The bones of the head consist of the =skull proper= together with a
number of separate bones forming part of the =visceral skeleton=; these
are the lower jaw, the hyoid, and the ear-bones.
The skull proper is considered as divided into =cranial= and =facial=
portions. The former includes all the bones which take part in bounding
the cranial cavity or cavity of the brain; the latter includes the bones
which support the face.
The cranial portion of the skull includes all that part enclosing the
large cavity which contains the brain. For convenience this portion may
be considered as made up of three segments, each of which forms a ring
surrounding a part of the cranial cavity. The first or caudal segment or
ring consists of the occipital bone (with the interparietal) surrounding
the foramen magnum. The second segment consists of the sphenoid ventrad,
the parietals laterad and dorsad. Between the first and second segments
are intercalated laterally the temporal bones containing the auditory
organ. The third segment or ring consists of the presphenoid ventrad, of
the frontals laterad and dorsad. The cranial opening of this ring is
closed by the lamina cribrosa of the ethmoid.
The cranial portion of the skull therefore contains eleven separate
bones: one occipital, one interparietal, two temporals, one sphenoid,
two parietals, one presphenoid, two frontals, and one ethmoid.
The facial portion of the skull is much smaller than the cranial, and
lies craniad of the latter; it encloses the nasal cavity. It contains
the following thirteen bones: two palatines, one vomer, two maxillaries,
two lachrymals, two premaxillaries, two nasals, two malar or zygomatic
bones. The two halves of the mandible or lower jaw are frequently
included in the facial portion of the skull, making in all fifteen
separate bones in this part of the skull.
In the following the bones of the skull are first described separately,
then an account is given of the skull as a whole.
=Occipital Bone.= =Os occipitale= (Figs. 17 and 18).--The occipital bone
forms the most caudal portion of the cranium, entering into the
formation of its caudal wall and of its base. It connects the cranium
with the vertebral column and surrounds a large opening, the =foramen
magnum= (_d_), by means of which the cranial cavity communicates with
the vertebral canal.
[Illustration: FIG. 17.--OCCIPITAL BONE, CAUDAL OR OUTER SURFACE.