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_.

[Footnote: 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.]