INSPIRATION 253 



THE MOVEMENTS OF THE RESPIRATORY MECHANISM. 



Respiratory movement consists of the alternate expansion and contrac- 

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

 the lungs. 



A movement of the side walls or floor of the chest to increase its diameter 

 or length will enlarge the capacity of the interior. By such an increase of 

 capacity there will be of course a diminution of the pressure of the air in the 

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

 to equalize the pressure on the inside and outside of the chest. This move- 

 ment is called inspiration. 



The movement which diminishes the capacity of the chest and increases 

 the pressure in the interior expels air until the pressure within and that without 

 the chest are again equal. This movement is called expiration. In both 

 cases the air passes through the trachea and larynx, whether in entering or 



FIG. 228. Schematic Representation of Diaphragm. In expiration (7), quiet inspiration (//), 

 and deep inspiration (///). (After Schaffer.) 



leaving the lungs, there being no other communication with the exterior of 

 the body. And the lung, for the same reason, remains closely in contact 

 with the walls and floor of the chest under all the circumstances described. 

 To speak of expansion of the chest is to speak also of expansion of the lung, 

 and vice versa. 



Inspiration. The enlargement of the chest during inspiration is 

 due to muscular action, which brings about an increase in the size of the 

 chest cavity through the contraction of the inspiratory muscles, the role 

 played by the lungs being a passive one. The chest cavity is increased in* 

 its three axes, the vertical, lateral, and antero-posterior diameters. The 

 muscles engaged in ordinary inspiration are: the diaphragm, the external inter- 

 costals, and the scaleni and levatores costarum. During forced inspiration 



