PARTURITION. 845 



centa and chorion ; in others there is a single chorion and a double amnion ; 

 but in some both foetuses are enclosed in the same amnion. As a rule 

 the two placentae are distinct; but sometimes there is a vascular com- 

 munication between them, or what appears to be a single placenta may 

 give origin to two umbilical cords. If there be but a single chorion and 

 amnion and a single placenta, it has been thought* that the two beings are 

 developed from a single ovum ; otherwise it would be necessary to assume 

 that there were originally two sets of membranes, which had become fused 

 into one. The instances on record of twins, one white and the other black, 

 show conclusively that two ova may be developed in the uterus at the same 

 time. While there can be no doubt upon this point, the question of the 

 possibility of the development of two beings from a single ovum remains un- 

 settled. 



As pathological conditions, extrauterine pregnancies occur, in which the 

 fecundated ovum, forming its attachments in the Fallopian tube (Fallopian 

 pregnancy) or within the abdominal cavity (abdominal pregnancy), under- 

 goes a certain degree of development. The uterus usually enlarges in these 

 instances and forms an imperfect' decidua. 



Cause of the First Contractions of the Uterus in Normal Parturition. 

 The cause of the first contraction of the uterus in normal parturition is un- 

 doubtedly referable to some change in the attachment of its contents, which 

 causes the foetus and its membranes to act as a foreign body. When for any 

 reason it is advisable to cause the uterus to expel its contents before the full 

 term of pregnancy, the most physiological method of bringing on the con- 

 tractions of this organ is to cautiously separate a portion of the membranes, 

 as is often done by introducing an elastic catheter between the ovum and the 

 uterine wall. A certain time after this operation, the uterus contracts to 

 expel the ovum, which then acts as a foreign body. 



In the normal state, toward the end of pregnancy, the cells of the decidua 

 vera and of that portion of the placenta which is attached to the uterus 

 undergo fatty degeneration, and in this way there is a gradual separation of 

 the outer membrane, so that the contents of the uterus gradually lose their 

 anatomical connection with the mother. When this change has progressed 

 to a certain extent, the uterus begins to contract ; each contraction then 

 separates the membranes more and more, the most dependent part pressing 

 upon the os internum; and the subsequent contractions are due to reflex 

 action. The first " pain " is induced by the presence of the foetus and its 

 membranes as a foreign body, a mechanism similar to that which obtains 

 when premature labor has been brought on by separation of the membranes. 



According to Korner, there exists in the spinal cord, at the site of the 

 first and second lumbar vertebrae, a reflex centre for parturition. This, like 

 other centres in the cord, is subordinate to a centre which is situated in the 

 medulla oblongata. 



The mechanism of parturition, although this is entirely a physiological 

 process, is considered elaborately in works upon obstetrics. The first con- 

 tractions of the uterus, by pressing the bag of waters against the os internum, 

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