Dr. Martin's notes

Monday September 19, 2016

The Skull

CRANIAL BONES. The skull, in the proper use of the term, includes both the facial and cranial bones. The bones of the cranium enclose and protect the brain and its associated structures, the special sense organs. The muscles of mastication as well as the muscles for head movements are attached to the cranium. At certain locations within the cranium, cavities, or air sinuses, are present.

The individual bones of the cranium are immovably united at sutures, or juncture lines. During infancy and early childhood, the articulation is formed by sheets of fibrocartilaginous tissue which gradually ossify. Union of the cranial bones continues as the bone itself grows by increments at its outside edges; thus, the bones grow toward each other, so to speak, and eventually meet at suture lines,

Frontal Bone. The frontal bone forms the forehead, roof of the nasal cavity, and orbits, the bony sockets which contain the eyes. It develops in two halves which fuse by the end of the second year of life. The notable markings of the frontal bone are the orbital margin, a definite ridge above each orbit, and the supraorbital ridge, a prominence overlying the frontal sinus.

Parietal Bone. The two parietal bones form the sides and roof of the cranium and are joined at the sagittal suture in the midline. The line of articulation between the frontal bone and the two parietal bones is called the coronal suture.

Occipital Bone. The occipital bone forms the back and base of the cranium and joins the parietal bones anteriorly. The inferior portion of the bone has a large opening, the foramen magnum (literally, great opening), through which the spinal cord passes. It is at the level of the foramen magnum that the spinal cord joins the medulla oblongata of the brain. On each lower side of the occipital bone is a process called the occipital condyle for articulation with the first vertebra. Other obvious projections are the external occipital crest and the external occipital protuberance. Inside of skull

Temporal Bone. The paired temporal bones help to form the sides and base of the cranium. Each encloses an ear and bears a fossa for articulation with the lower jaw. Projecting forward from the lower part of the squamous is the zygomatic process, forming the lateral part of the zygomatic arch or cheek bone. The petrous contains the middle and inner ear within its complexly fashioned cavities.The mastoid portion is located behind and below the meatus or opening of the ear. The mastoid process is a rounded projection of the temporal bone easily found behind the external ear. Several muscles of the neck are attached to the mastoid process.

Sphenoid Bone. The sphenoid bone forms the anterior portion of the cranium. It is a single, wedge-shaped bone having a central body and two expanded wings that articulate with the temporal bones on either side. Thus, it serves as a. kind of anchor, binding the cranial bones together.

Ethmoid Bone. The ethmoid is the principal supporting structure of the nasal cavity and contributes to the formation of the orbits. Each labyrinth exhibits two or three bony plates, the nasal conchae or turbinate bones, which project into the nasal cavity and allow for circulation and filtration of inhaled air.

Auditory Ossicles. Three bones of the ear--the malleus, incus, and stapes -- are highly specialized in both structure and function.

Wormian Bones. The so-called wormian bones are located within the sutures of the cranium. These wormian bones are inconstant in number. They are small and irregular in shape, and are not included in the total number of bones in the body. In summary, then, the cranial portion of the skull consists of the following bones:

1 frontal
2 parietal
1 occipital
2 temporal
1 sphenoid
1 ethmoid
6 auditory ossicles
wormian bones (variable number)

Like those of the cranium, the bones of the face are immovably united by sutures, with a single exception - the mandible.

Nasal Bone. The paired nasal bones join to form the bridge of the nose.

Palatine Bone. The two palatine bones form the posterior part of the roof of the mouth, or hard palate. This area is the same as the floor of the nose.

Maxillary Bone. The two maxillae constitute the upper jaw. Each maxillary bone consists of a body, a zygomatic process, a frontal process, a palatine process, and an alveolar process. The zygomatic process extends laterally to participate in the formation of the cheek. The alveolar process bears the teeth of the upper jaw.

Zygomatic Bone. The two bones forming the prominence of the cheek are also called malar bones. The zygomatic bone has a frontal process extending upward to articulate with the frontal bone and a smaller temporal process articulating laterally with the temporal bone, thus forming the easily identified zygomatic arch,

Lacrimal Bone. The paired lacrimal bones make up part of the orbit at the inner angle of the eye. The lateral surface of the bone presents a fossa which lodges the lacrimal sac and provides a groove or canal for the passage of the lacrimal duct.

Inferior Turbinate Bone. The two nasal conchae, or turbinate bones, are similar to those described with the ethmoid; the conchae of the ethmoid, however, occupy superior and middle portions.

Vomer. The single, flat vomer constitutes the lower posterior portion of the nasal septum.

Mandible . Although the mandible develops in two parts, the intervening cartilage ossifies in early childhood and the bone becomes fused into a single continuous structure. This is called the inferior maxillary bone. The lower jaw is the strongest and longest bone of the face.. On either side of the body are the rami which extend perpendicularly upward. Each ramus presents a condyle or condyloid process for articulation with the mandibuIular fossa of the temporal bone.

In summary, then, the facial portion of the skull consists of the following bones:

1 mandible
1 vomer
2 maxillary
2 zygomatic
2 nasal
2 lacrimal
2 inferior nasal conchae
2 palatine         .

ORBITS. The orbits are the two deep cavities in the upper portion of the face that serve to protect the eyes.

The Hyoid Bone
The single hyoid bone is the unique component of the axial skeleton since it has no articulations. Rather, it is suspended from the styloid process of the temporal bone by two stylohyoid ligaments.