I. With the muscles, they constitute the agents of all motion in us. Place your hand on the inside of your arm, and then bend your elbow. You perceive that cord, do you not? That is a tendon. You have observed them in animals, doubtless.
Ann. I have. They are round, white, and lustrous; and these are the muscular terminations.
I. Yes; this tendon which you perceive, is the termination of the muscles of the fore-arm, and it is inserted into the lower arm to assist in its elevation.
E. Now we are coming to it. Please tell me how I move a finger—how I raise my hand in this manner.
I. It is to the contractile power of the muscles that you are indebted for this power. I will read what Dr. Paley says of muscular contraction; it will make it clearer than any explanation of mine. He says, "A muscle acts only by contraction. Its force is exerted in no other way. When the exertion ceases, it relaxes itself, that is, it returns by relaxation to its former state, but without energy."
E. Just as this India-rubber springs back after extension, for illustration.
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.