The =bony pelvis= is composed of the ossa coxarum, the sacrum, and the
first three coccygeal vertebræ. The =dorsal wall= or =roof= is formed by
the sacrum and first three coccygeal vertebræ, and the =ventral wall= or
=floor= by the pubic and ischial bones. The =lateral walls= are formed
by the ilia and the acetabular part of the ischia. The defect in the
skeleton here is supplied in the fresh state by the sacro-sciatic
ligaments and semimembranosus muscles.
The =anterior aperture= or =inlet= (Apertura pelvis cranialis) is
bounded by the =terminal line= (Linea terminalis) or brim, composed of
the base of the sacrum dorsally, the ilio-pectineal lines laterally, and
the anterior border of the pubis ventrally. It is almost circular in the
mare, semi-elliptical in the stallion, and faces obliquely downward and
forward. It has two principal diameters. Of these, the =conjugate= or
=sacro-pubic diameter= (Conjugata) is measured from the sacral
promontory to the anterior end of the symphysis. The =transverse
diameter= (Diameter transversa) is measured at the greatest width, _i.
e._, just above the psoas tubercle.
The =posterior aperture= or =outlet= (Apertura pelvis caudalis) is much
smaller and is very incomplete in the skeleton. It is bounded above by
the third coccygeal vertebra and below by the ischial arch; in the fresh
state it is completed laterally by the sacro-sciatic ligament and the
semimembranosus muscle.
The =axis= of the pelvis is an imaginary line drawn through the centers
of the inlet, cavity, and outlet.
=Sexual Differences.=—Marked differences exist in the size and form of
the pelvis in the two sexes. The average =conjugate diameter= is about
9½ inches (ca. 23 to 24 cm.) in the mare, 7½ inches (ca. 18 to 20 cm.)
in the stallion. The =transverse diameter= of the inlet averages about 9
inches (ca. 22 to 23 cm.) in the mare, and 8 inches (ca. 20 cm.) in the
stallion. The =obliquity= of the inlet is greater in the female; the
difference is indicated by the fact that a vertical plane from the
pecten cuts the fourth sacral segment in the female, the second in the
male. The =outlet= is also larger in the mare, the =ischial arch= being
about one-third wider than in the stallion. The =cavity= is much more
roomy in the female; the transverse diameter between the middles of the
superior ischiatic spines is about 8 inches (20 cm.) in the mare, 6
inches (15 cm.) in the stallion. The pubic part of the =floor= in the
female is concave and lies considerably lower than the ischiatic part,
which is wide and relatively flat. In the stallion the pubis is very
thick centrally, and this part of the floor is convex, while the ischial
part is relatively narrow, and is concave from side to side. The
=obturator foramina= are correspondingly larger in the female. The ilium
is shorter, and the greater sciatic notch deeper and narrower in the
male. The pelvis of the gelding, when castration has been performed
early, resembles that of the mare; otherwise the male characters appear
to be retained to a large degree.