96 SCIENCE PRIMERS, * [§ vi. 



Stretched lungs return to their former size, carrying 

 the diaphragm up with them, and squeeze out the 

 extra quantity of air. This is called expiration. ' 



As the diaphragm descends it presses down on the 

 abdomen \ when it ceases to descend, the contents of 

 the abdomen help to press it up. If you place your 

 hand on your stomach, you can feel the abdomen 

 bulging out each time the diaphragm descends in 

 inspiration, and going in again each time the dia- 

 phragm returns to its place in expiration. 



41. But what causes the diaphragm to descend ? 



If you look at the diaphragm of the rabbit (or of any 

 other animal) a little carefully, you will see that it is in 

 reality a flat thin muscle, rather curiously arranged ; 

 for the red fleshy muscular fibres are on the outside all 

 round the edge (Fig. 12, ^ and C), while the centre B 

 is composed of a whitish transparent tendon. These 

 muscular fibres, like all other muscular fibres, have the 

 power of contracting. What must happen when they 

 contract and become shortened ? 



When these muscular fibres are at rest, as in the 

 dead rabbit, the whole diaphragm is arched up, as we 

 have seen, towards the thorax, somewhat as is shown 

 in Fig. 13, B, It is partly pushed up by all the 

 contents of the abdomen (for the cavity of the abdo- 

 men, you will remember, is quite filled by the liver, 

 stomach, intestines, and other organs), partly pulled ap 

 by the lungs, which, as we know, are always on the 

 stretch. When the muscular fibres contract, they pull 

 at the central tendon (just as the biceps pulls at its 

 lower tendon), and pull the diaphragm flat ; and 

 some of the fibres, such as those at C. Fig. 12, also 

 pull it down. The diaphragm during its con- 



