130 



DE VEL OP MEN T. 



distance from the heart, became fused together to form a single median artery, 

 which coursed down in front of the vertebrae to the bottom of the spinal column, 

 forming the permanent descending aorta. From the extremity of this the two 

 vitelline arteries, which were originally parts of the primitive main trunks, pass 

 to the area vasculosa. As the umbilical vesicle dwindles and the allantois grows, 



--vagus nerve. 



external 



- ' " carotid. 



^internal 



carotid. 



vertebral. 



right / 



subclariitii. 



innominate 



artery. 



ascending 

 aorta. 



descending aorta. 



FIG. 99 -Diagram to show the destination of the arterial arches in man and mammals. (Modified from 

 Rathke. From Quain's Anatomy, vol i. pt. 1, 1890.) The truncus arteriosus and the five arterial arches springing 

 from it are represented in outline only ; the permanent vessels in colors those belonging to the aortic system 

 red, to the pulmonary system blue. 



two large branches are formed as lateral offshoots of the median aorta. These are 

 the two umbilical or hypogastric arteries, and are concerned in the placental circu- 

 lation. The portion of the median aorta beyond this point becomes much dimin- 

 ished in size, and eventually forms the sacra media artery, and thus the two 

 umbilical branches become in appearance bifurcating branches of the main aorta. 

 The common and internal iliac arteries are developed from the proximal end of 

 these umbilical arteries ; the middle portion of the vessel, after birth, becomes 

 partially atrophied, but in part remains pervious as the superior vesical artery ; the 

 distal portion becomes obliterated, constituting part of the superior ligament of 

 the bladder. The external iliac and femoral arteries are developed from a small 

 branch given off from the umbilical arteries near their origin, and are at first of 

 comparatively small size. 



Development of the Veins. The formation of the great veins of the embryo 

 may be best considered under two groups, visceral and parietal. 



The visceral are derived from the vitelline and umbilical veins. In the earliest 

 period of the circulation of the embryo, we have seen that there were two veins 

 (vitelline or omphalo-mesenteric) returning the blood from the vitelline membrane. 

 These unite together to form a single channel, the sinus venosus, which opens into 

 the auricular extremity of the heart. As soon as the placenta begins to be formed 

 two umbilical veins appear and open into the sinus venosus, close to the vitelline 

 veins. The two vitelline veins enter the abdomen and run upward on either side 

 of the intestinal canal ; at the upper part of the abdomen, in the site of the future 

 liver, which now begins to form around them, transverse communications are 

 formed, which encircle the duodenum and enclose it in two vascular rings. The 



