RESPIRATION. 



243 



Fig. 3. 



Fig. 4. 



Fig. Ill, diaphragm in its state of greatest descent in inspiration ; 

 2, muscles of the abdomen, showing the extent of their protrusion in 

 the action of inspiration. Fig IV, diaphragm in the state of its 

 greatest ascent in expiration ; 2, muscles of the abdomen in action 

 forcing the viscera and diaphragm upwards. 



8. We have now seen how the capacity of the chest is 

 enlarged by inspiration, how is its capacity diminished by 

 expiration ? The descent of the ribs is occasioned by the 

 elasticity of the cartilages and ligaments which join them 

 to the sternum and the spine. Indeed the natural condition 

 of the chest is that which obtains after a full expiration ; 

 and therefore we may consider the bones which form the 

 walls of the chest, and the muscles which cover those bones, 

 as anatagonizing or opposite forces. fThe ribs, then, have a 

 natural tendency to fall, owing to their peculiar structure, 

 as well as their position ; while their expansion is effected 

 by the specific action of their muscles.) Besides this, when 

 the diaphragm relaxes, the abdominal muscles contract and 

 push the abdominal viscera, and the diaphragm also, up to- 



