2l8 ANATOMY AND PHYSIOLOGY 



tion with substances which were discarded by the tissue cells, are 

 all represented, dissolved in the water of the serum. 



It is serum which is found in the peritoneum, pleura, pericardium and 

 other serous-membrane cavities, and which fills the raised cuticle when a 

 blister " draws." Serum is the basis of exudates and indurations. Gases are 

 absorbed by serum, carbon dioxide chiefly, with a little nitrogen and oxygen. 



Fibrin by itself is a colorless, sticky protein substance, fibrous 

 and elastic. It may be obtained from freshly drawn blood by 

 whipping it with a glass rod or more quickly with small twigs. 

 (This leaves a red fluid called defibrinated blood.) 



Formation of fibrin. Fibrin is derived from the fibrinogen of 

 the blood by the action of an enzyme called thrombin. 



Source of thrombin. It is assumed that something has hap- 

 pened to so affect the leucocytes and blood plates that they have 

 liberated a special enzyme, thrombokinase, which causes the split- 

 ting off of thrombin from the pro-thrombin of the blood. The rest 

 follows: thrombin acts upon fibrinogen, fibrin is formed, the cor- 

 puscles are entangled, and coagulation is accomplished. 



Diagram of change from fluid to coagulated blood. 



Cor- I Red Red cells 



puscles \ White (and platelets) . White cells 



Fluid 

 blood 



Plasma 



Thrombokinase I 



f Prothrom- Throm- 

 bin J bin. 



Fibrinogin. 



Fibrin 



Coagu- 

 lum. 



Coagu- 

 lated 

 blood. 



[ Serum Serum 



The formation of thrombin takes place only in the presence of calcium 

 salts. (Further than this, the salts are not essential to coagulation.) 



For these processes to go on it is necessary that some unusual condition 

 be present. As has been said, normal coagulation takes place upon expos- 

 ure to the air. In order that the blood may be exposed to the air, the 

 vein or artery which contains it must be wounded, the blood flowing over 

 the injured tissues. 



So simple a change from the normal condition as this, is evidently 

 "unusual" enough to cause the liberation of thrombokinase from the leu- 

 cocytes and blood plates and appearance of thrombin, and the rougher the 

 edges of the wound or the more uneven the surface over which the blood 

 flows, the more rapidly does coagulation take place. 



But (we are told) if an artery or vein be opened in a clean cut with a 

 sharp knife and the blood received in an oiled tube under oil (thus prevent- 

 ing any possibility of friction), no coagulation follows; while the same blood 

 poured into an unoiled tube can be made to coagulate rapidly. 



