1 1 2 THE TISSUES 



and polymorphonuclear forms. Such cells can take up foreign sub- 

 stances, bacteria., degeneration products, etc., carry them to other 

 parts of the body or entirely outside the body (salivary corpuscles), 

 or apparently absorb or digest them. (See also p. 52.)^ 



3. Blood platelets (thrombocytes) are minute round or oval 

 bodies from 2« to 4/« in diameter. They are colorless and vary in 

 shape. In fixed preparations they often appear stellate. Their num- 

 ber has been variously estimated. The average is probably about 

 200,000 to 300,000 per cubic millimeter of blood. In the blood 

 stream they are separate, but show a marked tendency to aggluti- 

 nation directly the blood is drawn. Some comparatively recent 

 observations tend toward considering the platelets as true cells. 

 Thus they have been described as amoeboid, as having the same 

 chemical composition as cells, and as containing either granules 

 of chromatin or distinct nuclear structures. The so-called throm- 

 bocytes of Ovipara are larger than those of man and are unques- 

 tionably nucleated cells. Their appearance is, however, wholly un- 

 like human thrombocytes and the identity of the two forms is 

 doubtful. 



Various functions have been ascribed to the thrombocytes. Under 

 certain conditions — exposure to air passing over a roughened 

 vessel wall — the fibrinogen is precipitated as fine crystals of fibrin. 

 Thrombin, or prothrombin, probably a blood platelet derivative, is 

 apparently the active element in determining the precipitation of the 

 fibrinogen. The formed elements of the blood are carried down in 

 the precipitation and there is thus formed a solid mass, the blood 

 clot or thrombus, leaving a clear defibrinated fluid almost free from 

 cells, the blood serum. The time required for the clotting of human 

 blood outside the body is usually from two to eight minutes ac- 

 cording to conditions. In certain individuals otherwise apparently 

 normal, also in certain diseases, the blood clots more slowly, a matter 

 of some surgical importance. 



4. The blood dust (hasmatokonia) occurs in the form of small 

 refractive granules. 



In the blood of the lower mammals and in herbivorous animals 

 small droplets of fat derived from the chyle are found. They are 



^It is to be noted that phagocytosis is not confined to leucocytes but has been ob- 

 served in other cells, e.g., fixed connective-tissue cells including endothelium. It is 

 also to be noted that phagocytic cells are not equally phagocytic to all substances. In 

 other words phagocytes apparently have some powers of selection. Thus if two kinds 

 of bacteria be presented to them, they may take up only one kind, or one kind much 

 more readily than the other. 



