THE RESPIRATORY SYSTEM 151 



of this remain as with the hydrogen, and displaces a 

 similar amount of vitiated air, then the amount of re- 

 newal must be the ratio between the air in the lungs before 

 the inspiration that is, the reserve and the residual air 

 (3200 c.c.), and the fresh air (330 c.c.) remaining in the 

 lungs after each respiration (AW) = 0.103. This has 

 been termed the coefficient of ventilation, and the case 

 given shows that only about one-tenth of the air in the 

 lungs is changed at each breath. The intermixture of this 

 fresh air in the air-passages is due largely to the currents 

 produced by inspiration, but the residual air in the alveoli 

 and smallest air-passages must depend mostly on the jars 

 produced by the heart-beats and the pulsation of the 

 pulmonary arterioles, and also on the ciliary currents in 

 the bronchi, facilitated by the swaying to and fro of the 

 air as the chest alternately expands and contracts. 



It would seem reasonable to suppose, therefore, that the 

 air in the alveoli remains of a nearly uniform percentage 

 of carbon dioxid, and a less percentage of oxygen than the 

 expired air. The direct experiments of Wolfberg and 

 others, however, show there is but little difference in the 

 composition of the alveolar air and that which may be 

 expired by a full expiration. This seems to show that the 

 diffusion in the lungs is very rapid. 



The trachea and bronchi are kept in a distended state, 

 and their cylindric form maintained by the elastic annu- 

 lar cartilages of which the walls of these tubes are con- 

 structed. When in the lungs these cartilages are less 

 apparent, as the tubes ramify and diminish, until we find 

 the walls consisting simply of an elastic but very delicate 

 membrane. 



The accessory organs of respiration are the diaphragm 

 and the intercostal and abdominal muscles; in difficult 

 respiration other muscles of the chest are called into play. 

 These muscles are all under the control of the respiratory 



