88 A Manual of Veterinary Physiology. 



process just described is repeated. But the loss of oxygen 

 in the tissues is not the only change the blood undergoes, 

 for not only is its haemoglobin reduced, but as the outcome 

 of tissue activity increased quantities of another gas are 

 added to it, the gas alluded to is carbonic acid ; this is 

 largely taken up by the venous blood and conveyed to the 

 lungs, and the method by which it is got rid of will bo 

 presently explained. 



The changes occurring to the oxygen in the tissues are 

 quite unknown ; it is supposed to be stored up in some 

 way or other until required. It has been suggested that 

 the storing up in the tissues may be closely, allied to the 

 storing up of oxygen by haemoglobin, though with this 

 difference, that the tissue ox}^gen-holding substance is 

 more stable than the blood oxygen-holding substance. All 

 we do know of the fate of the oxygen is that it eventually 

 assists in producing certain changes in the tissues, which 

 lead to the production of carbonic acid and other sub- 

 stances ; but the changes which the oxygen undergoes 

 from the time, to use the words of Professor Foster, it slips 

 from the blood into the muscular substance, to the moment 

 it issues from the tissues united with carbon as carbonic 

 acid, constitute the whole mystery of life. We do know, 

 however, that the oxidations take place in the tissues, and 

 not in the blood as was formerly supposed. 



In the systemic capillaries the partial pressure of the 

 carbonic acid is lower than the partial pressure of this 

 gas in the tissues, the result of which is that it is hurried 

 into the blood by the process of diffusion ; but here, as 

 with oxygen, simple absorption of the gas by the plasma 

 would not be sufficient for the purpose of carrying off the 

 whole of the C0 2 resulting from tissue activity. Now, 

 although there is no compound of CO, and hcemoglobin 

 definitely known, still there is a substance in the blood 

 capable of fixing CO, until the lungs are reached. 



If the serum of blood be exposed to the vacuum of an 

 air-pump, it is found to yield little oxygen but a quantity 

 of C0 2 ; it yields but little O, because, as we have already 



