Anatomy and function. Diagram of cranial bones. Cranial bone conditions. Tips for healthy cranial bones. Read this next. The 12 Cranial Nerves. Medically reviewed by Deborah Weatherspoon, Ph. Cranial CT Scan. What Is a Craniectomy? Medically reviewed by Alana Biggers, M. Except for the mandible, all skull bones are joined together by sutures —synarthrodial immovable joints. The skull contains air-filled cavities called sinuses. Their functions are debatable, but may be related to lessening skull weight, contributing to voice resonance, and warming and moistening inspired air.
Key Terms viscerocranium : The skeleton that supports facial structure. Composition of the Skull The adult human skull is comprised of twenty-two bones which are divided into two parts of differing embryological origin: the neurocranium and the viscerocranium. Neurocranium The neurocranium forms the cranial cavity that surrounds and protects the brain and brainstem.
On the posterior skull, the sagittal suture terminates by joining the lambdoid suture at the intersection called lambda. The lambdoid suture extends downward and laterally to either side away from its junction with the sagittal suture. The lambdoid suture joins the occipital bone to the right and left parietal and temporal bones. The squamous suture is located on the lateral skull.
It unites the squamous portion of the temporal bone with the parietal bone see Figure 7. At the intersection of the frontal bone, parietal bone, squamous portion of the temporal bone, and greater wing of the sphenoid bone is the pterion , a small, capital-H-shaped suture line that unites the region.
It is the weakest part of the skull. Strong blows to the cranium can produce fractures. These may result in bleeding inside the skull with subsequent injury to the brain. The most common is a linear skull fracture, in which fracture lines radiate from the point of impact. Other fracture types include a comminuted fracture, in which the bone is broken into several pieces at the point of impact, or a depressed fracture, in which the fractured bone is pushed inward.
In a contrecoup counterblow fracture, the bone at the point of impact is not broken, but instead a fracture occurs on the opposite side of the skull.
Fractures of the occipital bone at the base of the skull can occur in this manner, producing a basilar fracture that can damage the artery that passes through the carotid canal. A blow to the lateral side of the head may fracture the bones of the pterion. The pterion is an important clinical landmark because located immediately deep to it on the inside of the skull is a major branch of an artery that supplies the skull and covering layers of the brain.
A strong blow to this region can fracture the bones around the pterion. If the underlying artery is damaged, bleeding can cause the formation of a hematoma collection of blood between the brain and interior of the skull. As blood accumulates, it will put pressure on the brain. Symptoms associated with a hematoma may not be apparent immediately following the injury, but if untreated, blood accumulation will exert increasing pressure on the brain and can result in death within a few hours.
View this animation to see how a blow to the head may produce a contrecoup counterblow fracture of the basilar portion of the occipital bone on the base of the skull. Why may a basilar fracture be life threatening? The facial bones of the skull form the upper and lower jaws, the nose, nasal cavity and nasal septum, and the orbit. The facial bones include 14 bones, with six paired bones and two unpaired bones.
The paired bones are the maxilla, palatine, zygomatic, nasal, lacrimal, and inferior nasal conchae bones. The unpaired bones are the vomer and mandible bones. Although classified with the cranial bones, the ethmoid bone also contributes to the nasal septum and the walls of the nasal cavity and orbit. The curved, inferior margin of the maxillary bone that forms the upper jaw and contains the upper teeth is the alveolar process of the maxilla Figure 7.
Each tooth is anchored into a deep socket called an alveolus. On the anterior maxilla, just below the orbit, is the infraorbital foramen. This is the point of exit for a sensory nerve that supplies the nose, upper lip, and anterior cheek. On the inferior skull, the palatine process from each maxillary bone can be seen joining together at the midline to form the anterior three-quarters of the hard palate see Figure 7. The hard palate is the bony plate that forms the roof of the mouth and floor of the nasal cavity, separating the oral and nasal cavities.
The palatine bone is one of a pair of irregularly shaped bones that contribute small areas to the lateral walls of the nasal cavity and the medial wall of each orbit. The largest region of each of the palatine bone is the horizontal plate. The plates from the right and left palatine bones join together at the midline to form the posterior quarter of the hard palate see Figure 7. Thus, the palatine bones are best seen in an inferior view of the skull and hard palate.
Cleft lip is a common developmental defect that affects approximately births, most of which are male. This defect involves a partial or complete failure of the right and left portions of the upper lip to fuse together, leaving a cleft gap. A more severe developmental defect is cleft palate, which affects the hard palate. The hard palate is the bony structure that separates the nasal cavity from the oral cavity. It is formed during embryonic development by the midline fusion palatine and maxilla bones.
Cleft palate affects approximately births and is more common in females. It results from a failure of the two halves of the hard palate to completely come together and fuse at the midline, thus leaving a gap between them.
This gap allows for communication between the nasal and oral cavities. In severe cases, the bony gap continues into the anterior upper jaw where the alveolar processes of the maxilla bones also do not properly join together above the front teeth. If this occurs, a cleft lip will also be seen. Because of the communication between the oral and nasal cavities, a cleft palate makes it very difficult for an infant to generate the suckling needed for nursing, thus leaving the infant at risk for malnutrition.
Surgical repair is required to correct cleft palate defects. The zygomatic bone is also known as the cheekbone. Each of the paired zygomatic bones forms much of the lateral wall of the orbit and the lateral-inferior margins of the anterior orbital opening see Figure 7. The short temporal process of the zygomatic bone projects posteriorly, where it forms the anterior portion of the zygomatic arch see Figure 7.
The nasal bone is one of two small bones that articulate with each other to form the bony base bridge of the nose. They also support the cartilages that form the lateral walls of the nose see Figure 7. These are the bones that are damaged when the nose is broken. Each lacrimal bone is a small, rectangular bone that forms the anterioromedial wall of the orbit see Figure 7.
The anterior portion of the lacrimal bone forms a shallow depression called the lacrimal fossa , and extending inferiorly from this is the nasolacrimal canal. The lacrimal fluid tears of the eye , which serves to maintain the moist surface of the eye, drains at the medial corner of the eye into the nasolacrimal canal.
This duct then extends downward to open into the nasal cavity, behind the inferior nasal concha. In the nasal cavity, the lacrimal fluid normally drains posteriorly, but with an increased flow of tears due to crying or eye irritation, some fluid will also drain anteriorly, thus causing a runny nose. The right and left inferior nasal conchae form a curved bony plate turbinate that projects into the nasal cavity space from the lower lateral wall see Figure 7. The inferior concha is the largest of the nasal conchae and can easily be seen when looking into the anterior opening of the nasal cavity.
The unpaired vomer bone, often referred to simply as the vomer, is triangular-shaped and forms the posterior-inferior part of the nasal septum see Figure 7. The vomer is best seen when looking from behind into the posterior openings of the nasal cavity see Figure 7. In this view, the vomer is seen to form the entire height of the nasal septum. A much smaller portion of the vomer can also be seen when looking into the anterior opening of the nasal cavity.
The mandible forms the lower jaw and is the only moveable bone of the skull. At the time of birth, the mandible consists of paired right and left bones, but these fuse together during the first year to form the single U-shaped mandible of the adult skull. The outside margin of the mandible, where the body and ramus come together is called the angle of the mandible Figure 7.
The ramus on each side of the mandible has two upward-going bony projections. The more anterior projection is the flattened coronoid process of the mandible , which provides attachment for one of the biting muscles. The posterior projection is the mandibular condyles , which is topped by the oval-shaped condyle. The condyle of the mandible articulates joins with the mandibular fossa and articular tubercle of the temporal bone.
Together these articulations form the temporomandibular joint, which allows for opening and closing of the mouth see Figure 7. The broad U-shaped curve located between the coronoid and condylar processes is the mandibular notch.
The orbit is the bony socket that houses the eyeball and contains the muscles that move the eyeball or open the upper eyelid. Each orbit is cone-shaped, with a narrow posterior region that widens toward the large anterior opening. The sphenoid bone, from the outside, appears to contribute to only a small portion of the cranium, but when the parietal bones are removed and the interior of the cranial cavity where the brain would be housed is viewed, you can see the butterfly-like shape of the sphenoid bone makes a large contribution to the floor of the cranial cavity.
The ethmoid bone, which from the outside is only visible in the eye sockets and as the upper conchae internal bumps of the nasal cavity, also contributes to the floor of the cranial cavity. The contributions of these two bones to the floor of the cranial cavity are shown in Figure 6. There are three prominent bone markings on the temporal bones. The external acoustic meatus is the opening that leads to the organs of the inner ear.
The styloid process is a thin, pen-like projection where muscles and ligaments of the neck are attached. The mastoid process is a wide and rough projection that serves as another attachment point for neck muscles.
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