DEVELOPMENT OF THE CIECULATORY SYSTEM. 



235 



(2) The tubular heart shows demarcations into four parts 

 (Fig. 194): (a) Sinus venosus; (b) Primitive auricle; (c) Primi- 

 tive ventricle ; (d) Conus or bulbus arteriosus. In this condition 

 (third week) the human heart is exactly like that of a fish, viz., a 

 tubular four-chambered heart which pumps blood into the 

 branchial or aortic arches. 



(3) The disappearance of the mesocardium allows the heart 

 to become twisted and bent. Two chief bends are formed which 

 materially help to give the heart its adult shape (Fig. 195): 



'st arch 

 2nd arch 

 ventral aorta 



conus arteriosus 



prim, ventricle 



prim, auricle 



sinus venosus 



vit. vein^-^ "^vit. vein 



Fig. 194. — The Primitive Divisions of the Heart. 



(a) The Ventricular Bend. — The ventricular part of the tube is 

 bent into a V-shaped piece, the apex of the V-shaped loop being- 

 turned towards the right. 



(b) The Auriculo-ventricular Bend. — The ventricular part is bent 

 in front of the auricular so that the auricle becomes dorsal to the 

 ventricle (Fig. 195). 



The Sinus Venosus. — The sinus venosus, the first chamber 

 of the foetal heart, is formed by the union of the vitelline 

 veins ; the umbilical veins and ducts of Cuvier come subse- 

 quently to open in it (Fig. 185). The sinus is imbedded in 

 the persistent posterior part of the mesocardium (Fig. 202). 



