336 MANUAL OF PHYSIOLOGY. 



30 mm., and when the lungs are in a position of expiration they 

 still tend to contract with a force of 6 mm. mercury. 



PRESSURE DIFFERENCES IN THE AIR. 



The immediate effect of the increase in capacity of the chest is 

 that a pressure difference is established between the interior of 

 the thoracic cavity and the atmosphere. 



The reduction in pressure produced in the lungs and air- 

 passages by inspiratory movements, or the increase of pressure 

 accompanying expiration, is very slight during ordinary quiet 

 breathing with free air-passages. But the least impediment to 

 the entrance or to the exit of the air at once makes the difference 

 very notable. 



It is very difficult to obtain an accurate experimental estimate 

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

 sages during quiet breathing, because even the most careful at- 

 tempt 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 towards 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 connect- 

 ing 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 expiration a posi- 

 tive 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 respectively 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. 



