Sir Henry Holland, in his chapter on Sleep, considers its investigation should include causes which prevent as well as those which favour or produce it. In the investigation of sleeplessness we assume the converse of this as true, and advert briefly to what experimental physiologists regard as causes and accompaniments of sleep before engaging on its pathological condition, for the therapeutics of sleeplessness imply a knowledge of both. Disturbances of sleep are so constantly the heralds and associates of mental disease as to make insomnia, more than any other condition, common to all, or pathognomonic. The object of sleep, says Sir Henry Holland, is reparation, and sleep itself repose of sensibility and volition, whose cause is a change of condition of the nervous substance closely related to these functions, probably never to be ascertained by investigation, “yet,” he continues, “not the less real as a change on that account.”