by different observers. The arterial and venous systems, treated as each a single tube, may be compared roughly to two funnels, each having its narrow end at the heart. The very wide and very short single tube of the capillary system may be imagined to connect the wide ends of the two funnels. Equal quantities of blood pass in equal times any two points of the collective blood-path between the left ventricle and the right auricle. Therefore where the blood-path is wide, these quantities must move slowly, and swiftly where the blood-path is narrow. It is owing, then, to the rapid widening of the arterial path that the speed declines, like the pressure, toward the capillaries. It is owing to the huge relative calibre of the path at the capillaries that in them the speed is by far the least while the same volume is passing that passes a point in the narrow aorta in the same time; it is owing to the steady narrowing of the venous path toward the heart that the venous blood is constantly quickening its speed while its pressure is falling. As the calibre of the venous system is greater than that of the arterial, the average speed in the veins is probably less than in the arteries. As the collective calibre of the veins which enter the right auricle is greater than that of the aorta, the blood probably moves into the heart less swiftly than out of it; though of course equal quantities enter and leave it in equal times provided those times are not mere fractions of a beat. In connection with this it is significant that the entrance of blood into the heart takes place during the long auricular diastole, while its exit is limited to the shorter ventricular systole. Time Spent by the Blood in a Systemic Capillary.-The width of the path, then, determines the slow movement of the blood in the areas where it is fulfilling its functions; the narrowness of the path, the swiftness of movement of the blood in leaving and returning to the heart. We have seen (p. 79) that a particle of blood may make the entire round of a dog's circulation in from fifteen to eighteen seconds. If we assume the systemic capillary flow to be at the rate of 0.8 millimeter in one second, the blood would remain about 0.6 of a second in a systemic capillary half a millimeter long. Slow as is the capillary flow, it thus appears that it is none too slow to give time for the uses of the blood to be fulfilled. F. THE FLOW OF BLOOD THROUGH THE LUNGS. The blood moves from the right ventricle to the left auricle under the same general laws as from the left ventricle to the right auricle. Certain differences, however, are apparent, and must be noted. One difference is that the collective friction is less in the pulmonary than in the systemic vessels, and that therefore the resistance to be overcome by each contraction of the right ventricle is less than that opposed to the left ventricle. Accordingly it appears from dissection that the muscular wall of the right ventricle is much thinner than that of the left. No accurate measurements can be made of the normal pressure and speed of the blood in the arteries, capillaries, and veins of the lungs, because they can be reached only by opening the chest and destroying the mechanism of respiration, and thereby disturbing the normal conditions of the pulmonary blood-stream. In the opened chest these cannot be entirely restored by artificial respiration. The thinness of the wall of the pulmonary artery, however, indicates that it has much less pressure to support than that of the aorta, which fact also is indicated by such roughly approximate results as have been obtained with the manometer after opening the chest. As the pulmonary artery and veins lie wholly within the chest, but outside the lungs, their trunks and larger branches all tend to be dilated continuously by the elastic pull of the lungs—a pull which increases at each inspiration. On the other hand, the pulmonary capillaries lie so close to the surface of each lung that they are exposed to the same pressure, practically, as that surface, and the full weight of the atmosphere may act upon them. These conditions all tend to unload the capillaries and the pulmonary veins, but to weaken the unloading of the pulmonary artery. The two effects can hardly balance one another, however. The wall of the pulmonary artery is so much stiffer than that of the vein, that the actual results should be favorable to the flow. The elasticity of the lungs and the contractions of the muscles of inspiration thus lighten, probably, the work of the right ventricle as well as of the left. The right ventricle, however, like the left, can accomplish its work without assistance; for the entire circulation, including, of course, the flow through the lungs, continues after the chest has been opened, if artificial respiration be maintained. G. THE PULSE-VOLUME AND THE WORK DONE BY THE VENTRICLES OF THE HEART. The Cardiac Cycle.-It is assumed that the anatomy of the heart is known to the reader. The general nature and effects of the heart's beat have been sketched already. Each beat has been seen to comprise a number of phenomena, which occur in regular order, and which recur in the same order during each of the succeeding beats. Each beat is therefore a cycle; and the phrase "cardiac cycle" has become a technical expression for "beat," as it conveys, in a word, the idea of a regular order of events. As each of the four chambers of the heart has its own systole and diastole, there are eight events to be studied in connection with each cycle. The systoles of the two auricles, however, are exactly simultaneous, as are their diastoles; and the same is true of the systoles and of the diastoles of the two ventricles. We may, therefore, without confusion, speak of the auricular systole and diastole, and of the ventricular systole and diastole, as of four events, each involving the narrowing or widening of two chambers, a right and a left. The heart of the mammal or bird consists essentially of a pair of pumps, the ventricles, each of which acts alternately as a powerful force-pump and as a very feeble suctionpump. To each ventricle is superadded a contractile appendage, the auricle, through which, and to some extent by the agency of which, blood enters the ventricle. The Pulse-volume.-The central fact of the circulation of the blood is the injection, at intervals, by each ventricle, against a strong resistance, of a charge of blood into its artery, which charge the ventricle has just received out of its veins through its auricle. This quantity must be exactly the same for the two ventricles under normal conditions, or the circulation would soon come to an end by the accumulation of the blood in either the pulmonary or the systemic vessels. The blood ejected from each ventricle during the systole must also be equal in volume to the blood which enters each set of capillaries, the pulmonary or systemic, during that systole and the succeeding diastole of the ventricles, provided the circulation be proceeding uniformly. The quantity just referred to is called the "contraction volume" or "pulse-volume" of the Were it always the same, and could we measure it, we should possess the key to the quantitative study of the circulation. heart. The pulse-volume may vary in the same heart at different times, as is easily shown by opening the chest, causing the conditions of the circulation to change, and noting that under certain conditions the heart during each beat varies in size more than before. This variation of volume is easily possible because the walls of the heart are of muscle, soft and distensible when relaxed. It is probable that at no systole is the ventricle quite emptied; that most of its cavity may become obliterated by the coming together of its walls, but that a space remains, just below the valves and above the papillary muscles, which is not cleared of blood. It is also probable that not only the blood which is ejected at the systole may vary in amount, but also the residual blood which remains in the ventricle at the end of the systole. It is therefore clear that it is useless to attempt the measurement of the pulse-volume by measuring the fluid needed to fill the ventricle, even if the heart be freshly excised from the living body and injected under the normal blood-pressure. Rough approximations to this measurement may, however, be attempted in at least two ways: In the first place, a modification of the stromuhr has been applied successfully to the aorta of the rabbit, between the origins of the coronary arteries and of the innominate. This operation requires that the auricles be clamped temporarily so as to stop the flow of blood into the ventricles, and to permit the aorta in its turn to be clamped and divided between the clamp and the ventricle, without serious bleeding. After the circulation has been re-established, the volume of the blood which passes through the instrument during the experiment, divided by the number of the heart-beats during the same period, gives the pulse-volume. The average result obtained, for the rabbit, 1 F. Hesse: "Beiträge zur Mechanik der Herzbewegung," Archiv für Anatomie und Physiologie (anatomische Abtheilung), 1880, S. 328. C. Sandborg und W. Müller: "Studien über den Mechanismus des Herzens,” Pflüger's Archiv für die gesammte Physiologie, 1880, xxii. S. 408. C.S. Roy and J. G. Adami: "Contributions to the Physiology and Pathology of the Mammalian Heart," Proceedings of the Royal Society of London, 1891-92, i. p. 435. J. E. Johansson und R. Tigerstedt: "Ueber die gegenseitigen Beziehungen des Herzens und der Gefässe;" "Ueber die Herzthätigkeit bei verschieden grossem Wiederstand in den Gefässen," Skandinavisches Archiv für Physiologie, 1891, ii. S. 409. is a volume of blood the weight of which is 0.00027 of the weight of theanimal.1 A second way of attempting to ascertain the pulse-volume is to measure the swelling and the shrinkage of the heart. This is called the "plethysmographic" 2 method. One application of it is as follows: The chest and pericardium of an animal are opened, and the heart is inserted into a brass case full of oil. The opening through which the great vessels pass is made water-tight by mechanical means which do not impede the movement of the blood into and out of the heart. The top of the brass case is prolonged into a tube, the oil in which rises as the heart swells and falls as it shrinks. Upon the oil a light piston moves up and down, and records its movements upon the kymograph. The instrument is called a "cardiometer." 3 The average pulse-volume of the human ventricle has been very variously estimated upon the basis of observations of various kinds made upon mammals of various species. The figures offered range, in round numbers, from 50 to 190 cubic centimeters. If we assume the human pulse-volume to weigh 100 grams, and the blood of a man who weighs 69 kilograms to weigh 5.308 kilograms, or of his body-weight, the pulse-volume will be about of the entire blood, and the entire blood will pass through the heart, from the veius to the arteries, in only fifty-three beats—that is, in less than one minute. The speed with which a man may bleed to death if a great artery be severed is therefore not surprising. The Work done by the Contracting Ventricles.-Uncertain as is this important quantity of the pulse-volume, the estimation of the work done by the heart in maintaining the circulation must be based upon it, and upon the force with which each ventricle ejects the pulse-volume. A small fraction of this force is expended in imparting a certain velocity to the ejected blood; all the rest serves to overcome a number of opposing forces. The force exerted by the muscular contraction is opposed by the weight of the volume ejected, and by the strong arterial pressure, which resists the opening of the semilunar valve and the ejection of the pulse-volume. Moreover, the elasticity of the lungs tends at all times to dilate the ventricles, with a force which is increased at each recurring contraction of the muscles of inspiration. Probably there is also in the wall of the ventricle itself a slight elasticity which must be overcome by the ventricle's own contraction in order that its cavity may be effaced. The strong arterial pressure, with which the reader is already familiar, is by far the greatest of these resisting forces-in fact, is the only one of them. which is not of small importance in the present connection. Are we obliged to measure the force of the systole indirectly? Can we not ascertain it by direct experiment? Manometers of various kinds have been placed in direct communication with the cavities of the ventricles. The fol 1 R. Tigerstedt: "Studien über die Blutvertheilung im Körper." Erste Abhandlung. "Bestimmung der von dem linken Herzen herausgetriebenen Blutmenge," Skandinavisches Archiv für Physiologie, 1891, iii. S. 145. From novoμóc, enlargement. 3 C. S. Roy and J. G. Adami, op. cit. lowing method, among others, has been employed: A tube open at both ends is introduced through the external jugular vein of an animal into the right ventricle, or, with greater difficulty, through the carotid artery into the left ventricle. In neither case is the valve, whether tricuspid or aortic, rendered incompetent during this proceeding, nor need the general mechanism of the heart and vessels be gravely disturbed. If the outer end of the tube be connected with a recording mercurial manometer, a tracing of the pressure within the right or left ventricle may be written upon the kymograph. It is found, however, that the pressure within the heart varies so much and so rapidly that the inert mercurial column will not follow the fluctuations, and that the attempt to learn the mean pressure by this method fails. A valve, however, may be intercalated in the tube between the ventricle and the manometer-a valve so made as to admit fluid freely to the manometer, but to let none out. The manometer will then record, and record not too incorrectly, the maximum pressure within the right or left ventricle during the experiment; in other words, it will record the greatest force exerted during that time by the ventricle in order to do its work. In this way the maximum pressure within the left ventricle of the dog has been found to present such values as 176 and 234 millimeters of mercury, the corresponding maximum pressure in the aorta being 158 and 212 millimeters respectively.2 The maximum pressures obtained from simultaneous observations upon the right and left ventricle of a dog are variously reported. It would perhaps be not far wrong to say that in this animal the pressure in the right ventricle is to that in the left as 1 to 2.6.3 The work done by each ventricle during its systole is found by multiplying the weight of the pulse-volume ejected into the force put forth in ejecting it. That force is equal to the pressure under which the pulse volume is expelled. If we use as a basis of calculation the pressures observed in the dog's heart with the maximum manometer, we may assume as the measure of a given pressure within the contracting human left ventricle 200 millimeters of mercury, and for the human right ventricle 77 millimeters. If for each column of mercury there be substituted the corresponding column of blood, the heights will be 2.567 meters and 0.988 meter respectively. The force exerted by the right or left ventricle upon the pulse-volume might therefore just equal that put forth in lifting it to a height of 0.988 or 2.567 meters. If we assume 100 grams as the weight of a possible pulse-volume ejected by a human ventricle, the work done at each systole of the left ventricle would be 100 × 2.567 = 256.7 grammeters, and at each systole of the right ventricle 100 × 0.988 98.8 grammeters; a grammeter being the work done in raising one gram to the height of one meter. The work of both ventricles together would be 256.7 +98.8 355.5 grammeters. The foregoing estimates are offered not as statements of what does occur, but as very rough indications of what may occur. Even = 1 F. Goltz und J. Gaule: "Ueber die Druckverhältnisse im Innern des Herzens," Archiv für die gesammte Physiologie, 1878, xvii. S. 100. 2 S. de Jager: "Ueber die Saugkraft des Herzens," Pflüger's Archiv s ologie, 1883, S. 504, 505. 3 Goltz und G si |