5i6 Vetermary Obstetrics 



the amniotic fluid is pushed out into the water-bag, until finally 

 the tension becomes too great and it ruptures. Slightly before 

 this period, in normal cases, the presenting portion or portions 

 of the fetus, usually the two anterior feet, followed closely by 

 the nose, appear in the "water-bag." 



Under more or less abnormal conditions, there are great varia- 

 tions in reference to the behavior of the fetal membranes at this 

 stage of labor. In the mare it is possible for the chorion to 

 become completely detached from the uterus without becoming 

 ruptured. Consequently the fetus is in some cases said to be 

 expelled completely encased within all its membranes. This is 

 apparently a general statement and we do not know of a well 

 authenticated instance where it has occurred in a fully developed 

 fetus, though it is frequently seen in the mare, as well as in other 

 animals, when abortion takes place at an early period in gesta- 

 tion. In one case which we observed, the chorion ruptured at 

 the anterior end and was pushed over the fetus and expelled, 

 along with the amnion, while the dead fetus remained in the 

 uterus, owing to some deviation in its extremities which inter- 

 fered with its expulsion. 



In cases of twin pregnancy, it is essential that but one fetus 

 should enter the birth passage at a time. In the cow and ewe 

 ordinarily, when there are twins, one occupies each horn in part, 

 while a portion of each fetus projects more or less into the body 

 of the uterus and, when labor pains come on, that fetus which 

 chances to be most advanced ordinarily gains the cervical canal 

 and pelvic inlet first and by its presence prevents the other from 

 entering. Thus the two fetuses are born one after the other. 



In practice, this favorable disposition of twin fetuses is occa- 

 sionally interrupted by some portion or portions of each fetus 

 simultaneously entering the pelvic canal and causing more or less 

 serious dystokia. 



We have no evidence to show that the uterine contractions in 

 the two horns may not be essentially equal, but apparently they 

 soon become concentrated upon that portion of the organ which 

 contains the fetus in the most advanced position, and consequently 

 where the contractions will prove most efficient. As soon as the 

 first fetus is expelled and the passage thus vacated and dilated, 

 the expulsion of the other one, as a rule, follows very promptly. 



In the mare, only a few minutes usually elapse between the 



