THE FCETUS. 



915 



From its anterior part spring two branches, the aortic arches (arcus aortce\ 

 which are directed towards the head of the embryo, and are afterwards in- 

 flected downwards and backwards. They join together to constitute the single 

 aorta, which, in its turn, divides into two trunks, the arterim vertebralis or 

 primitive aortce. These vessels pass along the lower surface of the embryo, 

 parallel to each other, and furnish during their course four or five divisions, 

 the omplialo-mesenteric arteries, which ramify in the area germinativa, and 

 open into a limitary vein named the sinus (or vena) terminalis. From the 

 network of the area and the sinus arise two vessels, the vence, omptialo- 

 mesentericce, which enter the posterior extremity of the heart. 



The circulation in the umbilical vesicle is somewhat ephemeral in 

 several species, and it has been already stated that the vesicle is atrophied at 

 an early period of foatal life. In birds it is most extensive ; and its presence 

 may be noted in the Carnivora during the whole term of uterine existence. 



2. DEVELOPMENT OF THE HEART AND VESSELS. PLACENTAL CIRCULATION. 

 The heart which, until now, was a cylindrical tube, is considerably modified 

 before it attains its complete development. In its different phases, it 

 successively offers all the forms known to exist in vertebrate animals. The 

 first change consists in an inflexion ; the tube curves in an S manner, so 



Fte. 441. 



HEART OF THE EMBRYO OP 

 RABBIT, SEEN FROM BEFORE. 



, Truncus arteriosus ; ?, Left ven- 

 tricle ; r, Right ventricle ; a, Au- 

 ricle ; v, Venous sinus. 



THE SAME, SEEN FROM BEHIND. 



a, Venae omphalo-mesentericae ; c?, 

 Right auricle ; e, Bulbus aortse ; /, 

 The six aortic arches ; c, Atrium ; 

 6, Auriculae 



that its inferior part becomes superior; it then dilates at three points: 

 the anterior and superior dilatation situated at the origin of the 

 aorta is named the aortic bulb (bulbus aortas) ; the middle dilatation, the 

 ventricular cavity ; and the posterior dilatation, the auricular cavity. Holler's 

 passage is the name given to the constriction between the auricle and 

 ventricle, which at this time are single. They do not remain long so, how- 

 ever. The ventricular cavity is the first to be divided into two compart- 

 ments, and the division is marked externally by a groove which appears on 

 the surface of the heart of the Ovine embryo towards the nineteenth day, and 

 on the twenty-fifth in the Equine foetus. This groove corresponds to an inter- 

 ventricular septum which insensibly rises from the bottom of the ventricles ; 

 when it reaches the auricles it concurs in forming the auriculo-ventricular 

 openings. The margins of these openings are provided with a small slightly 

 salient lip, which afterwards, in developing, originates the mitral and tricuspid 

 valves. The heart has now three cavities : two ventricles and an auricle , but 

 in a brief period the latter is doubled, and the compartments are then four in 

 number. Externally, there is observed a depression which shows the division 



