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. When the shoulder is dislocated the head of the humerus has been forced out of its socket. The lower end of the bone is grooved to help form a hinge joint at the elbow with the bones of the forearm (Fig. 27). 44. The Forearm. The forearm contains two long bones, the ulna and the radius. The ulna, so called because it forms the elbow, is the longer and larger bone of the forearm, and is on the same side as the little finger. It is connected with the humerus by a hinge joint at the elbow. It is prevented from moving too far back by a hook-like projection called the _olecranon process_, which makes the sharp point of the elbow. The radius is the shorter of the two bones of the forearm, and is on the same side as the thumb. Its slender, upper end articulates with the ulna and humerus; its lower end is enlarged and gives attachment in part to the bones of the wrist. This bone radiates or turns on the ulna, carrying the hand with it.