A Treatise on Painting
by Leonardo da Vinci, translated by John Francis Rigaud
Of the bending of Members, and of the Flesh round the bending Joint
3999921A Treatise on Painting — Of the bending of Members, and of the Flesh round the bending JointJohn Francis RigaudLeonardo da Vinci

Chap. L.Of the bending of Members, and of the Flesh round the bending Joint.

The flesh which covers the bones near and at the joints, swells or diminishes in thickness according to their bending or extension; that is, it increases at the inside of the angle formed by the bending, and grows narrow and lengthened on the outward side of the exterior angle. The middle between the convex and concave angle participates of this increase or diminution, but in a greater or less degree as the parts are nearer to, or farther from, the angles of the bending joints.