388 



THE HUMAN BOD7 



and the sternal ends of the ribs attached to it are raised, 

 and so the distance between the sternum and the vertebral 



a 



Ql 

 FIG. 128. The diaphragm seen from below. 



column is increased. That this must be so will readily be 

 seen on considering the diagram Fig. 129, where db repre- 

 sents the vertebral column, c and d two 

 ribs, and st the sternum. The continu- 

 jl \ s f ous 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 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 dorso-ventrally. 

 The inspirator? elevation of the ribs is 



FIG. 129. Diagram illus- . f - 



tratiug the dorso-ventrai mainly due to the action of the scalene 



increase in the diameter of , . 7 . . , , _ rri , 



the thorax when the ribs and external intercostal muscles. The 

 scalene muscles, three on each side, arise 

 from the cervical vertebras, and are inserted into the upper 

 ribs. The external intercostals (Fig. 113, A) lie between the 

 ribs and extend from the vertebral column to the costal carti- 

 lages; their fibres 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, in fact, tends to pull together 

 the pair of ribs between which it lies, but as the upper one of 

 these is held tight by the scalenes and other muscles above, 



