In from 2 to 7 minutes after withdrawal, the blood becomes solid; colourless drops of serum then ooze out, and the clot becomes firmer and smaller, till, at the end of from 24 to 48 hours, it is but the bulk of the serum. These changes, with the respective proportions of the clot and serum, are represented here in a bowl of blood, and may Coagulated Blood. be contrasted with the figure on page 122. The following conditions respectively favour or retard coagulation: Rest; though the fibrin will clot when the blood is whipped with twigs. Roughness of surface; as ca- Exposure to air; by flowing in Motion. An extreme amount of fibrin delays coagulationthus, the blood drawn in a case of pneumonia did not coagulate for a week, and putrefaction was proportionally retarded. It is prevented by neutral salts-hence their use in acute rheumatism to prevent clots on valves of heart-by an alkali, as soda, in the proportion of 1 to 1,000 parts of blood; by bile, solution of opium, acids, even that of the vaginal mucus; for if menstrual fluid is removed from the uterus by a speculum, it coagulates. The blood does not clot after death by lightning, electric shocks, excessive exercise, as in animals hunted to death, according to Hunter, but Gulliver doubts this conclusion; also after death by blows on the stomach, fits of anger, asphyxia, alcohol, hydrocyanic acid, and other poisons. The blood drawn by a leech, and even that flowing after it is removed, does not coagulate-hence the difficulty of stopping bleeding from its bite. The Cause of Coagulation has been a much debated question. Hunter vaguely said: "My opinion is, that it coagulates from an impression-that is, its fluidity under such circumstances being improper, or no longer necessary, it coagulates to answer now the necessary purpose of solidity." That it is not the lower temperature to which the blood is exposed when drawn into air, he showed by that of a fish, which is at 60° while the animal lives, clotting when exposed to air of a higher temperature. His opinion that it was "an operation of life" is expressed by Harvey when he calls blood "primum vivens, ultimum moriens." That some attempt at organization is made seems supported by the following facts-1. The clot assumes the definite form of very minute interlacing threads, such as we find in lowly organized structures, as the membrana putaminis, or lining of egg-shell, effused lymph, &c. 2. The clot is imperfect under the same diseased conditions in which lymph is aplastic. 8. It often coagulates, as the first step towards organization, as seen in the conical clots above and below a ligature upon an artery. The organization of cerebral clots and false membranes, and of blood effused into joints, which is supposed to give rise to foreign bodies, are analogous occurrences. Gulliver objects to regarding coagulation a phenomenon showing any evidence of vitality, for he says, after being exposed for 57 weeks to nitrate of potash or to a freezing temperature, blood will coagulate; and, he remarks, we cannot freeze and pickle life;" but the term vitality may be applied to blood in the same sense as we apply it to a seed, in which the capability of germination will lie dormant for centuries. Coagulation is prevented by contact with the surface of living vessels, for if a glass tube be inserted between the blood and the vessel, it at once occurs; whereas in the vessel between ligatures it remained fluid twenty-five hours. Coagulation, then, would seem to be a vital process, which, under more favourable circumstances, would proceed to organization. Draper regards it the same process as that which is constantly occurring in tissue-making, save that the cells are not entangled, the fibrin being picked out by muscular tissue as fast as presented. That coagulation is due to fibrin cannot be doubted, for if it is removed by disease, or artificially by twigging, the blood no longer clots. The red cells in the frog are so large that they will, as discovered by Müller, remain on a filter, and the plasma, which passes through, still clots. Strong solution of sulphate of soda will prevent the human blood-cells from passing through a filter. Bernard has lately stated that blood from which all fibrin is abstracted by whipping, and renal blood which contains none, may be made to coagulate, which may be due to the conversion of the white cells by oxidation into fibrin. The investigations of Dr. B. W. Richardson have demonstrated that the coagulation of blood depends on, or is attended by, the evolution of ammonia. It is remarkable that the Hon. Robert Boyle, in 1684, stated the blood evolved a 66 spirit," and discovered that an ammoniacal salt would preserve its fluidity, and that the blood coagulated when it was evolved. Haller proved the vapour from blood to be alkaline: "Deinde de effuso sanguine volatile aliquid et halituosum aufugit cum odore quodem inter fœtorem urinæ et sudoris odorem medio. Id, vasis idoneis receptum aquosum adparet cum levi alcalinæ indolis quasi tinctura. Hac halitu emisso sanissimi hominis sanguis sponte cogitur in tremulam scissilem massam. We know that albumen is kept fluid by another alkali, soda, which, however, not being volatile, requires to be removed by an acid. Richardson shows that a microscope glass wetted with hydrochloric acid, and held over recently drawn blood will be covered with crystals of hydrochlorate of ammonia. They may be also obtained if the cupping-glass be moistened with acid, before being placed over the incisions. Coagulation is prevented by adding ammonia, by preventing its escape, or by exposing one vessel of blood to the vapour of another. The clot will re-dissolve if treated with ammonia, and re-coagulation occur if it is allowed to volatilise. One part of ammonia will keep about 3,000 of blood fluid. It will be seen that nearly all the circumstances favouring or retarding coagulation are explicable on this hypothesis, which has been, however, opposed by Lister, as blood kept fluid by ammonia will still remain so if that alkali be neutralised by acetic acid. This objection does not possess much weight, as the neutral salt thus formed has the power of hindering coagulation. The resemblance of coagulation of fibrin and the contraction of muscle, and its arrangement in fibrillæ, were remarked by Hunter. The coincidence of coagulation, rigor mortis, &c., support this view. I regard the formation of urea (which is metameric with cyanate of ammonia) by muscles when contracting, and the evolution of ammonia when blood is coagulating, as also analogous. concave. The clot sometimes assumes a yellow colour for half an inch or so of its surface, and becomes also This condition is called the "cupped and buffed coat," and depends usually on the proportion of fibrin being absolutely or relatively increased, when coagulation takes place more slowly, and the red cells have time to sink to the bottom. It is most frequent in Blood-" Cupped and Buffed inflammatory blood, but no mistake founded on false deductions from physiology was ever more fatal in practice than that of supposing it afforded an argument for the lancet, as anemia will also produce it, the cells sinking readily through the thin blood. It may occur during pregnancy, and when the system is excited by nightwatching, &c. The serum which oozes from the buffy clot will often coagulate itself. The arrangement of the cells in piles tends to produce the buffy coat. Transfusion of blood was first performed by Lower, in 1665, and was used therapeutically by Blundell in cases of uterine hæmorrhage. Berard cites 15 cases where life was saved. Animals bled to syncope are revived by injecting blood still containing red cells, for mere serum produces no effect. The fibrin should be separated, as otherwise its coagulation would interfere with its introduction, and Dr. B. Ward Richardson advises that ammonia should be added to the blood about to be transfused. It is generally stated that blood containing circular cells is fatal to birds and other animals whose cells are oval, but Bischoff asserts this is only true of venous blood which acts like a rapid poison, but arterial blood may be transfused without injury. The greatest care should be taken to prevent the entrance of air. Saline solutions have been injected with impunity, and Stevens proposed it as a cure for cholera. The animal from which the blood is abstracted should be previously fed, as it will then sustain the loss more easily, and the proportion of the vivifying element, the red cells, will be increased that of the fibrin diminished; and, indeed, Bernard has found that the blood of a starving animal acts as a poison; for, if transfused into another animal it dies with symptoms of putrid resorption. QUESTIONS FOR EXAMINATION. JUNIOR. 1. What is the average amount of blood in man's body, and how has it been determined? |