248 



TEXT-BOOK OF EMBRYOLOGY. 



From the point of its bifurcation to its caudal end the aorta gives off paired, 

 segmental branches which accompany the segmental nerves. The last (eighth) 

 cervical branch and the first tyro thoracic branches undergo longitudinal anas- 

 tomoses, similar to those between the first seven cervical, to form the superior 

 intercostal artery (A. intercostalis suprema) which opens into the subclavian 

 (Fig. 222). The other thoracic branches persist as the intercostal arteries; the 

 lumbar branches persist as the lumbar arteries. At the same time anastomoses 

 are formed between the distal ends of the intercostal and lumbar arteries in 

 the ventro-lateral region of the body wall, which give rise, on the one hand, to the 

 internal mammary artery and, on the other hand, to the inferior epigastric artery. 

 Of these two the former opens into the subclavian, the latter into the external 

 iliac. By a further anastomosis the distal ends of the internal mammary and 

 inferior epigastric are joined, thus forming a continuous vessel from the sub- 



Post. cerebral vein 

 (sup. petrosal sinus) 



Circulus arteriosus 

 Transverse sinus 



Basilar artery 

 Int. jugular vein 



Confluence of sinuses 



Inf. sagittal sinus 

 Sup. sagittal sinus 

 Post, cerebral artery 



Ant. cerebral artery 

 Int. carotid artery 



Fig. 224. — Brain, arteries and veins of a human embryo of 33 mm. Mall. 



clavian to the external iliac (Fig. 225). It is interesting to note that while 

 originally all the lateral branches of the aorta are arranged segmentally, many 

 of them lose their segmental character and are replaced or supplemented by 

 longitudinal vessels. 



In addition to the lateral segmental branches of the aorta, which have been 

 described, other branches develop which carry blood to the viscera. A num- 

 ber of these, or possibly all, are also primarily segmental vessels, although they 

 lose every trace of their segmental character during development. The first of 

 the visceral branches to appear is the omphalomesenteric artery which arises 

 from the ventral side of the aorta and which has been mentioned in connection 

 with the vitelline circulation. Originally it passes out through the mesentery 

 and follows the yolk stalk to ramify on the surface of the yolk sac. But since 

 the yolk sac is of slight importance, the distal part of the artery soon disappears, 

 while the proximal part becomes the superior mesenteric artery (Fig. 226). The 



