180 ADVANCED PHYSIOLOGY 



ward toward the throat, to be finally expelled from the mouth 

 in the sputum, or to be swallowed. When working or riding 

 where it is very dusty, although one may have cleared the 

 throat of what mucus, saliva, or dust has collected there, 

 after waiting for a time he finds on clearing the throat again, 

 that more dust has accumulated. A service which can hardly 

 be overestimated is thus performed for us by these minute 

 ciliary projections from the cells in the air passages. 



Blood Vessels in the Lungs. A second set of passages in 

 the lungs is that of the blood vessels. We have already 

 noticed that the pulmonary artery from the right ventricle 

 divides, sending a branch into each lung. Each of these 

 branches separates in the lungs into smaller and smaller 

 divisions. These minute vessels finally break up into ex- 

 tremely complex sets of capillary vessels in the walls of the 

 alveoli; Fig. 99. The walls of these air sacs, as well as those 

 of the blood vessels, are extremely thin and the blood, flow- 

 ing in the capillaries, is brought into very close relation with 

 the air. There is, of course, no actual contact, for the blood 

 remains in the blood vessels and the air in the alveoli; but 

 the membranes that separate them are extremely thin, so 

 thin indeed, that they do not form any hindrance to gaseous 

 exchange between the air and the blood in the alveoli. In 

 other words, the gases in the alveoli pass with perfect 

 readiness into the blood, and gases that are dissolved in the 

 blood can with equal ease pass out of the vessels into the 

 alveoli. It is while the blood is flowing through the capillark 

 in the walls of these alveoli that changes, which constitute 

 an important part of the process of respiration, take place. 



The Pleura. On the outside of the lungs is a double fol< 

 of membrane called the pleura; Fig. 73, page 136. To under- 

 stand its relations, imagine a large, thin, flexible bag, com- 

 pletely closed at its top and bottom, to be wrapped around 

 the lungs. One side of the bag would thus be in contact with 



