52 2 Veterinary Obstetrics 



bency. Even here the position tends to vary to some degree, 

 especially in the mare, according to the stage of the act. The 

 mare generally lies upon her sternum until the head of the fetus 

 has protruded some distance beyond the vulva,, when, should 

 she continue this position, there would be a tendency for the 

 fetus to strike against the floor or ground and offer obstruction, 

 to its further expulsion. At this period she usually assumes 

 lateral recumbency, in which position she is capable of exerting 

 the maximum expulsive power, while the fetus may be expelled 

 without obstructing its long passage by contact with the ground. 



The standing position of large animals at the close of the ex- 

 pulsion of the fetus is unfavorable for the well-being of the young, 

 because of the possibility of its injury in falling. It is not the 

 best position for the mother, because she cannot exert the maxi- 

 mum degree of force. 



In the multiparous animals the contraction of the abdominal 

 muscles plays a comparatively unimportant part in the expulsion 

 of the fetus. This is brought about chiefly by the contractions 

 of the muscular walls of the uterus itself. 



The ' ' water bag ' ' performs a highly important office in the 

 expulsion of the fetus. We have already suggested that, in the 

 dilation of the os uteri, the bag of water tends to bring this 

 about, in the safest, gentlest, and yet most efEective manner 

 possible, by producing an equal pressure in every direction. 

 Later it dilates the vulvar opening also, although here it is not 

 so important because, when the water bag reaches this part, the 

 membrane protrudes through the vulva and the fetal liquid is 

 forced through the vulva in a stream, to accumulate on the out- 

 side till the amnion ruptures and the fluid escapes. Exception- 

 ally, the "water bag " is said to not rupture in the mare, and the 

 foal to be expelled inclosed within it. 



In normal parturition, the allantoic and amniotic fluids serve 

 to lubricate the passages throughout labor, and portions of the 

 fluids remain within the uterus after the fetus has been expelled. 

 The presence of this fluid in the uterus serves to fill out and effa'ce 

 all irregularities in the form of the fetus and prevents the uterus 

 from closely investing the body of the fetus in such a way as to 

 interfere with its expulsion. 



Should the uterus contract directly upon the irregular fetal 

 body and closely invest it throughout, it could not exert that 



