Usefulness of Human Studies.

... it follows that in every example only a single and specific case was shown: thus, in general painting art, one must not only understand what each muscle does in such an action but also in what degree and manner and subordination it performs the action; and when it does so more and when less, strongly or gradually: because one must be aware that every particular movement or disturbance does not only involve its own muscle or muscles to execute this or that joint to bend, to straighten, to lift, inwardly, outwardly and to rotate elsewhere and maintain it, but also that it occurs through many incidental stages. Introducing one muscle alternately allows the other to cooperate, and sometimes several together, but never all at once; and in few positions, no matter how simply observed, are all entirely still. For straightening and bending, moving inwards and outwards from a joint, are contradictory operations: similarly, greater or lesser change takes place, though it provides some variation in the action and observation: And therefore one may easily understand that while some muscles work around a joint, others remain still, and that while one performs an action, and appears swollen and visibly prominent in its resting state, others draw back inwardly and seemingly disappear or are at least very uncertain. Thus, we see again, that it is necessary, following these varied operations, to take notice of the diversity; if it is possible to reveal a single true and functioning act. But possibly, someone might say, one...

Translation Notes:

'Schilderkonst' translates to 'painting art'. 'Muskel' means 'muscle'. 'Werkingen' translates to 'operations'.