576 



THE HUMAN EMBRYO. 



In the early part of the fifth week, the left fourth, or systemic 

 arch, becomes distinctly larger than the corresponding arch of the 

 right side ; and this difference soon becomes more pronounced. 

 By the end of the fifth week the fourth right arch is not only 

 markedly smaller than the left arch, but has lost its connection 

 with the aorta, and now forms only the vertebral and subclavian 

 arteries of the right side. 



The fifth aortic arch of the right side disappears, beyond 

 the origin of the right pulmonary artery. The fifth left arch, 



A! 



A.+ 



AE 



RW 



RX 



AW 



FIG. 245. The aortic arches of a Human Embryo thirty-two dnys old, from 

 the left side. (From His.) x 18. 



A, dorsal aorta. A.4, fourth, or systemic aortic arch. A.5, fifth, or pulmonary 

 aortic arch. AE, external carotid artery. AT, internal carotid artery. AP, pulmonary 

 artery. AV, vert dn-al artery. A~W, inter vertebral or segmental arteries. LQ.lunp. 

 T-jR, trachea. MM", mandible, or lower jaw. PT, pituitary rliverticulam from mouth. 

 R"W, pulmonary trunk. RX, systemic trunk. TN, tongue. TO, fflsophagus. ^.^ 



however, remains of large size up to the close of foetal life ; the 

 portion of the arch between the root of the left pulmonary 

 artery and the dorsal aorta being known as the ductus arteriosus 

 (Figs. 245 and 246, A.S). 



The dorsal aorta and its branches. The point at which the 

 two aortas unite, to form the single dorsal aorta, is about the 

 junction of the cervical and dorsal regions, in embryos at the 

 end of the fourth week, but the exact position varies considerably 

 in different specimens. As the union proceeds backwards, the 

 dorsal aorta increases considerably in size, and its diameter in 



