RESPIHATION. 347 



that, in the course of twenty-four hours, about 2 lbs. 8 ozs. 

 of oxygen is consumed by a man. After an ordinary respi- 

 ration a considerable quantity of air still remains — perhaps 

 four-fifths, one-fifth having been expired. 



Having mentioned the changes that take place in the at- 

 mosphere, we must next consider in what manner the blood 

 becomes so altered by its passage through the lungs. 



The blood, as it traverses through the body, gradually be- 

 comes darker ; it is loaded with carbon, and is rendered 

 unfit for the circulation, and in this state it is called venous 

 blood. If venous blood, taken out of the body, be exposed 

 to oxygen, it quickly becomes red ; and so it does if ex])0sed 

 to the atmosphere, but not so rapidly. So, likewise, if ar- 

 terial blood be exposed to carbonic acid, it quickly acquires 

 the color and character of venous blood. In the same man- 

 ner is the color of the blood changed in the lungs ; thus the 

 principal use of respiration appears to be to free the blood 

 from its impurities ; and this is effected although the air and 

 the blood do not actually come in contact. It was found, 

 that if blood in a common bladder were exposed to the at- 

 mosphere for some time, it acquired a coating of florid 

 blood ; and thus, as the membrane lining the air-cells is by 

 no means so thick as that of the bladder, there is no longer 

 any difficulty in accounting for the change taking place. It 

 has been the subject of some dispute as to when the change, 

 or rather exchange, takes place, some contending that the 

 carbon unites with the oxygen in the air-cells, whilst others 

 maintain that the oxygen enters the blood, and there unites 

 with the carbon, forming carbonic acid gas, which is then 

 exhaled into the air-cells. It was found, however, that if 

 venous blood were put within the exhausted receiver of an 

 air-pump a quantity of carbonic acid escapes ; thus proving 

 the presence of this gas in the blood, and supporting the 

 second theory. And as there appears to be a greater quan- 

 tity of oxygen abstracted from the atmosphere than can be 

 accounted for by the formation of carbonic acid, we must 

 conclude that a portion mingles with the blood and enters 

 the circulation ; which theory agrees with the fact, that it 

 has recently been discovered, by correct analyses, that both 

 venous and arterial blood contains carbonic acid, nitrogen, 

 and oxygen ; but that the latter gas is most abundant in ar- 

 terial and the former in venous blood. 



Although the action of the heart is much more frequent 



