The Pelvis

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.