Animals are dependent on several external agents for the support of existence. The oxygen of the air is necessary for respiration. Animal heat, as will be shown presently, is in a great measure dependent upon it. The external heat is so regulated that animal existence is comfortably supported. Electricity is without doubt an essential element in the living processes; and, indeed, many physiologists have been inclined to refer vital force to the development of electricity by chemical action in the brain. This view has, however, no foundation in experiment beyond that afforded by the appearance of electric currents, when the brain is excited. This proves no more than that the operations of mind develope physical power in the matter with which it is mysteriously connected. The phenomena of the Torpedo and Gymnotus we have already noticed,[264] and there are other creatures which certainly possess the power of secreting and discharging electricity. Galvani’s experiments, and those of Aldini, appear to show--and the more delicate researches of Matteucci have satisfactorily determined--that currents of electricity are always circulating in the animal frame;--that positive electricity is constantly passing from the interior to the exterior of a muscle. Matteucci, by arranging a series of muscles, has formed an electric pile of some energy.[265] These currents have been detected in man, in pigeons, fowls, eels, and frogs. In the human body it is evident a large quantity of electricity exists in a state of equilibrium. Du Bois Raymond has shown that we may by mere muscular motion give rise to electric currents which can be measured by the galvanometer. This, however, may be said of every substance. It is perhaps more easily disturbed in the human system; indeed, the manifestation of sparks from the hair and other parts of the body by friction is not uncommon. Every chemical action, it has been already shown, gives rise to electrical manifestations; and the animal body is a laboratory, beautifully fitted with apparatus, in which nearly every chemical process is going on. It has been proved that acid and alkaline principles are constantly acting upon each other through the tissues of the animal frame; and we have the curious phenomena of endosmose and exosmose in constant effort, and catalysis or _surface force_, operating in a mysterious manner.[266] With the refined physiological questions connected with the phenomena of sensation we cannot deal, nor will any argument be adduced for or against the hypothesis which would refer these phenomena to some extraordinary development of electric force in the brain. The entire subject appears to stand beyond the true limits of science, and every attempt to pass it is invariably found to lead to a confused mysticism, in which the real and the ideal are strangely confounded. Science stops short of the phenomena of vital action.