MORPHOGENESIS OF THE ARTERIES 



635 



of convenience than as indicating a strict numerical correspondence between segments and 

 vessels. 



The dorsal segmental arteries primarily supply the central nervous system but later give 

 off two sets of vessels to the body wall; these persist in the adult as the anterior and posterior 

 main branches of the intercostal and lumbar arteries. The remainder of each segmental artery is 

 represented in the adult by the spinal ramus which accompanies the corresponding nerve root 

 through the intervertebral foramen. The tendency to form intersegmental anastomoses 

 between these vessels (and their branches) gives rise to many of the important longitudinal stems 

 of adult anatomy. Thus, the spinal ramus gives rise to a pre- and postneural anastomosing 

 channel on either side, the (primarily paired) anterior and posterior spinal arteries. The anterior 

 branches have each a longitudinal precostal anastomosis, and, as they grow forward with the 

 developing body wall, their ends are connected to form the mammary anastomosis. 



Between the posterior rami, a postcostal and a postvertebral anastomosis may be formed. 

 In addition, the anterior rami give off lateral and anterior perforating branches (fig. 507). 



Two dorsal segmental arteries have been recognized in the occipital region, the first dis- 

 appears and the second, the hypoglossus artery, follows the hypoglossal nerve to the ventral sur- 

 face of the brain where it is connected with the termination of the internal carotid of its own side 

 by means of a longitudinal stem the a. vertebralis cerebralis. The hypoglossus artery, by shift- 

 ing forward to the third aortic arch, itself acquires a secondary origin from the internal carotid. 



In the cervical region, the spinal ramus of segmental cervical I forms the third, or sub' 

 occipital, part of the vertebral artery. Cervical segmentals I to VI lose their connection with 

 the aorta and a postcostal anastomosis between them forms the second part of the same artery. 

 The first part of the vertebral is formed by the posterior ramus of cervical VI and its precostal 

 anastomosis with cervical VII (subclavian) (fig. 508). 



Fig. 507. — Scheme of the Typical Arrangement of the Branches of the Aorta. (After 



Quain.) 



Longitudinal anastomoses: 1, precostal; 2, postcostal; 3 postvertebral; 4, preneural; 5, post- 

 neural; 6, mammary. 



Posterior branch 



Anterior branch 



'i-^ - Lateral perforating branck 



Anterior perforating branch 



The anterior ramus of cervical VII forms the entire first part of the subclavian on the left, 

 and the distal portion of it upon the right (see system of aortic arches). The second part of 

 the subclavian is formed by the lateral branch of the anterior ramus of cervical VII, while the 

 portion of the anterior ramus ventral to this becomes the root of the internal mammary. The 

 anterior ramus of cervical VIII disappears, but the precostal anastomosis connecting it with the 

 subclavian (cervical VII) persists to form the costo-cervical of the adult. The posterior ramus 

 of cervical VIII forms the root of the deep cervical, and, by a postvertebral anastomosis with 

 the other posterior cervical rami and with the occipital, forms the remainder of the deep cervical 

 and the descending branch of the occipital artery. 



In the thoracic and lumbar regions, the embryonic conditions very largely persist (fig. 508). 

 The anterior rami of thoracic segmentals I and II, however, lose their connection with the 

 aorta and, by a precostal anastomosis with cervical VIII, become secondarily connected 

 (through the costo-cervical trunk) with the subclavian. The superior intercostal of the adult 

 is thus formed. The fifth lumbar segmental apparently joins the umbilical artery (of the 

 ventral segmental series) to form the external iliac which, in the adult, provides the chief 

 arterial supply to the lower extremity. The inferior gluteal (sciatic), which is the primitive 

 artery of supply for the lower extremity, if it is segmental at aU, belongs to the sacral region. 

 The free ends of the anterior rami of all the thoracic and the upper four lumbar segmentals 

 become united, as they grow out with the body wall, to form the longitudinal mammary 

 anastomosis (fig. 508). This anastomosis, by its connection with the anterior ramus of cervical 



