HEART 



175 



the heart has become thin and has ruptured in the hollow of the U- 

 shaped bend, forming the sinus transversus pericardii, which persists 

 throughout life as a small but very definite structure. 



While the heart is still a simple tube consisting of endothelium inter- 

 nally and mesothelium externally, with a space between them bridged 

 by protoplasmic strands, it beats regularly, although possessing neither 

 nerves nor muscles. Without causing any interruption of the circulation 

 the simple tube becomes divided into four chambers, namely the right 

 and left atria (or auricles 1 ) and the right and left ventricles. The process 

 of subdivision may be outlined as follows: 



When the tube becomes bent into a U, the venous end of the heart 

 is carried anteriorly, dorsal to the aortic end, as shown in Fig. 1 70, A-C. 



A B c 



FIG. 169. DIAGRAMS ILLUSTRATING THB FORMATION OF THE PERICARDIAL CAVITY. 



A.., Aortic end of heart; B. W., body wall; D., diaphragm; Ht., heart; Li., liver; Lu., lung; P. C., pericardia! 

 cavity: Per., peritoneal cavity; PI., pleural cavity; S.p-p., pleuro-pericardial septum, S. tr. p., sinus 

 transversus pericardii; V., venous end of the heart. 



At the same time the ventral or aortic limb of the U is carried to the right 

 of the median plane (C). The dorsal limb is divided into two parts by an 

 encircling transverse constriction, the coronary sulcus (s.c.}. Its thick- 

 walled portion, ventral to the sulcus, forms the ventricles; the thin-walled 

 dorsal portion becomes the atria. 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 freely communicate 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. 



In the heart of a i2-mm. pig embryo this septum has already formed 



1 According to the anatomical nomenclature adopted at Basle, the term auricle (diminutive 

 of auris, ear) is restricted to what was formerly called the auricular appendix, and the term 

 atrium (chamber) is used for the cavity as a whole. 



