276 CIRCULATORY SYSTEM. 



other veins, it is furnished with valves, of which there are two: 

 one at its termination in the sinus portarum, and the other at 

 the cardiac extremity of the ductus venosus.* The establish- 

 ment of respiration, by putting the circulation into other chan- 

 nels, likewise causes its obliteration and final conversion into a 

 ligamentous chord. The valve, at the sinus portarum, pre- 

 vents the blood from taking a retrograde course, and thereby 

 keeping the umbilical vein open; the valve of the ductus veno- 

 sus has the same effect upon the duct to which it belongs, and 

 is aided by the current of blood in the left branch of the sinus 

 portarum, setting across the mouth of the ductus venosus in- 

 stead of plunging into it from the umbilical vein, as in fcetal 

 life. 



It is worthy of remark, that the left branch of the sinus por- 

 tarum is bounded, on its right extremity, by the end of the 

 vena portarum, and receives, about its middle, the umbilical 

 vein. In the space, then, between the umbilical vein and the 

 portal, the circulation, from the predominance of umbilical 

 blood in foetal life, is conducted from left to right, but afterwards 

 from right to left, as the portal circulation is established and 

 the Other is arrested. 



The Umbilical Arteries discharge the important office of 

 conducting the effete blood of the foetus to the placenta. They 

 are the continuations of the internal iliacs, and are two in num- 

 ber, one on either side; they conduct off so much of the blood 

 of the primitive iliacs, as to leave the external iliacs of a very 

 small size. During the early months of uterine life, they are 

 rather, indeed, the continued trunks of the primitive iliacs, the 

 branches from the latter being then so little developed as to ap- 

 pear quite subordinate to the chief function of carrying the 

 blood out of the foetus to the placenta. But as the inferior ex- 

 tremities and the buttocks grow, these subordinate branches 

 are more and more evolved. 



At birth, the umbilical arteries, after dipping very superficially 

 into the pelvis, rise up at the sides of the bladder and converge 

 towards the navel. They emerge at the latter, cling together 

 and traverse the umbilical chord by twisting spirally around the 



* Bichat, Anat. Descrip. vol. v. p. 419. 



