204 RESPIRATION. 



The cells situated nearest to the centre of the lung are 

 smaller, and their networks of capillaries are closer than 

 those nearer to the circumference, in adaptation to the 

 more ready supply of fresh air to the central than the 

 peripheral portion of the lungs. The cells of adjacent 



Fig. 6 1.* 



lobules do not communicate ; and those of the same lobule, 

 or proceeding from the same intercellular passage, do so 

 as a general rule only near angles of bifurcation ; so that, 

 when any bronchial tube is closed or obstructed, the 

 supply of air is lost for all the cells opening into it or its 

 branches. 



Mechanism of Respiration. 



For the proper understanding of the mechanism by 

 which air enters and is expelled from the lungs, the follow- 

 ing facts must be borne in mind : 



The lungs form two distinct hollow bags (communicating 

 with the exterior through the trachea and larynx), and are 

 always closely in contact with the inner surface of the 



* Fig. 61. Capillary net-work of the pulmonary blood-vessels in the 

 human lung (from Kblliker) %. 



