THE INTERCOSTAL MUSCLES 



1.41 



muscle contracts, it tends to shorten the distance between its 

 tico ends be borne in mind. Let a and h in Fig. 46, A, 

 be two parallel bars, representing two consecutive ribs, 

 movable by their ends upon the upright c, which may be 

 regarded as the vertebral column at the back of the 

 apparatus ; then a line directed from xtoy will be inclined 

 downwards and forwards, and one from w to z will be 

 directed downwards and backwards. Now it is obvious 

 from the figure that the distance between x and y is 



Pig. 46. — Diagram of Models illustrating the Action of th4 

 External and Internal Intercostal Muscles. 



B, inspiratory elevation ; C, expiratory depression. 



shorter in B than in A and much shorter than in C ; 

 hence when x y \s shortened the bars will be pulled up 

 from the position C or A to or towards the position B. 

 Conversely, the shortening of w z will tend to pull the 

 bars down from the position B or the position A to or 

 towards the position C. 



If the simple apparatus just described be made of wood, 

 hooks being placed at the points x ;/, and w z ; and an 

 elastic band be provided with eyes which can be readily 

 put on to or taken off" these hooks ; it will be found that 



