Horner's syndrome is a eye disorder that consists of enophthalmos (sinking of the eyeball into its cavity), ptosis (droopy upper eyelid), swelling of the lower eyelid, miosis (abnormal contraction of the pupil), anhidrosis (absence of facial sweat) and heterochromia (difference in eye color).

Horner's syndrome is caused by paralysis of the cervical sympathetic nerves.

To better understand Horner's syndrome, one needs to understand how the nervous system works.

There are two major divisions within...