ar tissue; hence they are thin, and when empty
collapse. The inner surfaces of many of the veins are supplied with
pouch-like folds, or pockets, which act as valves to impede the backward
flow of the blood, while they do not obstruct blood flowing forward toward
the heart. These valves can be shown by letting the forearm hang down, and
sliding the finger upwards over the veins (Fig. 73).

The veins have no force-pump, like the arteries, to propel their contents
towards their destination. The onward flow of the blood in them is due to
various causes, the chief being the pressure behind of the blood pumped
into the capillaries. Then as the pocket-like valves prevent the backward
flow of the blood, the pressure of the various muscles of the body urges
along the blood, and thus promotes the onward flow.

The forces which drive the blood through the arteries are sufficient to
carry the blood on through the capillaries. It is calculated that the
onward flow in the capillaries is about 1/50 to 1/33 of an inch in a
second, while in the arteries the blood current flows about 16 inches in a
second, and in the great veins about 4 inches every second.

[Illustration: Fig. 74.--The Structure of Capillaries.

Capillaries of various sizes, showing cells with nuclei]

189. The Capillaries. The capillaries are the minute, hair-like
tubes, with very thin walls, which form the connection between the ending
of the finest arteries and the beginning of the smallest veins. They are
distributed through every tissue of the body, except the epidermis and its
products, the epithelium, the cartilages, and the substance of the teeth.
In fact, the capillaries form a network of the tiniest blood-vessels, so
minute as to be quite invisible, at least one-fourth smaller than the
finest line visible to the naked eye.

The capillaries serve as a medium to transmit the blood from the arteries
to the veins; and it is through them that the blood brings nourishment to
the surrounding tissues. In brief, we may regard the whole body as
consisting of countless groups of little islands surrounded by
ever-flowing streams of blood. The walls of the capillaries are of the
most delicate structure, consisting of a single layer of cells loosely
connected. Thus there is allowed the most free interchange between the
blood and the tissues, through the medium of the lymph.

The number of the capillaries is inconceivable. Those in the lungs alone,
placed in a continuo