286 HUMAN PHYSIOLOGY 



with fresh air. But we cannot say that the whole 

 volume of the tidal air is thus removed and replaced. 

 We have to make allowance for what is called the 

 dead space. 



This term is applied to the volume of air contained 

 in the nasal passages, the pharynx, the trachea, the 

 bronchi, and the branching bronchial tubes which lead 

 to the microscopic sacs. It amounts to about 10 cubic 

 inches. We are somewhat handicapped by the existence 

 of the dead space, as will now be shown. Suppose that 

 an expiration is just completed. The passages consti- 

 tuting the dead space contain air arrested on its way out 

 from the depths of the lungs. Inspiration begins. All 

 the vitiated air in the dead space must be borne back 

 again into the terminal sacs before any fresh, atmospheric 

 air can enter them. 



Again, at the height of inspiration the dead space is 

 occupied by fresh air. During expiration all this good 

 air must be rejected before the impoverished air from the 

 actual seat of the respiratory exchanges begins to escape 

 from the nostrils. It is easy to see that a man breath T 

 ing through a long rubber tube would be suffocated, 

 because he would merely move back and forth the same 

 air instead of obtaining fresh supplies. We are un- 

 avoidably hindered by the actual dead space, according 

 to the same principle, but the impediment is not a 

 serious one. It will appear that the deeper the breath- 

 ing the smaller the fraction of the total represented 

 by the dead space. On the other hand, when the breath- 

 ing becomes shoal from any cause the fraction correspond- 

 ing to the dead space becomes progressively larger. If 

 the tidal air sinks to 10 cubic inches there is little or no 

 useful surplus over the alternating movement within 

 the passages. 



The Vital Capacity. It is evident that we seldom 

 breathe as much as we can. The ordinary tidal air is 

 perhaps only one-eighth of the total volume which we 

 can take in after a forced expiration or expire after the 



