The veins carry the blood from the extremities to the heart They are divided like the arteries into two classes, according as, filled with red blood, they run from the capillaries of the lungs to the trunks of the pulmonary veins— which is the lesser circulation; or as they carry the black blood to the venae cavae—which is the general circulation. The veins of the liver and their principal trunk, the portal vein, have sometimes been considered as also a separate system; and this distinction has also been extended to those of the kidneys and other organs.
The walls of the veins are much thinner than those of the arteries, and are formed of four tunics, of which the fourth or internal one is like that of the arteries; the others are formed of elastic or cellular fibres, longitudinal in the third, circular in the second, in such a manner as to present the greatest resistance. The third and fourth coats form folds, which when extended partially close the vessel. These valves are disposed in such a manner as that when the blood moves toward the heart it presses them close to the walls of the veins, and they are no obstacle to its course; while, if it moves in the contrary direction, they close and prevent its return toward the extremities. The veins are disposed in two planes. Some are deeply placed, and accompany the arteries, of which they are called the companions (vena comites arteriarum); others are superficial, and creep along under the skin without any coincidence with the direction of the arterial vessels. Springing from the capillaries, by means of which they communicate with the arteries, the venous radicles unite more rapidly than those of the arteries into considerable branches, superior in numbers and total capacity to the arterial trunks. Many of the arteries, in fact, are accompanied by two veins as satellites, of at least equal calibre; and the superficial veins show still greater disproportion. In the interior of the cranium the veins are transformed into sinuses or canals formed by the dura mater, which receive the venous branches of the brain. The veins are enveloped in their course by numerous lymphatic vessels.
The superior and inferior vena cava are to the venous system what the aorta is to the arterial system.
The superior vena cava receives the blood from the head, neck, the upper extremities, and the walls of the chest; it opens into the right auricle of the heart It is formed by the two brachio-cephalic trunks and the azygos vein. Each of these brachio-cephalic venous trunks unites, like the arterial trunks of the same name, all the principal veins of the head and the arms, which are the two jugulars, internal and external, and the subclavian.
The internal jugular corresponds to the carotid artery; it receives the blood from the sinuses of the dura mater, from the veins of the head, of the neck, and part of the shoulder. The external jugular carries the blood from a part of the superficial veins of the head to the subclavian vein.
The subclavian vein corresponds to the artery of the same name, and receives the companion veins, and veins corresponding in name to the arteries of the upper limb; it is also the common trunk of the superficial veins of the hand, forearm, and arm, the principal of which are the cephalic and basilic. This last vein crosses the radial artery in the bend of the elbow, and is separated from it by a tendinous expansion of the biceps muscle. The cephalic and basilic veins are those most commonly opened in blood-letting; this operation is rendered delicate by the situation of the basilic, which exposes the artery to the danger of being wounded.
The azygos vein (azygos, without a fellow) forms the communication between the superior and inferior vena cava. It rises vertically in the posterior mediastinum and to the right of the spine, and about the level of the seventh rib it receives the lesser vena azygos, which comes from the abdomen.
The inferior vena cava opens into the right auricle under the superior vena cava. It is the common trunk of all the veins coming from the parts below the diaphragm, and is formed by the union of the two iliac trunks, companions of the iliac arteries; it runs up vertically to the right of the spine as companion vein to the aorta, and receives the veins of the abdomen. Its primitive branches, the iliac trunks, are formed by the union of the veins of the pelvic cavity and of the lower extremities, companions of the arteries of the same name. Among the superficial series of veins, which are also tributaries to the main iliacs, are the external and internal saphenous veins, which run from the foot to the top of the thigh. These two veins are plainly visible on the front of the ankle and on the calf.
An arrangement of veins peculiar to the abdomen, and especially to the liver, is designated by this term. The portal vein is formed by the union of the veins of the mesentery, spleen, stomach, and intestine; it transmits the blood from these organs to the liver, from whence it is poured into the inferior vena cava.
This is the name given to a special circulatory apparatus composed of very delicate vessels with transparent walls and of ganglia or glands, which appear to be formed by these vessels, some of which terminate in and others spring from them.
The lymphatic vessels have a very irregular course, and exhibit numerous swellings which are owing to valves. They exist in every part of the body, and transport the chyle and the lymph, drawn by their microscopical roots from the surface of the mucous membrame of the intestines, or from the tissues of the organs. They accompany the blood-vessels in their course, especially the veins, and they are also found in great numbers at the surface of the body in regions abounding in subcutaneous veins, as in the limbs, face, and neck. They are very numerous in the mesentery and around the intestines; they unite in two principal trunks or reservoirs, one of which is the thoracic duct, which rises through the chest at the left of the spine, and opens into the left subclavian vein, the other is called the great right lymphatic vessel (ductus lymphaticus dexter), which runs parallel with the first, and Opens into the right subclavian vein.
Galen was the first to discover that the arteries contained blood and that they communicated with the veins, but he went no farther than this. In 1553 Michael Servetus, guessing, so to speak, the phenomena of the pulmonary circulation, indicated very exactly the course of the blood and its elaboration in the lungs by contact with the air. But the doctrine of Servetus rested neither upon proof nor experiment, it resulted from a sort of intuitive perception of the facts, and he was not aware either of the impulsive force of the heart or the action of its valves.
Other physiologists, like Servetus, had glimpses of the truth and added new discoveries to his; in fact, most of the phenomena of the circulation had been suspected or indicated at the commencement of the seventeenth century, but all the knowledge on the subject was but a chaos of facts and reasonings without unity, and often contradictory. It required the genius of Harvey to extract from this chaos a simple and irrefutably demonstrated system.