What Dr. Sieveking said in 1854 holds equally good to-day; and, indeed, the position then taken has received substantial indorsement through the positive results of more recent experimental physiology. Conspicuous in this connection are the inductive researches of Durham, Fleming, and Hammond, touching the modifications in the cerebral circulation during sleep and wakefulness. By these experiments it has been conclusively proved that the amount of blood in the brain is decreased during sleep and increased during wakefulness. More, recently I have had occasion to confirm the experiments of Fleming in this direction, and have published the results of those researches in various papers and articles.[1] "What Hippocrates said of spasm," says Dr. Sieveking, "that it results either from fullness or emptiness, or, to use more modern terms, from hyperæmia or anæmia, applies equally to headache; but, to embrace all the causes of this affection we must add a third element, which, though most commonly complicating one of the above circumstances, is not necessarily included in them, namely a change in the constitution of the blood." While I agree with Dr. Sieveking as regards the importance to be ascribed to the first two factors--cerebral hyperæmia and anæmia, in the production of the group of symptoms known as "headache,"--I fail to perceive why especial prominence should be given to the third condition mentioned by Dr. Sieveking. Indeed, I am quite unable to imagine how the periodical, and more especially the intermittent form, of headache is to be explained by what Dr. Sieveking describes rather ambiguously as a "change in the constitution of the blood." It is quite evident, admitting that such a change is capable of producing an amount of cerebral irritation sufficient to develop well-marked cephalalgia, that the latter must of necessity be within certain limits continuous. This is not the case, as the causative factor is constant and not fluctuating. I am, therefore, not prepared to accept this third causative factor without question. Nevertheless I am perfectly willing to admit that other factors besides cerebral hyperæmia and anæmia may produce the functional variety of headache. There would seem to be ample ground for ascribing great causative importance to excessive irritation of the brain plasma itself. Hence those forms of headache which while, being unaccompanied by any especial circulatory derangements, succeed, oftentimes, with relentless regularity upon any considerable degree of mental work. It is not my purpose to discuss the treatment of the multifarious forms of cephalalgia on this occasion, did time permit. As regards the so-called "neuralgic" variety I content myself by referring to the admirable work on "Neuralgia and Kindred Diseases of the Nervous System," by Dr. John Chapman of London, in which will be found many interesting facts bearing on the question. Accepting the propositions,