Such is the history of the changes which blood-pigment undergoes within the living body. To a certain extent its chemistry can be followed in the laboratory; but it must be remembered, when we are treating of the chemistry of a substance as complex as hæmoglobin, that the products which can be obtained from it in the laboratory are not necessarily those into which it is transformed in the body. In the laboratory oxyhæmoglobin is easily changed into methæmoglobin, a substance of the same percentage composition, but with its oxygen more firmly fixed. Methæmoglobin can be decomposed into a proteid substance and hæmatin. Hæmatin, when acted on by reducing agents, becomes hæmochromogen. Hæmochromogen, when subjected to such a reducing agent as a mixture of tin and hydrochloric acid, gives rise to coloured bodies closely resembling bile-pigments—not as they are secreted by the bile, but as they appear in the urine. It is impossible to prove that the changing colours of a bruise indicate a sequence of chemical transformations from hæmoglobin to bile-pigment, but it is not improbable that such a description is correct. The test commonly used to ascertain the presence of bile-pigment, i.e., bilirubin, is the play of colours which it exhibits when oxidized by fuming nitric acid. From yellow it turns to green, to blue, and then to purple, more or less reversing the colours of the bruise. It is fairly certain that effused blood undergoes changes along lines which, if not identical with those through which blood passes on its road to bile-pigment, are at any rate very similar.
Coagulation of Lymph and Blood.—Two or three minutes after blood has been shed it begins to clot. In ten minutes the vessel into which it has been received may be inverted without spilling the blood. After a time the jelly, holding all the corpuscles, shrinks from the sides of the jar. It squeezes out a transparent, straw-coloured fluid—serum. The clot continues to contract until, in a few hours, about one-half of the weight of the blood is clot, the other half serum. Lymph coagulates like blood, but most specimens clot more slowly, and the product is less firm.
When the process is watched through the microscope—a few drops of the almost colourless, transparent blood of a lobster afford an excellent opportunity of studying the formation of the clot—innumerable filaments of the most delicate description are seen to shoot out from many centres. They multiply until they constitute a felt-work. In the case of blood obtained from a vertebrate animal, this felt-work holds the corpuscles in its meshes. Its filaments exhibit a remarkable tendency to contract. They shorten as much as the enclosed corpuscles allow.
The filaments may be prevented from entangling the corpuscles by whipping the blood, from the instant that it is shed, with a bundle of twigs or wires. The fibrin collects on the wires, while the corpuscles remain in the serum. If this fibrin is washed in running water until all adherent serum and corpuscles are removed, it appears as a soft white stringy substance which, when dried, resembles isinglass.
Clotting is a protection against hæmorrhage. As it oozes from a scratch or tiny wound, blood clots, forming a natural plaster which prevents continued bleeding. It has little if any influence in resisting a strongly flowing stream of blood. But a clean cut through a large vessel is an accident which rarely happens as the result of natural causes. It is not the kind of injury to which animals are liable. When an artery is severed by a blunt instrument, the muscle-fibres of its wall contract. They occlude the vessel. The blood clots at the place where the vessel is injured, and plugs it. This happens also when a surgeon ties an artery. He is careful to pull the ligature sufficiently tight to crush its wall. His sensitive fingers feel it give. He stops before the thread has cut it through. As will be explained later, the clotting of blood is promoted by contact with injured tissue. If in tying an artery its wall be not crushed, the blood in it may remain liquid. When it is skilfully tied, the blood clots, forming a firm plug which is practically a part of the artery, by the time that the silk thread used in tying it is thrown out, owing to the death of the ring of tissue which it compressed. After a tooth has been extracted, the cavity is closed and further bleeding stopped by clotted blood.
When large vessels have been severed, the copious hæmorrhage which follows induces fainting. For a short time the heart stops, or beats very feebly. The blood-pressure falls. The bloodvessels contract. A clot has time to form. An emotional tendency to faint at the sight of blood is a provision for giving the various causes which stop bleeding an opportunity of coming into play. It is a useful reflex action, always supposing that the person who is liable to it faints at the sight of his own blood. Amongst other reasons for the greater fortitude of women—they are far less subject to this emotional reflex than men—might be alleged the circumstances of life of primitive people. It was the part of their women-folk to dress wounds, not to receive them.
The phenomenon of coagulation has attracted attention from the earliest times. It was a phenomenon that needed explanation, and culinary experience suggested analogies close at hand. Hippocrates attributed the clotting of blood to its coming to rest and growing cold. The blood which gushed from a warrior’s wound formed a still pool by his side. It set into a jelly as it cooled. Until the second quarter of the nineteenth century this theory was deemed sufficient. It then occurred to two men of inquiring mind to institute control experiments. John Davy placed a dish of blood upon the hob. William Hunter kept one shaking. In both experiments the blood clotted more quickly than it did in vessels of the same size, containing the same amount of the same blood, left upon the table.
Even before this date an observation had been made regarding the circumstances in which clotting occurs, which has thrown much light upon the causes of the phenomenon. In 1772 Hewson gently tied a vein in two places. At the end of a couple of hours he opened the vein. The blood was still liquid, but clotted in a normal manner after it was shed. Scudamore showed that blood clots more slowly in a closed than in an open flask. A new theory, as little trustworthy as Hippocrates’, was based upon these observations. Blood clotted because it was exposed to air. A record of all observations of the circumstances of coagulation, and of all the theories to which they have given rise, would make an exceptionally interesting chapter in the history of human thought. It would bring into singular prominence stages in the development of what is now known as the “scientific method.” Not that Science has a method of her own. Philosophers of all classes would follow the same method if their data allowed of its application. The peculiarity of the data with which Science deals is that they can be brought to a test of which the data of historical, or political, or economic theory are not susceptible. They can be confronted with control experiments. The control experiment is the alphabet and the syntax of the scientific method. No hypothesis is admissible into the pyramid of theory until it has passed this test. A natural phenomenon is observed. Every measurement which is applicable is taken and recorded—time, weight, temperature, colour. Scientific observation implies the tabulation of all particulars which are capable of statistical expression. Reflecting upon the relation of the phenomenon to other phenomena of a like nature, the philosopher—it is the philosophy of physiologists which interests us—formulates an hypothesis as to its cause. At this point the real difficulty of applying the scientific method begins. It is easy to formulate hypotheses. It is very difficult to devise control experiments. An experiment must be arranged which will provide that, while all other conditions in which the phenomenon has been observed to occur are reproduced, the condition which was ex hypothesi its cause shall be omitted. This digression into the philosophy of science may seem to be somewhat remote from our line of march, but it may perhaps hasten our progress in the comprehension of the story of physiology. There is no other science in which the control experiment plays an equally important part. Unless this is realized, the whole trend of experimental work will be misunderstood. Scudamore explained coagulation as due to contact with air. Based on the observations we have cited, no hypothesis could have seemed more reasonable. With a view to checking this hypothesis, blood was received into a tube of mercury. It coagulated in the Torricellian vacuum. Scudamore’s hypothesis, like many earlier and later, when confronted with a control experiment, was turned away, ashamed.
Clotting is a property of plasma. Red corpuscles play no part in the process. Coagulation does not occur in a living healthy vessel. It occurs when the vessel, and especially when its inner coat, is injured. It is hastened by contact with wounded tissues, especially with wounded skin. Contact with a foreign body also starts coagulation. If a silk thread is drawn through a bloodvessel, from side to side, fibrin filaments shoot out from the thread, as well as from the wound inflicted on the vessel by the needle which was used to draw it through.
Plasma contains a substance which sets into fibrin. It has been termed “fibrinogen.” It is present in lymph, and in almost all forms of exuded lymph. If sodium chloride (common salt) is added to plasma until it is half saturated—until it has dissolved half as much as the maximum quantity which it can dissolve—fibrinogen is thrown down as a flocculent precipitate. It can be redissolved and reprecipitated until it is pure. When fibrinogen was separated from plasma a step was taken towards the explanation of coagulation. Under certain conditions fibrinogen sets into fibrin. The question which then presented itself for solution was as follows: What is the substance which, by acting upon or combining with fibrinogen, converts it into fibrin? The clue to the solution of this question was obtained from the consideration of certain observations made by Andrew Buchanan in 1830, but long neglected, because their significance was not understood. Buchanan had observed that some specimens of lymph exuded into a lymph-space—the peritoneal cavity, for example—will clot; others will not. He noticed that they clot when, owing to puncture of a small bloodvessel during the process of drawing them off, they are tinged with blood. Determined to ascertain which of the constituents of blood is effective in rendering non-coagulable effusions capable of clotting, he added to them in turn red blood-corpuscles, serum, and the washings of blood clot. Either of the two latter was found to contain the clot-provoking substance. Thirty years later a German physiologist prepared fibrinogen from effused lymph by precipitating it with salt. He also treated serum in a similar way, precipitating a protein which he termed fibrinoplastin. When these two substances were dissolved and the solutions mixed, he obtained a clot, which he regarded as a compound of fibrinogen and fibrinoplastin. Subsequently he found that the mixture did not always clot, but he discovered that if he coagulated blood with alcohol, and washed this residue, the washings added to the mixed solution just referred to invariably produced a clot. Thinking that the substance which he obtained from his alcohol-coagulated blood could not be proteid, he termed it “fibrin-ferment.” He neglected the control experiment. He failed to ascertain whether or not all three substances were needed. Had he tried adding fibrin-ferment to fibrinogen, he would have discovered that the further addition of fibrinoplastin was unnecessary. He did not ascertain, as he might have done, that the weight of fibrin formed is somewhat less, not greater, than the weight of fibrinogen used. (Fibrinogen gives off a certain quantity of globulin when it changes into fibrin.) He was also wrong in supposing that the water which he added to alcohol-coagulated blood dissolved no protein. His “fibrin-ferment” is always associated with a protein. Since it may also be obtained from lymphatic glands, thymus gland, and other tissues which contain lymphocytes, it has been inferred that it is itself a protein, of the class known as nucleo-proteins. The fact that it is destroyed at so low a temperature as 55° C. has been supposed to confirm the theory that it is a protein. But with regard to the chemical nature of fibrin-ferment, as of all other ferments, we are at present in the dark. Under ordinary circumstances, when blood clots, the fibrin-ferment, or plasmase, or thrombin—it has received various names—is set free by leucocytes. Fluids which contain fibrinogen clot on the addition of a “ferment” which is either secreted by leucocytes or set free from leucocytes when they break up—as they are very apt to do, as soon as the conditions upon which their health depends are interfered with.