RESPIRATION. 183 



MECHANISM OF RESPIRATION. 



Respiration consists of the alternate expansion and contraction of the 

 thorax, by means of which air is drawn into or expelled from the lungs. 

 These acts are called Inspiration and Expiration respectively. 



For the inspiration of air into the lungs it is evident that all that is 

 necessary is such a movement of the side- walls or floor of the chest, or of 

 both, that the capacity of the interior shall be enlarged. By such in- 

 crease of capacity there will be of course a diminution of the pressure of 

 the air in the lungs, and a fresh quantity will enter through the larynx and 

 trachea to equalize the pressure on the inside and outside of the chest. 



For the expiration of air, on the other hand, it is also evident 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 pressures 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. 



AVe 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 in- 

 ternal intercostals; the levatores costarum; and serratus posticus superior. 



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

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

 scending most, and the central tendon remaining comparatively unmoved; 

 while the intercostal and other muscles, by acting at the same time, pre- 

 vent 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 Skele- 

 ton in frontispiece). 



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



