VERTEBRATA : MAMMALIA. 65 I 



except the Cameltdce (in which they are oval) they are circular 

 and discoid, 



The lungs of Mammals differ from those of Birds in being 

 freely suspended in the thoracic cavity, the greater part of 

 which they fill, and in being enclosed freely in a serous sac 

 (pleura) which envelops each lung. The lungs are minutely 

 cellular throughout, and the bronchi never open on the sur- 

 face of the lung into a series of air-receptacles communicating 

 with one another, and placed in different parts of the body, 

 as is the case in Birds. 



There is no " inferior larynx," in any Mammal, and the 

 upper aperture of the true larynx is always protected by an 

 epiglottis. 



The kidneys in Mammals are situated in the lumbar region, 

 and exhibit a division of their substance into cortical and 

 medullary portions. 



There are two ovaries in the Mammals, and the oviducts 

 are known as the " Fallopian tubes." Each oviduct dilates 

 on its way to the surface into a uterine cavity, which opens 

 into the vagina. In the Monotremes and Marsupials this 

 primitive condition is retained throughout life, the uterus 

 remaining double, and opening by two apertures into the 

 cloaca or vagina. In most cases this condition is so far modi- 

 fied in the adult, that the two uteri have coalesced inferiorly, 

 so as to have only a single opening into the vagina, whilst 

 they separate into two horns or "cornua" superiorly. Only 

 in the Monkeys and in Man have the two uteri completely 

 coalesced to form a completely single cavity, into the " fundus " 

 of which the Fallopian tubes open. In male Mammals there 

 are always two testes present. In many Mammals the testes 

 are permanently retained in the abdominal cavity, and there 

 is no scrotum. This is the case in the Monotremes, the Ele- 

 phants, all the Cetacea, and many of the Edentata. Mostly, 

 however, the testes at an early period of life are transferred 

 from the abdomen to a pouch of integument called the 

 " scrotum." Usually the scrotum is placed beneath the pubic 

 arch and behind the penis, but this position is reversed in 

 the Marsupials. 



Mammary glands are present in all Mammals, and they are 

 regarded by Huxley as an extreme modification of the cutane- 

 ous sebaceous glands. In the male Mammals the mammary 

 glands are present, but, under all ordinary circumstances, they 

 remain functionally useless and undeveloped. Considerable 

 differences obtain as to the number and position of the mam- 

 mary glands in different cases : but they are always placed on 



