3i6 HISTORY OF THE HUMAN BODY 



ances. Although quite constant in number and arrangement 

 in a given mammal there is the greatest variation of the 

 lobes in forms not closely related ; and that these parts are of 

 slight physiological importance is shown by their complete 

 absence in mammals quite unlike structurally and occupying 

 different environments. Thus the lungs are without lobes in 

 the Cetacea, Sirenia, and some seals, thus suggesting a modi- 

 fication due to an aquatic life, but on the other hand the lungs 

 are similarly undivided in sloths and ant-eaters, and in cer- 

 tain rodents, as mice and squirrels. The left lung in the 

 elephant is also without lobes. 



The development of a diaphragm in mammals separates the 

 general body-cavity into thoracic and abdominal portions and 

 cuts off the pleura, which invests the lungs and lines the tho- 

 racic cavity, from the peritoneum, which stands in similar 

 relationship to the abdominal viscera. These changes cause 

 some variation in the mechanism of breathing, in which the 

 diaphragm becomes a powerful accessory organ. 



