REPRODUCTION 253 



of an opening in the auricular septum, known as the for amen ovale. 

 The flow of blood from the inferior vena cava through the fora- 

 men ovale and into the left auricle is facilitated by the fact that 

 the inferior vena cava points almost directly into the foramen 

 ovale. The Eustachian valve, consisting of a crescentic fold 

 of fibrous tissue covered with endocardium and extending from 

 a point between the opening of the superior and inferior venae 

 cavse over to the lower and anterior margin of the foramen 

 ovale, also favors this peculiar course of the blood. The base 

 of the fold lies on the right auriculoventricular ring, and the 

 concavity of the fold is directed upward. From its position the 

 Eustachian valve acts as a guiding groove or gutter for passing 

 the blood from the inferior vena cava to the foramen ovale. 

 On entering the left auricle the blood is passed into the left 

 ventricle and thence into the aorta, to be distributed all over 

 the body, but principally to the head and upper extremities. 

 From the latter regions the blood returns to the heart by the 

 superior vena cava. On entering the right auricle the blood 

 from the superior vena cava passes in front of the stream that 

 flows from the inferior vena cava to the foramen ovale, and 

 enters the right ventricle. The direction in which the superior 

 vena cava points (toward the auriculoventricular ring), and 

 also the Eustachian valve, are the factors that determine the 

 separation of the two streams. On entering the right ventricle 

 the blood from the superior vena cava is forced into the pul- 

 monary artery toward the lungs. Before reaching the lungs 

 this blood meets with a channel of communication between 

 the pulmonary artery and the aorta (ductus arteriosus), into 

 which the larger portion of the blood from the pulmonary artery 

 enters and mingles with the blood of the aorta; the remainder 

 passes along the pulmonary artery to the structure of the lungs, 

 which it nourishes, and thence back to the left auricle by means 

 of the pulmonary veins. 



The blood in the aorta that comes from the left ventricle 

 passes largely to the head, but that which enters from the 

 ductus arteriosus largely passes into the descending aorta. On 

 passing down the descending aorta, some of the blood enters the 

 mesenteric arteries, and thence back to the venous circulation 

 by means of the portal vein and the liver. Some of the blood 

 enters the iliac arteries and nourishes the lower extremities; 



