THE MOVEMENTS OF THE RESPIRATORY MECHANISM 275 



these irregular sinuses pass in toward the root of the lung to reach the bron- 

 chial glands. 



Nerves. The nerves of the lung are to be traced from the anterior 

 and posterior pulmonary plexuses, which are formed by branches of the 

 vagus and sympathetic. The nerves follow the course of the blood vessels 

 and bronchi, and many small ganglia are situated in the walls of the latter. 



FIG. 227. Capillary Network of the Pulmonary Blood Vessels in the Human Lung. X 60. 



(Kolliker.) 



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 with- 

 out 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 

 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. 



