512 URO-GENITAL SYSTEM. 



these will form at a later period, following a posterior direc- 

 tion, the two iliac arteries. Numerous arterial branches, 

 the most remarkable of which on account of size are those 

 which go to the intestine and umbilical vesicle (Fig. 146, 5), 

 are sent off from the posterior vertebral arteries, and distrib- 

 ute blood to the tissues of the embryo ; these two omp/ialo- 

 mesenteric arteries are most essential to the primary circula- 

 tion (146, 6). The blood goes through them to the walls of 

 the umbilical vesicle, and percolating there in a rich network, 

 which occupies only a portion of this vesicle (area vasculosa, 

 Fig. 146), it is charged with the nutritive elements of the yolk, 

 and is afterwards launched into a sinus, which occupies the 

 outside of the area vasculosa (terminal sinus, sinus venosus, 

 Fig. 146, 1) ; the blood then returns through two veins 

 (omphalo-mesenteric) to the posterior extremity of the car- 

 diac cylinder (Fig. 146, 2, 3). This primary circulation lasts 

 but a short time in the human embryo, as the functions of 

 the umbilical vesicle soon cease and the vesicle undergoes 

 atrophy (see p. 502) ; so also at this period the corresponding 

 portion of the omphalo-mesenteric vessels undergo the same 

 fate, the arteries being reduced to one mesenteric artery, the 

 veins to one mesenteric vein, and thus form the future vena 

 portce. 



2. The remainder of the primary circulation, thus modified, 

 and with the addition of new vessels, then forms the second- 

 ary o\' place ntal circulation. We will consider the formation 

 of the organs of this new system, by commencing with the 

 placenta, and the course of the blood towards the heart by 

 the venous system, and its return from the heart of the foetus 

 to the placenta through the arterial system. 



a. Placental Venous System. The blood that become 

 charged in the placenta with constructive elements received 

 from the blood of the mother (see p. 506) goes to the body 

 of the loetus by two veins which are developed on the pedi- 

 cle of the allantois ; these veins pass into the embryo with 

 the umbilicus, whence their name of umbilical veins. One 

 of these vessels immediately becomes atrophied, and only 

 one umbilical vein then remains, which unites with the 

 posterior extremity of the heart to form the central end of 

 the mesenteric vein ; so that this central end which was 

 primarily the omphalo-mesenteric (omphalo-mesaraic) trunk, 

 afterwards the trunk of the mesenteric vein, now repre- 

 sents the common trunk of the umbilical and mesenteric 

 veins (Fig. 149, 1 ; yet the transformations do not rest here. 



