The following pages contain an account of the microscopical examination of the central nervous systems of forty-one cases of idiopathic epilepsy, which form the data on which I base my thesis that epilepsy is a disease occurring in persons with a defect of the nervous system either congenital or involutional, and in whom also there is an abnormal state of the blood, characterised by a special tendency to intravascular clotting, and that the fits, whether of the nature of grand mal or petitmal,owe their exciting cause to sudden stasis of the blood stream in some (generally limited) portion of the cortex, resulting from the blocking of cerebral cortical vessels by these aforementioned intravascular clots. I have already in a paper read at the pathological meeting of the Neurological Society in December, 1905, and published in the British Medical Journal March 3rd, 1906, given a short account of my views, but it was impossible in the limit of time at my disposal when reading the paper to deal in any but a very cursory way with many interesting aspects of the question, nor could I then give sufficient details of the microscopical examination of the individual cases.