THE CIRCULATORY SYSTEM. 



During the second week in the life of the embryo, the heart is 

 recognizable as two symmetrical and distinct halves, which con- 

 sist at this time of two straight tubes, lying along the head-end 

 of the embryo between the yolk-sac and the neural folds and 

 connected at their posterior ends with the vitelline vessels. 

 These two tubes soon unite to form a single one, which becomes 

 twisted upon itself and forms a prominent swelling on the ventral 

 surface of the embryo in the region of the pharynx. It is 

 twisted in an S-shaped loop and is free in its middle portion, 

 while the ends are attached anteriorly and posteriorly to the 

 ventral surface of the fore-gut. The posterior, or dorsal, portion 

 of the heart represents the future auricles and is separated by a 

 somewhat marked constriction from the ventral portion, which 

 is to become the ventricles. The anterior end of the loop is 

 somewhat enlarged to constitute the triuiais arteriosus, which is 

 attached to the fore-gut in the vicinity of the mandibular arches. 



The heart increases rapidh' in size and, within a week from 

 the first traces of its formation, the constriction between the au- 

 ricular and ventricular portions has become very narrow, the 

 auricles have assumed their ear-like form, and the ventricular 

 portion lies transversely across the body, shaped somewhat like 

 the adult stomach. 



The Sinus Venosus. 



The blood is returned to the heart by three .symmetrical pairs 

 of veins : the Cuvierian vein from the body of the embryo, the 

 vitelline vein from the yolk-sac, and the allantoic vein from the 

 placenta. These three pairs unite to constitute the sinus veno- 

 sus, lying transversely across the body and opening into the 

 auricular portion of the heart. The sinus venosus ultimately 

 becomes a part of the auricle and disappears as a separate 

 structure. 



The auricular chamber becomes imperfectly divided into the 

 two auricles, at an early date, by outgrowths from the walls, 

 which finally separate the two chambers. The septum between 

 the two auricles does not become complete during fetal life, but 

 closes normally at the time of birth ; abnormally, it may persist 



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