114 ELEMENTARY PHYSIOLOGY less. 



antecedent of fibrin, and indeed it was on this account 

 that it first received the name of fibrinogen, or " fibrin 

 maker," having been obtained from serous fluids before it 

 was shown to be present in plasma. It is undoubtedly 

 present in the substance thrown down by salt from blood 

 plasma, but then it is mixed with other bodies ; and the 

 presence of some or other of these bodies seems to be the 

 reason why in this case it is always converted into fibrin, 

 and So gives a clot. Conversely the absence of this body 

 or these bodies from pericardial fluid is the reason wliy 

 pericardial fluid, or fibrinogen prepared from pericardial 

 fluid, does not clot spontaneously. 



We have previously descril)ed how this fibrinogen may 

 be separated from plasma. If, now, the yellow fluid or 

 serum which exudes from a blood-clot be treated with 

 salt in a similar way (see p. 109) no fibrinogen can ever 

 be obtained from it, while on the other hand it is found 

 to contain both paraglobulin and serum albumin. A 

 comparison of plasma and serum thus shows that during 

 clotting, i.e. during the formation of fibrin, one constituent 

 of the plasma, namely, fibrinogen, disappears, the other 

 two proteins being left to appear in the serum. 



Putting these facts together there can be no doubt that 

 when blood clots the fibrin is formed out of fibrinogen. 

 But there must also be some substance in blood after it is 

 shed which leads to the ccmversion of fibrinogen into 

 fibrin ; for pericardial and other serous fluids contain 

 fibrinogen, but do not usually clot, and pui'ified solutions 

 of fibrinogen never clot spontaneously. What is this 

 substance ? 



If serum be* precipitated with an excess of strong 

 alcohol and after some weeks the precipitate is collected 

 and extracted with distilled water, this watery extract 

 contains very little solid matter, but is found to be active 

 in causing the conversion of fibrinogen into fibrin. We 

 do not as yet know exactly what the substance is in this 

 extract which brings about the change of the fibrinogen, 



