66 THE SKELETON 



lodging unerupted teeth. Eaoli half is joined at the symphysis by fibrous tissue. 

 The upi)er eclge of the symphysis and the condyles are nearly on a level. The 

 mandiV)ular nerve lies in a shallow groove between the dentary and splenial plates. 

 During the first year the two halves ankylose, union taking place from below 

 upwards, but the ankylosis is not complete until the second year. After the first 

 dentition, the ranuis forms with the l)ody of the mandible an angle of al)out 140°, 

 and the mental foramen is situated midway between the upper and lower l)ordi'rs 

 of the bone opposite the second milk-molar. In the adult, the angle formed by the 

 ramus and body is nearly a right angle, and the mental foramen is opposite the 

 second bicuspid, so that its relative position remains unaltered after the first denti- 

 tion. In old age, after the fall of the teeth, the alveolar margin is absorbed, the 

 angle formed by the ramus and body becomes obtuse, and the mental foramen 

 approaches the alveolar margin. In a young and vigorous adult the mandible 

 is, with the exception of the petrosal, the densest boiie in the skeleton, and resists 

 decay longest; in old age it becomes exceedingly porous, and often so soft tliat it 

 may be broken easily. 



THE HYOID, THE STYLOID PROCESS, AND THE EAR-BOXES 



The hyoid or lingual bone consists of a body and four processes. The body 

 (basi-hyal) forms the central portion of the bone; it is somewhat oblong in shape. 

 Its anterior aspect is convex and divided by a transverse ridge into a suj)eri()r 

 and an inferiijr portion. Frequently it presents a median vertical ridge, and at tlie 



Fig. 77. — Thk Hvoin. 



GREATER CORNU 



lESSER CORNU 



point where the horizontal and vertical ridges intersect, a tubercle, sometimes 

 measuring four millimetres in length, is formed. The whole of the anterior surface 

 is crowded with the origin and insertions of muscles. The posterior surfacic is 

 deeply concave. 



The inferior border is free, the superior gives attachment to the thyro-hyoid 

 membrane. Between this membrane and the concavity of the hyoid there is a 

 large bursa. The lateral borders are in relation with the greater (•ornua, but 

 remain separated from them until late in life. 



The greater cornua (thyro-hyals) i)roject backwards and ui)wards. Their 

 upper and lower borders and anterior surfaces are occupied with muscles. Each 

 corner terminates posteriorly in a rounded tubercle, to which the thyro-hyoid 

 ligament is attached. 



The lesser cornua (cerato-hyals) are small conical pieces of bone occupying 

 the upper ])art of the suture between the body and the greater cormia. Their tips 

 are continuous with the stylo-hyoid ligaments. 



Muscles attached to the hyoid bone : — 



Lingualis. Digastric. INIylo-hyoid. 



Genio-hyo-glossus. Genio-hyoid. Hyo-glossus. 



Middle constrictor. Thyro-hy<Md. Hyo-epiglottideus 



Sterno-hyoid. Omo-hyoid. (when present). 



