The Cranium or Skull: A General Overview

The Cranium, also called the "Skull," describes the skeleton of the head, face and mandible.

It is a portion of the axial skeleton, or that portion associated with the central nervous system. Those portions of the skeleton not associated with the central nervous system, are associated with the appendicular skeleton or the extremities (i.e., the arms and legs).

The axial skeleton consists of the cranium, all the osseous elements of the vertebral column, the ribs, and the sternum.

In an adult, various of the bones of the cranium are paired left and a right, while others, which cross the mid-sagittal plane, are unpaired. Furthermore, the bones of the skull are classified as those which are called cranial Bones, or contribute to that portion surrounding the brain, or facial bones (i.e., those which do not assist in forming the braincase).

Detailed Description

The skull is one of the principle groups of bones in the human anatomy. The skull consists of twenty-six bones: eight bones form the cranium, which houses the brain and ear ossicles, plus fourteen facial bones, which form the front of the face, jaw, nose, orbits, and the roof of the mouth, three more bones make up the inner ear ossicles, and one more, the hyoid bone, is in the neck and is attached to the temporal bone by ligaments and anchors the tongue. The skull also contains a dental arcade of teeth, which are technically not bones, though they do share some of the compositional characteristics of bone tissue. Children may grow twenty deciduous (non-permanent) teeth, which will eventually fall out and be replaced by the permanent teeth (32 of them in the average adult). The bones of the skull include the frontal bone (which makes up the forehead and roof of the orbits), the occipital bone (which forms the back and base of the skull), two parietal bones (which form the roof and upper sides of the skull), and two temporal bones (which form the lower sides of the skull and house the inner ear ossicles). The lower rearmost part of each temporal bone is called the mastoid process, but because it is separated from the temporal bone, proper, by a suture, it is often considered a separate bone. The sphenoid bone forms the central base of the skull and spans the skull from side to side, the greater wings forming side plates of the skull. The sections of the ethmoid bone are positioned between the orbits, forming the walls and roof of the nasal cavity, while the three middle ear ossicles (stapes, malleus, and incus) are located within the temporal bones on each side of the skull. The U-shaped hyoid bone is found in the neck, and is attached by ligaments to the temporal bones. In the face, the two maxillary bones form much of the orbits, nose, upper jaw and roof of the mouth, while the malar (zygomatic) bones form the cheeks. The lachrymal bones are located on the inner sides of the orbits and are attached to the ethmoid and maxillary bones. Within the nasal cavity, the vomer is located in the low center and forms the thin flat bone of the nasal septum, while two inferior urbinates form the lower sides of the cavity and two palate bones form the floor of the nasal cavity as well as the roof of the mouth. The mandible is the only movable part of the skull, forming the lower jaw and mounting the teeth. The bones of the skull, with the exception of the mandible, are held together by very thin sutures, or seams, in which the periosteum of the individual bones interweave with each other, and are cemented by a fibrous, connective tissue. In the newborn, these sutures are not yet developed, with the bones being attached by cartilage which ossifies over time as the bones of the skull fuse together. The most evident external sutures of the cranium include the coronal suture, joining the frontal and parietal bones, the sagittal suture, joining the parietal bones to each other, the lambdoid suture, joining the occipital and parietal bones, the squamous suture, joining the temporal and sphenoid bones to the parietal bone on each side of the skull. The pterion is the short segment of the suture joining the squamous and parietal bones. The bones of the skull also feature a number of sinuses (cavities) and foramina (the plural of foramen, meaning hole or opening). Four pairs of sinuses flank the nasal cavity (and are therefore called paranasal sinuses). Two are found in the maxillary bone, and are called maxillary sinuses. The sphenoid bone forms two paranasal sinuses called the sphenoids, and the ethmoid bone forms the two paranasal sinuses called ethmoids. Additionally, the frontal sinuses are located in the frontal bone just behind the roof of each orbit. The foramen magnum is a large, round opening in the base of the skull which admits the spinal cord, while at the base of each temporal bone is the external auditory meatus, which serve as the auditory canals. Just above each orbit in the frontal bone is a small notch or hole, called a supraorbital foramen, and just below each orbit, in the maxillary bone, is an infraorbital foramen. Two more openings, one on each side of the skull, can be found in the frontal processes of the malar (zygomatic) bones, and are called, therefore, zygomatofacial foramina. On each side of the mandible, just below the lower canines, are the mental foramina. These facial foramina serve to admit blood vessels and nerves through and into the bone. The teeth are mounted in the maxillary bone and the mandible, and are brought together for chewing by the hinge-like motion of the mandible (the lower jawbone). An average adult will have thirty-two teeth, evenly arrayed on the maxilla and mandible.

The Individual Bones

Each of the bones of the cranium posses a number of distinctive features which not only allow the bone to be identified, but also permit its exact location and orientation in the body to be determined (i.e., as a left or right, medial- lateral, posterior-anterior, inferior-superior, etc.). Additionally, this article contains various QuickTime VR movies in order to assist you in viewing a real three dimensional view of the bones.

The following list shows how the bones in the cranium are arranged.

Paired Cranial Bones:

• Parietals
• Temporals

Unpaired Cranial Bones:

• Frontal
• Occipital
• Sphenoid
• Ethmoid

Paired Facial Bones:

• Lacrimals
• Nasals
• Zygomatics
• Maxillae
• Palatines
• Inferior Nasal Conchae

Unpaired Facial Bones:

• Vomer
• Mandible
• Hyoid

The following diagrams show the bones that this article will cover. Click on the name of the bone or the area of the skull on the bone you want information about. There are 15 pages to this article.

The Parietal Bones

The parietal bones form, by their union, the sides and roof of the cranium. Each bone is irregularly quadrilateral in form, and has two surfaces, four borders, and four angles.
 

To view a QuickTime VR movie of the parietal bone click

Surfaces.—The external surface is convex, smooth, and marked near the center by an eminence, the parietal eminence (tuber parietale), which indicates the point where ossification commenced. Crossing the middle of the bone in an arched direction are two curved lines, the superior and inferior temporal lines; the former gives attachment to the temporal fascia, and the latter indicates the upper limit of the muscular origin of the Temporalis. Above these lines the bone is covered by the galea aponeurotica; below them it forms part of the temporal fossa, and affords attachment to the Temporalis muscle. At the back part and close to the upper or sagittal border is the parietal foramen, which transmits a vein to the superior sagittal sinus, and sometimes a small branch of the occipital artery; it is not constantly present, and its size varies considerably.
The internal surface is concave; it presents depressions corresponding to the cerebral convolutions, and numerous furrows for the ramifications of the middle meningeal vessel; 27 the latter run upward and backward from the sphenoidal angle, and from the central and posterior part of the squamous border. Along the upper margin is a shallow groove, which, together with that on the opposite parietal, forms a channel, the sagittal sulcus, for the superior sagittal sinus; the edges of the sulcus afford attachment to the falx cerebri. Near the groove are several depressions, best marked in the skulls of old persons, for the arachnoid granulations (Pacchionian bodies). In the groove is the internal opening of the parietal foramen when that aperture exists.

Borders.—The sagittal border, the longest and thickest, is dentated and articulates with its fellow of the opposite side, forming the sagittal suture. The squamous border is divided into three parts: of these, the anterior is thin and pointed, bevelled at the expense of the outer surface, and overlapped by the tip of the great wing of the sphenoid; the middle portion is arched, bevelled at the expense of the outer surface, and overlapped by the squama of the temporal; the posterior part is thick and serrated for articulation with the mastoid portion of the temporal. The frontal border is deeply serrated, and bevelled at the expense of the outer surface above and of the inner below; it articulates with the frontal bone, forming onehalf of the coronal suture. The occipital border, deeply denticulated, articulates with the occipital, forming one-half of the lambdoidal suture.

Angles.—The frontal angle is practically a right angle, and corresponds with the point of meeting of the sagittal and coronal sutures; this point is named the bregma; in the fetal skull and for about a year and a half after birth this region is membranous, and is called the anterior fontanelle. The sphenoidal angle, thin and acute, is received into the interval between the frontal bone and the great wing of the sphenoid. Its inner surface is marked by a deep groove, sometimes a canal, for the anterior divisions of the middle meningeal artery. The occipital angle is rounded and corresponds with the point of meeting of the sagittal and lambdoidal sutures—a point which is termed the lambda; in the fetus this part of the skull is membranous, and is called the posterior fontanelle. The mastoid angle is truncated; it articulates with the occipital bone and with the mastoid portion of the temporal, and presents on its inner surface a broad, shallow groove which lodges part of the transverse sinus. The point of meeting of this angle with the occipital and the mastoid part of the temporal is named the asterion.

Ossification.—The parietal bone is ossified in membrane from a single center, which appears at the parietal eminence about the eighth week of fetal life. Ossification gradually extends in a radial manner from the center toward the margins of the bone; the angles are consequently the parts last formed, and it is here that the fontanelles exist. Occasionally the parietal bone is divided into two parts, upper and lower, by an antero-posterior suture.

Articulations.—The parietal articulates with five bones: the opposite parietal, the occipital, frontal, temporal, and sphenoid.