128 PRINCIPLES OF PHYSIOLOGY 



greater than the tension of the gas in the air 

 cells of the lung, and it may be supposed that 

 this tension would drive off the carbonic acid. 

 Still there are difficulties with regard to this 

 purely physical explanation, and it may be 

 that the cells found on the delicate wall 

 between the blood and the air may exert 

 selective action, and, in a manner analo- 

 gous to true secretion, excrete the carbonic 

 acid. 



67. Respiration, however, is a double 

 function. Not only is carbonic acid eliminated 

 in the air cells of the lung, but oxygen is 

 absorbed into the blood, and by the blood 

 it is carried to the tissues. There can be no 

 doubt the oxygen is taken up by the all 

 important constituent of the red cells called 

 haemoglobin. It combines to form a loose 

 union with this pigment, The red corpuscles, 

 laden with oxygen, are hurried to the tissues, 

 and there a reverse process occurs. The 

 oxygen leaves the oxy-haemoglobin probably 

 in successive small quantities, passes into 

 the lymph, and is at once taken up by 

 the living tissues. In the air cells of the 



