iz8 THE ENGINES OF THE HUMAN BODY 



when the ascending sheet, which is also interrupted, 

 contracts the front end of the rib is drawn downwards. 

 Thus the 7th rib is worked by two complex engines : 

 one raises it and the other lowers it ; but it is plain, from 

 what has already been said about muscular engines, that 

 the one cannot act without the other. As the upper 

 muscle shortens and raises the 7th rib, its opponent has 

 to yield to a corresponding degree and yet keep a grip 

 of the lever all the time. This harmony between the 

 opposing muscles is established by the elaborate nerve 

 mechanism already described (p. 43). At no part of a 

 breath do the elevators and depressors loosen hold of a 

 rib ; all the time the opponent muscular engines are kept 

 balanced and maintain a steady grip or balance on the 

 rib. We have thus a power to control the bellows, as in 

 singing and speaking. 



Having seen the manner in which the 7th rib is 

 hinged and the muscular engines which set this costal 

 lever in motion, we are now to look at the way it helps 

 in drawing in and expelling the breath. It is quite clear 

 as the rib is raised towards a horizontal position that its 

 front end will not only rise, but will also pass forwards ; 

 so will its cartilage and the breast-bone, which is carried 

 on the cartilage (fig. 31). The ribs above and below, 

 moving at the same time, will produce a similar effect. 

 The front wall of the chest is raised and pressed forwards 

 on the ends of the costal levers, thus enlarging the 

 respiratory bellows from back to front. At the same time 

 there is also an enlargement of the thorax from side to 

 side, one produced in a very simple manner, which can 

 be best studied in the 7th rib. Its axis or fulcrum is 

 obliquely fixed to the backbone, so that as the lever rises 

 it also moves outward ; as it sinks it moves inward. If 

 the axis on which the 7th rib moves had been set at a 

 right angle to the spine, it would have remained the same 

 distance apart from its companion of the opposite side as 

 they rose and fell. But as their axes slope downwards 

 they diverge as they rise and approach as they descend. 

 The outward and inward movements are most extensive 



