The veins are formed by the aggregation of the capillaries, and return the blood to the heart. They, like the arteries, are arranged in two sets - pulmonary and systemic. The pulmonary veins enter the left auricle of the heart by four openings, and the systemic veins - i.e., the coronary sinus, and superior and inferior venae cavae-enter the right auricle by three openings.

The Pulmonary veins are found in the alveoli of the lungs, and form a single large vessel for each lobe. In the root of the right lung the veins from the upper, and middle lobes join together, so that two veins pass out from the root of each lung to enter the left auricle.

Systemic Veins

The systemic veins, three in number, all enter the right auricle.

The coronary sinus returns the blood from the walls of the heart only. The superior vena cava returns the blood from the head, neck, upper limbs, thoracic wall, and a part of the posterior abdominal wall. The inferior vena cava returns blood from the lower limbs, and the walls and organs of the abdomen and pelvis.

The veins of the body wall and limbs are arranged in two sets-superficial and deep. The superficial veins run in the superficial fascia, and the deep veins accompany the arteries, usually as venae comites. The superficial veins ultimately pierce the deep fascia to unite with the deep veins. The visceral veins, of which there is usually one accompanying the artery, end in the deep systemic veins, with the exception of the portal vein.

The Coronary Sinus lies between the left auricle and left ventricle, and terminates in the lower and back part of the right auricle. It receives the blood from the walls of the heart.

The Superior Vena Cava is formed at the lower border of the first right costal cartilage by the junction of the two innominate veins, descends to the level of the third right costal cartilage, where it enters the right auricle. Anteriorly it is overlapped by the right lung and pleura and the ascending aorta.

Tributaries, - Vena azygos major.

Small pericardial veins.

The Vena azygos major is the upward continuation of a vessel known as the right ascending lumbar vein, which connects together the lumbar veins of the right side. It passes through the aortic opening in the diaphragm, up the posterior thoracic wall, and arches over the root of the right lung to enter the superior vena cava. It receives the right intercostal veins and the vena azygos minor superior and inferior, which receive the lumbar and intercostal veins of the left side.

The Innominate Vein of each side is formed behind the sternal end of the clavicle by the union of the internal jugular with the subclavian vein. The two innominate veins unite to form the superior vena cava behind the first right costal cartilage; consequently the left one is longer than the right.

The tributaries are the veins corresponding to the arteries in that region - viz., internal mammary, inferior thyroid, vertebral, pericardial, and bronchial.

The Internal Jugular vein commences as the direct continuation of the lateral sinus of the brain, and passes through the jugular foramen to reach the neck, uniting behind the sternal nd of the clavicle with the subclavian vein to form the -innominate vein. It lies on the outer side of the common carotid artery, and on the left side overlaps it in front at its lower end.

Tributaries

Sinus in the brain.

Veins from the wall of the pharynx, the tongue, and the veins accompanying the thyroid arteries.