CII. XXIV.] THE RESPIRATORY MECHANISM 351 



sinuses pass in towards the root of the lung to reach the bronchial 

 lymphatic 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 

 vessels and bronchi, and in the walls of the latter many small ganglia 

 are situated. 



The Respiratory Mechanism. 



Eespiration 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 increase of capacity there will be a diminution of the pressure 

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

 larynx and trachea to equalise 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 dimmish 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 com- 

 munication 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 move- 

 ments 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 on 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 in the vertical, and in the 

 lateral and antero-posterior diameters. The muscles engaged in 

 ordinary inspiration are the diaphragm; the external intercostals ; 

 parts of the internal intercostals; the levatores costarum; and ser- 

 ratus posticus superior. 



