Of The Hand

Between the knuckle of the first finger and the thumb is a bulging mass. This is the first interosseous muscle, large here on account of the exposed position of the finger, also because it aids the thumb. In clasping, it is perpendicular to the thumb and diagonal to the knuckle. It attaches to the phalanx at the knuckle, to the whole side of the thumb (first segment) and to the base of the metacarpal bone of the finger itself.

Beyond its edge is a fold of skin, alternately drawn into a half-moon blade, and dimpled and wrinkled, as the thumb changes its position.

Running the length of the thumb to the last joint, on its back, is seen the extensor tendon, pointing always to the top of the wrist. At the root of the thumb is seen another tendon, that of the short extensor, pointing always to the bottom of the wrist; the two converging on the second joint.. Between them at the wrist is a depression, quite deep when the thumb is extended.

This latter tendon marks the front border of the metacarpal bone of the thumb. Bulging in front of it are, first, the trapezium, marking the radial end of the wrist arch, then the thenar eminence, to the big joint of the thumb. Sometimes the basal joint of the thumb still farther bulges this eminence.

Anatomy The Hand Basal JointContracted Thenar EminenceThenar Space

The mass of the hand sets an an angle across the end of the forearm; the mass of the thumb sets at an angle across the base of the hand.

The Muscles of the Thumb 1 Long extensor of the thumb

The Muscles of the Thumb 1 Long extensor of the thumb

Contracted Thenar Eminence

The power of the thumb depends chiefly on its short muscles. Muscles must be long in proportion to the distance they have to contract. Muscles to the ends of fingers and thumb are therefore long reaching to the elbow. Those of the first and middle segments of the thumb (the latter with very little movement) are short and are developed about the segment and across the palm, where they act in direct line with the movement of the bone. The power produced by muscular action depends on the leverage and the angle at which it is applied. The long muscles act at an acute angle, with rapid movement but little power.

Anatomical Movement Photos

These short muscles being in direct line produce great power but are relatively slow. The fastest movement of the thumb is therefore slow compared to that of the fingers; its power is proportionately greater.

Thumb Anatomy Tendons Palma

Thumb Side of the Hand

Distinguishable under the skin of the thumb (palmar side) are three muscles, sometimes a fourth. These, from the back forward, are the fat opponens, hugging the bone; the broad abductor, forming the bulk of the mass; and the thin flexor brevis, inside. Deeper and reaching transversely across the hand is the adductor muscle, which throws the skin of the palm into a bulging wrinkle when the thumb is flattened back.

Abductor Pollicis Transversus

1 Opponens pollicis

2 Abductor pollicis

3 Adductor transversus

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