If, therefore, we conceive that entire muscles or each of the fibers composing them, like this balloon, have a capacity appropriate for receiving animal spirits; that the pores through which the spirits thread their way are perhaps proportionately smaller than the neck of a bladder or the hole of a balloon; that the spirits are retained and pushed in the nerves approximately like the breath in the blowgun, and that the spirits are more agitated than the air of the lungs, and pushed with more force in the muscles than the air is in balloons; then we will recognize that the motion of spirits that expand in the muscles can overcome the force of the heaviest loads we carry, and that if we cannot carry heavier ones, the lack of strength is due not so much to the spirits as to the fibers and skins composing the muscles, which would burst if we made too much effort.