RESPIRATION 317 



of the thorax and abdomen can open up the thoracic cage, and may 

 equal -70 mm. Hg. These measurements in themselves are not of much 

 importance, except as a measure of muscular development. 



Intrapulmonic pressures that are intermediate between the two ex- 

 tremes will be acquired in the lower air passages in cases in which there 

 is partial obstruction of the upper respiratory passages, as in bronchitis, 

 spasm of the glottis, diphtheria, etc. During coughing also, the intra- 

 pulmonic pressure may become very high. In this act the thorax is first 

 filled with air by a deep inspiration; the glottis is then closed, and a 

 forced expiration is made. "When a sufficiently high intrapulmonic pres- 

 sure is attained, the glottis opens and the sudden change in pressure 

 causes so forcible a blast of air that the offending foreign substance is 

 frequently carried with it out of the air passages. It is often assumed 

 that during coughing the sudden increase in pressure in the alveoli will 

 tend to cause their walls to rupture. This, however, is not the case. 

 The alveoli do not alone support the increase of pressure ; they merely 

 act as the inner layer, of a practically homogeneous structure cam- 

 posed of lung, pleura and thoracic cage. When the tissues of the lung 

 are partially degenerated or atrophied, as in old people, then it is pos- 

 sible that a rupture may take place, but under ordinary conditions it 

 is not likely to occur. 



Amount of Air in the Lungs 



Measurements of the amount of respired air have recently assumed a 

 considerable interest on account of the various applications which can 

 be made of them in the study of lung conditions. The tidal air is that 

 which enters and leaves the lungs with each respiration (about 500 c.c.) ; 

 the complement al air is that which we can take in over and above an 

 ordinary tidal respiration (about 1500 c.c.) ; and the supplemental air, 

 is that which we can give out after an ordinary tidal expiration (about 

 1500 c.c.). Taking these three together, we have what is known as the 

 vital capacity. It is usually about 3500 c.c., and is represented by the 

 amount of air which we can expel from the lungs after as deep an inspi- 

 ration as possible. The vital capacity is diminished in certain cardiac and 

 pulmonary diseases, (page 330). After all the supplemental air has been 

 expelled, there still remains in the lungs a large volume of air which 

 can not be voluntarily expelled. This is known as the residual air. To 

 measure it in a dead animal it is necessary to clamp the trachea, open 

 the thorax, remove the lungs to a vessel of water, and then allow the air 

 to collect from the opened trachea in an inverted graduated cylinder. 

 One part of the residual air is sometimes called the minimal air; it is 

 represented by that which is not expelled from the lungs of a dead 



