[Illustration:  Fig. 17.—­Thorax. (Anterior view.)]

The hip bones are large, irregularly shaped bones, very firm and strong, and are sometimes called the haunch bones or ossa innominata (nameless bones).  They are united to the sacrum behind and joined to each other in front.  On the outer side of each hip bone is a deep cup, or socket, called the acetabulum, resembling an ancient vinegar cup, into which fits the rounded head of the thigh bone.  The bones of the pelvis are supported like a bridge on the legs as pillars, and they in turn contain the internal organs in the lower part of the trunk.

41.  The Hyoid Bone.  Under the lower jaw is a little horseshoe shaped bone called the hyoid bone, because it is shaped like the Greek letter upsilon ([Greek:  u]).  The root of the tongue is fastened to its bend, and the larynx is hung from it as from a hook.  When the neck is in its natural position this bone can be plainly felt on a level with the lower jaw and about one inch and a half behind it.  It serves to keep open the top of the larynx and for the attachment of the muscles, which move the tongue. (See Fig. 46.) The hyoid bone, like the knee-pan, is not connected with any other bone.

The Bones of the Upper Limbs.

42.  The Upper Limbs.  Each of the upper limbs consist of the upper arm, the forearm, and the hand.  These bones are classified as follows: 

    Upper Arm: 
      Scapula, or shoulder-blade,
      Clavicle, or collar bone,
      Humerus, or arm bone,

    Forearm: 
      Ulna,
      Radius,

    Hand: 
      8 Carpal or wrist bones,
      5 Metacarpal bones,
     14 Phalanges, or finger bones,

making 32 bones in all.

43.  The Upper Arm.  The two bones of the shoulder, the scapula and the clavicle, serve in man to attach the arm to the trunk.  The scapula, or shoulder-blade, is a flat, triangular bone, placed point downwards, and lying on the upper and back part of the chest, over the ribs.  It consists of a broad, flat portion and a prominent ridge or spine.  At its outer angle it has a shallow cup known as the glenoid cavity.  Into this socket fits the rounded head of the humerus.  The shoulder-blade is attached to the trunk chiefly by muscles, and is capable of extensive motion.

The clavicle, or collar bone, is a slender bone with a double curve like an italic f, and extends from the outer angle of the shoulder-blade to the top of the breastbone.  It thus serves like the keystone of an arch to hold the shoulder-blade firmly in its place, but its chief use is to keep the shoulders wide apart, that the arm may enjoy a freer range of motion.  This bone is often broken by falls upon the shoulder or arm.

The humerus is the strongest bone of the upper extremity.  As already mentioned, its rounded head fits into the socket of the shoulder-blade, forming a ball-and-socket joint, which permits great freedom of motion.  The shoulder joint resembles what mechanics call a universal joint, for there is no part of the body which cannot be touched by the hand.