THE ENLARGEMENT OF THE THORAX. 



361 



marked in the lower ones. Daring inspiration the breast- 

 bone and the sternal ends of the ribs attached to it are 



Ql 



FIG. 110. The diaphragm seen from below. 



raised, and so the distance between the sternum and the 

 vertebral column is increased. That this must be so will 

 readily be seen on considering the diagram Fig. 111. where 

 ab represents the vertebral column, 

 c and d two ribs, and st the ster- 

 num. The continuous lines repre- 

 sent the natural position of the ribs 

 at rest in expiration, and the dotted 

 lines the position in inspiration. It 

 is clear that when their lower ends 

 are raised, so as to make the bars lie 

 in a more horizontal plane, the ster- 

 num is pushed away from the spine, 

 and so the chest cavity is increased 

 dorso- ventrally. The inspiratory 

 elevation of the ribs is mainly due to 

 the action of the scalene and exter- 

 nal intercostal muscles. The scalene muscles, three on each 

 side, arise from the cervical vertebrae and are inserted into 



FIG. 111. Diagram illus- 

 trating the dorso-ventral in- 

 crease in the diameter of the 

 thorax when the ribs are 

 raised. 



