duced, may prove a cause of inflammation; and particularly, that this may arise from an obstruction formed by a matter stopping up these vessels. But many difficulties attend this doctrine.

1, The opinion seems chiefly to have arisen from the appearance of the blood described in 237, when the separated gluten was considered as a preternatural and morbid matter: but we know now very certainly, that this gluten is constantly a constituent part of the human blood; and that it is only a peculiar separation of the parts of the blood, that happens in consequence of inflammation and some other circumstances, which gives occasion to the appearance that was falsely considered as a mark of the morbid lentor in the blood.

2, There are no experiments directly in proof of a preternatural lentor prevailing in the mass of blood; nor is there any evidence of certain parts of the blood occasionally acquiring a greater density and force of cohesion than ordinary; neither is there any proof of the denser, or more coherent parts, being present in the mass of blood in such greater proportion than usual, as to occasion a dangerous spissitude. The experiments of Dr. Browne Langrish on this subject afford no conclusion, having been made on certain parts of the blood, separated from the rest, without attending to the circumstances of blood-letting, which very much alter the state of separation and concretion of the blood drawn out of the veins.

3, The supposition of a preternatural lentor or viscidity of the blood, is not well founded; for it is probable, that nature has specially provided against a state of the fluids, so incompatible with the exercise of the most important functions of the animal economy. While motion continues to prevent any separation of parts, and heat continues to preserve the fluidity of the more viscid, there seems to be always so large a proportion of water present, as to give a sufficient fluidity to the whole. I must own, that this is not absolutely conclusive; but I still repeat it, as giving a probability to the general argument.

4, In the particular case of inflammation, there are several circumstances which render it probable, that the blood is then more fluid than usual.

5, I presume that no such general lentor, as Boerhaave and his disciples have supposed, does ever take place; because, if it did, it must shew more considerable effects than commonly appear.

6, Besides the supposition of an obstructing lentor, physicians have supposed, that an obstruction may be formed by an impermeable matter of another kind, and that such an obstruction may also be the cause of inflammation. This supposition is what is well known in the schools under the title of an error loci; but it is an opinion that I cannot find to be at all probable: for the motion of the blood in the extreme vessels is so weak and slow, as readily to admit a retrograde course of

it; and therefore, if a particle of blood should happen to enter a vessel whose branches will not allow of its passage, it will be moved backwards, till it meet with a vessel fit for transmitting it; and the frequent ramifications and anastomoses of the extreme arteries are very favourable to this. I must own, indeed, that this argument is not absolutely conclusive; because I allow it to be pretty certain, that an error loci does actually upon occasion happen: but, for the reasons I have given, it is probable that it seldom happens, and is therefore rarely the cause of inflammation; or, if it be, that it is not merely by the obstruction that it produces; as, among other reasons, I conclude particularly from the following argument.

7, Though an obstruction should be supposed to take place, it will not be sufficient for producing the effects, and exhibiting the phenomena, that appear in inflammation. The theory that has been commonly employed on this occasion is by no means satisfying; and in fact, it appears from many observations and experiments, that considerable obstructions may be formed and may subsist, without producing the symptoms of inflamination.

242. Obstruction, therefore, from a matter stopping up the vessels, Gaub. Pathol. 249, i, is not to be considered as the primary cause of inflammation; but, at the same time, it is sufficiently probable, that some degree of obstruction does take

place in every case of inflammation. The distension, pain, redness, and tumor, attending inflammation, are to be explained only by supposing, that the extremities of the arteries do not readily transmit the unusual quantity of blood impelled into them by the increased action in the course of these vessels. Such an obstruction may be supposed to happen in every case of an increased impetus of the blood; but it is probable, that in the case of inflammation, there is also a preternatural resistance to the free passage of the fluids.

243. From the doctrine of fever, we are led to believe, that an increased action of the heart and arteries is not supported for any length of time by any other means than a spasm affecting the extreme vessels; and that the same spasm takes place in inflammation seems likely, because that every considerable inflammation is introduced by a cold stage, and is accompanied with that and other circumstances of pyrexia. It seems also probable, that something analogous to this occurs even in the case of those inflammations which appear less considerable, and to be purely topical.

244. From all this, the nature of inflammation may in many cases be explained in the following manner. Some causes of inequality in the distribution of the blood, may throw an unusual quantity of it upon particular vessels, to which it must

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necessarily prove a stimulus. But, further, it is probable, that to relieve the congestion, the vis medicatrix naturæ increases still more the action of these vessels; and which, as in all other febrile diseases, it effects by the formation of a spasm on their extremities.

spasm

245. A 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.

246. That, in inflammation, there is the concurrence of a constriction of the extreme vessels, with an increased action in the other parts of them, seems probable, from the consideration of rheumatism. This is a species of inflammation, which is often manifestly produced, either by cold applied to over-distended vessels, or by causes of an increased impetus, and over-distension in vessels previously constricted. Hence the disease especially appears at seasons liable to frequent and considerable vicissitudes of heat and cold.

To this we may add, that the parts of the body most frequently affected with inflammation, are those exposed, both to over-distension, from a change in the distribution of the fluids, and, at the