It remains yet to know the causes why the spirits slide not from the brain into the muscles always after one manner, and wherefore they come sometimes more towards some than others. For besides the action of the soul, which in truth, is in us one of the causes, as I shall show hereafter, there are yet two besides, which depend not of anything but the body, which it is necessary to take notice of. The first consists in the diversity of motions, excited in the organs of the senses by their objects, which I have already amply enough explained in the Dioptrics. But that those who see this, may not need to have read ought else, I will here repeat, that there are three things to be considered in the sinews, to wit: their marrow or interior substance, which stretches itself out in the form of little threads from the brain, the original thereof, to the extremities of the other members whereunto these threads are fastened; next, the skins wherein they are lapped, which being continuous with those that envelop the brain, make up little pipes wherein these threads are enclosed; lastly, the animal spirits, which being conveyed through these very pipes from the brain to the muscles, are the cause that these threads remain there entirely unmolested, and extended in such a manner, that the least thing that moves that part of the body, whereunto the extremity of any one of them is fastened, does by the same reason move that part of the brain from whence it comes. Just as when a man pulls at one end of a string, he causes the other end to stir.