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The substance of this bone is externally hard and compact;
internally there is a cellular structure, through the centre of
which runs the canal, for the nerves and blood vessels.
From this canal smaller ones are detached containing the
vascular filaments which run to the roots of the teeth. The
maxilla inferior articulates with the temporal bones by means
of their glenoid cavities. Very marked differences in many
individuals exist in the size and formation of these cavities,
in consequence of which some persons are liable to a spon-
taneous dislocation of the jaw, in yawning, &c. when the
condyle of the lower jaw, is slipped over the anterior rim
of the glenoid cavity.*

SECTION II.

MUSCLES OF THE LOWER JAW.

Besides a very strong ligamentous connection of the lower
jaw to the temporal and sphenoidal bones, it receives attach-
ments from several of the strongest muscles of the system,
by which its motions are aifected and it becomes a powerful
lever ; the food being the resistance and the muscles the
moving powers.
These muscles may be divided into two classes :
Firstly, Those which elevate the jaw and keep it in ap-
position to the other.
Secondly, Those which depress the jaw.

* See the Anatomy of Doctor Horner, Adjunct Professor of Anatomy in
the University of Pennsylvania.