and nervous fluid mix, ferment, and by rarefaction and expansion, swell and blow up the cells, and thereby inflate and distend the muscle, and increase its thickness, while its length is shortned:—this is so perplexed and unreasonable an hypothesis, that I am astonished how men of sense ever came to think of such a doctrine. There is no such nervous fluid to be found, to cause this fermentation, rarefaction, etc; and if there was, expansive force must lengthen as well as thicken, and the muscle could not be shortned in length, and swelled in thickness. The natural action of the fluids upon the solids is, to increase dimensions proportionably every way, that is, in the direction of the axis and conjugate diameter equally. Beside, if there was expansion, circulation must stop. The distention of the vesicles, and the rapid exit of the rarifying fluid could not be at once. The plain account of the matter is then, that muscular motion is performed by the elasticity of the nervous fibrillae, contracting and restoring themselves against the stretching force of the circulating blood. The contraction of the muscle straitens and compresses the bloodvessels, and forces the blood with impetuosity thro' the heart; and this squeezing or propelling force gives the fluid an impetus, that makes it return with violence upon the muscle, in the course of its circulation; then by force and impulse, it stretches the transverse and spiral nervous fibres, and so extends the contracted muscle, that drove it by contraction from itself. Upon this, the bloodvessels having obtained their due extent and capacity, the distending force of the blood of consequence ceases: but the moment it does, the contractive power of the nerves begins to act again, and restores them to a contracted dense state, by a force exactly equal to that which extended them; till the returning propelled blood re-enters the muscle, and stretches it again, as before described. Such are the two wonderful counter-forces that produce the natural involuntary motion of the heart, and carry on the circulation of the blood. You see with your eyes, in the opened live dog, this alternate contraction and extension; and as the stretching power is but a consequence of the contracting power, contraction is the spring of this wonderful action, in which our will or free agency has no concern. And to what shall we ascribe this astonishing operation, this amazing contractive power, so exactly as to time, and so constantly continued on the muscles of natural or necessary motion; till the aequilibrium by some means or other be broken, and the motion is