ik fflkbtannJUps A syuiptom of mental exhaustion indioative of a very greet degree oi mental strain is persistent wakefulnesfi. The physiological cause of this ooudinou ís weïl understood. During axoessive labor ot' the brain' there is an inoreased rlow of blood to the working oigan, the vesseis of the head and neek beoonung j tended with blood, as is showu by the flushingof the face, lf thiseondition of distensión is long continued, the vessels are apt to lose the power of oon trac tin g when mental activity is diminished. Henee arises the impossibility of fultilling the physieal conditions of sleep, the most important of whioh is the duuinuition of the flow of the vital fluid to the brain. Soine extraordinary instances have been reeorded of prolongad wakeiuluess as a result of mental overstrain. Boerhaave mentions that when, on one occasion, intently engaged on a particular study, he did not close his eyes in sleep tbr six weeks. Sir Gilbert Blane was 111formed by General Fichergru that, for u. whole year, when engaged in active ■ campaign operations, he slept but one hour in twenty-four. These and other similar cases have probably been uriconsciously exaggerated, for people often sleep without au after consciousness ot the fact. It is certaiu enough that the continued deprivation of any considerable amount of th# normal sleep will be seriously detriinental to health. Dr. Hammond, in his work on sleep, mentions the case of a literary man in America, who, for nearly a year, while intently engaged in a favorite study, restricted his period of rest to four hours a day, and frequently less. At the end of that time the overtasking of his mental powers was manifested in a curious way. He told his physician that, though still able to maintain a connected line of reasoning, he fonnd that, though still able to record his ideas on paper the composition turned out to be simply as tissue of arrant nonsense. When in the act of writing, his thoughts flowed so rapidly that he was not conscious of the discentinued nature of what he was writing ; but as soon as he stopped to read it over he was aware how completely he had misrepresented his conceptions. If the language happened to be at all intolligible, it was sure to have no relation to the ideas he wished to express. Thus, he wished to obtain a book froin a tïiend, ho found that, instead of the request, he had written the prayer of Sócrates as given by Plato. Sir Isaac Newton, in the later years of his life, suffered greatlv froin wakefulness. The fact, well known to every medical man, that persistent sleeplessness is frequently the precursor or initiatory stage of several most intractable maladies, physieal and mental, always invests the presence of this indication of mental overstrain with grave interest. But a continued course of Mental labor generally manitVsts lts rtlVets on the mind itself in vaiiuus wnys, uil uio:e or less premonitory of approaching colla pse. The brain-worker begins to see u unwonted want of olearness in his ideas; work becomes gradually less easy to him ; he is alarmed at awkwar suddden failuresof memory, a feeling of surfeit or disgust will steal over him iu the inidst of work; he becomes unable lona; to flx his attention, nd latttrly feels as if all mental energy were crushed out of him. If these warnings of an over-wrought braiu - now speaking distinctly with the tongue of disease - are disregarded, the wonder frequently is, not that the inevitable retnbution follows, but that it should have been so long delayed. What particular form the Nemesis shall assume, whether of physieal or mental disoase, will be determined by accidents, partly of personal habits and temperaments, and partly of inherited predispositions. It is noteworthy, however, that the common opinión that excessive mental occupation gravitates towards insanity, does not appear to be verified by facts. Indeed, one of the foremost of living English physicians doubts whether alienation of miud is ever the result of simple mental overstrain. It is to physieal and not to mental derangement that excessive work of the brain generally gives rise. Insanity, he pointed out, finds the most suitable material for development among our cloddish, uneducatod classes, while the worst forms of physieal disease are originated and inteusified by oureducated overstraining braiu -workers.