Redness

Healthy inflammation is of a pale red, when less healthy it is of a darker shade; but, according to Dr. Hunter, the inflamed parts in every constitution will partake more of the healthy red the nearer they are to the source of the circulation. The redness is manifestly owing in a great measure to the increased quantity of blood in an inflamed part; more blood must necessarily be contained there, because the vessels which previously conveyed this fluid are preternaturally distended, and the small vessels, which naturally contained lymph, are now so enlarged as to be capable of receiving red blood. "I froze" (says Dr. Hunter) "the ear of a rabbit, and thawed it again; this occasioned a considerable inflammation, an increased heat, and thickening of the part. This rabbit was killed when the ear was in the height of inflammation, and the head being injected, the two ears were removed and dried. The uninflamed ear dried clear and transparent, the vessels were distinctly seen ramifying through its substance; but the inflamed ear dried thicker and more opaque, and its arteries were considerably larger."

Many have supposed that the redness of common inflammation is partly occasioned by the generation of new vessels. This doctrine, however, is losing ground; when coagulated lymph is extravasated upon the surface of a wound or an inflamed membrane, unquestionably it often becomes vascular, in other words furnished with new vessels. But in the extravasated lymph of a phlegmonous tumour we have no evidence that there is any formation of new vessels. Were the lymph to be rendered organised and vascular, the swelling and redness would probably be more permanent, and at least not admit so easily of resolution. When adhesions are formed between two inflamed surfaces, the organised substance forming the connection lives after the subsidence of the inflammation, and is a permanent effect. In the experiments detailed by Dr. Hastings, when the inflammation began and terminated without any lesions of the part affected, new vessels were never formed.

At the same time it must be confessed that great obscurity prevails on this very difficult part of the subject; for when suppuration happens in a phlegmonous tumour, the cavity is lined by a kind of cyst, or membraneous. layer of lymph, which is unquestionably furnished both with secreting vessels and absorbents; for otherwise how could the continued secretion of pus or its occasional sudden disappearance be at all explicable? It was probably the enlargement of the small vessels, and the circumstance of their being filled with red blood, that led to the theory of new vessels being usually formed in inflammation.

It has, however, been justly observed that the supposition easily admits of refutation; for heat and many other causes of inflammation operate so quickly that there can be no time for the formation of new vessels, and yet the redness is as great and the inflammation as perfect in one minute as it is in an hour, or a day, after the application of the exciting cause.

Another reason assigned for the redness of inflammation is, that the blood after it has become venous retains more or less its red colour; and in some late, very carefully conducted experiments, it was remarked that the weakened action of the smaller vessels was always accompanied with an alteration in the appearance of the blood. Now in the natural state of this fluid, globules can be distinctly seen, but after inflammation has commenced the globular structure disappears, the blood becomes redder, and the most minute capillaries are distended with it

Swelling

This effect arises from several causes, First, The increased quantity of blood in the vessels. Second, The effusion of coagulating lymph and serum, and the deposition of new matter; and, third, The interruption of absorption.

Pain

This is observed to be greatest during the diastole of the arteries. The affection is probably owing to the unnatural state of the nerves, and not to mere distension, as many have asserted: were the latter cause a real one, the pain would always be proportioned to it.

Heat

The heat or real increase of temperature in an inflamed part, when judged of by the thermometer, is generally much less than might be supposed from the sensations produced. It is said never to exceed the heat of the blood at the heart. This in health is usually about 100°, Fahrenheit's thermometer; but sometimes in diseases it rises to 1060, or even 1070. Dr. Hunter artificially excited inflammation in the chest of a dog, and in the abdomen, rectum, and vagina of an ass, without being able to discover any obvious rise of temperature in these parts; and as Dr. Hastings observes, the advocates for excited action of the vessels in an inflamed part have thought that the increase of temperature favours their hypothesis, and have called to their aid ingenious calculations. Daily experience convinces us (he says) that the temperature is not always proportional to the velocity of the circulation.

It appears certain, therefore, that the generation of animal heat, either in an inflamed or uninflamed part, can never be satisfactorily explained by any reference merely to chemical principles, for the process is essentially connected with, and influenced by, the state of the functions of the brain and nervous system, and no doubt also by the principle of life itself.