II 



The Circulatory System 



The nature of the parts and the pattern of the circulatory 

 system characterize the vertebrate, although the system is 

 suggested in the cephalochordate. This system is described 

 as being composed of a heart and a series of vessels, the ar- 

 teries carrying blood away from the heart and the veins 

 returning blood to the heart. Connecting the main arteries 

 and veins are vessels decreasing in diameter to the capillar- 

 ies and increasing again to the veins. All of the tissues of the 

 body are permeated by the capillaries, and no cell is at a 

 great distance from such a channel. 



Both the parts and the pattern of the circulatory system 

 show some variation from one group to another; much of 

 this variation is of a functional nature and has little relation- 

 ship to the phylogeny of the group. Several areas of this 

 system can be examined in search of modifications of a 

 phylogenetic nature. 



THE HEART 

 Mammals 



The heart of the mammal has four chambers; two ven- 

 tricles and two auricles (or atria — singular, atrium). Func- 

 tionally it represents two hearts, serving separate and parallel 

 pulmonary and systemic circulations, each with a contractile 

 auricle and ventricle. The heart and its history is best 

 understood in terms of its embryological development. 



The heart appears first as irregular clusters and chords of 

 mesenchymal cells lying between the splanchnic mesoderm 

 and the endoderm in the region below the pharyn.\. These 

 cells become organized into two strands lying to either side 

 of the anterior intestinal portal. Each strand acquires a 

 lumen and thus becomes an endocardial primordium. These 

 tubes extend beyond the cardiac region into the head fold 

 and posterolaterally on to the yolk sac. Meanwhile the 

 splanchnic mesoderm has become thickened in the region 

 of the heart and this thickening tends to wrap around the 

 tubes. As the anterior intestinal portal moves posteriorly, 

 these two tubes approach each other at the midline, meet, 

 and fuse to form a single channel, the endocardium or lining 



of the heart; the enclosing splanchnic mesoderm now forms 

 the myocardium, the muscular part of the heart. 



As the endocardial tube is formed, it becomes flexed in a 

 sigmoid curve. Anteriorly the first two aortic arches extend 

 outward and upward; posteriorly the vitelline veins (or the 

 omphalomesenteric veins) extend out onto the yolk sac. A 

 series of constrictions now appear dividing the tubular heart 

 into segments. Anteroventrally and to the right is the trun- 

 cus arteriosus (bulbus cordis) which widens into a conus at 

 the reflexed ventricle; the endocardial primordia first meet 

 and fuse at the anterior end of the truncus. The ventricle is 

 separated by an atrioventricular constriction from the an- 

 terodorsal atrium, and behind the atrium is the sinus veno- 

 sus formed by the junction of the two large vitelline veins 

 (Figure 11-41). 



Meanwhile other blood vessels have appeared: anterior 

 cardinals drain the head region, and posterior cardinals 

 drain the posterior part of the body. These unite to form 

 the paired common cardinals (or ducts of Cuvier) which de- 

 scend to enter the sinus venosus. The opening of the sinus ve- 

 nosus into the atrium now lies dorsal to the ventricular por- 

 tion. The atrium has pouches extending out to either side and 

 down around the truncus and the anterior end of the ven- 

 tricle. The latter now shows bilateral posterior bulges, pre- 

 cursors of right and left ventricles. 



Looking at the interior of the heart, one sees the beginning 

 of an interatrial septum between the two outpocketings of 

 the atrium; the sinus venosus opens through a slit to the 

 right of the interatrial septum (Figure 11-1). The two ven- 

 tricular pouches are slightly separated by an interventricular 

 septum. The wide atrioventricular canal is divided by dorsal 

 and ventral cushions into an I-shaped opening. As devel- 

 opment progresses, both the interatrial and interventrical 

 septa grow toward each other, and then the atrioventricular 

 cushions meet and divide the atrioventricular canal into 

 right and left passages. Osteum I, between the interatrial 

 septum and the atrioventricular cushion, closes as osteum II 

 develops in the upper part of the septum and a secondary 

 interatrial septum begins to appear. 



While the right and left sides of the heart are separating, 

 the truncus arteriosus begins to be divided into pulmonary 

 and systemic channels. Division begins by a ridge forming 



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