THE HEART 41 



at the time. An examination of this internal pressure not only 

 throws light upon the circulation, but also furnishes a better 

 understanding of the mechanism of the heart itself. 



It is interesting to note that the first experimental work done 

 in this connection was carried out by a French veterinary surgeon, 

 Chauveau, in conjunction with a physicist.. Marey. Their work, 

 for beauty, originality, and exactitude, has only recently been ex- 

 ceeded. Observations were made on the horse by means of an 

 instrument known as the cardiac sound, a diagram of which may 

 be seen in Fig. 17. It is a double tube, having at its extremity two 

 elastic balls separated so that when the apparatus is introduced 

 into the heart, on the right side through the jugular vein, and on 

 the left through the carotid artery, in each case one ball lies in 

 the auricle and one in the ventricle. The air in the apparatus 

 is compressed when the 

 heart's cavities contract, 

 and the compression 

 moves a lever placed in 

 connection with a record- 

 ing surface. The intro- 

 duction of the apparatus 

 causes no pain, and as 



there are no sensory nerves Fig. 17.— Diagram of Cardiac Sound. 



in the lining membranes A Elastic ampulla for auricle . v , for 



of the bloodvessels or ventricle. T, tubes connected with 



heart, its presence gives recording tambours. 



rise to no inconvenience. 



Chauveau, in one of his memoirs, states that the pulse-rate 

 was not disturbed, and the introduction of the instrument 

 did not cause the animal to cease feeding. 



A tracing taken by means of the apparatus just described is seen 

 in Fig. 18. In it may be seen a curve obtained simultaneously 

 from the auricle and ventricle ; the vertical dotted lines indicate 

 coincident periods in both chambers. Taking the auricular 

 curve, there is a sharp, sudden rise, indicating auricular systole, 

 followed by a sudden fall in pressure, and the contents of the 

 chamber are discharged. This is succeeded by two minor rises 

 and falls in pressure before the pause in the heart's cycle D is ■ 

 reached. The curve of intraventricular pressure shows a slight 

 and temporary rise at the moment the auricle reaches its maxi- 

 mum of pressure, and immediately afterwards a sharp, sudden 

 rise in the intraventricular pressure starts. The pressure is main- 

 tained for a short time when once it has reached il^maximum, 

 and the curve is in consequence flattened ; this flattening is 

 called the systolic plateau, and is followed by an abrupt fall the 

 moment the pressure within the ventricles is sufficiently low 



