HEART. 



133 



former there is a thick elastic network; the transverse cells rest on a 

 delicate fibered connective tissue. 



THE HEART. Development. The heart has already been described 

 as a median longitudinal vessel beneath the pharynx, formed posteriorly 

 by the union of the vitelline veins and terminating anteriorly in the two 

 ventral aortae. Such a heart from a rabbit embryo is shown in Fig. 159, 

 A. It soon becomes bent like a U, the venous opening being carried 

 forward dorsal to the aortic part as shown in B and C. The ventral or 

 aortic limb of the U at the same time is carried to the right of the median 

 plane (C). The dorsal limb is divided into two parts by an encircling 

 constriction, the coronary sulcus (s.c.). Its thick walled portion ventral 



FIG. 159. EMBRYONIC HEARTS. 



A and B, From rabbits 9 days after coitus; C, from a human embryo of 3 (?) weeks; D and E, from a 12 

 mm. pig (D sectioned on the left of the median septum, and E on the right of it) ; F, from a 13.6 

 mm. human embryo, sectioned like E. The hearts are all in corresponding positions with the left 

 side toward the observer, the anterior end toward the top of the page, the dorsal side to the right. 

 ao., Aorta ; c. s., coronary sinus ; f. o., foramen ovale ; i. f., interventricular foramen ; 1. a., left atrium ; 

 p. a., pulmonary artery ; p. v., pulmonary vein ; r. a., right atrium ; s. c., coronary sulcus ; v., ventricle ; 

 v. b., bicuspid valve; v. t., tricuspid valve; v.v., vitelline vein; v. v. s., valves of the venous sinus. 



to the sulcus is to form the ventricles of the heart; the thin walled dorsal 

 portion becomes the atria [auricles]. In the human embryo of three weeks 

 (C) the atria are represented by a single cavity subdivided into right and- 

 left parts only by an external depression in the median plane. The right 

 portion receives all the veins which enter the heart (the vitelline veins and 

 their tributaries) and is much larger than the left portion. The cavities of 

 the atria not only connect with each other but they have a common outlet 

 into the undivided ventricle. From the ventricle the blood flows out of 

 the heart through the aortic limb. In a complex manner, described in 

 text-books of embryology, a median septum develops, dividing the heart 

 into right and left halves. 



