VENOUS SYSTEM (in descriptive hu man anatomy).—Under this head it is in tended to give a connected and concise ac count of the descriptive anatomy of the several elements of the venous system, referring the reader to those articles which treat of the regional anatomy of the body, in which will These veins commence in the capillaries of the pulmonary lobules, and by successive junctions with adjacent branches enlarge in size, whilst they diminish in number. They traverse the lungs in company with the sub divisions of the bronchial tubes and of the pul monary arteries, the number of venous rami fications being identical with that of the arterial, and finally emerge from those organs, having formed into trunks, each of which corresponds to a lobe Of the lungs. There are consequently three venous trunks for the right lung, and two for the left. On the right side the branch from the middle lobe unites with that from the superior; and hence two pulmonary veins, one superior to the other, are usually found in the root of each lung, where on both sides they are placed below hut on a plane anterior to the pulmonary artery and bronchial tube.
In the substance of the lungs the relative po sition of these parts is different, for there the branches of the veins are behind, whilst those of the artery are in front, the bronchus being interposed between both.
The four pulmonary veins then perforate the pericardium, and after a short course enter the most posterior part of the left auricle, those of the left side opening very close to gether, and not unfrequently by a common orifice.
Within the pericardium these veins are in vested by its serous membrane, but on their anterior surfaces only. In passing to their destination, the right pulmonary veins lie behind the right auricle of the heart.