Читать книгу The Body at Work: A Treatise on the Principles of Physiology онлайн

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Coagulation of Lymph and Blood.

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.