196 RESPIRATIOX. 



tendon remaining comparatively unmoved while the in- 

 tercostal, and other muscles just mentioned, by acting at 

 the same time, not only prevent the diaphragm during its 

 contraction from drawing in the sides of the chest, but 

 increase the diameter of the chest in the lateral direction, 

 by elevating the ribs ; that is to say, by rotating them, to 

 speak roughly, around an axis passing through their 

 sternal and spinal attachments, somewhat after the 

 fashion of raising the handle of a bucket (fig. 62). This 

 is not all, however. Another effect of the contraction of 

 the intercostal muscles is to increase the antero-posterior 

 Fig. 62. 



diameter of the chest, by partially straightening out the- 

 angle between the rib and its cartilage, and thus lengthen- 

 ing the distance between its spinal and sternal attachments 

 (fig. 62, A). In this way, at the same time that the ribs 

 are raised, the sternum is pushed forward. This forward 

 movement of the sternum, which is accompanied by a 

 slight upward movement, is in part accomplished also by a 

 raising of the anterior extremities of the rib cartilages, 

 which of course, in any movement, carry the sternum with 

 them. The differences in shape and direction of the upper 

 and lower true ribs, and the more acute angles formed by 



