206 UMBILICAL VESSELS. 



The umbilical vein, in its progress through the fissure of the 

 liver, before it arrives at the sinus of the vena portarum, sends 

 off a considerable number of branches to each of the Ipbes of 

 that organ, but more to the left than to the right lobe. 



After birth, when \>lood ceases to flow through the umbilical 

 vein, it is gradually converted into a ligament ; and the venous 

 duct is also converted into a ligament in the same manner. 

 The vena portarum, which before appeared very small, when 

 compared with the umbilical vein, now brings all the blood 

 which fills its great sinus, and increases considerably in size. 



It has been ascertained by anatomical investigation, that the 

 umbilical arteries above mentioned, after ramifying minutely 

 in the placenta, communicate with the minute branches of the 

 umbilical vein ; and it is probable that the whole blood carried 

 to the placenta by these arteries, returns by the umbilical vein 

 to the foetus. 



It is clearly proved by the effects of pressure on the umbilical 

 cord, in cases of delivery by the feet, as well as by other similar 

 circumstances, that this circulation cannot be suspended for any 

 length of time without destroying the life of* the foetus. From 

 these circumstances, and from the florid color which the blood 

 acquires by circulating in the placenta, it seems probable that the 

 object of the circulation through that organ is somewhat analo- 

 gous to the object of the pulmonary circulation through the lungs 

 of adults.* 



* During the first four months of pregnancy a very small vesicle, which does 

 not exceed the size of a pea, is found between the chorion and the amnios, near 

 the insertion of the umbilical cord into the placenta. It is connected to the 

 fcctus by an artery and a vein, which pass from the abdomen through the 

 umbilicus, and proceeding along the cord to the plancenta, continue from it to 

 the vesicle. The artery arises from the mesenteric, and the vein is united to the 

 mesenteric branch of the vena portarum. It is probable that these vessels com- 

 monly exist no longer than the vesicle, namely, about four months ; but they 

 have been seen by Haller and Chaussier at the termination of pregnancy. They 

 are called Omphalo- Mesenteric vessels. The vesicle is denominated the 

 Umbilical Vesicle. 



This intricate structure is delineated in Hunter's Anatomy of the Gravid 

 Uterus, plate xxxiii. figures v. and vi. ; in the Academical Annotations of Albi- 

 nus, first book, plate i. figure xii. ; and also in the Icones Embryonum Humano- 

 rum of Soemmering, figure ii. 



