I317

A Painter is indispensibly obliged to be acquainted with Osteology: That is, with the several Bones serving as Props to bear up the Flesh, wherewith they are covered; and with the Jonctures, which occasion the Limbs in bending to enlarge and diminish; for the length of the Arm when extended, is not equal to that of the same Arm, when contracted; it always losing or gaining an eighth part of its Measure, as it is stretched out or drawn in. This shortening and lengthening of the Arm, is owing to the Bone between the Shoulder and the Elbow; which withdrawing out of its Cavity, when the Elbow is bent into an Acute Angle, [as you see in the Figure A B] adds to the length of that part of the Arm; and this Addition will be always the greater, as the Angle at the Elbow is more acute; and on the contrary, as the Angle at the Elbow grows more obtuse, this part of the Arm must become shorter.