These two great venous trunks are the inferior vena cava, bringing the blood from the trunk and the lower limbs, and the superior vena cava, bringing the blood from the head and the upper limbs.  These two large trunks meet as they enter the right auricle.  The four pulmonary veins, as we have learned, carry the arterial blood from the lungs to the left auricle.

[Illustration:  Fig. 73.

  A, part of a vein laid open, with two pairs of valves;
  B, longitudinal section of a vein, showing the valves closed.
]

A large vein generally accompanies its corresponding artery, but most veins lie near the surface of the body, just beneath the skin.  They may be easily seen under the skin of the hand and forearm, especially in aged persons.  If the arm of a young person is allowed to hang down a few moments, and then tightly bandaged above the elbow to retard the return of the blood, the veins become large and prominent.

The walls of the larger veins, unlike arteries, contain but little of either elastic or muscular 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.