DEVELOPMENT OF THE VEINS. 977 



third branchial or visceral arches or lobes, already described as appear- 

 ing in the facial and cervical regions of the embryo. These arterial 

 arches are thus modified: The fifth, or posterior arch, the* last one 

 formed, disappears on the right side ; whilst, on the left, it constitutes 

 the trunk of the future pulmonary artery, and joins the fourth arch. 

 This fourth arch, on the left side, forms the left aortic arch, which, 

 joining with the first, then descends and unites, in the Bird, with the 

 corresponding or fourth arch of the right side, to form the aorta ; but 

 in the Mammal, the fourth arch, on the right side, does not form a right 

 aortic arch, but remains pervious, as the innominate and the commence- 

 ment of the right subclavian arteries, from which the vertebral and 

 axillary arteries are given off. On the left side, the subclavian is given 

 off directly from the fourth, or left aortic arch. The commencement 

 or inner ends of the third arches, form the corresponding common 

 carotids, whilst the third arches themselves remain as the internal 

 carotids. The commencement or inner ends of the first and second 

 arches become the external carotids, the transverse parts of the last- 

 named arches entirely disappearing. (Rathke".) 



The veins of the embryo consist, at first, of four symmetrical cardinal 

 veins, two anterior and two posterior, one on each side. The anterior 

 and posterior vein of each side join to form the so-called right and left 

 ducts of Cuvier ; these again unite, in the middle line, into a very short 

 trunk, which, at an early period, opens into the single auricle, and, except 

 in Fishes, ultimately forms part of that cavity ; hence, the two ducts of 

 Cuvier open independently, as two superior venae cavae, into the auri- 

 cle. This condition remains permanent in Birds and some of the lower 

 Mammalia, which possess both a right and a left vena cava superior, 

 opening separately into the right auricle. Instances are occasionally 

 met with, from arrest of development, of two such veins in the human 

 body. The anterior cardinal veins remain on each side, as the internal 

 jugular vein. Between them, across the root of the neck, a transverse 

 branch is very early formed, below the future subclavian veins. This 

 cross branch, becoming enlarged, at the same time that the part of 

 the left cardinal vein, extending below it down to the coronary veins 

 on the back of the heart, is obliterated, conveys the blood from the 

 left side of the head and upper limb, over to the venous trunk of the 

 right side, and forms the future left innominate vein, the persistent 

 venous trunk of the right side becoming the vena cava superior. The 

 posterior cardinal veins originally return the blood from the Wolffian 

 bodies, and from the hinder part of the embryo. These afterwards 

 become the azygos veins, of which, however, the right one only remains 

 in Man and the higher Mammalia, the left vein disappearing up to the 

 back of the heart. Here, however, traces of the left cardinal trunk, 

 or left vena cava superior, remain as the cardiac sinus, or the short 

 dilated terminal portion of the great cardiac or coronary vein. Some- 

 times a smaller azygos vein, on the left side, ascending to the innomi- 

 nate, represents another portion of the primitive cardinal vein ; from 

 this point, however, down to the cardiac sinus, at the back of the heart, 

 vestiges of the course of the primitive vein may be found throughout 

 life. The left superior vena cava, when it exists, pursues the same 



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