THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



267 



open into the fibular in the caudal border; the other, the so-called connecting 

 branch, begins as twigs in the abdominal wall and tibial side of the extremity and 

 opens into the fibular just proximal to the opening of the anterior tibial (Fig. 242). 

 Later the distal part of the primitive fibular is broken up by the differentiation 

 of the digits (toes) and disappears almost up to the point of junction with the 

 anterior tibial. The latter enlarges and receives the digital branches, and 

 appears as a continuation of the proximal part of the primitive fibular. The 

 anterior tibial and primitive fibular together thus constitute the sciatic vein 

 (Fig. 243). Another vessel appears in embryos of fifteen days, which represents 

 the beginning of the femoral vein and opens into the cardinal, cranial to the 



Ext.j UJU Ur nif-^jLiftft yel \ 



veirj 



FlG. 242. 



FIG. 243. 



FIG. 242. Diagram of the veins in the extremities of a rabbit embryo of 14 days 



and 18 hours (14.5 mm.). Modified from Lewis. 



FIG. 243. Diagram of the veins in the extremities of a rabbit embryo of 17 days 

 (21 mm.). Modified from Lewis. 



opening of the sciatic (Fig. 243). From this time on the femoral, with its 

 branches, enlarges at the expense of the other veins and becomes the principal 

 vein of the lower extremity. In the human embryo the femoral anastomoses 

 with the sciatic near the knee and the proximal portion of the sciatic then 

 atrophies, the distal 'portion persisting as the small saphenous vein. The large 

 saphenous vein and the posterior tibial vein possibly are derivatives of the 

 femoral, but this question has not been settled. 



CHANGES IN THE CIRCULATION AT BIRTH. During fcetal life the course of 

 the blood is adapted to the placental circulation, since the placenta is the only 

 means by which the blood is purified and from which the foetus derives its 

 nutriment. The pure blood from the placenta passes through the umbilical 

 vein to the liver; there a part of it is distributed to the liver by some of the 



