Three bones make the pelvis; two innominate (without a name) bones and one sacrum (sacrificial bone).
The sacrum is a wedge about the size of the hand but more perfectly shaped, bent like a half-bent hand, and carrying a very small tip about as big as the last joint of the thumb (coccyx). It forms the central piece in the back, curving first back and down and then down and in.
The two innominate bones are formed like two propellers, with triangular blades twisted in opposite directions. The rear corners of the top blades meet the sacrum in the back, and the front corners of the lower blades meet in front to form the symphysis pubis. The hip socket itself forms the central point for the shaft. The two blades stand at right angles to each other.
The upper blade is called the ilium, the lower is called the pubis in front and the ischium behind, with an opening between. The only superficial parts are the top of the upper blade (iliac crest) and the front tip of the lower (symphysis pubis).
MASSES AND MARKINGS
The size of the pelvis is due to its position as the mechanical axis of the body; it is the fulcrum for the muscles of the trunk and legs, and is large in proportion. Its mass inclines a little forward, and is somewhat square as compared with the trunk above.
At the sides the ridge is called the iliac crest. It is the fulcrum for the lateral muscles and flares out widely for that purpose; rather more widely in front than behind.
Above the rim is a roll of muscle belonging to the abdominal wall; immediately below it a groove or depression, made by the sag of the hip muscles, obliterated when these are contracted in Action.
So great are the changes in surface form of the muscles in different positions of the hip that the iliac crest remains as the one stable landmark. It is a curve, but being beveled backward, it presents to the side view two lines and almost an angle between them at the top.
The posterior line is marked by two dimples where it joins the sacrum, and the line continues downward into the fold of the buttocks. From this whole line the gluteus maximus muscle passes down and forward, to just below the head of the thigh bone, making the mass of the buttocks and hip.
Just in front of this, from the top of the crest, descends the gluteus medius muscle, forming a wedge whose apex is at the head of the thigh bone. Between these two muscles is the dimple of the thigh.
Only part of the medius is superficial; its front portion is overlaid by the tensor fasciae femoris muscle, which rises from the edge of the front line of the crest and descends to form with the gluteus maximus the wedge filled in by the medius. The two fasten to the dense plate of fascia that guards the outside of the thigh (ilio-tibial band). This muscle is always prominent and changes its appearance greatly in different positions of the hip, forming a U-shaped wrinkle when the thigh is completely flexed.
On the front end of the crest is a small knob, from which descends the sartorius (tailor's) muscle, longest in the body. It forms a graceful curve as it lies in the groove of the inner side of the thigh, passing to under the knee.
From just below the knob, overlaid therefore by the sartorius, descends the rectus femoris muscle, straight to the knee cap.
From the knob, the line continues down and in to the symphysis, marking the boundary between abdomen and thigh.
The Pelvis and Hip
1 Tensor vaginse femoris.
3 Rectus femoris.
4 Gluteus medius.
5 Gluteus maximus.
Gluteus Medius: From ilium, outer surface, to femur, greater trochanter. Action: Abducts and rotates inward thigh.
Gluteus maximus: From crest of ilium, rear portion, sacrum and coccyx to femur. Action: Extends, rotates and turns out thigh.
The Lower Limbs
The lower limb is divided into three parts -- the thigh, the leg, and the foot. These parts correspond to the arm, the forearm, and the hand of the upper limb.
The thigh extends from the pelvis to the knee, and the leg from the knee to the foot.
The longest and strongest bone of the body is the femur (thigh bone). It is joined to the bones of the pelvis at the hip socket by a long neck, which carries the shaft itself out beyond the widest part of the crest. From there the femora (thigh bones) converge as they approach the knees, bringing the knee under the hip socket. At the knee, the femur rests on the tibia (shin bone), the main bone of the leg, and makes a hinge joint. The tibia descends to
form the inner ankle. Besides it, not reaching quite to the knee, is the fibula, the second bone of the leg, which descends to form the outer ankle. It is located on the outside, and is attached to the tibia at the top and bottom. These two bones are almost parallel. Above the juncture of the femur and tibia lies the patella (knee cap). This is a
small bone almost triangular in shape. It is flat on its under side, and convex on the surface.
The great trochanter of the femur is the upper tip of the shaft which reaches up slightly beyond where the neck joins.
The lower portion of the femur widens to form two great huge processes, known as tuberosities. They are on the outer and inner sides, and they are both visible.
From the head of the femur (trochanter) to the outside of the knee runs a band of tendon called the ilio-tibial band. It makes a straight line from the head of the thigh bone to the outside of the knee.
The rectus femoris muscle makes a slightly bulging straight line from just below the iliac crest to the knee cap.
On either side of the latter is a twin mass of muscles. That of the outside (vastus externus) makes one mass with it, and slightly overhangs the ilio-tibial band outside. That of the inside (vastus internus) bulges only in the lower third of the thigh, and overhangs the knee on the inside.
Behind and inside of this is the groove of the thigh occupied by the sartorius muscle, passing from the ilium above to the back of the knee below.
Behind the groove is the heavy mass of the adductors, reaching two-thirds of the way down the thigh.
Behind groove and adductors, around the back of the thigh and to the ilio-tibial band outside, is the mass of the hamstring muscles, whose tendons are found on either side of the knee at the back. It is a dual mass of muscle, dividing above the diamond-shaped popliteal space at the back of the knee, whose lower corner is formed by the gastrocnemius
muscle, similarly divided.
The mass of the thigh is inclined inward from hip to knee, and is slightly beveled toward the knee from front, back and outside.