S52 



RESPIRATION 



[CH. XXIV. 



The vertical diameter of the chest is increased by the contraction 

 and consequent descent of the diaphragm ; at rest, the diaphragm is 

 dome-shaped with the convexity upwards ; the central tendon forms 

 a slight depression in the middle of this dome. On contraction the 

 muscular fibres shorten, and so the convexity of the double dome is 

 lessened. The central tendon, which was formerly regarded as 

 remaining fixed, is drawn down a certain distance, but the chief 

 movement is at the sides. For the effective action of this muscle, 

 its attachment to the lower ribs is kept fixed by the contraction of 

 the cjuadratus lumborum. The diaphragm is supplied by the phrenic 

 nerves. 



The increase in the lateral and antero-posterior diameters of the 



FIG. 327. Diagram of axes of movement of ribs. 



chest is effected by the raising of the ribs, the upper ones being fixed 

 by the scaleni. The greater number of the ribs are attached very 

 obliquely to the spine and sternum. 



The elevation of the ribs takes place both in front and at the 

 sides the hinder ends being prevented from performing any upward 

 movement by their attachment to the spine. The movement of the 

 front extremities of the ribs is of necessity accompanied by an upward 

 and forward movement of the sternum to which they are attached, 

 the movement being greater at the lower end than at the upper end 

 of the latter bone. 



The axes of rotation in these movements are two : one correspond- 

 ing with a line drawn through the two articulations which the rib 

 forms with the spine (a, b, fig. 327) ; and the other with a line drawn 



