48 BLOOD PLATELETS. [BOOK i. 



When a needle or thread or other foreign body is introduced 

 into the interior of a blood vessel, they are apt to collect upon, and 

 indeed are the precursors of the clot which in most cases forms 

 around the needle or thread. They are also found in the thrombi 

 or plugs which sometimes form in the blood vessels as the result of 

 disease or injury. Indeed it has been maintained that what are 

 called white thrombi (to distinguish them from red thrombi which 

 are plugs of corpuscles and fibrin) are in reality aggregations of 

 blood platelets ; and for various reasons blood platelets have been 

 supposed to play an important part in the clotting of blood, carrying 

 out the work which in this respect is by others attributed to the 

 white corpuscles. But no very definite statement can at present 

 be made about this ; and indeed the origin and whole nature of 

 these blood platelets is at present obscure. 



