RESPIRATION. 



449 



lire may be crudely demonstrated by Hering's device ( Fig. 79). The chamber 

 a represents the thorax ) the rubber bottom B the diaphragm ; c, the opening 

 of the trachea; e d, a tube leading from the thoracic cavity to the manometer 

 I, by means of which intrathoracic pressure is measured ; <; is a vessel contain- 

 ing water, colored blue in imitation of venous blood, communicating bv means 

 of a tube with an oblong flaccid bag F, in imitation of the heart and the intra- 

 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. 79.— Hering'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 n, the valve v preventing a back-flow into G. The degri I" force 



exerted by the traction on the diaphragm is read from the scale <m 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 

 Vol. I.— 29 



