I004 A MANUAL OF ANATOMY 



Arterial Arches. — (i) The first, second, and fifth arches, on either side, 

 disappear; and the sixth right arch also disappears. (2) The third arch, 

 on each side, persists and forms part of the corresponding internal carotid 

 artery. (3) The fourth right arch forms part of the right subclavian artery. 



(4) The fourth left arch forms the part of the arch 0! the aorta between the 

 left common carotid artery and the point of entrance of the ductus arteriosus. 



(5) The ventral portion of the sixth left or pulmonic arch gives rise to the 

 right and left pulmonary arteries respectively. (According to some authorities, 

 the right pulmonary artery arises from the ventral part of the sixth right 

 arch, which would then be a pulmonic arch like the sixth left.) (6) The 

 dorsal part of the sixth left arch — :that is to say, the part beyond the origin 

 of the right and left pulmonary arteries — persists up to birth, and forms the 

 ductus arteriosus. 



Summary of the Permanent Aorta. — (i) The ascending aorta is developed 

 from the aortic bulb or bulbns arteriosus, along with the trunk of the pul- 

 monary artery. (2) The part Of the arch of the aorta between the innom- 

 inate and left common carotid arteries is developed from the root of the left 

 ventral aorta — that is to say, the part between the aortic bulb and the ventral 

 root of the fourth left arch. (3) The part of the arch of the aorta between 

 the left common carotid artery and the point of entrance of the ductus arte- 

 riosus is developed from the fourth left arch. (4) The descending aorta is 

 formed from above downwards by (a) a portion of the left dorsal aorta, and 

 {b) the fusion of the right and left dorsal aortae. 



Summary of the Pulmonary Artery. — (i) The trunk of the pulmonary 

 artery, otherwise spoken of as the common pulmonary artery, is developed 

 from the aortic bulb or bulbus arteriosus, along with the ascending aorta. 

 (2) The right and left pulmonary arteries are developed from the ventral portion 

 of the sixth left arch. 



Development of the Segmental, Vertebral, and Left Subclavian Arteries. — 

 Each primitive dorsal aorta in the cervical part of its course gives off seven 

 arteries, which are called the cervical segmental arteries. There are thus 

 seven pairs of these arteries, serially arranged. The first or highest lies 

 between the occiput and the atlas, and is therefore suboccipital ; and the seventh 

 or lowest corresponds to the interval between the sixth and seventh cervical 

 vertebrae. These seven vessels, on either side, are connected by interseg- 

 mental anastomotic vessels, which, by their continuity, establish a longi- 

 tudinal anastomotic chain. Superiorly this chain is prolonged through the 

 foramen magnum into the cranial cavity, where it terminates by anas- 

 tomosing with the corresponding internal carotid artery. 



The upper six cervical segmental arteries soon disappear, but the seventh 

 segmental artery and the longitudinal anastomotic chain persist, on either 

 side. 



The first part of the vertebral artery is developed from the seventh cervical 

 segmental artery. The second part of the vertebral artery, which traverses 

 the upper six costo-transverse, or vertebrarterial, foramina, represents the 

 persistent longitudinal anastomotic chain, except its cephalic part. The 

 third, or suboccipital, and fourth, or intracranial, parts of the artery are devel- 

 oped from the cephalic part of the longitudinal anastomotic chain, which, as 

 stated, is prolonged through the foramen magnum into the cranial cavity. 



The seventh cervical segmental artery, on either side, is of special importance 

 for two reasons. In the first place it gives origin to the vertebral artery, 

 which takes a course cephalicwards. In the second place, it gives origin, on 

 the left side, to the left subclavian artery, and on the right side it contributes 

 to the formation of the right subclavian artery. Each subclavian artery 

 takes an outward course to the corresponding upper limb-bud. The seventh 

 left cervical .segmented artery thus constitutes a common stem for the left 

 vertebral and left subclavian arteries, whilst the seventh right cervical seg- 

 mental artery forms a common stem for the right vertebral artery and a part 

 of the right subclavian artery. 



The origin of a vertebral artery precedes that of the corresponding sub- 



