THE BLOOD 



503 



FIG. 453. Human red blood corpuscles. Highly magnified, a. 

 Seen from the surface. 6. Seen in profile and forming rouleaux. 

 c. Rendered spherical by water, d. Rendered crenate by salt 

 solution. 





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General Composition of the Blood. Blood consists of a faintly yellow fluid, the 

 plasma or liquor sanguinis, in which are suspended numerous minute particles, 

 the blood corpuscles, the majority of which are colored and give to the blood its 

 red tint. If a drop of blood be placed in a thin layer on a glass slide and examined 

 under the microscope, a number of these corpuscles will be seen floating in the 

 plasma. 



The Blood Corpuscles are of 

 three kinds: (1) colored cor- 

 puscles or erythrocytes ; (2) color- 

 less corpuscles or leucocytes; (3) 

 blood platelets. 



1. Colored or red corpuscles 

 (erythrocytes} , when examined 

 under the microscope, are seen 

 to be circular disks, biconcave in 

 profile. The disk has no nucleus, 

 but, in consequence of its bicon- 

 cave shape, presents, according 

 to the alterations of focus under 

 an ordinary high power, a central 

 part, sometimes bright, sometimes 



dark, which has the appearance of a nucleus (Fig. 453, a). It is to the aggregation 

 of the red corpuscles that the blood owes its red hue, although when examined 

 >y transmitted light their color appears to be only a faint reddish yellow. The 

 corpuscles vary slightly in size even in the same drop of blood, but the average 

 diameter is about 7.5/*, 1 and the thickness about 2ju. Besides these there are 

 found certain smaller corpuscles of about one-half of the size just indicated; 

 these are termed microcytes, and are very scarce in normal blood; in diseased con- 

 ditions (e. g., anemia), however, they are more numerous. The number of red 

 corpuscles in the blood is enormous; between 4,000,000 and 5,000,000 are con- 

 tained in a cubic millimetre. Power states that the red corpuscles of an adult 

 would present an aggregate surface of about 3000 square yards. 



If the web of a living frog's foot be spread out and examined under the micro- 

 scope the blood is seen to flow in a continuous stream through the vessels, and the 

 corpuscles show no tendency to adhere to each other or to the wall of the vessel. 

 Doubtless the same is the case in the human body; but when human blood is drawn 

 nd examined on a slide without reagents the corpuscles tend to collect into heaps 

 like rouleaux of coins (Fig. 453, 6). It has been suggested that this phenomenon 

 may be explained by alteration in surface tension. During life the red corpuscles 

 may be seen to change their shape under pressure so as to adapt themselves, to 

 some extent, to the size of the vessel. They are, however, highly elastic, and 

 speedily recover their shape when the pressure is removed. They are readily 

 influenced by the medium in which they are placed. In water they swell up, lose 

 their shape, and become globular (endosmosis) (Fig. 453, c). Subsequently the 

 hemoglobin is dissolved out, and the envelope can barely be distinguished as a 

 faint circular outline. Solutions of salt or sugar, denser than the plasma, give 

 them a stellate or crenated appearance (exosmosis) (Fig. 453, d), but the usual 

 shape may be restored by diluting the solution to the same tonicity as the plasma. 

 The crenated outline may be produced as the first effect of the passage of an elec- 

 ric shock: subsequently, if sufficiently strong, the shock ruptures the envelope. 

 A solution of salt, isotonic with the plasma, merely separates the blood corpuscles 

 mechanically , without changing their shape. Two views are held with regard to 



i A micromillimetre (J l ) is 1/1000 of a millimetre or 1/25000 of an inch. 



