RESPIRA TION. 



557 



thoracic vessels, and finally with the vessel H ; v' and v are valves in imitation 

 of valves in the heart and pulmonary vein and aorta. If now the knob K 

 which is fastened to the centre of the diaphragm be pulled down, rarefaction 

 of the air within the chamber occurs, so that the greater external pressure 

 forces air through the tube c into the two rubber bags (lungs) ; at the same 

 time and for the same reason water is forced from the vessel G into F, which is 

 distended. The diaphragm upon being released is drawn up in part by virtue 

 of its own elasticity and in part by the negative pressure within the chamber. 

 The rubber bags are emptied by their own natural elastic reaction. At the 



FIG. 141. Bering's device to illustrate the influence of respiratory movements upon the circulation. 



same time the distended bag F contracts on its contained fluid, forcing it into 

 the vessel H, the valve v preventing a back-flow into G. The degree of force 

 exerted by the traction on the diaphragm is read from the scale on the man- 

 ometer. 



This simple contrivance teaches us that during the entire phase of inspira- 

 tion there is a condition of progressively increasing negative pressure within 

 the thorax, and that not only is air aspirated into the lungs, but that blood is 

 drawn into the large, flaccid vense cavae ; and that during expiration there is a 

 gradual diminution of negative pressure during which *air is expelled from the 

 lungs and blood from the expanded venae cava3. 



The increased flow into the thoracic cavity during inspiration is favored in 

 its progress through the pulmonary vessels by the attendant dilatation of the 

 lung-capillaries and by the fact that the increased negative pressure aifects the 



