CHAPTER XXXIV. 



THE ORGANS OF EXTERNAL RESPIRATION AND THE 

 RESPIRATORY MOVEMENTS. 



Anatomical Considerations. — Some of the anatomical ar- 

 rangements in the lungs which have an immediate physiological 

 interest may be recalled briefly. The structures of the trachea and 

 bronchi are admirably adapted to their functions as air tubes, in that 

 the walls possess flexibility combined with rigidity. The lining of 

 ciliated epithelium throughout the air passages is of importance, 

 primarily it may be assumed, in removing mucus and foreign 

 material from these passages. The smaller bronchi possess a dis- 

 tinct muscular layer, and, as we shall see, this musculature is under 

 the control of a special set of nerve fibers through whose reflex 

 activity the capacity and resistance of the bronchial system may b<j 

 modified. The smallest bronchioles are expanded into a system of 

 membranous air cells, and in the walls of these thin sacs the capil- 

 laries of the pulmonary artery are distributed. 



Fig. 262o. — The structure of a primary lung lobule as described by Miller: Br., Respiratory 

 bronchiole; D, alveolar duct; A, atrium; s, alveolar air-sacs; c, the alveoli or air-sacs; P, the 

 pleura. 



The finer structure of the lungs as described by Miller* is as follows (see 

 Fig. 262a). The bronchi divide into smaller and smaller branches forming the 



* W. S. Miller, "The American Review of Tuberculosis," 3, 11, 1919. 



