158 OSTEOLOGY. 



a carrot-like mass, at the condyle ; the large end forms the condyle ; the tapering enc 

 is wedged into the ossifying ramus under the root of the coronoid process. This cartilag* 

 appears about the eleventh week. About the thirteenth week a strip of cartilage appear; 

 along the anterior border of the coronoid process. Along the anterior end of the alveola] 

 walls close to the middle line, and turning down the symphysial surface of the mandibL 

 to end below in the region of the future digastric impression, another mass of cartilag< 

 appears about the fourteenth week. All the above cartilages are ossified by invasior 

 from the surrounding membrane bone and are not therefore independent centres. L 

 is possible that the symphysial cartilages may be occasionally independently ossifieq 

 and thus give rise to the ossa mentalia when they exist. From what has been statecl 

 it thus appears that under normal conditions each half of the mandible ossifies from om j 

 centre only. The above account is based on the researches of Low l and Fawcett. 2 



In a third or fourth month foetus the cartilage can be traced from the under surface 

 of the anterior part of the tympanic ring downwards and forwards to reach the jaw, tc] 

 which it is attached at the opening of the mandibular canal ; from this it may be tracec j 

 forwards as a narrow strip applied to the medial surface of the mandible, which it sensibl} 

 grooves. The proximal end of this furrow remains permanently as the mylo-hyoic 

 groove. The part of the cartilage between the tympanic ring and the mandible dis-l 

 appears, and its sheath becomes converted into fibrous tissue, and persists in the adult 

 as the spheno-mandibular ligament, its proximal end being continuous, through the* 

 petro-tympanic fissure, with the slender process of the malleus, with the development oij 

 which bone it is intimately associated. I. Chaine (Comptes fiendus, Biologie, 1903) 

 takes exception to this view and regards the spheno-mandibular ligament as thcj 

 remnant of a muscular slip. 



At birth the mandible consists of two halves united at the syrnphysis by fibrous! 

 tissue ; towards the end of the first, or during the second year, osseous union between the j 

 two halves is complete. In infancy the mandible is shallow and the rami proportionately 

 small ; further, owing to the obliquity of the ramus, the angle is large, averaging about 150. 

 The mental foramen lies near the lower border of the bone. Coincident with the eruption 

 of the teeth and the use of the mandible in mastication, the rami rapidly increases in size, 

 and the angle becomes more acute. After the completion of the permanent dentition iti 

 approaches more nearly a right angle varying from 110 to 120. The body of the bone 

 is stout and deep, and the mental foramen usually lies midway between the upper and> 

 lower borders. As age advances, owing to the loss of the teeth and the consequent! 

 shrinkage and absorption of the alveolar border of the bone, the body becomes narrow and j 

 attenuated, and the mental foramen now lies close to the upper border. At the same time 

 the angle opens out again (130 to 140), in this respect resembling the infantile condition. 

 In old age the coronoid process and the condyle form a more open angle with each other 

 than in the young adult. 



Os Hyoideum. 



The hyoid bone, though placed in the neck, is developmentally connected 

 with the skull. It lies between the mandible above and the larynx below, 



and is connected with the root of the tongue. Of 

 U-shaped form, as its name implies (Greek v and 

 etSos, like), it consists in the adult of a central 

 part, or body, with which are united two long pro- } 

 cesses extending backwards the greater cornua 

 one on each side. At the point where these 

 are ossified with the body, the lesser cornua, which 

 project upwards and backwards, are placed. 



The body is arched from side to side and 

 compressed from before backwards, so that its 

 surfaces slope downwards and forwards. Its 



~ " AS SEEN Anterior surface displays a slight median ridge, on 



either side of which the bone is marked by the 

 attachment of the mylo-hyoid muscles. Its posterior surface, deeply hollowed, is 

 concave from side to side and from above downwards. Herein lie a quantity of 



1 Journal of Anatomy and Physiology, vol. xliv. p. 82. 2 Graduation Thesis, Edinburgh, 1906. 



