RATIO OF INSPIRED AND EXPIRED OXYGEN. 167 



nearly equal to that which would be exhaled by respirations of three 

 times the normal frequency, and on examining a single respiration, he 

 demonstrates what, however, would obviously be foreseen from a consid- 

 eration of the circumstances of the case, that the last portions of the ex- 

 piration are the richest in carbonic acid. Thus the first half of a respi- 

 ration contained only 3.72 per cent, of carbonic acid, the last half 5.44 

 per cent.. 



With respect to the ratio between the quantity of oxygen inspired and 

 that contained in the expired carbonic acid, a variation will K at i ooflhein 

 be observed, depending on many conditions, as, for example, spired and ex- 

 on the nature of the food. Thus, with a carbohydrate, the pired oxygen ' 

 quantity of oxygen in the carbonic acid will always be less than that in- 

 spired, a portion being employed in the destruction of the systemic nitro- 

 genized material which is undergoing decay. This destruction of nitro- 

 genized material is not sufficient for the support of animal heat, and 

 hence either carbohydrates introduced by 'the food, or fat already exist- 

 ing in the system, must be resorted to for the purpose of making up the 

 deficiency. With such variations in the requirements of the system, 

 and variations in the nature of the food, the ratio of the oxygen intro- 

 duced to that in the carbonic acid removed must also vary. 



For the perfect oxidation of the different elements of food, very differ- 

 ent quantities of oxygen are required ; thus, for the oxidation of 100 

 parts of fat, it would require 292.14 of oxygen; for that of starch, 118.52; 

 for that of muscle, 147.04. 



For reasons to be considered when we treat of the production of heat, 

 the quantity of carbonic acid disengaged varies with external Variations in 

 circumstances. When the weather is cold it is greater than the respired 

 when it is warm. Thus at 68 there is twice as much lib- air ' 

 erated as at 106. It increases during exercise and after eating, but 

 diminishes during sleep. More is set free by men than by women ; it 

 also varies with age, the proportion rising from eight years to thirty, re- 

 maining stationary to forty, and then declining. It changes with the 

 frequency of the respirations. The total quantity of carbon daily re- 

 moved by respiration may be estimated at eight ounces. 



Besides the carbonic acid removed, a large quantity of water is ex- 

 creted by the lungs, for the expired air may be regarded as Water removed 

 saturated, or containing the maximum quantity of water for in espiration. 

 94. For the vaporization of this water much heat is consumed, as is 

 likewise the case for the warming of the introduced air, which, no mat- 

 ter wha"t the external temperature may have been, is brought to that of 

 the lungs. 



With respect to the absolute amount of air expired, and also the quan- 

 tity of water removed by the lungs, some experiments have recently been 



