THE FEMALE REPRODUCTIVE ORGANS 291 
(d) The angle of the axis from the promontory of the 
sacrum to the symphysis is always greater in the quad- 
rupeds than in man, averaging in the former from 70 to 
80 degrees, in the latter about 55 degrees, (e) The 
symphysis pubis is an extremely long joint, being fre- 
quently greater than half the length of the entire pelvis, 
(f) The sacroiliac joint is more or less movable in all 
quadrupeds, especially in young animals. The rotation 
of the sacrum on the ilia increases the anteroposterior 
(dorsoventral) diameter of the outlet and at the same 
time wedges apart the ilia, thus increasing the lat- 
eral diameter. 
In regard to the forces of labor it is to be pointed out 
that in the lower mammals the pregnant uterus hangs be- 
low the pubic arch so that the fetus must rise at an angle 
of about 45 degrees to pass over the pelvic brim. In so 
doing it meets the narrowest part of the triangular bony 
pelvic inlet, the anterior pubic angle. Since the sacrum 
is above and out of the way, the lateral diameter is the 
one which must be suitable to the passage of the pre- 
senting part. This is the head in homo, the largest part 
of the fetus, but in lower animals either head or breech 
often accompanied by one or more extremities, may 
present; the head is not the largest part in lower mam- 
malian fetuses. The uterine contraction proceeds as in 
man, the fundus and cornua acting alone until the cervix 
contains the fetus, at which time all parts contract. In 
the bicomate organ both sides must contract or the fetus 
might be forced from the gravid to the empty side. In 
muciparous animals with both uterine horns occupied, 
the fetuses lie head to head, breech to breech and are 
expelled alternately from each side. 
In uniparous animals rotation is in the nature of an 
accommodation of the greatest diameter of the fetal 
body in cross section to greater axis of the mother — the 
dorsoventral. Uterine contractions cause the fetus to 
unfold from its elliptical form and to assume an 
