EFFECTS OF RESPIRATION ON THE AIR. 523 



559. In estimating, however, the effects of the Respiratory function upon 

 the air which passes through the lungs, we are not so much concerned with the 

 quantity which may be drawn in and forced out, as with that actually exchanged 

 at each movement. There are many difficulties in arriving at any exact con- 

 clusion upon this point ; and hence it happens that the estimates of those who 

 have inquired into it are singularly discrepant. The following are the amounts 

 assigned by some of the most recent experimenters. 



Herbst 1 ..... . . 2030 cnbic inches. 



Valentin 2 ....... 1492 



Vierordt 8 . . . ... . 1042 



Coatkupe< ....... 16 



If we take 20 cubic inches as the average quantity exchanged at each respi- 

 ration, we cannot but observe how small a proportion it bears to the entire 

 amount which the lungs usually contain ; for the ft residual volume," which 

 cannot be expelled, is estimated by Dr. Hutchinson at from 75 to 100 cubic 

 inches, and the "reserve volume," which can only be expelled by a forced expi- 

 ration, is about as much more ; the sum of the two being from 150 to 200 cub. 

 in., or from 7 J to 10 times the " breathing volume." Now it is obvious that 

 if no provision existed for mingling the air inspired with the air already occu- 

 pying the lungs, the former would penetrate no further than the larger air- 

 passages ; and as this would be again thrown out at the next expiration, the 

 bulk of the air contained in the lungs would remain altogether without renewal, 

 and the expired air would not be found to have undergone any change. 6 That 

 a change is effected, however, in the whole volume of the air contained in the 

 lungs, with every inspiration, is indicated by the difference between the inspired 

 and expired air ; and this change must be attributed to the "mutual diffusion" 

 of gases, these (as discovered by Prof. Graham) tending to interpenetrate one 

 another, when of different densities or of different temperatures. 



560. The total amount of air which passes through the Lungs in twenty-four 

 hours, will of course vary with the extent and frequency of the respiratory 

 movements ; and these are liable to be affected by many circumstances, but 

 particularly by the relative degrees of repose and of exertion. Moreover, as 

 any such computation must be based upon the datum of the ordinary volume 

 of breathing or " tidal" air, it is obvious that the estimates of different observers 

 must vary with the amount they adopt. Thus Mr. Coathupe's estimate of the 

 diurnal total is 460,800 cub. in., or 366 cubic feet; that of Vierordt, from his 

 observations on his own person in a state of rest, is 530,026 cub. in., or 306f 

 cub. feet, but this, when corrected (by Scharling's experiments) for a moderate 

 amount of exertion, would be raised to 624,087 cub. in., or 361 cub. feet; and 

 that of Valentin is as high as 688,348 cub. in., or 398 J cub. feet. It is of 

 great practical importance to determine the quantity of air which ought to be 

 allowed for consumption by individuals confined in prisons, work-houses, schools, 

 &c. ; and for this, experience seems to have fixed 800 cubic feet as the minimum 

 that can be safely assigned, except where extraordinary provisions are in opera- 

 tion for its constant renewal by ventilation. The evil consequences of an insuf- 

 ficient supply of air will be noticed hereafter (Sect. 3). 



1 ' Meckel's Arcliiv.," 1828. 



2 'Lehrbuch der Physiologie," band i. p. 538. 



3 'Wagner's Handworterbuch," band ii. p. 835. 



4 'Philosophical Magazine," 1839, vol. xiv. p. 401. 



5 'Cyclop, of Anat. and Phys.," vol. iv. p, 1067. 



6 See Mr. Jeffreys's "Statics of the Human Chest," in which this important point first 

 received due consideration. 



