KESPIEA.TION. 179 



We have now to consider the means by which the respiratory move- 

 ments are effected. 



Respiratory Movements. 



A. Inspiration. The enlargement of the chest in inspiration is a 

 muscular act; the effect of the action of the inspiratory muscles being 

 an increase in the size of the chest-cavity (a) in the vertical, and (b) in 

 the lateral and antero-posterior diameters.. The muscles engaged in 

 ordinary inspiration are the diaphragm; the external intercostals; parts 

 of the internal intercostals; the levatores costarum; and serratus posti- 

 cus superior. 



(a. ) The vertical diameter of the chest is increased by the contraction 

 and consequent descent of the diaphragm, the sides of the muscle 

 descending most, and the central tendon remaining comparatively un- 

 moved; while the intercostal and other muscles, by acting at the same 



FIG. 155. Diagram of movement of a rib in inspiration. 



time, prevent the diaphragm, during its contraction, from drawing in 

 the sides of the chest. 



(b.) The increase in the lateral and antero-posterior diameters of the 

 chest is effected by the raising of the ribs, the greater number of which 

 are attached very obliquely to the spine and sternum . (see Figure of 

 Skeleton in frontispiece). 



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 extremi- 

 ties 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 corresponding 

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



