The present experimental investigations were suggested by an interesting clinical observation. One of us (F. C. G.)1recently reported a case in which chewing movements were accompanied by an involuntary elevation of the upper eyelid and the condition was completely relieved by section of the motor root of the fifth nerve. This reaction, known as the Marcus Gunn phenomenon, is characterized by ptosis, usually congenital, of the affected eye and by slow elevation of the upper eyelid synchronous with chewing movements. Two principal theories have been advanced as an explanation for the mechanism of the phenomenon. One theory suggests a cortical pattern, in analogy with the Bell phenomenon; the other implies a connection between the medullary nuclei of the third and those of the fifth cranial nerve. Neither of these theories was satisfactory to us, since it was demonstrated in the case reported that the associated movements described were