circulation. For example, the rhythmically contracting heart forces a
small quantity of blood into the arteries at short intervals. These
tubes are large near the heart, but smaller at their ends, where they
flow into the veins, so that the blood does not flow out into the veins
so readily as it flows in from the heart. The jet of blood that is sent
in with every beat of the heart slightly stretches the artery, and the
tension thus produced causes the blood to continue to flow between the
beats. But the heart continues beating, and there is an accumulation of
the blood in the arteries until it exists under some pressure--a
pressure sufficient to force it rapidly through the small ends of the
arteries into the veins. After passing into the veins the pressure is at
once removed, since the veins are larger than the arteries, and there is
no resistance to the flow of the blood. Hence the blood in the arteries
is under pressure, while there is little or no pressure in the veins.
Into the details of this matter we need not go, but this will be
sufficient to indicate that the whole process is a mechanical one.

We must not fail to see, however, that in this problem of circulation
there are two points at least where once more we meet with that class of
phenomena which we still call vital. The beating of the heart is the
first of these, for this is active muscular power. The second is a
contraction of the smaller blood-vessels which regulates the blood
supply. Both of these phenomena are phases of muscular activity, and
will be included under the discussion of other similar phenomena later.

[Illustration: FIG. 6.--A bit of muscle with its blood-vessels: _a_, the
muscle fibres; _b_, the minute blood-vessels. The fibres and vessels are
bathed in lymph (not shown in the figure), and food material passes
through the walls of the blood-vessels into this lymph.]