EXTERNAL RESPIRATION AND RESPIRATORY MOVEMENTS. 591 



the actual nature of physiological oxidations that is, the part 

 taken by the oxygen in the chemical reactions of living matter 

 is one of the great problems of nutrition which may need many years 

 for solution. 



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 be 

 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. The great efficiency 

 of this apparatus is evident when one recalls that every one of the 

 infinite number of red corpuscles is exposed separately to the air in 

 the air cells, so that although the time of transit is brief the entire 

 amount of hemoglobin is nearly completely saturated with oxygen. 

 Each lung is enveloped in its own pleural sac. The space between 

 the parietal and the visceral layer of each sac is the so-called 

 pleural cavity, but it must be borne in mind that under all normal 

 conditions this cavity is only potential, that is, the parietal and 

 visceral layers are everywhere in contact with each other. Under 

 pathological or accidental conditions air or exudations may enter 

 this space and form an actual cavity. Along the mid-line of the 

 body and around the roots of the lungs we have the mediastinal 

 spaces lying between the pleural sacs of the two sides, but entirely 

 filled with the various thoracic viscera, such as the heart, aorta and 

 its branches, pulmonary artery and veins, venae cavae, azygos vein, 

 trachea, esophagus, thoracic duct, various nerves, and lymph 

 glands. All these organs, therefore, lie outside the lungs. A 

 schematic view of these relations is represented in Fig. 242. 



