The evolution of man : a popular exposition of the principal points of human ontogeny and phylogeny . tsection, from the foremost end of the ventricle. It formsthe enlarged, hindmost end of the gill-artery stem (Fig. DEVELOPMENT OF THE VASCULAR SYSTEM. 375 299, abr). From each side of this, from five to seven gill-arteries proceed; these rise between the gill-openings (s)to the gill-arches, encircle the throat, and combine aboveinto a common aorta-stem, the continuation of which,passing backward above the intestine, corresponds to thedorsal vessel of Worms. As the arched arteries distributethe
The evolution of man : a popular exposition of the principal points of human ontogeny and phylogeny . tsection, from the foremost end of the ventricle. It formsthe enlarged, hindmost end of the gill-artery stem (Fig. DEVELOPMENT OF THE VASCULAR SYSTEM. 375 299, abr). From each side of this, from five to seven gill-arteries proceed; these rise between the gill-openings (s)to the gill-arches, encircle the throat, and combine aboveinto a common aorta-stem, the continuation of which,passing backward above the intestine, corresponds to thedorsal vessel of Worms. As the arched arteries distributethemselves in a respiratory capillary net over the gill-arches, they thus contain venous blood in their lower part(as arterial gill-arches), and arterial blood in their upperpart (as aorta-arches). The points at which separate aorta-arches unite, which occur on the right and left sides, arecalled aorta-roots. Of an originally greater number ofaorta-arches, only five pairs are retained, and from thesefive (Fig. 300), in all higher Vertebrates, the most im-portant parts of the arterial system Fig. 299.—Head of an embryonic Fish, with the rudiment and theblood-vessel system; seen from the left side : do, Cuverian duct (point ofunion of the front and hind main veins) ; sv, venous sinus (enlargedterminal portion of the Cuverian duct); a, auricle ; v, main chamber ;abr, gill-artery stem; s, gill-openings (between the arterial arches); ad,aorta; c, head-artery (carotis); n, nose-groove. (After Gegenbaur.) The appearance of the lungs, connected with the respi-ration of air, which first occurs in the Dipneusta, is mostimportant in the further developement of the arterial 3?6 THE EVOLUTION OF MAN. system. In Dipneusta, the auricle of the heart separatesinto two halves by the formation of an incomplete the right auricle now absorbs the venous blood of thebody-veins. The left auricle, on the other hand, absorbsthe arterial blood of the lung-veins; both auricles dis-
Size: 2183px × 1145px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., book, bookcentury1800, bookdecade1870, booksubjectembryologyhuman
