

RESPIRATION. 3J-4I 



tween them. These muscles, though thin, consist of a double 

 layer of fibres, the external and the internal, which pass in 

 inverse directions, one layer from above downwards, the other 

 from behind forwards, from edge to edge of the ribs, crossing 

 each other. Now, the first rib being fixed, the second move- 

 able, but less so than the third, the third less than the fourth, 

 and so on through the whole series ; the contraction of the 

 intercostals must consequently elevate the whole series, as 

 the upper ribs serve as fixed points for the action of the 

 muscles. ) 



6. The chief muscle, however, of respiration, is the 

 diaphragm. By the aid of this, the capacity of the chest is 

 enlarged downwards, as we have seen it to have been up- 

 wards and outwards by that of the ribs. fThe diaphragm, 

 or midriff, is a circular muscle, placed transversely across 

 the trunk, nearly at its centre, dividing the cavity of the 

 thorax from that of the abdomen. It is attached to the in- 

 side of the breast-bone and the cartilages of the false ribs, 

 and is fleshy all around its border, but tendinous towards its 

 centre ; the surface towards the abdomen is concave, and 

 that towards the chest convex. Even when it is not in 

 action, its upper surface forms an arch, the convexity of 

 which is towards the thorax, and reaches as high as the 

 fourth rib. The central, or tendinous portion of the dia- 

 phragm, is attached to the pericardium supporting the heart ; 

 and is nearly, or quite immoveable, in order to afford a fixed 

 point for the action of the muscular fibres, which constitute 

 its sides. Owing to this arrangement, its motions do not 

 interfere with those of the hearty 



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