"3. As the vital energy of animals is always diminished by reducing their temperature below the natural standard, so is the coagulation of the blood retarded by the same means, and wholly prevented by long-continued cold.

"4. The blood of individuals belonging to the sanguine or dynamic temperament, coagulates sooner and more firmly than in such as are of a weak and phlegmatic constitution, while its contractile power is diminished by whatever impedes the function of respiration, as in phthisis, asthma, disease of the heart, the cold stage of fever, and all diseases of long standing, by which the powers of life are greatly reduced."

Regarding the influence of temperature on the blood as it escapes from the body, Lehmann observes that we are still ignorant how far this may affect the period of coagulation, and he points out the difficulties that stand in the way in the investigation of this problem. The question, whether, during coagulation, heat is evolved from blood, has been discussed on many occasions; Hunter and Dr. Davy took the negative view of this question. Dr. Gordon and Sir Charles Scudamore were of opinion that a slight evolution does take place.

EXPOSURE TO THE AIR.

Having disproved the ancient notion concerning the effects of temperature, Hewson raised a theory of his own regarding coagulation, which attributed the occurrence of the phenomenon mainly to the air. He seems to have been led to this conclusion by discovering that blood, retained or tied up in a vessel-say a vein-of an animal, coagulates much more slowly than when ex

posed to the atmosphere, and by observing the negative effects of a low temperature. This theory has not been very favourably received by succeeding physiologists, who have shewn that blood, effused into serous cavities or cellular structures, does sometimes coagulate, though not exposed to the air; and that even in the veins themselves, this change may take place during life; a fact which Hewson admitted.

The condition of the blood when placed in vacuo has also been inquired into by various experimenters. Lehmann states that blood coagulates rapidly in vacuo, in consequence of the violent action induced in the molecules of the blood by the rapid development of vesicles of gas and of aqueous vapour. Scudamore (p. 27) came to three conclusions on the effect of exposure in vacuo, viz.: 1. That at a reduced temperature in vacuo, coagulation is hastened. 2. That atmospheric air being simply excluded in a stoppered bottle, the temperature not being reduced, coagulation is retarded. 3. That all communication with atmospheric air being prevented, the temperature not being reduced, coagulation is much retarded. This author was also of opinion that the extrication of gases, especially of carbonic acid gas, from the blood, influences materially the period of coagulation. Simple exclusion from the air, without the vacuum, returds coagulation, in the opinion of Dr. Carpenter, who observes, that if blood be received in a vessel under oil, its coagulation will take place slowly, a fact first noticed by Dr. Babington.

The question of the effect of exposure to the air on coagulation was further inquired into by Sir Humphry Davy, who found that the mere exposure of blood to the atmosphere, nitrogen, nitrous acid, nitrous oxide, and various other gases, does not materially affect the process.

REST AND MOTION.

Another theory on the process of coagulation has been that it arises simply from absolute rest. I do not know who originated this theory; but it has been referred to incidentally by various authors. Thus Lancisi, in speaking of aneurism, remarks on the separation and coagulation of glutinous chyle in aneurismal cavities; and implies that, as this is merely held in mechanical suspension in the blood, it is prone to separate and become deposited whenever the motion of the blood is slow. He compares an aneurism to a lake formed in the course of a river; and concludes, that as the insoluble substances, which are mechanically suspended in the natural current of the river, where the motion is rapid, are laid down on the sides of the lake, where the motion, from well known dynamical causes, is slow, so the blood, when flowing through the arteries, holds in suspension its solid constituents, while, in flowing through an aneurism, where the motion is impeded, it deposits these constituents on the sides of the sac. Rest and cold were assigned by Davies as the causes of coagulation.

Dr. S. Collins, in his work on Anatomy, written in 1685, contends that polypous concretions in the heart are identical in structure with the buffy coat of coagulated blood, and that they arise from a languid motion of the blood current, being deposited from the blood in a manner analogous to the formation of stone in the bladder. Dr. Brown, too, writing in 1684, remarks that depositions of fibrin and firm clots of blood are most commonly found on the right side of the heart, and most frequently in the right auricle; an effect due mainly to the slow motion of the returning blood through these parts of the circulation. Spallanzani

also taught indirectly that rest is the cause of coagulation; since blood stagnates and coagulates most quickly where its motion is slowest, as in the extreme parts of the circulation. The same view has been supported by more modern physiologists and pathologists, who have argued that the tendency to coagulate is greatest in weakened animals. Magendie, who entertained a similar opinion, made an experimental trial by introducing a tube into the two extremities of a divided artery, and found that the blood remained fluid only so long as the action of the heart continued to drive it forward. Magendie also, with J. Simon and Carswell, refer to the experiment of passing a ligature, or needle, transversely through an artery, and to the fact, that a coagulum of fibrin forms upon the foreign substance thus introduced into the circulating current. On a similar basis of reasoning, reference has often been made to the frequent coagulation of blood on the valve cords. of the heart, on indurated valves, and on any other points in the circulatory apparatus which, from retarding the blood current, form favourable points d'appui for the formation of fibrinous clots.

Dr. Bostock, in summing up the causes of coagulation, supports the view that the procees depends on cessation of motion. He remarks, "that, as fibrin is gradually added to the blood, particle by particle, while this fluid is in a state of agitation in the vessels, it has no opportunity of concreting; but when it is suffered to be at rest, either within or without the vessels, it is then able to exercise its natural tendency. In this respect," he adds, "the coagulation of the fibrin of the blood is very analogous to the formation of organised solids in general, which only exercise their property of concreting or coalescing under certain circumstances, and when

those causes, either chemical or mechanical, which would tend to prevent the operation, are not in action. Upon this principle, we shall be induced to regard the coagulation of the blood as analogous rather to the operation by which the muscular fibre is originally formed, than to that by which its contractile power is afterwards occasionally called into action."

On the other side, it has been forcibly urged that the mere statical condition of the blood has little, if anything, to do with its coagulation. The fact stated by Hewson, and supported by Scudamore, and lately by Stromeyer, that if a portion of blood be tied up in the vein of a living animal it does not coagulate for some hours, unless let out, when it may coagulate rapidly, is thought by some to settle negatively the stasis theory. Instances have been related, moreover, where blood effused into the cellular tissue, or other soft parts, has remained fluid for weeks, and has coagulated immediately on being let to escape into a vessel. There have thus been adduced, to oppose the theory of rest as a cause of coagulaion, the very arguments which have been brought forward by its supporters as fully substantiating their idea. It has been further urged, that if the particles of fibrin were merely mechanically suspended in the blood, they would be as easily distinguishable by the microscope in an isolated form as are the blood corpuscles. Some interesting inquiries on this point, favouring the mechanical suspension view, have been made by Dr. A. Buchanan; and Dr. Milne Edwards is inclined to believe that fibrin, like the proper globules, is merely suspended in the mass of the blood in a state of extreme subdivision, and possessed of transparency too perfect to be distinguished in the surrounding fluid.