Boerhaave entertained the theory that inflammation was caused by an impediment to the free circulation of the blood in the minute vessels; and this obstruction, he supposed, might be caused by heat, diarrhoea, too copious flow of urine, and sweat, or whatever could dissipate the thinner parts of the blood, and produce a thickness or viscidity of that fluid. He imagined that the larger globules of the blood passed into the small vessels, and thus plugged them up.

This theory of obstruction was too circumscribed and too mechanical; it reduced all inflammations to one species; the only distinctions which could have arisen must have proceeded from the nature of the obstruction itself; and it was a doctrine that never could account for the action of many specific diseases and morbid poisons.

The decided impossibility of giving a rational explanation of the immediate cause of inflammation by any supposed state of the blood alone, led pathologists to investigate how far a change in the bloodvessels themselves might account for the process, to explain the various facts and experiments in support of the opinion that the arterial tubes, and especially the capillaries, possess a high degree of vital contracti-bility, whereby the motion of the fluids in them, the process of secretion, and other local phenomena, may be importantly affected, in a manner not at all explicable by reference only to the action and power of the heart.

The bloodvessels through every part of the system are commonly believed to possess a considerable share of irritability, by which they contract and propel forwards their contents. Hence the blood, by the action of the vessels, receives a new impulse in the most minute tubes, and a well-regulated momentum is preserved in every part of its course. But of all parts of the sanguiferous system the capillaries seem most eminently endowed with this facility, and are least indebted to the presiding influence of the heart. Yet even in these vessels the action of the heart is of high importance in sustaining the healthy circulation, inasmuch as it gives the first impulse to the blood, and preserves the harmony of the system.

The vessels, then, are endowed with this vital property in order that each organ of the body may receive such a supply of blood as will enable it to exercise its functions in the manner due. Hence, a healthy state of this property is absolutely necessary for the preservation of the animal functions; for if the vital contraction of the bloodvessels be either increased or diminished, irregular distribution of the blood inevitably follows, and from this source numerous diseases arise, and none more frequently than inflammatioa Such was the doctrine favoured by Dr. Hastings, but I am not prepared to join in the opinion that inflammation is ever produced simply by an inequality in the distribution of the blood.

Dr. Cullen attributed the proximate cause of inflammation to a "spasm of the extreme arteries supporting an increased action in the course of them." This theory only differs from that of Boerhaave in the cause which is assigned for the obstruction, for he says again: -

"A spasm of the extreme arteries supporting an increased action in the course of them, may therefore be considered as the proximate cause of inflammation, at least in all cases not arising from direct stimuli applied, and even in this case the stimuli may be supposed to produce a spasm of the extreme vessels."

Let us now notice the celebrated and very original opinions promulgated on this subject by Dr. Hunter.

According to this authority, "inflammation is to be considered only as a disturbed state of parts which require a new but salutary mode of action to restore them to that state wherein a natural mode of action alone is necessary." Again, the same authority remarks: - "The act of inflammation is to be considered as an increased action of the vessels, which at first consists simply in an increase or distension beyond their natural size." This increase seems to depend upon a diminution of the muscular power of the vessels; at the same time the elastic power of the artery must he dilated in the same proportion. This is, therefore, something more than simply a common relaxation. We must suppose it an action in the parts to produce an increase of size for particular purposes, and this Dr. Hunter would call an act of dilatation. The whole is to be considered as a necessary operation of nature. Owing to this dilatation there is a greater quantity of blood circulating in the part, which is according to the common rules of the animal economy; for whenever a part has more to do than simply to support itself, the blood is there collected in large quantity. The swelling is produced by an extravasation of coagulable lymph, with some serum, but this lymph differs from the common lymph in consequence of passing through inflamed vessels. It is this lymph that becomes the uniting medium of inflamed parts, vessels shoot into it, and it has even the power of becoming vascular itselt The pain proceeds from spasm. The redness is produced either by the arteries being mora dilated than the veins, or because the blood is not changed in the veins.

As the vessels become larger, and the part becomes mora of the colour of blood, it is to be supposed that there is mora blood in the part, and as the true inflammatory colour is scarlet, or that colour which the blood has when in the arteries, one would conclude either that the arteries were principally dilated, or, at least, if the veins are equally distended, that the blood undergoes no change in such inflammation in its passage from the arteries into the veins (which Dr. Hunter thought most probably the case). When a part cannot be restored to health after injury by inflammation alone, or by adhesion, then suppuration, as a preparatory step to the formation of granulations, and the consequent restoration of the part, takes place. The vessels are nearly in the same state as in inflammation, but they are more quiescent, and have acquired a new mode of action.