HOW IT IS NECESSARY TO A PAINTER THAT HE SHOULD KNOW THE INTRINSIC
FORMS [STRUCTURE] OF MAN.

The painter who is familiar with the nature of the sinews, muscles,
and tendons, will know very well, in giving movement to a limb, how
many and which sinews cause it; and which muscle, by swelling,
causes the contraction of that sinew; and which sinews, expanded
into the thinnest cartilage, surround and support the said muscle.
Thus he will variously and constantly demonstrate the different
muscles by means of the various attitudes of his figures, and will
not do, as many who, in a variety of movements, still display the
very same things [modelling] in the arms, back, breast and legs. And
these things are not to be regarded as minor faults.

How to acquire practice.