Skull : Human Skeletal System

 

Skull

The Human Skull is a bony structure, the head in the skeleton, which supports the structures of the face and forms a cavity for the brain.

The adult skull is said to consist of two parts of different embryological origin–the neurocranium and the viscerocranium. The neurocranium (or braincase) is a protective cranial vault that surrounds the brain and brainstem. The viscerocranium (also splanchnocranium or facial skeleton) is formed by the bones supporting the face.

Except for the mandible, all of the bones of the skull are joined together by sutures, synarthrodial(immovable) joints formed by bony ossification, with Sharpey's fibres permitting some flexibility.

Cranial Bones

Surrounding the brain is a region of the skull known as the cranium. In this region we have eight cranial bones:

1. Frontal bone
2. Two parietal bones
3. Two temporal bones
4. Occipital bone
5. Ethmoid bone
6. Sphenoid bone

Collectively, these bones provide a solid bony wall around the brain, with only a few openings for nerves and blood vessels. Our occipital bone contains the foramen magnum, the hole through which the spinal cord enters the skull to attach to the brain. The occipital bone also forms the atlanto-occipital joint with the atlas (the first cervical vertebra in our spine). The frontal, ethmoid, and sphenoid bones contain small hollow spaces known as paranasal sinuses. The sinuses help to reduce the weight of these bones and increase the resonance of the voice during speech, singing, and humming. Facial Bones The 14 bones that support the muscles and organs of the face are collectively known as our facial bones. The facial bones consist of:

1. Mandible
2. Two maxillae (singular: maxilla)
3. Vomer
4. Two palatine bones
5. Two nasal bones
6. Two zygomatic bones
7. Two nasal conchae (singular: concha)
8. Two lacrimal bones

The mandible, or jaw bone, is the only movable bone of the skull, forming the temporomandibular joint with the temporal bone. The lower teeth are rooted into the mandible while the upper teeth are rooted in the two maxillae. The maxillae also contain paranasal sinuses like the frontal, ethmoid, and sphenoid bones of the cranium.

Temporal Styloid Process

Red portion of this Skull is "Temporal Styloid Process"

The styloid process is a pointed piece of bone that extends down from the human skull, just below the ear.

Structure

The styloid process is a slender pointed piece of bone just below the ear. It projects down and forward from the inferior surface of the temporal bone, and serves as an anchor point for several muscles associated with thetongue and larynx.

#Itsproximal part (tympanohyal) is ensheathed by the vaginal process of the tympanic

#Itsdistal part (stylohyal) gives attachment to the following:

1. stylohyoid ligament
2. stylomandibular ligament
3. styloglossusmuscle (innervated by the hypoglossal nerve)
4. stylohyoid muscle(innervated by the facial nerve)
5. stylopharyngeusmuscle (innervated by the glossopharyngeal nerve)

The stylohyoid ligament extends from the apex of the process to the lesser cornu of the hyoid bone, and in some instances is partially, in others completely, ossified.

A small percentage of the population will suffer from an elongation of the styloid process and stylohyoid ligament calcification. This condition is also known as Eagle syndrome. The tissues in the throat rub on the styloid process during the act of swallowing with resulting pain along the glossopharyngeal nerve. There is also pain upon turning the head or extending the tongue. Other symptoms may include voice alteration, cough, dizziness, migraines, occipital neuralgia, pain in teeth and jaw and sinusitis or bloodshot eyes.

Carotid Canal

Green marked portions of this Skull is "Carotid Canal"

The carotid canal is the passage way in the temporal bone through which the internal carotid artery enters the middle cranial fossa from the neck. The canal starts on the inferior surface of the temporal bone at the external opening of the carotid canal (also referred to as the carotid foramen). The canal ascends at first vertically, and then, making a bend, runs horizontally forward and medialward. The canal's internal opening is near the foramen lacerum above which the internal carotid artery passes on its way to the cavernous sinus.

The carotid canal is a V-shaped recession on the floor of which and the orifice of the aqueduct of the cochlea is near the posterior margin, inhabiting the gap posteriorly as well as medially in the middle of the jugular fossa.

Choanae

Green marked portions of this Skull are "Choanae"

The choanae is a funnel-shaped opening communicating between the nasal opening and the pharynx.

External Acoustic Meatus

Red marked portion of this Skull is "External Acoustic Meatus"

The ear canal (external auditory meatus, external acoustic meatus, EAM) (Latin: meatus acusticus externus), is a tube running from the outer ear to the middle ear. The adult human ear canal extends from the pinna to the eardrum and is about 2.5 centimetres (1 in) in length and 0.7 centimetres (0.3 in) in diameter.

External Occipital Protuberance

Position of external occipital protuberance (shown in red).

Near the middle of the occipital squama is the external occipital protuberance, the highest point of which is referred to as the inion. Extending laterally from it on either side is the superior nuchal line, and above it is the faintly marked highest nuchal line. It is thought to be caused by an unknown polymorphism of the H2NY gene.

A study of 16th-century Anatolian remains showed that the external occipital protuberance statistically tends to be less pronounced in female remains.

Foramen Magnum

Skull seen from below. The hole through which medulla(shown in red) is passing is foramen magnum.

In anatomy, the foramen magnum (Latin: "great hole") is a large opening in the occipital bone of the cranium. It is one of the several oval or circular apertures in the base of the skull (the foramina). It is the foramen magnum through which the spinal cord (an extension of the medulla oblongata) enters and exits the skull vault.

Apart from the transmission of the medulla oblongata and its membranes, the foramen magnum transmits the vertebral arteries, the anterior and posterior spinal arteries, the membrana tectoria and alar ligaments. It also transmits the spinal component of the accessory nerve into the cranial fossa.

Foramen Ovale

Position of Foramen Ovale(shown in green).

At the base of the skull the foramen ovale (Latin: oval window) is one of the larger of the several holes (the foramina) that transmit nerves through the skull. The foramen ovale is situated in the posterior part of the sphenoid bone, posterolateral to the foramen rotundum.

Frontal Bone

Position of the frontal bone (highlighted in green).

The frontal bone or os frontis is a bone in the human skull. The name comes from the Latin word frons (meaning "forehead"). The bone resembles a cockleshell in form, and consists of three portions.

Hyoid Bone

Position of the Hyoid Bone(highlighted in red).

Shape of hyoid bone.

The hyoid bone (lingual bone) (/ˈhaɪɔɪd/;Latin os hyoideum) is a horseshoe-shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies at the level of the base of the mandible in the front and the third cervical vertebra (C3) behind.

Unlike other bones, the hyoid is only distantly articulated to other bones by muscles or ligaments. The hyoid is anchored by muscles from the anterior, posterior and inferior directions, and aids in tongue movement and swallowing. The hyoid bone provides attachment to the muscles of the floor of the mouth and the tongue above, the larynx below, and the epiglottis and pharynx behind.

Its name is derived from Greek hyoeides, meaning "shaped like the letter upsilon (υ)".

Hypoglossal Canal

Position of Hypoglossal Canal(marked in blue)

The hypoglossal canal is a bony canal in the occipital bone of the skull. It transmits the hypoglossal nerve from its point of entry near the medulla oblongata to its exit from the base of the skull near the jugular foramen. It lies in the epiphyseal junction between the basiocciput and the jugular process of the occipital bone.

Lambdoid Suture

Lambdoid suture shown in red.

The lambdoid suture (or lambdoidal suture) is a dense, fibrous connective tissue joint on the posterior aspect of the skull that connects the parietal bones with the occipital bone. It is continuous with the occipitomastoid suture.

Its name comes from its lambda-like shape.

Mandible

This is Mandible.

The mandible (from Latin mandibula, "jawbone") or inferior maxillary bone is the largest, strongest and lowest bone in the face. It forms the lower jaw and holds the lower teeth in place. In the midline on the anterior surface of the mandible is a faint ridge, an indication of the mandibular symphysis, where the bone is formed by the fusion of right and left processes during mandibular development. Like other symphysis in the body, this is a midline articulation where the bones are joined by fibrocartilage, but this articulation fuses together in early childhood.

Mandibular Fossa

Blue color portions are Mandibular Fossa.

The mandibular fossa is the depression in the temporal bone that articulates with the mandibular condyle. In the temporal bone, the mandibular fossa is bounded, in front, by the articular tubercle; behind, by the tympanic part of the bone, which separates it from the external acoustic meatus; it is divided into two parts by a narrow slit, the petrotympanic fissure (Glaserian fissure). The mandibular fossa is also referred to as the glenoid fossa, especially in dental literature.

Mastoid Foramen

The portions in red circle and blue arrow are Mastoid Foramen

The mastoid foramen is a prominent opening in the temporal bone, which is located behind (or posterior) to the mastoid process. Generally, it provides the opening through which a vein and artery pass.

The mastoid foramen is a large hole in the posterior border of the temporal bone. It transmits a Mastoid emissary vein to the sigmoid sinus and a small branch of the occipital artery, the posterior meningeal artery to the dura mater.

The position and size of this foramen are very variable; it is not always present; sometimes it is situated in the occipital bone, or in the suture between the temporal and the occipital.

It transmits (1) an emissary vein connecting the sigmoid sinus with the posterior auricular vein and (2) a meningeal branch of the occipital artery

Mastoid Process

Mastoid process shown in red.

The mastoid process is a conical prominence projecting from the undersurface of the mastoid portion of the temporal bone. It is located just behind the external acoustic meatus, and lateral to the styloid process.

This part of the skull projects from the temporal bone and is roughly pyramidal or conical in shape. It is also filled with cavities called "air cells".

Maxilla

Maxilla shown in green.

The maxillae (plural: maxillae /mækˈsɪliː/) are the two maxilla bones forming the upper jaw and palate of the mouth. The two halves are fused at the intermaxillary suture to form the upper jaw. This is similar to the mandible (lower jaw), which is also a fusion of two halves at the mandibular symphysis.

Sometimes (e.g. in bony fish), the maxilla is called "upper maxilla," with the mandible being the "lower maxilla." Conversely, in birds the upper jaw is often called "upper mandible."

Median Palatine Suture

Median Palatine Suture in green.

The medial palatine suture develops to form the hard palate of the mouth and the floor of the nasal cavity. If not closed properly during development it can form a cleft palate.

Occipital Bone

Position of occipital bone (shown in green).

The occipital bone /əkˈsɪpɨtəl/, a saucer-shaped membrane bone situated at the back and lower part of the skull, is trapezoidal in shape and curved on itself. It is pierced by a large oval opening, the foramen magnum, through which the cranial cavity communicates with the vertebral canal.

The curved, expanded plate behind the foramen magnum is named the squama occipitalis.

The thick, somewhat quadrilateral piece in front of the foramen is called the basilar part of occipital bone, - the basioccipital.

On either side of the foramen are the lateral parts of occipital bone, -the exoccipitals.

The occipital, like the other cranial has outer and inner tables, between which is the cancellous tissue or diploë; 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.

Occipital Condyle

Position of occipital condyle (shown in red)

The occipital condyles are undersurface protuberances of the occipital bone in vertebrates, which function in articulation with the superior facets of the atlas vertebra.

The condyles are oval or reniform (kidney-shaped) in shape, and their anterior extremities, directed forward and medialward, are closer together than their posterior, and encroach on the basilar portion of the bone; the posterior extremities extend back to the level of the middle of the foramen magnum.

The articular surfaces of the condyles are convex from before backward and from side to side, and look downward and lateralward.

To their margins are attached the capsules of the atlanto-occipital joints, and on the medial side of each is a rough impression or tubercle for the alar ligament.

At the base of either condyle the bone is tunnelled by a short canal, the hypoglossal canal.

Parietal Bone

Left parietal bone (shown in green). Animation stops for a few seconds at inner and outer surface.

The parietal bones /pəˈraɪɨtəl/ are bones in the human skull which, when joined together, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin paries (-ietis), wall.

Sagittal Suture

Sagittal suture shown in red.

The sagittal suture is a dense, fibrous connective tissue joint between the two parietal bones of the skull. The term is derived from the Latin word sagitta, meaning arrow. The derivation of this term may be demonstrated by observing how the sagittal suture is notched posteriorly, like an arrow, by the lambdoid suture. The sagittal suture is also known as the interparietal suture and the sutura interparietalis.

In forensic anthropology, the sagittal suture is one method used to date human remains. The suture begins to close at age twenty-nine, starting at where it intersects at the lambdoid suture and working forward. By age thirty-five, the suture is completely closed. This means that when inspecting a human skull, if the suture is still open, one can assume an age of less than twenty-nine. Conversely, if the suture is completely formed, one can assume an age of greater than thirty-five.

Two anatomical landmarks are found on the sagittal suture: the bregma, and the vertex of the skull. The bregma is formed by the intersection of the sagittal and coronal sutures. The vertex is the highest point on the skull and is often near the midpoint of the sagittal suture.

Two anatomical landmarks are found on the sagittal suture: the bregma, and the vertex of the skull. The bregma is formed by the intersection of the sagittal and coronal sutures. The vertex is the highest point on the skull and is often near the midpoint of the sagittal suture.

At birth, the bones of the skull do not meet. If certain bones of the skull grow too fast then "premature closure" of the sutures may occur. This can result in skull deformities. If the sagittal suture closes early the skull becomes long, narrow, and wedge-shaped, a condition called scaphocephaly.

Sphenoid Bone

Position of sphenoid bone (shown in green)

The sphenoid bone (/ˈsfiːnɔɪd/; from Greek sphenoeides, "wedgelike") is an unpaired bone of the neurocranium. It is situated in the middle of the skull towards the front, in front of the temporal bone and basilar part of the occipital bone. The sphenoid bone is one of the seven bones that articulate to form the orbit. Its shape somewhat resembles that of a butterfly or bat with its wings extended.

Teeth

The white portions of this image are teeth

The teeth are a group of hard organs found in the oral cavity. We use teeth to masticate (or chew) food into tiny pieces. They also provide shape to the mouth and face and are important components in producing speech.

A tooth can be divided into two main parts: the crown and root. Found above the gum line, the crown is the enlarged region of the tooth involved in chewing. Like an actual crown, the crown of a tooth has many ridges on its top surface to aid in the chewing of food. Below the gum line is the region of the tooth called the root, which anchors the tooth into a bony socket known as an alveolus Roots are tapered structures resembling the roots of plants, and each tooth may have between one to three roots. The exterior surface of the root is covered in a bone-like mixture of calcium and collagen fibers known as cementum. Cementum provides grip for the periodontal ligaments that anchor the root to the surrounding alveolus.

Temporal Bone

The 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 and houses the structures of the organ of hearing. The lower seven cranial nerves and the major vessels to and from the brain traverse the temporal bone.

The temporal bone consists of five parts:[1][2] The Squama temporalis (squamous part), which is the biggest part of the temporal bone and lies superior to all parts. Posterior and inferior to the squamous part lies the Mastoid portion. Fused with the squamous and mastoid parts and between the sphenoid and occipital bones lies the Petrous portion (Petrosal ridge). It is recognizable because of it's pyramidial shape. The Tympanic part is a small portion which lies inferior to the squamous part, anteroir to the mastoid part and superior to the styloid process. As already mentioned lies the Styloid process inferior to the tympanic part. It is shaped like a thorn.

Zygomatic Bone

Position of Zygomatic Bone (green).

In the human skull, the zygomatic bone (cheekbone, malar bone) is a paired bone which articulates with the maxilla, the temporal bone, the sphenoid bone and the frontal bone. It is situated at the upper and lateral part of the face and forms the prominence of the cheek, part of the lateral wall and floor of the orbit, and parts of the temporal and infratemporal fossa. It presents a malar and a temporal surface; three processes, the frontosphenoidal, orbital, maxillary, and temporal; and four borders.