UTERUS. 255 



so that it attains a thickness of 6 mm. It then is called the 

 decidua menstrualis. Changes occur also in the glands. They 

 increase in length and become corkscrew-shaped. The 

 increased size of the mucosa is due largely to a cellular multi- 

 plication. Karyokinetic figures in large numbers have been 

 observed in the menstruating uterus by Mandl, not only in the 

 epithelium, but in the interstitial tissue as well. After these 

 changes have occurred there is an escape of blood in the sub- 

 epithelial layers, which is due partly to a bursting of capil- 

 laries, and partly to a diapedesis of red corpuscles through tjie 

 capillary walls. The epithelium covering these collections of 

 blood is broken away and the blood escapes. The bleeding 

 goes on for about four days, and then the regeneration of the 

 mucosa begins. In the course of five to ten days the epithe- 

 lium is quite restored and the glands regain their normal 

 relations. Following this are a few days of rest before the 

 next period begins. There has been considerable discussion 

 as to the extent of the tissue destruction which takes place 

 during menstruation. According to some, the whole mucosa is 

 cast off at each period. Others hold that none at all is 

 destroyed, and that pieces of the epithelium are lifted up 

 merely to allow the blood to escape. It seems certain, however, 

 that there is always some destruction of epithelium, and at the 

 same time there is never a complete destruction. Parts of the 

 gland tubules at least always remain uninjured, and from these 

 and the surface cells that remain the whole epithelium 

 regenerates. 



During pregnancy the whole uterine mucosa suffers very 

 marked changes in its structure. At the end of this time it is 

 nearly all lost, and forms the so-called decidua graviditatis, of 

 which there are three parts. The decidua basalis s. serotina is 

 the part of the mucosa to which the ovum attaches itself, and 

 in which later the placenta is developed ; the decidua capsularis 

 s. reflexa is that part which grows up to surround the ovum ; 

 while the decidua vera is the tissue which lines the rest of the 

 uterine cavity. 



In the part of the uterine mucosa where the decidua vera 



