given to the elucidation of the curve thus recorded; but, so far, too little agreement has been reached for the subject to be entered upon here.'

J. THE SOUNDS OF THE HEART.

If the ear be applied to the human chest, at or near the place of the apexbeat, the heart's pulsation will be heard as well as felt. This fact was known to Harvey. About two hundred years later than Harvey, in 1819, the French physician Laennec, the inventor of auscultation, made known the fact that each beat of the heart is accompanied not by one but by two separate sounds. He also called attention to their great importance in the diagnosis of the diseases of the heart.3

Relations of the Sounds.-The first sound is heard during the time when the apex-beat is felt; it therefore coincides with the systole of the ventricles. The second sound is much shorter, and follows the first immediately, or, to speak more strictly, after a scarcely appreciable interval. The second sound, therefore, coincides with the earlier part of the diastole of the ventricles. The second sound is followed in its turn by a period of silence, commonly longer considerably than the second sound, which silence lasts till the beginning of the first sound of the next ventricular beat. The period of silence, therefore, coincides with the later, and usually longer, portion of the diastole of the ventricles, and with the systole of the auricles. It is interesting that the great auscultator, Laënnec, offered no explanation of the cause of either sound, while he made and reiterated the incorrect and misleading statement that the second sound coincides with the systole of the auricles. When the heart beats oftener than usual, each beat must be accomplished in a shorter time; and it is found that, during a briefer beat, the period of silence is shortened much more than the period during which the two sounds are audible; which latter period may not be altered appreciably.

Characters of the Sounds.-The first sound is not only comparatively long, but is low-pitched and muffled. The second sound is comparatively short, and is high and clear. The two sounds, therefore, are sharply contrasted in duration, pitch, and quality. A rough notion of the contrasted characters of the sounds may be obtained by pronouncing the meaningless syllables "lubb dup." In other mammals the sounds have substantially the same characters as in man.

Cause of the Second Sound.-Since Laënnec's time, the cause of the second sound has been demonstrated by experiment. The second sound is due to the vibrations caused by the simultaneous closure of the semilunar valves of the pulmonary artery and of the aorta, when the diastole of the ventricles has just begun. This cause was first suggested by the French physician

1 M. von Frey: Die Untersuchung des Pulses, etc., 1892, S. 102; R. Tigerstedt: Lehrbuch der Physiologie des Kreislaufes, Leipzig, 1893, S. 112.

2 Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus, 1628, p. 30; Willis's translation, Bowie's edition, 1889, p. 34.

3 R. T. H. Laennec: De l'auscultation médiate, etc., Paris, 1819.

Rouanet in 1832; not long afterward it was conclusively proven by experiment by the English physician C. J. B. Williams.2

Dr. Williams's experiment was as follows: In a young ass the chest was opened and the heart was exposed. It was ascertained that the second sound was audible through a stethoscope applied to the heart itself. A sharp hook was then passed through the wall of the pulmonary artery, and was so directed as to make the semilunar valve incompetent temporarily. By means of a second hook, the aortic semilunar valve was likewise made incompetent. When both hooks were in position, the heart was auscultated afresh, and the second sound was found to have disappeared, and to be replaced by a hissing The hooks were withdrawn during auscultation, and at the moment of withdrawal the murmur disappeared and the normal second sound recurred. Subsequent clinical and post-mortem observations have shown that the second sound may be altered by disease which cripples the aortic valves.

Causes of the First Sound.-The causes of the first sound have not been proven so clearly by the available evidence, which is partly experimental and partly derived from physical diagnosis followed by post-mortem verification. The first sound, like the second, was ascribed by Rouanet3 to vibrations depending upon valvular closure,—the simultaneous closure of the tricuspid and mitral valves; but the persistence of the sound throughout the whole ventricular systole made this cause less probable than in the case of the second sound. Williams, on the other hand, ascribed the first sound to the contraction of the muscular tissue of the ventricles, an explanation consistent with the muffled quality of the first sound, and with its persistence throughout the systole of the ventricles. It is now believed by many that both of the foregoing explanations are correct, and that the first sound is composite in its origin, and due both to closure of the valves and to muscular contraction. The evidence in favor of these causes is, briefly, as follows:

4

In favor of a valvular element in the first sound, it is maintained: That if the ventricles of a dead heart be suddenly distended with liquid, the mitral and tricuspid valves produce a sound in closing; and that clinical and postmortem observations show that the first sound may be altered by disease which cripples the auriculo-ventricular valves.

In favor of an element in the first sound caused by muscular contraction it is maintained: That in a still living but excised heart, the first sound continues to be heard under circumstances which preclude the closure and vibration of the valves, and leave in operation no conceivable cause for the first sound except muscular contraction. Experiments upon the first sound of the excised heart were reported in 1868 by Ludwig and Dogiel," and were 1 J. Rouanet: Analyse des bruits du cœur, Paris, 1832.

2 C. J. B. Williams: Die Pathologie und Diagnose der Krankheiten der Brust, etc. Nach der dritten, sehr vermehrten Auflage aus dem Englischen übersetzt, Bonn, 1838. (The writer has not seen an English edition.) 3 Loc. cit. 4 Loc. cit.

5 J. Dogiel und C. Ludwig: "Ein neuer Versuch über den ersten Herzton," Berichte über die Verhandlungen der k. sächsischen Gesellschaft der Wissenschaften zu Leipzig, math.-physische Classe, 1868, S. 89.

performed upon the dog as follows: The heart was exposed during artificial respiration, and loose ligatures were placed upon the venæ cavæ, the pulmonary artery, the pulmonary veins, and the aorta. Next, the loose ligatures were tightened in the order above written, during which process the beating heart necessarily pumped itself as free as possible of blood. The vessels were then divided distally to the ligatures, and the heart was excised and suspended in a conical glass vessel containing freshly drawn defibrinated blood, in which the heart was fully immersed without touching the glass at any point. Under these conditions the excised heart might execute as many as thirty beats. The conical glass vessel was supported in a “ringstand." The narrow bottom of the vessel consisted of a thin sheet of indiarubber, with which last was connected the flexible tube and ear-piece of a stethoscope. By means of the latter any sound produced by the beating heart could be heard through the blood and the sheet of rubber. The second sound was not heard; but at each contraction of the ventricles the first sound was heard, not of the same length or loudness as normally, but otherwise unaltered. The conditions of experiment were held to preclude error resulting from adventitious sounds; moreover, the heart before excision had pumped itself free from all but a fraction of the amount of blood required to close the valves, and had been so treated that no more could enter. It was therefore believed to be practically impossible that the sound heard could have its origin at the valves; and no origin remained conceivable other than in the muscular contraction of the ventricular systole. Later experiments, in which the auriculo-ventricular valves have been rendered incompetent by mechanical means, have seemed to confirm the importance of muscular contraction as a cause of the first sound.'

By the use of a stethoscope combined with a peculiar resonator, the German physician Wintrich of Erlangen satisfied himself that he could analyze the first sound upon auscultation, so as to detect in it two components, one higher pitched, which he attributed to the vibration of the auriculo-ventricular valves, and a component of lower pitch, attributed to the muscular contraction of the heart. The other experiments above referred to, however, which sustain muscular contraction as a cause of the first sound, did not reveal a change of pitch following incompetence of the valves, but only a diminution in loudness and duration.

Both the closure of the cuspid valves and the contraction of the muscular tissue of the ventricles are rejected by a recent observer as causes of the first sound, which he ascribes to the opening of the semilunar valves.3

1 L. Krehl: "Ueber den Herzmuskelton," Archiv für Anatomie und Physiologie, Physiologische Abtheilung, 1889, S. 253; A. Kasem-Bek: "Ueber die Entstehung des ersten Herztones," Pflüger's Archiv für die gesammte Physiologie, 1890, Bd. xlvii. S. 53.

2 Wintrich: " Experimentalstudien über Resonanzbewegungen der Membranen,” Sitzungsberichte der phys.-med. Societät zu Erlangen, 1873; Wintrich: "Ceber Causation und Analyse der Herzetöne," Ibid., 1875.

3 R. Quain: "On the Mechanism by which the First Sound of the Heart is Produced,” Proceedings of the Royal Society, vol. Ixi. p. 331.

K. THE FREQUENCY OF THE CARDIAC CYCLES.1

In a healthy full-grown man, resting quietly in the sitting posture, the heart beats on the average about 72 times a minute. In the full-grown woman the average is slightly higher, perhaps 80 to the minute. The heart beats less frequently in tall people than in short ones. The difference between men and women largely depends upon this, but careful observation shows that in the case of men and women of the same stature the heart-beats are slightly more frequent in the women. There is, therefore, a real difference as to the pulse between the sexes. Shortly before and after birth the heart-beats are very frequent, from 120 to 140 to the minute. During childhood and youth, the frequency diminishes gradually, the average falling below 100 to the minute at about the sixth year, and below 80 to the minute at about the eighteenth year. In extreme old age the pulse becomes slightly increased in frequency. It must, however, be borne in mind that there are very wide differences between individuals as to the average frequency of the heart-beats. Pulses of 40 and even fewer strokes to the minute, or, on the other hand, of more than 100 to the minute, are natural to some healthy people.

In every individual the frequency of the pulse varies decidedly, and may vary very greatly, during each twenty-four hours. It is least during sleep, and less in the lying than in the sitting posture. Standing makes the heart beat oftener, the difference being greater between standing and sitting than between sitting and lying. During muscular exercise the pulse-rate is much increased, violent exercise carrying it possibly to 150 or even more. Thermal influences have a marked effect, a hot bath, for instance, heightening the frequency of the pulse and a cold bath diminishing it. The taking of a meal also commonly puts up the frequency. The influence of emotion upon the heart's contractions is well known. It may act either to heighten the rate or to lower it. Finally, the practising physician soon learns that the heart's rate is more easily affected by comparatively slight causes, emotional or otherwise, in women, and especially in children, than in men—a fact of some importance in diagnosis.

The causes of the differences referred to in this section are partly unknown, and partly belong to the subject of the regulation of the circulation.

L. THE RELATIONS IN TIME OF THE MAIN EVENTS OF THE CARDIAC CYCLE.

We have now considered the effects produced by the cardiac pump; its general mode of working; and the actual frequency of its strokes. We must next study certain important details relating to the individual strokes or beats of the ventricles and of the auricles. For this study the basis has already been laid in the sections headed "Causes of the Blood-flow" (p. 77), "Mode of Working of the Pumping Mechanism" (p. 78), "The Cardiac Cycle" (p. 104), and " Use and Importance of the Valves" (p. 108). These sections 1Tigerstedt: Lehrbuch der Physiologie des Kreislaufes, Leipzig, 1893, S. 25-35; Vierordt: Daten und Tabellen zum Gebrauche für Mediciner, 1888, S. 105–109, 259.

should now be read again in the order just given. Details can best be dealt with if we use, instead of the more familiar word "beat," the more technical one "cycle."

The Auricular Cycle; the Ventricular Cycle; the Cardiac Cycle.— Each systole and succeeding diastole of the auricles constitute a regularly recurring pair of events which may truly be spoken of as an "auricular cycle;" and so also it is exact to say that the ventricles have their cycle, consisting of systole and succeeding diastole. As soon, however, as we strive for clearness, we find that the useful phrase "cardiac cycle" is necessarily arbitrary and imperfect. A perusal of the account given on p. 78 of the "Mode of Working of the Pumping Mechanism" shows at once that each auricularcycle, consisting of systole followed by diastole, must begin shortly before the corresponding ventricular cycle begins, and must end shortly before the corresponding ventricular cycle ends. The pumping mechanism is such that the auricular systole is completed just before the ventricular systole begins. The phrase "cardiac cycle" implies a reference to both auricular and ventricular events; if now we assume that the beginning of the auricular systole marks the beginning of the cardiac cycle, this must end either with the end of the auricular diastole or with the end of the ventricular diastole. In the former case the cardiac cycle would coincide with the auricular cycle, but would begin before the end of one ventricular diastole and would end beforethe end of another, thus containing no one complete ventricular diastole. In the second case, the cardiac cycle would contain one complete ventricular diastole and a fraction of another, and would also contain two auricular systoles. The second case is clearly even more objectionable than the first. The cardiac cycle had best be defined as consisting of all the events both auricularand ventricular which occur during one complete auricular cycle. The above discussion deals with a phrase which is a constant stumbling-block to students; and the question may well be asked, Why should the expression "cardiac cycle" not be abolished? The answer is, that this phrase is indispensable in order to accentuate certain important relations of the auricular cycle to the ventricular. During a heart-beat there is a period when the auricles and ventricles are in diastole at the same time. During this period, as we have seen, blood is passing from the veins directly through the auricles into the ventricles, and all the muscular fibres of the heart are resting. This period is therefore called that of "the repose of the whole heart," or the pause." Whenever the heart is not wholly at rest, either auricles or ventricles must be in systole. We see, therefore, that each cardiac cycle must coincide with an auricular systole, the instantly succeeding ventricular systole, and a period of repose of the whole heart; and it is precisely these two systoles and the succeeding universal rest which most engage the attention when the beating heart is looked at in the opened chest. These three phenomena, it will be noted, exactly coincide with one complete auricular cycle, and so do not confuse the definition of the cardiac cycle which has been given already. We see, therefore, that the phrase which seemed at first so