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 design and movement of even the forearm is to give the freest sweep to the thumb; while, through the biceps muscle, its movement is seen to begin really at the shoulder.
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 pollicis 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 one 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.
Extensors of the Thumb:
I Extensor ossis metacarpi pollicis.
1 Extensor brevis pollicis.
3 Extensor longus pollicis.
Muscles of the Thumb, palmar view:
I Flexor brevis pollicis.
a Abductor pollicis.
3 Apponens pollicis.
Each of the four fingers has three bones (phalanges, soldiers). Each phalanx turns on the one above, leaving exposed the end of the higher bone. There are no muscles below the knuckles; but the fingers are traversed by tendons on the back, and are covered on the front by tendons and skin pads.
The middle finger is the longest and largest, because in the clasped hand it is opposite the thumb and with it bears the chief burden. The little finger is the smallest and shortest and most freely movable for the opposite reason. It may move farther back than the other fingers, and is usually held so, for two reasons; one is that the hand often "sits" on the base of the little finger; the other is that being diagonally opposite the thumb it is twisted
farther backward in any outward twisting movement, and so tends to assume that position.
All bones of the body are narrower in the shaft than at either end, especially those of the fingers. The joints are square, the shafts smaller but square, with rounded edges; the tips are triangular. The middle joint of each finger is the largest.
In the clenched fist it is the end of the bone of the hand (metacarpal) that is exposed to make the knuckle. The finger bone (first phalanx) moves around it, and bulges beyond. The extensor tendon makes a ridge on the knuckle and connects it with the first phalanx; but on the middle and the last joints the tendon makes a depression or groove in
the centre of the joint.
The masses of these segments are not placed end to end, as on a dead centre, either in profile or in back view. In the back view, the fingers as a whole arch toward the middle finger.
In the profile view, there is a step-down from each segment to the one beyond, bridged by a wedge.
A series of wedges and squares thus marks the backs ot the fingers. Into the square of the knuckles a blunt wedge is seen to enter from above. From it a long tapering wedge arises and enters the square of the middle joint, from which a blunt wedge also reaches backward. Another tapering wedge arises here and moves half way down the segment. The
whole finger tapers from the middle joint, to become embedded in a horseshoe form holding the nail. This form begins back of the root of the nail and bevels to below its end, at the tip of the finger. The whole last segment is a wedge.
The palmar webbing opposite the knuckles, which reaches to about the middle of the first segment of the finger, in front, bevels backward and points to the top of the knuckle in the back.
The segments of individual fingers are of different lengths, those of the middle finger being longest. From tip to base, and on into the bones of the hand, the segments increase in length by definite proportions.
Each joint moves about one right angle except the last, which moves slightly less; and limited to one plane, except the basal, which has also a slight lateral movement, as in spreading the fingers.
Pad Between Thumb and First Finger:
I First dorsal interosseus.
Mechanism of the First Finger
While the segments of any finger, seen on the back, are of different lengths, the pads seen on the palmar side are of the same length, including the pad of the base which is part of the palm, so that the creases between them are not all opposite the joints. The reason is immediately seen when the finger is viewed closed on itself. The creases are then seen to form a cross, the pads to meet in the common centre, filling in the four sides of a diamond.
In the first finger the creases are: short of the last joint; opposite the middle joint; half way between middle and basal joint, and opposite the basal knuckle (above the joint proper, which is considerably beyond the point of the knuckle).
In the second finger they are: opposite the last joint; beyond the middle joint; midway between middle and basal joint, and opposite the basal joint.
In the other fingers they vary in different individuals.
The creases are all transverse except that opposite the basal joint, which forms one long wavy crease on the palm; and those next beyond, on first and little fingers, which slope down on the outside, in the spread fingers making a curve around the base of the thumb.