296 THE BLOOD. 



coagulates more slowly than arterial, because of the lessened 

 amount of oxygen and the increased amount of carbon dioxid. 

 It is said that blood from the capillaries does not coagulate at all. 



It is the prevalent opinion that menstrual blood does not clot ; 

 this, strictly speaking, is not true. If the blood was collected as 

 it comes from the uterine vessels, it would doubtless coagulate as 

 does other blood ; but when it is mixed with the acid vaginal 

 mucus its coagulation is then impeded. Then, too, during the 

 menstrual period some of the blood undergoes clotting within the 

 uterine cavity or in the vagina : that which escapes and which is 

 regarded as menstrual blood is for the most part only serum, 

 which, of course, does not coagulate. 



Influences which Hasten Coagulation. Heat hastens coagulation, 

 as does agitation of the blood, as in whipping it with twigs or rods. 

 In general, anything which tends to break down the leukocytes or 

 plaques from which the nucleoproteid prothrombin is derived will 

 cause the latter to be set free and favor coagulation. 



Causes of Coagulation. Perhaps no physiologic question has 

 excited more controversy than that which deals with the cause of 

 blood-coagulation. Normally, blood remains fluid within the 

 blood-vessels, but within a few minutes after withdrawal it begins 

 to undergo coagulation. What is the explanation ? 



It has been suggested that blood-coagulation is due to exposure 

 to the air. It is true that contact with the air hastens coagula- 

 tion, but that this is unnecessary to the process is shown by the 

 fact that coagulation will take place under mercury when all air is 

 excluded. Nor can it be due to the cooling the blood undergoes 

 when it is exposed to the air, for, as already noted, cold retards 

 coagulation, while heat aids it. It has also been suggested that 

 the fluid condition of the blood in the circulation is due to its 

 motion, and that it clots when it comes to a state of rest. But 

 experiment shows that motion, such as the beating of blood with 

 wires, hastens coagulation. 



Experiments demonstrate that the fluidity of the blood is main- 

 tained only when the blood is in contact with the normal lining 

 membrane of the blood-vessels : when this relation is interrupted, 

 either by disease, or by death or injury of the membrane, or by 

 withdrawal of the blood from the vessel, the fluidity ceases and 

 the blood coagulates. 



The property of coagulation possessed by blood is of great 

 service in arresting hemorrhage. There are individuals in whom 

 bleeding, which in most people would be only slight, amounts to 

 a dangerous hemorrhage, often requiring surgical skill for its 

 arrest, and in some instances being so uncontrollable, even by the 

 most skilful treatment, that death results. Such persons are 

 called bleeders, and on them surgeons hesitate to perform any opera- 

 tion, however trivial, the extraction of a tooth even being often 



