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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 

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, chordæ tendineæ, and trabeculæ are developed. The simple tubular heart, already 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.: (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.


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



FIG. 460– Head of chick embryo of about thirty-eight hours’ incubation, viewed from the ventral surface. X 26 (Duval.) (See enlarged image)


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