RESPIRATION. 



This is, by weight, U5 grammes, or rather more than one pound and 

 a half avoirdupois. 



The absorption of oxygen by different animals varies according to 

 their functional activity ; and this difference exists even between those 

 of the same class. In the sparrow the amount of oxygen absorbed, 

 in proportion to the bodily weight, is ten times as great as in the 

 common fowl ; and in a carp the quantity consumed in an hour 

 would hardly be sufficient for the respiration of a pigeon for a single 

 minute. 



In the same individual a temporary increase of muscular activity 

 augments in a marked degree the absorption of oxygen. It was found 

 by Lavoiser and Seguin that a man, who in the ordinary condition 

 absorbed a little over 19,000 cubic centimetres of oxygen per hour, 

 consumed nearly 13,000 cubic centimetres during fifteen minutes of 

 active exercise ; the rapidity of absorption being increased to more 

 than 2^ times its former rate. On the other hand, the process is 

 diminished in activity during sleep ; and in hibernating animals, and 

 in insects undergoing transformation, at the time of their most pro- 

 found lethargy it is reduced to a mere vestige, as compared with their 

 usual condition. Spallanzani observed that in insects the oxygen con- 

 sumed in a given time by the chrysalis was far less than that absorbed 

 by the caterpillar or the butterfly; and in the experiments of Reg- 

 nault and Reiset on the marmot, the consumption of oxygen by this 

 animal at the commencement of the cold season was about 500 cubic 

 centimetres per hour for every kilogramme of bodily weight, while 

 after hibernation was fully established it was reduced to 26 cubic centi- 

 metres per kilogramme per hour. 



The absorption of oxygen, accordingly, in respiration, is directly 

 associated, in rapidity and amount, with the physiological activity of 

 the living organism. 



Owing to its diminution in oxygen, air which has once been breathed 

 is less capable of supporting respiration than before. When an animal 

 is confined within a limited space, the air becomes poorer in oxygen as 

 respiration goes on ; and when its proportion has been reduced to a 

 certain point, the animal dies, because a substance essential to life is no 

 longer present in sufficient quantity. Different animals are affected in 

 various degrees by a given diminution in the atmospheric oxygen. 

 Cold-blooded species, in which respiration is comparatively slow, may 

 still breathe when only a very small quantity of oxygen is present ; 

 and it has been found that electrical fishes, as well as slugs and snails, 

 may continue respiration until they have completely exhausted the 

 oxygen in the water or air in which they are confined. But where 

 respiration is active, as in birds, quadrupeds, and man, a partial reduc- 

 tion of the oxygen is sufficient to cause death. If the carbonic acid 

 exhaled be absorbed by an alkaline solution, so that the purity of the 

 air be maintained, it is found that a sparrow dies in an hour when the 

 proportion of oxygen has been reduced to 15 per cent. ; and a mouse 



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