Anaesthesia of the thumb suppresses servo action in the long flexor of the thumb during movements of the terminal phalanx. 2. It needs a greater subjective effort to flex an anaesthetized thumb than a normal one. 3. Anaesthesia of other digits is without effect on the thumb flexor. 4. In an anaesthetic thumb without servo responses the changes in force exerted when a mechanical perturbation is applied which are due purely to the passive mechanical properties of the muscle can be measured. Subtraction of these gives the active components of servo action in the normal thumb and thus an estimate of the mechanical gain of the servo. 5. Giving the subject a visual tracking task can partially restore servo action when the thumb is anaesthetic. 6. After some years subjects become resistant to the effect of peripheral anaesthesia. 7. Peripheral anaesthesia has no detectable effect on servo responses in the long flexor of the great toe, in infraspinatus or in pectoralis major. Servo action in these muscles is presumably based predominantly on muscle receptors. 8. Tendon jerks are not apparently influenced by peripheral anaesthesia. 9. The Discussion considers the possibility that, for the thumb, muscle afferents co-operate with a somatic and a visual input in a servo loop via the cerebral cortex.
