THE RESPIRATION 



279 



760 





rally becomes less and less ; as they are expanded, greater and 

 greater. Taken in the average condition of expansion in 

 which they exist in the chest, the elasticity of the excised 

 lungs of a man is capable of supporting a column of mercury 

 of about 30 mm. in height, so that they are constantly 

 tending to collapse with this force. 



But the inside of the lungs freely communicates with the 

 atmosphere, and this 

 at the sea level has 

 a pressure of about 

 760 mm. Hg. Dur- 

 ing one part of 

 respiration this pres- 

 sure becomes, a few 

 mm. smaller, during 

 another part a few 

 mm. greater ; but 

 the mean pressure 

 of 760 mm. of mer- 

 cury is constantly 

 expanding the lung, 



and acting against a pressure of only 30 mm. of mercury, 

 tending to collapse the lung. 



Obviously, therefore, the lungs must be kept expanded 

 and in contact with the chest wall. 



When a pleural cavity is opened, the distribution of forces 

 is altered, for now the atmospheric pressure tells also on the 

 outside as well as on the inside of the lung, and acts along 

 with the elasticity of the organ. So that now a force of 760 

 mm. + 30 mm. = 790 mm. act against 760 mm., causing a 

 collapse of the lungs. 



In the surgery of the thorax, as well as in the physiology 

 of respiration, these points are of great importance. 



FIG. 131. Shows the Distribution of Pressure in the 

 Thorax with the chest wall intact, and with an 

 opening into the Pleural Cavity, (j) Indicates 

 the Atmospheric Pressure of 760 mm. of Mercury : 

 30 is the elasticity of the Lungs also in -mm. Hg. 



II. Physiology. 



The process of respiration consists of two parts 

 1st. The passage of air into and out of the air sacs. 

 2nd. The interchange of gases between the air in the air 

 vesicles and the blood in the capillaries. 



