RESPIRATION IN MAMMALIA. 327 



The muscles which move the ribs conspire 

 also to produce dilatations and contractions of 

 the cavity 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 mo- 

 tion, assuming the chest to be in the erect posi- 

 tion, as in man, is chiefly upwards and down- 

 wards. 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 inter- 

 costal muscles, which produces an approxi- 

 mation 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 extremities 

 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 the diaphragm, and by the action of 

 the intercostal muscles. It is only in peculiar 

 exigencies that the whole power of this appa- 

 ratus is called into action ; for in ordinary res- 

 piration the diaphragm is the chief agent em- 

 ployed, and the principal effect of the action of 

 the intercostal muscles is simply to fix the ribs, 



