Temperature and its Effects. As far as written evidence goes, it proves, 1st, that the process of coagulation is retarded more or less by a decrease of temperature below the natural standard of the body, such retardation being more marked as the freezing point is approached; 2ndly, that blood brought down to a temperature of 25° Fahr. may become frozen, but not coagulated, coagulation being possible, however, when the frozen mass has been thawed by exposure to a higher temperature; 3rdly, that, at certain temperatures above the natural standard, the process of coagulation is hastened, i. e. it takes place more quickly at 110° than at 90° or 100°, and more quickly at 120° than at 110°. These conclusions are corroborated so fully, that they may be received with confidence; for, although the ancient philosophers took an opposite view, their opinions are here of no weight, since their arguments were based on a general and vague inference, while the modern view rests firmly on direct experiment. Why temperature influences coagulation, in the way we have seen, is a great question. Its effects do not in themselves explain the phenomenon to the observer; nor do they chime in with the other hypotheses and theories which have been advanced, save with one, that, namely, which relates to the escape of gaseous matter from the newly drawn blood. In relation to the speculation about a loss of vital force as a cause of coagulation, the experiments regarding temperature are, on the contrary, in direct opposition. For, as the vitality of an animal is impaired in proportion as its temperature is reduced below the natural standard, so, according to this hypothesis, should the tendency to coagulation of its blood be hastened by a similar reduction. But the reverse obtains.

Exposure to the Air. The experiments of Hewson on the exposure of blood to the air are at once striking, simple, and ingenious, and, as far as they go, convincing; they afford at the least some evidence of a circumstantial character. His hypothesis has failed to convey full conviction; first, because it does not attempt to explain the act of coagulation, but certain conditions only under which the act is possible; secondly, because it does not in truth include all those conditions; and, again, because it does not show the reasons why the conditions which it presumes to be necessary, are necessary. It is argued, that the exclusion of air from blood retards coagulation, and that exposure to air increases it. But it is clear that these observations, however accurate, prove but little when taken alone. For, in the mere act of enclosing blood in an air-tight vessel, certain physical or chemical changes may be prevented from taking place. On the other hand, the free exposure of blood to air admits, as seems well proved, of an escape of gaseous matters. When to these arguments there is added the further proof, that blood coagulates as rapidly in vacuo, or when exposed to carbonic acid gas, to carbonic oxide, or to oxygen, as it does in common air, the hypothesis becomes limited in its application and meaning. But as a fact observed is often far more valuable than the inference made from it by the observer, and even remains when the inference is disproved, so we may accept the observation of Hewson without binding ourselves strictly to his deduction. The effects of exposure to air, like those of temperature, chime in well with the hypothesis of gaseous evolution; and it is curious that Scudamore did not see the matter in this light, and give another reading to the vacuum experi

ment. For, as free gases act as vacua to each other, so the escape of a gas from the blood should, a priori, be favoured by exposure of blood to air or to any gas; the only additional effect of the vacuum being its assistance in extricating the gases from the blood itself.

The experimental facts which indicate that blood received, under oil, or in long and narrow vessels, or in vessels tightly closed, coagulates slowly, all go to support Hewson's view, when received with its necessary limitations.

The Theory of Rest as the main cause of Coagulation leads us at once into the sphere of physics. It offers two propositions: first, that by motion the blood is prevented from coagulating; in other words, that the molecules of fibrin are prevented from coming together: secondly, that when this motion is suspended, the mechanical force, by which the molecules of fibrin are separated, is removed; that the mutual attraction which they possess for each other is permitted; and that the first step in the process of coagulation is thus set up. In this line of argument we rest, at all events, upon simple physical principles, which admit of plain discussion, and of direct experiment.

We have already seen that so powerful was the strength of this mode of reasoning felt to be by some of the older physiologists, that they adopted it as the only satisfactory explanation of coagulation; in which view they have been in part followed by Dr. Bostock. The circumstances favouring the view are important.

In the first place, pathology has shewn very clearly, that whenever the circulation of blood through the heart, or other part of the circulating system, is diminished in force, a decided tendency to coalescence

amongst the particles of fibrin ensues, giving rise to the formation of what are now called fibrinous concretions. It has been shewn, also, that when the circulation of blood is completely arrested in its course through the vessels, as when a ligature is applied, the formation of a coagulum of blood at the obstructed point is the ordinary fact; again, that in aneurism, where the current is impeded from its passing through a dilated vessel, a deposition of the fibrin is common at the most depending part, as a result of the languid motion; nay, more, that any direct obstacle to the course of the blood in its vessels, say an indurated valve, or a thread stretching across an artery or vein, is sufficient, under some conditions, to produce at the point where it obstructs a kind of coagulation.

cess.

In the second place, it has been very properly urged, that the mere act of suspending coagulation for a time by the agitation of blood in a closed vessel, is a proof that absolute rest has to do with the coagulating proFor, although the fibrin may either eventually cling to the side of the vessel in which the agitation is kept up, or float to the top of the fluid, these results are slowly induced, and depend, indeed, on the fact, that the fluidity of the blood has been kept up for a length of time sufficient to permit the red corpuscles to subside, and the fibrin to exert at leisure its specific property of molecular attraction.

Thirdly, the experiments which shew that the blood in a glass tube affixed by its two extremities to the free ends of a divided artery, does not coagulate, so long as it is passing through the tube, have been adduced in favour of the correctness of the theory now being considered.

Against it there stand out in bold relief; first, the valid objections derived from the facts Hewson discovered and dwelt upon so forcibly, viz., that blood retained in a state of rest in a vein coagulates very slowly so long as it is thus confined. Secondly, that blood extravasated into the living cellular tissue may remain fluid for weeks, though at perfect rest, but being let out will coagulate while still at rest in the receiving vessel. Thirdly, that blood drawn from the body cannot, by any kind of motion or agitation, be held in the same fluid condition as during life; i. e. with its fibrin distributed throughout the whole fluid as a part of the homogeneous liquor sanguinis. To all these arguments double force is added by the overwhelming evidence in favour of the view, that, in the circulating system, during healthy states, the fibrin is as certainly in solution as is the albumen or the soluble salts. If so, then, the force which can thus hold fibrin soluble is sufficient of itself, is chemical rather than physical in kind, and is independent of motion.

Such is the conflicting evidence on the subject of the stasis theory of coagulation. Fact meets fact in collision. Taking it all in all, however, unless something more can be said in its favour, the theory fails to account in full for the phenomenon of coagulation. But like the theory relating to exposure to air, it may comprise so much of truth, that, in the presence of the whole truth, the arguments and facts on which it has been founded would be easily read off and understood.

Hypothesis of the Vital Principle. If writers who speak of a vital force, or principle, could define what they mean by the term, or could even agree to a kind of