88 



VALSALVA'S EXPERIMENT. 



blood ; nevertheless, in consequence of the limited action of the auricles, only small 

 pulse-beats are observed. In several individuals Donders found the pulse to be 

 smaller and slower ; afterwards it became larger and faster. 



(3) When the chest is in a position of moderate rest, whereby the elastic traction 

 is moderate = 7 *5 mm. Hg, we have the condition most favourable to the action of the 

 heart sufficient diastolic dilatation of the cavities of the heart, as well as un- 

 hindered emptying of them during systole. 



Voluntary increase or diminution of the intra-thoracic pressure affects the 

 action of the heart. 



(1) Valsalva's Experiment (1740). If the thorax is fixed in the position of 

 deepest inspiration, and the glottis be then closed, and if a powerful expiratory 

 effort be made by bringing into action all the expiratory muscles, so as to contract 



Fig. 65. 

 Apparatus for demonstrating the action of inspiration, II., and expiration, I., on the heart and 

 blood-stream P, p, lungs ; H, h, heart ; L, I, closed glottis ; M, m, manometers ; E, e, 

 ingoing blood-stream, vein ; A, a, outgoing blood-stream, artery ; D, diaphragm during 

 expiration ; d, during inspiration. 



the chest, the .cavities of the heart are so compressed that the circulation of the 

 blood is temporarily interrupted. In this expiratory phase the elastic traction is 

 very limited, and the air in the lungs being under a high pressure also acts upon 

 the heart and the intra-thoracic great vessels. No blood can pass into the thorax 

 from without ; hence the visible veins swell up and become congested, the blood in 

 the lungs is rapidly forced into the left ventricle by the compressed air in the 

 lungs, and the blood soon passes out of the chest, so that the heart and lungs contain 

 little blood, thus leading to a greater supply of blood in the systemic than in the 

 pulmonary circulation and the heart. The heart-sounds disappear, and the pulse 

 is absent {E. H. Weber, Donders). 



