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TEXT-BOOK OF PHYSIOLOGY 



ratus posticus superior. Among the muscles active in extraordinary inspira- 

 tions may be mentioned, in addition to the foregoing, the sterno-cleido- 

 mastoideus, the trapezius, and the pectorales minor and major. 



The vertical diameter is increased by the contraction and descent of the 

 diaphragm, and more especially of its lateral muscular portions. At the 

 end of an expiration the diaphragm is relaxed, and the lower portion closely 

 applied to the walls of the thorax. At the beginning of an inspiration the 

 muscle-fibers contract, 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 

 triangular in shape, termed the complementary pleural space, into which 

 the lateral and posterior portions of the lungs at once descend. The attach- 

 ment of the central tendon of the diaphragm to the pericardium prevents 



any marked descent of this portion 

 except in forcible inspiratory efforts. 

 The vertical diameters are thus en- 

 larged, though unequally in different 

 regions of the thorax. 



As the diaphragm descends it 

 displaces the abdominal viscera, 

 forcing them downward 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. 

 Coincidently with the descent of the diaphragm and the partial removal 

 of the pressure on the under surface of the lung, the intra-pulmonic air ex- 

 pands. As it expands it distends the lungs in the vertical direction, causing 

 them to follow the diaphragm and to occupy the so-called complementary 

 pleural space. With the expansion of the intra-pulmonic air there is a fall 

 in its pressure below the atmospheric pressure, to be followed immediately 

 by an inflow of air until atmospheric pressure is again established. This 

 occurs at the end of the inspiration. 



The antero-posterior and transverse diameters are increased by the eleva- 

 tion and outward rotation of the ribs and an advance of the sternum, both 

 movements made possible by the construction and arrangement of the costo- 

 vertebral and costo-chondral and chondro-sternal articulations. The 

 construction of these articulations is such as to permit at the first a slight 

 elevation and depression of the head of the rib, and at the second a glid- 

 ing of the tubercle 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 horizontal, in the lower part some- 

 what 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 anterior extremity 



FIG. 189. DIAGRAM SHOWING THE POSITION 

 AND SHAPE or THE DIAPHRAGM AT REST a AND 

 DURING INSPIRATION a' AND b. (Boruttau.) 



