Complex sentence: Ethmoid hematoma is a progressive and locally destructive disease of horses, showing as a mass in the paranasal sinuses, the cause and startings of which is generally unknown, that resembles a tumor but is not neoplastic by any means.
Split into simpler sentences: Ethmoid hematoma is a progressive and locally destructive disease of horses. It is indicated by a mass in the paranasal sinuses, that resembles a tumor but is not neoplastic by any means.