1032 



THE VASCULAE SYSTEM. 



part of the cephalic limb of the loop become placed transversely (Fig. 827), but after a short 

 period its ventricular extremity again passes caudally until the original cranial limb of the 

 loop lies to the right and somewhat ventral to the original dorsal limb. Still later, the bulbus 

 cordis is partly absorbed into the truncus arteriosus and partly into the ventricle ; the right 

 segment of the sinus venosus is absorbed into the atrium, and the left segment forms the 

 coronary sinus. At a still later period the atrium is divided, by intra-atrial septa, into right 

 and left atria. The atrio- ventricular canal becomes converted into the right and left a trio - 

 ventricular apertures. The ventricle, including the absorbed portion of the bulbus cordis, is 



Ventricle 



Truncus arteriosus 



Truncus arteriosus 



Ventricle 



Atrium 



Anterior 

 cardinal vein 



Bulbus cordis 



Atrio-ventrieular 

 canal 



Sinus venosus 



Ventricle 



FIG. 827. VENTRAL VIEW OF A MODEL OF THE 

 HEART OF A HUMAN EMBRYO 2*5 MM. LONG. 

 (Meyer's collection. Modelled by Professor P. 

 Thompson.) 



_ Anterior 

 cardinal vein 



..Right horn of 

 sinus venosus 



Left horn of 

 sinus venosus ! Intermediate part of sinus venosus 



Ventricle 



FIG. 828. DORSAL VIEW OF A MODEL OF THE 

 HEART OF A HUMAN EMBRYO 2 '5 MM. LONG. 

 (Meyer's collection. Modelled by Professor P. 

 Thompson.) 



separated into the right and left ventricles of the adult heart ; and the truncus arteriosus, and 

 the part of the bulbus cordis which becomes incorporated with it, is separated into the ascending 

 aorta and the pulmonary artery. Thus, from the embryonic heart are eventually produced the 

 adult heart, the ascending aorta, the pulmonary artery, and the coronary sinus. 



The main outlines of the transformation of the simple tubular heart of the young embryo 

 into the four-chambered heart of the adult are, therefore, comparatively simple, but the details 

 of the process are intricate and some of them are still imperfectly understood. 



The Sinus Venosus. In embryos possessing fourteen mesodermal somites the primitive single 

 heart, formed by the fusion of the caudal portions of the primitive ventral aortae, receives at its 

 caudal end two venous stems, each of which is the common termination of the lateral umbilical 

 vein, the vitelline vein, and the anterior cardinal veins of the same side (Fig. 819). At a later 

 period, after the heart has begun to fold on its longitudinal axis, the two common venous stems 

 are absorbed into the heart and form its most caudal section, the sinus venosus, which consists 

 of a right and a left cornu, united by an intermediate segment. In the meantime, however, the 

 posterior cardinal veins have appeared and have united with the anterior cardinals. After this 

 union occurs the portion of the anterior cardinal vein ventral to the point of union becomes the 

 duct of Cuvier. At this period, therefore, each cornu of the sinus venosus receives three veins 

 the vitelline vein, the lateral umbilical vein, and the duct of Cuvier. 



As development proceeds the left vitelline vein and the left lateral umbilical vein lose their 

 connexion with the left horn of the sinus venosus and pour their blood into the liver, through 

 which it reaches the cranial end of the right vitelline vein. The latter in the meantime has 

 become the only outflow from the liver to the heart, and it ultimately forms the terminal part 

 of the inferior vena cava. In association with these changes the right horn of the sinus venosus 

 becomes considerably larger than the left horn (Fig. 828), and the left horn and the intermediate 

 portion of the sinus become reduced to the form of a comparatively narrow channel which opens 

 into the right horn, whilst the latter opens into the dorsal part of the atrial chamber, by a cleft- 

 like aperture which is bounded by right and left lateral lips called the right and left venous 

 valves (Figs. 829, 830). 



As the right horn of the sinus venosus is absorbed into the right part of the dorsal portion of 

 the atrium the caudal or lower portion of right venous valve becomes the valve of the inferior 

 vena cava and the valve of the coronary sinus. The cephalic or upper portion is united with 

 a septal projection into the cavity of the atrium called the septum spurium, and forms with it 

 the crista terminalis of the completed heart, which indicates, therefore, the line of union of the 

 right horn of the sinus with the atrium proper. The left venous valve becomes incorporated with 

 the interatrial septum, and the angle in the posterior wall of the sinus venosus which indicates 

 the line of union of the right cornua and the intermediate part of the sinus appears in the dorsal 

 wall of the right atrium, where it unites with the dorsal pa'rt of the caudal or inferior portion of 

 the septum secundum of the atrium, and takes part in the formation of the limbus ovalis. 



Whilst these changes have been proceeding the right duct of Cuvier has become the lower 

 part of the superior vena cava, the intermediate part of the sinus venosus and the left cornu are 

 transformed into the coronary sinus, and the left duct of Cuvier has become the oblique vein of 

 the left atrium. Thus when the changes mentioned are completed the right part of the atrium 

 receives the superior and the inferior vena cava and the coronary sinus. 



