i62 THE TRUNK. 



towards the front on their upper and lower ends, as on pivots, 

 when air is drawn into the lungs, and then revolving back 

 again when the air is expelled from them. I may explain that 

 the (tidal) air is expelled from the lungs by the elastic recoil 

 of the ribs, which takes place the moment the muscles which 

 drew the ribs forward become relaxed. Youatt's statement 

 is altogether incorrect; for the rounder the ribs are, other 

 things being equal, the greater will be the difference of 

 chest capacity when the lungs are full to what it would be 

 when they are comparatively empty. 



We may piove the foregoing remarks as follows : Let the shaded oval in 

 Figs. 265, 266 and 267 diagrammatically lepresent the space respectively 

 •enclosed between three pairs of iibs of different degiees of convexity, but of 

 the same depth, viewed from behind, at the end of an expiration. Let a b 

 {-a b^) be, respectively, the distance of the centre of each nb from its 

 vertical axis, if a c the angle at which the ribs m all three figures are inclined 

 to the rear, and I)^ a h the angle through which they respectively turn 

 -during an inspiration. We shall then see that the difference of capacity — 

 shown by the difference of aiea between the shaded oval and the one which 

 circumscnbes it — is greatly m favour of the round barrel. If it were possible 

 to have a chest perfectly circular (as in Fig. 267) at the end of an expiration, 

 the transverse axis of the chest, when the lungs were fully inflated, would 

 exceed in length the vertical axis. 



The second desirable condition — namely, good length of 

 ribs — should, as we have seen, be obtained rather by rotundity 

 than by the distance which the ends of the ribs respectively 

 are from each other. Given ribs sufficiently round, we need 

 not trouble ourselves much as to their length, except as 

 regards the farthest back ones, which should be as long and 

 directed as much outwards as possible, so as to afford a broad 

 attachment to the diaphragm (p. 38). It would be easy to 

 prove that the more inclined the ribs are to the rear, the 

 greater will be the difference in capacity of the chest when 

 empty to what it would be when full of air, and, conse- 

 quently, the better the breathing power ; but I do not think 

 that it is possible to estimate this inclination with sufficient 

 accuracy to make it a practical guide. My experience leads 

 me to conclude that all useful purposes in this respect will be 



