172 LOCOMOTION. [CHAP. vn. 



caused by an active contraction excited by the stimulus of the 

 injury. It has been also styled tonicity. It is well exemplified in 

 all those contracted states of muscles which follow paralysis of their 

 antagonists, as when the features are drawn towards the healthy 

 side in hemiplegia. The passive contraction of muscles is continu- 

 ally opposed to their elongation by the active or passive contrac- 

 tion of antagonists, and restores them when that subsides. By it 

 they are accommodated to an attitude artificially given, when no 

 muscular effort is required to maintain it. When no active con- 

 traction is present in a limb, the passive contraction remains ; and 

 being brought to a state of equilibrium in all the muscles, by their 

 mutual antagonism, the limb is said to be at rest. This is the 

 general condition during sleep, in which the posture assumed by the 

 limbs is determined by the relative power of antagonist muscles : 

 the flexors overcome the extensors, and hence the limbs are bent. 



Active Contraction is attended with those striking manifestations 

 of power that specially characterize muscle. It is always excited 

 by a local or partial stimulus, arid is always exerted in opposition 

 to another force within the body, which it is able more or less com- 

 pletely to master. The opposing force is generally the passive con- 

 traction of antagonist muscles, as well as the weight or resistance of 

 some part upon which the muscles act directly ; but it may be the 

 elasticity of parts, or, in the case of hollow muscles, the resistance 

 of their own contents. Active contractions are also frequently 

 opposed to one another in the maintenance of a fixed posture. 

 Active contraction is partial and interrupted, both in extent and 

 duration. It requires intervals of rest, being attended with exhaus- 

 tion of the power which produces it; which exhaustion, in the 

 voluntary muscles, is attended with the sensation called muscular 

 fatigue. 



The contractility of muscles, therefore, is being ever exerted, in 

 obedience to the equable stimulus of tension, without fatigue, in the 

 production of what we have termed passive contraction ; when it is 

 affected by a powerful, partially-applied stimulus, active contraction 

 results, inducing the necessity for subsequent rest. But there seems 

 no good ground for supposing the contractile force to differ in its 

 nature, when exhibited under these different modes of action. 



Stimuli to Muscular Contraction. Whatever is capable of in- 

 ducing contraction in the muscles, when either naturally or un- 

 naturally applied to them, is termed a stimulus. In the living 

 body, the muscular fibres are in most instances made to con- 



