PLACENTA (NORMAL ANATOMY). 



717 



Partly within its substance is formed an in- 

 complete sinus, the circular vein or sinus. 

 This constitutes an interrupted channel, which 

 more or less encircles the placenta. Several 

 orifices are observed in its walls. Some com- 

 municate directly with the interior of the pla- 

 centa, and others with the uterine sinuses. 



Substance. When a clean section has been 

 made through the placenta (fig. 4-84.), the two 

 surfaces already described are observed to 

 enclose between them a soft spongy sub- 

 stance, which is made up principally of count- 

 less ramifications of the fcetal villi. These 

 are attached at their base to the chorion, from 



Fig. 484. 

 J 



Vertical section of the walls of the uterus ivith (lie placenta attached. From a woman in the thirtieth week of 



gestation. (After Wagner.) 



The length of the lines uu, serves to distinguish the uterus; p, the placenta, and dd, the decidua. To 

 ihe right of the figure the decidua is separated into ud, uterine decidua, and dp, decidual prolongations 

 which form the dissepiments dividing the placenta into lobes ;/, funis ; am, amnion ; ch, chorion; vf, 

 fo?tal blood-vessels (divided) upon the surface of the placenta; vv, villi; us, uterine sinuses; a a, curling 

 arteries in the substance of the uterus. 



which they spring, while their opposite extre- 

 mities are united to the deridual layer form- 

 ing the uterine boundary of the placenta. 

 The interspaces left between the villi and 

 their ramifications form what have been 

 termed the cells of the placenta. They are 

 widest between the roots of the villi, and 

 much smaller between their extremities. In 

 these spaces the maternal blood circulates. 

 When injections are thrown into the placenta 

 from the uterine arteries or veins, these spaces 

 become filled, and the mass, when broken, 

 exhibits a peculiar granular appearance. Dip- 

 ping down among the vilii, and reaching in 

 some cases as far as the fcetal surface of the 

 placenta, are numerous sheet-like prolonga- 

 tions of decidua (fg. 484. dp). These con- 

 stitute the dissepiments which separate the 

 entire mass into its several lobes or cotyle- 

 dons. At the placental margin, the decidual 

 layer generally dips under the villi, forming a 

 return end or border, which is directed in- 

 wards, and is attached at a distance of 3 4'" 

 from the margin to the outer surface of the 

 chorion. The exact relation of the decidua 

 to the villi, in various parts of the placenta, 

 will be better understood after a more minute 



description has been given of each of these 

 structures. 



The tufts and villi. A placental tuft 

 has been often compared to a tree. It con- 

 sists of a trunk giving off numerous branches, 

 which ultimately end in finer subdivisions 

 or villi (fig. 484. vv and fig. 485. a). The 

 trunks may be said to take root in the cho- 

 rion, from which they spring, while the 

 branches and finer subdivisions spread la- 

 terally and upwards, until they come into 

 contact, at their sides, with the adjacent tufts 

 and villi, and above with the decidua which 

 bounds the placenta towards the uterus. 

 Many of the villi, instead of branching like 

 trees, proceed thread-like from the floor to 

 the roof of the placenta, only sending off 

 short knotty side branches. The tufts are 

 so closely set, that their forms cannot be 

 readily discerned until they are floated out 

 in water. The stems are tough and fibrous, 

 or coriaceous, while the branches and finer 

 villi, though strong, are of a more brittle 

 texture. When one of these is broken off, 

 and examined by the microscope, it presents 

 the following characteristics the subdivi- 

 sions are abrupt, contorted, and singularly 



