576 EMBRYOLOGY. 



convey the blood into the unpaired aorta. From each of them/ 

 there arises, as in the embryo, a separate carotis communis and 

 subclavia. 



Another abnormality is brought about by the development of 

 the aortic arch of the right side of the body instead of that of the 

 left, a condition which is met with in the class of Birds (fig. 319) as 

 the normal state. This malformation is always connected with an 

 altered position of the organs of the chest, a situs inversus viscerum. 

 Of the other changes in the region of the arterial system the- 

 metamorphosis of the primitive aorta is to be mentioned before all 

 others. As in the other Vertebrates (fig. 127 ao\ so in Man, them 

 are formed a right and a left aorta; but they subsequently move 

 close together and fuse. This, again, explains an abnormality, which, 

 it is true, has very rarely been observed in Man. The aorta is 

 divided into right and left halves by means of a longitudinal 

 partition ; the process of fusion, therefore, has not been fully 

 effected. 



The aorta gives off at an early period as branches the unpaired 

 mesenterica sup. and mesenterica inf. to the intestinal canal ; 

 furthermore, near its posterior end, the two voluminous navel 

 vessels, arterise umbilicales (fig. 139 Al). These run from the dorsal 

 wall of the trunk along the sides of the pelvic cavity ventrally to- 

 that part of the allantois which is subsequently differentiated into- 

 urinary bladder and urachus, here bend upward and pass on either side 

 of the latter in the abdominal wall to the navel, enter the umbilical 

 cord, and are resolved in the placenta into a capillary network, from 

 which the blood is re-collected into the venae umbilicales. During 

 their passage through the pelvic cavity the umbilical arteries give 

 off lateral branches that are at first inconspicuous, the iliacse 

 internse, to the pelvic viscera, the iliacae externse to the posterior 

 limbs now sprouting forth from the trunk as small knobs. The 

 more the latter increase in size in older embryos, the larger do the 

 iliacse externse and their continuations, the femorales, become. 



After giving off the two umbilical arteries, the aorta becomes 

 smaller and is continued to the end of the vertebral column as an 

 inconspicuous vessel, the aorta caudalis or sacralis media. 



At birth an important alteration occurs in this part of the 

 arterial system also. With the detachment of the umbilical corcT r 

 the umbilical arteries can 110 longer receive blood; they therefore 

 waste away with the exception of the proximal portion, which has 

 given off as lateral branches the internal and external iliacs, and is 



