RESPIRATION. 517 



It will be observed that during quiet respiration iutrapulmonary pressure 

 (pressure within the lungs) oscillates between negative and positive and vice 

 versd, whereas intrathoracic pressure (pressure outside the lungs) is persistently 

 negative, the amount by which it differs from atmospheric pressure becoming 

 greater during inspiration and diminishing to the previous level during expi- 

 ration (p. 505). Under forced expiration, however, when the air-passages are 

 obstructed intrathoracic pressure may become positive. This may be demon- 

 strated in this way: If a manometer be connected with the mediastinum of 

 a cadaver, and the chest be pulled upward in imitation of deep inspiration, 

 intrathoracic pressure will be found to be about 30 millimeters. If now a 

 second manometer be connected with the trachea, and air be forced into the 

 lungs through a tracheal tube, as intrapulmonary pressure rises intrathoracic 

 pressure falls, so that when the former reaches +30 millimeters the intratho- 

 racic negative pressure exerted by the elastic traction of the lungs is counter- 

 balanced and the pressure within and outside the lungs is equal. If intra- 

 pulmonary pressure now rise above this limit, intrathoracic pressure must 

 proportionately become positive. During violent coughing, when the expira- 

 tory blast is obstructed and the muscular effort is powerful, intrapulmonary 

 pressure may rise to -f-80 millimeters or more. 



The intercostal tissues tend to be drawn inward as long as negative intra- 

 thoracic pressure exists, and to be forced outward when there is positive intra- 

 thoracic pressure ; hence during inspiration the traction becomes more marked 

 with the rise of intrathoracic pressure, and during expiration the reverse; 

 while during forced expiration with obstructed air-passages the pressure exerted 

 by the effort of the expiratory muscles, together with the weight of the chest 

 and the elastic reaction of the costal cartilages, etc., may be, as above stated, 

 far more than sufficient to counterbalance the traction exerted by the distended 

 elastic lungs, and thus cause positive intrathoracic pressure. 



The influences exerted by changes in intrathoracic and intrapulmonary 

 pressure upon the circulation are marked and important, and may be so pro- 

 nounced as to cause an obliteration of the pulse. 



Respiratory Sounds. During the respiratory acts characteristic sounds 

 are heard in the lungs. A study of these sounds, however, properly belongs 

 to physical diagnosis. 



The Value of Nasal Breathing 1 . Nasal breathing has a value above 

 breathing through the mouth, inasmuch as the air is warmed and moistened 

 and thus rendered more acceptable to the lungs, more or less of the foreign 

 particles in the air are removed, and noxious odors may be detected. 



B. THE GASES IN THE LUNGS, BLOOD, AND TISSUES. 



Alterations in the Gases in the Lungs. The object of respiratory 

 movements is to renew the air within the lungs, which air is constantly being 

 vitiated, and thus supply O and remove CO 2 and other effete substances. The 

 lungs of the average adult man after quiet expiration contain about 2800 cubic 

 centimeters (170 cubic inches) of air. During quiet respiration there is <m 



