The colorless cells of the blood, commonly called the white corpuscles, differ in no essential respect from the pale round cells which are found in most of the tissues of the body. They exist in large numbers in that fluid, namely, the lymph, which is drained back from the tissues into the blood, and occupy a great part of the lymphatic glands and spleen. They are often spoken of as lymphoid cells, leucocytes, indifferent, or formative cells, on account of their being so widely distributed throughout the tissues.

When fresh blood is examined with the microscope these cells can generally be seen adhering to the glass slide or cover glass and lying singly, apart from the groups of red discs. They can be recognized by their absence of marked color, finely granular structure, spherical shape, and the nuclei which may often be recognized near the centre of the cell. Though not always visible in fresh preparations, the nuclei can be brought to light by the action of many reagents - e.g., acetic acid. If examined while being moved by the blood current to the capillary vessels, they are seen to pass slowly along in contact with the vessel wall, while the red corpuscles rush rapidly past them down the centre of the channel (Fig. 98). This may partly be due to their peculiar adhesiveness, which also causes them to stick to the glass slide, while the red discs are washed away when a gentle stream of saline solution is allowed to flow under the cover glass. These cells show all the manifestations of activity characteristic of independent living beings. If kept in a medium suitable to them, and at the temperature of the body, they will soon be seen to alter their appearance; their outline becomes faint, they are no longer spherical, but very irregular in shape, and constantly change their form by sending out and retracting processes, by means of which they change their position, so that they may be said to perform locomotion. These movements are rendered more active by a slight increase of temperature, and are checked by cold. For continued observation, about 380 C. is the best temperature for mammalian blood. The blood of the frog is generally used to see the motion of the white corpuscles, as warming is unnecessary in the case of cold-blooded animals. They respond to many other influences, such as electricity, etc., even for a considerable time after removal from the body.

No doubt they continually absorb fluid nutriment from the surrounding medium, as is shown by the effect of poisons on them; and, by the repeated contractions and relaxations of parts of their substance in the form of pseudopodia, they appear to take into the inner parts of the protoplasm solid particles, which after some time are ejected after the manner of the small unicellular animals known as amoebae (p. 91).

While in motion in the circulation none of these amoeboid movements appear to take place, but when an arrest of the flow of blood in the capillaries occurs, they not only change their form, but also their position; and if there be no onward flow of blood for some little time, they creep out of the capillaries, passing through the delicate vessel walls. This emigration of the blood cells is possibly a common event when a tissue is in need of textural repair. When excessive, it forms one of the most striking items of the series of events occurring in inflammation.

Vessels of the Frog's Web.

Fig. 98. Vessels of the Frog's Web.

(a) Trunk of vein, and (b b) its tributaries passing across the capillary network. The dark spots are pigment cells.

These cells differ much in size; generally they are somewhat larger than the red discs. Nothing like a cell wall can be seen to surround them, and from the movements above described it would appear certain that they are free masses of active protoplasm.

The number of white cells that can be collected is too small to allow of accurate chemical analysis, but there is no reason to suppose that they differ from other forms of protoplasm.