The Da Vinci Notebooks at sacred-texts.com
Three are the principal muscles of the shoulder, that is b c d, and two are the lateral muscles which move it forward and backward, that is a o; a moves it forward, and o pulls it back; and bed raises it; a b c moves it upwards and forwards, and c d o upwards and backwards. Its own weight almost suffices to move it downwards.
The muscle d acts with the muscle c when the arm moves forward; and in moving backward the muscle b acts with the muscle c.
188:174 : See Pl. XXI. In the original the lettering has been written in ink upon the red chalk drawing and the outlines of the figures have in most places been inked over.