306 THE BLOOD 



(Chap. VIII. and Part II.), or, according to Tait, it may be the 

 membrane siirroimding one type of the cells in the blood (thigmo- 

 cytes or platelets). The addition of a phospholipin, thrombo- 

 plastin, with the aid of the calcium of the blood, overcomes the 

 protective power of the anti-prothrombin, neutralises the inhibi- 

 tory solute, or causes the membrane of the thigmocyte to be 

 ruptured, depending on which view one adopts. 



(1) Surface. Howell presents experimental proof that the 

 thromboplastic substance is either kephalin or some other phospho- 

 lipin of similar composition and structure. Pure kephalin has not 

 been prepared in sufficient quantity to allow of complete analysis, 

 but its molecule probably consists of : 



m J. , , -1 ( Stearic (saturated) 

 i wo ratty acids ]-r ■ , • ; , / -,. 

 •^ iLinolic (unsaturated), 



+ 

 an amino alcohol, NH2-CH2-CH2-OH 



(amino ethyl alcohol), 



+ 

 a glycero-phosphoric acid. 



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

 the right (p. 126). 



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

 tissues is its colloidal and fatty 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 first step in the formation of the clot is the liberation of 

 kephalin or some such thromboplastic substance which, by inter- 

 action with calcium soaps or other colloidal calcium complex, 

 offers water-wettable surfaces to which the thigmocytes (Tait) or 

 other blood cells may adhere and over which they Mill spread. 

 The spreading process causes mechanical rupture of the cells, with 

 the liberation of thrombin (a proteose) from their contents. Then, 

 as we have seen, thrombin acts on fibrinogen and a clot of fibrin is 

 produced. 



That the introduction of a hydrophilic surface can initiate the 

 process of clot formation can easily be demonstrated. Almost any 

 hydrophilic substance with an effective surface (p. 71) will answer 

 as well as thromboplastin, e.g. powdered glass, quartz particles, 

 bubbles of carbon-dioxide gas, etc. Emboli (bubbles of gas) are 

 apt to cause clotting in the vessels where they stick, i.e. in the 

 smaller vessels. Other foreign substances whose surfaces can be 

 wet by plasma have the same effect. 



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



