gins to the wall of the artery at its junction with the ventricle. The straight borders of the valve are unattached and are directed upwards in the course of the vessel, against the sides of which they are pressed during the passage of blood along its canal. The free margin of each valve is somewhat thicker than the rest of the valve and is strengthened by a bundle of tendinous fibers. During the passage of blood along the pulmonary artery, these valves are pressed against the sides of its cylinder. During ventricular diastole (rest), when the current of blood along the pulmonary artery is checked and partly thrown back by its elastic walls, these valves become immediately expanded and close the entrance of the tube. 3 The left atrium is smaller but thicker than the right atrium. It consists of two parts; a principle cavity/sinus and an appendix auriculae. The sinus is cuboidal in form and is covered in the front by the pulmonary artery and the aorta. Internally, it is separated from the right atrium by the septum auricularum. Behind the sinus on each side, it receives the pulmonary veins. The appendix auriculae in the left atrium is narrower and more curved than the same structure in the right atrium. Its margins are more deeply indented, presenting a kind of foliated appearance. Its direction is forwards towards the right side, overlapping the root of the pulmonary artery. There are two main openings in the left atrium; the openings of the four pulmonary veins and the atrial-ventricular opening. Two of the four pulmonary veins open into the right side of the atrium and two open into the left side. The two veins on the left exit into the atrium through a common opening. None of the pulmonary veins have valves. The atrial-ventricular opening is the large oval opening of blood flow between the atrium and the ventricle. It is smaller than the same opening between the right atrium and ventricle.3 The...
