THE HYOID BOXE 



177 



After birth (Fig. 183) the two segments of the bone become joined at the symphysis, from 

 below upward, in the first year; but a trace of separation may be visible in the beginning of the 

 second year, near the alveolar margin. The body becomes elongated in its whole length, but 

 more especially behind the mental foramen, to provide space for the three additional teeth devel- 

 oped in this part. The depth of the body increases owing to increased growth of the alveolar 

 part, to afford room for the roots of the teeth, and by thickening of the subdental portion which 

 enables the jaw to withstand the powerful action of the masticatory muscles; but the alveolar 

 portion is the deeper of the two, and, consequently, the chief part of the body lies above the 

 oblique line. The mandibular canal, after the second dentition, is situated just above the level 

 of the mylohyoid line; and the mental foramen occupies the position usual to it in the adult. 

 The angle becomes less obtuse, owing to the separation of the jaws by the teeth; about the fourth 

 year it is 140. 



In the adult (Fig. 184) the alveolar and subdental portions of the body are usually of equal 

 depth. The mental foramen opens midway between the upper and lower borders of the bone, 

 and the mandibular canal runs nearly parallel with the mylohyoid line. The ramus is almost 

 vertical in direction, the angle measuring from 110 to 120. 



In old age (Fig. 185) the bone becomes greatly reduced in size, for with the loss of the teeth 

 the alveolar process is absorbed, and, consequently, the chief part of the bone is below the oblique 

 line. The mandibular canal, with the mental foramen opening from it, is close to the alveolar 

 border. The ramus is oblique in direction, the angle measures about 140, and the neck of the 

 condyle is more or less bent backward. 



The Hyoid Bone (Os Hyoideum; Lingual Bone). 



The hyoid bone is shaped like a horseshoe, and is suspended from the tips of the 

 styloid processes of the temporal bones by the stylohyoid ligaments. It consists 

 of five segments, viz., a body, two greater cornua, and two lesser cornua. 



The Body or Basihyal (corpus oss. hyoidei). The body or central part is 

 of a quadrilateral form. Its anterior surface (Fig. 186) is convex and directed 

 forward and upward. It is crossed in its upper half by a well-marked transverse 

 ridge with a slight downward 

 convexity, and in many cases 

 a vertical median ridge divides 

 it into two lateral halves. 

 The portion of the vertical 

 ridge above the transverse line 

 is present in a majority of 

 specimens, but the lower por- 

 lion is evident only in rare 

 cases. The anterior surface 

 gives insertion to the Genio- 

 hyoideus in the greater part 



Body 



MYLOHYOIDEUS 

 GENIOHYOIDEUS 



Fia. 186. Hyoid bone. 



\ 



DIGASTRICTTS & 

 STYLOHYOIDEUS 



THYKEOHYOIDECS 

 OMOHYOIDEUS 



STERNOHYOIDEUS 



Anterior surface. Enlarged. 



of its extent both above and 

 below the transverse ridge; a 

 portion of the origin of the 

 Hyoglossus notches the lateral 



margin of the Geniohyoideus attachment. Below the transverse ridge the Mylo- 

 hyoideus, Sternohyoideus, and Omohyoideus are inserted. The posterior surface is 

 smooth, concave, directed backward and downward, and -separated from the epi- 

 glottis by the hyothyroid membrane and a quantity of loose areolar tissue; a bursa 

 intervenes between it and the hyothyroid membrane. The superior border is 

 rounded, and gives attachment to the hyothyroid membrane and some aponeurotic 

 libers of the Genioglossus. The inferior border affords insertion medially to the 

 Sternohyoideus and laterally to the Omohyoideus and occasionally a portion of the 

 Thyreohyoideus. It also gives attachment to the Levator glandulse thyreoidese, 

 when this muscle is present. In early life the lateral borders are connected to 

 the greater cornua by synchondroses; after middle life usually by bony union. 

 " 



