CHEST AND RIBS. 



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

 a^ain when the air is expelled from them. I may explam 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. 



Fig. 176. Fig. 177- ^^°- ^78- 



Diagrams showing different Degrees of Expansion of Chest. 



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

 Figs. 176, 177 and 178, diagrammatically represent the space respectively 

 enclosed iDetween pairs of ribs of different degrees of convexity, but of the 

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

 {= ab^) be, respectively, the distance of the centre of each rib from its 

 vertical axis'; ^ a ^ the angle at which the ribs in all three figures are inclined 



