CHEST AND RIBS. 229 



angles to the length of the body. Instead of this being the 

 case, the difference in capacity of the chest is due to the 

 fact of the ribs, which are inclined to the rear, turning 

 round 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. 

 We may here note 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 between the chest capacity 

 when the lungs are full, and its capacity when they are 

 comparatively empty. 



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

 oval in Figs. 315, 316 and 317 diagrammatically represent the space 

 respectively enclosed between three 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 { = a ¥ ) he the distance of the centre of each 

 rib from its vertical axis, b a c the angle at which the ribs in all three 

 figures are inclined to the rear, and b^ a b the angle through which they 

 respectively turn during an inspiration. We shall then see that the 

 difference of capacity — shown by the difference of area between the 

 shaded oval and the one which circumscribes it — is greatly in favour 

 of the round barrel. If it were possible to have a chest perfectly circular 

 (as in Fig. 317) 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 respective ends 

 of the ribs are from each other. If the ribs are 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. 50). It would be easy to prove that the more inclined 

 the ribs are to the rear, the greater will be the difference 



