RESPIRATORY ORGANS 99 



about 7 cubic centimeters of oxygen, 15 cubic centimeters 

 of nitrogen, and varying small quantities of carbon dioxid. 

 In sea-water these substances are slightly less in amount. 



In respiration the passage of oixygen from the air or 

 water into the blood or through the surface of a cell is 

 not a peculiarity of living things but follows the law of 

 the diffusion of gases that is essentially the same as the 

 diffusion of liquids. If we take a closed jar divided by 

 a thin moist membrane, such as a sheet of parchment, 

 into two equal compartments in one of which there is 

 twice as much gas as in the other, the molecules will 

 pass through the membrane until the pressure on each 

 side is the same. The net result is a passage of gas from 

 the compartment of higher pressure or greater amount 

 into that with less, until the density in both compart- 

 ments is equal. 



In the cell where oxygen is continually being used and 

 the pressure is accordingly less than in the surrounding 

 air or water, or in the lungs where the amount per unit 

 volume of this gas in the blood is less than in .the air 

 outside the thin moist lung-wall, diffusion brings a fresh 

 supply. In the same way the abundance of carbon di- 

 oxid in a cell leads to its diffusion into the water or blood 

 where the amount is less. 



Respiratory Organs. — In unicellular organisms 

 where the body surface is relatively large and comes in 

 immediate contact with the surrounding medium (air or 

 water), no special respiratory organs exist. Even in 

 higher types, such as the hydra, jelly-fish, sponges, and 

 many worms living in moist places, respiration takes place 

 through the general body surface, and the needs of the 

 underlying cells are ministered to by diffusion, with or 

 without the aid of a circulatory system. Even in animals 

 of large size, where respiratory organs are present, the 

 skin may continue to take an active part in the process. 

 In the frog, for example, the skin is always active in this 

 regard and during hibernation, or the winter sleep, is the 



