It is not to be denied, however, that many individuals are able to live in comparative health for long periods with but little or no sleep. Thus it is stated[130] that Boerhaave did not “close his eyes in sleep for a period of _six_ weeks, in consequence of his brain being overwrought by intense thought on a profound subject of study.” Sir Gilbert Blane[131] says he was informed by General Pichegru, that for a whole year, while engaged in active campaign operations, he slept but one hour out of the twenty-four. Such statements as these, however, and others to the same effect which have been made, must be accepted with some allowance. Many persons sleep unconsciously, and we all know how common it is for individuals to deny having slept when we have been eye-witnesses of their somnolency. I should consider it impossible for a person to enjoy good health if deprived for even a few weeks of half his ordinary amount of sleep; and it is very probable that Boerhaave’s standard of health, never high, was very much lowered by his protracted vigils. So long as the attention is kept fully aroused, the blood-vessels of the brain are distended, and it is possible for an individual to remain awake while this condition exists. When the attention begins to flag, the tendency is for the vessels to contract, and for sleep to ensue. This disposition may not, however, be strong enough to restore the full measure of contractility to vessels that have been long overdistended, and then insomnia results. To this increase in the amount of blood circulating in the brain, many instances of hallucination have been due. It has already been shown that strong mental emotions determine an augmented flow of blood to the cerebral vessels, and cause the production of spectral illusions. In all such cases there is a marked tendency to insomnia present. The account given by Nicolai, a celebrated German bookseller of the last century, of his own disorder, is so interesting and appropriate that I quote it in full. It has never to my knowledge been published in this country.