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.