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liquid itself is seen to be almost as clear as water. This liquid is called the plasma. Floating in it are millions of biconcave disks containing a pigment (hemoglobin) which gives the red color to the blood. The disks are called red corpuscles (Fig. 60). A few irregularly shaped bodies, nucleated and almost transparent, and called white corpuscles, are also found in the blood. The red corpuscles go only where the plasma carries them (Exps. 3, 4). The white corpuscles sometimes leave the blood vessels entirely. At times one may be seen shaped like a dumb-bell, half of it through the wall of the blood vessel and half still in the blood vessel. After the corpuscle is out, no hole can be found to account for its mysterious passage. The white corpuscles consist of protoplasm. The red corpuscles contain no protoplasm. Hence the latter are not really alive.

Fig. 60.—Human Blood Cells (magnified 40,000 areas), showing many red cells and a single white blood cell on left, larger than red cells. (Peabody.)

Fig. 61.—Side and Front Views of Frog's and Man's Red Corpuscles, drawn to same scale. Compare outline, concavity, diameters.

The Use of Each Part of the Blood.—The plasma keeps the blood in a liquid state, so that it may flow readily; the plasma also transports the food that has been eaten and digested, and carries carbon dioxid to the lungs and other waste material to the kidneys. The red