OF MOVEMENTS OF CHEST-WALL IN RESPIRATION. 255 



the latter half they are 0*4 inch forward to 075 inch up- 

 ward. In the female and the youth the curves are much 

 more equable in the two directions of motion. 



It may also be noticed in these healthy curves that the 

 lines of the tracing enclose an irregular but yet a percep- 

 tible space — in other words, that the anterior end of the rib 

 takes a different course in its ascent from that of its expi- 

 ratory descent. 



In most cases the uppermost line is that of inspiration ; 

 and when it reaches its highest point it descends somewhat 

 more abruptly than it rose, being afterwards drawn inwards 

 more horizontally. In other instances, however, this track 

 is reversed, and the descent of the rib is more gradual than 

 its ascent (see figs. 9 and 10) ; and in some the two tracks 

 will interlace with one another once or twice ; or they may 

 occasionally, but very rarely in healthy breathing, follow 

 the same line. 



These variations are doubtless produced by the varying 

 degree to which the will interferes with the action. The 

 expiratory act in most persons is at first automatic, and 

 mainly due to the elasticity of the parts; but when a 

 forced effort of expiration has to be made, the voluntary 

 muscles of respiration are used to a greater or less extent 

 according to the will of the subject. 



Now it is important to observe that the actual shape of 

 this respiratory curve is very different from what it would 



