1. The persistent undulatory rhythm of the spinal dogfish can be reversibly augmented, accelerated and arrested by peripheral stimulation.

2. These reflexes are a property of the whole spinal cord, and are not localised in any particular level.

3. In a state of fatigue the reflex responses become modified, in particular a stimulus which, in the fresh preparation, caused excitation, inhibits the rhythm in the fatigued fish. The significance of this is discussed.

4. The behaviour of the fatigued intact dogfish approximates to that of the spinal fish. A striking resemblance in the response to inhibitory stimuli is described, and the thesis is developed that the persistent rhythm of the spinal fish is the expression of a basic rhythmic activity of the spinal cord.

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