CHAP, i.] BLOOD. 49 



fresh but apt to assume a faintly granular aspect. They are 

 called blood platelets. They have been supposed by some to become 

 developed into and indeed to be early stages of the red corpuscles, 

 and hence have been called haematoblasts ; but this view has not 

 been confirmed, indeed, as we have seen ( 27), the real hsemato- 

 blasts or developing red corpuscles are of quite a different nature. 



They speedily undergo change after removal from the body, 

 apparently dissolving in the plasma ; they break up, part of their 

 substance disappearing, while the rest becomes granular. Their 

 granular remains are apt to run together, forming in the plasma the 

 shapeless masses which have long been known and described as 

 " lumps of protoplasm." By appropriate reagents, however, these 

 platelets may be fixed and stained in the condition in which they 

 appear after leaving the body. 



The substance composing them is peculiar, and though we 

 may perhaps speak of them as consisting of living material, their 

 nature is at present obscure. They may be seen within the living 

 blood vessels, and therefore must be regarded as real parts of the 

 blood and not as products of the changes taking place in blood 

 after it has been shed. 



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. 



F. 



