THE HUMAN PLACENTA. 337 



demonstrate only the coarser or all the branches of the vessels. The injection 

 should be made through one of the arteries of the umbilical cord. As there is 

 almost invariably a cross-anastomosis between the two arteries close to the 

 placenta, it is sufficient to inject one of them in order to fill the entire system of 

 vessels. The starch mass may be injected in the cold specimen. If the gelatin 

 mass is used, the specimen must be submerged in warm water until it is suffi- 

 ciently heated to keep the gelatin mass melted during the process of injection. 

 After the gelatin injection is completed, the placenta may be preserved in 70 per 

 cent, alcohol, to every 100 c.c. of which 2 c.c. of hydrochloric acid have been 

 added. After twenty-four hours the acidulated alcohol may be replaced by 

 fresh alcohol of 70 per cent., which should be again changed after another twenty- 

 four hours. Specimens will then keep indefinitely. Such specimens may be 

 used either for sections of the placenta to be made from pieces imbedded in cel- 

 loidin, or for the study of isolated fragments of the villi, which are pulled out of 

 the placenta by forceps. 



The human placenta is a disc of tissue to which the umbilical cord of the 

 child is attached by its distal end. As a result of normal labor the amnion 

 and chorion, by which the foetus in utero is surrounded, are ruptured; the child 

 is then expelled, but by means of the long umbilical cord remains attached to the 

 uterus ; after an interval the placenta, with which the cord retains its connection, 

 is loosened from the uterine wall and expelled, together with the foetal envelopes 

 and portions of the decidual membranes (uterine mucosa) of the mother; the 

 parts thus thrown off secondarily constitute the so-called, after-birth of obstetri- 

 cians. 



The placenta at full term, as thus obtained by natural expulsion, is a moist 

 mass, containing a great deal of blood, spongy in texture, about seven inches in 

 diameter, but very variable in size, being roughly proportionate to the bulk of 

 the child ; usually oval, sometimes round, but not infrequently irregular in shape. 

 One surface is smooth and covered by a pellucid membrane (the amnion), and 

 reddish-gray in color; to this surface the umbilical cord is attached, and it shows 

 the arteries and veins branching out irregularly from the cord over the surface of 

 the placenta (Fig. 196). The opposite surface is rough, lacerated, and usually 

 covered irregularly with more or less blood, which is often dark and clotted. 

 When the blood is removed, the surface is seen to be crossed by a system of 

 grooves which divide the placental tissue into irregular areas, each perhaps an 

 inch or so in diameter; these areas are called cotyledons. The placenta is about 

 25 or 30 mm. thick, but thins out rapidly at the edges, and its tissue passes over 

 from the margin of the placenta. 



When in situ, the placenta is fastened to the walls of the uterus by its rough 



