It is this specific appearance alone which characterises a specific ulcer; and this, in each peculiar ulcer, is different; and a knowledge of it can only be obtained by an attentive examination of many sores. On this account, it is extremely difficult, in many instances, to distinguish a specific ulcer, because the discrimination depends altogether upon the recollection of the practitioner, and the improvement which he has made of his former observations. It is indeed, it may be thought, an easy matter to distinguish a simple ulcer by negative characters, or the want of the peculiar aspect; but, as this aspect is very arbitrary, and as the appearance of simple ulcers is, as has been already described, very various, it is difficult to say, without much judgment, whether the sore be simple or specific; for the appearance of the one and of the other run imperceptibly into each other. Even if it be ascertained to be specific, it is difficult often to distinguish betwixt particular specific diseases, in so much, that many are forced to take mercury for the cure of syphilis, who never had that disease.
Besides the appearance of the sores, specific action likewise produces a perceptible effect upon the scab which covers them, or the cicatrix which is formed. Thus, scrophula is marked by a particular appearance of the cicatrix, or of the scab. The venereal ulcer has likewise a particular scab, and many cutaneous ulcers are best distinguished by the scab. Other actions produce no considerable ulceration, but only successive desquamation of the cuticle.
We may also sometimes discover specific action by the sensation of which the action is productive. Thus, for instance, cancer produces a burning kind of pain, which never attends simple ulceration.
Specific ulceration is also always surrounded with more or less simple inflammation, or erysipelas, of the surrounding skin. In some cases the margin is hard, in others ragged, &c.
It were much to be wished, that some more certain, and less arbitrary criteria, than those which we possess of the presence of specific action could be discovered; but, as yet, we know of no other which can be applied universally. Some kinds, indeed, are so well marked, and so peculiarly distinguished from simple sores, that they can be tolerably well described, and easily discovered to be specific; but, there are others which it is more difficult to ascertain, owing to the difficulty of fixing the character of each individual action.
The number of specific inflammations is very great, and the causes which produce them are often obscure. In the preliminary dissertation it was mentioned, that, whenever any action existed strongly in any one part, it tended to induce an inflammatory state. There are, therefore, no general, or febrile diseases, which may not be attended with peculiar inflammations. That typhus fever is attended with local inflammatory action is pretty certain; but the presence of specific inflammation is still more evidently seen in the different exanthematous diseases. The diseases called cutaneous, afford us also numerous instances of specific inflammation.
From the difficulty of discriminating betwixt diseases, which, although essentially different from each other, yet possess a very great similarity, we find, that the number of specific inflammations is confined much within the true limits; for we find many confounded under the name of herpetic, &c. which are radically different from each other.
Phagedena has been used by medical writers in a very extensive sense, and has been made to comprehend diseases, which, strictly speaking, cannot be considered phagedenic.