THE AETEEIES. 



577 



the lumbar region is more than double that in the anterior 

 thoracic region. At the hinder end of the lumbar region the 

 aorta divides into the right and left allantoic arteries, which run 

 along the allantoic stalk to the placenta, and which, at any rate 

 in the early stages, appear as direct continuations of the aorta 

 rather than as branches of it. 



The proximal ends, or roots, of the allantoic arteries persist 

 throughout life as the common iliac arteries, from which the 

 external iliac arteries arise as branches, on the formation of the 



Fig. 246. — The aortic arches of a Human Embryo thirty-five days old, from 

 the left side. (From His.) x 30. 



A.4, fourth, or systemic aortic arch. A.5, fifth, or pulmonary aortic arch. A]B» 

 common carotid artery, at its point of division into internal and external carotid arteries. 

 AI, internal carotid artery. AP, pulmonary artery. AS, suhclavian artery. AV, 

 vertebral artery. CH, notochord, LR, trachea. Rw, pulmonary trunk. BX» 

 systemic trunk. TH, thyroid body. TW", tongue. TO, oesophagus. 



hind limbs. The hypogastric arteries are the abdominal, or 

 intra-fcetal, portions of the allantoic arteries, beyond the origin of 

 the internal iliac arteries ; their cavities become obliterated after 

 birth, but their walls persist as solid cords, crossing the sides of 

 the bladder obliquely, and running forwards and upwards to the 

 umbilicus. 



The vertebral arteries appear, about the twenty-fourth day, 

 as a pair of longitudinal trunks running along the sides of the 

 brain, and extending from the level of the ears to the commence- 



PP 



