results for a robust than for a weak individual.

Most of the changes which take place after an injury and their
sequence can be followed under the microscope. If the thin membrane
between the toes of a living frog be placed under the microscope the
blood vessels and the circulating blood can be distinctly seen in the
thin tissue between the transparent surfaces. The arteries, the
capillaries and veins can be distinguished, the arteries by the
changing rapidity of the blood stream within them, there being a
quickening of the flow corresponding with each contraction of the
heart; the veins appear as large vessels in which the blood flows
regularly (Fig. 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. In a short while the rapidity of flow in the
dilated vessels diminishes, becoming slower than the normal, and the
separation between the red and white corpuscles is not so evident. In
the slowly moving stream the white corpuscles move much more slowly