Final answer:
Mitosis is not always present in Horner's syndrome; it is a condition with symptoms caused by damage to sympathetic nerve pathways, potentially by a tumor in the thoracic cavity.
Step-by-step explanation:
False, mitosis is not always present in Horner's syndrome. Horner's syndrome is a collection of signs associated with disruption to the sympathetic nerves supplying the eye and part of the face. This syndrome can be caused by various issues including tumors, trauma, or vascular diseases affecting the sympathetic nervous system pathway. A tumor in the thoracic cavity could impact these autonomic functions because it could compress or damage the sympathetic nerve fibers that travel from the hypothalamus, down the spinal cord, and exit at the thoracic level to eventually reach the eye and face.