THE THORAX 1003 



artery. Two other factors, however, are to be taken into account in connection 

 with this vessel — namely: (i) the part of the right dorsal aorta between the 

 dorsal roots of the fourth and sixth right arches, and (2) the seventh right 

 cervical segmental artery, which arises from the right dorsal aorta at the level 

 of the dorsal root of the sixth right arch. This segmental artery furnishes 

 the root of the right vertebral artery. The vertebral artery is at first larger 

 than the subclavian, which latter appears then to be a branch of the vertebral, 

 but as the right subclavian arter\' grows out into the corresponding upper 

 limb-bud to become the right axillary artery, it becomes larger than the 

 vertebral artery. 



The orgin of the left subclavian artery is much more simple than that of 

 the right vessel. The left subclavian artery is a lateral branch of the seventh 

 left cervical segmental artery, which arises from the left dorsal aorta at the 

 level of the dorsal root of the sixth left arch. This segmental artery furnishes 

 the root of the left vertebral artery, which latter vessel is, as on the right side, 

 at first larger than the subclavian. As the left subclavian vessel, however, 

 grows out into the left upper limb-bud to become the left axillary, it becomes 

 larger than the vertebral artery. Another subsequent factor in the forma- 

 tion of the left subclavian is the root of the seventh left cervical segmental 

 artery, which thus brings the left subclaNnan vessel on to the arch of the 

 aorta, and makes the left vertebral now a branch of it, though originally the 

 vertebral artery was really its parent. 



Developmental Differences between the two Subclavian Arteries. 



(Fourth right arterial arch. 

 Portion of right dorsal aorta. 

 Seventh right cervical segmental artery. 

 Left Artery = Seventh left cervical segmental artery. 



The part of each ventral aorta between the ventral roots of the third and 

 fourth arches forms the corresponding common carotid artery. 



The part of each ventral aorta above the ventral root of the third arch gives 

 rise to the corresponding external carotid artery. 



The third arch and the part of the dorsal aorta above its dorsal root give 

 rise, on either side, to the internal carotid artery. 



Summary. 



Right Ventral Aorta. — (i) The root, or part between the aortic bulb and 

 the ventral root of the fourth right arch, gives rise to the innominate artery. 

 (2) The part between the ventral roots of the third and fourth right arches 

 forms the right common carotid artery. (3) The part above the ventral root 

 of the third right arch gives rise to the right external carotid artery. 



Left Ventral Aorta. — (i) The root, or part bet^veen the aortic bulb and 

 the ventral root of the fourth left arch forms the part of the arch of the aorta 

 between the innominate and left common carotid arteries. (2) The part 

 between the ventral roots of the third and fourth left arches gives rise to the 

 left common carotid artery. (3) The part above the ventral root of the third 

 left arch forms the left external carotid artery. 



Ri^nt Dorsal Aorta. — (i) The part between the dorsal root of the third 

 right arch and the place of junction of the two dorsal aortae disappears, except 

 the small portion which contributes to the formation of the right subclavian 

 artery. (2) The part above the dorsal root of the third right arch persists 

 and forms part of the right internal carotid artery. 



Left Dorsal Aorta. — (i) The part between the dorsal roots of the third and 

 fourth left arches disappears. (2) The part above the dorsal root of the third 

 left arch persists and forms part of the left internal carotid artery. (3) The part 

 between the dorsal root of the sixth left arch and the place of junction of the 

 two dorsal aortae forms the commencement of the descending aorta. 



