THE PLACENTA. (513 



substance of the umbilical cord. On reaching the placenta the 

 arteries branch freely, and very irregularly ; the branches spread 

 over the surface of the placenta, between the amnion and the 

 chorion, and dip rather suddenly into the substance of the 

 placenta. Within this they branch freely, and approach the 

 uterine surface of the placenta by a series of terrace-like steps, 

 spreading out horizontally, and then dipping suddenly towards 

 the surface, two or three times in succession ; ultimately they 

 enter the villi, and follow their branches to their finest rami- 

 fications. , 



The capillaries of the villi, into which the arteries finally 

 pass, are variable in diameter, and exhibit irregular dilatations 

 and constrictions ; their average size is, however, very large, as 

 they frequently admit from four to six red-blood corpuscles 

 abreast. 



The capillaries unite at their further ends to form veins, 

 which follow generally the same paths as the arteries, and finally 

 leave the placenta as the allantoic vein, which, running along 

 the umbilical cord, returns the blood from the placenta to the 

 foetus. 



The fcetal vessels thus form a closed set of blood-vessels, 

 which have proper walls of their own, of the usual structure, 

 along their entire course, and which show no special pecu- 

 liarities except the large size of the capillaries in the villi. 



The maternal blood-vessels are derived directly from the 

 uterine arteries and veins. The arteries, which are of com- 

 paratively small size, run with a very tortuous course, from 

 which their name ' curling arteries ' is derived, through both the 

 spongy and compact layers of the decidua, and open suddenly 

 into the great sinuses or loculi of the placenta. From these 

 sinuses the blood is returned by veins, which run very obliquely 

 through the decidua, and eventually join the veins of the muscular 

 wall of the uterus. 



The foetal villi are thus bathed by a slowly moving stream 

 of maternal blood ; and the necessary nutritive and respiratory 

 interchanges must be effected in this middle or villous zone of 

 the placenta. Nothing is known as to the exact mode in which 

 these are Carried on ; whether by simple diffusion, or whether 

 aided or modified by active participation of epithelial or other 

 cellular elements. The one thing that is quite clear is, that in the 



