Latissimus Dorsi
The latissimus dorsi is a broad, flat muscle that covers approximately one-third of the back on each side (figs. 1-28 and 1-29). It rotates the arm inward and draws the arm down and back. It originates from the upper thoracic vertebrae to the sacrum and the posterior portion of the crest of the ilium. Its fibers converge to form a flat tendon that has its insertion in the humerus.

Gluteus
The gluteus (maximus, minimus (not shown), and medius) are the large muscles of the buttocks, which extend and laterally rotate the thigh, as well as abduct and medially rotate it (fig. 1-29). They arise from the ilium, the posterior surface of the lower sacrum, and the side of the coccyx. Their points of insertion include the greater trochanter and the gluteal tuberosity of the femur. The gluteus maximus is the site of choice for intramuscular injections.

Figure 1-29.-Posterior view of superficial skeletal muscles.

Quadriceps
The quadriceps is a group of four muscles that make up the anterior portion of the thigh. The four muscles of this group are the rectus femoris that originates at the ilium; and the vastus lateralis, v. medialis, v. intermedius (not shown), that originate along the femur (fig. 1-28). All four are inserted into the tuberosity of the tibia through a tendon passing over the knee joint. The quadriceps serves as a strong extensor of the leg at the knee and flexes the thigh. Additionally located in the quadriceps area is the adductor longus that adducts, rotates, and flexes the thigh.

Biceps Femoris
The biceps femoris (often called the hamstring muscle) originates at the tuberosity of the ischium (the lowest portion of the coxal bone, part of the pelvic girdle) and the middle third of the femur (fig. 1-29). It is inserted on the head of the fibula and the lateral condyle of the tibia. It acts, along with other related muscles, to flex the leg at the knee and to extend the thigh at the hip joint.

Gracilis
The gracilis is a long slender muscle located on the inner aspect of the thigh (figs. 1-28 and 1-29). It adducts the thigh, and flexes and medially rotates the leg. Its origin is in the symphysis pubis, and its insertion is in the medial surface of the tibia, below the condyle.

Sartorius
The sartorius is the longest muscle in the body. It extends diagonally across the front of the thigh from its origin at the ilium, down to its insertion near the tuberosity of the tibia (fig. 1-29). Its function is to flex the thigh and rotate it laterally, and to flex the leg and rotate it slightly medially.

Gastrocnemius and Soleus
The gastrocnemius and soleus (together commonly called the calf muscles) extend the foot at the ankle (figs. 1-28 and 1-29). The gastrocnemius originates at two points on the femur; the soleus originates at the head of the fibula and the medial border of the tibia. Both are inserted in a common tendon called the calcaneus, or Achilles tendon.

Tibialis Anterior
The tibialis anterior originates at the upper half of the tibia and inserts at the first metatarsal and cuneiform bones (fig. 1-28). It flexes the foot.

Diaphragm
The diaphragm (not shown) is an internal (as opposed to superficial) muscle that forms the floor of the thoracic cavity and the ceiling of the abdominal cavity. It is the primary muscle of respiration, modifying the size of the thorax and abdomen vertically. It has three openings for the passage of nerves and blood vessels.