The object of all this is the restoration of the part to its healthy condition, and it is effected by a principle existing in the system—it matters little comparatively by what name you call it. The name is simply expressive of a great, general fact, as the term gravitation is, and is not intended, any more than that term is, as an explanation of the _nature_ of the fact indicated. This same principle is in operation in all diseases, resisting them, hemming them in, and as they retreat, following hard upon their footsteps, repairing their injuries as well as it can. It is true that its efforts are often ineffectual, that they are sometimes overpowered by disease, that they are frequently perverted by injudicious interference, and that they are sometimes stimulated to a higher degree than is necessary, producing over-action, and thus making this conservative principle an instrument of injury, perhaps destruction. It would be interesting and profitable to illustrate these several points in the operation of this principle, but it is not essential to our purpose. We will pass now to the consideration of the principle of self-limitation,[3] which we find existing in many diseases. These diseases have a regular rise and decline, including a set of processes, and a succession of symptoms peculiar to themselves. When they have once fairly begun, they cannot be abridged; neither are they prolonged beyond their natural limits, though they may, and often do, leave results behind them, which are sometimes mistaken for a continuance of the disease itself. The period of continuance is more definite and fixed in some of these diseases than in others, and there is a similar difference also in regard to uniformity of shape. Thus small pox runs through its course with more regularity of period, and with a more uniform series of phenomena than scarlet fever, which, though having a certain general character and average period, is extremely diversified in its degree of severity, and in its accompanying circumstances. The more simple and regular and definite any disease is, the more accurate can our observations be in regard to it, and the less apt are we to confound the effects of remedies with the natural changes that take place in its progress.