Frontal Bone

The frontal bone may be divided into two main portions, a vertical squamous portion which articulates with the paired parietals along the Coronal Suture and forms the forehead, and two orbital plates, which contribute to the ceiling and lateral walls of the left and right eye orbits. On the external surface the squamous portion frequently possesses a left and right Frontal Eminence. Additionally, the bone possesses two Supra-Orbital Ridges (i.e., Superciliary or Brow Ridges) which are bumps above each of the eye orbits. In early hominids these ridges formed a Torus or large shelf-like process protruding from above the eyes. Associated with each Superior Orbital Margin of the eye orbit the frontal bone may posses a Supra-Orbital Notch or if completely surrounded by bone, a Supra-Orbital Foramen. Above the fronto-nasal suture which allows articulation between the frontal and nasal bones there is generally a trace of the vertical Metopic Suture. In early life the metopic suture divided the frontal bone into left and right halfs. With in the bone, and above and the metopic suture, is the Frontal Sinus. The left and right Frontal Crest, begins at each Zygomatic Process of the frontal bone, and provides the anterior origin of the Temporal Line to which the left and right temporal muscle is attached.

Internally, the frontal bone possesses the Median Sagittal (i.e., Sagittal-Frontal) Crest which separates the two frontal hemispheres of the brain.


Parietal Bone

The Parietals are paired left and right. Externally, each possess a Superior, and Inferior Temporal Line, to which the temporal muscle is attached. The lines run from the Frontal Crest of the anterior frontal bone to the Supra-Mastoid Crest on the posterior portion of the temporal bone. The parietals articulate with each other by way of the Mid-Sagittal Suture, and with the frontal bone anteriorly by way of the Coronal Suture. These two sutures generally form a right angle with one another. Posteriorly, the parietals articulate with the Occipital Bone by way of the Lambdoid Suture. The intersection of the Lambdoid and Sagittal Sutures approximate a 120 degree angle on each of the parietals and the occipital bone. Among the sutures the Lambdoid is by far more serrated than either the Sagittal or the Coronal. Inferiorly the Parietal articulates with the temporal bone by way of the Squamosal and Parieto-Mastoid Sutures. On the external surface near the center of the bone is the Parietal Eminence. Slightly posterior to the eminence there may be a Parietal Foramen.

Internally, the bones possess a number of Meningeal Groves as well as perhaps some number of Arachnoid Foveae. The groves generally branch from the inferior/anterior edge of the bone to superior/posterior, while the foveae are freqquenly found along the sagittal suture. At the area of intersection of the lambdoid and parieto-mastoid sutures there is a brief portion of the Sigmoid (i.e., Transverse) Sulcus.


Temporal Bone

The Temporal Bone is another paired cranial bone which is difficult to describe due to its various features, and projections. It consists of two major portions, the Squamous Portion, which is flat or fan-like and projects superiorly from the other, very thick and rugged portion, the Petrosal Portion.

The squamous portion assists in forming the Squamous Suture which separates the temporal bone from the adjacent and partially underlaying parietal bone. The petrosal portion contains the cavity of the middle ear and all the ear ossicles; the Malleus, Incas and Stapes. This portion projects anterior and medialy beneath the skull. Projecting inferiorly from the petrosal portion is the slender Styloid Process which is of variable length. The styloid process serves as a muscle attachment for various thin muscles to the tongue and other structures in the throat. Externaly the petrosal portion possesses the External Auditory Meatus while internally there is an Internal Auditory Meatus. Anterior to the external meatus the Zygomatic Process has its origin. This process projects forward toward the face and its articulation with the temporal process of the zygomatic. Just anterior of the external meatus and inferior of the origin of the zygomatic process is the Glenoid or Mandibular Fossa which assists in forming the shallow socket of the Tempro-Mandibular Joint. Posterior to the external auditory meatus is the inferiorly projecting Mastoid Process which serves as an attachment for the sternocleidomasotid muscle. Above the mastoid process is the Supramastoid Crest to which the posterior portion of the temporal muscle is attached.


Sphenoid Bone

The Sphenoid is one of the more difficult bones to describe and invision. It has a number of features and projections, which allow it to be seen from various views of the skull. It is a single bone that runs thrugh the mid-sagittal plane and aids to connect the cranial skeleton to the facial skeleton. It consists of a hollow body, which contians the Sphenoidal Sinus, and three pairs of projections: the more superior Lesser Wings, the intermediate Greater Wings, and the most inferior projecting Pterygoid Processes. Internally upon the body is the Sella Turcica where the pituitary gland rests in life. The smaller lesser wings posssesses the Optic Foramen through which the optic or second cranial nerve passes before giving rise to the eye. The Supra-Orbital Fissure separates the lesser wing superiorly from the greater wing below and can best be viewed on the posterior wall of each eye orbit. The left and right greater wings assist in forming the posterior wall of each of the eye orbits where it forms an Orbital Plate. In addition the external surface of the greater wing can be viewed in the the lateral view of the cranium in an area called the Pterion Region. Just inferior to the supra-orbital fissure near the body of the sphenoid, each of the greater wings also possess a Foramen Rotundum which in life transmits the maxillary branch of the fifth, or trigeminal, cranial nerve. Each of these wings also possesses a much larger Foramen Ovale more laterally, which transmits the the mandibular branch of the same nerve. More posteriorly is the smallest of the three pairs of foramena, the Foramen Spinosum which transmits the middle meningial vessels and nerve to the tissues covering the brain.

The left and right pterygoid processes project inferiorly from near the junction of each of the greater wings with the body of the sphenoid. These processes run along the posterior portion of the nasal passage toward the palate. Each process is formed from a Medial and Lateral Pterygoid Plate to which the respective medial and lateral pterygoid muscle is attached during life. The muscles run from these attachments to the internal, or medial surface, of the mandible in the area of the gonial angle. In life the muscles assist in creating the grinding motion associated with chewing.


Occipital Bone

The Occipital Bone consists of a large squamous, or flattened portion separated from a small thick basal portion by the Foramen Magnum on either side of which is a left or right Occipital Condyle. The occipital condyles articulate with the first cervical vertebrae (the Atlas). Externally, the squamous portion of the bone possesses Superior, Middle, and Inferior Nuchal Lines to which the muscles at the back of the neck are attached. The External Occipital Protuberance lies on the superior nuchal line in the mid-sagittal plain. Lateral to each occipital condyle are the Condylar Fossae and Foramen while the Hypoglossal Canal is medial to them.

Internally, are the Sagittal and Transverse Sulci, or grooves which converge at the Confluence of Sinuses. A single internal Occipital Protuberance or Cruciform Eminence is also found in this area. Running inferior from the eminence to the foramen magnum is the Internal Occipital Crest which separates the Cerebellar Fossae. The transverse sulci assist in directing the developing jugular vein to the Jugular Notch on either side of the basilar portion of the occipital.

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