Some histologists hold that the neuroglia is able to contract or expand; that, when expanded, it takes or receives impressions from without, and, when it contracts, it shuts out such impressions, thus inducing sleep. Dr. J. Leonard Corning, of New York, says that “Sleep may be defined as that state of the central nervous system in which the higher centers are, to a great extent, in a condition of physiological quiescence, with all the consequences thereby implied.” Dr. Corning means that the nervous centers of the brain are inactive, as a result of contraction, and that this state results in drowsiness and in consequent loss of the consciousness. He recognizes, however, that this purely physical condition does not always produce sleep; that there may be disturbing causes within. He says: “Those who suffer from sleeplessness are, almost without exception, beset by a variety of disagreeable mental symptoms during the day, dread of impending evil, irritability, depression, dread of society, etc.” Although these are often the result of wrong states of mind or heart, he recommends for such cases warm baths, Turkish baths, massage, and for obstinate cases he even suggests the use of drugs, because he regards the formation of habits of insomnia as more likely than the formation of the drug habit. This suggestion is not generally favored by the investigators of sleep, the most of whom discountenance the use of drugs. Almost everyone has known somebody who contracted the “drug habit,” or has heard of somebody who died from the effects of an overdose of some poison taken to induce sleep. Nobody should dose himself with drugs, hoping to get good results from sleep thus secured. It is wiser by far to discover the cause of sleeplessness and remove it, rather than merely to stupefy ourselves for the time being. It was as a worker along histological lines that Henry Hubbard Foster of Cornell University became convinced that sleep is induced by the absence of stimuli: that is, of things that attract and hold our attention. It may be that the individual withdraws from all stimulating conditions and creates conditions to cause sleep, as we do when we prepare for bed; or it may be that, because of fatigue, our senses do not respond to the things that would otherwise stimulate us. In either event, the result is the same—there is an absence of stimuli.