desire to avoid particularising the causes of coagulation, says, in a word, "it is a vital process," and Dr. Carpenter, in one place, speaks of the coagulation of blood as an evidence of vitality, and says that a deficiency of coagulating power in blood is due "to the want of due elaboration in the fibrin alone, or to the destruction of its vital endowments by some agent which has a noxious influence upon it."

NERVOUS INFLUENCE.

Thackrah was of opinion from his experiments, that blood, placed in an organised structure removed directly from the body of a living animal, coagulated less speedily than when it was placed in a similar structure from which the vitality had absolutely passed away. These conclusions receive opposition from certain experiments of Scudamore directed to ascertain the same point.

But Thackrah's views being in harmony with the hypothesis of the existence of a nervous force, generated in the medullary substance of the nerve-centres, and communicated by the nerve-cords to all parts of the economy, supplying or governing nutrition, secretion, circulation, and all the organic processes, it has been argued from them that the fluidity of the blood in the living body is kept up by this nervous force, which has been supposed by some to be communicated to the blood in the lungs by the nerve-filaments. To this hypothesis a kind of force has been added, by the argument that the blood is found fluid in cases of death from electricity, and where life has been destroyed by sheer exhaustion— in the parlance of the neuro-physiologists, by nervous exhaustion. This hypothesis, which is now well-nigh exploded, has not unfrequently been confounded with

the vital hypothesis of Hunter, whose writings, indeed, tend not a little to support such a confusion of ideas. Thackrah's summary runs thus: "I conclude that the vital or nervous influence is the source of the blood's fluidity." (Essay, p. 91.)

CHEMICAL HYPOTHESES.

The advance of modern chemistry has led to not a few theories, or rather hypotheses, regarding the cause of coagulation. One point that has been forcibly urged, is, that the blood, when drawn from a vessel, is prevented from coagulating by the addition of certain salts and other chemical substances. Thus it has been stated, that the fixed alkalies completely prevent coagulation; and that some of the vegetable acids, as the acetic, have the same effect. The various salts which are said to have this property are, common salt, the tartrates and borates, nitrate of potash, acetate of potash, chloride of potassium, sulphate of copper, acetate of lead, tartrate of potash, ammoniacal sulphate of copper, nitrate of lime, chloride of iron, sulphate of potash, sulphate of magnesia, and chloride of ammonium. Among the vegetable salts are nitrate of strychnine and nitrate of morphine, and the vegetable alkaloids, nicotine and morphine. Mr. Blake found that the blood was not coagulated after injection into the veins of caustic soda, carbonate of soda, sulphate of soda, ammonia, nitrate of silver, sulphate of zinc, sulphate of iron, phosphoric acid, arsenic acid, arsenious acid, oxalic acid, infusion of galls, infusion of digitalis, and alloxan. Magendie found a fluid condition after injection of putrid matters. into the circulation.

It has been stated that coagulation is favoured in

C

drawn blood by the addition of acetate of morphine, ⚫ solution of sugar, alcohol, ether, tannin, starch, alum, benzoic acid, or iodine. Magendie observed that the injection of a solution of bichloride of mercury into the veins was followed by the coagulation and separation of fibrin. In the hands of Gaspard and Mr. Henry Lee, the injection of pus into veins, or its addition to blood drawn from the body, has produced rapid coagulation. The injection of brain substance into the veins has led to a similar result; and Mr. Blake found the blood coagulated after the injection of the following matters: liquor potassa (firmly), carbonate of potash (firmly), nitrate of potash (firmly, blood scarlet), nitrate of soda, nitrate of ammonia, nitrate of lime, nitrate of baryta, chloride of calcium, chloride of barium, chloride of strontian, sulphate of magnesia, sulphate of copper, acetate of lead, arsenite of potash, nitric acid, narcotine, tobacco, strychnia, conium, hydrocyanic acid, euphorbium, and water in quantity. On the subject of salines and their effects on the blood, Nasse comes to a general conclusion, differing, indeed, in a great degree from that of any other observer. He argues that almost all salts, when not used in too large quantities, accelerate coagulation, though in small quantities they may retard it. Nasse believes, too, that water in small quantities hastens coagulation, and in large quantities retards it. (Lehmann, vol. ii, pp. 197-8.) Gulliver points out that blood nfay be kept fluid for weeks by the neutral salts, and may then be coagulated by dilution with water.

It will be observed, that various contradictions exist in these statements; and that the effect which is produced by a given agent in drawn blood in one experiment, seems to be just the reverse of what occurs in

other experiments when the same agent has been injected into the living circulation. Franz Simon, in fact, seems to experience so much difficulty in accounting for coagulation on the chemical process, that he refers it to the old doctrine of vitality, as I have before shown. Other physiologists and pathologists, however, more daring than Simon, seizing on the fact that the blood, when drawn from the body, may be kept from coagulating by some alkaline solutions, have argued that upon alkalinity of the liquid part of the blood depends the non-coagulability of that fluid during life: in short, that fibrin is chemically dissolved in the living blood. Dr. Fuller, in his excellent work on Rheumatism (1st edition), accepts this theory, and argues that the cause of the deposition of fibrin on the valves of the heart is due partly to its presence in unusual quantity, and partly to the weak state of solution in which it is held, in consequence of that extreme acidity of the system, which is so well marked in acute rheumatic affections.

Further experimental researches of a chemical kind. relate to the effects of gaseous substances in retarding or increasing the coagulation of the blood. I have already referred to the experiments of Sir Humphry Davy on this point, which shewed that the exposure of drawn blood to nitrogen, nitrous acid gas, oxygen, nitrous oxide, carbonic acid, carburetted hydrogen, and atmospheric air, did not modify coagulation. Other experimentalists have shewn that the process is not retarded by the fluid being placed in vacuo. From a long series of experiments performed by Dr. Polli of Milan, with great care, it would seem that coagulation takes place with equal freedom in pure oxygen, nitrogen, and atmospheric air; but that the presence

of carbonic acid always impedes it. Carbonic acid gas, according to this author, is always given off from blood during coagulation; and the more quickly this occurs, the more quickly the blood coagulates. The more carbonic acid gas the blood contains, the more slowly it coagulates, according to Polli, and the greater chance is there of a buffy coat being formed; while a buffy coat, formed over a dark clot without froth, is always a sign of the blood being surcharged with carbonic acid.

On the subject of the effects of gases on the blood, Lehmann also remarks that, since the blood contains gases, the different quantities in which they occur modify coagulation; hence blood rich in carbonic acid coagulates less rapidly than when the contrary is the case. Scudamore, in his Essay on the Blood, in referring to carbonic acid, states that he feels himself fully authorised in concluding that the period of time in which the blood coagulates depends, in a great measure, on the quick or slow extrication of carbonic acid gas. The evolution of this gas takes place most freely as the blood begins to concrete, and ceases when coagulation is completed. It is evidently, he presumes, an essential circumstance. in the process of coagulation, as the same causes which retain the carbonic acid in the blood delay coagulation.

In relation to the point now before us, there is an interesting and curious paper in the Philosophical Transactions (vol. for 1818, p. 81), from the pen of Sir Everard Home, which I notice here incidentally only, but to which Scudamore refers at length. Sir Everard, in giving an account of the evolution of carbonic acid. from the blood during coagulation, relates some experiments made at his request by Professor Brande, from which it was inferred, not only that carbonic acid