The beat of the heart is one of those great and elemental features of man’s life which, in spite of our familiarity with it and its momentary recurrence, never loses its quality of mystery and isolation. The ceaseless accompaniment to our lives which the heart is always beating, like the inexorable stroke of an unseen pendulum, fills even the stoutest and bravest at times with a sense of awe. It seems now and then as though an independent living thing were in our breasts, and when it quickens and struggles, as it were, with its work, or languishes and hesitates in its efforts we have a sense of helpless domination by an existence—a living thing—over whose vagaries we have no control.

The heart of man is no special endowment of the human race, nor even of the higher animals. As I mentioned a few pages back, the oyster and other shell-fish have a heart which keeps time and beats the seconds for their uneventful lives, as does that of man for his more varied career. Not only the molluscs, but the insects, the spiders, the crabs, lobsters, and shrimps, and even the worms, have each a rhythmically beating heart. In all of them the significance of this heart and its beat are the same—it is driving the nourishing, oxygen-carrying blood through the great vessels (arteries), which branch from it like a tree into the living tissues of the body, whence it returns by other vessels (the veins) back to the heart.

In man and the warm-blooded quadrupeds, in birds, reptiles, and fishes, the blood is of a splendid red colour, and the transparent vessels can be easily traced in their graceful ramifications and intricate networks, in consequence of the red blood showing through their walls. The red colour is due to a peculiar body, which can be easily separated from the blood as crystals. It has the special duty of carrying oxygen gas dissolved and attached to it; and of giving up that essential element to cause slow burning or oxydation in all parts of the body whilst taking up fresh supplies of oxygen on its passage through the lungs or the gills. In many of the lower animals (for instance, the oyster) the blood is devoid of this red crystalline substance (which, by the bye, is called hÆmoglobin), and accordingly we cannot easily catch sight either of the heart or the blood-vessels (see, however, Fig. 30). But in shell-fish the blood has a very pale blue tint, and this colour is due to a substance like hÆmoglobin, which also can be crystallised, and is the oxygen-carrier. Some sea-worms have a green substance of a similar nature dissolved in their blood, and one can trace their blood-vessels as a beautiful green network. A good many worms, for instance the common earth-worm and the leeches (a discovery made by Cuvier, and referred to by him on his deathbed), and many sea-worms have deep-red-coloured blood, due to the presence of the same crystalline substance which we find in man’s blood. And even a snail, common in the ponds at Hampstead and such places—the flat coiled snail known as Planorbis—has blood of a fine crimson colour, due to the presence of the same red oxygen-carrier, as an exception to the colourless or pale-blue blood found in most shell-fish. Perhaps if oysters, too, had red blood, there would be a prejudice against eating them in the uncooked condition.

The heart is essentially an enlargement of the great stem or main blood-vessel which, like the trunk of a tree, has branching roots at one end of it and ordinary branches at the other. The trunk branches, and roots of the “heart-tree” are, of course, hollow blood-holding tubes, not solid fibrous structures, as are the woody branches and trunk of a vegetable tree. Further, the finest rootlets and the finest terminal branches in the case of the heart-tree are connected to one another by the network of very fine branches or by great blood-holding cavities, which occupy all parts of the body of an animal. The enlarged part of the trunk of the tree-like system of blood-vessels—the heart—has powerful muscles forming its walls, the fibres disposed so as to surround the contained chamber. When these muscular fibres contract, they squeeze the walls of the chamber together and drive the blood out of it into the forward branches, called “arteries.” It is prevented from going backwards into the hinder branches called “veins” (which we compare to the roots of a tree) by flaps which are so set on the inside of the great vessel at the entrance to those branches that the flaps are made to move out across the space by the backward current, and thus prevent any backward flow, whilst a forward current merely presses them flat against the wall of the vessel, and thus no obstruction to a forward flow is presented. These flaps are called the valves of the heart. The consequence of this arrangement is that whilst blood flows freely into the heart from the veins or hinder (root-like) set of vessels, it is driven by the muscular contraction of the heart—only in one direction—namely, forwards into the arteries. This movement in one direction is helped in some elongated hearts by the contraction of the wall of the heart beginning behind and spreading quickly forward like a wave. The heart of the common earth-worm and of small transparent worms with red blood like it, which are common in the mud of ponds and rivers and can be easily watched with the microscope so that one can see through their glass-like skin what is going on inside them, shows very beautifully this wave of contraction. The heart in these worms is a long contractile vessel which runs the whole length of the body along the back. You can watch the red blood flowing into it through the veins in each ring or segment of the worm’s body—slowly swelling it out—so that it looks like a long red cord. Then, suddenly, there is a movement like a flash in its rapidity, passing from behind forwards! The walls of the red cord-like heart contract so as to drive the blood forward into the arteries, which also are present in every ring of the worm’s body. At the same time you can see the valves, which hang at the entrance of the veins to the heart, swing with a sudden “chuck” and close those vessels against the driven blood. The red cord becomes colourless progressively from behind forwards, owing to the squeezing out of the blood, and by the time the movement has reached the head of the worm, the hinder part of the cord-like heart is beginning slowly to dilate again with the influx of red blood from the veins.

What causes the muscles of the heart to contract at regular intervals? There is no doubt that the “stimulus” which excites the heart muscles to contraction is in these simpler animals merely the tension or strain produced by the presence of a sufficient quantity of blood which has flowed into the heart from the veins. The heart muscle, after its rapid contraction, rests; it has no other rest, no sleep, as have all the other parts of the body. It must rest and take refreshment after each effort. Whilst it rests the blood quietly flows in and dilates the heart’s cavity; then the rested muscular wall of the heart, gently stretched by the recovery after compression of its elastic components, nourished and oxygenated by the blood, is ready for another “stroke,” and again it contracts tightly, emptying its cavity of blood, which is driven into the arteries. So it goes on—effort and rest, effort and rest alternating without cease. Whilst it is the stroke of the heart which causes the blood to flow through the arteries into the finest network of hair-like vessels, what is it that causes the blood to flow on through the collecting veins, to reach the heart, and actually to distend that collapsed cavity after its stroke? It must be remembered that a very low pressure is enough to effect this. In the simplest arrangements of worms and such-like animals, there is probably some pressure transmitted to the blood in the veins by the heart-stroke; but the elasticity of the heart-wall and its necessary tendency to resume its dilated condition after its squeezing by its rings of muscle, is what is chiefly effective in drawing on the blood in the veins into the heart.

In man and the higher animals the whole mechanism of the heart is greatly complicated by the action of the nervous system upon it and upon the contraction or expansion of the blood vessels. In this way the rate of the beat of the heart is affected and brought into relation with the needs of the blood circulation in remote parts of the body. The beat of the heart in the human species is more rapid in children than in adults, and more rapid in women than in men, and it differs in all individuals under differing conditions. Before birth it is 140 per minute, in the first month after birth 130, and gradually diminishes to 90 at nine years of age, and at twenty-one to 70 in man and to 80 in woman. But these figures only represent a general average; there are healthy men whose pulse usually is less than 45 per minute, and there are individuals who, without being invalids, yet have the movement of the heart so liable to increase in rapidity through mental or other excitement, acting by nerves directly on the heart muscle, that the pulse often goes up to 120. In the horse and the ox the pulse or heart beat is 36 to 40 a minute; in the sheep 60 to 80; in the dog 100 to 120; in the rabbit 150; and in small creatures, like mice and moles, 200, and even more! I do not know what is the record for the elephant, but as it seems that the larger the mammal the slower the pulse, one would not expect more than 20 to 25 beats a minute in his case.

It is easy to watch the beating of the heart of a flea or other small insects—under the microscope—since the skin is sufficiently transparent. It is not usually much more rapid than in man, but in the very transparent little fresh-water shrimps which are called water-fleas (Entomostraca) I have seen the heart beating so rapidly that I could not count its rate. The heart in insects and shrimps and their like is remarkable for the fact that whilst it pumps out blood through arteries both in front and behind, it has no actual veins opening into it. All the veins, which in their ancestors entered the heart in a row on each side of it, have united, and their walls broken down, so that the heart lies in a sac full of venous blood from which it draws its fill, when it dilates, through a series of valve-bearing openings on its surface, openings which, in an earlier stage of development, were connected with individual veins.

The heart of the Ascidians or sea-squirts, common sac-like marine creatures of most varied form, size, and colour, is perhaps the most extraordinary in the whole animal series. I have often watched it in transparent individuals of this group. It is an oblong sac with branching vessels at either end. It beats for some thirty or forty strokes so as to drive the blood forwards; it then pauses, and the onlooker is astounded to see the wave of movement changed, and the heart steadily beating the same number of strokes in the reversed direction. What were arteries become veins, and the veins become arteries. Then again there is a pause—which seems like a moment of hesitation and doubt—and the original direction of movement is resumed; then again there is a pause and a reversal, and so on, with absolute regularity. It is still a matter for investigation as to why and how this altogether exceptional alternating reversal of the heart’s action is brought about.

It is a curious fact in illustration of the essential character of the heart and its beat that “hearts” are produced in some animals by dilatation of the lymph-vessels—a system of delicate vessels, difficult to see, which take up the colourless fluid which the blood-vessels exude into the tissues and return it to the heart. The eel has a pair of these “lymph-hearts” in its tail, and the common frog has a pair near the shoulder-blades and another pair at the hips. These sacs have muscular walls, and pulsate rhythmically like the blood-heart, driving on the lymph fluid through the lymph vessels to join the blood-stream.

The simplest thing in the animal world which can claim the name of a heart—or, at any rate, be compared with that organ—is found in those microscopic animalcules which consist of only a single “cell” or corpuscle of living protoplasm. These animalcules may be compared to a single brick or unit of structure, whereas all other animals consist of thousands, or even millions, of such corpuscles or units aggregated and fitted together as are the bricks and planks of a house. In most of these uni-cellular animalcules you may observe with a high-power microscope a little spherical liquid-holding cavity, which slowly enlarges, then bursts at the surface and collapses. After a brief interval it forms again, and again bursts to the exterior. In the “bell-animalcule”—a beautiful active little creature only one-thousandth of an inch in diameter—it may be seen to form, swell, collapse, and re-form as often as twenty times in a minute (see Fig. 41). Soluble colouring matter taken in by the animalcule with food is excreted by the liquid accumulated in and ejected to the exterior by this spherical chamber. It is called the “pulsating” or “contractile” vacuole, and by its rhythmical pulsating movement of dilatation and collapse presents definite points of similarity to the alternately dilating and contracting hearts of higher animals. The entering flow of liquid here, as in the veins and heart of higher animals, is continuous. The rhythm is due, as is the rhythm of the heart, to the alternation of a brief period of activity or contraction, and a brief period of consequent exhaustion, rest, and repair on the part of living contractile substance.