OUTPUT OF WASTE MATTER 127 



with the haemoglobin of the red cells. The 

 lymph surrounding the living tissues is, as 

 already explained, a respiratory as well as a 

 nutritive medium. The living elements, in a 

 sense, breathe in the lymph. They receive 

 oxygen from it and they give up oxygen to it. 

 The tension of the oxygen in lymph is high, 

 whereas that of carbonic acid is low, as it 

 is quickly removed. This facilitates the 

 interchange of gases, which we may regard 

 as an internal respiration. The lymph ulti- 

 mately reaches the blood and carries the 

 carbonic acid with it. Further, in the tissues 

 themselves there is an absorption of carbonic 

 acid by the small capillaries and veins. The 

 blood thus becomes venous. It is carried 

 away to the right side of the heart by the 

 veins, from thence it passes by the pulmonary 

 circulation to the lungs, and there the carbonic 

 acid is got rid of into the air cells of the lungs, 

 and, finally, it reaches the outer air in the 

 air of expiration. How the carbonic acid 

 passes out of the capillaries into the air cells 

 has not yet been clearly made out. The 

 tension of the carbonic acid in venous blood is 



