Stretch Receptors in the Muscles of Fishes

1. The presence of stretch receptors in the muscles of Raja and Scyllium has been physiologically demonstrated. Their morphology is unknown. 2. The behaviour of a single end-organ in the radial muscles of the pelvic fin of Raja clavata has been examined in detail. 3. The adaptation curve and the tension-frequency relationship have been determined. 4. Under constant tension a single end-organ has continued to function rhythmically for over 1 hour. 5. When tension is suddenly reduced, there follows a silent period before the frequency of discharge corresponding to the reduced tension appears. 6. The quantitative aspects of the behaviour of the stretch receptor of Raja bear a closer resemblance to those of the receptors of the carotid sinus than to those of mammalian and amphibian limb muscles. 7. The response of muscle receptors during a passive undulatory movement of the pectoral fin has been recorded, and their role in the swimming rhythm is discussed.