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
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.
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
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
T h e g a s t r o c n e m i u s a n d s o l e u s ( t o g e t h e r
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.
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.
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.
THE INTEGUMENTARY SYSTEM
LEARNING OBJECTIVE: Identify skin, its
functions, structure, and appendages.
Organ systems are comprised of tissues grouped
together to form organs, and groups of organs with
specialized functions. Since the skin acts with hair
follicles, sebaceous glands, and sweat glands, these
organs together constitute the integumentary system.
The skin covers almost every visible part of the
human body. Even the hair and nails are outgrowths
from it. It protects the underlying structures from
injury and invasion by foreign organisms; it contains
the peripheral endings of many sensory nerves; and it
has limited excretory and absorbing powers. The skin
also plays an important part in regulating body
temperature. In addition, the skin is a waterproof
covering that prevents excessive water loss, even in
very dry climates.
The skin, or integument, consists of two layers, the
epidermis and the dermis, and supporting structures
and appendages (fig. 1-30).
The epidermis is the outer skin layer (fig. 1-30). It
is made up of tough, flat, scalelike epithelial cells. Five
sublayers or strata of epidermal cells have been
identified, and, listed from superficial to deep, they are