DEVELOPMENT OF THE VASCULAR SYSTEM. 



797 



Fig. 301. 



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

 body assumes the arrangement which belongs to the latter periods of 

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

 arteries. These divide into the external iliacs supplying the lower ex- 

 tremities, and the internal iliacs supplying the pelvis ; and this division 

 is so placed that the umbilical or hypogastric arteries arise from the 

 internal iliacs, of which they now appear to be secondary branches. 



After the birth of the foetus and the separation of the placenta, the 

 hypogastric arteries become partially atrophied, and are converted, in 

 the adult condition, into solid cords, running upward to the umbilicus. 

 Their lower portion, however, remains pervious, 

 and gives off arteries supplying the urinary blad- 

 der. The terminal continuation of the original 

 abdominal aorta, is the arteria sacra media, which, 

 in the adult, runs downward on the anterior sur- 

 face of the sacrum, supplying branches to the 

 rectum and to the anterior sacral nerves. 



Development of the, Venous System. According 

 to the observations of Coste, the principal veins of 

 the body consist at first of two long venous trunks, 

 the vertebral veins (Fig. 301), which run 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 of equal size, the canals of Cu- 

 vier. When the inferior extremities become de- 

 veloped, their two veins, returning from below, 

 join the vertebral veins near the posterior portion 

 of the body ; and, crossing them, afterward unite 

 with each other, thus constituting another vein of 

 new formation (Fig. 302, 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, by means of which the veins of the lower extremi- 

 ties thus unite, become afterward the common iliac veins ; while the 

 single trunk (a) resulting from their union becomes the vena cava in- 

 ferior. Subsequently, the vena cava inferior becomes very much larger 

 than the vertebral veins ; and its two branches of bifurcation are after- 

 ward represented by the iliac veins. 



Above the level of the heart, the vertebral and intercostal veins re- 

 tain their relative size until the development of the superior extremities 

 has commenced. Then, two of the intercostal veins increase in diameter 

 ( Fig. 302), and become converted into the right and left subclavians ; 

 while those portions of the vertebral veins situated above the subcla- 

 vians become the right and left jugular veins. Just below the junction 

 of the jugulars with the subclavians, a small branch of communication 

 now appears between the two vertebrals (Fig. 302, 6), passing over from 



Diagram of the VE- 

 NOUS SYSTEM in its 

 early condition ; show- 

 ing the vertebral veins 

 emptying into the heart 

 by two lateral trunks, 

 the " canals of Ouvier." 



