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 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. 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. 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. SKIN FUNCTION 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. SKIN STRUCTURE The skin, or integument, consists of two layers, the epidermis  and  the  dermis,  and  supporting  structures and appendages (fig. 1-30). Epidermis 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 1-22