VENOUS SYSTEM. 703 



On the right side the auterior root of the fourth arch, and the 

 part of the aortic trunk leading to it, persists as the innominate 

 artery, the fourth arch being represented by the right subclavian 

 artery. 



The part of the primitive aortic trunk joining the fourth and 

 third right anterior roots becomes the common carotid artery of 

 the same side, whilst arising from this is the internal carotid, 

 which, taking the position of the third arch, passes to the poste- 

 rior roots, and occupies the trunk of the primitive aorta from the 

 third to the first arches. 



The external carotid, arising from the common carotid at the 

 third anterior root, occupies the position of the vessel joining this 

 root to those of the second and first arch. 



On the left side the common carotid and its branches are 

 developed similarly to those on the right, the only difference being 

 that the common carotid arises from the aorta and not from the 

 innominate. 



The iliac arteries are developed from the hypogastric. At 

 first they appear as branches, but with the growth of the limbs 

 they become so much larger that after birth they appear to be 

 the main branches from the point of division of the aorta, the 

 hypogastric arteries now being merely small branches of the iliac 

 vessels. 



With the development of the organs and limbs, vessels in con- 

 nection with those above described arise in the mesoblast. It is, 

 however, beyond the scope of this work to describe in detail the 

 origin of the lesser vessels. 



Venous System. The blood is returned from the head by the 

 two primitive jugulars, which unite with the cardinal veins con- 

 veying the blood from the trunk and lower extremities to form a 

 vessel on each side, called the duct of Cuvier. 



From the lower extremity of the embryo the inferior vena cava 

 commences by the union of the external iliac veins; this passes 

 up and opens into the venous trunk common to the left vitelline 

 and left umbilical veins. 



The left vitelline becomes continuous with the vessels from the 

 common trunk going to the right side of the liver (the right vena 



