3 8o 



HISTOLOGY 



without branching, into the septa of the placenta, and before they empty 

 into the intervillous spaces, their walls are reduced to mere endothelium. 

 The veins which drain the intervillous spaces are not found in the septa, 

 except at the placental margin. They pursue an oblique course downward 

 from the floor of the cotyledons, beginning as large thin-walled tubes, into 

 which free ends of villi may project (Fig. 378). 



^Decidual cells. 



"_ .Connective tissue 



Cell knots. 



FIG. 381. FROM A SECTION OF THE HUMAN PLACENTA AT TERM. Xa6o. 



UMBILICAL CORD. 



The umbilical cord is a translucent, glistening, white or pearly rope 

 of tissue about 2 feet in length, extending from the umbilicus to the 

 placenta. It consists of mucous tissue (p. 62) covered with epithelium, 

 and contains at birth three large blood vessels, two umbilical arteries and 

 one umbilical vein (Fig. 382, B). The parallel arteries generally wind 

 around the vein making sometimes forty revolutions. The surface of the 

 cord shows corresponding spiral markings and often irregular protuberances 

 called false knots. (True knots, tied by the intrauterine movements of the 

 embryo, are very rare.) There are no lymphatic vessels or capillaries in 

 the cord, and the large blood vessels do not anastomose. The walls of the 

 arteries contain many muscle fibers but very little elastic tissue, and they 

 are usually found collapsed in sections; their contraction is of interest since 

 nerves have been traced into the cord for only a very short distance. The 

 vein generally remains open. 



