260 ANATOMY, PHYSIOLOGY, &C. 



lungs at each respiration. For breathing freely, a 

 man requires a gallon of atmospheric air per minute. 

 Fifty-two pounds and a quarter avoirdupois are 

 drawn in arid thrown out of the lungs per diem, equal 

 in bulk to 1,152,000 cubic inches. 



The air in breathing loses its oxygen, and is re- 

 placed by an equal quantity of carbonic acid gas, 

 which is ejected from the lungs at a temperature of 

 about 90.* 



The condition of the organs of respiration and 

 digestion appear so intimately connected with the 

 comfortable continuance and attainment of old age, 

 that existence may be said to depend on the due 

 exercise of the functions they perform. 



Amphibious Animals. 



To enable an animal to exist equally in air and 

 water, it should have lungs and gills, that is, it 

 should have the power of breathing air like the mam- 

 malia and birds, and of breathing water like fishes, 

 and it should be able to use either, to the exclusion 

 of the other. But we know of no such animal. 



Air Bladders. 



Flat fish, which have no air-bladder, have great 

 power in their pectoral fins, which enable them to 

 strike the water from above downwards with consi- 

 derable force. 



Cod-sounds are the swimming-bladders of the 

 large cod. 



* Respiration is the principal cause of the developeraent of 

 animal heat ; assimilation, the motion of the blood, the friction 

 of the various parts, produce the small remaining portiou. Be- 

 sides the oxygen employed in the production of carbonic acid, 

 another portion disappears, possibly in assisting the combustion 

 of the hydrogen of the blood. 



