. A laboratory manual and text-book of embryology. Embryology. 262 THE DEVELOPMENT OF THE VASCULAR SYSTEM dorsal wall of the right atrium is guarded by two valves (Fig. 253). Along the dorsal and cephalic wall of the atrium these unite to form the septum spurium. Caudally the valves flatten out on the floor of the atrium, but the left valve later becomes continuous with the atrial septum II. In embryos of 10 to 20 mm. the atria increase rapidly in size and the right horn of the sinus venosus is taken up into the wall of the right atrium. The superior vena cava now opens directly into the cepha
. A laboratory manual and text-book of embryology. Embryology. 262 THE DEVELOPMENT OF THE VASCULAR SYSTEM dorsal wall of the right atrium is guarded by two valves (Fig. 253). Along the dorsal and cephalic wall of the atrium these unite to form the septum spurium. Caudally the valves flatten out on the floor of the atrium, but the left valve later becomes continuous with the atrial septum II. In embryos of 10 to 20 mm. the atria increase rapidly in size and the right horn of the sinus venosus is taken up into the wall of the right atrium. The superior vena cava now opens directly into the cephalic wall of the atrium, the inferior vena cava into its caudal wall. Crista lerminalis Jept. U I valve of Sinus venasus 5 up. Vena ecu/a-{opened.). Tor am en ovale Aorta VaLve of Coronary smu Tricuspid valve Oemi lunar valves of Fig. 256.—Dissection of the heart of a 105 mm. fetus from the right side. X 7. The transverse portion of the sinus venosus, as the persisting coronary sinus, opens into the posterior wall of the atrium. The right valve of the sinus venosus is very high in 10 to 65 mm. embryos (first to third month) and nearly divides the atrium into two chambers (Fig. 255). It becomes relatively lower during the third and fourth months. Its cephalic portion becomes the rudimentary crista terminalis (Fig. 256); the remainder is divided by a ridge into two parts, of which the larger cephalic division persists as the valve of the inferior vena cava (Eustachian valve) located at the right of the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Prentiss, Charles William, 1874-1915. Philadelphia, London, W. B. Saunders
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