INFLUENCE OF TEMPERATURE AND SEASON. 223 



active exercise, etc. By diminishing the frequency, and 

 increasing the depth of respiration, the per-centage pro- 

 portion of carbonic acid in the expired air is diminished ; 

 being in the deepest respiration as much as 1*97 per cent, 

 less than in ordinary breathing. But for this proportionate 

 diminution also, there is a full compensation in the greater 

 total volume of air which is thus breathed. Finally, the 

 last half of a volume of expired air contains more carbonic 

 acid than the half first expired ; a circumstance which is 

 explained by the one portion of air coming from the 

 remote part of the lungs, where it has been in more im- 

 mediate and prolonged contact with the blood than the 

 other has, which comes chiefly from the larger bronchial 

 tubes. 



c. Influence of external Temperature. The observations 

 made by Vierordt at various temperatures between 38 F. 

 and 75 F. show, for warm-blooded animals, that within 

 this range, every rise equal to i o F. causes a diminution 

 of about two cubic inches in the quantity of carbonic 

 acid exhaled per minute. Letellier, from experiments 

 performed on animals at much higher and lower tempera- 

 tures than the above, also found that the higher the tem- 

 perature of the respired air (as far as 104 F.), the less is 

 the amount of carbonic acid exhaled into it, whilst the 

 nearer it approaches zero the more does the carbonic acid 

 increase. The greatest quantity exhaled at the lower tem- 

 peratures he found to be about twice as much as the 

 smallest exhaled at the higher temperatures. 



d. Season of the Year. Dr. Edward Smith has shown 

 that the season of the year, independently of temperature, 

 also materially influences the respiratory phenomena ; for 

 with the same temperature, at different seasons, there is 

 a great diversity in the amount of carbonic acid expired. 

 According to his observations, spring is the season of the 

 greatest, and autumn of the least activity of the respira- 

 tory and other functions. 



