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. THE FEMUR The =femur= or thigh bone (Os femoris) is the largest and most massive of the long bones. It extends obliquely downward and forward, articulating with the acetabulum above and the tibia and patella below. It presents for examination a shaft and two extremities.