regularly recurring changes of size in the auricles, the ventricles, and the arteries. These changes of size are accompanied by corresponding changes in the form and position of the heart, which are both interesting in themselves and important in relation to the diagnosis of disease. The basis of their study consists in opening the chest and pericardium of an animal, and seeing, touching, and otherwise investigating the beating heart. The changes in the beating heart, moreover, underlie the production of the so-called cardiac impulse, or apex-beat, which is of interest in physical diagnosis.

Observation of the Heart and Vessels in the Open Chest.-The beating heart may be exposed for observation in a mammal by laying it upon its back, performing tracheotomy, and completely dividing the sternum in the median line, beginning at the ensiform cartilage. Artificial respiration is next established, a tube having been tied into the trachea before the chest was opened. The two sides of the chest are now drawn asunder and the pericardium is laid open to expose the heart.

If, in any mammal, the ventricles be lightly taken between the thumb and forefinger, the moment of their systole is revealed by the sudden hardening of the heart produced by it, as the muscular fibres contract and press with force upon the liquid within. On the other hand, the ventricular diastole is marked by such flaccidity of the muscular fibres that very light pressure indents the surface, and causes the finger to sink into it, in spite of care being taken to prevent this. Commonly, therefore, at the systole the thumb and finger are palpably and visibly forced apart, no matter where applied, in spite of the fact that the volume of the ventricles is diminishing. This sinking of the finger or of an instrument into the relaxed wall of the heart has given rise to many errors of observation regarding changes during the beat. The time when the ventricles are hardened beneath the finger coincides with the up-stroke of the arterial pulse near the heart, and, as shown by Harvey, with the time when an intermittent jet of blood springs from a wound of either ventricle. The hardening is proven thus to mark the systole of the ventricles. Those changes of size, form, and position of the exposed heart which accompany the hardening of the ventricles beneath the finger are therefore the changes of the ventricular systole; and the converse changes are those of the ventricular diastole. To interpret all the changes correctly by the eye alone, without the aid of the finger or of the jet of blood, is a task of surpassing difficulty in a rapidly beating heart, as was eloquently set forth by Harvey.2

Changes of Size and Form in the Beating Ventricles.—In a mammal, lying upon its back, with the heart exposed, the ventricles evidently become smaller during their systole. Their girth is everywhere diminished and their length also, the latter much less than the former; indeed the diminution in length is a disputed point. Not merely a change of size, but a

1 Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus, 1628, p. 23; Willis' translation, Bowie's edition, 1889, p. 23.

Op. cit., 1628, p. 20; Willis' translation, Bowie's edition, p. 20.

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change of form is thus produced; the heart becomes a smaller and shorter, but a more pointed cone. The narrowing from side to side is very conspicuous. In the opened chest of a mammal lying on its back this narrowing is accompanied by a change which probably does not occur in the unopened chest, viz., by some increase in the diameter of the heart from breast to back, so that the surface of the ventricles toward the observer becomes more convex (see p. 116). Thus the base of the ventricles, which tended to be roughly elliptical during their relaxation, tends to become circular during their contraction; and the diameter of the circle is greater than the shortest diameter of the ellipse, which latter diameter extends from breast to back. At the same time, the area of the base when circular and contracted is much less than when elliptical and relaxed.1 Naturally, none of these comparisons to mathematical figures makes any pretence to exactness. At the same time that the contracting heart undergoes these changes, the direction of its long axis becomes altered. In animals in which the heart is oblique within the chest, the line from the centre of the base to the apex, that is, the long axis, while it points in general from head to tail, points also toward the breast and to the left. In an animal lying on its back, the ventricles when relaxed in diastole tend to form an oblique cone, the apex having subsided obliquely to the left and toward the tail. As the ventricles harden in their systole, they tend to change from an oblique cone to a right cone; the long axis tends to lie more nearly at right angles to the base; and consequently the apex, unfettered by pericardium or chest-wall, makes a slight sweep obliquely toward the head and to the right, and thus rises up bodily for a little way toward the observer. This movement was graphically called by Harvey the erection of the heart. It is accompanied by a slight twisting of the ventricles about their long axis, in such fashion that the left ventricle turns a little toward the breast, the right ventricle toward the back.

Changes of Position in the Beating Ventricles.-The changes in form imply changes in position. The oblique movement of the long axis implies that in systole the mass of the ventricles sweeps over a little toward the median line and also a little toward the head. The shortening of the long axis implies that either the apex recedes from the breast, or the base of the ventricles recedes from the back, or both. Of these last three possible cases, the second is the one that occurs. The oblique movement of the apex is accompanied by no recession of it; but the auriculo-ventricular furrow and the roots of the aorta and pulmonary artery move away from the spinal column as the injected arteries lengthen and expand, and, as the auricles swell, during the contraction of the ventricles. During their diastole the ventricles are soft; they swell; and changes of form and position occur which are simply converse to those of the systole and have been indicated already in dealing with the latter.

1C. Ludwig: "Ueber den Bau und die Bewegungen der Herzventrikel," Zeitschrift für rationelle Medizin, 1849, vii. S. 189.

2 Op. cit., 1628, p. 22. Translation, 1889, p. 22.

Changes in the Beating Auricles.-Except in small animals, the walls of both the ventricles are so thick that the color of the two is the same and is unchanging, namely, that of their muscular mass; but the walls of the auricles are so thin that their color is affected by that of the blood within, so that the right auricle looks bluish and dark and the left auricle red and bright. During the brief systole of the auricles they are seen to become smaller and paler as blood is expelled from them, while their serrated edges and auricular appendages shrink rapidly away from the observer. The changes of the auricular systole are seen to precede immediately the changes of the systole of the ventricles and to succeed the repose of the whole heart. During the relatively long diastole of the auricles these are seen to swell, whether the ventricles are shrinking in systole or are swelling during the first and greater part of their diastole.

Changes in the Great Veins.—In the venæ cave and pulmonary veins a pulse is visible, more plainly in the former than in the latter, which pulse has the same rhythm as that of the heart's beat. The causes of this pulse are complex. It depends in part upon the rhythmic contraction of muscular fibres in the walls of the veins near the auricles, which must heighten the flow into the latter, and which contraction the auricular systole immediately follows. This venous pulse will be mentioned again in discussing the details of the events of the cycle (see p. 138).

Changes in the Great Arteries. It is interesting to note that even in so large an animal as the calf the pulse of the aorta or of the pulmonary artery can hardly be appreciated by the eye, so far as the increase in girth of either vessel is concerned. The expansion of the artery affects equally all points in its circumference, and being thus distributed, is so slight in proportion to the girth of the vessel that the profile of the latter scarcely seems to change its place. The lengthening of the expanding artery can be more readily seen.

Effects of Opening the Chest.-Such are the changes observed in the heart and vessels when exposed in the opened chest of a mammal lying on its back. The question at once arises, Can these changes be accepted as identical with those which occur in the unopened chest of a quadruped standing upon its feet, or of a man standing erect? It will be most profitable to deal at once with the case of the human subject. What are the possible, indeed probable, differences between the changes in the heart in the unopened upright chest and in the same when opened and supine?

When air is freely admitted to both pleural sacs, all those complex effects upon the circulation are at once abolished which we have seen to be caused by the elasticity of the lungs and the movements of respiration. The artificial respiration will have an effect upon the pulmonary transit of the blood and so upon the circulation; but the details of this effect are not the same as those of natural respiration, and, for our present purpose, may be disregarded.

1 T. Lauder Brunton and F. Fayrer: "Note on Independent Pulsation of the Pulmonary Veins and Vena Cava," Proceedings of the Royal Society, 1876, vol. xxv. p. 174.

What has been abolished is the continual suction, rhythmically increased in inspiration, exerted by the lungs upon the heart and all the vessels within the chest, which suction at all times favors the expansion and resists the contraction of the cavities of the heart and of the vessels. On the opening of both pleural sacs the heart and vessels are exposed to the undiminished and unvarying pressure of the atmosphere. Moreover, the heart has ceased to be packed, as it were, between the pleuræ and lungs to right and left, the spine, the front of the chest-wall, and the diaphragm. From these considerations it follows that the heart must be freer to change its form and position in the opened than in the unopened chest; and that these changes must be more modified by simple gravity in the former case than in the latter. Even in the open chest we have studied these changes only in an animal lying on its back. But if we turn the creature to either side, or place it upright in imitation of the natural human posture, the ventricles of the exposed heart in any case tend to assume, in systole, the same form, which has been compared roughly to a right cone with a circular base. This is the form proper to the hardened structure of branching and connected fibres of which the contracting ventricles consist. But if the exposed ventricles be noted in diastole, it will appear that their form depends very largely upon the effects of gravity upon the exceedingly soft and yielding mass formed by their relaxed fibres. We have seen them, in diastole, to flatten from breast to back, to spread out from side to side, to gravitate toward the tail and to the left. If the animal is laid on its side, they flatten from side to side, they spread out from breast to back, and gravitate to the right or left, as the case may be.1

Probable Changes in the Heart's Form and Position in the Unopened Chest. It is fair to conjecture that the increase of the relaxed ventricles in girth and in length which is seen in the open chest would not be greatly different in the closed chest of a man in the upright posture. But it is probable that the flattening of the exposed heart from breast to back, which is seen in diastole, would not occur if the chest were closed. It is precisely in this direction that the flaccid heart exposed in the supine chest would be flattened unduly by its own weight, when deprived of many of its anatomical supports and of the dilating influence of the lungs. The flattening from breast to back must cause an exaggerated spreading out from side to side and hence an unduly elliptical form of the base, inasmuch as, at the same time, the girth of the ventricles is increasing as they enlarge in their diastole. Conversely, it is probable, both a priori and from experimental evidence, that in the chest, when closed and upright, the diminution in size of the contracting ventricles proceeds more symmetrically; that their girth everywhere diminishes through a diminution of the diameter from breast to back as well as of that from side to

1 J. B. Haycraft: "The Movements of the Heart within the Chest-cavity, and the Cardiogram," The Journal of Physiology, vol. xii., Nos. 5 and 6, December, 1891, p. 448; J. B. Haycraft and D. R. Paterson: "The Changes in Shape and in Position of the Heart during the Cardiac Cycle," The Journal of Physiology, vol. xix., Nos. 5 and 6, May, 1896, p. 496.

side, and not through an exaggerated lessening of the latter and an actual increase of the former. In this case, too, the base would tend to become more circular during the systole by means of a less marked change from the diastolic form.1

It has been said that in systole the ventricles are somewhat shortened in the exposed heart, and probably also in the unopened human chest. In the open chest the apex does not recede at all in virtue of this shortening; on the contrary, the base of the ventricles is seen to move toward the apex, and away, therefore, from the spine. Experiment has proven that the foregoing is true also of the unopened chest. It has been noted already that this movement of the base, which in the upright chest would be a descent, is accompanied by a lengthening of the aorta and pulmonary artery as their distention takes place. Very probably it is the thrust of the lengthened arteries which largely causes the descent of the base of the contracting ventricles, which descent compensates for the shortening of the ventricles and retains the apex in contact with the chest-wall.

The Impulse or Apex-beat.-It must always have been a matter of common knowledge that, in man, a portion of the heart lies so close to the chestwall that, at each beat, the soft parts of that wall may be seen and felt to pulsate over a limited area. This is commonly in the fourth or fifth intercostal space, midway between the left margin of the sternum and a vertical line let fall from the left nipple. A similar pulsation may be observed in other mammals. The protrusion of the chest-wall at the site of this "impulse" or "apexbeat" occurs when the arteries expand, and the up-stroke of their pulse is felt; and the recession of the chest coincides with the shrinking of the arteries away from the finger. The impulse proper, that is the protrusion of the chest-wall, occurs, therefore, at the time of the systole of the ventricles. By far the most important factor of the apex-beat is probably the effort of the hardening ventricles to change the direction of their long axis against the resistance of the chest-wall. A heart severed from the body and bloodless, if laid upon a table, lifts its apex as it hardens in systole and assumes its proper form. If a finger be placed near enough to the rising apex to be struck by it, the same sensation is received as from the impulse.

It is interesting to note that around the point where the soft parts of the chest are protruded by the impulse, they are found to be very slightly drawn in at the time of its occurrence. This drawing-in is called the "negative impulse," and must be caused by the diminution in size of the contracting ventricles. These are air-tight within the chest, and so their forcibly lessened surface must be followed down, in varying degrees, under the pressure of the atmosphere, by the elastic and yielding lungs and by the far less yielding soft parts of the chest-wall.

The apex-beat can be brought to bear in various ways upon a recording lever, and thus be made to inscribe upon the kymograph a rhythmically, fluctuating trace, which is called a cardiogram. Considerable attention has been 1 1 J. B. Haycraft: loc. cit.

Haycraft: loc. cit.