THE DEVELOPMENT OF THE VASCULAR SYSTEM. 253 



time on, the femoral and its direct continuation, the popliteal, increase in size; 

 and at the same time the sciatic loses its primary connection and becomes much 

 reduced to form the inferior gluteal artery. The direct continuation of the 

 sciatic in the leg, which is now the direct continuation of the popliteal, becomes 

 reduced to form the peroneal artery. The branch of the original sciatic, which 

 was given off just below the knee, unites with the branch which was given off 

 just above the ankle to form a continuous vessel, the anterior tibial artery. A 

 new branch arises from the proximal portion of the peroneal, extends down the 

 back of the leg, anoj unites with the distal part of the embryonic saphenous to 

 form the posterior tibial artery. The proximal part of the saphenous then 

 atrophies, leaving but one of the small genu branches of the popliteal (Fig. 228) 

 (McMurrich). 



The Veins. The changes which occur during the development of the 

 venous system are so complicated, and in some cases so varied, that the scope of 

 this book permits only a brief outline of the growth of the more important of the 

 venous trunks. 



Corresponding to the arterial system, the first veins to appear are the 

 omphalomesenteric veins. These vessels, which carry blood from the yolk sac 

 to the heart, arise in the area vasculosa, enter the embryonic body at the sides of 

 the yolk stalk, pass cranially along the intestinal tract, and join the caudal end 

 of the heart (Figs. 213, 215, 216, and 231). Next in point of time to appear 

 are the umbilical veins which carry back to the heart the blood which has been 

 carried to the placenta by the umbilical arteries. These also are paired veins 

 within the embryo, although they form a single trunk in the umbilical cord. 

 They extend cranially on each side through the ventro-lateral part of the body 

 wall and join the duct of Cuvier (see below) in the septum transversum (Figs. 

 217, 216 and 231). Very soon after the appearance of the umbilical veins 

 two other longitudinal vessels develop, one on each side of the aorta. In 

 the cervical region they lie dorsal to the branchial arches and are called the 

 anterior cardinal veins (Figs. 215 and 231). The more caudal parts of the 

 vessels are situated in the region of the developing mesonephros and are called 

 the posterior cardinal veins (Figs. 215 and 231). At a point about opposite the 

 heart the anterior and posteiior cardinals on each side unite to foim a single 

 vessel, the duct of Cuvier, which turn? medially through the septum transversum 

 and opens into the sinus venosus (Figs. 215 and 216). Thus three primary 

 sets of veins are formed at a very early stage of development: (i) The omphal- 

 omesenteric veins; (2) the umbilical veins; (3) the cardinal veins. 



The veins of the head and neck regions are derivatives of the anterior cardinals. 

 The proximal parts of these vessels are present in embryos of 3.2 mm.; later they 

 extend cranially along the ventro-lateral surface of the brain on the medial side of 

 the roots of the cranial nerves. The position relative to the nerves is only tern- 



