Nor do the sufferings of studious men end here, since from another law, which holds equally in the animal œconomy, a fourth cause of the disorders of the learned springs from an over violent labour of the mind; for the animal fibre is indurated by being used, the whole machine grows callous as it grows old; the parts that are exercised by labour become hardened in workmen; the brain of studious men contracts a callosity, so that becoming unequal to the task of forming ideas, they fall into a premature dotage: thus the too great softness of an infant’s brain, and the too great hardness of a brain occasioned by study, are equally ill adapted to produce those vibrations, without which the force of thought is lost. Let the fibre be moisten’d with water, or stiffened with lime and sand, it in both cases becomes equally incapable of performing its function. At first the memory is impair’d, as GALEN justly observes, and then reason itself is disturb’d[23]. Nor is there any necessity that the mind should meditate great and sublime objects, to impair the strength of the nervous system; a too protracted use of the eyes has often given rise to innumerable nervous disorders; concerning which, the testimony of GUNZIUS is of the utmost authority[24]. And every weak man must know by experience, how much too great a use of the eyes weakens the head. Of this I have already given an example, and have often found it by experience; for when I contemplate any object a considerable time, after the fit of a fever, or any other disorder, and before my health is quite recovered, I am seiz’d with a vertigo, a sickness in the stomach, and a disagreeable listlessness of the whole body.