THE BONES OF THE HEAD I2I 



part of the ramus as low as the inferior dental foramen. The eoronoid cartilage 

 gives rise to (i) the eoronoid process, and (2) the anterior part ot the ramus 

 as low as the inferior dental foramen. The symphysial cartilage gives rise to 

 the hmited symphysial part of the mandible. 



At birth the mandible consists of two halves, connected at the symphysis by 

 fibrous tissue. In the course of theirs/ year osseous union takes place, which 

 is completed towards the end of the first year or beginniog of the second year. 



Meckel's cartilage extends on either side downwards and forwards from the 

 periotic cartilaginous capsule to the median Une, where it meets its fellow. 

 It is surrounded by a fibrous investment. The proximal end of the cartilage 

 gives rise to the malleus and incus, two of the three ossicles of the tympanum. 

 The part of the cartilage between the periotic cartilaginous capsule and 

 the inferior dental foramen disappears, and the membranous investment of 

 this part persists as the spheno-mandibular ligament. The part of the cartilage 

 between the inferior dental foramen and the mental foramen also disappears, 

 and its membranous investment undergoes ossification from a single centre. 

 and gives rise to (i) the greater part of one-half of the body of the mandible 

 (incisor and symphj'sial parts excepted), and (2) the lower half of the ramus 

 as high as the inferior dental foramen. The mesial part of Meckel's cartilage, 

 when ossified, becomes the incisor part of the mandible. 



At birth the inferior border is but little developed, and the body is 

 consequently shallow. The rami are very short, so that each condyle is 

 nearly on a level with the upper border of the symphysis, and the eoronoid 

 process is rather longer than the condyle. The mental foramen is nearer the 

 inferior than the superior border, and the angle amounts to 1 50 degrees or more. 

 Subsequently the body increases in depth, the rami lengthen, the angledecreases, 

 and the mental foramen gradually assumes a position midway between the 

 superior and inferior borders. In old age, after the bone becomes edentulous, 

 the alveolar border undergoes absorption, the body consequently becomes 

 shallower, the mental foramen lies near the superior border, the rami droop 

 backwards, and each angle becomes increased to about 140 degrees. For the 

 development of the alveolar border, and its relation to the milk-teeth, see the 

 superior maxilla. 



The Hyoid Bone. 



The hyoid bone is situated in the median line of the neck, 

 between the chin and the thyroid cartilage of the larynx, 

 with which latter it is connected 

 by means of the thyro-hyoid 

 membrane and thyro-hyoid liga- 

 ments. It is closelj' connected 

 with the base of the tongue, and 

 is hence known as the os linguce. 

 In its development it is associated 

 with the skull, and it is sus- 

 pended from the lower ends of 

 the styloid processes of the tem- 

 poral bones by two fibrous bands. Great Comn' 

 called the stylo-hyoid ligaments Smaiicomii 

 (epi-hyals). It consists of a central " | v 



portion or body and two pairs of Body and Hyoid Tubercle Superior Border 



cornua, great and small. tt,^ ,a t„„ tt t, 



Ti,„ v^j„ • 1 i J . '^^^- 76. — The Hyoid Bone 



The body is elongated trans- (Anterior View). 



versely, compressed from before 



backwards, and quadrilateral. Its surfaces, which are anterior and 



posterior, occupy an obUque plane, being sloped downwards and for- 



