. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 242 DEVELOPMENT OF THE ARTERIAL SYSTEM aortae fuse as far forward as the region of the eighth cervical segment to form a single trunk from which segmental branches arise. It will be convenient to consider first the history of the vessels which pass dorsally in the branchial arches. Altogether, six of these vessels are developed, the fifth being rudimentary and transitory, and when fully formed they have an arrangement which may be under- stood from the diagram (Fig. 147). This arrangement rep
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 242 DEVELOPMENT OF THE ARTERIAL SYSTEM aortae fuse as far forward as the region of the eighth cervical segment to form a single trunk from which segmental branches arise. It will be convenient to consider first the history of the vessels which pass dorsally in the branchial arches. Altogether, six of these vessels are developed, the fifth being rudimentary and transitory, and when fully formed they have an arrangement which may be under- stood from the diagram (Fig. 147). This arrangement repre- sents a condition which is per- manent in the lower vertebrates. In the fishes the respiration is performed by means of gills developed upon the branchial arches, and the heart is an organ which receives venous blood from the body and pumps it to the gills, in which it becomes arte- rialized and is then collected into the dorsal aortae, which distrib- ute it to the body. But in terres- trial animals, with the loss of the gills and the development of the lungs as respiratory organs, the capillaries of the gills disappear and the afferent and efferent branchial vessels become con- tinuous, the condition repre- sented in the diagram resulting. But this condition is merely temporary in the mammalia and numerous changes occur in the arrangement of the vessels before the adult plan is realized. The first change is a disappearance of the vessel of the first arch, the ventral stem from which it arose being continued forward to form the temporal arteries, giving off near the point where the branchial vessel originally arose a branch which represents the internal maxillary artery in part, and possibly also a. Fig. 147.—Diagram Illustrating the Primary Arrangement of the Bran- chial Arch Vessels. a, aorta; db, aortic bulb; ec, external carotid; ic, internal carotid; sc, subclavian; I-VI, branchial arch Please note that these images are extracted from scanned page images th
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