INVOLUTION OF THE UTERUS. 453 



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 probably 

 due to reflex action. The first "pain" is induced by the 

 presence of the foetus and its membranes as a foreign body, 1 

 a mechanism similar to that which obtains when premature 

 labor has been brought on by separation of the membranes. 



We shall not describe the mechanism of parturition, al- 

 though this is entirely a physiological process, for the reason 

 that it is necessarily considered elaborately in works on ob- 

 stetrics. 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 upon the outlet ; and, the uterine contractions 

 becoming 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 con- 

 traction of the muscular walls of the uterus, which becomes 

 a hard, globular mass, easily felt through the flaccid abdom- 

 inal 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 the haemorrhage 

 from the mother. The changes which then take place in the 

 respiration and the circulation of the infant have been fully 

 considered in connection with the development of the circu- 

 latory system. 2 



Involution of the Uterus. At from four to six days, and 



1 HENNIG, Studien uber den Ban der mcnschlichen Placenta, Leipzig, 1872, S. 

 26. 2 See page 441. 



