1054 PHYSIOLOGY 



from the lungs. The effects of this discovery were to transfer the chief seat 

 of oxidation to the tissues of the body, and to show that the blood acts 

 simply as a carrier of the oxygen from the lungs to the tissues, and of the 

 carbon dioxide from the tissues to the lungs. We thus learnt to distinguish 

 between external and internal respiratory processes. A consideration of the 

 chemical mechanisms involved in the process of external respiration in- 

 cludes therefore an investigation of the manner in which gases are held by 



FIG. 493. Barcroft's modification of the Topler pump. 



the blood and of the factors which are responsible for the transfer of oxygen 

 and carbon dioxide from blood to alveolar air, and from alveolar air to blood. 

 If blood be exposed to a Torricellian vacuum at the ordinary temperature, 

 the whole of its contained gases is given off. For the purpose of extracting 

 the blood gases a great variety of pumps have been devised. In every case 

 a glass vessel is evacuated by means of the mercury pump, and is then put 

 into connection with a reservoir containing blood which has been denbrinated, 

 or has been prevented from clotting by the addition of oxalate or citrate. 

 In all these pumps the main difficulty arises in the exclusion of atmospheric 

 air, and it is therefore important to dispense so far as possible with taps. 



