KEPHALIN 237 



It is optically active, rotating the plane of polarised light to 

 the right. 



The main point of interest for us apart from its presence in all 

 tissues is its colloidal character. It is an extremely hydrophilic 

 emulsoid with a strong negative charge. On exposure to air it 

 readily absorbs oxygen (a property dependent on its unsaturated 

 fatty acid) and undergoes partial decomposition. 



The addition of calcium ions to a solution of kephalin causes it 

 to form a calcium soap with very little water-holding power 

 (see p. 82). Calcium seems also to combine readily with the 

 products of decomposition mentioned above. 



One must not consider for a moment that the last word on 

 clotting has been spoken. The subject has recently been investi- 

 gated from the standpoint of colloidal physics. The scheme 

 given above does not account for the experiment of Lord Lister 

 already quoted. Kephalin does not seem to be necessary. 

 Freund noted that blood would not clot if it came into contact 

 with a vaselined surface. Placed with due precautions on a 

 nasturtium leaf, which is not wetted by water, blood does not 

 clot. Clotting is induced by contact with a water- (i.e. plasma) 

 wettable surface. This is normally afforded by colloidal kephalin, 

 but may be produced artificially by the introduction of powdered 

 glass, etc., or even by bubbling CO 2 gas through the plasma. 



Tait and his colleagues have shown that the precursors of 

 thrombin are the thigmocytes of the blood. These thigmocytes 

 or platelets are spindle-shaped cells which have a strong phago- 

 cytic action. In place of prothrombin we may write thigmocyte. 

 Now in calcium-free plasma the" thigmocytes may come in contact 

 with a surface, but will not stick. The introduction of an ionisable 

 calcium salt causes them to adhere to the surface, and, on account 

 of the extreme lowering of surface tension, the covering membrane 

 of the thigmocyte is ruptured, i.e. it undergoes cytolysis and the 

 cytoplasm is sent into the blood stream. Thrombin comes from 

 this cytoplasm. 



To summarise : (a) Shedding of blood sets free kephalin from 

 the cells of the blood or of the tissues, (b) this forms a surface on 

 which, in the presence of ionised calcium, the thigmocytes may 

 spread themselves and rupture, setting free thrombin, which 

 causes the formation of fibrin-gel from fibrinogen sol. 



It is worthy of note that kephalin particles of a size just visible 

 under an oil-immersion microscope are capable of producing 

 intra vascular clotting when injected into' the blood stream, while 



