II BODY CAVITY AND DIAPHRAGM 69 



into lobules ; this is the case with the Whales, the Bears, the 

 Oxen, and a few other forms. A curious fact about the kidneys 

 of the Mammalia is their very general asymmetry of position. 

 One of them usually lies in a more advanced position than the 

 other. The ureters lead from the kidneys to the urinary bladder, 

 which in its form and relations is quite distinctive of the 

 Mammalia. The bladder is formed out of the remains of the 

 allantois, and is therefore not the exact homologue of the bladder 

 of the frog, which is the equivalent of the entire sac which grows 

 out of the cloaca in the mammal, and is the foetal allantois. 

 The ureters open into the bladder in the higher ]\'Ia,mmalia, but 

 lower down in the urino-genital passage in the more primitive 

 mammals. 



The Body Cavity. — The ilammalia differ from all other living 

 vertebrates by the arrangement of the body cavity in which lie 

 the viscera. That cavity is divided into two by a partly muscular 

 and partly tendinous partition,»the diaphragm. No other verte- 

 brate has this precise disposition of the coelom. The diaphragm 

 lies usually transversely to the longitudinal axis of the body, 

 but gets a much more oblique arrangement in the Cetacea and 

 the Sirenia, whose needs demand a more expanded chamber for 

 the lungs. For in front of the diaphragm lie the lungs and 

 heart ; behind it the stomach, liver, intestines, and the organs 

 of reproduction and excretion. The diaphragm is used in re- 

 spiration ; when its muscles contract, the surface directed toward 

 the pleural cavity becomes less convex, and the cavity of the lungs 

 is thus increased, allowing them to expand under the pressure of 

 the entering air. 



The Lungs. — The lungs of the ^Mammalia differ from those 

 of animals lying lower in the series by the fact, just referred to, 

 that they occupy a pleural cavity completely shut off from the 

 abdomen by the diaphragm. As a rule the lungs of the Mammalia 

 are to be distinguished by their more or less extensive lobation. 

 In the Whales, however, and in the Sirenia, they are not much 

 divided, but present the appearance of the simple sac-like lungs 

 of the reptiles. In some mammals there is a median and 

 posterior unpaired lobe of the lung, which lies in the post- 

 pericardial cavity behind the pericardium. This is not uni- 

 versally present. The lungs are very frequently not symmetrical 

 in their lobation, the number of separate lobes on the right side 



