sugar, or fat, or proteids, it has already undergone complete combustion. It can not, like them, unite any further with oxygen and thus supply energy.
And yet it is more essential to life than any of them, for without it the products of waste can not be removed from the tissues, and the vital fires, so to speak, are smothered in their own ash. If we take the excised muscle of a frog and stimulate it to repeated contraction, the contractions become feebler and feebler, until at last they cease altogether. But this is not because the fuel which the muscle contains in itself has been so completely burned up that none of it is left to furnish the requisite energy to the muscle; it is because the chemical processes necessary to the contraction of the muscle are arrested by the accumulation of the products of its own waste. If we wash these out of the muscle by sending through its vessels a weak solution of-common salt, which supplies to it no new material, but which removes these waste products, the contractile power of the muscle will be restored.
This restoration takes place still more quickly and thoroughly if we employ a fluid which will supply oxygen, such as a solution of permanganate of potash, instead of a simple solution of salt, which merely washes out the muscular waste. The muscle is like a fire in the grate, which goes out long before the coal is entirely consumed, on account of the ash which smothers it, and just as we can revive the smoldering embers by supplying them with oxygen by the use of bellows, so the muscle revives more quickly when its supply of oxygen is increased. The quicker the fire burns the sooner will it be choked in ash, and the more rapidly the muscle contracts the sooner will it lose its powers.
The same is the case with the heart. The slowly beating heart of a crocodile will pulsate for a day or more after it has been cut out of the body, but the rapidly pulsating heart of a mammal will very soon cease to beat; and, the more rapidly it has been beating before the animal's death, the sooner will it cease to contract afterward. If the vagi are cut in the living animal so that the cardiac pulsations become excessively rapid, the heart's movement ceases almost as soon as the animal dies; but, if during life the vagi are irritated so as to make the heart contract very slowly indeed, it comes to resemble more nearly the heart of the crocodile, and continues to pulsate for a considerable time after the animal's death. The heart, too, resembles voluntary muscles, inasmuch as, if we wash out of it the products of its own waste, it will continue to beat for a much longer time than if we allow them to accumulate. By simply allowing a saline solution to circulate through the heart of a frog it may be kept beating for many hours longer than if it were left to itself. Both voluntary muscles and involuntary ones, such as the heart, cease to act, almost invariably, not by exhaustion of their energy-yielding substance, but by accumulation of the waste products within them; and muscles, both voluntary and involuntary,
