CH. XXIV.] MECHANISM OF RESPIRATION. 347 



that, by an opposite movement which shall diminish the capacity 

 of the chest, the pressure in the interior will be increased, and 

 air will be expelled, until the pressure within and without the 

 chest are again equal. In both cases the air passes through the 

 trachea and larynx, whether in entering or leaving the lungs, 

 there being no other communication with the exterior of the 

 body ; and the lung, for the same reason, remains, under all the 

 circumstances described, closely in contact with the walls and 

 floor of the chest. To speak of expansion of the chest, is to speak 

 also of expansion of the lung. The movements of the lung are 

 therefore passive, not active, and depend on the changes of shape 

 of the closed cavity in which they are contained. A perforation 

 of the chest-wall would mean that the lung in that side would no 

 longer be of use ; a similar injury on the other side (double 

 pneumothorax) would cause death. If the two layers of the 

 pleura were adherent, those portions of the lung would be expanded 

 most where the movements of the chest are greatest. The 

 existence of the two layers prevents this, and thus the lung is 

 equally expanded throughout. 



Inspiration. The enlargement of the chest in inspiration is 

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

 is an increase in the size of the chest cavity (a) in the vertical, 

 and (Ij) in the lateral and antero-posterior diameters. The 

 muscles engaged in ordinary inspiration are the diaphragm ; 

 the extemal intercostals ; parts of the internal intercostals ; the 

 levatores costarum ; and serratus posticus superior. 



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

 traction 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 quadratus lumborum. The 

 diaphragm is supplied by the phrenic nerves. 



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

 the 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 



