REPRODUCTIVE ORGANS 75 



spongiosum not being present. In many Marsupials the glans penis 

 is bifurcated. In most Primates, Carnivora, Rodentia, Insectivora, 

 and Chiroptera, but in no other orders, an os penis is present. 



Ovaries and Oviduct. In the female, the ovaries permanently retain 

 their original abdominal position, or only descend a short distance 

 into the pelvis. They are of comparatively smaller size than in 

 other vertebrates, have a definite flattened oval form, and are 

 enclosed in a more or less firm " tunica albigenia." The oviduct 

 has a trumpet-like, and usually fimbriated abdominal aperture, and 

 is more or less differentiated into three portions : (1) a contracted 

 upper part, called in Man and the higher mammals the " Fallopian 

 tube "; (2) an expanded part with muscular walls, in which the 

 ovum undergoes the changes by which it is developed into the 

 foetus, called the " uterus "; (3) a canal, the " vagina," separated 

 from the last by a valvular aperture, and terminating in the urino- 

 genital canal, or common urinal and genital passage, which in 

 higher mammals is so short as scarcely to be distinct from the vagina. 

 The complete distinction of the oviducts of the two sides through- 

 out their whole length, found in all lower vertebrates, only occurs 

 in this class in Monotremes ; a prevailing mammalian characteristic 

 being their more or less perfect coalescence in the middle line to form 

 a single median canal. In the Marsupials this union only includes 

 the lower part of the vagina ; but in most Placentals it extends to the 

 whole vagina and a certain portion of the uterus, which cavity is 

 then described as "bicornuate." In the higher mammals, as in 

 Man, and also in some of the Edentates, the whole of the uterus is 

 single, the contracted upper portion of the oviducts or Fallopian 

 tubes, as they are then called, entering its upper lateral angles by 

 small apertures. In certain lower forms the urino- genital canal 

 opens with the termination of the rectum into a common cloaca, 

 as in other vertebrates ; but it is characteristic of the majority 

 of the class that the two orifices are more or less distinct exter- 

 nally. 



Mammary Glands. Mammary glands secreting the milk by 

 which the young are nourished during the first portion of their 

 existence after birth, are present in both sexes in all mammals, 

 though usually only functional in the female. In the Monotremes 

 alone their orifices are mere scattered pores in the skin, but in all 

 other forms they are situated upon the end of conical elevations, 

 called mammillae or teats, which, taken into the mouth of the 

 young animal, facilitate the process of sucking. These are always 

 placed in pairs upon some part of the ventral surface of the body, 

 but vary greatly in number and position in different groups. In 

 the Cetacea, where the prolonged action of sucking would be incom- 

 patible with their subaqueous life, the ducts of the glands are 

 dilated into large reservoirs from which the contents are injected 



