The oviducal apparatus of the mammal is more specialised than that of lower vertebrates. It is most simple, as might be imagined, in the egg-laying Monotremes, where, indeed, it is on the same level as that of reptiles. But in the Eutheria the fimbriated mouth of the oviduct passes into a narrow and winding tube, the Fallopian tube; this widens into a uterus, and the two uteri combine into a single tube in the higher forms. They are called the Monodelphia on this account. In the Marsupials the uteri are distinct though they often join above, and from this junction depends a median "uterus." After the uterus or the uteri follows in every case a single vagina. The testes of the Mammalia, like those of other vertebrates, occupy primitively a position within the body cavity precisely corresponding to that of the ovaries. And in the lowly-organised Monotremata, and some other forms, such as the Whales, they retain that primitive position within the body. It is, however, distinctive of the Mammalia as opposed to lower vertebrates that the testes descend later into a scrotum, which is simply a protrusion of the skin of the body surrounded by muscles, and, of course, containing a section of the body cavity in which lie the testes. The penis of the Mammalia, represented by the clitoris and associated structures in the female, is of a structure entirely peculiar to this group. [Illustration] FIG. 49.--Brain of Dog. A, ventral; B, dorsal; C, lateral aspect. _B.ol_, Olfactory lobe; _Cr.ce_, crura cerebri; _Fi.p_, great longitudinal fissure; _HH_, _HH_^1, lateral lobes of cerebellum; _Hyp_, hypophysis; _Med_, spinal cord; _NH_, medulla oblongata; _Po_, pons Varolii; _VH_, cerebral hemispheres; _Wu_, middle lobe (vermis) of cerebellum; _I-XII_, cerebral nerves. (From Wiedersheim's _Comparative Anatomy_.)