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. XVII SLEEP An enterprising journalist has recently published the replies of a number of well-known men to an inquiry as to how many hours’ sleep they are in the habit of taking, and what they find to be the best remedy for sleeplessness. Such an inquiry naturally leads on to further thoughts about “Sleep.” What a mysterious, yet sweet and lovable thing it is! How strange it is that we all regularly and gladly abandon ourselves to it! How terrible is the state of those who cannot do so! And then one is led to ask, what is it? and why is it? Do all living things sleep for some part of the twenty-four hours? How does it differ from mere resting, and in what does its virtue consist? Shakespeare has said the most beautiful words that have ever been uttered about sleep, and that because he knew what it was to seek for it in vain— “Methought I heard a voice cry, ‘Sleep no more! Macbeth does murder sleep,’ the innocent sleep; Sleep, that knits up the ravell’d sleave of care, The death of each day’s life, sore labour’s bath, Balm of hurt minds, great Nature’s second course, Chief nourisher in life’s feast.” And again, when the strenuous life of the great Bolingbroke has at last overtaxed his brain, and he can no more find rest and unconsciousness at night, Shakespeare makes him say—