THE FCETUS. 



915 



Prom 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 arteries vertehralis or 

 j)rimitive aortm. These vessels pass along the lower surface of the embryo, 

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

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

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

 network of the area and the sinus arise two vessels, the venm omphalo- 

 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 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 or 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 



Fi5. 441. 



HEART OF THE EMBRYO OF 

 RABBIT, SEEN FROM. BEFORE. 



ta, Truncus arteriosus ; I, Left ven- 

 tricle ; r. Right ventricle ; a. Au- 

 ricle ; V, Venous sinus. 



THE SAME, SEEN FROM BEHIND. 

 a, Venae omphalo-mesentericse ; d, 

 Riglit auricle ; e, Bulbus aorta ; /, 

 The six aortic arches ; c, Atrium ; 

 6, Auriculae 



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

 +bA nntPrior and superior dilatation situated at. the origm ol tne 

 SrtaL named the 2"c hulb (Ulhus aorta.); the middle di atation the 

 L« rVirSv; and the posterior dilatation, the a«nc«Zarca.*^2/. Ealler s 

 ventricular caviiy , d, ^ constriction between the auricle and 



passage is «^«, i^^^f.f^|^°^ !re single They do not remain long so, how- 

 ir'Thlvl^iS ca^^^^^^^^ fi-tto L divided into, two compart- 



m^nis and the division is marked externally by a groove which appears on 

 ments, and ™e ^ivisio towards the nineteenth day, and 



the surface of f .f. ^^^f^^f ^J'.^^^^^^^ This groove corresponds to an inter- 

 on the twenty-fifth i^*^;,^^^^"^;^ rises from the bottom of the ventricles ; 

 ventricular ^f^Tv, ief ft concurs in forming the auriculo-ventricular 

 when It r«-«t'ItinsTf tLse oTenTng are proved with a small sHghtly 

 :Sfp wMc\TSl>dVvLpin'g,orig^^^ 



salient ^V^^^ ^ ^^^Ities : two ventricles and an auricle , but 



LtteWperiStt latter is doubled, and the compartments are tjen four in 

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



