138 



RESPIRATION. 



The following table shows the mean results of the immense number ot observations 

 on which this conclusion is based : 



Progression of the Vital Capacity Volume icith the Stature. 



Age has an influence, though less marked than stature, upon the extreme breathing 

 capacity. As the result of 4,800 observations (males), it was ascertained that the volume 

 increases with age up to the thirtieth year, and progressively decreases, with tolerable 

 regularity, from the thirtieth to the sixtieth year. These figures, though necessarily sub- 

 ject to certain individual variations, may be taken as the basis for examinations of the 

 extreme breathing capacity in disease, which frequently give important information. 

 Of course, the breathing capacity is modified by any abnormal condition which interferes 

 with the mobility of the thorax or the dilatability of the lungs. 



Relations in Volume of the Expired to the Inspired Air. A certain proportion of the 

 inspired air is lost in respiration, so that the air expired is always a little less in volume 

 than that which is taken into the lungs. All the older experimenters, except Magendie, 

 were agreed upon this point. The loss was put by Davy at T V, and by Cuvier at -^ of 

 the amount of air introduced. Observations on this point, to be exact, must include a 

 considerable number of respiratory acts ; and, from the difficulty of continuing respira- 

 tion in a perfectly regular and normal manner when the attention is directed to that 

 function, the most accurate results may probably be obtained from experiments on the 

 lower animals. Despretz caused six young rabbits to respire for two hours in a confined 

 space containing two hundred and ninety-nine cubic inches of air, and ascertained that 

 the volume had diminished sixty-one cubic inches, or a little more than one-fiftieth. We 

 may take the approximations of Davy and Cuvier, as applied to the human subject, as 

 nearly correct, and assume that, in the lungs, from -fo to -fa of the inspired air is lost. 



Diffusion of Air in the Lungs. When it is considered that, with each inspiration, but 

 about twenty cubic inches of fresh air is introduced, sufficient only to fill the trachea and 

 larger bronchial tubes, it is evident that some forces must act by which this fresh air 

 finds its way into the air-cells and the vitiated air is brought into the larger tubes, to be 

 expelled with the succeeding expiration. The expired air may become so charged with 

 noxious gases, by holding the breath for a few seconds, that, when collected in a receiver 

 under water, it is incapable of supporting combustion. 



