850 



DEVELOPMENT 



[CH. LIX. 



pericardium and on the ventral wall of the foregut; they are the 

 primitive heart tubes, and their anterior ends run into the first 

 cephalic aortic arches, which pass round the sides of the anterior end 

 of the foregut into the primitive dorsal vessels. A little later the 

 parts of the anterior ventral vessels in front of the heart are con- 

 nected with the dorsal vessels by four additional arches, one in each 

 visceral arch that is, there are now five aortic arches on each side 

 connecting the anterior parts of the ventral with the anterior parts of 

 the dorsal vessels. The portions of the ventral vessels which lie behind 



the arches in the dorsal wall of the peri- 

 cardium rapidly enlarge, and they fuse 

 together to form the single heart, which 

 is thus for a time a single longitudinal 

 vessel. The parts of the ventral and 

 dorsal vessels immediately behind each 

 arch are called the roots of the arch. 



In mammals, the first and second 

 arches disappear, and their ventral roots 

 become the external carotid artery. The 

 third arches and the dorsal roots of the 

 first and second arches form the internal 

 carotids. The dorsal root of the third 

 arch disappears on each side, and the 

 ventral root forms the common carotid 

 artery. The ventral root of the right 

 fourth arch becomes the innominate 

 artery, and the arch itself takes part in 

 the formation of the right subclavian 

 artery. The dorsal roots of the right 

 fourth and fifth arches and the dorsal 

 part of the fifth arch itself disappear, 

 and the ventral part of the fifth arch 

 becomes the right pulmonary artery. 

 The left fourth arch, with its dorsal and 

 ventral roots, and the dorsal root of the 

 left fifth arch, take part in the formation of the arch of the aorta. 

 The left fifth arch persists till birth, then its dorsal part becomes a 

 fibrous cord, the ligamentum arteriosurn, and its ventral part forms 

 the left pulmonary artery (fig. 647). 



The five aortic arches correspond with the gill arteries of fishes, 

 but in mammals they never break up into capillaries, as in the fishes' 

 gills. In amphibia three pairs persist throughout life. In reptiles 

 the fourth pair remains throughout life as the permanent right and 

 left aortse. In birds the right fourth remains as the permanent aorta, 

 curving over the right bronchus, whereas in mammals, the left 



Fio. 645. Diagram representing the 

 primitive blood-vessels of the embryo. 

 1, First cephalic aortic arch ; 2, anterior 

 ventral part of primitive vessel ; 8, 

 dorsal part of primitive vessel ; 4, 

 vascular area of yolk-sac ; 5, posterior 

 ventral part of primitive vessel ; 6, 

 caudal aortic arch ; 7, allantoic or 

 umbilical branch ; 8, umbilical or 

 allantoic vein ; 9, placenta. 



