HEART 



177 



wall is incomplete, the blood from either ventricle may pass out through 

 either artery as shown in E. In the more advanced human embryo, F, 

 the partition between the aorta and pulmonary artery has extended so 

 that it joins the interventricular septum, and causes the interventricular 

 foramen to open into the root of the aorta only (s). This portion of the 

 interventricular wall which is the last to form, is translucent in the adult, 

 and is known as the septum membranaceum. 



As previously noted all the veins come together to enter the right 

 atrium. The original vitelline veins are no longer directly connected 

 with the heart, and their persistent cardiac outlet becomes the terminal 

 part of several large branches. These are the superior vena cava from 

 the head and arms, the inferior vena cava from the trunk and legs (receiv- 

 ing as branches the hepatic vein draining the portal system from the in- 

 testine, and the umbilical vein from the placenta) ; and the coronary sinus 

 which, as it passes across the heart in the coronary sulcus, receives 

 branches from the wall of the heart. All these veins come together in a 

 cavity, ill denned in mammals, known as the sinus venosus, and this sinus 

 empties into the right atrium through an orifice guarded by a valve with 

 right and left flaps. With further growth the sinus venosus becomes a 

 part of the atrium, and the superior and inferior venae cavas and coronary 

 sinus open separately, guarded by imperfect valves derived from the 

 valves of the sinus venosus. The left flap of this valve is said to assist 

 in closing the foramen ovale; the right flap becomes subdivided into the 

 rudimentary valve of the vena cava inferior (Eustachian valve) and the 

 valve of the coronary sinus (Thebesian valve). The degeneration of the 

 valve of the venous sinus seems to take place after the bicuspid and 

 tricuspid valves have become well formed, and have rendered it super- 

 fluous. In early stages it must be regarded as the principal valve of the 

 heart. The tricuspid valve, between the right atrium and right ventricle, 

 develops from the cardiac walls in the same way as the bicuspid valve. 

 Their formation is discussed by Mall (Amer. Journ. Anat., 1912, vol. 13, 

 pp. 249-298). 



In the embryonic heart, the left atrium receives most of its blood 

 through the foramen ovale, but the pulmonary veins early grow out from 

 it as a small vessel (Fig. 170, D) which sends four branches to the lungs. 

 These are given off near the heart, and with the enlargement of the atrium 

 they come to open into it separately. After birth they are the only 

 supply of the left atrium, and they convey the same quantity of blood as 

 the veins which enter the right atrium. 



LAYERS OF THE HEART. Early in the development of the heart a 

 third layer, consisting of mesenchyma, forms between the endothelium 

 and mesothelium. It gives rise to the cardiac musculature, and toward 

 the primary layers it produces connective tissue. The wall of the heart 



