VI 



FORM AND FUNCTION 



87 



man 



To return to the subject of breathing, 

 breathes not only by means of the diaphragm, but by 

 raising the ribs. This is effected by means of muscles 

 called the external intcrcostals, which pass downwards 

 and outwards from each rib to the one below it. The 

 contraction of these muscles will raise both ribs, as 

 may be shown by an easy experiment. Take two 

 thin rods of deal and screw them on to a third piece, 

 nail a fourth piece to their other ends to keep them 

 parallel/ Join them by an india rubber band, sloping 



Fig. 26. . , . ,. ,, 



R K represent ribs ; B the backbone ; S the breastbone ; E is the india-rubber 

 band representing the external intercostal muscle. 



downwards and outwards and too short to reach 

 without stretching. This will raise the two ribs. 

 There are muscles fastened to the inside of the ribs 

 and from that called the Internal Intercostals which 

 slope downwards and inwards towards the backbone, 

 and therefore act just in the opposite way— i.e., they 

 lower the ribs. When a bird is standing or walking, the 

 breast rises and falls in breathing much in the same 

 way as it does in man, though the great weight of the 



