394 



THE HUMAN BODY 



num. The continuous lines represent 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 



Ql 

 FIG. 118. The diaphragm seen from below. 



as to make the bars lie in a more horizontal plane, the sternum is 

 pushed away from the spine, and so the chest cavity is increased 

 dorsiventrally. The inspiratory elevation of the ribs is mainly 

 due to the action of the scalene and external intercostal muscles. 

 The scalene muscles, three on each side, arise 

 from the cervical vertebrae, and are inserted 

 into the upper ribs. The external intercos- 

 tals (Fig. 120, A) lie between the ribs and 

 extend from the vertebral column to the 

 costal cartilages; their fibers slope downwards 

 and forwards. During an inspiration the 

 scalenes contract and fix the upper ribs 

 firmly; then the external intercostals shorten 

 and each raises the rib below it. The muscle, 



lustrat'mg the dorsi- in fact, tends to pull together the pair of ribs 



ventral increase in the . . . , . ,. 



diameter of the thorax between which it lies, but as the upper one 



when the ribs are raised. Qjf thege ig 



tight by the gcalenes and 



other muscles above, the result is that the lower rib is pulled up, 

 and not the upper down. In this way the lower ribs are raised 

 much more than the upper, for- the whole external intercostal 

 muscles on each side may be regarded as one great muscle with 

 many bellies, each belly separated from the next by a tendon, 



