Muscles of Animals and the Hollows Between Them
How skin and spongy fat fill the concavities between muscles (c, f, i; n m o)
Under the red-chalk heading "Muscles of animals," Leonardo analyses the concavities between muscles, aided by three small lettered diagrams at the upper right. The hollows should not make the skin look as though it clothes two rods meeting in contact (as c), nor two rods drawn apart with the skin sagging in a wide curve (as f), but should rest as at i on the spongy fat that nature sets in the angle n m o where the muscles' contact ends. Because the skin cannot descend into that angle, nature fills it with a little spongy, bladder-like fat whose tiny air-filled sacs condense or rarefy as the muscle's substance grows or shrinks, so the concavity i always curves more than the muscle.
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The right and wrong hollows between two muscles (c, f, i)
The concavity between muscles must not look like skin clothing two rods in common contact, as at c, nor like two rods drawn apart with the skin sagging in a wide curve, as at f. It should sit like i, resting on the spongy fat in the angle where the muscles' contact ends.
Air-filled spongy fat fills the intermuscular angle n m o
The angle n m o arises where the contact of the muscles ends, and the skin cannot descend into it. Nature fills the angle with a little spongy, bladder-like fat of minute air-filled sacs, which condense or rarefy as the muscle's substance grows or shrinks, so the concavity i always has a greater curvature than the muscle.