BREATHING 285 



vertebral column. If the conditions are successfully 

 visualized, it will be seen that when the first ribs are raised 

 they must carry the breast bone with them and at the 

 same time thrust it forward. The result is a deepening 

 of the upper chest. 



As we follow the series of the ribs downward we find 

 that there is progressive shifting in the direction of 

 their axes of rotation. These axes never become quite 

 dorso-ventral (front to back) but they tend more and 

 more toward this limit. Our attention may be fixed 

 upon a selected pair, perhaps the ninth or tenth. 

 These ribs are long and they are attached to the lower 

 part of the breast bone only indirectly and by means of 

 flexible cartilages. Their borders are much lower than 

 their articulations with the spine and lower, also, than 

 their final connection with the breast bone. When 

 ' these ribs are raised their lateral portions are swung 

 apart to the right and left. Thus the lower part of the 

 chest is widened more than it is deepened in inspiration. 



The Diaphragm. This dome-shaped partition be- 

 tween the thorax and the abdomen has already been 

 mentioned. Its central part is a flat tendon and its 

 borders are muscular, the general direction of the fibers 

 being radial. It is one of the muscles concerning which 

 it is not easy to distinguish origin and insertion. As 

 these terms have been defined, the origin of a muscle is 

 the stationary and the insertion the movable attach- 

 ment. When the muscle fibers of the diaphragm are 

 thrown into contraction it cannot always be predicted 

 that one part or another will be moved. There are at 

 least two distinct possibilities. 



We may picture the margin as fixed. If this is the 

 case the curve of the dome will be flattened and the 

 chest will gain in capacity by the lowering of its floor. 

 The borders of the diaphragm will be bent inward and, 

 with a more intense contraction, there will be an actual 

 depression of the center. The lungs will be stretched 

 vertically, their bases descending. A movement such 



