When a frog's gastrocnemius at 20° with active circulation is stimulated through its nerve by a single break induction shock, the individual muscle fibres contract synchronously within ±1σ throughout their entire length. The response of the whole muscle is, therefore, an accurate index course of contraction in the individual fibres.

The latent period of shortening may be divided into four phases, which in a gastrocnemius at 20°C. have the following characteristics :

1. Conduction Time in the Nerve.—With 3 cm. average conducting distance as in these experiments the interval would be 1σ (30 m. per sec.).

2. Delay at the End Plate.—An interval 4σ was observed between the stimulus and the muscle action current. The end-plate delay was therefore 3σ.

3. “True” Latency.—The interval after the beginning of the electrical response during which the mechanical conditions are unaltered.

This is the same whether thickening is recorded, or whether determined by the response of the tendon, and has an absolute value 1·5 to 2·0σ.

4. Rigidity. When a second response falls during the relaxation of response I., the muscle ceases to relax 1·5 to 2· 0σ after the active current, and becomes “rigid” ‘for a period 4 to 5σ before shortening.

This accounts for the supposed difference between the latent period as determined by thickening and by shortening of the tendon. The duration of the rising phase of the muscle action current corresponds closely with the duration of the period of “true” latency as well as with the absolute refractory period.

I wish to acknowledge my indebtedness to Professor Sherrington and to offer my thanks for his invaluable and kindly criticism.