MORPHOGENESIS OF CIRCULATORY SYSTEM 747 



the posterior cardinal system function to drain the blood from the caudal 

 end of the body and posterior appendages, while the anterior cardinal veins 

 and common cardinal veins function to drain the blood from the head, neck, 

 and forelimb areas. 



d. The Developing Converging Veins of the Mammalian Heart 



(e.g., Human) 



The formation of the hepatic portal system in the human embryo is shown 

 in figure 338G, H, and that of the inferior and superior venae cavae is 

 shown in figure 338 A-F. The general principles of venous development, de- 

 scribed in the previous pages of this chapter, apply here, and descriptive 

 matter is not needed to supplement the accompanying figures. It is worthy 

 of mention, however, that two additional veins are introduced in the abdominal 

 area of the embryo, namely, the two supracardinal veins. These veins persist 

 as a part of the vena cava inferior and azygos veins. Anteriorly, the two 

 precavae, so prominent in the lower vertebrates, including the birds, are dis- 

 placed partially by the formation of an anastomosing vein from the left to 

 the right side with the dropping out, to a considerable extent, of the proximal 

 portion of the left precava. Thus, the common cardinal vein on the right 

 side comes to function as the proximal portion of the single superior or 

 anterior vena cava, while the common cardinal vein on the left side comes 

 to form the coronary sinus of the heart, and occasionally as a variant, the 

 oblique vein of the left atrium. 



3. Development of the Heart 

 a. General Morphology of the Primitive Heart 



In the vertebrate group, two types of hearts are present, namely, lymph 

 hearts (fig. 335H) and the heart of the arteriovenous system. The heart of 

 the arteriovenous system is a centralized, well-muscularized mechanism, placed 

 ventral to the esophageal segment of the gut in the anterior extremity of the 

 coelomic cavity. Its function is to receive blood from the veins of the body 

 and to propel it forward toward the anterior or head region. Fundamentally, 

 the embryonic heart of the arteriovenous system is a tubular affair, composed 

 of four segments: 



( 1 ) a thin-walled sinus venosus or caudal portion of the heart, connect- 

 ing with a series of converging veins, 



(2) the atrium, a segment lying anterior to the sinus, 



(3) the ventricle, lying anterior to the atrium, and 



(4) the bulbus cordis. 



The ventricle and, to some extent, the bulbus cordis of the embryonic heart 

 later develop the structures which act as the main propulsive mechanism of 

 the heart, while the sinus and atrium give origin to the blood-receiving areas. 



