The hyoid is part of the skull and it is made up of lots of long smaller bones that are joined end to end with each end of the apparatus connecting to the skull. The stylohyoid is the largest bone of the rather complicated hyoid. It forms a bony frame work that helps support the tongue, the larynx, and the pharynx. Diseases of the hyoid apparatus, referred to as temporohyoid osteoarthropathy, can result in neurologic deficits of the cranial nerves of horses. Depending on the location of the disease and the nerves affected clinical signs vary form horse to horse. Horses have been presented with complaints of head shaking, incoordination (ataxia), head tilt, facial muscle paralysis, ear infections, and corneal ulceration. Recently a retrospective study of cases of admitted to Washington State University and Texas A&M University between 1993 and 2000 were reviewed and the result presented to the American Association of Equine Practitioners in 2001, 21 cases were found. This report summarizes those findings and adds information from other reports of this disease.

Hyoid bone of horse green...............basihyoid purple..............lingual process of basihyoid light blue........thyrohyoid dark blue.......cartilage of thyrohyoid red...................ceratohyoid junction of red and yellow......epihyoid light yellow....stylohyoid dark yellow...tympanohyoid a cartilagenous bone that forms the hyoid side of the temporohyoid joint (From Ellenberger and Baum, 1908.)

The hyoid apparatus sits under the skull and between the mandibles. It forms a bony frame work that helps support the tongue, the larynx, and the pharynx and helps coordinate the movements during swallowing. It attaches to the temporal portion of the skull by the cartilagenous tympanohyoid. This joint is called the temporohyoid joint. From this point the stylohyoid travels ventrally and toward the mouth. It sits within the fascia and has fibrous attachments to the thyroid cartilage of the larynx. The lingual process of the basihyoid is embedded in the root of the tongue.