IMPORTANT FOODSTUFFS, IMPORTANT TISSUES 125 



thrombin, which, is present in the blood as prothrombin. Pos- 

 sibly prothrombin is combined with a substance "anti- 

 thrombin." When blood is shed, as in injury, the blood platelets 

 yield a substance thrombokinase which combines with anti- 

 thrombin, liberating the prothrombin. The prothrombin then 

 reacts with calcium to form thrombin. Thrombin acts on fibrino- 

 gen, which is converted into the insoluble fibrin. 



Various things delay or prevent clotting, such as substances 

 which precipitate calcium, strong salt solutions, alkalies, cool- 

 ing, and a substance contained in the head of leeches, also some 

 snake venoms and other poisons. 



Lymph. The lymph is a fluid much resembling blood plasma 

 in composition. It fills the spaces between the tissues and or- 

 gans, and serves as a medium through which the interchange of 

 material between blood and cells takes place. 



The Skin. The skin, hair, nails, etc., are made up chiefly of 

 the protein keratin, and contain also various salts. The skin 

 also contains various pigments, some of those of dark color 

 being classed as melanins. 



