In the hand as in the figure there is an action and an inaction side. The side with the greatest angle is the action side, the opposite is the inaction or straight side.
With the hand turned down (prone) and drawn toward the body, the thumb side is the action side, the little finger the inaction side. The inaction side is straight with the arm, while the thumb is almost at right angles with it.
The inaction construction line runs straight down the arm to the base of the little finger. The action construction line runs down the arm to the base of the thumb at the wrist, from there out to the middle joint, at the widest part of the hand; thence to the knuckle of the first finger, then to that of the second finger, and then joins the inaction line at the little finger.
With the hand still prone, but drawn from the body, the thumb side is the inaction side, and is straight with the arm, while the little finger is at almost right angles with it. The inaction construction line now runs straight to the middle joint of the thumb, while the action line runs to the wrist on the little finger side, thence to the first joint, etc., etc.
These construction lines, six in number, are the same with the palm turned up. according as it is drawn in or out. They place the fingers and indicate the action and proportions of the hand.
The Hand. Turning of the Masses of Hand and Wrist.
The Hand. Masses of Fingers, Hand and Wrist: Step-down, Wedging, Interlocking.
The Hand. Interlocking of Hand and Wrist: Little Finger Side.
Drill master to the fingers, the hand and the forearm, is the thumb.
The fingers, gathered together, form a corona around its tip. Spread out, they radiate from a common centre at its base; and a line connecting their tips forms a curve whose centre is this same point. This is true of the rows of joints (knuckles) also.
Bent, in any position, or closed as in clasping, the fingers form arches, each one concentric on this same basal joint of the thumb. Clenched, each circle of knuckles forms an arch with the same common centre.
The mass of the thumb dominates the hand.
The thumb has three segments and as many joints. Its bones are heavier than those of the fingers, its joints more rugged.
Its last segment has a nail and a heavy skin pad. The middle segment has only tendons. The basal segment is a pyramidal mass of muscle reaching to the wrist, the "line of life" of the palm, and the base of the first finger.
The superficial muscles of this mass are a fat one, a broad one, and a thin one. The fat muscle hugs the bone (opponens), the broad one forms the bulk of the pyramid (abductor) and the thin one lies inside, toward the index finger (flexor brevis). Between the thumb and first finger the skin is raised into a web, which is bulged, especially when the thumb is flattened, by the adductor policis muscle.
The thumb is pyramidal at the base, narrow in the middle, pear-shaped at the end. The ball faces to the front more than sideways. It reaches to the middle joint of the first finger.
The last segment bends sharply back, carrying the nail. Its skin pad, broad at the base, gives it an appearance not unlike a foot, expressing its pressure-bearing function.
The middle segment is square with rounded edges, smaller than the other two, with a small pad.
The basal segment is rounded and bulged on all sides except where the bone is superficial at the back.
The last joint has about one right angle of movement, in one plane, and may by pressure be twisted toward the fingers.
The heavy middle joint moves less freely, also limited to one plane.
The joint of the base is a saddle joint, with movement like one in a saddle, that is, with easy bending sideways, less easy forward and back; which two in combination give some rotary movement, but giving a twisting movement only with difficulty and strain.
The Thumb. Extensors of the Thumb.
1 Extensor ossis metacarpi pollicis.
2 Extensor brevis pollicis.
3 Extensor longus pollicis.
The Thumb. Muscles of the Thumb, palmar view.
1 Flexor brevis pollicis.
2 Abductor pollicis.
3 Apponens pollicis.