Constructing these masses first as blocks, we will have the mass of the shoulder, or deltoid muscle, with its long diameter sloping down and out, beveled off at the end; its broad side facing up and out; its narrow edge straight forward.
This mass lies diagonally across and overlaps the mass of the arm, whose long diameter is vertical, its broad side outward, its narrow edge forward.
The mass of the forearm begins behind the end of the arm and passes across it at an angle forward and out. It is made of two squares. The upper half of the forearm is a block whose broad side is forward, its narrow edge sideways. The lower half, smaller than the upper, has its narrow edge forward, and its broad side facing out (with the hand held thumb up).
These blocks are joined by wedges and wedging movements, and to the straight lines are wedded the curved lines of the contour of the muscles. The deltoid is itself a wedge, whose apex sinks into the outer groove of the arm half way down. The mass of the biceps ends in a wedge which turns outward as it enters the cubital fossa.
The mass of the forearm overlaps the end of the arm on the outside by a wedge (supinator longus) that arises a third of the way up the arm, reaches a broad apex at the broadest part of the forearm and tapers to the wrist, pointing always to the thumb; and on the inside by a wedge that rises back of the arm and points to the little finger (flexor-pronator muscles).
In the lower half of the forearm, the thin edge of the mass, toward the thumb, is made by a continuation of this wedge
from the outside. The thin edge toward the little finger is made by the end of the wedge from the inside.
When the elbow is straight and the hand turned in, the inner line of the forearm is straight with that of the arm. When the hand is turned out, this line is set out at an angle that corresponds with the width of the wrist. The little finger side (ulna) is the hub of its movement.
The flexor tendons on the front of the forearm point always to the inner condyle; the extensor tendons on the back point always to the outer condyle.
The breadth of the hand corresponds with that of the lower mass, not joining it directly, but with a step-down toward the front.
In the back view of the arm, the mass of the shoulder sits across its top as in the front view. The back edge of this mass is seen to be a truncated wedge arising under the deltoid and focusing on the elbow. The upper end resolves itself into the three heads of the triceps; the lower or truncated end is the triceps tendon, to which is to be added the tiny wedge of the anconeus (donkey's foot) muscle bridging from outer condyle to ulna.
Wedging of the Arm into the Forearm Back View
Wedging of Arm into the Forearm at the Elbow
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