NATURAL THEOLOGY. 135 



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 motion 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 brachialis internus 

 muscles placed in the front part of the upper arm, 

 by their contraction, bend the elbow, and with such 

 degree of force as the case requires or the strength 

 admits of. The relaxation of these muscles after 

 the eftbrt would merely let the forearm drop down. 

 For the hack 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 brachialis externus, and the an- 

 conseus, placed on the hinder part of the arms, by 

 their contractile twitch, fetch back the forearm 

 into a straight line with the cubit, with no less force 

 than that with w^hich it was bent out of it. The 

 same thing obtains in all the limbs, and in every 



minute correspondence of the muscles. The muscles are antago- 

 nists certainly, but there is a fine combination and adjustment in 

 tlieir action, which is not illustrated by the two saM'yers dividing a 

 log of wood. The muscle having finished what we call its action 

 or contraction, is not in the condition of a loose rope, but on the 

 contrary tliere is always a perfe.ct balance of action preserved be- 

 tween the extent of relaxation of the one class of muscles, and the 

 contraction of the other; and there is a tone in both by which the 

 limb may be sustained in any posture that is willed. Tliis subject 

 is treated in the Philosophical Transactions, and also in the Trea- 

 tise on the Hand, under the head of the " Muscular Sense." 



