The horse has 7 pairs of paranasal sinuses, including the large caudal maxillary sinus (CMS) that communicates dorsally with the frontal and indirectly with the dorsal conchal sinus (DCS), with the latter two sinuses being collectively termed the concho-frontal sinus (CFS). The sphenopalatine sinus consists of two small inter-connected sinuses, that runs below the base of the cranium and drain into the CMS by an ostium that lies on the medial aspect of the caudal cheek tooth (upper lis) alveoli. Another group of small sinuses i.e. the ethmoidal sinuses, drain into the sphenopalatine and then into the CMS. The larger (2nd) ethmoturbinate (endoturbinate) that is visible on nasal endoscopy (Figure 1), contains one of the ethmoidal sinuses. The above anatomically connected caudal group of 5 sinuses (i.e. CMS, frontal, DCS, sphenopalatine and ethmoidal sinuses) act as a single functional sinus and all drain via the CMS through a rather tortuous, narrow pathway into the caudal aspect of the middle meatus.
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