CHAPTER VIII 

 BLOOD 



Blood may be regarded as a tissue in which the intercellular material 

 is a fluid, the plasma. The plasma contains fibrin in solution (fibrino- 

 gen) ; on exposure to air, as in case of cuts, the fibrin is precipitated in 

 the form of delicate needle-like crystals (Fig. 217), leaving a clear straw- 

 colored liquid, the serum. The active element in causing this precipita- 

 tion is thrombin (or prothrombin), probably liberated by the blood- 

 platelets on disintegration under the influence of air or the stimulus 

 of a roughened area on the wall of the blood-vessels. The blood-cells 

 become entangled in this fibrin net forming a clot or thrombus. Plasma 

 is very similar to, but not identical with, the lymph of the lymph 

 vessels. 



The formed elements, or blood-cells, are of two main classes, red 

 and white, or erytliroplastids (erythrocytes) and leukocytes. The red 

 blood corpuscles or plastids owe their characteristic color to the presence 

 of hemoglobin. Single corpuscles have a light yellowish-green color, in 

 masses they appear red. The function of the hemoglobin is to carry 

 oxygen in weak combination as oxyhemoglobin for transportation through 

 the blood-vessels from the lungs to the tissues, where it is employed in 

 the oxidation processes upon which life depends. 



THE RED BLOOD CELL 



The erythroplastid is a circular biconcave disk, of 7.5 microns (a 

 micron is 1-1000 of a millimeter) diameter. They are present in prac- 

 tically all histologic preparations, hence they serve well as a ready 

 scale for approximate determination of size of cellular elements in the 

 same microscopic field. Every section; blood preparation, as of a drop 

 under a cover-slip; and even the blood-vessels of living mammals, con- 

 tain also a larger or smaller number of cells of saucer shape (cup shape; 



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