RESPIRATORY SOUNDS. 343 



bid state of*the lung, renders it inapplicable as a gauge of pul- 

 monary disease. 



From the foregoing it appears that the volume of air habitu- 

 ally sojourning in the lungs during natural respiration, or station- 

 ary air, is about 3500 cc. (nearly 220 cubic inches), while the 

 fresh air introduced by each inspiration is only a little over 500 

 cc. (30 cubic inches), or, in other words, about one-seventh of the 

 air in the lungs is changed at each breath. Indeed, the 500 cc. 

 of air is only just sufficient to fill the trachea and larger bron- 

 chial passages, so that the fresh air does not reach the pulmonary 

 alveoli, or directly replace any of the air they contain. The tidal 

 stream is, however, brought into immediate relation with the sta- 

 tionary air, and the thbracic movements cause them to mix me- 

 chanically, so that rapid diffusion takes place in the minute 

 bronchi. Diffusion is also constantly occurring between the air 

 of the small tubes and the terminal sacks, and it alone suffices 

 to maintain the necessary standard of purity in the air of the 

 alveoli. If, during breathing, a harmless gas, such as hydrogen, 

 be inhaled during one inspiration, it requires 6 to 10 respirations 

 to get rid of the impurity from the expired air. From this it has 

 been inferred that this number of respiratory acts would be 

 necessary to render the air in the alveoli quite pure even if no 

 fresh impurities were allowed to enter from the blood. 



RESPIRATORY SOUNDS. 



As the streams of air enter the air passages and lungs they 

 produce sounds which are of the greatest importance to the phy- 

 sician, owing to the manner in which they are altered by disease. 



A sound called " bronchial breathing " is produced in the large 

 bronchi and trachea, and is like the noise of air blowing through 

 a tube. This can normally be heard over the trachea, or at the 

 back between the shoulder blades over the entrance of the large 

 bronchi into the root of the lung. 



Another sound called "vesicular" can be heard all over the 

 chest, being most distinct where the lung is most superficial, and 

 where other sounds are absent, as in the subaxillary region. It 

 is a gentle rustling sound caused by the air passing into the 



