The animal contractility is always the same in whatever part of the body it is situated. But there exist in the organic contractility two essential modifications, which would seem to indicate a difference in their nature, though there be only diversity in outward appearances. This difference is sometimes visible, at other times though really existing, it cannot be seen by inspection. The sensible organic contractility may be observed in the heart, in the stomach, intestines, bladder,[31] and other organs. It is exercised upon very considerable quantities of the animal fluids. The insensible organic contractility is that, by virtue of which the excreting tubes re-act upon their respective fluids, the secreting organs upon the blood, which flows into them, the parts where nutrition is performed upon the nutritive juices, and the lymphatics upon the substances which excite their open extremities; upon all these occasions, wherever the fluids are disseminated in small quantities, or are very much divided, this second species of contractility is brought into exercise. A tolerably precise idea may be given of both, by comparing the one with attraction, a power which is exercised upon the great aggregate of matter, and the other with the chemical affinities, the phenomena of which take place in the molecules of different substances. For the purpose of explaining this difference, Barthez has compared the one to the second hand of a watch, which traverses the circumference in a very apparent manner, and the other to the hour hand, which moves also, but whose motion is not distinguishable. The sensible organic contractility nearly answers to the irritability of authors; the insensible organic contractility to what is called tonicity. But these words seem to suppose in the properties, which they indicate a difference of nature, while this difference exists only in appearance. I therefore prefer employing for both a common term. It designs their general character, that of appertaining to the interior life, and their independence with regard to the will. To this term I join an adjective expressive of the particular attribute of each. In fact we should possess a very inaccurate idea of these two modes of action, were we to consider them as proceeding from different principles. The one is but the extreme of the other; they are both connected by insensible gradations. Between the obscure but real contractility, which is necessary to the nutrition of the nails, and the hair, &c. and that which we see in the motions of the stomach, and intestines, there exist innumerable shades of this property, which serve as transitions betwixt its perceptible degrees; such are the motions of the dartos,[32] of the arteries, and of certain parts of the cutaneous organs.