516 



VETERINARY PHYSIOLOGY 



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. + 80 mm. = 790 mm. acts against 760 mm., 

 causing a collapse of the lung, which comes to occupy a 

 small space posteriorly round the bronchus and pulmonary 

 vessels (fig. 208). 



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

 of respiration, these points are of great importance. 



It is possible that a small opening may not immediately 

 lead to collapse, because the surface tension between the 



1760 



760^ 



760 



1^-^ 



Fig. 208. — 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. 



parietal and pulmonary pleura may be sufficient to overcome 

 the atmospheric pressure. 



II. The Passage of Air into and out of the Lungs. 



This is brought about — 



\st. By the movements of respiration — breathing. 



^nd. By diffusion of gases. 



The air is made to pass into and out of the lungs by 

 alternate insjnration and expiration. 



1. Movements of Respiration. 

 1. Inspiration. — During this act, the thoracic cavity is 

 increased in all directions — lateral, vertical, and antero- 



