282 RESPIRATION. 



they were to be entered without precaution. A lighted candle is accord- 

 ingly first let down into the suspected cavity, and if a sufficient quantity 

 of oxygen be present, it continues to burn ; if not, it is immediately 

 extinguished. 



This test is the more valuable, because it is found that the proportion 

 of oxygen necessary to support the combustion of a candle is a little 

 greater than that required for the immediate continuance of respiration. 

 A candle is extinguished when the air contains only It per cent, of its 

 volume of oxygen, while a little less than this may still serve a short 

 time for respiration. According to Milne-Edwards, a man may still 

 keep up respiration in an atmosphere which is insufficient to support 

 combustion ; and we have repeatedly seen pigeons continue to breathe, 

 though with difficulty, in air in which a candle flame was immediately 

 extinguished. 



Although, however, an atmosphere containing from 10 to IT per cent, 

 of oxygen is not immediately fatal to man by suffocation, it is still unfit 

 for continued breathing. The deficiency is not sufficient to stop respira- 

 tion at once, but after a time its deleterious effects become manifest, 

 and increase in intensity with each repetition. A complete renewal of 

 the deteriorated atmosphere is essential to the perfect performance of 

 the respiratory process. 



The absorption of oxygen by different species of animals varies 

 according to their general state of functional activity ; and this differ- 

 ence may be manifested even between species belonging to the same 

 class. Thus it has been found that in the sparrow the amount of 

 oxygen absorbed, in proportion to the weight of the body, is ten times 

 as great as in the common fowl ; and in a carp the quantity consumed 

 in the course of an hour would hardly be sufficient for the respiration 

 of a pigeon for a single minute. 



In the same individual, also, a temporary increase of muscular activity 

 augments in a marked degree the absorption of oxygen by the lungs. 

 In the human subject it was found by Lavoisier and Seguin that a man, 

 who in the ordinary quiescent condition absorbed a little over 19,000 

 cubic centimetres of oxygen per hour, consumed nearly 13,000 cubic 

 centimetres of the same gas during fifteen minutes of active muscular 

 exercise ; the rapidity of absorption being thus increased to more than 

 2J times its former rate. On the other hand, the same process is dimin- 

 ished in activity during sleep ; and in the hibernating animals, and in 

 insects which undergo transformation, at the time of their most pro- 

 found lethargy is reduced to a mere vestige as compared with its usual 

 activity. Spallanzani observed that in insects the amount of oxygen 

 consumed in a given time by the chrysalis was far less than that ab- 

 sorbed before or afterward by the caterpillar or the butterfly ; and in 

 the experiments of Kegnault and Reiset upon the marmot, at the com- 

 mencement of the cold season, the consumption of oxygen by this 

 animal was about 500 cubic centimetres per hour for every kilogramme 



