CHAPTEE VI 

 BLOOD 



THE blood is a bright red, semi-opaque fluid, which circulates 

 within a closed system of vessels, the circulatory system. The 

 blood may be considered as a primary tissue, whose peculiar cell 

 elements are the blood corpuscles, and whose intercellular sub- 

 stance is the fluid blood plasma. Its corpuscles, according to their 

 color, are of two varieties : those which are colored, the red blood 

 cells, and those which are colorless, the white blood cells. To 

 these must now be added the blood platelets, minute protoplasmic 

 masses of definite form which are constantly present, and are 

 therefore true structural elements of the blood, but which can as 

 yet scarcely be classed as tissue cells. 



When blood is removed from the body it immediately assumes 

 a viscid consistence, and in a short time will solidify into a jelly- 

 like mass, the blood clot. This peculiar property is part of the 

 phenomenon of coagulation a phenomenon which results from a 

 rearrangement of the chemical constituents of the blood with the 

 formation of a new histological element, the fibrin. Coagulated 

 blood consists of corpuscles, fibrin, and a fluid serum which differs 

 from the plasma in its chemical composition, though the two fluids 

 are very similar in their histological appearance. 



RED BLOOD CORPUSCLES (colored blood corpuscles, red blood 

 cells, erythrocytes). The red blood cells are minute circular disks 

 with biconcave surfaces and rounded or convex edges. The color 

 of the corpuscle differs according as they are viewed by reflected 

 or by transmitted light. By reflected light they have a bright red 

 color, by transmitted light a faint greenish-yellow or amber shade. 

 This color is entirely due to the presence of hemoglobin within 

 the corpuscle ; the so-called shadows which remain after the hemo- 

 globin has been dissolved out of the cell are absolutely devoid of 

 color, and possess very little affinity for stains. Since all the 

 hemoglobin of the blood is contained within these red cells, the 



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