92 BLOOD AND LYMPH. 



about 9 per cent, of albuminous substances, mineral salts, 

 and other constituents. These substances serve as pabulum 

 for the nourishment of the tissues. It readily undergoes 

 coagulation, separating into clot and serum. The plasma 

 originates by absorption from the alimentary tract. Being 

 homogeneous, it ordinarily exhibits no structural features to 

 the microscope; its composition is a matter for chemical 

 study. 



Under certain circumstances, however, substances or par- 

 ticles visible to the microscope appear in the plasma. After 

 standing for some time specimens of fresh blood exhibit in 

 the plasma numerous delicate colorless straight filaments of 

 fibrin interlacing and running in different directions; they 

 often radiate from points or centres, and at these centres 

 blood-plates are often situated. These filaments consist of 

 fibrin which has undergone coagulation. The plasma under 

 the microscope often has a yellow tinge, due to haemoglobin 

 dissolved out of the red corpuscles. 



Rhombic crystals of haemoglobin appear in blood under 

 proper conditions or treatment. Small particles of fat are 

 sometimes present in the plasma, especially after a fatty 

 diet. 



In the blood-plasma are frequently to be seen minute 

 granules, or "blood-dust," both in normal and abnormal 

 conditions. Ordinarily they are not numerous, but occur 

 singly here and there. Some of these may be particles of 

 fat, or foreign particles accidentally introduced. In most 

 cases, however, they seem to be minute bits of protoplasm, 

 or neutrophile or oxyphile granules extruded from leuko- 

 cytes ; their nature is not yet fully determined. These are 

 very minute in size, variable in shape, colorless and refractile, 

 and are in constant very active oscillating or Brownian move- 

 ment. 



Red blood-corpuscles (Fig. 40) are also called erythrocytes, 

 and, most appropriately, colored blood-corpuscles. 



The red blood -corpuscles of man are circular disc-shaped 

 cells, between 7 and 8 /j. in diameter (averaging 7.5 /./), and 

 about 2 p. thick. They are biconcave, and thinner in 'the centre 

 than at the edges, so that their color is lighter and paler in 

 the middle and deepens toward the periphery. In appearance 



