The function of muscle is to pull and not to push, except in the case of the genitals and the tongue.
Leonardo da VinciTo speak of this subject you must... explain the nature of the resistance of the air, in the second the anatomy of the bird and its wings, in the third the method of working the wings in their various movements, in the fourth the power of the wings and the tail when the wings are not being moved and when the wind is favourable to serve as guide in various movements.
Leonardo da VinciThere are many occasions when the muscles that form the lips of the mouth move the lateral muscles that are joined to them, and there are an equal number of occasions when these lateral muscles move the lips of this mouth, replacing it where it cannot return of itself, because the function of muscle is to pull and not to push except in the case of the genitals and the tongue.
Leonardo da VinciNature is so delightful and abundant in its variations that among trees of the same kind there would not be found one which nearly resembles another, and not only the plants as a whole, but among their branches, leaves, and fruit, will not be found one which is precisely like another.
Leonardo da Vinci