181.  Colorless Corpuscles.  The colorless corpuscles are larger than the red, their average diameter being about 1/2500 of an inch.  While the red corpuscles are regular in shape, and float about, and tumble freely over one another, the colorless are of irregular shape, and stick close to the glass slide on which they are placed.  Again, while the red corpuscles are changed only by some influence from without, as pressure and the like, the colorless corpuscles spontaneously undergo active and very curious changes of form, resembling those of the amoeba, a very minute organism found in stagnant water (Fig. 2).

The number of both red and colorless corpuscles varies a great deal from time to time.  For instance, the number of the latter increases after meals, and quickly diminishes.  There is reason to think both kinds of corpuscles are continually being destroyed, their place being supplied by new ones.  While the action of the colorless corpuscles is important to the lymph and the chyle, and in the coagulation of the blood, their real function has not been ascertained.

[Illustration:  Fig. 66.—­Blood Corpuscles of Man.

  A, red corpuscles;
  B, the same seen edgeways;
  C, the same arranged in rows;
  D, white corpuscles with nuclei.
]

Experiment 85. To show the blood corpuscles.  A moderately powerful microscope is necessary to examine blood corpuscles.  Let a small drop of blood (easily obtained by pricking the finger with a needle) be placed upon a clean slip of glass, and covered with thin glass, such as is ordinarily used for microscopic purposes.

The blood is thus spread out into a film and may be readily examined.  At first the red corpuscles will be seen as pale, disk-like bodies floating in the clear fluid.  Soon they will be observed to stick to each other by their flattened faces, so as to form rows.  The colorless corpuscles are to be seen among the red ones, but are much less numerous.

182.  The Coagulation of the Blood.  Blood when shed from the living body is as fluid as water.  But it soon becomes viscid, and flows less readily from one vessel to another.  Soon the whole mass becomes a nearly solid jelly called a clot.  The vessel containing it even can be turned upside down, without a drop of blood being spilled.  If carefully shaken out, the mass will form a complete mould of the vessel.

At first the clot includes the whole mass of blood, takes the shape of the vessel in which it is contained, and is of a uniform color.  But in a short time a pale yellowish fluid begins to ooze out, and to collect on the surface.  The clot gradually shrinks, until at the end of a few hours it is much firmer, and floats in the yellowish fluid.  The white corpuscles become entangled in the upper portion of clot, giving it a pale yellow look on the top, known as the buffy coat.  As the clot is attached to the sides of the vessel, the shrinkage is more pronounced toward the center, and thus the surface of the clot is hollowed or cupped, as it is called.  This remarkable process is known as coagulation, or the clotting of blood; and the liquid which separates from the clot is called serum.  The serum is almost entirely free from corpuscles, these being entangled in the fibrin.