viii THE VOCAL AND RESPIRATORY ORGANS 173 



of gases between the blood and the surrounding medium, 

 and (2) internal respiration, or the exchange of gases be- 

 tween the blood and the tissues. The metabolism of every 

 cell of the body involves the consumption of oxygen which 

 is received from the blood, and as the result of the oxidation 

 of compounds of carbon which occurs throughout the body 

 every cell produces carbon dioxide, which is given off into 

 the blood. The blood, therefore, acts as a means of 

 transporting oxygen from the organs of respiration to 

 the tissues and of carbon dioxide from the tissues to the 

 organs of respiration. It thus serves as the medium 

 between internal and external respiration. The greater 

 portion of oxygen in the blood is carried by the red cor- 

 puscles in combination with hemoglobin. This peculiar 

 substance has the power of forming a weak and unstable 

 chemical union with oxygen. As the blood passes through 

 the capillaries of the respiratory organs, oxygen diffuses into 

 it and combines with the hemoglobin ; when the blood 

 reaches the tissues where the partial pressure of the oxygen 

 is diminished, the hemoglobin parts with its oxygen to the 

 surrounding cells. Hemoglobin is a proteid compound 

 containing iron ; it is readily soluble in water and may be 

 obtained by evaporation from its solution in the form of 

 crystals. When combined with oxygen, it assumes a bright 

 red color, but when it loses its oxygen, it becomes a much 

 darker and more bluish tinge. It is to this change in the 

 hemoglobin that the difference in color between arterial and 

 venous blood is due. Blood that has been oxygenated is 

 bright red, while blood that has not been purified has a 

 much darker color. 



The Respiratory Function of the Skin. — The skin of the 

 frog is an organ of respiration of the utmost importance. Dur- 

 ing the winter when the frog lies buried in the mud it becomes 



