But those are affected in the most dangerous manner, who dwell too long upon one and the same thought; for thus one part of the sensorium being longer stretch’d than the rest, without being ever reliev’d by the others in their turn, is the sooner broke; for as the body suffers more if one or but a few muscles alone act, so the brain is the less fatigued when various parts act successively; the part which discontinues recovers its strength, whilst the others are at work; and that is durable which does not want alternate rest. Many years ago I knew a woman, who appear’d to have very good sense for five and twenty years; but having, unhappily for her, embrac’d the sect of the Herrenhutters, she became entirely inflam’d by the love of our blessed Saviour, and of him alone; and she dwelling entirely upon this idea both day and night, it so broke the tone of her brain, that in a few months time she became an idiot; but still she was so mindful of her Lamb, that visiting her every day for half a year together, and frequently addressing my discourse to her, I could never hear any other words from her than _my sweet Lamb_; and this she utter’d every half hour, with downcast eyes, and was the only word she spoke for a year and a half; which time being elapsed, she pin’d away and died. And to pass by other examples, there was not long ago in this academy, a young man of bright hopes, who being too earnestly solicitous about squaring the circle, died mad in the Paris hospital. The third cause of the disorders of the brain, is, that law to which the human machine is subjected; that a new quantity of blood should be added to the part that acts. The illustrious MORGAGNI knew a man of learning at Boulogne, who whenever he gave his mind to abstract meditations in the morning, before he rose, was sure to bleed at the nose for a short time after[19]. Therefore as often as the brain increases its action, it is moisten’d with an additional quantity of blood, which, giving a new tension and a quicker motion to the vessels, brings on both the sense of pain and heat already mentioned, and many more grievous symptoms; namely, according to the different state of the brain, of the blood, and external objects, it occasions dilatations of the veins and arteries, obstructions of the nerves, inflammations, suppurations, scirrhous tumours, ulcers, dropsies, and thence head-achs, deliriums, drowsiness, convulsions, lethargy, apoplexy, and continual obstinate watchings, which cruelly torment the studious, and afflict them with new diseases; for what diseases, what perturbations of the mind do not protracted watchings give rise to? The great BOERHAAVE was six weeks without sleep, after profound study and meditation; and at the same time so indifferent to every thing, that scarce any object whatever affected him[20]. And which of you, learned auditors, has not found by experience, that a slight and disturb’d sleep, a sleep that scarce gives refreshment, generally follows