A rat has four sacral vertebrae. The two anterior sacral vertebrae are attached to the hip bones.
The two coxal bones (os coxae) or hip bones comprise the pelvic girdle. Each bone is firmly united dorsally to the two anterior sacral vertebrae. Ventrally the two are held together by connective tissue at the pubic symphysis. The lateral surface of each bone bears a deep pit, the acetabulum, for the reception of the head of the thigh bone (femur). Unlike the pectoral girdle the direct attachment of the pelvic girdle to the backbone furnishes a rigid support for the strong and active hind legs of the animal. The coxal bone consists of three fused elements, the ilium, ischium, and pubis.
Femur or thigh bone
The femur, or thigh bone, consists of a long body with processes at each end for attachment of muscles, or for articulation with other bones. The proximal end is Y-shaped. The medial branch of the Y is capped with a hemispherical knob, the head, which fits into the acetabulum. The head is held in place by the round ligament (ligamentum teres femoris) of the femur, attached to the center of the articular surface. The head is joined to the shaft by the narrow cylindrical neck. Head and neck together resemble a small "toad stool," the neck corresponding to the stalk of the "stool."
The massive lateral branch of the Y, the great trochanter, is continued as a high thin ridge down the lateral surface of the femur. This ridge is sometimes referred to as the third trochanter, and the lateral branch of the Y as the first trochanter, the two being considered components of the greater trochanter. The lesser or second trochanter is a knob ventral to the base of the neck on the flexor surface of the femur. The intertrochanteric crest connects the greater and lesser trochanters. The trochanteric fossa is a deep depression at the base of the medial side of the greater trochanter. The lateral and medial condyles are large ventral swellings at the distal end of the femur. They are separated from each other by the deep intercondyloid fossa, which is continued on to the extensor surface of the bone as the grooved patellar surface for articulation with the patella. The two condyles articulate with the proximal end of the tibia.
The patella is a sesamoid bone lying in front of the knee joint.
The tibia extends from the knee to the ankle. The fibula is a slender bone posterior to the tibia. The two are anchylosed in the lower third of the tibia. These two bones resemble an archer's bow, the fibula representing the cord, the forward bending tibia the wooden bow. The shaft or body of the tibia decreases in thickness distally. Beyond the anchylosis it is cylindrical in cross section, but triangular proximal to this point. Thus the bone presents three surfaces:
a concave anterolateral surface,
a convex antermnedial surface,
and a posterior surface with two pronounced longitudinal grooves.
A ridge, the anterior crest, is located proximally on the anterior side of the bone at the angle formed by the lateral and medial surfaces. The bone articulates with the femur proximally. The articular surface is divided into two large concave condyles, separated from each other by a longitudinal groove, the intercondyloid fossa. Each of these is opposed to the convex surface of one of the femoral condyles. The tibia and fibula articulate distally with the talus (astragalus) bone of the heel. The tibial part of the articular surface contains two longitudinal grooves, separated by a ridge. The medial malleolus forms the medial boundary of the inner groove. The fibula bears the lateral malleolus upon its lateral surface. The slender fibula expands dorsally, where it is attached by a ligament to the external condyle of the tibia.
The tarsus, or ankle, contains eight bones arranged in two rows, one distal and one proximal, with one bone between the rows. The proximal row contains the talus and calcaneus. The talus articulates with the tibia and fibula. The calcaneus, or heel bone, is shovel-shaped and is the largest bone in the tarsus. The "handle," or heel process, is directed posteriorly, forming the support for the heel. When the toes are depressed the trochlea of the talus acts as a fulcrum, the heel process as a lever moved by the posterior leg muscles. The length of the heel process makes it a relatively efficient lever. There are also four sesa-mold bones in the tarsus.
There are five long, cylindrical metatarsal bones in the foot, those of the great toe, or hallux, and of the little toe being shorter than the remaining three.
Each toe, or digit, possesses three phalanges, except the hallux, which has two. Each terminal phalanx ends in a curved claw.