THE HEART BEAT. 485 



by simultaneous measurement of the pressure in the aorta, it 

 being evident that the pressure will begin to rise in this latter 

 vessel the moment that the valves open. It is interesting to find 

 that the yielding of the valves to the rising pressure in the ventricle 

 is not indicated on the curve itself by any variation, a fact which 

 indicates that the valves open smoothly, and are not thrown back 

 with a sudden shock. A very characteristic feature of the ventric- 

 ular curve is its flat top, or plateau as it is called. In some cases 

 the plateau slopes more or less upward, in other cases downward, 

 depending, doubtless, on" the respective values of the force of the 

 heart contraction and the aortic tension, for during the whole 

 time of the plateau the semilunar valves are open and the ven- 

 tricle is discharging a column of blood into the aorta. The 

 different features of the ventricular systole as gathered from these 

 pressure curves are expressed by Hiirthle * as follows : 



I. Systole, phase of contraction of the muscle fibers (0 to 3 in Fig. 205, V). 



(a) Period of tension (0 to 1), during which the auriculo- ventricular and 

 semilunar valves are both closed and the heart muscle is squeezing 

 upon the contained blood. This period ends at the opening of the 

 semilunar valves. 



(b) Period of emptying (1 to 3). During this time the heart is empty- 



ing itself into the aorta and the intraventricular pressure remains 

 above aortic pressure. It ends with the cessation of the contrac- 

 tion of the muscle and the beginning of the rapid relaxation. 

 II. Diastole, phase of relaxation and rest of the muscle fibers. 



(a) Period of relaxation from 3 until the 'curve reaches a horizontal. 

 At the beginning of the relaxation the semilunar valves are closed, 

 and from comparison with the aortic curve the instant of the occur- 

 rence of this closure is placed at 4. 



(b) Period of filling. This period begins as soon as the auriculo-ventric- 



ular valves open and the stream of blood, which had been flowing 

 into the auricle throughout the ventricular systole, is permitted to 

 enter the ventricle. During this period of filling the ventricular 

 pressure rises slightly as the heart becomes turgid with blood. This 

 increase of pressure is indicated in most cardiograms by a gradual 

 rise of the curve during this period. It is shown in the curve of 

 Chauveau and Marey, given in Fig. 201. 



The Heart Sounds. An interesting and important feature of 

 the heart beat is the occurrence of the heart sounds. Two sounds 

 are heard, one at the beginning, the other at the end of the ventricu- 

 lar systole. The first sound has a deeper pitch and is longer than 

 the second, and their relative pitch and duration are represented 

 frequently by the syllables lubb-dup. According to Haycraft,f 

 both tones from a musical standpoint fall in the bass clef, and are 

 separated by a musical interval of a minor third. The sounds are 

 readily heard by applying the ear to the thorax over the heart, but 

 for diagnostic purposes the stethoscope is usually employed, and 



* Hiirthle, "Archiv f. d. gesammte Physiologie," 49, 84, 1891. 

 t Journal of Physiology/' 11, 486, 1890. 



