The
occipital bone, a saucer-shaped membrane bone situated at the back and lower part of the
craniumThe skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
, is trapezoidal in shape and curved on itself. It is pierced by a large oval
apertureIn optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
, the
foramen magnum, through which the
cranial cavityThe cranial cavity, or intracranial space, is the space formed inside the skull. The brain occupies the cranial cavity, which is lined by the meninges and which contains cerebrospinal fluid to cushion blows....
communicates with the vertebral canal.
- The curved, expanded plate behind the foramen magnum is named the squama occipitalis
The squama of the occipital bone, situated above and behind the foramen magnum, is curved from above downward and from side to side.- External surface :...
.
- The thick, somewhat quadrilateral piece in front of the foramen is called the basilar part of occipital bone
The basilar part of the occipital bone extends forward and upward from the foramen magnum, and presents in front an area more or less quadrilateral in outline....
.
- On either side of the foramen are the lateral parts of occipital bone
The lateral parts of the occipital bone are situated at the sides of the foramen magnum; on their under surfaces are the condyles for articulation with the superior facets of the atlas....
.
Foramen magnum
- See also Foramen magnum
The foramen magnum (Latin for large hole) is a large oval aperture with its long diameter antero-posterior; it is wider behind than in front where it is encroached upon by the condyles.
It transmits the
medulla oblongataThe medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
and its membranes, the accessory nerves, the vertebral arteries, the anterior and posterior spinal arteries, and the membrana tectoria and alar ligaments.
Angles
The
superior angle of the occipital bone articulates with the occipital angles of the parietal bones and, in the fetal skull, corresponds in position with the
posterior fontanelleThe posterior fontanelle is a gap between bones in the human skull, triangular in form and situated at the junction of the sagittal suture and lambdoidal suture. It generally closes in 6-8 weeks from birth. A delay in closure is associated with congential hypothyroidism....
.
The
inferior angle is fused with the body of the sphenoid. The lateral angles are situated at the extremities of the grooves for the
transverse sinusesThe transverse sinuses , within the human head, are two areas beneath the brain which allow blood to drain from the back of the head. They run laterally in a groove along the interior surface of the occipital bone...
: each is received into the interval between the mastoid angle of the parietal and the mastoid part of the
temporalThe temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebrum.The temporal bone supports that part of the face known as the temple.-Parts:The temporal bone consists of four parts:* Squama temporalis...
.
Borders
The
superior borders extend from the superior to the lateral angles: they are deeply serrated for articulation with the occipital borders of the parietals, and form by this union the lambdoidal suture.
The
inferior borders extend from the lateral angles to the inferior angle; the upper half of each articulates with the mastoid portion of the corresponding
temporalThe temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebrum.The temporal bone supports that part of the face known as the temple.-Parts:The temporal bone consists of four parts:* Squama temporalis...
, the lower half with the petrous part of the same bone.
These two portions of the inferior border are separated from one another by the jugular process, the notch on the anterior surface of which forms the posterior part of the
jugular foramenThe jugular foramen is a large aperture in the base of the skull. It is located behind the carotid canal and is formed in front by the petrous portion of the temporal, and behind by the occipital; it is generally larger on the right than on the left side....
.
Structure
The occipital, like the other cranial has outer and inner tables, between which is the cancellous tissue or
diploëDiploë refers to the spongy bone structure of the internal part of short, irregular, and flat bones.In the cranial bones, the layers of compact tissue are familiarly known as the tables of the skull; the outer one is thick and tough; the inner is thin, dense, and brittle, and hence is termed the...
; the bone is especially thick at the ridges, protuberances, condyles, and anterior part of the basilar part; in the inferior fossæ it is thin, semitransparent, and destitute of diploë.
In other animals
The occipital bone is part of the
endocraniumFor internal cast of the cranium, see Endocast.The endocranium in comparative anatomy is a part of the skull base in vertebrates and represent the basal, inner part of the cranium. The term is also applied to the outer layer of the dura mater in human anatomy.-Basic structure:Structurally, the...
, the most basal portion of the skull. In
ChondrichthyesChondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
and
AgnathaAgnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....
ns, the occipital do not form as a separate element, but remain part of the
chondrocraniumThe chondrocranium is the primitive cartilaginous skeletal structure of the fetal skull that grows to envelop the rapidly growing embryonic brain....
throughout life. In most higher vertebrates, the foramen magnum is surrounded by a ring of four bones. The
basioccipital lies in front of the opening, the two
exoccipitals lie to either side, and the larger
supraoccipital lies to the posterior, and forms at least part of the rear of the cranium. In many
bony fishOsteichthyes , also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species...
and
amphibianAmphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s, the supraoccipital is never ossified, and remains as
cartilageCartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
throughout life. In primitive forms the basioccipital and exoccipitals somewhat resemble the centrum and neural arches of a vertebra, and form in a similar manner in the embryo. Together, these latter bones usually form a single concave circular condyle for the articulation of the first vertebra.
In
mammalMammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, however, the condyle has divided in two, a pattern otherwise seen only in a few amphibians. Most mammals also have a single fused occipital bone, formed from the four separate elements around the foramen magnum, along with the paired postparietal bones that form the rear of the
cranial roofThe skull roof , or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land living vertebrates. The bones are derived from dermal bone, hence the alternative name dermatocranium...
in other vertebrates.
See also
- Bone terminology
- Comparative anatomy
Comparative anatomy is the study of similarities and differences in the anatomy of organisms. It is closely related to evolutionary biology and phylogeny .-Description:...
- Neanderthal
The Neanderthal is an extinct member of the Homo genus known from Pleistocene specimens found in Europe and parts of western and central Asia...
- Occipital bun
Occipital bun is a morphological term used to describe a prominent bulge, or projection, of the occipital bone at the back of the skull. The term is most often used in connection with scientific descriptions of classic Neanderthal crania...
- Terms for anatomical location
- Ossification of occipital bone
thumb|right|Figure 3 : Occipital bone at birth.The planum occipitale [Fig. 3] of the squama of the occipital bone is developed in membrane, and may remain separate throughout life when it constitutes the interparietal bone; the rest of the bone is developed in cartilage.The number of nuclei for the...