by cerebral excitement.  Whoever has seen repeated, as we have, the experiment first performed by Weber, showing the consequence of irritating the vagus nerve, which connects the brain with the viscera—­whoever has seen the action of the heart suddenly arrested by irritating this nerve; slowly recommencing when the irritation is suspended; and again arrested the moment it is renewed; will have a vivid conception of the depressing influence which an over-wrought brain exercises on the body.  The effects thus physiologically explained, are indeed exemplified in ordinary experience.  There is no one but has felt the palpitation accompanying hope, fear, anger, joy—­no one but has observed how laboured becomes the action of the heart when these feelings are violent.  And though there are many who have never suffered that extreme emotional excitement which is followed by arrest of the heart’s action and fainting; yet every one knows these to be cause and effect.  It is a familiar fact, too, that disturbance of the stomach results from mental excitement exceeding a certain intensity.  Loss of appetite is a common consequence alike of very pleasurable and very painful states of mind.  When the event producing a pleasurable or painful state of mind occurs shortly after a meal, it not unfrequently happens either that the stomach rejects what has been eaten, or digests it with great difficulty and under protest.  And as every one who taxes his brain much can testify, even purely intellectual action will, when excessive, produce analogous effects.  Now the relation between brain and body which is so manifest in these extreme cases, holds equally in ordinary, less-marked cases.  Just as these violent but temporary cerebral excitements produce violent but temporary disturbances of the viscera; so do the less violent but chronic cerebral excitements produce less violent but chronic visceral disturbances.  This is not simply an inference:—­it is a truth to which every medical man can bear witness; and it is one to which a long and sad experience enables us to give personal testimony.  Various degrees and forms of bodily derangement, often taking years of enforced idleness to set partially right, result from this prolonged over-exertion of mind.  Sometimes the heart is chiefly affected:  habitual palpitations; a pulse much enfeebled; and very generally a diminution in the number of beats from seventy-two to sixty, or even fewer.  Sometimes the conspicuous disorder is of the stomach:  a dyspepsia which makes life a burden, and is amenable to no remedy but time.  In many cases both heart and stomach are implicated.  Mostly the sleep is short and broken.  And very generally there is more or less mental depression.