326 RESPIRATION 



in obstructing the current of blood through the lungs, as seen in the con- 

 gestion in the exaggerated condition of straining, this condition being pro- 

 duced by pressure on the entire group of pulmonary vessels. 



There are other mechanical factors, such, for example, as the effect of 

 the abdominal movements, both in inspiration and in expiration, upon the 

 arteries and veins within the abdomen and of the lower extremities. Also 

 the influence of the varying intrathoracic pressure upon the pulmonary 

 vessels, which ought to be taken into consideration. The effect of the 

 abdominal movements during inspiration is twofold. On the one hand, 

 blood is sent upward into the chest by compression of the vena cava inferior; 

 on the other hand, the passage of blood downward from the chest through 

 the abdominal aorta, and upward in the veins of the lower extremity, is to 

 a certain extent obstructed. 



