The =occipital bone= (Os occipitale) is situated at the posterior part
of the cranium, of which it forms the posterior wall and part of the
ventral wall or base.[6]
Its lower part is perforated centrally by a large, almost circular
opening, the =foramen magnum= (Foramen occipitale magnum), at which the
brain and spinal cord join. The foramen is bounded laterally and
dorsally by the =lateral parts= of the bone, and ventrally by the
=basilar part= or =process=. Above the lateral parts—but not entering
into the formation of the foramen magnum—is the =squamous part=.
[Illustration:
FIG. 27.—LINE DRAWING OF POSTERIOR HALF OF BASE OF SKULL OF HORSE,
WITHOUT MANDIBLE. (Key to Fig. 28.)
1, Incisura carotica; 2, incisura ovalis; 3, incisura spinosa; 4,
external orifice of parieto-temporal canal; 5, Eustachian canal; 6,
petro-tympanic fissure; 7, external auditory canal; 8, hyoid
process; 9, Vidian groove; 10, supraorbital process; _A_, basilar
part of occipital; _B_, body of sphenoid; _C_, temporal wing of
sphenoid; _D_, squamous temporal bone; _E_, petrous temporal bone;
_F_, orbital part of frontal bone.
]
The =lateral parts= (Partes laterales) bear the =occipital condyles=
(Condyli occipitales), which articulate with the atlas. The condyles are
obliquely placed, wide apart dorsally, and separated by a small interval
ventrally. The articular surface is curved so sharply in the
dorso-ventral direction as to form a blunt ridge externally. The cranial
surface is concave and smooth. External to the condyle is the
=paramastoid= or =styloid process= (Processus jugularis), a strong
flattened bar of bone which projects downward and backward; its external
surface is convex and roughened for muscular attachment. Between the
root of this process and the condyle is a smooth depression, the
=condyloid fossa= (Fossa condyloidea inferior); in this is the
=hypoglossal foramen= (Foramen hypoglossi), which transmits the nerve of
like name.
The =basilar part= or =process= (Pars basilaris) is a strong, somewhat
prismatic bar which extends forward from the ventral margin of the
foramen magnum. It is much narrower in front than behind. The ventral
surface is rounded. The cranial surface is concave and smooth; its
posterior part, supports the medulla, and its anterior part has a
shallow cavity on which the pons rests. The lateral borders are thin and
sharp, and form the inner margin of the =foramen lacerum= (Foramen
lacerum et jugulare). The anterior end has, in the young subject, a
semicircular, flat, pitted surface which is attached to the body of the
sphenoid bone by a layer of cartilage; in the adult there is complete
fusion. On the ventral aspect of the junction are tubercles for the
attachment of the ventral straight muscles of the head.
[Illustration:
FIG. 28.—VENTRAL SURFACE OF SKULL OF HORSE, POSTERIOR HALF WITHOUT
MANDIBLE.
The skull is inclined slightly. (Notation on key Fig. 27.)
]
The =squamous part= (Squama occipitalis) is the somewhat quadrilateral
mass situated above the lateral portions, from which it remains distinct
till the second year. The =outer surface= is crossed by a very prominent
ridge, the =occipital crest=; the middle part of this is thick,
transverse in direction, and forms the highest point of the skull when
the head is in the ordinary position; laterally it becomes thinner and
runs downward and forward to join the temporal crest.[7] The crest
divides the surface into two very unequal parts; the small anterior area
(Planum parietale) presents a median ridge which is the posterior part
of the external sagittal crest; the large area below the crest (Planum
nuchale) also has a central eminence, the =external occipital
protuberance=, on the sides of which the funicular part of the
ligamentum nuchæ is attached. The =internal surface= is concave and
presents a deep central depression and two shallower lateral ones which
adapt it to the surface of the cerebellum.
The occipital bone is connected by suture with the interparietal, two
parietals, and two temporals, and by synchondrosis with the sphenoid;
the condyles articulate with the atlas.
=Development.=—The occipital ossifies in cartilage from =four centers=,
and consists at birth of four pieces as described above.[8] The lateral
parts unite with the basilar at three to four months, and with the
squama in the second year, when the bone is consolidated.
The parieto-occipital suture and the spheno-occipital synchondrosis are
obliterated about the fifth year usually. The temporo-occipital suture
partially ossifies in old subjects.