150 ANATOMY FOR NURSES [Chap. IX 



and running together, the drops after a while form a superficial 

 layer of pale straw-colored fluid. Later on, similar layers of the 

 same fluid are seen at the sides, and finally at the bottom of the 

 jelly, which, shrunk to a smaller size and of firmer consistency, 

 now forms a clot, floating in a liquid. The upper surface of 

 the clot is generally slightly concave. If a portion of the clot 

 be examined under the microscope, it is seen to consist of a net- 

 work of fine fibrils, in the meshes of which are entangled the red 

 and some of the white corpuscles of the blood. The fibrils are 

 composed of the fibrin ; and the liquid in which the clot is sus- 

 pended is blood minus corpuscles and fibrin, and is called serum. 

 The relation between plasma and serum is shown in the following 



scheme : — 



Serum 



^, ^ Plasma 

 Blood I 



Corpuscles 



Fibrin 



Clot 



. The formation of insoluble fibrin from soluble fibrinogen is an 

 instance of enzyme action and is comparable to the clotting of 

 milk under the influence of rennin. When blood leaves the vessels, 

 the blood-plates and some of the white corpuscles luidergo prompt 

 disintegration, and give rise to a substance called prothrombin, 

 which is acted upon by the calcium salts of the plasma and con- 

 verted into thrombin. It is this substance called thrombin which 

 acts upon the fibrinogen and converts it into fibrin. 



Value of clotting. — This property is of very great im- 

 portance in the arrest of hemorrhage. The clot formed closes 

 the openings of wounded vessels, and the procedures used to check 

 hemorrhage are directed toward hastening the formation of a clot, 

 and stimulating the blood-vessels to contract so that a smaller- 

 sized clot will be sufficient. 



The coagulability of the blood differs in different individuals, 

 and in rare cases is so slight that the most trivial operation involv- 

 ing hemorrhage is attended with great danger. This condition 

 is known as hemorrhagic diathesis or hemophilia, and is thought 

 to be due to a lack of fil)rinogen or of calcium salts. 



Conditions affecting clotting. — Clotting is hastened by : — 



(1) A temperature higher than that of the body, 110-120° F. 



(2) Contact with any rough surface. 



(3) Contact with any foreign substance, such as gauze. • 



