296 



VETERINAEY PHYSIOLOGY 



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. During one part of respiration this pressure 

 becomes a few mm. less, during another part a few mm. 

 more; but the mean pressure of 760 mm. of mercury is 

 constantly expanding the lung, and acting against a pressure 

 of only 30 ILHI. of mercury, tending to collapse the lung. 



760 



FIG. 135. Shows the Distribution of Pressure in the Thorax with the chest wall 

 intact, and with an opening into the Pleural Cavity. ( ^ ) indicates the 

 atmospheric pressure of 760 mm. of mercury ; 30 is the elasticity of the 

 lungs also in mm. of mercury. 



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 lungs and acts aloncr 



o o 



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

 mm. + 30 mm. = 790 mm. acts 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. 



II. Physiology 



The process of external 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. 



