CHAPTER XV. 



COAGULATION OF THE BLOOD. 



In speaking of the chemical relationship of the plasma (see p. 

 223), the formation of fibrin has been mentioned as the essential 

 item in coagulation, and the relation of fibrin to its probable pre- 

 cursors has been discussed. If the points there explained be borne 

 in mind, and the presence of the corpuscles be taken into account, 

 the various characteristics of the clot which forms when blood is 

 shed into a vessel can be easily understood, and should require no 

 further description. The great importance of the coagulation of 

 the blood in pathological processes makes it expedient, however, 

 to consider more closely the steps of the process as well as the 

 various circumstances under which it occurs after its removal, as 

 well as in the living vessels. 



Before the formation of a perfect clot, blood may be seen after 

 it is shed to pass through three stages : 1 , viscous ; 2, gelatinous ; 

 3, contraction of clot and separation of serum. 



The first stage is very short and in thin layers of blood passes 

 immediately into the second. With considerable quantities of 

 blood, contained in deep vessels, the central parts take some little 

 time to turn into a firm jelly, so that the completion of the second 

 stage may occupy from one to thirty minutes. 



After a variable time, sometimes as soon as ten to fifteen min- 

 utes, the third stage begins ; clear drops of serum appear about 

 the clot. In some hours this contracts until it forms but a com- 

 paratively small mass floating in the serum. If the jelly-like 

 clot be disturbed, the serous fluid makes its appearance much 

 sooner than the time just stated. 



During the formation of the clot under ordinary circumstances 

 the corpuscles are entangled in the meshwork of fibrin, so that 

 the gelatinous mass has throughout a dark, red color. 



If the coagulation takes place slowly as it does in very cold 

 weather, in horses' blood, or in human blood if removed from a 



247 



