Till: KKTUS. Hi:. 



it* anterior part spring two branches, the ..///< un-ln'* (arcus aort'< . 



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



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



. \\hich. in its turn, divides into two trunks, the <trt> rim vertebralis or 



'tin- ti.-rtii: These vessels pass alonjj the lower surface of the embryo, 



parallel t> cadi other, and furnish during their course four or five divisions, 



tli<- ompkalo-metmtenc <u-fi riex, which ramify in the area germiuativa, and 



open into a limitary vein named tho sinus (or vena ) t< rmimiH*. From the 



network of tho area and tho sinus arise two vessels, tho vence ompltnln- 



metnit- rir.i . which enter the posterior extremity "of the heart. 



Tho circulation in tho umbilical vesicle is somewhat ephemeral in 

 several species, and it has been already stated that tho 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. 



-. 1 >KVELOPMENT OF THE HEABT AND VESSELS. PLAOENTAL CIRCULATION. 



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

 liel'i.re it attains its complete development. In its different phases, it 

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

 first change consists in an inflexion ; the tube curves in an 8 manner, so 



Fi<r. 441. 



III:AI:I "i mi. I.MI.I:V.I 01 \ mi-: SAMK. si:i-;x IT.OM IIKIIINK 

 I:VI l:l; - V,.,,;, omphalo-mesentericjp ; /. 



is; /,!.> v.-n- Right auricle; c, Bulbus aovt 



: r, Right ventri-l- : '. An- The six aortic arches; c, Atriuni : 



ricle; r, Venous sinn^. ', Auricula?. 



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

 the anterior and superior dilatation situated at the origin of the 

 aorta is named the </<.///> lill> (Inillnix tmrtu); the middle dilatation, the 

 MfriV'/A/j- i-in-ity ; and tlie posterior dilatution, tho auricular cavity. Hnll> / '< 

 postage 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 ca\ 'ty is the first to bo divided into two compart- 

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

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

 on the twenty-fifth in the Equine foetus. ThU -roovo corresponds to an inter- 

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

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

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

 'it lip, which afterwards, in developing, originates tho mitral and tricuspid 

 valves. 'I'ln heart has now three cavities: two ventricles and an auricle ; but 

 in a brief peril, d tin latter is doubled, and the compartments are then four in 

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



8 N -2 



