The older authors supposed that pus was derived from the solids—or that it was formed by the melting of dead animal matter—or that it was the result of putrefaction; in accordance with which latter opinion, the term pus was given to the fluid; but such opinions have long since been justly exploded. Pus is generally supposed to be separated from the blood by the secreting power of the bloodvessels of an exposed and inflamed part, in consequence of their having assumed a new mode of action. The secretion from exposed surfaces is not at first purulent, but is transparent, serous in fact, and is somewhat of a gelatinous appearance; and it is only, it is said, after exposure to the atmosphere for some time, and when drying, that it presents the appearance of globules. Pus is often formed where the secreting surface has not been exposed to the air; on opening an abscess, the parietes of which had been previously entire and not much attenuated, purulent matter of the usual properties is discharged. It has been asserted that pus globules may be formed independently of any vital action; and that, if the serous fluid be removed immediately after its secretion by a granular surface, and kept in a temperature similar to that of the inflamed part, and be at the same time freely exposed to the air, globules will appear in as short a period as when the secretion is allowed to remain in contact with the sore. Some have also supposed that the mere admission of air into the cellular substance causes suppuration; but this is far from being correct. In chronic purulent depôts, however, the admission of air, by favouring putrefaction probably, often produces most serious results; other causes are generally in operation at the same time, as will be afterwards more fully explained. Pus was formerly regarded as irritating and corroding, and was therefore carefully removed from every granulating sore; but purulent matter, though it may prove a source of irritation to the neighbouring parts, does not disturb the surface which secretes it, but, on the contrary, protects the tender granulations, and acts as a temporary cuticle. A crust is formed by the evaporation of the thinner part of the fluid; and we frequently see small sores healing rapidly when thus protected. In some instances, we adopt the hint given to us by nature, and produce a scab by the application of powders, lunar caustic, &c.