The flow of blood in the arteries is caused by the muscular force of the heart, aided by the elastic tissues and muscular fibers of the arterial walls, and to a certain extent by the muscles themselves. Most of the great arterial trunks lie deep in the fleshy parts of the body; but their branches are so numerous and become so minute that, with a few exceptions, they penetrate all the tissues of the body,—so much so, that the point of the finest needle cannot be thrust into the flesh anywhere without wounding one or more little arteries and thus drawing blood.
188. The Veins. The veins are the blood-vessels which carry the impure blood from the various tissues of the body to the heart. They begin in the minute capillaries at the extremities of the four limbs, and everywhere throughout the body, and passing onwards toward the heart, receive constantly fresh accessions on the way from myriad other veins bringing blood from other wayside capillaries, till the central veins gradually unite into larger and larger vessels until at length they form the two great vessels which open into the right auricle of the heart.
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.
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.