Arm bones and joints of upper limb
The upper limb is designed for mobility and skilled work. The upper limb joins the trunk at the shoulder joint which is formed between the bone of the arm (humerus) and the shoulder blade (scapula). The humerus has a ball like smooth surface at the upper end (head of humerus). The shoulder blade has a shallow socket into which the head of humerus fits. This is called a ball and socket type of joint. The shoulder joint enjoys a wide range of movements. You can lift your hand above the head turn it inward, outward and also rotate the upper limb. Because of this the joint is less stable than the corresponding joint of lower limb-the hip joint. Shoulder dislocation is commonly seen.
There are two bones in the forearm. The one which is medial is called ulna. This bone forms a joint with the lower end of humerus (elbow joint). The lower end of humarus has a pulley like area into which the notch of the upper end of ulna fits. This is a hinge type of joint with limited mobility compared to shoulder joint. Flexion (bending at the elbow joint) and extension (straightening of elbow ) are the only movements at this joint. Arm bone please click on the graphic to see enlarged image
Upper end of ulna
Ever wondered how you drive in a screw? Apart from holding the screw driver in the hand you have to alternately turn your forearm clockwise and anticlockwise. Clockwise movement in medical terminology is called supination and anticlockwise movement pronation. This movement cannot take place at the elbow joint. That is why nature has placed two bones in the forearm. The other bone is called radius. This bone articulates both with the humerus and ulna. The upper end of radius has a slightly concave surface which articulates with a spherical part (this is called capitulum) on the lower end of humerus. This joint is unstable because in any position only a small area of radial head is in contact with the capitulum of humerus. Hence there is another mechanism to keep the head of radius in place. It is held by a nearly circular ligament attached to upper end of ulna. The head of radius rotates within this ligament which is called annular ligament. The two forearm bones articulate with each other at both the upper and lower ends. The shafts of radius and ulna are held together by a fibrous sheet called interosseous membrane (Interosseous=in between bones). Pronation and supination take place at the radio ulnar joints.
Bones of the hand: The human hand is one of the most versatile and sophisticated tools in creation. It enables man to perform a multitude of tasks. It has a total of 27 bones. Scientists know how difficult it is to design a robot which can simulate a few functions of the hand. There are eight small bones is the wrist arranged in two rows. They are called carpal bones. The forearm bone radius articulates with the carpal bones in the proximal row. In the hand there are five short bones. They are called metacarpal bones. Every finger has 3 small bones except the thumb. We are able to bend our fingers because of joints in between these bones. The thumb has two bones. These bones are called phalanges and the joints imter-phalangeal joints.
Just a few facts about the thumb can reveal the marvelous intelligence involved in the creation of hand. Look at your hand with all the fingers straight and close together, palm facing up and the forearm at right angles to the arm. Observe the plane of the thumb. It is different from that of other fingers. The other fingers are directly looking up but the thumb is facing not only up but also towards the body. This rotation is responsible for the functionality of the thumb. Just imagine what would happen if the thumb and other fingers were in the same plane. Imagine threading a needle with this arrangement.
The thumb is shorter than other fingers. Had it been of the same length the functionality of the hand would have been severely impaired. Try to imagine holding a pen and writing, with your thumb as long as the middle fingers. Back to human body from arm bones
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