558 VEINS. 



the heart, and opens into the coronary sinus, its aperture being guarded by two 

 valves. It receives the posterior cardiac vein, and the left cardiac veins from 

 the left auricle and ventricle, one of which, ascending along the left margin of 

 the ventricle, is of large size. The branches joining it are provided with 

 valves. 



The posterior cardiac vein commences, by small branches, at the apex of the 

 heart, communicating with those of the preceding. It ascends along the pos- 

 terior interventricular groove to the base of the heart, and terminates in the 

 coronary sinus, its orifice being guarded by a valve. It receives the veins 

 from the posterior surface of both ventricles. 



The anterior cardiac veins are three or four small branches, which collect the 

 blood from the anterior surface of the right ventricle. One of these (the vein 

 of Galen), larger than the rest, runs along the right border of the heart. They 

 open separately into the lower part of the right auricle. 



The venae Thebesii are numerous minute veins, which return the blood 

 directly from the muscular substance, without entering the venous current. 

 They open, by minute orifices (foramina Thebesii], on the inner surface of the 

 right auricle. 



The Coronary Sinus is that portion of the great cardiac vein which is 

 situated in the posterior part of the left auriculo-ventricular groove. It is 

 about an inch in length, presents a considerable dilatation, and is covered by 

 the muscular. fibres of the left auricle. It receives the great cardiac vein, the 

 posterior cardiac vein, and an oblique vein from the back part of the left 

 auricle, the remnant of the obliterated left innominate trunk of the foetus, 

 described by Mr. Marshall. The coronary sinus terminates in the right auricle, 

 between the inferior vena cava and the auriculo-ventricular aperture, its orifice 

 being guarded by a sernilunar fold of the lining membrane of the heart, the 

 coronary valve. All the branches joining this vessel, excepting the oblique 

 vein above mentioned, are provided with valves. 



THE PULMONAEY YEIN-S. 



The Pulmonary Yeins return the arterial blood from the lungs to the left 

 auricle of the heart. They are four in number, two for each lung. The 

 pulmonary differ from other veins in several respects. 1. They carry arterial, 

 instead of venous blood. 2. They are destitute of valves. 3. They are only 

 slightly larger than the arteries they accompany. 4. They accompany those 

 vessels singly. They commence in a capillary network, upon the parietes of 

 the bronchial cells, where they are continuous with the ramifications of the 

 pulmonary artery, and, uniting together, form a single trunk for each lobule. 

 These branches, 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 unites with that from the upper lobe, in most cases, forming two 

 trunks on each side, which open separately into the left auricle. Occasionally 

 they remain separate; there are then three veins on the right side. Not 

 unfrequently, the two left pulmonary veins terminate by a common opening. 



Within the lung, the branches of the pulmonary artery are in front, the veins 

 behind, and the bronchi between the two. 



At the root of the lung, the veins are in front, the artery in the middle, and the 

 bronchus behind. 



Within the pericardium, their anterior surface is invested by the serous layer 

 of this membrane. The right pulmonary veins pass behind the right auricle 

 and ascending aorta ; the left pass in front of the thoracic aorta, with the left 

 pulmonary artery. 



