THE CIRCULATION OF THE BLOOD. 



283 



heard most distinctly in the neighborhood of the third rib to the right 

 of the sternum. 



The causes of the heart-sounds have enlisted the attention of 

 clinicians' and physiologists for, years, and many factors have been 

 assigned for their production.// At present it is generally believed 

 that the first sound is the product of at least two, possibly three, 

 factors: viz., the contraction of the muscular walls of theyentricles, 

 the simultaneous closure and subsequent vibration of tEe trTcuspid 

 and mitral valves, and the sudden increase of pressure of the apex of 

 the heart against the chest-wall. l| 



That the contraction of the ventricular muscle gives rise to a 

 sound is certain from the fact that it is perceptible in an excised heart 

 when the cavities are free from blood and when the valves are pre- 

 vented from closing. The explanation of this sound is extremely 

 difficult, as the contraction, though pro- 

 longed, is not of the nature of a tetanus 

 and therefore not characterized by rapid 

 variations of tension. The apex element 

 may be eliminated by placing the individual 

 in the recumbent position. 



The second sound is the product of the 

 simultaneous closure and subsequent vibra- 

 tion of the aortic and pulmonary valves 

 which occurs at the beginning of the ven- 

 tricular diastole as the blood surges back 

 against the closed valves. This has been 

 definitely proved by the fact that the sound 

 disappears when the valves are destroyed 

 or held back by hooks introduced into the 

 aorta and pulmonary artery. It is also 

 possible that the vibration of the column of 



blood produces an additional tone which adds itself to that produced 

 by the valves. 



The relation of the sounds to the systole and diastole of the heart 

 is represented in Figs. 124 and 121. 



The Blood-supply to the Heart. The nutrition of the heart, 

 its contractility, the force and frequency of the beat, are dependent 

 on and maintained by the introduction of arterialized blood into and 

 the removal of waste products from its tissue. This is accomplished 

 by the coronary arteries, on the one hand, and the coronary veins, on 

 the other. viThe arteries, two in number, the right and left, arise from 

 the aorta in the pouches of Valsalva just above the right and left 

 semilunar valves. ^Turning in opposite directions, they ultimately 

 anastomose, forming a circle around the base of the ventricles. 

 From both the right and left artery branches are given off which run 



Av 



FIG. 124. SCHEME or A 

 CARDIAC CYCLE. The 

 inner circle shows what 

 events occur in the 

 heart, and the outer, 

 the relation of the 

 sounds and silences to 

 these events. 



