Observe how each organ has its peculiar kind of sensibility; how false an idea we should have of the existence of this property, if we judged of it only from mechanical and chemical agents, and observe particularly how nature accommodates to the uses of each organ its kind of animal sensibility. In phlegmasia of the peculiar muscular texture, the animal sensibility is very often raised to a very great height; the least touch on the skin becomes painful; the patient can hardly bear the weight of the clothes, and frequently the least jar produces in the limbs the most acute pains. But in general these pains are wholly different from the painful sensation which we call lassitude; thus the pain of a ligament stretched in a sound state, is not the same as that which arises from the inflammation of a ligament or any other fibrous organ. I would add to what I have said above upon this sensation, that some organs are fatigued like the muscles, from too long continuance of their functions; for example, the eyes by the contact of light, the ears by that of sounds, the brain by thinking, &c. and in general all the organs of animal life; it is even this general lassitude which brings on sleep, as I have proved in my Researches upon Life. But observe that the sensation which the eye, the ear, the brain and all the external organs produce when thus fatigued is not the same as that which arises from the over-exertion of the muscles; another proof of the peculiar kind of their sensibility, and in general of that of every living part. _Animal Contractility._ This animal property, upon which all the phenomena of locomotion and voice depend, which assists those of the internal and external functions, has its seat exclusively in the animal muscular system; it is this which distinguishes it from the organic, and from all the others. It consists in the faculty of moving under the cerebral influence, whether the will or other causes produce this influence. The animal contractility has then, like the sensibility of the same species, a peculiar character, differing from the two organic contractilities, a character which consists in this, that its exercise is not concentrated in the organ which is moved, but that it requires also the action of the brain and the nerves. The brain is the principle from which, if we may so say, this property goes, as it is that to which all the sensations come; the cerebral nerves are the agents which transmit it, as they are, though in an opposite direction, the conductors of the sensitive phenomena. Whence it follows, that in order to understand this property well, it is necessary to examine it in the brain, in the nerves and in the muscle itself. _Animal Contractility considered in the Brain._ Every thing in the phenomena of animal contractility proclaims the influence of the brain.