This paper has for its object the placing on record of a considerable number of observations made during the past eighteen months upon the blood of the insane. It was after the perusal of Dr. Lewis Bruce's work upon the clinical aspect of mental diseases that this special subject suggested itself to me as one deserving minute and extended study. Whereas Bruce deals somewhat minutely with the various constituents of theblood, I have confined my observations to the changes that occur in the number and variety of the white blood-cells, and the relationship these alterations have to the acute mental diseases in which they are found. These researches, though still in their infancy, hold out great encouragement to the belief that in such directions we may yet succeed in arriving at a true knowledge of the pathology of mental diseases, and at the same time advance at least one step in what has hitherto baffled all research-the conditions governing the mutual relations of mind and matter.