Chap. XIII.] KESPIRATION. 169 



the walls of the bronchioles and air-sacs. The force of this 

 first inspiratory effort, spent in opening out and unfolding, as it 

 were, the inner recesses of the lungs, is considerable. In the 

 succeeding expiration, most of the air introduced by the first 

 inspiration remains in the lungs, succeeding breaths unfold the 

 lungs more and more, until finally the air-sacs and bronchioles 

 are all opened up and filled with air. The lungs thus once 

 filled with air are never completely emptied again until after 

 death. 



The air remaining in the lungs after expiration is called the 

 old or stationary air into which fresh air is introduced with 

 every inspiration, the fresh or tidal air, as it is called, giving 

 up its oxygen to, and taking carbon dioxide from, the old or 

 stationary air. Thus the stationary air transacts the business 

 of respiration, receiving, on the one hand, constant supplies of 

 oxygen from the tidal air which it delivers to the blood in the 

 capillaries on the walls of the air-sacs ; and, on the other hand, 

 returning, in exchange to the tidal air, the carbon dioxide it 

 has received from the blood in these capillaries. 



In ordinary respiration the lungs are not distended to their 

 fullest extent, but by more forcible muscular contraction the 

 capacity of the chest can be further enlarged, and a certain 

 additional amount of air will rush into the lungs. This addi- 

 tional amount is often spoken of as complemental air. In 

 laboured breathing the contraction of the respiratory muscles 

 not usually brought into play, such as the muscles of the throat 

 and nostrils, becomes very marked. 



The entry and exit of the air are accompanied by respiratory 

 sounds or murmurs. These murmurs differ as the air passes 

 through the trachea, the larger bronchial tubes, and the bron- 

 cliioles. They are variously modified in lung disease, and are 

 then often spoken of under the name of " rales." 



The effects of respiration upon the air outside the body. — With 

 every inspiration a well-grown man takes into his lungs about 

 thirty cubic inches (492 cubic centimetres) of air. The air 

 he takes in differs from the air he gives out mainly in three 

 particulars : — 



1. Whatever the temperature of the external air, the expired 

 air is nearly as hot as the blood ; namely, of a temperature 

 between 98° and 100° F. (36.7° and 37.8° C). 



