TEE FCETUS. 



1025 



6. The Umbilical Coed (Figs. 556, 557). 



The cord is formed by the vessels which, in the foetus, caiTj the blood to the 

 envelopes, and chiefly to the placenta. It is divided into two portions — an 

 amniotic, the longest, which is always twisted on itself like a cord, and covered 

 externally by the amnion that is prolonged on its surface, to be continued with 

 the skin around the umbilicus ; the other — the allantoid portion (Fig. 557, B) — 

 much shorter and less twisted, is enveloped by 

 the sheath that continues the two layers of the 

 allantois, and is inserted into the superior wall 

 of the chorion, between the two cornua.^ 



Three vessels compose the cord — two arteries 

 and a vein; these are covered by a layer of 

 embryonic tissue — the gelatine of Wharton — 

 which makes them appear much larger than 

 they really are. 



The Umbilical Arteeies ^ arise from the 

 internal iliac, and pass along the sides of the 

 bladder ; escaping by the umbilicus, they arrive 

 at the terminal extremity of the amniotic portion 

 of the cord, and giving off some branches to 

 the amnion, they are continued to the extremity 



of the allantoid portion, where they end in an 

 expansion of placental ramifications. These 



arteries run parallel to the median plane of 



the body of the foetus, to the umbilicus ; they 



are perpendicular at the infundibulum, which 



proves that the allantois undergoes a twist in 



the early period of development. The amniotic 



divisions of these arteries are few, and extremely 



flexuous ; they are included between the allantoid 



layer and the amniotic membrane, within which 



they may be seen projecting. 



The placental or cliorial divisions — infinitely 



larger and more numerous — leave the end of 



the cord, and pass in every direction between the chorion and external lamina 



of the allantois, beneath which they are very prominent. By their anastomoses 



they form a very rich network, from which proceed the capillary twigs that 



enter the villosities of the placenta. Observation demonstrates that these twigs 



do not communicate with the maternal vessels, and that they are continued by 



venous radicles, the origin of the vessel now to be described. 



The Umbilical Vein commences by these capillary radicles of the placental 



villi, which unite between the chorion and amnion to form a network of more 



voluminous divisions and complexity than that of the arteries. Two principal 



PORTION OF ONE OF THE FOETAlj 

 VILLI, ABOUT TO FORM PART OP 

 THE PLACENTA (HIGHLY MAGNI- 

 FIED). 



a, a, Its cell covering; 6, 6, 6, its 

 looped vessels; c, c, its basis of con- 

 nective tissue. 



' Goubaux has remarked that the relations between these two portions are not constant. 

 In a six months' foetus, the amniotic part measured -40 m., and the allantoid part -24 m. In a 

 foetus of ten months, the first was -30 m. long, and the second -20 m. 



2 The -walls of the umbilical artery are very rich in muscular fibres, which are longitudinal 

 and transversal, but are irregularly distributed. There is no internal elastic tunic in any of tha 

 branches external to the umbilicus. 



