THE HYOID BONE 99 



The process of ossification of the lower jaw commences very early, between the sixth and 

 eighth week, and proceeds rapidly, so that by the fourth month the various parts are formed. 



Age-changes. — At birth the mandible is represented by two nearly horizontal troughs of 

 bone, lodging unerilpted teeth, and joined at the symphysis by fibrous tissue. The body 

 is mainly alveolar, the basal part being but little developed; the condyle and the upper edge 

 of the symphysis are nearly on a level; the mental foramen is nearer the lower than the upper 

 margin, and the angle is about 175°. The inferior alveolar nerve lies in a shallow groove 

 between the splenial and dentary plates. 



During the first year osseous union of the two halves takes place from below upward, but 

 is not complete until the second year. After the first dentition, the ramus forms with the body 

 of the mandible an angle of about 140°, and the mental foramen is situated midway between 

 the upper and lower borders of the bone opposite the second milk-molar. In the adult, the 

 angle formed by the ramus and body is nearer to a right angle, and the mental foramen is oppo- 

 site the second bicuspid, so that its relative position remains unaltered after the first dentition. 

 In old age, after the fall of the teeth, the alveolar margin is absorbed, the angle formed by the 

 ramus and body is again increased, and the mental foramen approaches the alveolar margin. 

 In a young and vigorous adult the mandible is, with the exception of the petrous portion of the 

 temporal, the densest bone in the skeleton; in old age it becomes exceedingly porous, and often 

 so soft that it may easily be broken. • 



THE HYOID BONE 



The hyoid bone [os hyoideum] or os linguae (fig. 125), situated in the anterior 

 part of the neck between the chin and the thyreoid cartilage, supports the tongue 

 and gives attachment to numerous muscles. It is suspended from the lower 

 extremities of the styloid processes of the temporal bones by two slender bands 

 known as the stylo-hyoid ligaments, and is divisible into a body and two pairs of 

 processes, the greater and lesser cornua. 



The body, constituting the central portion of the bone, is transversely placed 

 and quadrilateral in form. It is compressed from before backward and lies 

 obliquely so that the anterior surface looks upward and forward and the posterior 

 surface in the opposite direction. 



The anterior surface is convex and divided by a horizontal ridge into a superior 

 and an inferior portion. Frequently it also presents a vertical ridge crossing the 

 former at right angles, and just above the point of intersection is the glosso-hyal 



Fig. 125. — The Hyoid Bone. A, Male, B, Female (Natural Size) 



Less 

 cornu 



Body 



process, the vestige of a well-developed process in this situation in the hyoid 

 bone of some of the lower animals (reptiles and the horse). In this way four 

 spaces or depressions for muscular attachments are marked off, two on either 

 side of the middle line. The posterior surface is deeply concave and separated 

 from the epiglottis by the thyreo-hyoid membrane, and by some loose areolar 

 tissue. The membrane passes upward from the thyreoid cartilage to be attached 

 to the superior border, and interposed between it and the concavity on the 

 back of the body is a small synovial bursa. The inferior border, thicker than 

 the upper, gives insertion to muscles. The lateral borders are partly in relation 

 with the greater cornua, with which they are connected, up to middle life, by 

 synchondrosis, but after this period, usually by bone. 



