RESPIRATION. 



1*5 



angle whicli the bony part forms with its cartilage (Fig. 156, A); and 

 thus an additional means is provided for increasing the antero-posterior 

 diameter of the chest. 



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

 are the external inter costals, and that portion of the internal intercostal* 

 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, arising 

 from the spine as a fixed point, pass obliquely downward and forward 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 fixed point 

 toward which they can pull the bones to which they are attached. 



A very simple apparatus will explain this apparent anomaly and make 

 their action plain. Such an apparatus is shown in Fig. 157. A B is an 

 upright bar, representing the spine, with which are jointed two parallel 

 bars, C and D, which represent two of the ribs, and are connected in 

 front by movable joints with another upright, representing the sternum. 



> 



FIG. 157. 



FIG. 158. 



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

 FIG. 158. Diagram of apparatus showing the action of the internal intercostal muscles. 



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

 the intercostal muscles, it will be found that when stretched 011 the bars 

 after the fashion of the external intercostal fibres (Fig. 157, C D), i.e., 

 passing downward and forward, they raise them (Fig. 157, C' D'); while 

 on the other hand, if placed in imitation of the position of the internal 

 intercostals (Fig. 158, E F), i.e., passing downward and backward, they 

 depress them (Fig. 158, E' F'). 



, The explanation of the foregoing facts is very simple. The intercostal 

 muscles, in contracting, merely do that which all other contracting fibres 



