misleading has a real value, and will cease to confuse if its limitations be carefully noted.

The Brevity and Variability of Each Cycle.-From the frequency with which the cycles recur, it follows at once that each one, with its complex changes in the walls, chambers, and valves, is very rapidly performed. If, for instance, the heart beat 72 times in one minute, each cycle occupies only a little more than 0.83 of a second. The brevity of each cycle is both an important physiological fact and a cause of difficulty in studying details. Each cycle, however, necessarily is capable of completion in much less time if the pulse-rate rise; for instance, during exercise. If repeated 144 times a minute instead of 72 times, each cycle would occupy only one-half of its previous time of completion. With a pulse of less than 60, again, each cycle would occupy over one second.

Relative Lengths of the Ventricular Systole and Diastole.-An important question is whether or no there is any fixed relation between the time required for a systole of the ventricles and the time required for a diastole. When the length of the cycle changes from one second to one-half a second, will the length of the systole be diminished by one-half, and that of the diastole also by one-half? Or is a nearly invariable time required for the ventricles to do their work of ejection, while the period of rest and of receiving blood can be greatly shortened, for a while at least? The answer is that, while both systole and diastole may vary in length, the length of the systole is much the less variable, while the diastole is greatly shortened or lengthened according as the heart beats often or seldom.

These facts have been ascertained as follows: A trained observer auscultated the sounds of the human heart during a number of cycles, and, at the instant when he heard the beginning either of the first or of the second sound, made a mark upon the revolving drum of a kymograph by means of a signalling apparatus. Of course, careful account was taken of the time lost between the occurrence of a sound and the recording of it. It was found that the time between the beginning of the first and that of the second sound did not vary to the same degree as the frequency of the beats. Although the interval in question may not be an exact measure of the period of ventricular systole, it is sufficiently near it for the purposes of this observation.

2

A second method depended upon the interpretation of the curve inscribed by a lever pressed upon the skin over a pulsating human artery. Such a curve exhibits two sudden changes of direction, which were taken to indicate approximately the beginning and end of the injection of blood by the ventricle, and, therefore, to afford a rough measure of the duration of its systole. While the interpretation of the curve in question is not wholly settled, it seems, neverthe

1 F. C. Donders: "De Rhythmus der Hartstoonen," Nederlandsch Archief voor Genees- en Natuurkunde, 1865, p. 141.

2 E. Thurston: "The Length of the Systole of the Heart as Estimated from Sphygmographic Tracings," Journal of Anatomy and Physiology, 1876, vol. x. p. 494.

less, to give a fair basis for conclusions as to the present question. The figures resulting from the second method are especially instructive. It was found that, with a pulse of 47 to the minute, the approximate length of the ventricular systole was 0.347 of a second; of the diastole, 0.930 of a second. With a pulse of 128 to the minute, while the systole was only moderately diminished, viz. to 0.256 of a second, the diastole was reduced to 0.213 of a second-an enormous decline.

These results upon the human subject have been confirmed upon animals by experiments in which were registered the movements of a lever laid across the exposed heart; or the fluctuations of the pressures within the ventricles.2

By whatever means investigated, the ventricular systole is found to be shortened with the cycle, and to be lengthened with it; the diastole is shortened or lengthened much more, however. In fact, if the pulse become very frequent, the diastole may be so shortened that the "pause" nearly disappears, and the systole of the auricles follows speedily after the opening of the cuspid valves. This signifies that, for a time, the cardiac muscle can do with very little rest, and that effective means exist for a very rapid "charging" of the ventricular cavity when necessary. For the working period of the ventricle, however, a more uniform time is required. For the average human pulse-rate this time of work is decidedly shorter than the time of rest-viz. about 0.3 of a second for the former as against about 0.5 for the latter.

Lengths of Auricular Events and of the Pause.-The systole of the auricles is very brief, being commonly reckoned at about 0.1 of a second, as the result of various observations.3 At the average pulse-rate, therefore, the auricular systole is only about one-third as long as the ventricular, and the length of the auricular diastole is to that of the ventricular as seven to five. Consequently, a cardiac cycle of 0.8 of a second would comprise an auricular systole of 0.1 of a second; a ventricular systole of 0.3 of a second; and a pause, or repose of the whole heart, of 0.4 of a second-one-half of the cycle.

Practical Application. The observations above described upon the interval between the beginnings of the sounds have a practical bearing upon physical diagnosis; for they show how faulty are the statements often made which assign regular proportions to the lengths of the sounds and the silences of the heart. The length of the "second silence" must be very fluctuating, as it comprises the longer part of the fluctuating ventricular diastole. The length of the first sound and of the very brief first silence together must be very constant, as they nearly coincide with the ventricular systole.

1 N. Baxt: "Die Verkürzung der Systolenzeit durch den Nervus accelerans cordis," Archiv für Anatomie und Physiologie, Physiologische Abtheilung, 1878, S. 122.

* M. von Frey und L. Krehl: "Untersuchungen über den Puls," Archiv für Anatomie und Physiologie, Physiologische Abtheilung, 1890, S. 31. W. T. Porter: "Researches on the Filling of the Heart," Journal of Physiology, 1892, vol. xiii. p. 531.

3

3 H. Vierordt: Daten und Tabellen zum Gebrauche für Mediciner, 1888, S. 105.

M. THE PRESSURES WITHIN THE VENTRICLES.'

We must now approach the study of further details of the working of the ventricular pumps, which details depend for their elucidation upon the measuring and recording of the pressures within the ventricles.

Absolute Range of Pressure within the Ventricles and its Significance. In dealing with the work done by the contracting ventricles (p. 106) we have seen that the mercurial manometer, as used for studying the pressure within the arteries, is quite unable to follow the changes of the intra-ventricular pressure; but that, by the intercalation of a valve, this instrument can be converted into a useful "maximum manometer" for the measuring and recording of the highest pressure occurring within the ventricle during a given time -that is, during a certain number of cycles. It must now be added that by a simple change of valves this same instrument can at any moment be changed into a "minimum manometer." 2 We can thus, by means of the modified mercurial manometer, learn with fair correctness the extreme range of pressure within the ventricles. As instances of the extent of this range, two observations may be cited upon the left ventricle of the dog, the chest not having been opened. In one animal the maximum was found to be 234 millimeters of mercury, the maximum pressure in the aorta being 212 millimeters; and the minimum in the left ventricle was -38 millimeters-that is to say, 38 millimeters less than the pressure of the atmosphere, the minimum pressure in the aorta.

1 The matters connected with the ventricular pressure-curve may best be studied in the following writings, in which citations of other papers may be found: K. Hürthle, in Pfluger's Archiv für die gesammte Physiologie, as follows: "Zur Technik der Untersuchung des Blutdruckes," 1888, Bd. 43, S. 399. "Technische Mittheilungen," 1890, Bd. 47, S. 1. "Ueber den Ursprungsort der sekundären Wellen der Pulscurve," Bd. 47, S. 17. "Technische Mittheilungen," 1891, Bd. 49, S. 29. "Ueber den Zusammenhang zwischen Herzthätigkeit und Pulsform," Bd. 49, S. 51. "Kritik des Lufttransmissionsverfahrens," 1892, Bd. 53, S. 281. “Vergleichende Prüfung der Tonographen von Frey's und Hürthle's,” 1893, Bd. 55, S. 319. J. A. Tschuewsky: "Vergleichende Bestimmung der Angaben des Quecksilber-und des FederManometers in Bezug auf den mittleren Blutdruck," Pflüger's Archiv für die gesammte Physiologie, 1898, Bd. Ixxii. S. 585. "Technische Mittheilungen," Ibid., 1898, Bd. lxxii. S. 566. K. Hürthle : "Orientirungsversuche über die Wirkung des Oxyspartein auf das Herz, Archiv für experimentelle Pathologie und Pharmakologie, 1892, Bd. xxx. S. 141. W. T. Porter: Researches on the Filling of the Heart,” The Journal of Physiology, 1892, vol. xiii. p. 513. "A New Method for the Study of the Intracardiac Pressure Curve," Journal of Experimental Medicine, 1896, vol. i., No. 2. M. von Frey und L. Krehl: " Untersuchungen über den Puls," Archiv für Anatomie und Physiologie, Physiologische Abtheilung, 1890, S. 31. M. von Frey: "Die Untersuchung des Pulses," Berlin, 1892. "Das Plateau des Kammerpulses," Archiv für Anatomie und Physiologie, Physiologische Abtheilung, 1893, S. 1. "Die Ermittlung absoluter Werthe für die Leistung von Pulsschreibern," Archiv für Anatomie und Physiologie, Physiologische Abtheilung, 1893, S. 17. "Zur Theorie der Lufttonographen," Archiv für Anatomie und Physiologie, Physiologische Abtheilung, 1893, S. 204. "Die Erwärmung der Luft in Tonographen," Centralblatt für Phys iologie vom 30 Juni, 1894, Heft 7. O. Frank: "Ein experimentelles Hilfsmittel für Eine Kritik der Kammerdruckcurven," Zeitschrift für Biologie, 1897, Bd. xxxv. S. 478. R. Rubbrecht: "Recherches cardiographiques chez les Oiseaux," Archives de Biologie, 1898, t. xv. p. 647. J. Waroux: "Du tracé myographique due coeur exsangue," Ibid., 1898, t. xv. p. 661.

2F. Goltz und J. Gaule: "Ueber die Druckverhältnisse im Innern des Herzens," Pflüger's Archiv für die gesammte Physiologie, 1878, xvii. S. 100.

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being 120 millimeters. In a second dog the figures were 176 and -30 millimeters for the ventricle, the aortic range being from 158 to 112 millimeters.1 In the right ventricle of the dog such ranges as from 26 to 8 millimeters, from 72 to 25, and various intermediate values, have been noted, both in the unopened and the opened chest. For reasons already stated (p. 103) no trustworthy figures can be given for the pressures in the pulmonary artery; but they can never fail to be less than the highest pressures within the right ventricle.

The range of pressure, therefore, within either ventricle is in sharp contrast to that within the artery which it supplies with blood; for the arterial pressure, although it fluctuates, is at all times far above that of the atmosphere, and is able, as we have seen, to maintain the circulation while the semilunar valve is closed and the ventricular muscle is at rest. On the other hand, the pressure within the ventricle, when at its highest, rises decidedly above the highest arterial pressure, and thus the ventricle can overcome this and other opposing forces, open the valve, and expel the blood. These facts have been stated already. In falling, however, the pressure within the ventricle not only sinks below that in the artery, and so permits the semilunar valve to close, but sweeps downward to a point, it may be, below the pressure of the atmosphere, and, in so doing, falls below the pressure in the auricle, and permits the opening of the auriculo-ventricular valve and the entrance of blood out of the auricle and the veins. As such a great range of pressure occurs in either ventricle of a heart which is repeating its cycles with entire regularity, it is presumable that at every cycle the pressure not only rises above that in the arteries but may sink below that of the atmosphere.

Methods of Recording the Course of the Ventricular Pressure.—It now becomes of interest to ascertain, if possible, not only the range, but the exact course, of these swift variations of pressure; the causes of them, and the effects which accompany them. It is hard to obtain, by the graphic method, a correct curve of the pressure within either ventricle. We have seen that the mercurial manometer is useless for this purpose; and it is very difficult to devise any self-registering manometer which shall truly keep pace with fluctuations at once so great and so rapid. The true form of this pressure-curve, therefore, still is partially in doubt, and is the subject of controversies which largely resolve themselves into contests between rival instruments. following characters are common to the manometers with which the most serious attempts have lately been made to obtain a true and minute record of the fluctuations of pressure, even if great and rapid, within the heart or the vessels (see Fig. 21). As in the case of the mercurial manometer, a cannula, open at the end and charged with a fluid which checks the coagulation of the blood, is tied into a vessel, or, if the heart is under observation, is passed down into it through an opening in a jugular vein or a carotid artery. If the chest

The

1S. de Jager: "Ueber die Saugkraft des Herzens," Pflüger's Archiv für die gesammte Physiologie, 1883, Bd. xxxi. S. 491.

2S. de Jager: Loc. cit., S. 506, 507; Goltz und Gaule: Loc. cit., S. 106.

have been opened, the cannula may also be passed into the heart through a small wound in an auricle or even through the walls of the ventricle itself. The end of the cannula which remains without the animal's body is connected, air-tight, with a rigid tube of small, carefully chosen calibre, and as short as the conditions of the experiment permit. The other end of this tube is not, as in the mercurial manometer, left as an open mouth, but is connected, air-tight, with a very small metallic chamber, which constitutes, practically, a dilated blind extremity of the system formed by the tube and the cannula together. The roof of this small metallic chamber is a highly elastic disk either of thin metal or of india-rubber. Except for this small disk, all parts of the chamber, tube, and cannula are rigid. In the instruments of some observers, the entire cavity of the system formed by the chamber, tube, and cannula is filled with liquid, viz. the solution which checks coagulation. Other observers introduce this

FIG. 21.-Diagram of the elastic manometer: A, auricle; V, ventricle; D, drum of the kymograph, revolving in the direction of the arrow, and covered with smoked paper; L, recording lever in contact with the revolving drum. (The working details of the instrument are suppressed for the sake of clearness.)

liquid only into the portion of the system nearest the blood; the terminal chamber, and most of the rest of the system, containing only air. In every case the blood in the vessel or in the heart is in free communication, through the mouth of the tied-in cannula, with the cavity common to the tubes and to the terminal chamber. At every rise of blood-pressure a little blood enters this cavity, room being made for it by a displacement of liquid or of air, which in turn causes a slight bulging of the elastic disk. At every fall of blood-pressure a little blood mixed with liquid leaves the tubes as the elastic disk recoils. If the disk is of the right elasticity, its rise and fall are directly proportional to the rise and fall of the blood-pressure, and can be used to measure it. With the centre of the disk is connected a delicate lever which rises and falls with the disk. The point of this lever traces upon the revolving drum of the kymograph a curve which records the fluctuations of the disk and therefore those of the blood-pressure. The elastic disk and the contents, together, of such an apparatus possess less inertia than mercury, and therefore follow far more closely rapid fluctuations of pressure. Such instruments may be called "elastic manometers," and are often called "tonographs," i. e. "tension-writers." They are of several forms.