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TEXT-BOOK OF PHYSIOLOGY. 



shorten, and approximate a straight line, whereby not only is the 

 convexity of the diaphragm diminished, but that portion in contact 

 with the thorax is drawn away, thus making a large free space into 

 which the lateral and posterior portions of the lungs at once descend. 

 The attachment of the central tendon of the diaphragm to the peri- 

 cardium prevents any marked descent of this portion except in forci- 

 ble inspiratory efforts (Fig. 168). The vertical diameters are thus 

 enlarged, though unequally in different regions of the thorax. 



As the diaphragm descends it displaces the abdominal viscera, 

 forcing them downward and outward against the abdominal walls, 

 which advance and become more convex. In forcible inspiration 

 the diaphragm, acting from the central tendon as the more fixed 

 point, would draw the lower portion of the 

 thorax inward were this not prevented by 

 the outward pressure of the displaced 

 viscera. 



The antero- posterior and transverse di- 

 ameters are increased by the elevation and 

 outward rotation of the ribs and an advance 

 of the sternum, both movements made pos- 

 sible by the construction and arrangement 

 of the costo-vertebral and costo-chondral 

 and chondro - sternal articulations. The 

 former articulations are two in number, the 

 first being formed by the beveled head of 

 the rib and the bodies of the vertebrae, the 

 second by the tubercle of the rib and the 

 transverse process. The construction of 

 these articulations is such as to permit at 

 the first a slight elevation and depression, 

 and at the second a gliding of the tuber- 

 cle on the transverse process. The axis 



around which the rib rotates practically coincides with the axis of 

 the rib neck, which in the upper part of the thorax is almost hori- 

 zontal, in the lower part somewhat sagittal in direction. Hence 

 when the ribs are elevated the upper part of the thorax increases in its 

 antero-posterior, the lower part in its transverse diameters. At the 

 same time, the lower portion of the sternum is pushed forward and 

 upward by the elevation of the costo-chondral articulation and the 

 widening of its angle. With the elevation of the ribs there goes an 

 eversion or outward rotation which gives an additional increase to 

 the transverse diameters. This elevation and outward rotation of 

 the ribs is the resultant of the cooperation of the muscles previously 

 mentioned with the exception of the diaphragm. 

 VThe scaleni muscles, anticus, medius, and posticus, arise from the 



FIG. 1 68. DIAGRAM SHOW- 

 ING INTERVAL BE- 

 TWEEN THE POSITION 

 OF THE DIAPHRAGM 

 IN EXPIRATION (e, e) 

 AND INSPIRATION (i, i} . 

 THE INCREASE IN 

 CAPACITY is SHOWN 

 BY THE WHITE 

 AREAS. (Yeo.) 



