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Reasons for Performing Study: Disorders of the equine sphenopalatine sinus, including empyema and neoplasia, have been reported to cause damage to cranial nerves II and V. However, the clinical anatomy of these sinuses is not well described in horses., Objective: To examine the anatomy of the sphenopalatine sinuses in a range of equidae and, in particular, to examine the relationship of these sinuses to adjacent major nerves and vessels., Methods: The anatomy of the sphenoidal and palatine paranasal sinuses was examined in 16 equidae, primarily using transverse skull sections. Relevant structures were documented and photographed., Results: There was much variation between individual horses in sphenopalatine sinus anatomy. The sphenoidal sinuses were small in young horses and appeared to become larger and more complex with age. Variation was present in the extent that the sphenopalatine sinus extended into the basisphenoid bone. The septum dividing left and right sphenoidal sinuses was frequently not midline, but was intact in all cases. The sphenoidal and palatine sinuses communicated in most horses. In such cases, what could accurately be termed the (combined) sphenopalatine sinuses usually drained directly into the caudal maxillary sinuses. Additionally, in 5 out of 16 cases, some compartments of the sphenoidal sinus also drained into the ethmoidal sinus. The dorsal and lateral walls of the sphenoidal sinus were very thin and directly adjacent to cranial nerves II, III, IV, V and VI and major blood vessels., Conclusions: The equine sphenoidal and palatine sinuses are very variable in their anatomy, but are always in close proximity to multiple cranial nerves and major blood vessels., Potential Relevance: Many cranial nerves and blood vessels could be damaged with disorders involving the sphenopalatine sinus, potentially causing major and variable neurological syndromes, haemorrhage and extension of sepsis.