J042 EMBRYOLOGY. 



arise two vessels— the venoe omphalo-mesenteric(B—vf\iich enter the posterior 

 extremity of the heart. 



In Birds there are only two omphalo-mesenteric or vitelline arteries. 



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 foetal 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 

 inflection ; the tube curves in an S-shaped manner, so 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 aortcn) ; the 

 middle dilatation, the ventricular cavity; and the posterior dilatation, the auri- 



Fig. 576. 



HEART OF THE EMBRYO OF A RABBIT, THE SAME, SEEN FROM BEHIND. 



SEEN FROM BEFORE. ^^ Venae omphalo-mesentericae ; d, right 



ta, Truncus arteriosus ; U left ventricle ; r, auricle; e, bulbus aortse -, /, the six aortic 



right ventricle; a,auricle; t;, venous sinus. arches; c, atrium ; 6, auricula?. 



cular cavity. Nailer'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, 

 however. The ventricular cavity is the first to be divided into two compartments, 

 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 coiTesponds to an interventricular 

 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 in the auricles ; at a point 

 corresponding to it, a septum is developed in their interior which remains incom- 

 plete during the whole of foetal life, being perforated by the foramen of Botal. 

 With regard to the aortic bulb, it contracts and divides into two vessels — the aorta 

 and pulmonary artery. 



' This septum is sometimes arrested in its development, whence results an abnormal com- 

 munication between the ventricles. Some instances have been given when describing the 

 heart. 



