126 THE ENGINES OF THE HUMAN BODY 



answer well as an example for study ; the 7th rib and its 

 companion the 8th are the longest, strongest, and most 

 important of the series (fig. 31). In our study of levers 

 we found three points which required attention : (1) the 



Scalenus? --- 

 anf: 



Scalenus med. 



ns 



pi. 



ition 



Expiration — j- 



Exf Oblique - 



Pelvis 



Fig. 31. — To show the manner in which the 7th rib and the muscular sheets 

 attached to it (only parts of which are shown) help in expanding the thorax. 



fulcrum or hinge on which the lever works ; (2) the mus- 

 cular engines which move it ; (3) the load which has to be 

 lifted, or the resistance which has to be overcome. We 

 are dealing, as far as ribs are concerned, with levers of the 

 third order — the forearm being the example of that order, 



