O Anatomical Painter! beware lest the too strong indication of the
bones, sinews and muscles, be the cause of your becoming wooden in
your painting by your wish to make your nude figures display all
their feeling. Therefore, in endeavouring to remedy this, look in
what manner the muscles clothe or cover their bones in old or lean
persons; and besides this, observe the rule as to how these same
muscles fill up the spaces of the surface that extend between them,
which are the muscles which never lose their prominence in any
amount of fatness; and which too are the muscles of which the
attachments are lost to sight in the very least plumpness. And in
many cases several muscles look like one single muscle in the
increase of fat; and in many cases, in growing lean or old, one
single muscle divides into several muscles. And in this treatise,
each in its place, all their peculiarities will be explained--and
particularly as to the spaces between the joints of each limb &c.
Again, do not fail [to observe] the variations in the forms of the
above mentioned muscles, round and about the joints of the limbs of
any animal, as caused by the diversity of the motions of each limb;
for on some side of those joints the prominence of these muscles is
wholly lost in the increase or diminution of the flesh of which
these muscles are composed, &c.

[Footnote: DE ROSSI remarks on this chapter, in the Roman edition of
the Trattato, p. 504: "_Non in questo luogo solo, ma in altri ancora
osservera il lettore, che Lionardo va fungendo quelli che fanno
abuso della loro dottrina anatomica, e sicuramente con cio ha in
mira il suo rivale Bonarroti, che di anatomia facea tanta pompa_."
Note, that Leonardo wrote this passage in Rome, probably under the
immediate impression of MICHAELANGELO'S paintings in the Sistine
Chapel and of RAPHAEL'S Isaiah in Sant' Agostino.]