414 THE EABBIT. 



The later changes undergone by the heart ai'e of very con- 

 siderable interest, and have been described with great care by 

 Born. It will be convenient to deal with the several cavities 

 in order, beginning at the hinder or venous end of the heart. 



The sinus venosus, on the tenth day, is a vessel running 

 transversely aci-oss the body, and slightly enlarged at its two 

 ends to form the right and left cornua or horns. Each horn 

 receives three veins: — (i) the Cuvierianvein, which is formed by 

 the junction of the anterior and posterior cardinal veins, return- 

 ing venous blood from the body of the embryo generally ; (ii) the 

 vitelline vein, returning blood from the yolk-sac ; (iii) the allan- 

 toic vein, returning blood from the allantois. The sinus venosus 

 is at this stage nearly symmetrical, and opens into the auricular 

 cavity by a wide median aperture. 



By the eleventh or twelfth day, the right horn of the sinus 

 venosus has become much larger than the left horn. The allan- 

 toic and vitelline veins, in place of opening separately into the 

 sinus venosus, now unite before reaching the heart, and discharge 

 into the sinus through a single vein, the posterior vena cava. 

 The posterior vena cava and the right Cuvierian vein, or right 

 anterior vena cava as it is now termed, open into the larger or 

 right horn of the sinus venosus ; while the smaller left horn 

 receives only the left Cuvierian vein, or left anterior vena cava. 

 The opening from the sinus venosus into the auricle has now 

 become more slit-like, and leads distinctly into the right half 

 of the auricular chamber ; the slit-like opening is bounded by 

 two valve-like folds of the endocardial lining of the heart, which 

 may be termed the right and left venous valves respectively. 



At a later stage the sinus venosus becomes absorbed into 

 the right auricle, of which it now forms part ; the three venae cavse 

 opening separately into the auricular cavity. Of the two venous 

 valves, the left one disappears, while the right one becomes the 

 Eustachian valve, by which the blood from the posterior vena 

 cava, and for a time that from the right anterior vena cava as 

 well, is directed into the left auricle. 



The auricular portion of the heart. The originally single 

 auricular chamber becomes divided into right and left auricles 

 by a septum, which arises during the twelfth day from the dorsal 



