11).

Between the veins and arteries is a large number of capillaries with thin transparent walls and a diameter no greater than that of the single blood corpuscles; they receive the blood from the arteries and the flow in them is continuous.

The white and red blood corpuscles can be distinguished, the red appearing as oval discs and the white as colorless spheres.

In the arteries and veins the red corpuscles remain in the centre of the vessels appearing as a rapidly moving red core, and between this core and the wall of the vessels is a layer of clear fluid in which the white corpuscles move more slowly, often turning over and over as a ball rolls along the table.
If, now, the web be injured by pricking it or placing some irritating substance upon it, a change takes place in the circulation.

The arteries and the veins become dilated and the flow of blood more rapid, so rapid, indeed, that it is difficult to distinguish the single corpuscles.