Vascular lesions, haemorrhage or thrombosis, are the most important factors in the production of grave nervous disease between the ages of 50 and 70; among 500 cases of cerebral haemorrhage 321, or 64 per cent, and of 110 cases of cerebral thrombosis 67, or 61 per cent, occurred in the sixth and seventh decades (Michell Clarke[207]). _Cerebral haemorrhage_ increases with frequency from the fourth decade and the largest number of cases occur between 50 and 60. From analysis of 154 cases at St. Bartholomew’s Hospital F. W. Andrewes[208] found that the apparent maximum is in the middle of the sixth decade, but that correction for the age distribution of the population shows that the liability of the individual to this form of death increases steadily up to old age. _Thrombosis of atheromatous vessels_ is an accident of later incidence than cerebral haemorrhage, and thus contrasts with hemiplegia due to syphilitic endarteritis which occurs about the prime of life. The physiological involution of the mind and accompanying organic changes in the brain gradually shade off into senile dementia. A regression to the mental state of childhood, which Dupré[209] called puerilism, may occur in widely different conditions, such as structural change of the brain, hysteria, and toxaemia. It may be acute and be transient or come on slowly and be permanent. Just as an old man may relapse into the speech and accent of his youth, so if he had a hard pecuniary struggle in his early days may he become miserly in the evening of his life. _Senile dementia_ is an exaggeration of the mental changes occurring in old age and due to further changes in the brain from vascular disease or toxic influences. It varies much in its features; some patients are maniacal, others depressed and melancholic, some feeble, some delusional, and a few immoral. The senile mania may be mainly nocturnal and was compared by Clouston[210] to the night delirium of a febrile neurotic child; it may pass into dementia. Of the melancholic group in which suicide may occur Clouston found that 30 per cent recovered.