I 9 8 



A MANUAL OF PHYSIOLOGY 



aneurism, or by air in the condition known as pneumo- 

 thorax. Between the two pleural sacs lies a mesial space, 

 the mediastinum, commonly divided into an anterior medias- 

 tinum in front of the heart, and a posterior mediastinum 

 behind it. The pleural and pericardial sacs and the medias- 

 tinum constitute together the thoracic cavity. The external 

 surface of the chest-wall and the alveolar surface of the lungs 

 are subjected to the pressure of the atmosphere, to which 



the pressure in the thoracic 

 cavity (intra - thoracic pres- 

 sure) would be exactly equal if 

 its boundaries were perfectly 

 yielding. But in reality the 

 intra - thoracic pressure is 

 always normally something 

 less than this. For even the 

 lungs, the least rigid part of the 

 boundary, oppose a certain 

 resistance to distension, and 



so hold off, as it were, from 

 FIG. 76. SCHEME TO ILLUSTRATE ,i_ ,1 , 



THE MOVEMENTS OF THE LUNGS the thoracic cavity a portion 



of the alveolar pressure ; and 



IN THE CHEST. 



T is a bottle from which the bottom i n any given position of the 



has been removed ; D a flexible and i , 



elastic membrane tied on the bottle, and chest the intra-thoraCIC pres- 



^i 

 to the atmo- 



1S 

 prir 



capable of being pulled out by the string 



S so as to increase the capacity of thf sure 



bottle. L is a thin elastic bag represent- 



ing the lungs. It communicates with the 



external air by a glass tube fitted airtight elastic tension of the lunS. 



through a cork in the neck of the bottle. _. . . . 



When D is drawn down, the pressure of The object of the TCSpira- 



wlerthTstring iTieTgo. I? contracts torv movements is the renewal 



again, in virtue of its elasticity. Q f t h e aif f n con tact with the 



rrp>ccnrf 



af 



alveolar membrane in other 



words, the ventilation of the lungs. Two main methods are 

 followed by sanitary engineers in the ventilation of buildings: 

 they force air in, or they draw it in. In both cases the 

 movement of the air depends on the establishment of a 

 slope of pressure from the inlet to the interior. In the first 

 method, this is done by increasing the pressure at the inlet ; 

 in the second, by diminishing the pressure at the outlet. In 

 certain animals Nature, in solving its problem of ventilation, 



