756 THE CIRCULATORY SYSTEM 



to the sixth pair of aortal arches, a spiral septum begins to form within the 

 caudal portion of the ventral aortal sac and the bulbus cordis. This septum 

 grows caudalward within the bulbus in a spiral manner, separating the pul- 

 monary trunk ventrally and the root of the systemic aorta dorsally. It con- 

 tinues backward toward the interventricular septum and there unites with 

 a similar septum at the caudal end of the bulbus. The original bulbus cordis 

 thus becomes divided at about the seventh day of incubation into two separate 

 vessels which course spirally around each other, namely, a pulmonary trunk 

 which unites with the right ventricle and an aortal root which is continuous 

 with the left ventricle (fig. 339J). 



Coincident with the above changes, the valves of the heart are developed. 

 As the spiral septum is developed in the region of the bulbus cordis, three 

 semilunar or cup-shaped valves appear at the base of each of the divisions of 

 the bulbus. That is, at the base of the aortic root and also at the base of the 

 pulmonary trunk. These valves prevent the backward flow of the blood from 

 the aortic root into the left ventricle and from the pulmonary trunk to the 

 right ventricle. When the original atrioventricular opening is divided into two 

 atrioventricular openings by the formation of the cushion septum, the atrio- 

 ventricular or cuspid valves are formed in the two atrioventricular openings. 

 These valves prevent the backflow of blood into the atria from the ventricles. 

 At the opening of the sinus into the right atrium, the right and left sides of 

 the opening enlarge and produce folds which project inward into the atrium. 

 These folds form the sinu-atrial (sinu-auricular) valves. During the last week 

 of incubation, a third valve, the Eustachian valve or sinus septum, arises as 

 a fold from the dorsal aspect of the sinus which projects into the right atrium 

 between the openings of the vena cava inferior and the right and left venae 

 cavae superior (precavae). It divides the sinu-atrial opening. 



As hatching time approaches, the sinus becomes incorporated almost com- 

 pletely into the walls of the right atrium. A small portion of the sinus probably 

 is incorporated into the cardiac end of the left precaval vein. The sinu-atrial 

 valves also disappear and the fenestrae of the atrial septum gradually close. 



b) Mammalian Heart; 1) Early Features. The early development of 

 the mammalian heart (fig. 339) follows the general pattern of the developing 

 heart of lower vertebrates. A primitive tubular heart composed of a sinus 

 venosus, atrium, ventricle and bulbus cordis is evolved. This simple tubular 

 heart is followed by a typical sigmoid-shaped structure in which the two 

 atrial lobes hang ventrally, one on either side of the bulbus cordis, while the 

 ventricular region projects caudo-ventrally (fig. 339N). The sinus venosus 

 is much smaller, relatively speaking, than that formed in lower vertebrates and 

 tends to be placed toward the right side of the heart in relation to the future 

 right atrium. By the fifth and sixth weeks in the human (fig. 3390), the 

 heart attains outwardly the general appearance of the four-chambered heart. 



2) Internal Partitioning. The internal divisions of the heart begin to appear 



