Rapid alternation of contraction and relaxation, or tremor, may be observed to affect either - (a) a few bundles of muscular fibres, (b) a single muscle, or (c) groups of muscles.

The tremors affecting a few bundles of fibres, or fibrillary twitchings, may occur in excised muscles, and are probably due to some conditions of the muscular fibre allied to those which have already been considered (p. 132). They may occur also in muscles which still remain in the living animal after the nerve has been cut, more especially in the muscles of the tongue after section of the hypoglossal nerve, or in the muscles of the face after section of the facial nerve.4

Tremors affecting groups of muscles occur, in some cases, when the limbs are at rest, and cease during voluntary movement, as in paralysis agitans; or may cease entirely when the limb is at rest, and only come on when the muscles are put in action, as in disseminated sclerosis and in mercurial tremor. As already mentioned, a certain number of motor impulses per second are required to keep a muscle steadily contracted.

1 Sitzungsber. d. Wiener Akad., Abth. lxxxii. p. 257-275. 2 Schonlein, du Bois Beymond's Archiv, 1882, p. 357. 3 Sitzungsber. d. Wien. Akad., Bd. lxxxvii., Abt. iii., March 1883. 4 They may possibly be regarded as due to disturbance of the normal relations between longitudinal and transverse contraction in muscular substance.

It is evident that, if the stimuli proceeding to the muscles from the nerve-centre should be too few, tremor, and not steady contraction, of the muscle will occur. And the same will be the case if any change in the muscle itself should render the duration of each single contraction less than usual.

But in all co-ordinated movements a number of muscles, the actions of which are antagonistic to each other, are brought into play; and it is by the proper adjustment of these antagonistic actions that the performance of delicate movements becomes possible. Unless the amount of contraction of each of these muscles is exactly graduated, there will be a tendency to oscillatory movement. As the amount of contraction in each muscle, or group of muscles, is regulated by the stimuli sent down to it from the nerve-centres, it is evident that if the motor cells supplying one group of muscles be affected more than those which supply the antagonistic or regulating muscles, inco-ordi-nation, and possibly tremor, will occur. The pathology of tremor is still, however, very obscure.