Involuntary muscles, with the exception of the heart, differ from voluntary not only in their anatomical structure but in their functional activity: instead of contracting or relaxing rapidly, both their contraction and relaxation are slow. We have seen that although voluntary muscle occasionally exhibits spontaneous rhythmical contractions, yet these occur only under exceptional circumstances, and but rarely. Involuntary muscle, on the other hand, has a much greater tendency to rhythmical contraction, although it may be regarded as doubtful whether some stimulus, however slight, is not required to induce this rhythm even in involuntary muscle. It has been already mentioned that the contractile tissue of medusa will beat rhythmically so long as it is connected with motor ganglia. When these ganglia are removed, the contractions cease, but will again reappear, notwithstanding the absence of the ganglia, if a constant stimulus be applied to the contractile tissue itself. This shows that the conditions for rhythm are contained in contractile tissue itself - that the rhythm may be independent of the ganglia with which the contractile tissue is connected (p. 113). The same appears to be the case with involuntary muscular fibre generally.

The ventricle of the frog's heart, containing ganglia, will beat rhythmically for a length of time after its removal from the body. If the ganglia which lie close to the auriculo-ventricular groove be cut off, the rhythmical action will cease just as in the medusa when the marginal ganglia are removed; but if a constant stimulus be applied to the apex of the heart, as for example by passing a constant current through it, or by distending it with serum, its rhythmical movement will again commence, mechanical distension appearing to have upon it the same exciting action that a little acid added to the water has upon the nerveless bell of the medusa.

The excitability of involuntary muscular fibre appears to be increased by small doses of atropine; for when the ganglia of the frog's heart are. removed the apex, instead of stopping immediately, will give a few beats before it stops if atropine has been previously given, and mechanical stimuli cause more beats in the atropinised than in the normal apex.1