MECHANISM OF THE THORAX. 119 



the anterior extremities of the ribs were not attached to the 

 sternum by cartilages, which have to ascend in order to reach 

 it; for it is obvious that the angle of the cartilage and rib, during 

 their elevation by the intercostal muscles, has a tendency to en- 

 large itself; and will, in doing so, increase the horizontal dis- 

 tance between the anterior end of the rib and the sternum, and 

 consequently increase the transverse diameter of the thorax. 

 The upper ribs, from the shortness as well as direction of their 

 cartilages, can do little or nothing in increasing this diameter. 



According to some anatomists, the capacity of the thorax is 

 also augmented by a rocking motion of the rib, in which, the 

 two extremities being stationary, the middle is drawn upward 

 and outward. It is not, however, very clear, that this motion 

 exists to much extent, in the adult, as the posterior articulations 

 of the thorax are opposed to it. 



While the transverse enlargement of the thorax is going on, 

 a simultaneous motion occurs in the sternum, and in consequence 

 of the oblique direction in which the ribs run to it, the sternum 

 is caused, by the elevation of their bodies, to recede from the 

 spine. But, as the ribs increase successively in length from the 

 first to the seventh, each lower one, in its elevation from the 

 oblique towards the horizontal line, has its anterior extremity 

 carried proporlionably farther off from the spine ; hence, the 

 sternum has a combined movement resulting from its several 

 attachments to the ribs: one motion elevates it as a whole, ano- 

 ther causes it to recede from the spine as a whole: and the 

 third causes its lower end, from the increased length of the ribs 

 there, to be pushed farther from the spine than the upper; giving 

 it, thereby, during respiration, a slight motion backwards and 

 forwards, resembling that of a pendulum. This latter motion, 

 however, though its existence is clear, is riot very considerable, 

 from the sternum being kept in check by the tendinous centre 

 of the diaphragm, as one may prove by examining his own 

 body. The enlargement of the thorax, in its antero-posterior 

 diameter, is much more considerable at the anterior extremi- 

 ties of the ribs, because there they are comparatively free. In 

 this case, the cartilages of the ribs are bent forwards, besides 

 being elevated. 



In expiration, the movements of the thorax are exactly the 

 reverse of what they are in inspiration, and alUts diameters are, 



