How it is necessary for the painter to know the inner structure of man

The painter who has a knowledge of the nature of the sinews, muscles, and tendons will know very well in the movement of a limb how many and which of the sinews are the cause of it, and which muscle by swelling is the cause of the contraction of that sinew; and which sinews expanded into most delicate cartilage surround and support the said muscle.
Thus he will in divers ways and universally indicate the various muscles by means of the different attitudes of his figures; and will not do like many who, in a variety of movements, still display the same things in the arms, the backs, the breasts, and legs. And these things are not to be regarded as minor faults.
In fifteen entire figures there shall be revealed to you the microcosm on the same plan as before me was adopted by Ptolemy in his cosmography; and I shall divide them into limbs as he divided the macrocosm into provinces; and I shall then define the functions of the parts in every direction, placing before your eyes the representation of the whole figure of man and his capacity of movements by means of his parts. And would that it might please our Creator that I were able to reveal the nature of man and his customs even as I describe his figure.
Remember, in order to make sure of the origin of each muscle to pull the tendon produced by this muscle in such a way as to see this muscle move, and its attachment to the ligaments of the bones. You will make nothing but confusion in demonstrating the muscles and their positions, origins and ends, unless you first make a demonstration of thin muscles after the manner of threads; and in this way you will be able to represent them one over the other as nature has placed them; and thus you can name them according to the limb they serve, for instance the mover of the tip of the big toe, and of its middle bone or of the first bone, etc. And when you have given this information you will draw by the side of it the true form and size and position of each muscle; but remember to make the threads which denote the muscles in the same positions as the central line of each muscle; and so these threads will demonstrate the shape of the leg and their distance in a plain and clear manner.
source: encarta encyclopedia