250 MANUAL OF PHYSIOLOGY. 



which protect the corpuscles and impede the stages in fibrin for- 

 mation must retard coagulation. 



These may be arranged categorically, viz. : 



A. Circumstances promoting coagulation : 



1. Contact with foreign bodies is of the first importance in 



hastening coagulation. The greater the surface of con- 

 tact with the vessel or the air, the more the corpuscles 

 are exposed to injury, and the more rapid are the 

 destructive chemical changes inducing fibrin formation. 

 Thus a drop or two of blood falling on any surface so 

 as to spread out in a thin layer clots almost instantly. 



2. Motion, by renewing the points of contact between the 



blood and the moving agent, hastens coagulation. Thus, 

 by whipping fresh blood, all the fibrin can be removed 

 in a few minutes, and the defibrinated blood left without 

 a clot. 



3. Moderate heat. The formation of the fibrin generators and 



the action of the ferment seem to go on more rapidly at 

 38-40 C. than any other temperature. 



4. A watery condition of the blood causes rapid coagulation, 



but a soft clot. This is seen in repeated bleedings or 

 hemorrhages ; the blood which flows last clots first. 



5. The addition of a small quantity of water by setting 



up rapid changes in the corpuscles -accelerates coagu- 

 lation. 



6. A supply of oxygen. Oxygen is used up in the chemical 



changes attendant upon the death of the blood, and its 

 presence aids the formation of firm clots, such as are 

 produced in arterial blood. Exposure to the air in a 

 shallow vessel facilitates coagulation, partly by exten- 

 sive contact and partly by a free supply of oxygen. 

 But exposure to air is not necessary, for blood collected 

 in mercury, without ever coming in contact with the 

 air, coagulates very rapidly. 



B. Circumstances which retard coagulation : 



1. Constantly renewed and close inter-relationship with the 

 lining of healthy blood vessels alone affords the require- 



