SECT. I. THE CLOTTING OF BLOOD. 



14. Blood, when shed from the blood vessels of a living body, 

 is perfectly fluid. In a short time it becomes viscid : it flows less 

 readily from vessel to vessel. The viscidity increases rapidly until 

 the whole mass of blood under observation becomes a complete 

 jelly. The vessel into which it has been shed can at this stage be 

 inverted without a drop of the blood being spilt. The jelly is of 

 the same bulk as the previously fluid blood, and if carefully shaken 

 out will present a complete mould of the interior of the vessel. 

 If the blood in this jelly stage be left untouched in a glass vessel, 

 a few drops of an almost colourless fluid soon make their appearance 

 on the surface of the jelly. Increasing in number, and running 

 together, the drops after a while form a superficial layer of pale 

 straw-coloured 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 or crassamentum, floating in a perfectly fluid serum. The 

 shrinking and condensation of the clot, and the corresponding 

 increase of the serum, continue for some time. The upper surface 

 of the clot is generally slightly concave. A portion of the clot 

 examined under the microscope is seen to consist of a feltwork of 

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

 and white corpuscles of the blood. In the serum nothing can be 

 seen but a few stray corpuscles chiefly white. The fibrils are 

 composed of a substance called fibrin. Hence we may speak 

 of the clot as consisting of fibrin and corpuscles ; and the act 

 of clotting is obviously a substitution for the plasma of fibrin 

 and serum, followed by a separation of the fibrin and corpuscles 

 from the serum. 



In man, blood when shed becomes viscid in about two or 

 three minutes, and enters the jelly stage in about five or ten 

 minutes. After the lapse of another few minutes the first drops 

 of serum are seen, and clotting is generally complete in from one 



