79G DEVELOPMENT OF THE VASCULAR SYSTEM. 



which they permanently occupy ; but upon the right side, as the fourth 

 arch and the aortic root are obliterated posteriorly, the passage for 

 blood from the aortic stem into the subclavian trunk is formed by the 

 persistence of the forepart of the fourth right arch as far as the place 

 where it meets the origin of the subclavian and vertebral arteries. 



The common carotid trunks, occupying the region which afterwards 

 becomes the neck, but which is at first absent or extremely short, are 

 formed by the anterior divisions of the aortic roots ; while the external 

 carotid artery is due to the persistence of a channel in the continuation 

 of each anterior aortic root, and the internal carotid artery arises from 

 the persistence of the crossing third arch and the upper part of the 

 posterior aortic root. 



Thus it falls out that, in man and a certain number of mammals, an 

 innominate artery is formed on the right side by the union of the first 

 part of the fourth right aortic arch leading into the right subclavian, 

 and the right anterior aortic root which forms the common carotid ; 

 while, on the left side, the carotid and subclavian vessels rise separately 

 from the permanent aortic arch in consequence of the distance lying 

 between them in the original foetal condition. 



It does not come within the scope of this chapter to describe the 

 further steps of development of these vessels, nor to enter into an 

 explanation of the manner in which abnormal position of the arch of 

 the aorta and its branches, or of the pulmonary arteries, may be sup- 

 posed to arise. For further information on this subject the reader is 

 referred to the short account of the varieties given in the description of 

 the blood-vessels in the first volume of this work, as well as to the third 

 volume of Henle's Handbuch, and to the special works of Tiedemann 

 and Eichard Quain on the Arteries. 



DEVELOPMENT OF THE GREAT VEINS. 



In the early embryo, before the development of the allantois, a right and a 

 left omphalo-mesenteric vem bring- back the blood from the walls of the nm- 

 bilical vesicle, and unite to form a short trunk, the meatus venosus, which is 

 continued into the auricular extremity of the rudimentary heart. 



In the first commencement of the placental circulation, or in the fourth week 

 of foetal life, two umbilical veins are seen coming from the placenta, and uniting 

 to form a short trunk, which opens into the common omphalo-mesenteric vein. 

 Very soon the right omphalo-mesenteric vein and right umbilical vein disappear. 

 In connection with the common trunk of the umbilical and omphalo-mesenteric 

 veins, two sets of vessels make their appearance in the yomig liver. Those 

 furthest from the heart, named vena; Iwpatlca advehentcs, become the right and 

 left divisions of the portal vein ; the others are the hepatic veins, vencv kcpaticce. 

 Tcvclwnifs. The portion of vessel intervening between those two sets of veins 

 forms the ductnn rcnoKiix, and the part above the hepatic vein, being subse- 

 quently joined by the ascending vena cava, forms the upper extremity of that 

 vein. Into the remaining or left omphalo-mesenteric vein, open the mesen- 

 teric and splenic veins. The part above the latter forms the trank of the portal 

 vein ; and the portion of vessel between the union of this with the umbilical 

 vein and the origin of the venaj hepaticaj advehentes is so altered that the portal 

 tiimk opens into the commencement of the right vena advehens. 



At the time of the commencement of the placental cu-culation, two short trans- 

 verse venous trunks, the ductii of Ci/ricr, open, one on each side, into the auricle 

 of the heart. Each is formed by the union of a superior and an inferior vein, 

 named the itrimitive jugular and the cardinal. 



The primitive jugtdnr vem receives the lilood from the cranial cavity by 

 channels in front of the ear, which are subsequently obliterated : in the greater 



