Rhythmical contraction is frequently regarded as a function of involuntary muscular fibre only; this, however, is not the case, for it is observed also in voluntary muscles. Rhythmical contraction of involuntary muscle is seen in the trachea,3 and is well marked in the heart and blood-vessels. It is very distinct in the intestines and bladder, and becomes still more marked after the influence of the central nervous system has been destroyed. In the case of the sphincter ani, for example, the rhythm is strong and regular, especially after the nerves have been divided and the muscle subjected to some mechanical distension by the introduction of the finger.

1 Zabludowski, Central.f, d. med. Wiss., 1883, No. 14, p. 241.

2 Aeby, Untersuchungen iiber die Fortpflanzungsgeschwindigkeit der Reizungen der quergestreiften Muskelfaser. Braunschweig, 1862, p. 52.

3 Horwath, Pfliiger's Archiv, 1875, vol. xiii. p. 508.

In voluntary muscle the tendency to large rhythmical pulsations is slight, although we see rapid contractions occurring in tetanus.

The number of impulses sent down to the muscles along the motor nerves, from the spinal cord, is about 10 per second in the dog. If more numerous impulses are sent down from the cerebral cortex, or corona radiata, or if more numerous stimuli are applied to the spinal cord itself, summation appears to occur in the cells of the spinal cord, and only 10 impulses per second are sent out.1

From the observations of Wedenskii, that irritation of the motor nerve of a muscle by exceedingly rapid stimuli still produces the same number of contractions in the muscle, it seems probable that this rate of contraction is due to the constitution either of the muscle itself, or of the nerve-endings within it. Under certain circumstances, however, the voluntary muscle may be made to contract with a slow rhythmical movement of considerable extent, and closely resembling that of involuntary muscular fibre.

Fig. 50.   Tracing of the contraction curve of a muscle poisoned by veratrine and exposed to a high temperature. The poison tends to cause prolonged contraction, and the high temperature to cause rapid relaxation of the muscle.

Fig. 50. - Tracing of the contraction curve of a muscle poisoned by veratrine and exposed to a high temperature. The poison tends to cause prolonged contraction, and the high temperature to cause rapid relaxation of the muscle. The result is a somewhat rhythmical spontaneous contraction. The muscle was only irritated at the very beginning of the first contraction.

Thus voluntary muscle treated by veratrine tends to remain contracted for a length of time like an involuntary muscle : heat has a tendency to cause its relaxation, and sometimes, as is seen in the accompanying figure (Fig. 50), these contending influences produce in the voluntary muscle a tendency to marked rhythmical contraction.

A still more remarkable phenomenon has been noticed by Kuhne,2 who finds that when the uninjured sartorius of a frog is placed in a solution of 5 grammes NaCl, and 2.5 grammes of common alkaline crystallised phosphate of sodium in a litre of water, it begins to contract at once, and after it has been transversely divided it beats with the regularity of the heart.

1 Horsley and Schafer, Proc. Roy. Soc, vol. xxxix. p. 406.

2 Untersuchungen aus dem Physiologischen Institute der Universitat Heidelberg. Sonderabdruck, 1879, p. 16.

The effect of various substances on the rhythmic action of muscle treated in this way has been investigated by Biedermann. He finds that the best fluid for the sartorius is 5 grammes NaCl, 2-2.5 grammes of Na2HP04, .04-.05 gramme of Na2CO3. A low temperature, not rising above 10° C, is best. The lower the temperature the slower is the rhythm and the more extensive the contraction. Heat quickens the rhythm and lessens the contraction. At about 18° to 20° C. the contractions become rapid and indistinct. When caustic soda is used instead of carbonate, the effect is similar, but the muscle dies much more quickly. Potassium carbonate and other potassium salts only cause pulsations when greatly diluted. Lactic acid stops the pulsations; alkaline NaCl solution again restores them. Veratrine and digitalin in a solution of NaCl also cause pulsations.1

Schonlein finds that, with a certain strength of current interrupted about 880 times in a second, the muscles of the water beetle are not tetanised, but contract rhythmically from two to six times in a second.2

Biedermann has succeeded in making a voluntary muscle, such as the sartorius, contract rhythmically by applying a solution of sodium bicarbonate (2 per cent.) to the tibial end, and then passing a constant ascending current through the muscle.3