276 HUMAN PHYSIOLOGY. 



teriorly to these first aortic arches four others are developed, so 

 that there are five altogether running along the visceral arches. The 

 two anterior soon disappear. The third arch becomes the internal 

 carotid and the external carotid; a part of the fourth arch, on the 

 right side, becomes the subclavian artery, and the remainder atrophies 

 and disappears, but on the left side it enlarges and becomes the 

 permanent aorta ; the fifth arch becomes the pulmonary artery on the 

 left side. The communication between the pulmonary artery and the 

 aorta, the ductus arteriosus, disappears at an early period. 



Veins. The venous system appears first as two short, transverse 

 veins, the canals of Cuvier, formed by the union of the vertebral 

 veins and the cardinal veins, which empty into the auricle. The 

 inferior vena cava is formed, as the kidneys develop, by the union 

 of the renal veins, which, in a short time, receive branches from 

 the lower extremities. The subclavian veins join the jugular as the 

 upper extremities develop. The heart descends in the thorax, and 

 the canals of Cuvier become oblique ; they shortly communicate by 

 a transverse duct, which ultimately becomes the left innominate 

 vein. The left canal of Cuvier atrophies and becomes a fibrous cord. 

 A transverse branch now appears, which carries the blood from the 

 left cardiac vein into the right, and becomes the vena azygos minor ; 

 the right cardiac vein becomes the vena azygos major. 



Circulation of Blood in the Fetus. The blood returning from the 

 placenta, after having received oxygen and being freed from car- 

 bonic acid, is carried by the umbilical vein to the under surface of 

 the liver ; here a portion of it passes through the ductus venosus into 

 the ascending vena cava, while the remainder flows through the 

 liver and passes into the vena cava by the hepatic veins. When the 

 blood is emptied into the right auricle, it is directed by the Eustachian 

 valve through the foramen ovale, into the left auricle, thence into 

 the left ventricle, and so into the aorta and to all parts of the 

 system. The venous blood returning from the head and upper ex- 

 tremities is emptied, by the superior vena cava, into the right 

 auricle, from which it passes into the right ventricle, and thence 

 into the pulmonary artery. Owing to the condition of the lung 

 only a small portion flows through the pulmonary capillaries, the 

 greater part passing through the ductus arteriosus, which opens into 

 the aorta at a point below the origin of the carotid and subclavian 

 arteries. The mixed blood now passes down the aorta to supply the 



