552 THE RESPIRATORY EXCHANGE 



the shed blood, and the bacteria rapidly consume oxygen and 

 produce carbon dioxide. 



Experiments show that the living cells of the tissues, not the 

 blood, are the seats of oxidation. A frog will live a day or two 

 in an atmosphere of oxygen, after its blood has been washed out 

 and replaced by normal saline solution ; it absorbs oxygen which 

 is in solution in the fluid and produces carbon dioxide. The 

 respiratory exchange of rabbits deprived by bleeding of one-half of 

 their haemoglobin is equal to that of the same animals before the 

 loss of blood ( 25 ). A mouse can live in two atmospheres of oxygen 

 when the oxygen- carrying powers of its red corpuscles have been 

 thrown out of action by carbon monoxide ; it obtains the oxygen 

 which it needs from the gas which is in solution in the plasma. 



Tissues removed from a recently killed animal survive for a 

 time, take up oxygen and discharge carbon dioxide. This gaseous 

 exchange varies in different tissues, is increased by heat and 

 diminished by cold ; of all tissues muscle appears to be the most 

 active seat of such respiration. The criticism applied to these 

 experiments is that putrefactive changes occur and vitiate the 

 results. This no doubt holds for many of the earlier experiments, 

 but more recent work by Tissot and Fletcher ( 26 ) shows that it is 

 possible to eliminate the influence of bacteria and to investigate 

 that portion of the respiratory exchange which is to be considered 

 as the survival respiration of the tissues. The latter observer 

 has made numerous experiments upon the conditions which 

 influence this survival respiration in frog's muscle. The discharge 

 of carbon dioxide from an excised muscle is increased during con- 

 traction, if there be present an abundant supply of oxygen, the 

 increase being roughly proportional to the number and strength 

 of contractions. If, however, the muscle be made to contract 

 in air or in nitrogen, the additional yield of carbon dioxide is 

 incomplete or absent. Oxygen possesses great power of delaying 

 the progressive loss of irritability, which is seen in an excised 

 muscle, and postpones almost indefinitely the onset of rigor mortis. 



The power of tissues to act as reducing agents is well known. 

 Ehrlich found that alizarin-blue was decolourised by the living 

 tissues, but regained its colour when the tissues were exposed 

 to air. Tissues placed in a solution of oxy-haemoglobin in normal 

 saline quickly reduce it, but show different rates of action ; thus 

 Bernstein found muscle to be the most effective, and, if its value 



