Vascular lesions, identical in appearance with the sequelae of organic diseases of the blood-vessels, have been found in a great variety of conditions (for instance in epilepsy, hypertension and cardiac disease, psychoses associated with infectious and toxic conditions, head injuries, certain groups of mental deficiency) in which no organic lesion of the blood-vessels themselves could be demonstrated. The theory has been put forward (Ricker, Spielmeyer and others) that these lesions owe their origin to functional disorder of the vascular system, and that this common factor accounts for the identity in histological appearance in spite of the profound ætiological differences.