HOW ANIMALS BREATHE. 



75i 



A rude apparatus to illustrate the mechanism of breathing in mam- 

 mals is easily made by suspending the lungs of some small animal in a 

 glass bell-jar closed below by an elastic membrane a sheet of rubber, 

 for instance the only access for air to the interior being through the 

 windpipe. Then the forced enlargement of the cavity, by pulling 

 down the membrane, causes the inflation of the lungs. This apparatus 

 is deficient in mobility of the walls. 



Fig. 11. Air-Cells of Lung, with Intervening Tissues, a, epithelium ; b, elastic trabecular ; 



c, membranous wall, with fine elastic fibers. 



The rapidity of the respiratory movements in man is about one 

 inspiration to four heart-beats, or fifteen to twenty-five per minute ; 

 greatly varying, however, according to age, sex, and circumstances. 

 In animals with high temperature, breathing is much faster, becoming 

 almost a tremor in birds. In the w T hale, on the contrary, breathing is 

 suspended while the animal is under water ; it being provided with 

 reservoirs of pure blood. When the latter is exhausted, the creature 

 comes to the surface and puffs and " blows " to obtain air and refill 

 the reservoirs. 



The difference in color of the blood of vertebrates is chiefly due to 

 the varying amount of oxygen in chemical combination with the haemo- 

 globin of the red corpuscles the brightness of color being proportion- 

 ate to the oxygen. An essential part of the haemoglobin is iron ; and 

 it has been supposed that the change in color is due to a chemical 

 change from a ferrous to a ferric salt. But this simple and plausible 

 explanation is now denied by eminent physiologists, who, however, 

 admit that the iron has some essential but unknown influence. A 



