A 3.5-5 mm passive stretch of dorsal neck muscles at velocities of 50 mm/sec, a stimulus adequate to excite most neck muscle spindles, was usually ineffective in eliciting unit discharge in the superior colliculus. The sudden release of muscle tension was effective and excited fifty-seven of sixty-seven units tested. 2. When electrical stimulation of neck muscle nerves was used, a stimulus strength sufficient to excite Group III muscle afferents was usually required to elicit unit discharge in the superior colliculus. 3. It is concluded that the projection from neck muscles to the superior colliculus largely takes origin in Group III afferent whose function remains to be determined. Some contribution to the projection to the superior colliculus from proprioceptive afferents served by larger fibres cannot be ruled out.
