The fifth arch always gives origin to the pulmonary artery ([fig. 365], p) and is continuous with one of the divisions of the truncus arteriosus. In Lizards ([fig. 366] A, i), Chelonians and Birds ([fig. 366] C, i) and probably in Crocodilia, the right and left pulmonary arteries spring respectively from the right and left fifth arches, and during the greater part of embryonic life the parts of the fifth arches between the origins of the pulmonary arteries and the system of the dorsal aorta are preserved as ductus Botalli. These ductus Botalli persist for life in the Chelonia. In Ophidia ([fig. 366] B, h) and Mammalia ([fig. 366] D, m) only one of the fifth arches gives origin to the two pulmonary arteries, viz. that on the right side in Ophidia, and the left in Mammalia.
The ductus Botalli of the fifth arch (known in Man as the ductus arteriosus) of the side on which the pulmonary arteries are formed, may remain (e.g. in Man) as a solid cord connecting the common stern of the pulmonary aorta with the systemic aorta.
The main history of the arterial arches in the Amniota has been sufficiently dealt with, and the diagram, [fig. 366], copied from Rathke, shews at a glance the character of the metamorphosis these arches undergo in the different types. It merely remains for me to say a few words about the subclavian and vertebral arteries.
The subclavian arteries in Fishes usually spring from the trunks connecting the branchial veins with the dorsal aorta. This origin, which is also found in Amphibia, is typically found in the embryos of the Amniota. In the Lizards this origin persists through life, but both subclavians spring from the right side. In most other types the origin of the subclavians is carried upwards, so that they usually spring from a trunk common to them and the carotids (arteria anonyma) (Birds and some Mammals); or the left one, as in Man and some other Mammals, arises from the systemic aorta just beyond the carotids. Various further modifications in the origin of the subclavians of the same general nature are found in Mammalia, but they need not be specified in detail. The vertebral arteries usually arise in close connection with the subclavians, but in Birds they arise from the common carotids.
Fig. 366. Diagrams illustrating the metamorphosis of the arterial arches in a Lizard A, a Snake B, a Bird C and a Mammal D. (From Mivart; after Rathke.)
A. a. internal carotid; b. external carotid; c. common carotid; d. ductus Botalli between the third and fourth arches; e. right aortic trunk; f. subclavian; g. dorsal aorta; h. left aortic trunk; i. pulmonary artery; k. rudiment of ductus Botalli between the pulmonary artery and the system of the dorsal aorta.
B. a. internal carotid; b. external carotid; c. common carotid; d. right aortic trunk; e. vertebral artery; f. left aortic trunk of dorsal aorta; h. pulmonary artery; i. ductus Botalli of pulmonary artery.
C. a. internal carotid; b. external carotid; c. common carotid; d. systemic aorta; e. fourth arch of right side (root of dorsal aorta); f. right subclavian; g. dorsal aorta; h. left subclavian (fourth arch of left side); i. pulmonary artery; k. and l. right and left ductus Botalli of pulmonary arteries.
D. a. internal carotid; b. external carotid; c. common carotid; d. systemic aorta; e. fourth arch of left side (root of dorsal aorta); f. dorsal aorta; g. left vertebral artery; h. left subclavian artery; i. right subclavian (fourth arch of right side); k. right vertebral; l. continuation of right subclavian; m. pulmonary artery; n. ductus Botalli of pulmonary artery.
Bibliography of the Arterial System.
(496) H. Rathke. “Ueb. d. Entwick. d. Arterien w. bei d. Säugethiere von d. Bogen d. Aorta ausgehen.” Müller’s Archiv, 1843.
(497) H. Rathke. “Untersuchungen üb. d. Aortenwurzeln d. Saurier.” Denkschriften d. k. Akad. Wien, Vol. XIII. 1857.
Vide also His (No. [232]) and general works on Vertebrate Embryology.