142 ) Gia OSTEOLOGY. 
The proximal end of this furrow remains permanently as the mylo-hyoid groove. The 
part of the cartilage between the tympanic ring and the jaw becomes converted into 
fibrous tissue, and persists in the adult as the so-called internal lateral ligament of the 
temporo-maxillary articulation, its proximal end through the Glaserian fissure being con- 
tinuous with the slender process of the malleus. The part which is applied to the inner 
surface of the lower jaw disappears. In the tissue overlying the cartilage ossification 
begins by several centres as early as the sixth or seventh week of foetal life, in this 
respect resembling the clavicle, by which it is alone preceded.’ The dentary or basal 
centre forms the outer wall and lower border. With this is united the splenial portion, 
which appears somewhat later, forming the inner table from near the symphysis back- 
wards towards the opening of the inferior dental canal where it terminates in the lingula. 
By the union of these two parts a groove is formed, which ultimately becomes covered in, 
and in which the inferior dental nerve and vessels are lodged. As has been already stated, 
the part of the body between the symphysis and the mental foramen is regarded as 
directly developed from the fore part of the Meckelian cartilage. As will have been 
gathered from the above description, the upper part of the ramus and its processes have 
no connexion with Meckel’s cartilage. The condyle and the coronoid process are each 
developed from a separate centre, preceded by a cartilaginous matrix. These several 
centres are all united about the fourth month. 
At birth the lower jaw consists of two lateral halves united at the symphysis by 
fibrous tissue; towards the end of the first or during the second year osseous union 
between the two 
halves is complete. 
In infancy the jaw 
is sballow 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. Coinci- 
dent with the erup- 
tion of the teeth and 
the use of the jaw in 
mastication, the rami 
rapidly increase in 
size, and the angle 
becomes more acute. 
After the completion 
of the permanent 
Fia. 112.—Lowek Jaw at birth. A, As seen from above ; dentition it epee 
B, Outer side ; C, Inner side. proaches 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 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. 
a, Mental foramen ; 6, Inferior dental canal; c, Lingula; d, Sockets for the dental sacs, 
THe Hyorp BONE, 
The hyoid bone (os hyoideum), or os lingue, 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 inphes (Greek v and edos, like), it consists in the adult of a 
central part, or body, with which on either side are united two long processes which — 
extend backwards—the great cornua. At the point where these are ossified with the 
body, the lesser cornua, which project upwards and backwards, are placed. 
