In surgical third nerve palsy, ptosis and mydriasis are evident and the eye looks 'down and out'. These features reflect loss of innervation to all the major structures supplied by the oculomotor nerve.

Oculomotor nerve palsy causes ptosis due to paralysis of levator palpebrae superioris. The eye is rotated down and out due to the unopposed actions of the superior oblique and lateral rectus muscles.

Surgical (compressive) causes of a third nerve palsy disrupt the parasympathetic pupillomotor fibres on the periphery of the nerve causing mydriasis.

In contrast, medical (ischaemic) causes of a third nerve palsy leave the superficially located parasympathetic fibres relatively unaffected and the pupil is spared.

Ptosis, anhydrosis and miosis is the classic triad of Horner's syndrome and reflects loss of sympathetic innervation to the tarsal muscle of the upper lid, facial skin and dilator pupillae, respectively.