THE PEIMITIVE VASCULAK SYSTEM. 



67 



3rd arches 



4th arches 



5th arches 



6th arches 



Dorsal aorta 



'Pulmonary arteries 



External carotids / / 

 Ventral root of 3rd arch / 

 Ventral root of 4th and 5th arches i 

 Truncus arteriosus 



85 __ ScHEMA OF AoBTIC ARCHES OF AN EMBRYO, 9 MM. LONG. (After 

 Tandler, modified.) The second and third arches have atrophied and 

 the transitory fifth has appeared. 



advanced considerably in development. Two aortic arches, on each side, now connect 

 the cephalic end of the heart with the primitive dorsal aorta. The umbilical artery 

 and vitelline arteries are quite separate, and each umbilical artery springs, by a 

 number of roots which anastomose together, from the caudal part of the corre- 

 sponding dorsal aorta. The vitelline arteries are still numerous, but that which rises 

 opposite the twelfth mesodermal somite is becoming the main artery of the yolk-sac ; 

 eventually its proximal 2nd arches atrophied 



part is transformed into 

 the superior mesenteric 

 artery of the foetus. 



When the embryo 

 has attained a length of 

 5 mm., and is about five 

 weeks old, it possesses 

 about thirty-eight 

 mesodermal somites, and ist arches atropwe 

 five aortic arches are 

 present on each side. 

 Commencing from the 

 cranial end, they are 

 the first, second, third, 

 fourth, and sixth; the 

 fifth arch appears sub- 

 sequently between the 



fourth and the sixth. All five arches pass to the corresponding dorsal aorta, but 

 the three most caudal, on each side, spring from the cranial end of the heart, which 

 is now called the aortic trunk, whilst the two most cranial rise from a common stem 

 which constitutes their ventral roots, and which springs, also, from the aortic trunk 

 (Fig. 84). A little later the aortic trunk gives off only two branches on each 

 side, (1) a stem common to the first five arches, for the fifth has now appeared, and 

 (2) the sixth arch (Fig. 85). The fifth arch is very transitory. Whilst it is present 



it runs from the 

 common ven- 

 tral stem, caudal 

 to the fourth 

 arch, to the 

 dorsal part of 

 the sixth arch. 

 It soon disap- 

 pears, and no 

 traces of it are 

 left in the adult 

 (Fig. 85). 



The portion 

 of the common 

 ventral stem 

 which lies 

 caudal to each 

 of the arches is 



Internal carotid 

 Internal carotid ! 

 [nternal carotid ; 



External carofTid y ' 

 External carotid 



Arch of aorta 



j Right subclavian artery 



i Left subclavian artery 

 i 



Right subclavian artery 



| Union of ductus arteriosus 



- with aorta 



' Union of dorsal roots of 

 6th arches 



Left 6th arch 



. Right pul- 



"monary artery 



-Left pulmonary artery 



"Innominate artery 

 -Right 6th arch 



Left common carotid 

 Right common carotid 



Left 6th arch 



Ascending aorta 



FIG. 86. SCHEMA OF PART OF THE ARTERIAL SYSTEM OF A F<ETUS SEEN FROM THE LEFT 

 SIDE. Parts of the first and second arches, the dorsal roots of the third arches, the 

 dorsal part of the right sixth arch, and the dorsal roots of the right fourth and fifth called the 

 arches have atrophied. The position of the fifth arch is not indicated ; see Fig. 84. 



root of the 



arch, and the parts of the primitive dorsal aortae which lie caudal to the dorsal 

 ends of the arches are called their dorsal roots. 



The first two arches, on each side, disappear, and their ventral roots become the 

 external carotid arteries of the adult. The ventral root of the third arch becomes 

 the common carotid, whilst the third arch and the dorsal roots of the first and 

 second arches are transformed into the internal carotid. The ventral root of the 

 fourth arch on the right side becomes the innominate artery, and the right fourth 

 arch forms the proximal part of the right subclavian artery. 



The remainder of the 



