182 



NORMAL HISTOLOGY. 



whole of the air-passages, possess a motion that urges particles 

 lodging in the mucus covering them toward the larynx, whence 

 they are either coughed out or are swallowed. Such solid particles 

 as pass beyond the regions guarded by ciliated epithelium are taken 

 up by leucocytes, which frequently migrate into the alveoli and the 

 air-passages, and are conveyed by them into the lymphatic vessels 

 or glands. Because of this the lymphatics and bronchial lymphatic 

 nodes are apt to be blackened by the deposition of carbon, except 

 in young individuals. The flow of air into the lung is the result of 

 atmospheric pressure, which tends to fill the thoracic cavity when the 



Fig. 158. 



Section of the lung of a dog, killed by ether-narcosis. The lung was hyperaemic at the time 

 of death, and the capillaries retain their blood in the section, o, alveolus in cross-sec- 

 tion, communicating with the infundibulum, b. A portion of the wall of the alveolus is 

 seen, in surface-view, at c. d, e, other alveoli opening into the same infundibulum : /. 

 cross-section of an infundibulum with alveoli opening into it; g, surface-aspect of an 

 alveolar wall, showing capillary plexus filled with red blood-corpuselrv 



chest is expanded through the action of the muscles of respiration. 

 The air is expelled from the lungs when those muscles relax, partly 

 because of the pressure exerted by the thoracic walls, but ehieflv 

 because of the contraction of the elastic fibres in the alveolar walls. 



