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Hyoid bone Hyoid bone is situated chiefly between the rami of the mandible, built dorsal part extend some what further caudal, it is attached to the styloid process of the petrous part of the temporal bone by the rods of cartilage, the tympanohyoid and supports the root of the tongue, the pharynx and the larynx ,it consist of many parts: 1-the basihyoid (body): is short transverse bar, compressed dorsoventrally. They basihyoid, lingual process and thyrohyoid are fused to spur or a fork with very a short handle. 2-lingual process: a project rostral medially from the basihyoid and it is embedded in the root of the tongue during life. it is compressed laterally and has a blunt pointed free end. 3-The thyrohyoid: these correspond to the great cornea of extend caudally and dorsally from the lateral parts of the basihyoid. They are compressed laterally and the caudal end has a short cartilaginous prolongation which is connected with the rostral cornu of the thyroid cartilage of the larynx. 4-The ceratohyoids (small cornu): are short rods which are directed dorsally and rostrally from either end of the body, the ventral end has a small concave facet which articulate with the basihyoid, the dorsal end articulate with stylohyoid or with epihyoid when present. 5-The stylohyoid: are much the largest parts of the bone .they are directed dorsally and caudally are connected dorsally with the base of the petrous part of the temporal bones: A-the dorsal extremity: is large and forms two angles: 1-the articulate angle is connected by a rod of cartilage (the tympanohyoid), with the styloid process of the petrous part of the temporal bone. 2-the muscular angle is thickened and rough for muscular attachment. B-the ventral extremity is small and articulate with ceratohyoid or epihyoid. 6-The epihyoid: are small, wedge shaped pieces or nodules interposed between the ceratohyoids and stylohyoids. They are usually transitory and unite with the styloids in the adult. Hyoid muscles: This is group consisting of eight muscles, one of which, the hyoideus transversus is un paired: 1-The mylohyoideus muscle: together with its fellow from a sort of sling between the molar parts of the mandible, in which the tongue is supported. Origin: the medial surface of the alveolar border of the mandible. Insertion: the lingual process, basihyoid bone and thyrohyoid bone. Action: it raises the floor of the mouth, the tongue and the hyoid bone. 2-Stylohyoideus muscle: is a slender and fusiform muscle having direction nearly parallel to that of the thyroid bone. Origin: the muscular angle of the dorsal extremity of the thyrohyoid bone. Insertion: the rostral part of thyrohyoid bone. Action: it draws the base of the tongue and the larynx dorsally and caudally. 3-The occiptohyoideus muscle: is a smaller triangular muscle, which lies in the space between the jugular process and thyrohyoid bone. Origin: the jugular process of the occipital bone. Insertion: the dorsa extremity and ventral edge of the thyrohyoid bone Action: it carries the ventral extremity of the thyroid bone caudally. 4-The geniohyoideus muscle: is long spindle shaped muscle, which lies under the tongue. Origin: medial surface of the molar part of the mandible close to the symphysis. Insertion: the extremity of lingual process of the hyoid bone. Action: it draws the hyoid bone and tongue rostrally. 5-The ceratohyoideus muscle: a small triangular muscle lays in the space between the thyrohyoid and ceratohyoid bones, under cover the hypoglosseus. Origin: the caudal edge of the ceratohyoid bone and ventral border of the thyrohyoid bone. Insertion: the dorsal edge of the thyrohyoid bone. Action: it raises the thyrohyoid bone and the larynx. 6-The hyoideus transversus muscle: is small unpaired muscle, which extends transversely between the two ceratohyoid bones. Origin: ceratohyoid bones. Insertion: thyrohyoid bone. Action: when it contracts, it elevates the root of the tongue. 7-The sternothyrohyoideus muscle: is long, slender and digastrics muscle applied to the ventral surface of the trachea. Origin: the cartilage of manubrium. Insertion: the basihyoid bone and lingual process of the hyoid bone. Action: to retract and depress the hyoid bone, the base of tongue and the larynx. 8-The omohyoideus muscle: is thin, ribbon like. Origin: the sub scapular fascia close to the shoulder joint. Insertion: the basihyoid bone and lingual process of the hyoid bone. Action: to tact the hyoid bone and the root of the tongue. Muscles of mastication (mandibular muscle): The muscles of this group are six in number in the horse, they are extending from the maxilla and the cranium, and are all inserted into mandible. 1-The masseter muscle: it is semi-elliptical in out line. Origin: zygomatic arch and facial crest. Insertion: lateral surface of the ramus of the mandible. Action: to bring the jaws together. 2-The temporal muscle: Occupies the temporal fosse. Origin: the temporal fosse Insertion: the coroniod process of the mandible. Action: raise the mandible. 3-Pterygoidus medialis muscle: occupies the medial surface of the ramus of the mandible similar to that of the masseter laterally. Origin: the crest formed by the pterygoid process of the basisphenoid and palatine bone. Insertion: medial surface of the ramus of the mandible. Action: to raise the mandible. 4- Pterygoidus lateralis muscle: is similar than the preceding one .it is situated lateral to its dorsal part. Origin: the lateral surface of pterygoid process of the basisphenoid bone. Insertion: the mandible. Action: to draw the mandible rostrally. 5-The digastricus muscle: is composed of two fusiform flattened bellies united by round tendon. The occiptomandibular part of the caudal belly extends from jugular process of occipital bone to the caudal border of the mandible. It covered by parotid gland. Origin: jugular process of occipital bone. Insertion: medial surface of the ventral border of molar part of mandible. Action: it assists in depressing the mandible and opening mouth.