THE VASCULAR SYSTKM 



207 



descending limb which passes from left to right, from the left dorso-anterior to the 

 right ventro-posterior extremity of the pericardium ; (2) of a transverse portion 

 directed dorsally ; and (3) of a smaller ascending limb which, passing forwards, 



d an m.p. 



FIG. 259. CONDITION OF THE HEART IN THE HUMAN EMBRYO OP ABOUT FIFTEEN DAYS, 

 RECONSTRUCTED FROM SERIAL SECTIONS. (His.) 4 ^>. 



A, from before, showing external appearance of heart ; B, the same with the muscular substance of 



heart removed showing the endothelial tube ; C, from behind. 



mn, mandibular arch with maxillary process ; hy, hyoidean arch ; b.a., bulbus aortee ; v, right 

 ventricle; v', left ventricle; au, auricular part of heart; c.a., canalis auricularis; sr, horn of sinus 

 venosus with umbilical vein (u.v.), superior vena cava (V.G.S.), and vitelline vein entering it; 

 d, diaphragm ; ?n.j0., mesocardium posterius ; Z, liver ; b.d., bile-duct. 



gradually narrows into the aortic bulb. This, finally inclining inwards, reaches the 

 mid-dorsal line at its point of attachment to the floor of the fore-gut. The ascending 

 limb of the loop is separated from the bulb by a slight constriction (the (return 



J.au. 



b.a. 



FIG. 260. HEART OF A SOMEWHAT MORE ADVANCED HUMAN EMBRYO. (His.) *->. 

 A, from before ; B, from behind. 



r.v., right ventricle; l.v., left ventricle; b.a., bulbus aortse; r.au., right auricle; l.au., left auricle; 

 v.c.s., vena cava superior; u.v., umbilical vein; v.v., vitelline vein ; d, diaphragm. 



Halleri), and although it ultimately becomes a portion of the right ventricle, it is 

 to be regarded at this stage as the proximal segment of the aortic bulb, and a 

 distinct chamber of the primitive heart (Greil). 1 In the following account we 



1 Morphol. Jahrbuch. xxxi. 1903. See also Hochstetter, in Hertwig's Handbuch der Entwickelungs- 

 lehre, III. Teil II 



