CHAP, vii.] ANTAGONISM OF MUSCLES. 169 



material, and from all immediate intercourse with it. How then 

 shall we explain the transmission of the nervous influence to a ma- 

 terial thus enclosed ? If it were wise or safe to go a single step in 

 advance of pure observation on so abstruse a question, we might 

 suggest, resting on the seemingly sure ground of exact anatomy, 

 that this influence must be of a nature capable of emanating beyond 

 the limits of the organ which furnishes it. But further than this, 

 as to how, or to what extent this influence may so emanate, or as 

 to what may be its nature, it would, perhaps, in the present state 

 of knowledge, be hardly warrantable even to speculate. 



The number of fibres in a muscle is always exactly proportioned 

 to the power demanded, and their length to the amount of shortening 

 required of them ; but, these circumstances being secured, muscular 

 parts are subject to great variety of form, being short, thick, or 

 rounded, long, slender, or flat, according to their position relatively 

 to particular bones, joints, other muscles, or the like. Thus, all are 

 compactly knit and adapted to work in concert, without any me- 

 chanical interference with one another; and perfect symmetry, 

 both of shape and action, is provided for. 



We notice that the muscles are arranged on the skeleton in a 

 great measure in sets, having opposite actions ; as, for example, the 

 flexors and extensors, pronators and supinators of the fore-arm. It 

 is evident that the action of every muscle depends solely on its 

 mechanical attachments, and that a tendon running round to a dif- 

 ferent side of a limb might quite reverse a given action. But we 

 find that in general the muscles of the same set are designed to act 

 together, not only by their attachment on the same side of a joint, 

 but by a supply of nerves from the same source (congeneres). 

 Yet even this can confer no special action of extension or flexion, 

 but only an association of action which may be both at once ; for 

 example, the flexors of the toes extend the ankle, and the extensors 

 of the toes bend it. Even a single muscle, the rectus femoris, 

 flexes the thigh, and by the same action extends the leg. Mus- 

 cles opposed in action are termed antagonists. This antagonism is 

 in most cases required by the necessity there is for an active 

 moving power in opposite directions; but it serves the important 

 accessory purpose of elongating muscles when they cease to be con- 

 tracted, as we see illustrated by the presence of elastic or some 

 other force for the same purpose, when there is no antagonist 

 muscle. When antagonists act together, the part is fixed. 



The locomotive framework may be regarded as a series of levers, 



