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PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



increase the lateral diameters of the thorax ; for, since the ribs form 

 arches, each increasing in its radius from above downward, it is evident 

 that when the lower rib is so elevated as to occupy the position pre- 

 viously held by an upper rib the lateral diameter of the thorax at 

 that point will be greater than before. Still further, the articulations of 

 the ribs with the sternum are more or less fixed, and the resistance, 

 therefore, which these articulations offer to the movement of the ribs 



Fig. 248.— The Action of the Bibs of Man in Inspiration. (Btdard.) 



The shaded parts represent the positions of the ribs in repose. The line A B represents a horizontal 

 plane passing through the stomal extremity of the seventh rib ; the line C D represents a horizontal plane 

 touching the superior extremity of the sternum ; the line H G indicates the linear direction of the sternum. 

 When the ribs are elevated, as indicated by the dotted lines, the line A B becomes the plane a b, the line CD 

 the line c d, and the line II G becomes the line h g, the projection of the sternum being the more marked 

 inferiorly. The distance which separates the line M N from the line m n measures the increase in the 

 antero-posterior diameter of the thorax. 



will in the elevation of the ribs tend to open out the angles between the 

 ribs and their cartilages. 



By the elevation, consequently, of the ribs, the diameters of the 

 thorax are increased both laterally and in an antero-posterior direction. 

 Hence, everything that tends to elevate the ribs will produce an inspira- 

 tory movement; everything which depresses the ribs will produce an 

 expiration. All muscles, therefore, which in any way may produce 

 elevations of the ribs are inspiratory muscles. 



