Morbid variations of structure and function present three leading phases of action, the acute, chronic, and diathetic. The primary causes of each are the same, a faulty personal hygiene and an unfavorable environment, but a derivative cause very generally predominates in the heritage of a defect or taint in the blood. In acute variations the struggle between the tendency to death and the tendency to reversion is usually short and severe, but decisive. The contending forces are often so nearly equal that a very little will turn the scale one way or the other. Chronic variations only differ from acute in this, that they are milder, or are more tolerable to the vital autonomy. The same inherent principle struggles in each for the restoration of health, although the process is not so impressively realized in the chronic as in the acute form, by a rapid and well-defined course. What dissimilarity there is in the two series of actions is wholly one of time and degree, not of nature, the struggle between the tendency to health and the tendency to disease presenting almost every conceivable phase of activity, and lasting from a day to months or even years.
As no one can say precisely when acute diseases end and chronic begin, the distinction being arbitrary and accepted for convenience, no more can any one say exactly when a slow disease is changed into a diathesis. In fact, there are no sound reasons in support of the opinion that such a change ever sharply occurs; but there are in support of this, that a slow disorder is often gradually toned down until it merges into a diathesis, when it is then known as a predisposition. During the gradual disappearance of a slow disease one of untutored and gross perceptions may decide recovery to be complete, while the acute adept may be able to discern the manifestations of the disorder subsequently for months. Indeed, there is scarcely any room for doubt that, if our senses were far more acute, a diathesis or tendency to some special disease would be disclosed in definite signs of imperfection, or of disorder. A diathesis is often seen gradually merging into development, and development into a diathesis, and the frequency with which this is seen to recur in the same person gives tone to the conclusion that the letter state is only a very mild or occult phase of the former. Any ordinary observer, with a predisposition to some disorder, is aware that very slight exciting causes usually serve sufficiently to increase the imperceptible imperfection as to bring it within the cognizance of the senses. But there is yet more indubitable evidence that a series of morbid changes may be going on in the body of which no cognizance can be taken. During the incubative stages of scarlet fever, measles, small-pox, and syphilis, the contagium of each has a definite period of maturative action before the signs of disease are in any way apparent. When a wave of cold air sweeps over a continent it develops in one a pneumonia, in another a rheumatism, in another a fever or an attack of neuralgia, upon the great majority no bad effects whatever. Can it be doubted that some definite imperfection which
