The white tongue is produced by a change in the secretion which takes place in that organ. Its yellow colour is the effect of bile; its dryness is occasioned by an obstruction of secretion, or by the want of action in the absorbents; and its dark and black colour, by a tendency to mortification.
It will be difficult to account for the variety in the degrees and locality of heat in the body in a fever, until we know more of the cause of animal heat. From whatever cause it be derived, its excess and deficiency, as well as all its intermediate degrees, are intimately connected with more or less excitement in the arterial system. It is not necessary that this excitement should exist only in the large blood-vessels. It will be sufficient for the purpose of creating great heat, if it occur only in the cutaneous vessels; hence we find a hot skin in some cases of malignant fever in which there is an absence of pulse.
Eruptions seem to depend upon effusions of serum, lymph, or red blood upon the skin, with or without inflammation, in the cutaneous vessels.
I decline taking notice in this place of the symptoms which are produced by the debility from action and abstraction, and by the depression of the system. They appear not only in the temperature of the body, but in all the different symptoms of fever. It is of importance to know when they originate from the former, and when from the latter causes, as they sometimes require very different and opposite remedies to remove them.
It remains only to explain the cause why excess in the force or frequency of the action of the blood-vessels should succeed debility in a part, or in the whole of the body, and be connected for days and weeks with depression and preternatural debility in the nerves, brain, muscles, and alimentary canal. I shall attempt the explanation of this phenomenon by directing the attention of the reader to the operations of nature in other parts of her works.
1. A calm may be considered as a state of debility in the atmosphere. It predisposes to a current of air. But is this current proportioned to the loss of the equilibrium of the air? By no means. It is excessive in its force, and tends thereby to destroy the works both of nature and art.
2. The passions are given to man on purpose to aid the slow and uncertain operations of reason. But is their action always proportioned to the causes which excite them? An acute pneumony, brought on by the trifling injury done to the system by the fatigue and heat of an evening spent in a dancing assembly, is but a faint representation of the immense disproportion between a trifling affront, and that excess of passion which seeks for gratification in poison, assassination, or a duel. The same disproportion appears between cause and effect in public bodies. A hasty word, of no mischievous influence, has often produced convulsions, and even revolutions, in states and empires.
If we return to the human body we shall find in it many other instances of the disproportion between stimulus and action, besides that which takes place in the excitement of fever.
3. A single castor oil nut, although rejected by the stomach upon its first effort in vomiting, has, in one instance that came within my knowledge, produced a vomiting that continued nearly four and twenty hours. Here the duration of action was far beyond all kind of proportion to the cause which excited it.
4. A grain of sand, after being washed from the eye, is often followed by such an inflammation or excess in the action of the vessels of the eye, as to require bleeding, purging, and blistering to remove it.