THE CIRCULATION 291 



sequence or beat. During nearly all this time the semilunar valves are 

 shut and the mitral and tricuspid open. This allows the blood constantly 

 returning to the heart by the veins to pour through the auricles directly, 

 it is likely, into the ventricles. This flow is urged by the suction of the 

 expanding auricles during ventricular systole, and then by the suction of 

 the expanding ventricles themselves. In those beats which occur during 

 inspiration, the enlargement of the thorax helps the expansion of all 

 the heart's chambers, but the influence exerted on the thin-walled 

 auricles is much greater than that on the ventricles whose walls are many 

 times as thick. During expiration, on the contrary, when the highly 

 elastic lungs, etc., are compressing the contents of the chest, the great 

 veins and the auricles share in the compression, auricular systole being 

 aided more than ventricular. 



The various valves of the heart are marvels of mechanical perfection. 

 We do not need to go into the functional details of these organs, however, 

 for there is little about them, unless it be the exact mode of action of the 

 tendinous cords, and papillary muscles, which their detailed anatomy 

 does not at once suggest. When one remembers that these complicated 

 valves act perfectly for the average individual more than a hundred 

 thousand times every day, the wonder is that valvular heart-disease is 

 not more common than it is, and sudden death not much more frequent 

 from their laceration or obstruction. 



The Sounds of the Heart may be heard plainly by the ear placed on any 

 part of the thorax, but most clearly in front on either side of the sternum. 

 In number these sounds are two, separated by a short pause, while a 

 longer pause separates those of one beat from the first sound of the next. 

 If we try to represent these sounds by letters, we may use flub and dup 

 better than any others perhaps (the vowel in the first being long and that 

 in the second very short) , while the interval between them is shorter even 

 than the second sound. 



The first sound has given rise to considerable discussion, especially 

 as to its mechanical cause in the heart. With a pulse-rate of 75, this 

 sound lasts about the same time as does the ventricular systole, 0.3 

 second, and coincides with it in time. Its cause, then, is doubtless, in 

 part at least, the powerful contraction of the three layers of muscle 

 composing the ventricles as they sharply slide over and compress one 

 another and the mass of blood within them. 



Within the ventricles are numerous muscular and tendinous struc- 

 tures extending nearly all through the cavity of the ventricles, and these 

 are crowded together and by their vibrations produce sound as they 

 squeeze the blood out from between them. Careful analysis of the sound 

 with resonators has made obvious two elements: a sort of flapping and 

 string-like tone of higher pitch, and beneath and subduing it a longer 

 noise more like a rumble. The former part probably comes from the 

 closure of the auriculo- ventricular valves, and especially from the vibra- 

 tion of the tendinous cords attached to them and stretched in both direc- 

 tions by the forced closure of the valve-flaps and the contraction of the 



