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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
septal cusps of the valves are developed as downward prolongations of the septum intermedium (Fig. 467). The aortic and pulmonary semilunar valves are formed from four endocardial thickenings—an anterior, a posterior, and two lateral—which appear at the proximal end of the truncus arteriosus. As the aortic septum grows downward it divides each of the lateral thickenings into two, thus giving rise to six thickenings—the rudiments of the semilunar valves—three at the aortic and three at the pulmonary orifice (Fig. 471).

Further Development of the Arteries.—Recent observations show that practically none of the main vessels of the adult arise as such in the embryo. In the site of each vessel a capillary network forms, and by the enlargement of definite paths in this the larger arteries and veins are developed. The branches of the main arteries are not always simple modifications of the vessels of the capillary network, but may arise as new outgrowths from the enlarged stem.


FIG. 473– Scheme of the aortic arches and their destination. (Modified from Kollmann.) (See enlarged image)

  It has been seen (page 506) that each primitive aorta consists of a ventral and a dorsal part which are continuous through the first aortic arch. The dorsal aortæ at first run backward separately on either side of the notochord, but about the third week they fuse from about the level of the fourth thoracic to that of the fourth lumbar segment to form a single trunk, the descending aorta. The first aortic arches run through the mandibular arches, and behind them five additional pairs are developed within the visceral arches; so that, in all, six pairs of aortic arches are formed (Figs. 472, 473). The first and second arches pass between the ventral and dorsal aortæ, while the others arise at first by a common trunk from the truncus arteriosus, but end separately in the dorsal aortæ. As the neck elongates, the ventral aortæ are drawn out, and the third and fourth arches arise directly from these vessels.
  In fishes these arches persist and give off branches to the gills, in which the blood is oxygenated. In mammals some of them remain as permanent structures while others disappear or become obliterated (Fig. 473).

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