514 Veterinary Obstetrics 



It is well to note that the os uteri is very much more dilatable 

 when the force is exerted from before backward — intra- uterine 

 force— than from behind forward — extra-uterine force — as the 

 obstetrist must largely do when applying forcible means for dila- 

 tion of the OS uteri. As the os uteri becomes dilated and the 

 cervix effaced, the pressure within the uterus presses the fetal 

 membranes, with the contained fluids, out through the os uteri, 

 vagina and vulva, until they appear externally. 



The relations of the various fetal membranes to each other, and 

 to the contained liquids, bring about very marked differences in 

 the various animals. The chorion ruptures without having 

 moved from its original position, or becoming extensively de- 

 tached from the uterus. Normally it can not protrude far 

 through the os uteri. It is essential that this should be so be- 

 cause it is through the maintenance of the intimate relation be- 

 tween the chorion and the uterus that the life of the fetus is 

 maintained during the time of parturition. If the chorion should 

 become so detached from the uterus as to interrupt the inter- 

 change of nutriment or excretions between the mother and the 

 fetus, the life of the latter must quickly terminate unless the act 

 of birth is abruptly completed. Normally, therefore, the chorion 

 may push out only a little way from the os uteri before rupturing, 

 without interfering materially with its general attachments to 

 the uterus through the placenta. After the rupture of the 

 chorion, some of the allantoic fluid may escape into the vagina 

 and later externally. Through this rupture in the chorion, the 

 posterior extremity of the amnion regularly protrudes until it 

 reaches the vulva and projects out between the vulvar lips for 

 some distance. 



The allantoic sac of ruminants, that is, the endoblastic portion 

 of the allantois, is so disposed that it may protrude through the 

 rupture in the chorion to constitute the first "water-bag." In 

 the mare this cannot occur independently of the chorion. Rupture 

 of the protruding allantoic sac occurs early, and is followed by 

 the appearance of the second or true ' ' water-bag ' ' which consists, 

 as in other animals, of a portion of the amniotic sac enclosing 

 some amniotic fluid. 



In the cow and mare the protruding portion of the amnion, or 

 "water-bag," appears as an elongated bladder with a thin, color- 

 less membrane filled with a clear fluid. The bag may extend for 



