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.

[Illustration: Fig. 18.--Left Scapula, or Shoulder-Blade.]