180 



HANDBOOK OF PHYSIOLOGY. 



with the spine (a b, Fig. 154); and the other, with a line drawn from 

 one of these (head of rib) to the sternum (A B, Fig. 154 and Fig. 155); 

 the motion of the rib around the latter axis being somewhat after the 

 fashion of raising the handle of a bucket. 



The elevation of the ribs is accompanied by a slight opening out of 

 the angle which the bony part forms with its cartilage (Fig. 15^ A); 

 and thus an additional means is provided for increasing the antero-poste- 

 rior diameter of the chest. . 



The muscles by which the ribs are raised, in ordinary quiet inspira- 

 tion, are the external intercostals , and that portion of the internal inter - 

 costals which is situate between the costal cartilages; and these are 

 assisted by the levatores costarum, and the serratus posticus superior. 

 The action of the levatores and the serratus is very simple. Their fibres, 



Ns. 



FIG. 156. 



FIG. 157. 



FIG. 156. Diagram of apparatus showing the action of the external intercostal muscles. 

 FIG. 157. Diagram of apparatus showing the action of the internal intercostal muscles. 



arising from the spine as a fixed point, pass obliquely downwards and 

 forwards to the ribs, and necessarily raise the latter when they contract. 

 The action of the intercostal muscles is not quite so simple, inasmuch as, 

 passing merely from rib to rib, they seem at first sight to have no filed 

 point towards which they can pull the bones to which they are attached. 



A very simple apparatus will make their action plain. Such an 

 apparatus is shown in Fig. 156. A B is an upright bar, representing the 

 spine, with which are jointed two parallel bars, and D, which repre- 

 sent two of the ribs, and are connected in front by movable joints with 

 another upright, representing the sternum. 



If with such an apparatus elastic bands be connected in imitation of 

 the intercostal muscles, it will be found that when stretched on the bars 

 after the fashion of the external intercostal fibres (Fig. 156, C D), i. e., 

 passing downwards and forwards, they raise them (Fig. 156, C' D'); 



