Henry Gray (1825–1861). Anatomy of the Human Body. 1918.
2. The Pulmonary Veins
The pulmonary veins return the arterialized blood from the lungs to the left atrium of the heart. They are four in number, two from each lung, and are destitute of valves. The commence in a capillary net-work upon the walls of the air sacs, where they are continuous with the capillary ramifications of the pulmonary artery, and, joining together, form one vessel for each lobule. These vessels uniting successively, form a single trunk for each lobe, three for the right, and two for the left lung. The vein from the middle lobe of the right lung generally unites with that from the upper lobe, so that ultimately two trunks from each lung are formed; they perforate the fibrous layer of the pericardium and open separately into the upper and back part of the left atrium. Occasionally the three veins on the right side remain separate. Not infrequently the two left pulmonary veins end by a common opening.