THE VASCULAR SYSTEM 



283 



arch, the first. In a 2.6 mm. embryo, a second aortic arch, the hyoid, is 

 added. Others are added in succession until, in a 4. 2 embryo, there are five 

 aortic arches. But the fifth or last of these is really the sixth arch, the 

 true fifth aortic arch being a rudimentary vessel which appears only 

 transiently in the 7.0 mm. embryo. By this stage, however, the first two 

 aortic arches have degenerated. Consequently, while all six aortic arches 

 arise, they are not present simultaneously in the human embryo. 



Only three of the six embryonic aortic arches are represented in the 

 adult, the third, fourth, and part of sixth. The right and left third aortic 

 arches become respectively the root of the right and left internal carotid 

 arteries, the external carotids coming from the ventral aorta. The left 

 fourth aortic arch becomes the arch of the aorta of the adult, while the 

 right forms the right subclavian artery. The dorsal aorta between the 



LUNG ANLAGE 

 PHARYNGEAL POUCHES 

 MYELENCEPHALON 



METENCEPH 



ECTODERM 



SPINAL CORD 



■STOMACH 



NCrroCHORD 



DORSAL AORTA 



'LIVER 



YOLK STALK' 



ALLANTOIS' 

 OPTIC VESICLE '' ^^5ai^5?\T-ELENCEPHALON CLOACA' 



Fig. 256. — A diagram of a 4.2 mm. human embryo showing five aortic arches. 

 (Redrawn after W. His.) Why does the human embryo have six aortic arches of which 

 only three persist in the adult, unless man's ancestors had six func<-.ional arches? 



third and fourth arches degenerates and disappears. Posterior branches 

 of the sixth pair of aortic arches connect with the lungs and form the 

 pulmonary arteries. Until birth, the remainder of the left sixth aortic 

 arch persists as the ductus arteriosus connecting the pulmonary artery 

 with the dorsal aorta. The vessel which connects the right pulmonary 

 artery with the right dorsal aorta degenerates. The fifth pair of arches 

 degenerate soon after their appearance. 



The Heart in Man. The human heart is a hollow muscular organ 

 about the size of the closed fist, weighing from nine to eleven ounces. 

 It is shaped like a truncated cone with its apex pointing downwards to 

 the left. It is placed asymmetrically behind the sternum, with its apex 

 just above the left fifth rib. A muscular partition extending from apex 

 to base divides it into right and left cavities, each of which is subdivided 

 into an anterior atrium or auricle and a posterior ventricle. Externally 

 the division of the heart into atria and ventricles is indicated by a groove, 

 the sulcus coronarius, and the two are separated internally by atrio- 



