294 HUMAN PHYSIOLOGY 



process. The common teaching has been that they 

 are indifferent to what is going on as if they were lifeless 

 structures but we may be obliged to credit them with 

 a definite participation in the performance. 



Internal Respiration. Emphasis has been placed 

 on the preliminary character of the exchanges occurring 

 in the lungs. It has been insisted that respiration in 

 the best sense of the term is a function of the tissues at 

 large in proportion as they share in the general metabo- 

 lism. We sometimes distinguish this genuine biologic 

 respiration as internal, contrasting it with the pul- 

 monary give and take which we call external respira- 

 tion. From this point of view external respiration makes 

 venous blood arterial and internal respiration makes 

 arterial blood venous. 



While the blood is flowing slowly through the systemic 

 capillaries, it is separated from the surrounding lymph 

 by a partition of the most delicate description. The 

 living cells, contractile or glandular or other, are in 

 contact with the lymph. They keep the oxygen con- 

 tent of the lymph at a low level but do not absolutely 

 exhaust it because its very poverty in this gas is an 

 invitation to bring in fresh supplies from the passing 

 blood. The immediate source is the plasma but this 

 has a very limited oxygen capacity and when drawn 

 upon it takes more from the corpuscles, turning oxyhemo- 

 globin into reduced hemoglobin. 



Meanwhile the active cells are thrusting the carbon 

 dioxid which they produce into the lymph and raising 

 the concentration of the waste in this fluid to a point 

 favorable to its transfer into the blood. It enters the 

 plasma and is carried mainly by that part of the blood 

 in various obscure combinations. A moderate share is 

 united with the hemoglobin of the corpuscles which can 

 contain carbon dioxid without losing their power to 

 hold oxygen. 



Carbon Monoxid. This is a gas (CO) which is not to be 

 confused with carbon dioxid (CO 2 ). It is formed when 

 carbon is burned with an insufficient supply of ox3 r gen gas, 



