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.
To this proposition there is a seeming objection. Cold, sleep, immoderate evacuations, and the debilitating passions of grief and fear (all of which abstract excitement) appear to induce fever without the interposition of a stimulus. In all these cases, the sudden abstraction of excitement destroys the equilibrium of the system, by which means the blood is diverted from its natural channels, and by acting with preternatural force in its new directions, becomes an irritant to the blood-vessels, and thus a stimulating and exciting cause of fever. When it is induced by cold alone, it is probable so much of the perspirable matter may be retained as to co-operate, by its irritating qualities, in exciting the fever.
VI. There is but one fever. However different the predisposing, remote, or exciting causes of fever may be, whether debility from abstraction or action, whether heat or cold succeeding to each other, whether marsh or human miasmata, whether intemperance, a fright, or a fall, still I repeat, there can be but one fever. I found this proposition upon all the supposed variety of fevers having but one proximate cause. Thus fire is a unit, whether it be produced by friction, percussion, electricity, fermentation, or by a piece of wood or coal in a state of inflammation.
VII. All ordinary fever being seated in the blood-vessels, it follows, of course, that all those local affections we call pleurisy, angina, phrenitis, internal dropsy of the brain, pulmonary consumption, and inflammation of the liver, stomach, bowels, and limbs, are symptoms only of an original and primary disease in the sanguiferous system. The truth of this proposition is obvious from the above local affections succeeding primary fever, and from their alternating so frequently with each other. I except from this remark those cases of primary affections of the viscera which are produced by local injuries, and which, after a while, bring the whole sanguiferous system into sympathy. These cases are uncommon, amounting, probably, to not more than one in a hundred of all the cases of local affection which occur in general fever.
In my 4th proposition I have called the action of the arteries irregular in fever, to distinguish it from that excess of action which takes place after violent exercise, and from that quickness which accompanies fear or any other directly debilitating cause. The action of the arteries here is regular, and, when felt in the pulse, affords a very different sensation from that jerking which we feel in the pulse of a patient labouring under a fever.