[29] Although, in general, an abscess has little tendency to extend itself deep down, but rather moves toward the surface, although so far from it originally, that, had the action extended equally in all other directions, the size of the abscess must have been immense; yet, when it is situated over a cavity, it may proceed toward that as if to an external surface. Even, however, in this case, the general law, of all actions tending to the skin, obtains; for the progress outward is much quicker than that inward; but, if the action commenced near the surface of the cavity, as is commonly the case, the difference of the distance will compensate for the superior tendency to extend outward; and, therefore, the abscess may burst at the internal surface into the cavity. [30] In abscesses seated on the thorax, I have known the intercostal muscles and pleura continue the suppurative action, after the external surface had opened, and thus an opening came to be formed into the thorax. In healthy abscesses, where we do not apprehend any detriment to the neighbouring parts, the question comes to be, whether opening them will accelerate the cure? Perhaps much of the diversity of opinion on this subject, has arisen from not attending to the condition of the abscess which we have been managing, and thus we apply the prognosis and treatment of one kind of abscess to different ones. When an abscess has been formed slowly, and runs its course rather tediously, we may suppose, that the action shall continue for a considerable time without being converted into the ulcerative one; and, therefore, the abscess shall remain long without healing. In this case, a free incision, or the irritation of a foreign body, may excite the ulcerative action, and thus accelerate the cure; for these abscesses have come to approach toward the nature of common encysted tumors, and require the same treatment. But, where abscesses are running their progress with due celerity, and the action is proceeding through its proper course, there is not the same cause for interference. If, in this case, we open them before the action has gained the surface, we derive no benefit; because the action still proceeds, and the same events and circumstances take place as if we had allowed it to burst. If we make a large aperture, when the abscess is ready to burst, we, by the irritation, interfere with the process which was going on, and delay the cure. The admission of the air to the abscess, owing to the free exposure, is one cause of this delay; for it changes the nature of the purulent action, and, if the ulcerative action takes place, frequently renders it unhealthy; the consequence of which, if the abscess be large, or situated in vital parts, is hectic. We likewise, in large abscesses, by the sudden evacuation of the matter, and removal of the distension, sink the action of the parts, and make covery more tedious[31]. When the abscess, then, is healthy, and the action strong,