This view of the nature of headache explains at once why it is so extraordinarily frequent and so extraordinarily varied in causation. It is not too much to say that _any_ influence that injuriously affects the body may cause a headache. It would, of course, be idle even to attempt to enumerate the different causes and kinds of this pain, as it would involve a review of the entire environment of the human species, internal and external. It makes not the slightest difference how the poison gets into the blood, or where it starts. A piece of tainted meat or a salad made from spoiled tomatoes will produce a headache just as promptly and effectively as an over-exposure to the July sun or an attack of influenza. It is even practically impossible to pick out from such a wealth of origins two or three, or even a score of, conditions which are the most frequent, most important, or the most interesting causes. The most exasperating thing about dealing with a headache is that we never know, until its history has been most carefully examined, whether we have to do with a mere temporary expression of discomfort and unbalance, due to overfatigue, errors in diet, a stuffy room, lack of exercise, or what-not, which can be promptly relieved by removing the cause; or whether we have to deal with the first symptoms of a dangerous fever, the beginning of a nervous breakdown, or an early warning of some grave trouble in kidneys, liver, or heart. The one thing, however, that stands out clearly is that _headache always means something_; that it should be promptly and thoroughly investigated with a view to finding and removing the cause,--never as something which is to be cured as quickly as possible, as the police cure social discontent, by clubbing it over the head, with some narcotic or other symptom-smotherer. Nor should it be regarded as a malady so trifling that it is best treated with contempt, and still less as a mere "thorn in the flesh," whose ignoring is to be counted a virtue, or whose patient endurance without sign a mark of saintship. Martyrdom is magnificent when it is necessary, but many forms of it are sheer stupidity. Don't either gulp down some capsule, or "grin and bear it." Look for the cause. The more trivial it is, the easier it will be to discover and remove before serious harm has been done. The less easy you find it to put your finger upon it, the more likely it is to be serious or chronic, and the more necessary it is to remove it.