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BONES. : 109

The Inferior Border is thicker than the superior, and gives attach-
ment to the sterno-hyoid muscles anteriorly and the thyro-hyoid muscles
posteriorly. The omo-hyoid muscles are attached at the junction of
the body with the great cornua.
The Great Coniua (thyrohyals) project backward from the body of
the bone on each side. They are compressed from above downward,
the ends being rounded for the attachment of the thyro-hyoid ligaments.
Their outer surfaces give attachment to the hyo-glossus muscles. The
superior borders of these horns give attachment to the superior con-
strictor of the pharynx, while on their inferior borders are inserted the
thyro-hyoid muscles.
The Lesser Cornua (ceratohyals) are short and conical ; they accom-
pany the great cornua, and project upward and backward from the
body of the bone. Their extremities give attachment to the stylo-
hyoid ligaments.
Development.—The hyoid bone is developed from the second pair
of visceral arches, and ossified from five centres of deposit—one for the
body and one for each of the cornua. The first centres for the body and
the great cornua are deposited during the last period of foetal life, those
for the lesser cornua not appearing until the first year. Ossific unions
between the greater cornua and the body of the bone take place during
middle life, while unions between the lesser cornua and the body do not
take place until advanced age. Occasionally the stylo-hyoid ligaments
are partially ossified.
The Skull as a Whole.
The study of the skull as a whole includes a consideration of all the
bones of the head and face articulated, described under three heads
I. General Development ; II. Articulation ; III. Regional Anatomy.
GENERAL DEVELOPMENT.
The entire bony structure of the head is developed from the meso-
blastic layer of the embryo. The axis around which the first parts of
the foetus are formed is called the notochord or chorda clorsalis. The
anterior or superior portion of this chord extends forward into the
mass of tissue which forms the principal matrix of the future bony
walls of the base of the brain-case, and terminates at the posterior
border of the pituitary fossa, its extreme anterior portion forming the
dorsum sellse of the sphenoid bone.
From the anterior portion of the dorslim sellse, in close proximity
to the posterior clinoid ])rocesses, two cartilaginous rudiments (known
as the trabeculse cranii of Rt)thke) are thrown out and pass forward,
uniting in front of the olfactory depressions. As these rudiments pass
forward they unite and separate from each other at intervals, enclosing
small interspaces between them. The nasal cartilage is developed
directly from these trabecule at or near their union in front of the ollhc-
tory fossse.
This axis, or line of origin, at the base of the brain-case is divided
into two portions, anterior and posterior.