Movable Skull Bones

The skull represents a bony structure that helps support the face and forms a protective cavity for the brain. It is composed of many bones that are formed by intramembranous ossification, and joined by sutures (i.e. fibrous joints). The bones of the skull can be divided into two groups: like that of the cranium (it consists of the cranial roof and the cranial base) and that of the face.

Here in this article, we will take a look at the anatomy of the bones of the skull – their orientation, articulations, and clinical importance.

Cranium

The cranium, also called the neurocranium, is generally formed by the superior aspect of the skull. It encases and protects the brain, meninges, and other cerebral vasculature. Anatomically, the cranium is subdivided into a roof and a base:

• Cranial roof: it comprises the frontal, occipital and two parietal bones. It is also called the calvarium.
• Cranial base: this comprises six bones such as the frontal, sphenoid, ethmoid, occipital, parietal and temporal. These bones articulate with the first cervical vertebra (i.e. atlas), the facial bones, and the mandible (jaw).

Facial Bones

The facial skeleton is also called viscerocranium ; it supports the soft tissues of the face. The facial bones consist of 14 bones, that fuse together to house the orbits of the eyes, the nasal and oral cavities, and the sinuses. The frontal bone, mainly the bone of the calvaria, is sometimes included as a part of the facial skeleton.

The facial bones are discussed below

• Zygomatic: they are generally two in number and form the cheekbones of the face and also  articulate with the frontal, sphenoid, temporal and maxilla bones.
• Lacrimal: these are also two in number, it is the smallest bone of the face. Lacrimal forms the part of the medial wall of the orbit.
• Nasal: are also two in number, these are two slender bones that are located at the bridge of the nose.
• Inferior nasal conchae: they are two in number and are located within the nasal cavity, these bones help to increase the surface area of the nasal cavity, thereby increasing the amount of inspired air that could come into contact with the cavity walls.
• Palatine: these are usually two in number and are situated at the rear of the oral cavity and forms part of the hard palate.
• Maxilla: they are two in number. It comprises part of the upper jaw and the hard palate.
• Vomer: are single and form the posterior aspect of the nasal septum.
• Mandible: these are present in the jaw, and it articulates with the base of the cranium at the temporomandibular joint (TMJ).

Sutures of the Skull

Sutures are a type of fibrous joint that is unique to the skull. They are immovable joints and fuse completely at the age of 20. These joints have a significance in the context of trauma, as they represent points of potential weakness in the skull. The main sutures in the adult skull are follows:

• Coronal suture: it fuses the frontal bone along with the two parietal bones.

• Sagittal suture: this fuses both parietal bones with each other.

• Lambdoid suture: it fuses the occipital bone to the two parietal bones.

In neonates, the incompletely fused suture joints usually give rise to membranous gaps between the bones, which is called the fontanelles. The two main fontanelles are as follows:

• Frontal fontanelle – these are located at the junction of the coronal and sagittal sutures.
• Occipital fontanelle – it is located at the junction of the sagittal and lambdoid sutures.

Movable Skull Bone

The mandible, or the lower jaw, represents the bone that forms the lower part of the skull, and this along with the maxilla (or the upper jaw), forms the so-called mouth structure. Movement of the lower jaw helps in opening and closing of the mouth and also allows for the chewing of food. The lower set of teeth in the mouth is basically rooted in the lower jaw. Mandible is the only movable skull bone in the body. 

Four different muscles connect with the lower jaw in order to facilitate its movement. These muscles include the masseter, the temporalis, the medial pterygoid, and the lateral pterygoid. Each of these muscles are present in pairs, with one of each muscle appearing on either side of the skull. All these muscles work in combination in order to pivot the lower jaw up and down and also to facilitate movement of the jaw from side to side.

Fractures in this bone can be minor or severe, the more severe fractures require the jaw to be wired shut so that the movement is prevented during healing. Other injuries and infections which affect the lower jaw includes tendinitis, infections stemming from tooth decay or other tooth injuries, and temporomandibular joint disorder (TMJD), this leads to painful swelling where the mandible meets the cheekbone.

Conclusion

The left and right halves of the lower jaw, or the mandible, emerge originally as two different bones, but in the second phase of life both the bones fuse together at the midline to form one. The horizontal central part on each side of the body is the mandible. The upper portion of the body represents the alveolar margin, similar to the alveolar margins of the maxillae. The projecting chin, situated at the lower part of the body in the midline, is said to be a distinctive characteristic of the human skull. On either side of the chin is present the mental foramen. Here, we come to the topic. Hopefully this article was helpful in clearing all your concepts regarding the movable skull bone.