164 HUMAN PHYSIOLOGY. 



by veins which enter the heart. These vessels are known as the omphalo- 

 mesenteric arteries and veins. The vitelline circulation is of short dura- 

 tion in the mammals, as the supply of nutritious matter in the vitellus soon 

 becomes exhausted. 



The Placental circulation becomes established when the blood vessels 

 in the allantois enter the villous processes of the chorion and come into 

 close relationship with the maternal blood vessels. This circulation lasts 

 during the whole of intra-uterine life, but gives way at birth to the adult 

 circulation, the change being made possible by the development of the 

 circulatory apparatus. 



The Heart appears as a mass of cells coming off from the anterior por- 

 tion of the intestine ; its central part liquefies, and pulsations soon begin. 

 The heart is at first tubular, receiving posteriorly the venous trunks and 

 giving off anteriorly the arterial trunks. It soon becomes twisted upon 

 itself, so that the two extremities lie upon the same plane. 



The heart now consists of a single auricle and a single ventricle. A 

 septum growing from the apex of the ventricle divides it into two cavities, 

 a right and a left. The auricles also become partly separated by a septum 

 which is perforated by the foramen ovale. The arterial trunk becomes 

 separated by a partition, into two canals, which become, ultimately, the 

 aorta and pulmonary artery. The auricles are separated from the ventri- 

 cles by incomplete septa, through which the blood passes into the ventricles. 



Arteries. The aorta arises from the cephalic extremity of the heart and 

 divides into two branches which ascend, one on each side of the intestine, 

 and unite posteriorly to form the main aorta ; posteriorly 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 upp^r extremities develop. The heart descends 

 in the thorax, and the canals of Cuvier become oblique ; they shortly 



