VI FORM AND FUNCTION 87 
To return to the subject of breathing, a man 
breathes not only by means of the diaphragm, but by 
raising the ribs. This is effected by means of muscles 
called the external intercostals, 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 
Fic. 26. - 
R, R represent ribs; B the backbone ; S the breastbone; £ 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—z.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 
