70 THE BODY AT WORK 



few drops of the almost colourless, transparent blood of a 

 lobster afford an excellent opportunity of studying the forma- 

 tion 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 cor- 

 puscles 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 cor- 

 puscles are removed, it appears as a soft white stringy sub- 

 stance which, when dried, resembles isinglass. 



Clotting is a protection against haemorrhage. 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 suffi- 

 ciently tight to crush its wall. His sensitive fingers feel it give. 

 He stops before the thread has cut it through. As will be ex- 

 plained 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 haemor- 

 rhage which follows induces fainting. For a short time the 

 heart stops, or beats very feebly. The blood-pressure falls. 



