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Human skull

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Skull.jpg

The skull is a critically important part of the skeletal system. It supports sensory system, forms the framework of the face, serves as points of attachment for muscles, and of course, it protects the brain.[1] The human skull, at first glance, may appear to consist of only one large bone, but that is not the case. The skull is made up of twenty-two bones. Eight of these bones are cranial bones and fourteen of these bones are facial bones. Each category exhibits unique anatomy.[2] The cranial bones are bonded by sutures, or joints, in the skull formed from ossified fibrous tissue.[3]The facial bones affect the physical appearance of the head.[1] Although most of the bones of the skull are fully grown at birth, the development of the different elements are important and intriguing.[4]

Anatomy

Features front view of skull.

The anatomy of the human skull can be seen from three views: the posterior view (from the back), the lateral view (from the side), and the anterior view (from the front). [5] The skull can also be broken down into three main parts: cranial bones, facial bones, and sutures. There are a total of twenty-two bones in the human skull. Eight of these bones are cranial bones and fourteen of these bones are facial bones. The sutures are what holds the entire compilation together. Cranial and facial bones are the foundation for soft tissues covering the head. The quality and shapes of these bones are what form the physical appearance of the face. [2]

Cranial

There are eight bones that make up the cranial bone structure: the ethmoid bone, frontal bone, occipital bone, parietal bone, sphenoid bone, and temporal bone. You may notice that only six bones are listed here. The reason for this being that both the parietal bone and temporal bone are made up of two small bones grouped together under a single name. The ethmoid bone is a cubically shaped bone located at the top of the nasal cavity in between the orbitals (eye sockets) and forms the round structure of the orbitals. The frontal bone is a curved bone anterior to (towards the front of) the skull roof [1] and consists of two parts: the vertical and horizontal portions. The vertical portion of the frontal bone is anatomically known as the squamus frontalis which forms the forehead. The horizontal portion of the frontal bone is anatomically known as the pars orbitalis which forms the top of the orbits and nasal cavity.[6] The occipital bone is a trapezoid shaped bone which is divided into three parts: the squama occipitalis, basilar part, and lateral part. The occipital bone forms the back of the cranium vault. [6] The parietal bone is a flat bone posterior to (towards the back of) the skull roof [1] which consists of two connected bones. Each bone has four borders (saggital border, squamous border, frontal border, and occipital border) and four angles (front angle, occipital angle, sphenoidal angle, and mastoid angle).[6] The sphenoid bone is a butterfly shaped bone located behind the orbitals at the base of the cranium. Its unique shape originates from the six parts it is divided into: the bone body, two winged bones that extend up, and two pterygoid processes that extend down. Lastly, the paired temporal bones are placed behind the ears on the sides of the skull.[1] Each temporal bone contains four parts: the squama temporalis, mastoid portion, petrous portion, and the tympanic portion. [6]

Facial

The facial structure is composed of fourteen bones: the inferior nasal conchae, lacrimal bones, mandible, maxilla, nasal bones, palatine bones, the vomer, and zygomatic bones. As you can see, there are only eight bones listed. The reason for this being that the inferior nasal conchae, lacrimal bones, maxilla, nasal bones, palatine bones, and zygomatic bones are all composed of two bones grouped together under the same title. The inferior nasal conchae are two horizontal bones located in the nasal cavity and can be observed as two parts better known as the nasal conchae and the turbinator. Both of these bones have two surfaces (medial and lateral) and two borders (upper and inferior). [6] The lacrimal bones are the smallest bones of the face. They can be found behind, and lateral to, the nasal cavity. [1] Each lacrimal bone has two surfaces (orbital and nasal) and four borders (anterior, posterior, superior, and inferior) which help form the orbital wall. [6] The mandible, which is the largest and strongest facial bone, consists of two bones that are fused together and able to be categorized into five parts: the body, two rami, the alveolar process, condyle, and coronoid process. [6] The mandible forms the lower jaw and chin. [1] The maxilla is composed of two bones that are also fused together and able to be divided into separate sections. The maxilla contains seven parts: the body, zygomatic process, frontal process, palatine process, infraorbital foramen, and the maxillary sinus. This bone forms the upper jaw and sides of the face.[1] The nasal bones are made up of two separate, touching bones. Each of these bones have two surfaces (inner and outer) and four borders which help form the bridge of the nose. [6] Palatine bones consist of two fused bones [6] which form the back of the roof of the mouth. [1] The vomer is a triangular bone plate located on the base of the nasal cavity [1] with four borders (anterior, posterior, inferior, and superior) [6] which form the nasal septum. [1] Finally, the zygomatic bones. The zygomatic bones are two separate bones each having four borders (orbital, maxillary, temporal, and zygomatic) [6] which prominently form the cheeks. [1]

Sutures

Sutures can be defined as the narrow space between two bones that is filled by fibrous tissue which, overtime, develop into bone and may also be referred to as joints or lines. [3] Once sutures are closed, bone growth will stop on that line. Sutures rarely exceed any wider that 2.5 cenimeters in diameter. [7] In a mature human skull, there are three prominent sutures readily apparent: the sagittal suture, coronal suture, and lambdoid suture.[3] Suture titles are relatively simple. After all, sutures are named for the bones between which they are formed. The sagittal suture is where the parietal bones meet the midline of the skull. The coronal suture is where the parietal bones fuse with the frontal bone. The lambdoid suture is where parietal bones fuse with the occipital bone. This particular suture resembles the Greek letter lambda and was appropriately named in accordance. [3]

Function

The skull serves many purposes when it comes to the body. The cranial bones first and foremost guards the brain by attaching its inner surface to membranes that help maintain the position of the brain. By the attachment of the inner surface of the cranium to stabilizing membranes, blood vessels and nerves are also kept in their correct locations. The outer surface of the cranium allows itself to be used as places of attachment for muscles which allow head movement. The facial bones provide a framework for the soft tissues of the head which affect the physical appearance of the face. These bones protect and support important entry points for both the respiratory and digestive systems. The facial bones also allow a place of attachment for muscles controlling facial expression. Both the cranial and facial bones house, protect, and support sensory organs which regulate vision, taste, smell, hearing, and balance.[1]

Cranial

Cranial bone function can be broken down more specifically by the contribution of individual bones. The ethmoid bone separates the orbitals from the nasal cavity, divides the nasal cavity into the left and right sides, and includes a porous plate (cribriform plate) which helps the nerves controlling smell communicate with the brain. The frontal bone protects the frontal lobe of the brain. The occipital bone houses a cavity (foramen magnum) which allows the brain to connect to the spinal cord and allows movement of the head. The sphenoid bone supports the position of the brain. The temporal bone contains many bones of the ear which enable us to hear. [8]

Facial

As well as the cranial bones, the functions of the facial bones can vary amongst individual bones. The lacrimal bones house the tear ducts. The mandible houses the lower teeth plus the motion of this particular bone is absolutely necessary for the process of chewing, the first step of digestion. The maxilla houses the upper teeth and consists of sinuses that drain fluid into the nasal cavity. The turbinator and nasal conchae form the nasal cavity. [1]

Sutures

Sutures are important for multifarious reasons. When an infant is being born, the sutures of the skull are unjoined (also called fontanelles). This allows the plates of the skull to overlap, making it easier for the newborn to pass through the birthing canal without causing brain damage. As the child ages, sutures become conducive to brain growth or development and protecting the brain from injury as the infant learns to sit up, roll over, and hold up its own head. [4]

Development

The development of the human skull is intriguing. As previously discussed, there are three main sutures visible on a mature human skull: the sagittal suture, the coronal suture, and the lambdoid suture. However, these sutures have developed over time. [3] When a human is first born the gaps in their skull do not begin joined. These unfused, immature spaces are known as fontanelles. Each person is born with six fontanelles but only two of them are readily evident: the anterior fontanel and the posterior fontanel. The anterior fontanel is a diamond shaped gap where the parietal bones meet the frontal bones.[7] This fontanel usually fuses between nine to eighteen months after birth.[4] When this joint ossifies, it is referred to as the coronal suture. The posterior fontanel is where the parietal bones meet the occipital bone.[7] If the posterior fontanel is not already fused at birth, it normally fuses one or two months after birth. [4] When this line ossifies, it is referred to as the lambdoid structure.[7]

Conclusion

Even though the body is made up of multiple types of elements such as tissue, organs, and muscle, the human skull provides various fascinating facts. Each category of the skull provides unique anatomy, important functions, and interesting development. Theses twenty-two bones protect necessary organs that provide sensation (vision, smell, hearing, taste, and balance), provide a foundation for our physical appearance, enables key functions (movement of the head), and supports other systems within the body (respiratory and digestive systems). Residing at the top of the body, the human skull - a small physique for the large roll, remains vitally significant in anatomy.

Video

"Anatomy: Skull Bones" - This video reviews the cranial and facial bones of the skull; it is a thorough description using correct terms.

References

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 Unknown. Bones of the Human Cranium and Face "IvyRose Holistic". Web. February 27, 2013 (accessed).
  2. 2.0 2.1 Unknown. Dataface: Anatomy of the Human Skull A Human Face. Web. February 27, 2013 (accessed).
  3. 3.0 3.1 3.2 3.3 3.4 Romanes, G.J. The Human Skull: Anatomy of Skull Bones, Skull Sutures, Foramina "101". Web. February 27, 2013 (accessed).
  4. 4.0 4.1 4.2 4.3 Unknown. Cranial Suture - Overview "University of Maryland Medical Center". Web. January 24, 2011 (last updated).
  5. Wile, Jay L., and Shannon, Marilyn M. The Human Body: Fearfully and Wonderfully Made!. Cincinnati: Apologia Educational Ministries, Inc., 2001. 83. Print.
  6. 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 Unknown. Skull Bones | Skull Anatomy "LearnBones". Web. 2003 (last updated).
  7. 7.0 7.1 7.2 7.3 Darling, David. Skull "The Encyclopedia of Science". Web. February 27, 2013 (accessed).
  8. Unknown. Anatomy of Skull Bones: Cranial Bones Names, Structure, Function "101". Web. February 27, 2013 (accessed).