thought and feeling can be estimated in heat-units; he even asserts that there is no common measure between intelligence and heat; but less timid physiologists are not wanting who reduce every kind of vital manifestation to the strict laws of thermo-dynamics. A few succinct remarks may perhaps show that such physiologists err.
A comparison between the muscular and the nervous systems from the point of view of their connection with heat is a bold one for many reasons. Between nerve and muscle there exists this enormous difference—that the former is endowed with a spontaneity denied to the latter. Muscular fibre never contracts of its own accord; it needs a stimulus—its energy is borrowed. The nerve-cell, on the contrary, has in itself an ever-present, never-exhausted power of action, of which the energy is its peculiar property. Both evidently derive the principle of the activity that marks them from the same external and internal media; but, while the muscle, a mechanical organ, is limited to the obedient transformation of the force assigned to it, under the form of heat, into a measurable amount of work, the nerve, a vital organ, remains impenetrable and inaccessible to our calculations, and exerts its characteristic and sovereign powers in its own way, through a series of operations that escape all estimates of their force and heat. On the part of the muscular system, every thing can be measured; on the part of the nervous system, nothing. Impressions, sensations, affections, thoughts, desires, pleasures, and pains, make up a world withdrawn from the common conditions of determination. That superior force which, ruling all the highest animal activities, decides, suspends, checks, and governs the very transformation of heat into movement; which, asserting its independence within us, call it by what oldest name we may—soul, will, or freedom—remains the most undeniable, though the most mysterious certainty of our consciousness, this force protests against the degradation of cerebral life to mechanism. Such is the conviction, moreover, of Claude Bernard and of Helmholtz.
II.
Independently of the slight and usual variations that heat may present in the same species, and those it exhibits in passing from one zoological group to another, we may consider the changes it undergoes in the same individual, influenced by the various disturbances of the system. Although it remains almost insensible to modifications of the surrounding temperature, it is not the same when the complete equilibrium of the organs is affected. The concord between the different parts of the organism and the functions they discharge is so perfect that the least trouble is reflected among them, and sends disorder everywhere. The nervous system, charged with keeping up harmonious communication between all points of the living being, first takes note of the change befalling, and transmits its abnormal impression into all quarters. It is not the generator, but it is the regulator, of
