3C ANIMAL MECHANICS 



faction, that this pliancy and elasticity have full 

 effect. When old age approaches, the cartilages 

 of the ribs become bony. They attach themselves 

 firmly to the breastbone, and the extremities of 

 the ribs are fixed, as if the whole arch were 

 formed of bone unyielding and inelastic. Then 

 every violent blow upon the side is attended with 

 fracture of the rib, an accident seldom occurring 

 in childhood or in youth. 



But there is a purpose still more important to 

 be accomplished by means of the elastic structure 

 of the ribs, as partly formed of cartilage. This 

 is in the action of breathing, or respiration ; espe- 

 cially in the more highly raised respiration which 

 is necessary in great exertions of bodily strength, 

 and in violent exercise. There are two acts of 

 breathing expiration, or the sending forth of 

 the breath ; and inspiration, or the drawing in 

 of the breath. When the chest is at rest, it is 

 neither in the state of expiration nor in that of 

 inspiration ; it is in an intermediate condition 

 between these two acts. And the muscular effort 

 by which either inspiration or expiration is pro- 

 duced is an act in opposition to the elastic pro- 

 perty of the ribs. The property of the ribs is 

 to preserve the breast in the intermediate state 

 between expiration and inspiration. The muscles 

 of respiration are excited alternately, to dilate or 

 to contract the cavity of the chest, and, in doing 



