112 VALSALVA AND MULLEIl's EXPERIMENTS. 



amount of blood. Hence the systole must also be small, which further 

 gives rise to a small pulse-beat. 



(2.) On taking the greatest possible inspiration, and therefore causing 

 the greatest stretching of the elastic tissue of the lungs, the elastic 

 traction of the lungs is, of course, greatest 30 mm. Hg. (Bonders). 

 This force may act so energetically as to interfere with the contraction 

 of the thin-walled atria and appendices, in consequence of which these 

 cavities do not completely empty themselves into the ventricles. The 

 heart is in a state of great distension in diastole, and is filled with 

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

 only small pulse-beats are observed. In several individuals Bonders 

 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. Bonders), we have the condi- 

 tion most favourable to the action of the heart sufficient diastolic 

 dilatation of the cavities of the heart, as well as unhindered emptying 

 of them during systole. 



A very important factor, is the influence exerted upon the action of 

 the heart, by the voluntary increase or diminution of the intra-thoracic 

 'pressure. 



(1.) Valsalva's Experiment. 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 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. 

 Hence the lungs and the heart contain little blood. Hence also there 

 is 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, Bonders). 



(2.) J. Miiller's Experiment. Conversely, if after the deepest pos- 

 sible expiration the glottis be closed, and the chest be now dilated with 

 a great inspiratory effort, the heart is powerfully dilated, the elastic 

 traction of the lungs, and the very attenuated air in these organs act 

 so as to dilate the cavities of the heart in the direction of the lungs. 

 More blood flows into the right heart, and in proportion as the right 

 auricle and ventricle can overcome the traction outwards, the blood- 



