foetus to the womb, the normal structures being gradually destroyed by a
fatty degeneration. This results in a separation between the fetal and
maternal placenta. The contents of the womb begin to affect the organ in
the same manner as a foreign body, irritating the nerve endings and
producing contractions of the muscles. These contractions of the muscles
help greatly in breaking down the attachments until finally the labor pains
begin in earnest, and the foetus is gradually forced out of the womb,
through the dilated os and into the vagina and vulva.

_A normal birth_ is possible, only when the expelling power of the womb is
able to overcome the resistance offered by the foetus and its membranes,
the pelvic walls and the vagina and vulva.

[Illustration: FIG. 18.--Photograph of model of uterus of cow containing a
foetus: foetus; umbilical cord; placenta; horn containing foetus; and
opposite horn. Note the difference in the development of the two horns.]

The relative size of the foetus to the inlet of the pelvic cavity and its
position are the most important factors for the veterinarian and stockman
to consider (Fig. 18). On leaving the womb, the foetus passes into the
vagina and vulva. This portion of the maternal passages is situated in the
pelvic cavity which continues the abdominal cavity posteriorly. The pelvic
walls are formed by bones and ligaments that are covered by heavy muscles.
As previously mentioned, the ligaments and muscles relax toward the end of
pregnancy in order to prepare the way for the passage of the foetus. Before
entering the pelvis it is necessary for the foetus to be forced through the
inlet to this cavity. This is the most difficult part of the birth, as the
bones that form the framework of the pelvis completely enclose the entrance
to it. It is only in the young mother that the pelvic bones give way
slightly to the pressure on them by the foetus. It can be readily