DEVELOPMENT OF THE AKTEEIES. 



1027 



7th pair of inter- 

 segmental arteries 



Vertebral 

 arteries 



1st pair of inter- 

 segmental arteries 



1st cephalic aortic arch 

 2nd cephalic aortic arch 

 3rd cephalic aortic arch 

 4th cephalic aortic arch 

 6th cephalic aortic arch 

 Bulbus cordis 

 Ventricle 



The Aortic Arches and their Ventral and Dorsal Roots. The aortic arches are the vessels 

 which connect the ventral portions of the primitive ventral aortse with the primitive dorsal 

 aortas. Six such arches are formed on each side. They spring from the ventral aortse or from 

 the heart, they pass round the side walls of the fore-gut, in the branchial arches, and they 

 terminate, in relation with the dorsal wall 

 of the pharynx, in the primitive dorsal 

 aortae. All six arches are not present at 

 the same time, for as the more caudally 

 situated arches are formed those situated 

 more cranialwards disappear (Figs. 821, 822). 

 Five pairs of arches, the first four and the 

 sixth, are present in embryos 5 mm. long ; 

 but by the time the length of the embryo 

 has increased to 7 mm. the first two arches 

 on each side have begun to disappear, and the 

 very transitory fifth arch has been formed 

 (Fig. 822). 



The first aortic arch, on each side, is part of 

 the primitive aorta, and is formed as the head 

 fold is developed and the fore-gut is enclosed. 

 It passes through the mandibular or first 

 branchial arch, and connects the cephalic parts 

 of the ventral and dorsal portions of the 

 primitive aortae together (Fig. 821). When 

 the number of mesodermal somites has in- 

 creased to twenty-three pairs and the embryo 

 is about 2-5 mm. long, a second pair of aortic 

 arches has appeared. They spring from the Atrium 

 cephalic end of the heart, dorsal to the ventral sinus venosus 

 roots of the first arches, and pass through the 

 hyoid or second pair of branchial arches to FIG. 821. SCHEMA OF THE STAGE OF FIVE AORTIC 

 the dorsal aortae ARCHES. The cardinal veins and ducts of Cuvier 



In embryos 5 mm. long the number of are not shown - 



aortic arches has increased to five on each 



side the first four and the sixth, the fifth appearing later, between the fourth and the sixth. At 

 the period when five pairs of arches are present only four vessels spring from the cephalic end of 

 the heart, which is now called the bulbus cordis ; they are, on each side, an anterior stem which 

 forms the ventral roots of the first and second arches, and a posterior stem common to the third, 

 fourth, and sixth arches (Fig. 821). Up to this period the head lies quite close to the thorax, 

 and a distinct neck can scarcely be said to exist. 



As the neck appears and the head is moved away from the thorax the third and fourth aortic 



arches also move headwards 

 and are transposed from the 



I 4th arches posterior to the anterior 



[ ; 5th arches stem, which is simultane- 



*^ ^^^^^^^^ j 6th arches ously elongated. When the 



T^MHM|M^^^^ transposition is completed 



^^^MW^^^MfMnBj ^Rll^B ^ 1C con dition depicted in 



^^^^^^^^P! . Fig. 822 is attained ; the 



J Vjf most cephalic, or anterior, 



I/ ^~-^4<t\"rm tirH^\ P art ^ eac k Dorsal aorta is 



T f^^^...\ connected with the cephalic 



1st arches atrophi^,^ ^^^^^l '*"'" Pulmonary arteries end of the heart by the sixth 



External caJtids / / ^^ first, second, third, and 



Ventral root of 3rd arch r^^ lourtn arcnes it is connected 



Ventral root of 4th and 5th arches with a ventral vessel which 



Truncus arteriosus springs from the cephalic 



,.. o n * A . IK ,~ n /A^._ end of the heart. The 



of each dorsal aorta 

 is caudal to any 

 given arch is called the 

 dorsal root of the arch, 



i and the portion of the ventral stem which lies caudal to any arch is the ventral root of 

 ' that arch. The sixth arch has no ventral root, for it arises directly from the heart. Its 

 dorsal root converges towards and meets its fellow of the opposite side about the level of 

 the tenth mesodermal somite, where the two fuse together to form the single dorsal aorta, 

 which becomes the descending aorta of the adult, and which extends caudalwards to the level 

 : of the twenty-third somite, where it gives off the umbilical arteries and becomes the middle 

 sacral artery. 



By the time the embryo has attained a length of 7 mm. the first two arches have partly dis- 

 appeared, only their dorsal and ventral extremities being left, and the transitory fifth arch has 



66 a 



2nd arches atrophied 



Ch 4th arches 

 ' ; 5th arches 



k 6th arches 



External carotids j ' ^Hft 



Ventral root of 3rd arch 7^^ 



Ventral root of 4th and 5th arches / 



Truncus arteriosus 



, FIG. 822. SCHEMA OF AORTIC ARCHES OF AN EMBRYO, 9 MM. LONG. (After 

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

 the transitory fifth has appeared. 



