The circulation will give us a very good idea of this graduated enchainment of the two kinds of organic contractility. The sensible organic contractility presides over this function in the heart and large vessels,[33] by degrees it become less apparent, in proportion as the diameter of the vascular system decreases; and lastly, it is insensible in the capillary tubes, where tonicity only is observed. Should we consider irritability as a property inherent exclusively in the muscles, as being one of the characters by which they are distinguished from other organs, and should we call this property by a name expressive of its peculiar seat in the muscle, we should conceive it, if I mistake not, in a very different way from that in which it naturally exists. It is true, that in this respect the muscles occupy the first rank in the scale of the animal solids; they possess the maximum of the organic contractility; but every living organ acts, as they do, though in a manner less apparent upon the excitant when artificially applied, or on the fluid, which in the natural way is carried to it for the purpose of supplying the matter of secretion, nutrition, exhalation, or absorption. Nothing in consequence is more uncertain than the rule, which is commonly adopted for pronouncing upon the muscularity of any doubtful part; for the rule consists in ascertaining whether such part does or does not contract under the action of stimuli. It is thus, that a muscular tunic is admitted in the arteries, although their organization entirely differs from that of the muscles; it is thus, that the womb is pronounced to be fleshy, however foreign to such structure; it is thus, that a muscular texture is admitted in the dartos, in the iris, and other parts, although no such structure be observable there. The faculty of contracting under the action of irritating substances like that of the sensibility, is unequally distributed among the organs; they enjoy it in different degrees. We do not properly conceive it, if we suppose that it belongs exclusively to some of them. It does not, as some have imagined, possess its peculiar seat in the muscular fibre. Life is the sole condition necessary to all the fibres for enjoying it; their peculiar texture influences the sum only, which they receive of it; it appears that to such an organic texture, is attributed, if I may so express myself, such a dose of contractility; to such another texture, such another dose, and so on; so that to employ the expressions, which I have used in treating on the subject of sensibility (however improper they may be, yet capable alone of rendering my ideas) the differences in the organic contractility of our different parts, consist in the quantity only, and not in the nature of this property: indeed it is with respect to quantity only that this property varies, accordingly as it is considered in the muscles, the ligaments, the nerves, or the bones.