VENTILATION OF AIR PASSAGES. 341 



It is very difficult to obtain an accurate experimental estimate 

 of the variations in the pressure in different parts of the air 

 passages during quiet breathing, because even the most careful 

 attempt to measure the pressure causes an increase, which is still 

 further magnified by the sensitive muscular mechanism of the 

 air passages. 



The variations in pressure occurring in the pulmonary air are 

 greatest in the alveoli, and gradually diminish toward the larger 

 air tubes, so that they disappear at the nasal orifice, where, if no 

 impediment be placed to the course of the air, the pressure will 

 remain very nearly equal to that of the atmosphere. By con- 

 necting one nostril with a manometer, and breathing through 

 the nose with the mouth shut, it can be shown that inspiration 

 causes a negative pressure of about 1 mm. mercury, and expi- 

 ration a positive pressure of 2 to 3 mm.; these results must be 

 divided by two, since by plugging one nostril they shut off half 

 the normal inlet. Forced inspiration and expiration give re- 

 spectively 57 and + 87 mm. 



This great difference depends on the elastic forces against 

 which the inspiratory muscles act in distending the thorax, all 

 of which assist in expiration. 



THE VOLUME OF AIR. 



During ordinary respiration the volume of the inspiratory and 

 expiratory stream of air is surprisingly small when compared 

 with the volume of air sojourning in the lungs. 



After an ordinary expiratory act we can force out a great 

 quantity of air by a voluntary effort ; but even after this is got 

 rid of the lungs are still well filled. Some of this re&idual air, 

 which never leaves the chest during the life of the animal, is 

 pressed out by the elasticity of the lungs when the pleura is 

 opened. But a certain amount of air cannot be removed in any 

 way from the alveoli. Even when the lung is cut out of the 

 chest and divided into pieces, enough air is retained in the air 

 cells to render it buoyant. This fact is relied on by medical 

 jurists as an evidence that an infant has breathed afterbirth and 

 distended the lungs with air, for, except breathing has been well 



