RESPIRATION. 261 



in the nutrition of the pulmonary tissues through which it passes. (6) 

 The branches of the bronchial arteries ramify for nutrition's sake in the 

 walls of the bronchi., of the larger pulmonary vessels, in the interlobular 

 connective tissue, etc. ; the blood of the bronchial vessels being returned 

 chiefly through the bronchial and partly through the pulmonary veins. 



Lymphatics. The lymphatics are arranged in three sets: 1. Irreg- 

 ular lacunae in the walls of the alveoli or air-cells. The lymphatic 

 vessels which lead from these accompany the pulmonary vessels toward 

 the root of the lung. 2. Irregular anastomosing spaces in the walls of 

 the bronchi. 3. Lymph-spaces in the pulmonary pleura. The lym- 

 phatic vessels from all these irregular sinuses pass in toward the root 

 of the lung to reach the bronchial 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. 



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 

 increase 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 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 communication with the ex- 

 terior of the body; and the lung, for the same reason, remain sunder 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 ex- 

 pansion of the lung. 



We have now to consider the means by which the respiratory move- 

 ments are effected. 



Inspiration. The enlargement of the cnest 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 (&) 



