_I._ Very well, Ellinora. He adds, "This is the nature of the muscular fibre; and being so, it is evident that the reciprocal _energetic_ motion of the limbs, by which we mean _with force_ in opposite directions, can only be produced by the instrumentality of opposite or antagonist muscles--of flexors and extensors answering to each other. For instance, the biceps and brachiæus _internus_ muscles, placed in the front part of the upper arm, by their contraction, bend the elbow, and with such a degree of force as the case requires, or the strength admits. The relaxation of these muscles, after the effort, would merely let the fore-arm drop down. For the _back stroke_ therefore, and that the arm may not only bend at the elbow, but also extend and straighten itself with force, other muscles, the longus, and brevis brachiæus _externus_, and the aconæus, placed on the hinder part of the arms, by their contractile twitch, fetch back the fore-arm into a straight line with the cubit, with no less force than that with which it was bent out. The same thing obtains in all the limbs, and in every moveable part of the body. A finger is not bent and straightened without the _contraction_ of two muscles taking place. It is evident, therefore, that the animal functions require that particular disposition of the muscles which we describe by the name of antagonist muscles." _A._ Thank you, Isabel. This does indeed make the subject very plain. These muscles contract at will. _E._ But how can the will operate in this manner? I have always wished to understand. _I._ And I regret that I cannot satisfy you on this point. If we trace the cause of muscular action by the nerves to the brain, we are no nearer a solution of the mystery; for we cannot know what power sets the organs of the brain at work--whether it be foreign to or of itself. We will come now, if you please to _sensibility_, which belongs to the nerves. _A._ I have a very indefinite idea of the nerves. _E._ My _ideal_ is sufficiently definite in its shape, but so droll! I do not think of them as "being flesh of my flesh," but as a _species_ of the _genus_ fairy. They are to us, what the Nereides are to the green wave, the Dryades to the oak, and the Hamadryades to the little flower. They are quite omnipotent in their operations. They make us cry or they make us laugh; thrill us with rapture or woe as they please. And, my dear Isabel, I shall not allow you to cheat me out of this pleasing fancy. You may tell us just what they are, but I shall be as incredulous as possible. _I._ They are very slender white cords, extending from the brain and spinal marrow--twelve pairs from the former, and thirty from the latter. These send out branches so numerous that we cannot touch the point of a pin to a spot that has not its nerve. The mucous membrane is-- _F._ Oh, these technicals! What is the mucous membrane?