But how shall we account for the production of fever from the measles and small-pox, which attack so uniformly, and without predisposing debility from any of its causes which have been enumerated? I answer, that the contagions of those diseases seldom act so as to produce fever, until the system is first depressed. This is obvious from their being preceded by languor, and all the other symptoms formerly mentioned, which constitute the forming state of fever. The miasmata which induce the plague and yellow fever, when they are not preceded by the usual debilitating and predisposing causes, generally induce the same depression of the system, previously to their exciting fever. Even wounds, and other local irritants seldom induce fever before they have first produced the symptoms of depression formerly mentioned. I shall presently mention the exceptions to this mode of producing fever from contagious miasmata and local injuries, and show that they do not militate against the truth of the general proposition that has been delivered. It may serve still further to throw light upon this part of our subject to take notice of the difference between the action of stimuli upon the body predisposed by debility and excitability to fever, and their action upon it when there is no such predisposition to fever. In health there is a constant and just proportion between the degrees of excitement and excitability, and the force of stimuli. But this is not the case in a predisposition to a fever. The ratio between the action of stimuli and excitement, and excitability is destroyed; and hence the former act upon the latter with a force which produces irregular action, or a convulsion in the arterial system. When the body is debilitated, and its excitability increased, either by fear, darkness, or silence, a sudden noise occasions a short convulsion. We awake, in like manner, in a light convulsion, from the sudden opening of a door, or from the sprinkling of a few drops of water in the face, after the excitability of the system has been accumulated by a night's sleep. In a word, it seems to be a law of the system, that stimulus, in an over-proportion to excitability, either produces convulsion, or goes so far beyond it, as to destroy motion altogether in death. V. There is but one exciting cause of fever, and that is stimulus. Heat, alternating with cold[3], marsh and human miasmata, contagions and poisons of all kinds, intemperance, passions of the mind, bruises, burns, and the like, all act by a stimulating power only, in producing fever. This proposition is of great application, inasmuch as it cuts the sinews of the division of diseases from their remote causes. Thus it establishes the sameness of a pleurisy, whether it be excited by heat succeeding cold, or by the contagions of the small-pox and measles, or by the miasmata of the yellow fever.