The hyoid bone (fig. 342) is U-shaped and is suspended from the tips of the styloid processes of the temporal bones by the stylohyoid ligaments. It consists of a body, two greater and two lesser cornu.
The body or central part of the hyoid bone is of a quadrilateral form. Its anterior surface is convex and directed forwards and upwards. Its upper part is crossed by a well-marked ridge which has a slight downward convexity, and in many cases a vertical median ridge divides the body into lateral halves, The portion of the vertical ridge above the transverse line is present in the majority of specimens, but that below the transverse line is rarely seen. The posterior surface is smooth, concave, directed backwards and downwards, and separated from the epiglottis by the thyrohyoid membrane and a quantity of loose areolar tissue ; a bursa intervenes between the bone and the membrane. In early life the lateral extremities of the body are connected to the greater cornu by primary cartilaginous joints, but after middle life they are usually united by bone.
The lesser cornu of the hyoid bone are two small, conical eminences, attached by their bases at the angle of junction of the body and greater cornu. They are connected to the body of the bone by fibrous tissue and occasionally to the greater cornu by synovial joints, which usually persist throughout life, but occasionally become ankylosed.
The lesser cornu are situated in line with the transverse ridge on the body and appear to be morphological continuations of it.
Particular features.-The inferior surface of the body gives insertion to the geniohyoid muscle in the greater part of its extent both above and below the transverse ridge ; a portion of the origin of the hyoglossus muscle invades the lateral margin of the geniohyoid area (fig. 343). The lower hart of this surface gives insertion to the mylohyoid muscle, and below that to the sternohyoid medially and the omohyoid laterally. The superior border of the body is rounded and gives attachment to the lowest fibers of the genioglossi, to the hyo-epiglottic ligament and to the thyrohyoid membrane. The inferior border gives insertion to the sternohyoid medially and the omohyoid laterally and sometimes to the medial fibers of the thyrohyoid muscle. It also gives attachment to the levator glandulae thyroideae, when that muscle is present.
The upper surface of the greater cornu gives origin to the. middle constrictor of the pharynx and, more laterally, to the hyoglossus muscle, both of which extend throughout its whole length. Near the junction of the cornu with the body the stylohyoid muscle is inserted lateral to the hyoglossus muscle; and a little posterior to this insertion the fibrous loop through which the tendon of the digastric muscle runs is attached to the bone. The medial border gives attachment to the thyrohyoid membrane ; the lateral border receives, anteriorly, the insertion of the thyrohyoid muscle. The inferior surface, which is oblique is directed towards the thyrohyoid membrane.
The posterior and lateral aspects of the lesser cornu give origin to fibers of the middle constrictor muscle of the pharynx. Its apex gives attachment to the stylohyoid ligament which is often ossified in part. The medial aspect of its base gives origin to the chondroglossus muscle.
Ossification.-The hyoid bone is developed from the cartilages of the second and third branchial or visceral arches—the lesser cornu from the second, the greater cornu from the third and the body from the fused ventral ends of. It is ossified from six centers two for the body, and one for each cornu. Ossification commences in the greater cornu, towards the end of fetal life, in the body before or shortly after birth; and in the lesser cornu during the first or second year, or later.
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