. The development of the chick; an introduction to embryology . thisday, but soon disappears. The blood passing up the third orcarotid arch is directed forward through the internal and externalcarotid arteries to the head; that passing up the fourth andsixth arches turns backwards to enter the dorsal aorta, so thatthere is an intermediate area of stagnation in the roots of thedorsal aorta between the carotid and aortic arches; though thisis more or less problematical, the arrangement of the vessels ren-ders such a condition very probable. A very small jjroportion ofthe blood enters the rudimen
. The development of the chick; an introduction to embryology . thisday, but soon disappears. The blood passing up the third orcarotid arch is directed forward through the internal and externalcarotid arteries to the head; that passing up the fourth andsixth arches turns backwards to enter the dorsal aorta, so thatthere is an intermediate area of stagnation in the roots of thedorsal aorta between the carotid and aortic arches; though thisis more or less problematical, the arrangement of the vessels ren-ders such a condition very probable. A very small jjroportion ofthe blood enters the rudimentary pulmonary arteries from thesixth nrch. The blood in the dorsal aorta passes l)ackwaids andenters (1) the segmental arteries, (2) the omphalomesentericarteries, (.3) the (rudimentary) umbilical arteries, and behindthe latter passes into the narrow contiiuuition of the tloisal aortae,still separate in this region, known as the caudal arteries. The blood is rclui-ncd to the heart thi-ouuh the sinus venosus LATER DEVELOPMENT OF VASCULAR SYSTEM 373 —. Fig. 214. — Reconstruction of the venous system of a sparrow embryo,corresponding to a chick of about 14 days. (After Miller.)V. H., Intra-hepatic part of the vena cava inferior. V. c. i. 8C.,Part of the vena cava inferior derived from the subcardinal vein. V. v. g.,Genital veins. V. i. e. d., s., Right and left vena iliaca externa. V. i. i.,^ena iliaca interna. V. i. 1. d., s., Right and left vena intervertebralis lum-balis. \. r. m. d., s., Right and left great renal veins. almost exclusively, the pulmonary veins beino; very rudimentaryat this stage. The veins entering the sinus venosus are the ductsof Cuvier, and the meatus venosus. The former are made upon each side by (1) the anterior cardinal vein, returning bloodfrom the head, (2) the posterior cardinal vein returning bloodfrom the veins of the Wolffian body, and the intersomitic veins,(8) the umbilical veins retiu*ning blood mainly from the body- 374 THK ])
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