HISTOLOGY. 



FIG. 172. BLOOD 

 PLATES BESIDE 

 A RED COR- 

 PUSCLE. 



Blood plates or platelets are round or irregular protoplasmic structures, 

 2 to 4 fi in diameter. From 245,000 to 778,000 have been estimated to occur 

 in a cubic millimeter of human blood. They are readily reduced to gran- 

 ular debris in ordinary sections but when well preserved and stained with 

 Wright's blood stain it appears that they have dark granular centers and 

 clear peripheral zones (Fig. 172). They have formerly 

 been interpreted as small nucleated cells, and as fragments 

 of leucocytes. Dr. J. H. Wright has recently shown that 

 they are fragments of elongated pseudopodia of the giant 

 cells in the bone marrow. Their peripheral zone is ecto- 

 plasm and their inner granular part is endoplasm. Con- 

 sequently they are non-nucleated. The giant cells are not 

 always producing blood plates. Only certain of them 

 show the pseudopodia, which have been observed extending 

 into the blood vessels. In the blood the plates exist for some time, as they 

 are found in clots several days old. The function of the plates is unknown. 

 In drawn blood they rapidly adhere to one another forming masses, but not 

 rouleaux. Sometimes they present irregular projections and so have been 

 described as amoeboid. 

 In the clotting of blood 

 the plasma separates 

 into a solid part, the 

 fibrin and a thin fluid, 

 the serum. The blood 

 clot or thrombus con- 

 sists of fibrin with the 

 entangled corpuscles, a 

 mass which contracts 

 after it forms, squeezing 

 out the serum. The 

 fibrin is deposited (pre- 

 cipitated ?) in slender 

 threads which radiate 

 from the blood plates 

 and form nets shown in 

 Fig. 173. Therefore 

 the plates have been 

 considered active agents 



in the clotting of blood and have been called thrombocytes. In the blood 

 of amphibia, spindle shaped nucleated cells smaller than their red corpus- 

 cles possess adhesive properties and are also named thrombocytes. Since 



FIG. 173. RED CORPUSCLES FORMING ROULAEUX. FIBRIN IN 

 FILAMENTS RADIATING FROM BLOOD PLATES. (From Da 

 Costa's Clinical Hematology.) 



