108 THE TISSUES 



are usually present. Red blood cells are biconcave, with rounded edges. 

 Seen on the flat, the difference in thickness between centre and pe- 

 riphery is evidenced by the difference in refraction (Fig. 53, i). Seen 

 on edge, the shape resembles that of a dumb-bell (Fig. 53, 2). Singly 

 or in small numbers, red blood cells have a pale straw color, due to 

 the presence of haemoglobin. Redness is apparent only when the 

 cells are seen in large numbers. If fresh blood be allowed to stand 

 for a moment, the red cells are seen to adhere to one another by their 

 flat surfaces, forming row^s or rouleaux (Fig. 53, 3). 



Subjected to the usual technic, the red blood cell appears homo- 

 geneous. By the use of special methods, this apparently homogene- 

 ous substance can be separated into (a) a color-bearing proteid — 

 hcEmoglohin, and {h) a stroma, the latter representing the protoplasm 

 of the cell. Peripherally the stroma probably forms an extremely 

 delicate cell membrane, although the presence of any membrane 

 w^hatever is denied by some. It is the haemoglobin which gives color 

 to the corpuscles. Haemoglobin is a complex proteid, which can be 

 resolved into a globulin and a pigment, h(B?natin. It is held in solu- 

 tion or in suspension in the stroma. 



Red blood cells are soft and elastic, and are easily twisted to 

 accommodate themselves to the smallest capillaries. This elasticity 

 results in their assuming many shapes, the most common next to 

 their disc shape being that of a cup or cap. 



The red blood cell is extremely susceptible to changes in the plasma. 

 Thus even shght evaporation of the plasma results in osmosis between 

 the now denser surrounding fluid and the contents of the cell. This 

 causes fluid to leave the cell, with the result that the latter becomes 

 spheroidal and irregularly shrunken, with minute knob-like projec- 

 tions from its surface. This is known as crenation of the red cell. 

 The addition of water to blood, thus decreasing the specific gravity 

 of the plasma, has the opposite eft'ect, resulting in swelling of the cell. 

 It also causes solution of the haemoglobin, w^hich leaves the cell, the 

 latter then appearing colorless, with a faint circular outhne — the 

 membrane of the cell. This separation of the haemoglobin from the 

 corpuscle is also caused by freezing and thawing, by heat (6o°C.), by 

 the addition of dilute acids, ether, or chloroform. 



Dilute alkaline solutions and bile first cause the red corpuscles to 

 swell and become spherical, and then to dissolve. This is known 

 as hcemolysis-, and may also be cfl'ected by mixing the blood of one 

 species with that of another. Dilute aceti9 acid causes swelling and 



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