ITS 



HANDBOOK OF PHYSIOLOGY. 



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 vessels and 

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



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 



FIG. 154. Diagram of axes of movement of ribs. 



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 pressure within and without the chest are again equal. In 

 both cases the air passes through the trachea and larynx, whether in en- 

 tering 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. 



