104 A TEXTBOOK OF PHYSIOLOCY 



thromhogcn and thrombokinar,e become in a stale of unstable 

 equilibrium, and unite together to form fibrin and thrombin. The 

 presence of calcium ions is necessary for this to take place. The 

 1 1 i-t i port ion of fibrin produced, compared to thrombin, varies according 

 to the proportion of the three colloids taking part in the process. 

 The exciting cause of this unstable equilibrium may be any of the 

 physical or chemical agents known to facilitate clotting, such as 

 contact with the walls of a glass vessel, tissue extracts, and so 

 forth. 



Haemophilia. This is a disease characterized by the great tendency 

 to severe bleedings in those afflicted with it; hence these are known 

 as " bleeders." The characteristic bleedings are into joints, and 

 subcutaneous haemorrhages following slight injuries or strains. It is 

 an hereditary disease, confined to the male sex; women transmit the 

 disease, but never suffer from it. The cause of the condition is not 

 known ; it has been wrongly attributed to abnormal thinness or brittle- 

 ness of the vessel wall. The most generally accepted view is that 

 some agent (thrombokinase) is missing from the tissue fluids, so that 

 these do not cause the blood to clot. This view accords with the 

 fact that certain tissues of a bleeder may bleed, and not others; and 

 that tissues may bleed at certain times, and not at other times. 

 The condition is very rare. Genealogical trees, showing male bleeders 

 and transmission through females, have been constructed from records 

 going back to many generations. 



