294 THE VITAL FUNCTIONS. 



contain, even in their most compressed state, a cer- 

 tain quantity of air ; and this air will tend, by its 

 elasticity, to expand the cells : the lungs will con- 

 sequently be dilated, and will continue to fill the 

 chest ; and the external air will rush in through 

 the trachea in order to restore the equilibrium. 

 This action is termed inspiration. The air is again 

 thrown out when the diaphragm is relaxed, and 

 pushed upwards, by the action of the large muscles 

 of the trunk ; the elasticity of the sides of the 

 chest concurs in producing the same effect ; and 

 thus expiration is accomplished. 



The muscles which move the ribs conspire also 

 to produce dilatations and contractions of the ca- 

 vity of the chest. Each rib is capable of a small 

 degree of motion on that extremity by which it is 

 attached to the spine ; and this motion, assuming 

 the chest to be in the erect position, as in man, is 

 chiefly upwards and downwards. But, since the 

 inclination of the ribs is such that their lower 

 edges form acute angles with the spine, they bend 

 downwards as they proceed towards the breast; 

 and the uppermost rib being a fixed point, the 

 action of the intercostal muscles, which produces 

 an approximation of the ribs, tends to raise them, 

 and to bring them more at right angles with the 

 spine ; the sternum also, to which the other extre- 

 mities of the ribs are articulated, is elevated by 

 this motion, and consequently removed to a greater 

 distance from the spine. The general result of all 

 these actions is to increase the capacity of the 

 chest. 



Thus there are two ways in which the cavity of 

 the thorax may be dilated ; namely, by the action of 



