242 



BIOLOGY. 



[Book ii. 



to the .calculations of Menzies, Goodwin, Davy, &c., this portion 

 may be valued at a fifth, a seventh, even an eighth of the pul- 

 monary capacity. The absolute volume of gaseous contents varies 

 remarkably, like the pulmonary capacity, according to individuals 

 and ages. Hutchinson has given the following averages of this 

 capacity : — ^ 



In a gigantic and athletic American the pulmonary capacity rose 

 to 7-082 litres. 



The pulmonary capacity, then, bears a very obvious and con- 

 stant relation to youth and sti-ength, or more generally to the 

 degree of vitality. Of this Hutchinson's American is a striking 

 example. After two years of idle and dissolute life, his enor- 

 mous pulmonary capacity fell first to 6-364 litres, then to 5-222 

 litres, and finally, a little later, he succumbed to a tubercular 

 malady. 



The mean volume of gas inhaled at each inspiration may be 

 estimated at about one-third of a litre, or from rdne to ten cubic 

 metres in twenty-four hours. As to the volume expired, it 

 naturally is much the same as the volume inspired, with a slight 

 diminution of a fiftieth to a seventieth ; for there are other 

 outlets. 



Evidently the pulmonary gases, expelled at each inspiration, 

 are especially those nearest to the outside, those which fill the 

 trachea and the large bronchise. As to the air inclosed in the 

 pulmonary cells and in the fine bronchial ramifications, it renews 

 itself by diffusion, by the gaseous mingling from point to point ; 

 so that at last the respiring surface, the vascular wall of the 



' Hutchinson, On the Spirometer, 1846 (analysed inArch.'Oen. de Med., 

 1847.) 



