THE SKELETON: THE ARM AND FOREARM

(All descriptions of position are based on a supinated position of the arm)

This is one of the most complicated areas of the body. When the arm is in a supine position the bones in the lower arm are parallel. When the arm is pronated the radius pivots across the ulna. Pronation changes the shape of the arm.

The HUMERUS is the only bone of the arm proper. It articulates with the scapula. The head of the humerus is roughly spherical.

The GREATER TUBEROSITY, a knobby projection on the head of the humerus, comes into contact with the acromion process when the arm is raised to approximately shoulder height. The BICIPITAL GROOVE, between the greater and lesser tuberosities, forms a channel for the major tendons of the biceps. The groove keeps the tendons from slipping back and forth.

The shaft of the humerus is roughly cylindrical about 2/3 of the way down. About ½ way down is a projection: the DELTOID IMPRESSION where the deltoid muscle inserts. The shaft of the humerus is deeply buried but the deltoid impression can be felt between the biceps and triceps muscles.

The humerus flattens out near the inferior (distal) end and curves slightly forward. As the shaft fans outward it creates two CONDYLES: the internal or medial and the external or lateral. The MEDIAL CONDYLE is subcutaneous and is an important construction point.

The distal end of the humerus articulates in a special way with the two forearm bones, the ULNA and RADIUS. It is most directly associated with the ulna which swings up and down on an oblique spool: the TROCHLEA (L. pulley). To the lateral side of the trochlea is a rounded knob: the CAPITULUM. The radius glides and revolves on this rounded surface. In front, above the trochlea, is the CORONOID FOSSA or DEPRESSION that allows the far upward bending of the ulna. In back the OLECRANON FOSSA receives the OLECRANON PROCESS of the ulna when the arm is fully extended. (A frequent feminine characteristic is the ability to extend the arm beyond a straight line because of a deeper olecranon fossa.)

The elbow is one of the few places in the body where two bones articulate with one bone.

In the supine position the radius (thumb side) and ulna (little finger side) angle laterally from the lower end of the humerus. They are somewhat flattened in front and have high, blade-like edges in the rear. They have reverse curves so their lower halves are bowed outward creating an appreciable space between them below.

The ulna is wide above and narrow below, at its head. The large end of the bone is shaped like a claw which grasps the trochlea. The large squarish mass at the back, the OLECRANON PROCESS, is easily felt and seen in life. The forward curve at the bottom of the humerus prevents the olecranon process from becoming a pronounced bump at the back of the arm. At the back of the shaft of the ulna is a ridge, the ULNAR CREST, which is subcutaneous from the elbow to the little-finger side of the wrist.

The radius is slender at the top, its head shaped like a thick button which rotates on the capitulum of the humerus. When the arm is extended the radius does not quite touch the capitulum. just below the head is the prominent RADIAL TUBEROSITY which serves as the attachment point for the biceps. The lower end of the radius is broad and wide, scooped out to receive the wristbones. The ulna does not interact with the wrist bones.

Subcutaneous spur-like projections, STYLOID PROCESSES, are found on the medial side of the head of the ulna (just above the wrist on the little-finger side) and the lateral, lower end of the radius. These serve to guide tendons and are important construction points.

The movement of pronation and supination occurs at the elbow. Pronation is largely the job of the radius. The head of the radius pivots around the capitulum. When the arm is extended it can be rotated almost 360 degrees but shoulder rotation accounts for part of this movement. When the arm is bent the lower arm can be rotated 180 degrees. This is true pronation.

Important construction points on the arm:

 

THE WRIST AND HAND

The wrist consists of 8 little CARPAL BONES articulating in sliding joints. These create a springy mass with a rounded upper surface which articulates with the scooped out, inferior end of the radius. One carpal bone projects more on the thumb side (the major multangular). The carpal mass has the form of an arch with a bone spur on either side of the palmar face of the wrist. A ligament, the ANTERIOR ANNULAR LIGAMENT, attaches to these spurs forming a bracelet which binds the tendons at the wrist and somewhat obscures them.

There are 5 METACARPALS which articulate with the carpal mass. The four metacarpals, which articulate with the phalanges of the fingers, are essentially immovable. These radiate fan like from an imaginary center above the lower end of the radius. The 5th metacarpal is attached to the side of the carpal mass. The highest and longest metacarpal is the middle one.

There is almost no movement between the carpals and the metacarpals. The only flexible carpal-metacarpal joint is the thumb.

Each finger is the same length as the metacarpal it articulates with plus the length of the carpal mass at that point.

The fingers consist of 14 bones called PHALANGES. The thumb has 1 metacarpal and 2 phalanges.The fingers each have 1 metacarpal and 3 phalanges. The articulation between the metacarpals and the first phalanges is a ball and socket joint (the knuckle). The joints between the phalanges are hinge ‘joints.

 

 

ARM AND HAND: SUMMARY

Proportions:

humerus to lower arm = 5:4

lower arm to hand and wrists = 3:2

upper am. (acromium process to under surface of bent elbow) = 1 ½ heads

tip of Iong finger to olecranon process = 2 heads

Parts that directly influence the surface:

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