9 8 NORMAL HISTOLOGY. 



one third the size of the red cells. They possess a minute chromatin mass, 

 the assumed equivalent of a nucleus, and are said to exhibit amoeboid move- 

 ments. They may be directly observed as free bodies circulating within the 

 blood-vessels. On withdrawal from the latter, without precautions for their 

 preservation, the blood-plates collect in irregular masses and undergo disin- 

 tegration, their remains often being centres from which radiate the threads 

 of fibrin. The blood-plates are supposed to assist in arresting hemorrhage. 



They have been variously attributed to 

 disintegration of the leucocytes, to ex- 

 trusion from the red cells, to destruction 

 of the endothelium of the vessels, and, 

 recently by Wright, to fragmentation of 

 o> long processes sent out by the giant cells 



(megakaryocytes) of the bone-marrow. 

 In view of their constant presence 

 and large normal quota an average of 

 ^ 400,000 or more in one cubic millimeter of 



blood none of these suggested sources 

 of the blood-plates seems satisfactory. 



^ Blood-Crystals. The most im- 



portant constituent of the red cells, the 

 FIG. i34--Human blood, showing erythro- hemoglobin, exists within the stroma of 



cytes and blood-plates. X 625. ', 



the corpuscles as a complex but loose 



chemical combination of iron with a globulin. The hemoglobin must be freed 

 by solution before crystallization occurs. After liberation, or "laking," as it 

 is called, as by the addition of a weak solution of ammonium oxalate, the col- 

 oring matter of the blood, in the form of oxyhemoglobin, separates as micro- 

 scopic crystals. The investigations of Reichert have shown that the forms of 

 these blood-crystals bear a constant and definite relation to genus and species, 

 so that differentiation between many animals is possible, since the crystals 

 of the species of any genus belong to the same crystallographic system and 

 generally to the same crystallographic group. The crystals derived from the 

 blood of man are elongated rhombohedra; from the horse, rhombic plates; 

 from the guinea pig, tetrahedra; and from the squirrel, hexagons. On mix- 

 ing dried blood with a few grains of sodium chloride and a small quantity of 

 glacial acetic acid, and heating until bubbles appear, minute brown crystals 

 are formed in large numbers. These are hemin crystals and derived from 

 the reduction of hemoglobin. They indicate only the presence of blood and 

 are valueless in differentiating the blood of man from that of other animals. 

 In blood-clots of long standing, minute crystals of hematoidin often appear 

 as yellowish-red plates. This substance is likewise a reduction-product of 

 hemoglobin, and is seemingly identical with bilirubin of the bile. 



After death, or upon standing after withdrawal from the body, blood 

 undergoes coagulation, whereby the corpuscles become entangled among the 

 innumerable delicate filaments of fibrin. In microscopical preparations of 

 fresh blood, the fibrin appears after a time within the plasma in the form 

 of innumerable delicate threads, which cross and interlace in all directions 

 and radiate from centres marked by groups of blood-plates. The entangle- 

 ment of the corpuscles in the fibrin-net results in the production of a dark- 

 red, jelly-like mass, the blood-dot or crassamentum, that separates from the 

 surrounding clear straw-colored fluid, the serum. In stained sections, the 

 white cells within the clot are readily identified as deeply tinted bodies, 

 particularly along the free border. 



