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A Latin word meaning “leg,” crus is a segment of the lower leg. Referred to as crura in the plural form, it is an anatomical term that may refer to the lower leg itself or to another structure in the body that is leg-like in shape, such as the crura of the heart and diaphragm, which are simply paired bundles of cells and fibers arranged in a linear pattern. Crus may also be known as a gaiter, a term that is more commonly used outside of anatomy to describe garments worn over the lower leg when fishing, hiking, mountain climbing, or riding horseback. In general terms, however, crus is used to describe either the entire lower leg or to refer to a structure contained therein, such as the tibia and fibula bones.
The shank or lower leg is designed exclusively for locomotion and is made up of, beneath skin and subcutaneous fat tissue, 13 muscles, numerous nerves and blood vessels of the leg and foot, and two major long bones, the tibia and fibula. As the larger of the two, the tibia absorbs a more significant portion of the body’s weight between the knee and the ankle. It is the bone that articulates with the femur in the thigh — the only bone in the body that is larger than the tibia — to form the knee joint, and also with the talus in the foot to form a majority portion of the ankle joint. Alongside the tibia in the crus is the fibula, a slightly smaller bone that attaches to the tibia at its top end and to a minor portion of the ankle joint at the bottom end.
Muscles of the crus are classified as belonging to either the anterior or posterior compartment of the leg, compartments that are divided by a sheath of white connective tissue known as fascia. Each compartment and the tissues contained in it are supplied by their own nerves and blood vessels, which run down either the front or back side of the leg. The anterior compartment is often referred to as the shin and contains the tibialis anterior, extensor digitorum longus, and extensor hallucis longus, muscles that lift upward the ankle, four smaller toes, and big toe, respectively. Also sometimes included in the anterior compartment are the three peroneus muscles on the outside of the shin, the longus, brevis, and tertius, which aid in tilting the foot outward.
In the posterior compartment of the leg, known as the calf, is made up of seven muscles. These include the large gastrocnemius and soleus and smaller plantaris muscles, which are plantarflexors of the ankle, meaning that they point the foot downward. Beneath these are the popliteus, which aid in flexion and rotation of the knee, the flexor digitorum longus and flexor hallucis longus, which plantarflex the four smaller and big toes, and the tibialis posterior, which tilts the foot inward.
Supplying these compartments are several major blood vessels and nerves, which are responsible for bringing oxygen and nutrients to the tissues of the leg and foot. In the anterior compartment of the crus are the anterior tibial artery and deep fibular nerve, each of which branches into numerous smaller tributaries. Similarly, the posterior compartment is supplied by the posterior tibial artery and the tibial nerve.