Müller's muscle can be thought of as a large serial type of muscle spindle of the levator muscle. Effective stretching of the mechanoreceptor in the proximal part of Müller's muscle by voluntary phasic contraction of the levator muscle for initial opening of the eye induces involuntary tonic contraction of the levator muscle as a stretch reflex via the mesencephalic trigeminal nucleus, to maintain an adequate visual field. After disinsertion of the levator aponeurosis from the tarsus by habitual rubbing, elongation of Müller's muscle secondary to thinning (aponeurotic blepharoptosis) or paralysis (Horner syndrome) desensitises the mechanoreceptor of Müller's muscle, resulting in blepharoptosis. Shortening of the elongated and thinned Müller's muscle by instillation of phenylephrine, and surgical shortening, and fixation of the disinserted, elongated, and thinned aponeurosis using the orbital septum, restored involuntary tonic contraction of the levator muscle in nearly all of 2000 patients with aponeurotic blepharoptosis and in 11 patients with Horner syndrome.