854 EMBRYOLOGY 



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 attached to the uterus 

 undergo fatty degeneration, and in this way there is a progressive separa- 

 tion of the outer membrane, so that the contents of the uterus gradually 

 lose their anatomical connection with the mother. When this change 

 has advanced to a certain extent, the uterus begins to contract ; each con- 

 traction then separates the membranes more and more, the most depend- 

 ent part pressing on 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 situated in 

 the bulb. 



The mechanism of parturition, although this is entirely a physiologi- 

 cal process, is considered elaborately in works on obstetrics. The first 

 contractions of the uterus, by pressing the bag of waters against the os 

 internum, gradually dilate the cervix ; the membranes usually rupture 

 when the os is pretty fully dilated, and the amniotic fluid is discharged ; 

 the head then presses on the outlet; and the uterine contractions be- 

 coming more and more vigorous and efficient, the child is brought into 

 the world, this being followed by the expulsion of the membranes and 

 placenta. There then follows a tonic contraction of the muscular walls 

 of the uterus, which becomes a hard globular mass, easily felt through 

 the flaccid abdominal walls. The very contractions of the muscular 

 fibres of the uterus which expel the foetus close the vessels ruptured by 

 the separation of the placenta and arrest haemorrhage from the mother. 

 The changes which then take place in the respiration and circulation of 

 the infant have been considered in connection with the development of 

 the circulatory system. 



Involution of the Uterus. At four to six days, and seldom later than 

 eight days after parturition, the uterus has sensibly advanced in the 

 process of involution ; and it is then gradually reduced to the size and 

 structure which it presents during the non-pregnant condition, though 

 it never becomes quite so small as in the virgin state. The new mucous 

 membrane, which has been developing during the latest periods of preg- 

 nancy, becomes perfect at about the end of the second month after 



