DEVELOPMENT OF THE ARTERIES. 



847 



Diagrams illustrating primary arrangement (A) and second- 

 ary modifications {B) in branchial arch vessels. TA, truncus 

 arteriosus; /-K/, aortic bows; yA, DA, ventral and dorsal 

 aorta; A, aorta; AA, aortic arch; /, innominate artery; CC, 

 CE, CI, common, external and internal carotids; .S, subclavian ; 

 P, pulmonary artery ; da, ductus arteriosus. 



the ventral longitudinal stem divides in the frontal plane into two portions, one of which is con- 

 nected with the sixth branchial vessels, while the other retains the remaining ones ; and (5) the 

 posterior portion of the right dorsal longitudinal stem disappears, so that the dorsal aorta is 

 formed only by the left stem (Fig. 678). As the result of these changes the anterior portion of the 

 ventral longitudinal stem becomes the external carotid artery ; the anterior portion of the dorsal 

 longitudinal stem the internal carotid ; 



the third branchial vessel becomes the Fig. 744. 



connection between the two carotids ; A. B. 



the fourth branchial vessel of the left 

 side, together with the left dorsal longi- 

 tudinal stem, becomes the arch of the 

 aorta ; the right fourth branchial vessel 

 and the persisting portion of the right 

 dorsal longitudinal stem become the 

 proximal portion of the right subclav- 

 ian artery ; the sixth branchial vessels 

 become the pulmonary arteries, the 

 persisting connection of the left one 

 with the aortic arch being the ductus 

 arteriosus ; the proximal portion of the 

 ventral longitudinal trunk which re- 

 mains connected with the sixth vessels 

 becomes the pulmonary aorta, while 

 the other portion becomes the prox- 

 imal part of the aortic arch. These 

 changes are shown diagrammatically 

 in Fig. 744, A and B. 



From the forward prolongations 

 of the carotid arteries the vessels which 

 supply the cranial structures are de- 

 veloped, and lateral branches also pass from the carotids to the structures which are formed 

 from the branchial arches. Of these branches the superior thyroid, lingual, and facial arteries are 

 probably from the beginning connected with the external carotid, but the greater part of the 

 internal maxillary takes its origin from the internal carotid and only secondarily becomes con- 

 nected with the external one (page 743). 



From the dorsal longitudinal stems, posterior to the point at which the sixth branchial 

 vessels join them, branches pass off laterally to each of the cervical segments, the most anterior 

 pair accompanying the hypoglossal nerve and passing to the occipital segments with which the 

 nerve is associated. Later, as the heart recedes towards its final position in the thorax, carrying 



with it the dorsal longitudinal stems, the majority of 

 the cervical lateral branches separate from the stems 

 and are represented in the adult by the segmental 

 muscular and spinal branches which arise from the 

 vertebral artery. The seventh branches, however, 

 retain their connection with the longitudinal stems 

 and become the subclavian arteries of the adult. 



Throughout the entire length of the dorsal aorta 

 segmental branches are distributed not only to the 

 body-wall, but also to the viscera, and in each seg- 

 ment two typical sets of visceral branches may be 

 distinguished, — a pair of lateral branches which pass 

 laterally beneath the peritoneum to the paired viscera, 

 and a single median branch which passes ventrally in 

 the mesentery and is supplied to the digestive tract 

 and its derivatives (Fig. 745). The lateral branches 

 to the body-wall persist in the adult as the inter- 

 costal lumbar and lateral sacral branches, the fifth 

 lumbar branches entering into the formation of the 

 iliac arteries. The visceral branches belonging to 

 both sets, however, undergo much modification, 

 some disappearing and others fusing, so that little trace of their primary segmental arrangement 

 is to be recognized in the adult. Representatives of the paired visceral branches are to be 

 found in the bronchial, suprarenal, renal, and spermatic (ovarian) arteries, and in the fcetus 

 the umbilical arteries represent the paired branches of the third lumbar segment. At an 

 early stage, however, these vessels make connections with branches of the iliac arteries and 



F1G.745. 



Diagram showing fundamental arrangement 

 of branches from aorta (A)\ B, lateral branches 

 to body-wall ; C, paired visceral, D, unpaired 

 visceral branch ; E. peritoneum. 



