth. The same arrangement occurs also in the 

 Marsupiata, so that the fluid necessary for their nourishment can be 

 supplied to the small and imperfectly developed embryos which hang 

 on to the elongated nipples. The mamma appears in general to be 

 a conglomerate gland with arborescently divided excretory ducts, 

 which terminate in clusters of small bladders like bunches of grapes. 

 An exception, however, to this is presented by the Monotremata. 

 Thus in the Ornithorynchus each mammary gland consists of a mass 

 of coeca of considerable size, and varying in length, and either single 

 or divided at their extremities, which converge toward the nippleless 

 external openings. 



In the Marsupiata a peculiar external organ of generation occurs. 

 There is found, namely, in front of the pelvis, supported by a pair 

 of peculiar bones, a sac or pouch (in many genera, e. g. in Didelphis, 



