can still be readily excited by touching the heart with the point of a steel needle. If the heart be now held by its base between the thumb and finger, with its point directed upward, it will be seen to have a pyramidal or conical form, representing very nearly in its outline an equilateral triangle (Fig. 85); its base, while in a condition of rest, bulging out laterally, while the apex is comparatively obtuse.

Fig. 85.

Fig. 86.

HEART OF FROG in a state of relaxation.

HEART OF FROG in contraction.

When the heart, held in this position, is touched with the point of a needle (Fig. 86), it starts up, becomes instantly narrower and longer, its sides approximating and its point rising to an acute angle. This contraction is immediately followed by a relaxation; the point of the heart sinks down, and its sides again bulge outward.

Fig. 87.

Let us now see in what manner this change in the figure of the ventricles during contraction is produced. If the muscular fibres of the heart were arranged in the form of simple loops, running parallel with the axis of the organ, the contraction of these fibres would merely have the effect of diminishing the size of the heart in every direction. This effect can be seen in the accompanying hypothetical diagram (Fig. 87), where the white outline represents such simple looped fibres in a state of relaxation, and the dotted internal line indicates the form which they would take in contraction. In point of fact, however, FIBRES, in relaxation and connone of the muscular fibres of the heart

Diagram of SIMPLE LOOPED

traction.

run parallel to its longitudinal axis. They are disposed, on the contrary, in a direction partly spiral and partly circular. The most superficial fibres start from the base of the ventricles, and

toward the apex, curling round the heart in such a manner as to pass over its anterior surface in an obliquely spiral direction, from above downward, and from right to left. (Fig. 88.) They converge

Fig. 88.

toward the point of the heart, curling round the centre of its apex, and then, changing their direction, become deep-seated, run upward along

BULLOCK'S HEART, anterior view, showing the superficial muscular fibres.

the septum and internal surface of the ventricles, and terminate in the columnæ carneæ, and in the inner border of the auriculoventricular ring. The deeper layers of fibres, on the contrary, are wrapped round the ventricles in a nearly circular direction (Fig. 89); their points of origin and attachment being still the auriculoventricular ring, and the points of the fleshy columns. The entire arrangement of the muscular bundles may be readily seen in a heart which has been boiled for six or eight hours, so as to soften the connecting areolar tissue, and enable the fibrous layers to be easily separated from each other.

By far the greater part of the mass of the fibres have therefore a circular instead of a longitudinal direction. When they contract, their action tends to draw the lateral walls of the ventricles together, and thus to diminish the transverse diameter of the heart; but as each muscular fibre becomes thickened in direct proportion to its contraction, their combined lateral swelling necessarily pushes out the apex of the ventricle, and the heart elongates at the same time. that its sides are drawn together. This effect is illustrated in the accompanying diagram (Fig. 90), where the white lines show the figure of the heart during relaxation, with the course of its circular

Fig. 90.

fibres, while the dotted line shows the narrowed and elongated figure necessarily produced by their contraction. This phenomenon, therefore, of the protrusion of the apex of the heart at the time of contraction, is not only fully established by observation, but is readily explained by the anatomical structure of the organ.

3. Simultaneously with the hardening and elongation of the heart, its apex moves slightly from left to right, and rotates also upon its own axis in the same direction. Both these movements result from the peculiar spiral arrangement of the cardiac fibres. If we refer again to the preceding diagrams, we shall see that, provided the fibres were arranged in simple longitudi- traction. nal loops (Fig. 87), their contraction would

Fig. 91.

merely have the effect of drawing the point of the heart directly upward in a straight line toward its base. On the other hand, if they were arranged altogether in a circular direction (Fig. 90), the apex would be simply protruded forward, also in a direct line, without deviating or twisting either to the right or to the left. But in point of fact, the superficial fibres, as we have already described, run spirally, and curling round the point of the heart, turn inward toward its base; so that if the apex of the organ be viewed externally, it will be seen that the superficial fibres converge toward its central point in curved lines, as in Fig. 91. It is well known that every curved muscular fibre, at the time of its shortening, necessarily approximates more or less to a straight line. Its curvature is diminished in exact proportion to the extent of its contraction; and if arranged in a spiral form, its contraction tends in the same degree to untwist the spiral. During the contraction of the heart, therefore, its apex rotates on its own axis in the direction indicated by the arrows in Fig. 91, viz., from left to right anteriorly, and from right to left posteriorly. This produces a twisting movement of the apex in the above direction, which is

CONVERGING FIBRES OF THE APEX OF THE HEART.

very perceptible to the eye at every pulsation of the heart, when exposed in the living animal.

4. The protrusion of the point of the heart at the time of contraction, together with its rotation upon its axis from left to right, brings the apex of the organ in contact with the parietes of the chest, and produces the shock or impulse of the heart, which is readily perceptible externally, both to the eye and to the touch. In the human subject, when in an erect position, the heart strikes the chest in the fifth intercostal space, midway between the edge of the sternum and a line drawn perpendicularly downward from the left nipple. In a supine position of the body, the heart falls away from the anterior parietes of the chest so much that the impulse may disappear for the time altogether. This alternate recession and advance of the point of the heart, in relaxation and contraction, is provided for by the anatomical arrangement of the pericardium, and the existence of the pericardial fluid. As the heart plays backward and forward, the pericardial fluid constantly follows its movements, receding as the heart advances, and advancing as the heart recedes. It fulfils, in this respect, the same purpose as the synovial fluid, and the folds of adipose tissue in the cavity of the large articulations; and allows the cardiac movements to take place to their full extent without disturbing or injuring in any way the adjacent organs.

5. The rhythm of the heart's pulsations is peculiar and somewhat complicated. Each pulsation is made up of a double series of contractions and relaxations. The two auricles contract together, and afterward the two ventricles; and in each case the contraction is immediately followed by a relaxation. The auricular contraction is short and feeble, and occupies the first part of the time of a pulsation. The ventricular contraction is longer and more powerful, and occupies the latter part of the same period. Following the ventricular contraction there comes a short interval of repose, after which the auricular contraction again recurs. The auricular and ventricular contractions, however, do not alternate so distinctly with each other (like the strokes of the two pistons of a fire engine) as we should be led to believe from the accounts which have been given by some observers. On the contrary, they are connected and continuous. The contraction, which commences at the auricle, is immediately propagated to the ventricle, and runs rapidly from the base of the heart to its apex, very much in the manner of a peris taltic motion, except that it is more sudden and vigorous.

William Harvey, again, gives a better account of this part of the heart's action than has been published by any subsequent writer. The following exceedingly graphic and appropriate description, taken from his book, shows that he derived his knowledge, not from any secondary or hypothetical sources, but from direct and careful study of the phenomena in the living animal.

1

"First of all," he says, "the auricle contracts, and in the course of its contraction throws the blood (which it contains in ample quantity as the head of the veins, the storehouse and cistern of the blood) into the ventricle, which being filled, the heart raises itself straightway, makes all its fibres tense, contracts the ventricles, and performs a beat, by which beat it immediately sends the blood supplied to it by the auricle, into the arteries; the right ventricle sending its charge into the lungs by the vessel which is called vena arteriosa, but which, in structure and function, and all things else, is an artery; the left ventricle sending its charge into the aorta, and through this by the arteries to the body at large.

"These two motions, one of the ventricles, another of the auricles, take place consecutively, but in such a manner that there is a kind of harmony or rhythm preserved between them, the two concurring in such wise that but one motion is apparent, especially in the warmer blooded animals, in which the movements in question are rapid. Nor is this for any other reason than it is in a piece of machinery, in which, though one wheel gives motion to another, yet all the wheels seem to move simultaneously; or in that mechanical contrivance which is adapted to fire-arms, where the trigger being touched, down comes the flint, strikes against the steel, elicits a spark, which falling among the powder, it is ignited, upon which the flame extends, enters the barrel, causes the explosion, propels the ball, and the mark is attained; all of which incidents, by reason of the celerity with which they happen, seem to take place in the twinkling of an eye."

The above description indicates precisely the manner in which the contraction of the ventricle follows successively and yet continuously upon that of the auricle. The entire action of the auricles and ventricles during a pulsation is accordingly as follows: The contraction begins, as we have already stated, at the auricle. Thence it runs immediately forward to the apex of the heart. The entire ventricle contracts vigorously, its walls harden, its apex

'Op. cit., p. 31.