EXHALATION OF CARBON DIOXIDE 127 



It has been observed, however, that when muscular exertion is 

 carried so far as to produce great fatigue and exhaustion, the exhalation 

 of carbon dioxide is notably diminished. 



Influence of Moisture and Temperature. It has been shown that 

 the exhalation of carbon dioxide is greater in a moist than in a dry 

 atmosphere. It has also been ascertained that the exhalation is much 

 greater at low than at high temperatures, within the limits of heat and 

 cold that are easily endured, amounting, according to the experiments 

 of Vierordt on the human object, to an increase of about one-sixth, 

 under the influence of a moderate diminution in temperature. It was 

 found, also, that the quantity of air taken into the lungs was slightly 

 increased at low temperatures. 



Influence of the Season of the Year, etc. It has been shown by the 

 researches of Edward Smith, that spring is the season of the greatest, 

 and fall the season of the least activity of the respiratory function. 



The months of maximum are January, February, March and April. 



The months of minimum are July, August and a part of September. 



The months of decrease are June and July. 



The months of increase are October, November and December. 



Observations on the influence of barometric pressure have not been 

 sufficiently definite in their results to warrant any exact conclusions. 



Some physiologists have attempted to fix certain hours of the day 

 when the exhalation of carbon dioxide is at its maximum and at its 

 minimum ; but the respiratory activity is influenced by such a variety of 

 conditions that it is impossible to do this with any degree of accuracy. 



RELATIONS BETWEEN THE OXYGEN CONSUMED AND THE CARBON DIOX- 

 IDE EXHALED 



Oxygen unites with carbon in a certain proportion to form carbon 

 dioxide, the volume of which is equal to the volume of the oxygen that 

 enters into its composition. It is possible, therefore, to study the rela- 

 tions of the volumes of these gases in respiration, by simply comparing 

 the volumes of the inspired and expired air. It is now commonly recog- 

 nized that the volume of air expired is less, at a given temperature, 

 than the volume of air inspired. Assuming, then, that the changes in 

 the expired air, as regards nitrogen and all gases except oxygen and 

 carbon dioxide, are insignificant, it must be admitted that a certain 

 quantity of the oxygen consumed is unaccounted for by the oxygen that 

 enters into the composition of the carbon dioxide exhaled. It has 

 already been noted that ^ to -^ (1.4 to 2 per cent) of the inspired air 

 is lost in the lungs ; or it may be said in general terms that the oxygen 



