The physical means by which the heat thus generated is rapidly given off, on occasion, to meet the varying conditions of muscular activity, is largely dependent upon the control of the blood supply, in which involuntary muscles, similar to those of the heart, are concerned. In times of great muscular activity, when the production of heat is relatively enormous, the arterioles that supply the surface of the body are rapidly dilated so that a preponderance of blood circulates at the surface of the body, where it may readily radiate its heat into space; the vast system of perspiratory ducts, with which the skin is everywhere supplied, aiding enormously in facilitating this result, through the secretion of a film of perspiration, which in evaporating takes up large quantities of heat.
The flushed, perspiring face of a person who has violently exercised gives a familiar proof of these physiological changes; and the contrary condition, in which the peripheral circulation is restricted, and in which the pores are closed, is equally familiar. Moreover, the same cutaneous mechanism is efficient in affording the organism protection from the changes of external temperature; though the human machine, thanks to the pampering influence of civilization, requires additional protection in the form of clothing.
APPLICATIONS OF MUSCULAR ENERGY
Having thus outlined the conditions under which the muscular machine performs its work, we have now to consider briefly the external mechanisms with the aid of which muscular energy is utilized. Of course, the simplest application of this power, and the one universally employed in the animal world is that in which a direct push or pull is given to the substance, the position of which it is desired to change. We have already pointed out that there is no essential difference between pushing and pulling. The fact receives another illustration in considering the muscular mechanism. We speak of pushing when we propel something away from a body, of pulling when we draw something toward it, yet, as we have just seen, each can be accomplished merely through the contraction of a set of muscles, acting on differently disposed levers. All the bodily activities are reducible to such muscular contractions, and the diversified movements in which the organism constantly indulges are merely due to the large number and elaborate arrangement of the bony levers upon which these muscles are operated.
We may well suppose that the primitive man continued for a long period of time to perform all such labors as he undertook without the aid of any artificial mechanism; that is to say, without having learned to gain any power beyond that which the natural levers of his body provided. A brief observation of the actions of a man performing any piece of manual labor will, however, quickly demonstrate how ingeniously the bodily levers are employed, and how by shifting positions the worker unconsciously makes the most of a given expenditure of energy. By bending the arms and bringing them close to the body, he is able to shorten his levers so that he can lift a much greater weight than he could possibly raise with the arms extended. On the other hand, with the extended arm he can strike a much more powerful blow than with the shorter lever of the flexed arm. But however ingenious the manipulation of the natural levers, a full utilization of muscular energy is possible only when they are supplemented with artificial aids, which constitute primitive pieces of machinery.
These aids are chiefly of three types, namely, inclined planes, friction reducers, and levers. The use of the inclined plane was very early discovered and put into practise in chipped implements, which took the form of the wedge, in such modifications as axes, knives, and spears of metal. All of these implements, it will be observed, consist essentially of inclined planes, adapted for piercing relatively soft tissues of wood or flesh, and hence serving purposes of the greatest practical utility.
The knife-blade is an extremely thin wedge, to be utilized by force of pushing, without any great aid from acquired momentum. The hatchet, on the other hand—and its modification the axe—has its blunter blade fastened to a handle; that the principle of the wedge may be utilized at the long end of a lever and with the momentum of a swinging blow. Ages before anyone could have explained the principle involved in such obscuring terms as that, the implement itself was in use for the same purpose to which it is still applied. Indeed, there is probably no other implement that has played a larger part in the history of human industry. Even in the Rough Stone Age it was in full favor, and the earliest metallurgists produced it in bronze and then in iron. The blade of to-day is made of the best tempered steel, and the handle or helve of hickory is given a slight curve that is an improvement on the straight handle formerly employed; but on the whole it may be said that the axe is a surviving primitive implement that has held its own and demonstrated its utility in every generation since the dawn, not of history only, but of barbarism, perhaps even of savagery.
The saw, consisting essentially of a thin elongated blade, one ragged or toothed edge, is a scarcely less primitive and an equally useful and familiar application of the principle of the inclined plane—though it requires a moment's reflection to see the manner of application. Each tooth, however minute, is an inclined plane, calculated to slide over the tissue of wood or stone or iron even, yet to tear at the tissue with its point, and, with the power of numbers, ultimately wear it away.
THE WHEEL AND AXLE
The primitive friction reducer, which continues in use to the present day unmodified in principle, is the wheel revolving on an axle. Doubtless man had reached a very high state of barbarism before he invented such a wheel. The American Indian, for example, knew no better method than to carry his heavy burdens on his shoulders, or drag them along the ground, with at most a pair of parallel poles or runners to modify the friction; every move representing a very wasteful expenditure of energy. But the pre-historic man of the old world had made the wonderful discovery that a wheel revolving on an axle vastly reduces the friction between a weight and the earth, and thus enables a man or a woman to convey a load that would be far beyond his or her unaided powers. It is well to use both genders in this illustration, since among primitive peoples it is usually the woman who is the bearer of burdens. And indeed to this day one may see the women of Italy and Germany bearing large burdens on their backs and heads, and dragging carts about the streets, quite after the primitive method.