The development of the human body; a manual of human embryology . ches, which pass out along with the allantois into thebelly-stalk and so to the chorionic villi, forming the allan-toidean or umbilical arteries, while anteriorly each aorta sends branches ventrallyin the anterior branchialarches and these, unitingtogether, pass backwardalong the floor of thepharynx to become con-tinuous with the aorticbulb (Fig. 136). Laterthe two dorsal aortaefuse together as far for-ward as the region of theeighth cervical segmentto form a single trunk(Fig. 137), and the leftomphalo-mesenteric ar-tery disappe
The development of the human body; a manual of human embryology . ches, which pass out along with the allantois into thebelly-stalk and so to the chorionic villi, forming the allan-toidean or umbilical arteries, while anteriorly each aorta sends branches ventrallyin the anterior branchialarches and these, unitingtogether, pass backwardalong the floor of thepharynx to become con-tinuous with the aorticbulb (Fig. 136). Laterthe two dorsal aortaefuse together as far for-ward as the region of theeighth cervical segmentto form a single trunk(Fig. 137), and the leftomphalo-mesenteric ar-tery disappears, the rightone persisting to formthe superior mesentericartery of the adult. It will be convenientto consider first the his-tory of the vessels whichpass ventrally in the branchial arches. Altogether, sixof these vessels are developed, the fourth branchialarch possessing a rudimentary one in addition tothat which properly belongs to it (Zimmermann), andwhen fully formed they have an arrangement which maybe understood from the diagram (Fig. 137), in which the. Fig. 137.—Diagram Illustratingthe Arrangement op the Bran-chial Vessels. ab, Aortic bulb; da, dorsal aorta; I toVI, branchial arch vessels. THE ARTERIES. 263 vessels are represented as spread out upon a plane surface,the lateral trunks being the dorsal aortae. This arrange-ment, represents a condition which is permanent in thelower vertebrates. In the fishes the respiration is per-formed by means of gills developed upon the branchialarches, and the heart is an organ which receives venousblood from the body and pumps it to the gills, in which itbecomes arterialized and is then collected into the dorsalaortae, which distribute it to the body. But in terrestrialanimals, with the loss of the gills and the development ofthe lungs as respiratory organs, the capillaries of the gillsdisappear and the afferent and efferent branchial vesselsbecome continuous, the condition represented in the dia-gram resulting. But this conditi
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
