70 HUMAN EMBKYOLOGY. 



the sinus venosus, three on each side the two ducts of Cuvier, the two vitelline 

 veins, and the two lateral umbilical veins (Fig. S3). 



The anterior cardinal veins and their tributaries, and cross anastomoses which 

 form between them, are transformed into some of the cranial blood sinuses, the 

 internal jugular veins, the innominate veins, and the cephalic (upper) part of the 

 superior vena cava. The right duct of Cuvier becomes the caudal part of the 

 superior vena cava, and the left is converted into the oblique vein of the left 

 atrium (O.T. oblique vein of Marshall) (Fig. 88). 



A portion of the abdominal part of the right posterior cardinal vein is replaced 

 by the right subcardinal vein, and from this and a transverse anastomosis between 

 it and the opposite subcardinal vein is formed that part of the inferior vena cava 

 which extends from the renal veins to the liver, and a part of the left renal vein. 

 From the remains of the cardinal veins and transverse anastomoses between them 

 are formed (1) the azygos, the hemiazygos, and the accessory hemiazygos veins ; 

 (2) the inferior vena cava, caudal to the renal veins ; (3) the common iliac veins ; 

 (4) the hypogastric veins; and (5) the parts of the left lumbar veins which 

 pass dorsal to the aorta (Fig. 88). 



The cephalic end of the inferior vena cava is formed from the cephalic extremity 

 of the right vitelline vein and a caudal outgrowth from it which unites with the 

 right subcardinal vein (Fig. 88). 



Details of the history of the transformations of the cardinal veins, the vitelline 

 and umbilical veins, and the formation of the cranial part of the inferior vena cava 

 are given in the account of the further stage of the development of the vascular 

 system. 



The Primitive Heart. The primitive heart is formed in the dorsal wall of the 

 pericardium, ventral to the fore-gut, by the fusion of the caudal parts of the 

 primitive ventral aortse, and shortly after its formation it is divided into five 

 primitive chambers. The most caudal of the five, which receives the main primitive 

 veins, is the sinus venosus, the second is the atrium, the third the ventricle, the 

 fourth is the bulbus cordis, and the fifth and most cranial is the truncus aorticus, 

 which discharges its contents into the ventral roots of the aortic arches (Fig. 84). 



During the period which intervenes between the time when the embryo is 

 8 mm. and 1*7 mm. long, that is between the fifth and the eighth weeks, the greater 

 part of the sinus venosus is absorbed into the atrium ; the ventricle and the atrium 

 are each divided into right and left chambers by the formation of an interatrial 

 and an interventricular septum ; the bulbus cordis is absorbed partly into the 

 ventricle and partly into the truncus aorticus, and the truncus aorticus is separated 

 into the ascending part of the aorta and the stem of the pulmonary artery. When 

 these changes are completed the heart consists of right and left atria and right 

 and left ventricles. The ventricles are entirely separated from one another by 

 the interventricular septum, but there is an orifice of communication between the 

 right and left atria (Fig. 88). 



The right atrium receives blood from the superior and inferior vense cavse, and 

 from the walls of the heart, by the coronary sinus, which is a remnant of the 

 transverse part and left horn of the sinus venosus. The blood which enters through 

 the superior vena cava and by the coronary sinus, passes through the right atrio- 

 ventricular orifice into the right ventricle, but the whole, or the greater part, of the 

 blood which enters by the inferior vena cava passes through the foramen ovale, 

 which lies in the interatrial septum, into the left atrium. 



The blood which enters the right ventricle is ejected into the pulmonary artery. 

 A small portion of it passes by the right and left branches of the artery into the 

 lungs, and is returned to the left atrium by the pulmonary veins, but by far the 

 greater part passes through the ductus arteriosus into the aorta, which it enters at 

 a point immediately beyond the origin of the left subclavian artery (Fig. 88). 



The blood which enters the left atrium, through the foramen ovale, mixes, 

 in the left atrium, with the blood which is returned by the pulmonary veins; 

 then it passes through the left atrio-ventricular orifice into the left ventricle, 

 by which it is forced into the aorta. Some of this blood passes into the 

 innominate artery, and so, by its right subclavian branch, to the right upper 



