482 PROFESSOR TURNER ON THE GRAVID UTERUS AND 



the villi. When these structures were in rows, or scattered, the terminal 

 branches which entered the villi arose independently ; but, when the villi were 

 in clusters, several terminal twigs arose close together as from an axis. The 

 arteries did not, like the branches from which they arose, lie parallel to the 

 surface of the chorion, but immediately after their origin entered the villi at 

 the base of attachment, and in the case of a compound villus divided, within 

 its substance, into twigs for the secondary villi. These twigs ascended towards 

 the tip of the villus, and ended in a very compact capillary jjlexus, situated 

 beneath the free surface of the villus, and in close relation to the sub- epithelial 

 layer of spheroidal cells already described. This plexus may conveniently be 

 termed the intra- villous capillary network (fig. 14, a). From the intra- villous 

 plexus capillaries proceeded, which passed out of the villus at its base, and at 

 once joined a capillary network situated immediately beneath the general plane of 

 the chorion. This plexus may be termed, from its position, the sub-chorionic or 

 extra- villous capillary network (fig. 14, b). The extra- villous plexus was much 

 less compact than that within the villi ; its meshes were, as a rule, elongated, 

 but occasionally more irregular in form. Numerous minute veins arose directly 

 from it, which joined together to form the rootlets of the umbilical vein. 



Occasionally I saw a vessel, which was somewhat larger than the capillaries 

 in its immediate neighbourhood, and might therefore be regarded as a direct 

 rootlet of an umbilical vein arising within, and leaving a villus at its base ; 

 but this was quite exceptional, for repeated examinations have satisfied me that 

 the arrangement which most commonly prevailed was for the capillaries of the 

 intra- villous plexus to be directly continuous with those of the sub-chorionic net- 

 work, and for the latter to give origin to an umbilical vein. Hence the blood 

 in its passage from the terminal twigs of the umbilical artery to the rootlets of 

 the umbilical vein has to flow through a complicated capillary system, one 

 subdivision of which lies within the villi, the other beneath the chorionic 

 membrane which connects them at their bases. When the chorion lay in its 

 proper position within the uterus, the first subdivision would have been in 

 relation to the maternal system of capillaries lining the walls of the crypts, the 

 other to the capillaries situated immediately beneath the free surface of those 

 parts of the mucous membrane which separated the trenches or pits into 

 which the crypts opened from each other. This greatly diffused capillary area 

 has undoubtedly a special relation to the comparatively simple mode of union 

 between the maternal and foetal surfaces in the diffused form of placenta. 



The three bare spots on the chorion were almost entirely destitute of blood- 

 vessels, and, after the chorion was injected, contrasted strongly with the sur- 

 rounding highly vascular villous portion.* 



* It may not be out of place to state that some portions of the chorion were injected with a blue- 

 coloured gelatine from branches of the umbilical artery only, "when the intia villous plexus was readily 



