PLACENTA 



77 



chcricn. 



The changes which occur in the decidua basalis have been already (p. 69) 

 alluded to. It becomes invaded by masses of syncytium, which penetrate it 

 even to the muscular layer, and it gradually undergoes a process of degeneration 

 until it is reduced to a thin layer. This lamella may even be incomplete, for the villi 

 are sometimes found in direct contact with the muscular tissue. The final term of 

 the degeneration is the development of a fibrinous layer which serves to mark off 

 the foetal and maternal tissues. A similar degeneration affects the ends of the 

 villi, and in the later months this extends to the villi themselves, so that in many 

 parts the cellular elements have in great part disappeared, as will immediately 

 be noticed in the description of the full- 

 time placenta. 



The shed placenta. At full 

 time the placenta is a discoidal plate 

 measuring from 16 to 21 cm. in 

 diameter and 3 to 4 cm. in thickness, 

 but in shape and dimensions it is 

 subject to considerable variations. It 

 is thickest in the centre, and thins 

 away at the margin where it is con- 

 tinuous with the chorion and portions 

 of the decidua. 



The surface which has been de- 

 tached from the uterus shows a number 

 of irregular areas (cotyledons) sepa- 

 rated by shallow fissures. The detach- 

 ment generally takes place through 

 the remains of the decidua basalis, so 

 that a thin layer of decidual tissue 

 covers the ends of the villi. The foetal 

 surface is covered by the amnion, and 

 under it are seen the vessels radiating 

 outwards from the umbilical cord before 

 they dip into the substance of the 

 organ. The cord is usually attached 

 near the centre. It conveys two 

 arteries to the placenta. They branch 

 freely but irregularly, and extend out- 

 wards in the connective tissue of the 

 chorion to reach the villi in a series 

 of terrace-like steps, spreading out 

 horizontally and dipping in vertically 

 several times. They end. in the villi 

 in capillary loops (fig. 106), from which 

 the blood is gathered into veins which, closely following the arteries, finally unite 

 to form the single umbilical vein round which the arteries coil spirally. 



A section across the placenta (fig. 107) shows that the mass of the organ between 

 the amniotic and chorionic membranes on the foetal side, and the thin covering 

 of decidua on the uterine side, is made up of immense numbers of arborescent 

 villi, hanging free in a great space filled with maternal blood. Here and there 

 the root of a villus is seen springing from the chorion, while the larger stems 

 and finer branches are seen cut in every direction. A certain number of the 

 villi are attached to the decidua, but the greater number hang free into the inter- 

 villous space. Projecting into the placenta from the decidua there are certain 



FIG. 106. DIAGRAM OF THE PLACENTA. 

 (E. A. Schiifer.) 



s, placental sinus ; d.s. decidua basalis ; sp, 

 spongy layer ; m, muscularis ; a, v, uterine artery 

 and vein opening into placental sinus. 



