The  hip,  or  INNOMINATE,  is  a  large  ir- regularly  shaped  bone  composed  of  three  parts: the  ilium,  ischium,  and  pubis.  In  children  these three parts are separate bones, but in adults they are  firmly  united  to  form  a  cuplike  structure, called  the  ACETABULUM,  into  which  the  head of  the  femur  fits.  The  ILIUM  forms  the  outer prominence  of  the  hip  bone  (crest  of  the  ilium), the ISCHIUM forms the hard lower part, and the PUBIS  forms  the  front  part  of  the  pelvis. The area where the two pubic bones meet is called the SYMPHYSIS PUBIS and is often used in  anatomical  measurements.  The  largest  foramen (opening) is located in the hip bone, between the ischium  and  the  pubis,  and  is  called  the  OB- TURATOR  FORAMEN.  The  crest  of  the  ilium is   used   in   making   anatomical   and   surgical measurements   (e.g.,   location  of  the  appendix, which  is  approximately  halfway  between  the  crest of  the  ilium  and  the  umbilicus). The  FEMUR,  or  thigh  bone,  is  the  longest bone  in  the  body.  The  proximal  end  is  rounded and has a head supported by a constricted neck that fits into the acetabulum. Two processes called the  GREATER  and  LESSER  TROCHANTERS are  at  the  proximal  end  for  the  attachment  of muscles. The neck of the femur, located between the head and the trochanters, is the site most fre- quently fractured. At the distal end are two bony prominences called the LATERAL and MEDIAL CONDYLES, which articulate with the tibia and the patella. The  PATELLA  is  a  small  oval-shaped  bone overlying the knee joint. It is enclosed within the tendon  of  the  quadriceps  muscle  of  the  thigh. Bones like the patella that develop within a ten- don  are  known  as  SESAMOID  bones. The TIBIA, or shin bone, is the larger of the two  leg  bones  and  lies  at  the  medial  side.  The proximal end articulates with the femur and the fibula. Its distal end articulates with the talus (one of  the  foot  bones)  and  the  fibula.  A  prominence easily felt on the inner aspect of the ankle is called the   MEDIAL   MAL.LEOLUS. The  FIBULA,  the  smaller  of  the  two  leg bones,  is  located  on  the  lateral  side  of  the  leg, parallel to the tibia. The prominence at the distal end  forms  the  outer  ankle,  known  as  the LATERAL   MALLEOLUS. The  TARSUS,  or  ankle,  is  formed  by  seven tarsal  bones.  The  strongest  of  these  is  the  heel bone  or  CALCANEUS. The  sole  and  instep  of  the  foot  is  called  the METATARSUS   and   is   made   up   of   five METATARSAL  bones.  They  are  similar  in  ar- rangement  to  the  metacarpal  of  the  hand. The  PHALANGES  are  the  bones  of  the  toes and are similar in number, structure, and arrange- ment  to  the  bones  of  the  fingers. JOINTS Whenever  two  bones  are  attached  to  each other, a joint is formed. In a freely movable joint, such as the knee or elbow joint, the ends of the bones  are  covered  with  a  smooth  layer  of  car- tilage. The whole joint is enclosed in a watertight sac  or  membrane  containing  a  small  amount  of lubricating  fluid.  This  enables  the  joint  to  work with little friction. The ligaments that reach across the  joints  from  one  bone  to  another  keep  them from getting out of place. When ligaments are ac- cidentally torn, we call the injury a sprain; when bones are out of place, there is a dislocation; and when bones are chipped or broken, the injury is called  a  fracture. Joints are classified according to the amount of movement they permit (fig. 3-23). They may be: 1.  IMMOVABLE.  Bones  of  the  skull  are rigidly  interlocked  along  immovable  joint lines known as sutures. Figure  3-23.—Typical  joints. 3-13