4. When examined under the microscope, the blood no longer appears a simple fluid, and its color is no longer red. It is then seen to be made up of two distinct parts: first, a clear, colorless fluid, called the _plasma_; and secondly, of a multitude of minute solid bodies, or corpuscles, that float in the watery plasma. The plasma, or nutritive liquid, is composed of water richly charged with materials derived from the food, viz., albumen, which gives it smoothness and swift motion; fibrin; certain fats; traces of sugar; and various salts. [Illustration: FIG 25.--THE BLOOD CORPUSCLES.] [Sidenote: 5. State what you can of the little bodies called corpuscles.] 5. THE BLOOD CORPUSCLES.--In man, these remarkable "little bodies," as the word _corpuscles_ signifies, are of a yellow color, but by their vast numbers impart a red hue to the blood. They are very small, having a diameter of about 1/3500 of an inch, and being one-fourth of that fraction in thickness; so that if 3,500 of them were placed in line, side by side, they would only extend one inch; or, if {103} piled one above another, it would take at least 14,000 of them to stand an inch high. Although so small in size, they are very regular in form. As seen under the microscope, they are not globular or spherical, but flat, circular, and disc-like, with central depressions on each side, somewhat like a pearl button that has not been perforated. In freshly-drawn blood they show a disposition to arrange themselves in little rolls like coins (Fig. 25). [Illustration: FIG. 26. _a_, Oval Corpuscles of a fowl. _b_, Corpuscles of a frog. _c_, Those of a shark. The five small ones at the upper part of the figure, represent the human corpuscles magnified 400 times.] [Sidenote: 6. The size and shape of corpuscles? Why is the fact important?] 6. The size and shape of the blood corpuscles vary in different animals, so that it is possible to discriminate between those of man and the lower animals (Fig. 26). This is a point of considerable practical importance. For example, it is sometimes desirable to decide in a court of justice the source, whether from man or an inferior animal, of blood stains upon the clothing of an accused person, or upon some deadly weapon. This may be done by a microscopical examination of a minute portion of the dried stain, previously refreshed by means of gum-water. Certain celebrated cases are recorded in which the guilt of criminals has been established, and they have been condemned and punished upon the evidence which science rendered on this single point, the detecting of the human from other blood. [Sidenote: 7. The character of the blood of dead animals? Means of detecting such blood?]