In general I would observe that it is infinitely better to make the experiments of the contact of the gases and the different fluids upon the muscles, by blowing the first, and injecting the others into the intermuscular texture of a living animal, than by drawing out a muscle, and afterwards plunging it all alive into the one or the other; or than laying a muscle bare, in order to direct upon it the current of a gas, or to moisten it with a fluid, for the purpose of observing the phenomena of the contact. It follows from all that we have just said, 1st, that to answer to the cerebral excitement by contracting, the muscle should be in general in a state determined by the laws of its organization; that out of this state it is not capable of contracting, or at least that it does it feebly and irregularly; 2d, that the contact of different foreign substances produces upon the muscle a very variable effect. Moreover many causes besides those stated above, appear to me also to alter the contractions, by acting directly upon the muscles, such as mercury taken by friction for the venereal disease, the influence of this metal, of copper and of lead, upon those who work in them, the action of cold, that of certain fevers, &c. The muscular tremor arising from these different causes, does not appear to arise from the brain; this organ at least does not most commonly give any sign of affection in this case; I confess however that in these different species of tremors, it is not easy to distinguish that which belongs to the peculiar affection of the muscle, from what arises from that of the nerves; perhaps these are especially affected, but the brain certainly does not participate. _Causes which bring into action Animal Contractility._ We have just seen that in the natural state this property constantly requires three actions, 1st, that of the brain; 2d, that of the nerves; 3d, that of the muscles; that it is from the brain that the principle of motion goes which is propagated by the nerves, and which the muscles receive. But it is necessary that some agent should excite the brain to determine it to exert its influence. In fact, the animal contractility being essentially intermittent in its exercise, so that each time after it has been exerted it is suspended, it is necessary that a new cause should place it again in activity; now this cause acts at first upon the brain in the natural state. I refer to two classes the causes that excite the brain in order to produce animal contractility. In the first is the will, in the second are all the impressions which this organ receives, and which are not under the control of the mind.