172 RESPIRATION. 



would, probably, be very injuriously charged with carbonic 

 acid ; for the respiratory movements alone are not enough to 

 empty the air-cells, and perhaps expel only the air which lies 

 in the larger bronchial tubes. Probably also the change is 

 assisted by the different temperature of the air within and 

 without the lungs ; and by the action of the cilia on the mu- 

 cous membrane of the bronchial tubes, the continual vibra- 

 tions of which may serve to prevent the adhesion of the air to 

 the moist surface of the membrane. 



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 cap- 

 illaries 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 suc- 

 cessive 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 junction, and incapable of closing the orifice of either 

 of the veins between which they are placed. The pulmonary 

 circulation also is unaffected by changes of atmospheric pres- 

 sure, 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 pulmonary 

 arteries is distributed to these organs to be purified and made 

 fit for the nutrition of all other parts of the body. The capil- 

 laries of the pulmonary vessels are arranged solely with refer- 

 ence 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 they are in intimate connection for 

 another purpose. For the nutrition of the rest of the lungs, 

 including the pleura, interlobular tissue, bronchial tubes and 

 glands, and the walls of the larger bloodvessels, a special 

 supply of arterial blood is furnished through one or two bron- 

 chial arteries, the branches of which ramify in all these parts. 

 The blood of the bronchial artery, when, having served for 

 the nutrition of these parts, it has become venous, is carried 



