COURSE OP NORMAL BIRTH 71 



Although parturition often terminates favorably, nevertheless 

 the posterior presentation frequently gives rise to dystokia. 



We noticed already that dilatation of the cervix uteri 

 depends on the pressure exerted upon the internal opening of 

 the cervix, by the segment of the chorion and allantois filled 

 with foetal liquor. This factor is also prominent in a birth with 

 posterior presentation. As the result of the extension of the 

 hind legs, the chorion-allantoic bladder is immediately followed 

 by the amniotic bladder, which contains the claws of the hind 

 feet and which now glide easily iuto the cervix uteri. But as 

 the calf's pelvis presents itself before the inlet, the dilatation 

 caused by the water-bag is often insufficient to permit the 

 passage of the foetal pelvis, and expulsion is thus interfered 

 with. In the anterior presentation the larger amniotic bladder 

 containing head and fore legs produces complete dilatation. 

 The contractions now increase in violence, aud since the cervix 

 is now dilated mechanically by the foetus, this obstacle may be 

 soou removed. After the hind-quarters are born, thorax and 

 head meet with no difficulties. Those complications which 

 may yet arise will be discussed later. 



During normal birth, with either anterior or posterior 

 presentation, the calf lies in a postero-anterior plane ; primary 

 transverse presentations do not occur in the cow. The intra- 

 uterine position of the calf is the result of its accommodation 

 to the form of the uterus. The back of the calf lies toward the 

 convexity of the uterus. The largest portion of the calf, its 

 hind- quarters, lies in the most roomy part of the horn, the head 

 in the narrower part and in the body of the uterus. This 

 theory, also called the accommodation theory, readily explains 

 why a head presentation in the cow is the rule. 



In man, one tries to explain frequency of the head presen- 

 tations by the fact that the point of gravity is nearer to the 

 head than to the breech or lower extremities. This theory— 

 gravity -theory— ha.s also been applied to explain the frequently 

 occurring anterior presentations in the uniparous mamalia. 

 (Battlehner, Kehrer, I.e. page 105). 



