OF THE MUSCLES OF ANIMALS.
The hollow spaces interposed between the muscles must not be of such a character as that the skin should seem to cover two sticks laid side by side like c, nor should they seem like two sticks somewhat remote from such contact so that the skin hangs in an empty loose curve as at f; but it should be like i, laid over the spongy fat that lies in the angles as the angle n m o; which angle is formed by the contact of the ends of the muscles and as the skin cannot fold down into such an angle, nature has filled up such angles with a small quantity of spongy and, as I may say, vesicular fat, with minute bladders [in it] full of air, which is condensed or rarefied in them according to the increase or the diminution of the substance of the muscles; in which latter case the concavity i always has a larger curve than the muscle.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.