The brain is only intermediate between the mind and the nerves, as the nerves are between the muscles and the brain; the principle, which wills, acts at first upon this organ, which afterwards re-acts. When they are thus produced, our motions are sometimes precise and regular; that is when the intellectual functions are sound, when the memory, imagination and perception are clearly exerted, when the judgment being correct, directs with regularity the acts of the will; sometimes they are irregular and singular, it is when the intellectual functions disturbed and agitated in various ways, produce a singular and irregular volition, as in the various mental alienations, in dreams, in the delirium of fevers, &c. But in all these cases, these are always voluntary motions; they go from the immaterial principle that animates us. In the second class of causes which influence the brain, the animal contractility becomes involuntary; it is exerted without the participation of the intellectual principle, often even against its will. Observe an animal whose brain is artificially irritated in experiments; it tries to stiffen itself to prevent the contractions, they take place in spite of it; prick a nerve in an operation, the muscle contracts suddenly below, without the mind's participating in this movement; the patient has not even a consciousness of it; he has only that of the pain. When much blood flows to the brain in the violence of inflammatory fevers, this organ excited by the fluid, re-acts immediately upon the muscles, without the will's partaking in it. All the phenomena of contraction and relaxation, arising from different accidents which accompany wounds of the head, cerebral inflammations, &c. are equally involuntary, although having their seat in the muscles which the will habitually directs. These are the different circumstances in which the action of any agent upon the brain is direct and immediate, and in which there is a mechanical cause applied to the brain.