Measurements of the Body and the Bending of the Arm
Why an outstretched arm and a bent arm cannot share one measure
This leaf of the Trattato continues Leonardo's rules on the outward figure, warning the painter to shun monstrous proportions—long legs, short torsos, narrow chests, long arms—and to vary the measures of the joints as nature does. A new chapter, 'On the measurements of the human body and the bending of the limbs,' insists the painter must know the supporting bones and the flesh's armature, since a straightened arm and a bent arm do not agree in measure. Leonardo states the arm gains or loses an eighth of its length between full extension and full flexion, illustrated by a small pen diagram of a bent arm at the right margin, lettered a, b and c.
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Flee monstrous proportions and vary the measures of the joints
The imitator should attend to the variety of features and avoid monstrous things such as long legs, short torsos, narrow chests and long arms. He should take the measures of the joints and their thicknesses, which vary greatly, and vary them likewise; for figures all built to one measure could not be told apart, which is never seen in nature.
The painter must know the supporting bones and flexing joints
Necessity constrains the painter to know the supporting bones and the armature of flesh that rests upon them, and the joints that increase and diminish in their bendings. For this reason the measure of the outstretched arm does not agree with the measure of the bent arm, marked c.
The arm shortens by an eighth between extension and flexion
Between the extremes of full extension and full bending the arm grows and diminishes by an eighth part of its length. The bone that projects from the elbow joint makes a long span from shoulder to elbow, marked a b in the figure; it lengthens the more as the elbow angle falls below a right angle, and shortens the more as that angle grows greater.