812 GENERATION. 



month. The placenta then rapidly assumes the anatomical characters ob- 

 served after it may be said to be fully developed. 



The fully formed placenta occupies about one-third of the uterine mu- 

 cous membrane, and generally is rounded or ovoid in form, with a distinct 

 border connected with the decidua and the chorion. It is seven to nine 

 inches (18 to 23 centimetres) in diameter, a little more than an inch (2-5 

 centimetres) in thickness at the point of penetration of the umbilical cord, 

 slightly attenuated toward the border, and weighs fifteen to thirty ounces 

 (425 to 850 grammes). Its foetal surface is covered with the smooth, amni- 

 otic membrane, and its uterine surface, when detached, is rough, and 

 divided into irregular lobes, or cotyledons, half an inch to an inch and a 

 half (12'7 to 38-1 mm.) in diameter. Between these lobes, are membranes, 

 called dissepiments, which penetrate into the substance of the placenta, and 

 at its border extend as far as the foetal surface. 



Upon the uterine surface of the placenta, is a thin, soft membrane, the 

 decidua serotina. This is composed of amorphous matter, a large number of 

 granulations, and colossal cells with enlarged and multiple nuclei. A portion 

 of this membrane is not thrown off with the placenta in parturition, but pro- 

 cesses extend into the placenta and closely surround the foetal tufts. 



The two arteries of the umbilical cord branch upon the foetal surface of 

 the placenta, beneath the amnion, and finally penetrate the substance of the 

 organ. The branches of the veins, which are about sixteen in number, con- 

 verge toward the cord and unite to form the umbilical vein. Upon the 

 uterine surface of the placenta are oblique openings of a large number of 

 veins which return the maternal blood to the uterine sinuses. There are also 

 the small, spiral arteries, which pass into the substance of the organ, to supply 

 blood to the maternal portion. These are the " curling arteries," described 

 by John Hunter. If the umbilical arteries be injected, the fluid is returned 

 by the umbilical vein, having passed through the vascular tufts of the foetal 

 portion of the placenta. 



According to Winkler, there are three kinds of foetal villi : 1. Those 

 which terminate just beneath the chorion, without penetrating the vascular 

 lacunae. 2. Longer villi, which hang free in the lacunae. 3. Long, branch- 

 ing villi, which penetrate more deeply into the placenta, some extending as 

 far as its uterine surface. 



The great vascular spaces, or lacunae of the maternal portion of the pla- 

 centa, present a number of trabeculae, which extend from the uterine to the 

 foetal surface ; and between these trabeculae, are exceedingly delicate trans- 

 verse and oblique secondary trabecular processes. The blood-vessels of the 

 foetal tufts are surrounded with a gelatinous, connective-tissue structure, and 

 as late as the sixth month (Heinz) are covered with a layer of chor ionic cells. 



The mode of formation of the vascular spaces in the placenta has been a 

 subject of much discussion. The following, however, seems to be the most 

 reasonable view with regard to this question : That portion of the uterine 

 mucous membrane which becomes the maternal portion of the placenta ex- 

 tends from the decidua serotina and surrounds the villi, which are embedded 



