Lower Limb.—The lower limb consists of a proximal part or haunch, a distal part or foot, and an intermediate shaft sub divided into thigh and leg. Each part has its appropriate skeleton (the thigh-bone in man being longer than the leg-bones). The bone of the haunch (os innominatum) forms an arch or pelvic girdle, which articulates behind with the side of the sacrum, and arches forward to articulate with the opposite haunch-bone at the pubic symphysis. It is the direct medium of connection between the axial skeleton and the shaft and foot, which form a free di vergent appendage.
The os innominatum, or haunch-bone, is a large irregular plate-like bone, which forms the lateral and inferior boundary of the cavity of the pelvis. In early life it consists of three bones— ilium, ischium and pubis—which unite about the twenty-fifth year into a single bone. These bones converge, and join to form a cup, the acetabulum, on the outer surface of the bone, which lodges the head of the thigh-bone at the hip-joint. At the bottom of the acetabulum is a depression, to the sides of which the ligamentum teres of the hip-joint is attached. From the aceta bulum the ilium extends upward and backward, the ischium down ward and backward, the pubis forward, inward and downward. Below the acetabulum is a large hole, the obturator foramen, which is bounded by the ischium and pubes; behind and above the acetabulum is the deep sciatic notch, which is bounded by the ischium and ilium, and below this is the small sciatic notch.
Ilium.—The ilium (fig. 16) in man is a broad plate-like bone, the lower end of which aids in forming the acetabulum, while the upper end forms the iliac crest, which, in man, is elongated into the sinuous crest of the ilium. This crest affords attachment to the broad muscles which form the wall of the abdominal cavity. One surface of the ilium is external, and marked by curved lines which subdivide it into areas for the origin of the muscles of the buttock; another is anterior, and hollowed out to give origin to the iliacus muscle ; the third, or internal, surface articulates posteriorly with the sacrum, whilst anteriorly it forms a part of the wall of the true pelvis. The external is separated from the anterior surface by a border which joins the anterior end of the crest, where it forms a process, the anterior superior spine. Between the anterior and internal surfaces is the ilio-pectineal line, which forms part of the line of separation between the true and false pelvis.
Pubis.—The pubis (fig. 16) is also a three-sided, prismatic, rod-like bone, the fundamental form of which is obscured by the modification in shape of its inner end. In human anatomy it is
customary to regard it as consisting of a body and of two branches, an upper and a lower ramus. Projecting forward from the junction of the body and upper ramus is the pubic spine, a landmark in surgery, and to this the ilio-pectineal line may be traced.
Ischium.—The ischium (fig. 16) also has the fundamental form of a three-sided prismatic rod. One extremity (the upper) com pletes the acetabulum, whilst the lower forms the large promi nence, or tuber ischii. The tuberosity, a thick, rough and strong process, gives origin to several powerful muscles : on it the body rests in the sitting posture; a flattened ramus ascends from it to join the lower ramus of the pubis, and completes both the pubic arch and the margin of the obturator foramen.
Pelvis.—By the articulation of the two innominate bones with each other in front at the pubic symphysis, and with the sides of the sacrum behind, the osseous walls of the cavity of the pelvis are formed. This cavity is sub divided into a false and a true pelvis. The false pelvis lies be tween the expanded wing-like portions of the two ilia. The true pelvis lies below the two ilio pectineal lines and the base of the sacrum, which surround the upper orifice or brim of the true pelvis, or pelvic inlet, whilst its lower orifice or outlet is bounded behind by the coccyx, laterally by the ischial tuberosities, and in front by the pubic arch. In the erect attitude the pelvis is so in clined that the plane of the brim forms with the horizontal plane an angle of from 6o° to 65°. The axis of the cavity is curved, and is represented by a line dropped perpendicularly from the planes of the brim, the cavity and the outlet ; at the brim it is directed downward and backward, at the outlet downward and a little for ward. Owing to the inclination of the pelvis, the base of the sacrum is nearly 4 in. higher than the upper border of the pubic symphysis. The female pelvis is dis tinguished from the male by certain sexual characters. The bones are more slender, the ridges and processes for muscular attach ment more feeble, the breadth and capacity greater, the depth less, giving the greater breadth to the hips of a woman; the inlet more nearly circular, the pubic arch wider, the distance between the tuberosities greater, and the acetabulum smaller in the female than in the male. The greater capacity of the woman's over the man's pelvis is to afford greater room for the expansion of the uterus during pregnancy, and for the expulsion of the child at the time of birth.