CONTRACTION OF BRONCHI. 2OQ 



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 vessels 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 

 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 blood vessels, a special supply of arterial 

 blood is furnished through one or two bronchial arteries, 

 the branches of which ramify in all these parts. The blood 

 of the bronchial artery, when, having served for the nutri- 

 tion of these parts, it has become venous, is carried partly 

 into the branches of the bronchial vein, and thence to the 

 right auricle, and partly into the small branches of the 

 pulmonary artery, or, more directly, into the pulmonary 



