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 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 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 being to increase the capacity of the 

 chest. 



Thus there are two ways in which the cavity 

 of the thorax can 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 simi)ly to fix the ribs. 



