60 STRUCTURE AND ECONOMY OF THE HORSE. 



Other symptoms of excitement are produced ; but if the oxygen 

 is diminished, fi sense of languor, and disposition to sleep, is pro- 

 duced. A given quantity of oxygen will, however, support life 

 longer than the same quantity of atmospheric air. It has been 

 computed that, in the course of twenty-four hours, about 2 lb. 

 8 oz. of oxygen is consumed by a man. 



After an ordinary respiration, a considerable quantity of au" 

 still remains, perhaps five fourths more than has been expired. 



Having mentioned the changes that take place in the atmo- 

 sphere, wc must next consider in what manner the blood becomes 

 so altered bv its passage through the lungs. 



The blood, as it traverses through the body, gradually becomes 

 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 exposed to the atmosphere, but not 

 so rapidly. So, likewise, if arterial blood be exposed to carbonic 

 acid, it quickly acquires the colour and character of venous 

 blood. In the same manner is the colour of the blood changed 

 in the lungs ; thus the principal nse 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 connnon bladder were exposed 

 to the atmosphere 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 dif- 

 ficulty in accounting for the change taking place. 



It has been the subject of some dispute, as to where 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 was 

 put within the exhausted receiver of an air pump, a quantity of 

 carbonic acid escaped, thus proving the presence of this gas in 

 the blood, and supporting the second theory. And as there 

 appears to be a greater quantity of oxygen abstracted from the 

 atmosphere than can be accounted for by the formation of car- 

 bonic acid, we must conclude that a portion mingles with the 

 blood and enters the circulation ; which theory agrees with the 

 fiict, that it has recently been discovered, by correct analysis, that 

 both venous and arterial blood contains carbonic acid, nitrogen, 

 and oxygen, but that the latter gas is most abundant in arterial, 

 and the tbrmer in venous, blood. 



Although the action of the heart is much more frequent than 

 that of the chest in respiration, yet there is a most intimate con- 

 nection between the one and the other ; for besides the changes 



