PULMONARY CIRCULATION. 219 



is assisted by the different temperature of the air 

 within and without the lungs ; and by the action of the 

 cilia on the mucous membrane of the bronchial tubes, 

 the continual vibrations of which may serve to prevent 

 the adhesion of the air to the moist surface of the mem- 

 brane. 



Movement of Blood in the Respiratory Organs. 



To be exposed to the air thus alternately moved into and 

 out of the air-cells and minute bronchial tubes, the blood is 

 propelled from the right ventricle through the pulmonary 

 capillaries in steady streams, and slowly enough to permit 

 every minute portion of it to be for a few seconds exposed 

 to the air, with only the thin walls of the capillary vessels 

 and air-cells intervening. The pulmonary circulation is 

 of the simplest kind : for the pulmonary artery branches 

 regularly; its successive branches run in straight lines, 

 and do not anastomose ; the capillary plexus is uniformly 

 spread over the air-cells and intercellular passages ; and 

 the veins derived from it proceed in a course as simple and 

 uniform as that of the arteries, their branches converging 

 but not anastomosing. The veins have no valves, or only 

 small imperfect ones prolonged from their angles of junc- 

 tion, and incapable of closing the orifice of either of the 

 veins between which they are placed. The pulmonary cir- 

 culation also is unaffected by changes of atmospheric 

 pressure, and is not exposed to the influence of the pressure 

 of muscles : the force by which it is accomplished, and the 

 course of the blood are alike simple. 



The blood which is conveyed to the lungs by the pul- 

 monary arteries is distributed to these organs to be purified 

 and made fit for the nutrition of all other parts of the 

 body. The capillaries of the pulmonary vessals are ar- 

 ranged solely with reference to this object, and therefore 

 can have but little to do with the nutrition of the lungs ; 

 or at least, only of those portions of the lungs with which 



