508 



ANGIOLOGY 





Further Development of the Heart. Between the endothelial lining and the 

 outer wall of the heart there exists for a time an intricate trabecular network of 

 mesodermal tissue from which, at a later stage, the musculi papillares, chordae 

 tendmese, and trabeculse carnese are developed. The simple tubular heart, already 



Dorsal aorta 



Primitive jugular 

 vein 



Amnion 



Cardinal vein 



Dorsal aorta 



Belly-stalk - 



(Jhorionic villi 



FIG. 459. Human embryo of about fourteen days, with yolk-sac. (After His.) 



described, becomes elongated and bent on itself so as to form an S-shaped loop, 

 the anterior part bending to the right and the posterior part to the left (Fig. 460) . 

 The intermediate portion arches transversely from left to right, and then turns 

 sharply forward into the anterior part of the loop. Slight constrictions make their 

 appearance in the tube and divide it from behind forward into five parts, viz.: 



Fore-brain 



Bulbus cordis 



A trium I 



Optic vesicle 



Ventricle 



Vitelline vein 



FIG. 460. Head of chick embryo of about thirty-eight hours' incubation, viewed from the ventral surface. X 26 



(Duval.) 



(1) the sinus venosus; (2) the primitive atrium; (3) the primitive ventricle; (4) the 

 bulbus cordis, and (5) the truncus arteriosus (Figs. 461, 462). The constriction 

 between the atrium and ventricle constitutes the atrial canal, and indicates the site 

 of the future atrioventricular valves. 



