100 THE BLOOD AND THE LYMPH 



clot or collect between it and the walls of the container, so that after 

 some time the clot breaks away from the container and comes to float 

 in the serum. The latter may be perfectly clear, but usually is more or 

 less opalescent, partly because of the presence of fat, and partly be- 

 cause of leucocytes which have migrated out of the clot on account of 

 their power of diapedesis. 



If a drop of freshly shed blood is examined under the microscope, it 

 will be observed that the first step in clotting consists in the formation 

 of fine threads radiating from foci, which are undoubtedly the blood 

 platelets. The fine threads are called fibrin. They multiply rapidly, 

 so as to form an interlacing meshwork which entangles the red blood 

 corpuscles and leucocytes. By the use of the' ultramicros,cope (page 52), 

 Howell 1 and others have observed that the fibrin (produced by adding 

 thrombin to oxalated plasma) is really deposited in the form of fine 

 crystalline needles " fibrin needles" which become packed together 

 as they increase rapidly in numbers. Although the process of clotting 

 consists therefore in the conversion of a hydrosol into a hydrogel (see 

 page 61), it is a unique process; a solution of the blood protein which 

 is responsible for the formation of the fibrin (fibrinogen) may, like other 

 colloidal solutions, be precipitated in a variety of ways, but it is only 

 when the conditions are favorable for blood clotting that fibrin needles, 

 and therefore fibrin threads, are formed. The blood of invertebrates 

 forms a structureless gel when it clots (Howell).- 



Methods of Retarding Clotting of Drawn Blood 



To understand the nature of the clotting process and the factors that 

 are responsible for its occurrence, it is advantageous to simplify the 

 conditions somewhat by getting rid of the red corpuscles and most of 

 the other formed elements of the blood and then using the fluid in 

 which these are suspended in living blood namely, the plasma. This 

 separation of blood into corpuscles and" plasma is readily effected either 

 by sedimentation or by centrifuging after measures have been taken to 

 inhibit or greatly delay the clotting process. The methods used for this 

 purpose are numerous. A few of the most important are as follows: 

 (1) Keeping the blood at a temperature very slightly above freezing 

 point. This method is, however, not very effective unless the blood is 

 immediately received into narrow vessels placed in ice and the tempera- 

 ture kept most strictly at the low level. In the case of horses' blood and 

 other slowly clotting bloods, the method succeeds without these precau- 

 tions. (2) Eeceiving the blood through a strictly clean and smooth can- 

 nula, coated with a layer of paraffin or vaseline, into a vessel similarly 

 coated. This method is of practical importance when it is necessary to 



