692 REPRODUCTION. 



Corresponding changes take place, during this time, in the lower part 

 of the body. Here the abdominal aorta runs undivided, on the median 

 line, to the end of the spinal column ; giving off lateral branches to the 

 intestine and the abdominal walls. When the allantois is developed, 

 two of these branches accompany it, and become, consequently, the 

 umbilical arteries. These vessels increase so rapidly in size, that they 

 soon appear as main divisions of the aorta ; while its original continua- 

 tion, running to the end of the spinal column, appears as a small branch 

 given off at the point of bifurcation. The lower limbs are supplied by 

 two small branches from the umbilical arteries near their origin. 



Up to this time, the pelvis and lower limbs are but slightly developed. 

 Subsequently they grow more rapidly, in proportion to the rest of the 

 body, and their arteries enlarge to a corresponding degree. That por- 

 tion of the umbilical arteries lying between the bifurcation of the aorta 

 and the branches going to the lower limbs, becomes the common iliac 

 arteries, which afterward divide into the umbilical arteries proper and 

 the femorals. Subsequently, in accordance with the growth of the 

 pelvis and lower limbs, the relative size of their blood-vessels is still 

 further increased ; and at last the arterial system in this part of the 

 body assumes the arrangement belonging to the latter periods of ges- 

 tation. The aorta divides, as before, into the two common iliac arteries. 

 These divide into the external iliacs, supplying the lower limbs, and the 

 internal iliacs, supplying the pelvis ; and this division is so placed that 

 the umbilical arteries arise from the internal iliacs, of which they now 

 appear to be secondary branches. 



After birth, the umbilical arteries become for the most part atro- 

 phied, and are converted, in the adult condition, into solid cords, run- 

 ning upward to the umbilicus. Their lower portions, however, remain 

 pervious, under the name of the " hypogastric arteries," and give off 

 branches supplying the urinary bladder. The terminal continuation 

 of the original abdominal aorta is the arteria sacra media, which, in 

 the adult, runs downward on the anterior surface of the sacrum, sup- 

 plying the rectum and the anterior sacral nerves. 



Development of the Venous System. According to Coste, the prin- 

 cipal veins of the body consist at first of two long venous trunks, the 

 vertebral veins (Fig. 238), running along the sides of the vertebral 

 column, parallel with the vertebral arteries. They receive in succession 

 all the intercostal veins, and empty into the heart by two lateral trunks, 

 the canals of Guvier. When the lower limbs become developed, their 

 two veins join the vertebral veins in the posterior portion of the body; 

 and, crossing them, afterward unite with each other, constituting a 

 third vein of new formation (Fig. 239, a), which runs upward a little 

 to the right of the median line, and empties by itself into the lower 

 extremity of the heart. 



The two branches which thus unite become afterward the common 

 iliac veins ; and the trunk resulting from their union is the vena cava 

 inferior. Subsequently, the vena cava inferior becomes very much 



