102 RESPIRATION AND DISPOSAL OF WASTES 



on the body and is breathed in during the time of sub- 

 mergence. The young of many flies and beetles are also 

 aquatic, and in the intervals of feeding come to the sur- 

 face and breathe in a supply of air. Where such species 

 are a pest, as in the case of the mosquito, a thin layer of 

 oil spread over the surface of the water cuts off the air 

 supply and destroys the insect. 



Lungs. — Lungs are ingrowths of the body surface or 

 of the digestive tract, which amounts to the same thing, 

 that communicate with the outside by comparatively 

 narrow openings and on the interior expand to form a 

 sac of varying degrees of complexity. In some of the 

 salamanders, lizard-like scaleless animals related to the 

 frog, the lung is merely a smooth-walled bag. In the 

 frog and toad its inner surface is increased by the develop- 

 ment of a few folds. In still higher species these folds 

 become extraordinarily numerous and of complicated 

 arrangement, exposing a large amount of surface to the 

 air. In man, for example, the surface of the lung meas- 

 ures about 2000 square feet, or 125 times that of the 

 general surface of the body. When it is remembered 

 that the lungs are exceedingly thin-walled, are kept con- 

 tinually moist, and are penetrated by a very extensive 

 capillary net-work, it will be seen that the high absorp- 

 tion of oxygen meets with a ready explanation. 



Lung or pulmonary respiration is associated with rhyth- 

 mical movements of the chest or thorax. During inspira- 

 tion the thoracic cavity, in which the lungs are located, 

 is enlarged, the lungs expand to fill the increased space, 

 and a supply of oxygen-containing air is thus breathed in. 

 In expiration the chest cavity decreases in volume and 

 presses the air to the outside. Such movements, as has 

 been noted, are purely accessory and do not constitute 

 respiration. 



Respiratory Pigments. — Blood plasma is capable of 

 absorbing about as much oxygen as water, and its ca- 

 pacity in this regard is far less than that of hemoglobin. 



