Two or three broad generalizations, however, can be made from the confused and enormous mass of data at our disposal, which are of both philosophic interest and practical value. One of these is that, while headache is felt in the head, and particularly in those regions that lie over the brain, the brain has comparatively little to do with the pain. Headache is neither a mark of intellectuality, nor, with rare exceptions, a sign of cerebral disturbance. Indeed, it is far more a matter of the digestion, the muscles, and the ductless glands, than it is of the brain, or even of the nervous system. It is, therefore, idle to endeavor either to treat or try to prevent it by measures directed to the head, the brain, or even the nervous system as such. Secondly, it is coming to be more and more clearly recognized that, while its causes are legion, a very large percentage of these practically and eventually operate by producing a toxic, or poisoned, condition of the blood, which, circulating through certain delicate and sensitive nerve-strands in the head and face, give rise to the sensation of pain. Thirdly, the tissues which give out this pain-cry under the torture of the toxins in the blood are, in a large majority of cases, neither the brain, nor the nerves of the eye, nor other special senses, but the nerves of common sensation which supply the face, the scalp, and the structures of the head generally, most of them derived from one great pair of nerve-trunks, the so-called _Trigeminus_, or fifth pair of cranial nerves. Strange as it may seem, the brain substance is comparatively insensitive to pain, and the acutest pain of an operation upon it, such as for the removal of a tumor, is over when the skin and scalp have been cut through. These poisons, of course, go all over the body, wherever the circulation goes, but they produce their promptest and loudest pain outcry, so to speak, in the region where the nerves are most exquisitely sensitive. When your head aches, nine times out of ten your whole body is suffering, but other regions of it are not able to express themselves so promptly and so clearly. These newer and clearer views of the nature of headache dispose at once of some of the most time-honored controversies in regard to its nature. In my student-days one of the most hotly debated problems in medicine was as to whether headaches were due to lack of blood (anaemia) or excess of blood (hyperaemia) in the brain. Few things could have been more natural for both the sufferer in, and the observer of, a case of throbbing, bursting headache, where every pulse-beat is registered as a thrill of agony, than to draw the conclusion that the pain was due to a huge engorgement and swelling of the brain with blood, resulting in agonizing pressure against its rigid, bony skull-walls.