Good everyone,Today we going to discuss fully on axial skeleton;
Learning outcomes
After studying this section, you should be able to:
- identify the bones of the skull (face and cranium)
- list the functions of the sinuses and fontanelles of the skull
- outline the characteristics of a typical vertebra
- describe the structure of the vertebral column
- explain the movements and functions of the vertebral column
- identify the bones forming the thoracic cage.
The bones of the skeleton are divided into two groups: the axial skeleton and the appendicular skeleton.
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The axial skeleton consists of the skull, vertebral column, ribs and sternum. Together the bones forming these struc- tures constitute the central bony core of the body, the axis. The appendicular skeleton consists of the shoulder and pelvic girdles and the limb bones.
Skull
The skull rests on the upper end of the vertebral column and its bony structure is divided into two parts: the cranium and the face.
Sinuses
Sinuses containing air are present in the sphenoid, ethmoid, maxillary and frontal bones. They all communicate with the nasal cavity and are lined with ciliated mucous membrane. They give resonance to the voice and reduce the weight of the skull, making it easier to carry.
Cranium
The cranium is formed by a number of flat and irregular bones that protect the brain. It has a base upon which the brain rests and a vault that surrounds and covers it. The periosteum lining the inner surface of the skull bones forms the outer layer of dura mater. In the mature skull the joints (sutures) between the bones are immovable. The bones have numerous perfo- rations (e.g. foramina, fissures) through which nerves, blood and lymph vessels pass. The bones of the cranium are:
- 1 frontal bone
- 2 parietal bones
- 2 temporal bones
- 1 occipital bone
- 1 sphenoid bone
- 1 ethmoid bone.
Frontal bone
This is the bone of the forehead. It forms part of the orbital cavities (eye sockets) and the prominent ridges above the eyes, the supraorbital margins. Just above the supraorbital margins, within the bone, are two air-filled cavities or sinuses lined with ciliated mucous membrane, which open into the nasal cavity.
The coronal suture joins the frontal and parietal bones and other sutures are formed with the sphenoid, zygomatic, lacrimal, nasal and ethmoid bones. The frontal bone originates in two parts joined in the midline by the frontal suture.
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Parietal bones
These bones form the sides and roof of the skull. They articulate with each other at the sagittal suture, with the frontal bone at the coronal suture, with the occipital bone at the lambdoidal suture and with the temporal bones at the squamous sutures. The inner surface is concave and is grooved to accommodate the brain and blood vessels.
Temporal bones
These bones lie one on each side of the head and form sutures with the parietal, occipital, sphenoid and zygomatic bones. The squamous part is the thin fan-shaped area that articulates with the parietal bone. The zygomatic process articulates with the zygomatic bone to form the zygomatic arch (cheekbone).
The mastoid part contains the mastoid process, a thickened region easily felt behind the ear. It contains a large number of very small air sinuses that communicate with the middle ear and are lined with squamous epithelium.
The petrous portion forms part of the base of the skull and contains the organs of hearing (the spiral organ) and balance.
The temporal bone articulates with the mandible at the temporomandibular joint, the only movable joint of the skull. Immediately behind this articulating surface is the external acoustic meatus (auditory canal), which passes inwards towards the petrous portion of the bone.
The styloid process projects from the lower process of the temporal bone, and supports the hyoid bone and muscles associated with the tongue and pharynx.
Occipital bone
This bone forms the back of the head and part of the base of the skull. It forms sutures with the parietal, temporal and sphenoid bones. Its inner surface is deeply concave and the concavity is occupied by the occipital lobes of the cerebrum and by the cerebellum. The occiput has two articular condyles that form condyloid joints with the first bone of the vertebral column, the atlas. This joint permits nodding movements of the head. Between the condyles is the foramen magnum (meaning ‘large hole’) through which the spinal cord passes into the cranial cavity.
Sphenoid bone
This bone occupies the middle portion of the base of the skull and it articulates with the occipital, temporal, parietal and frontal bones. It links the cranial and facial bones, and cross-braces the skull. On the superior surface in the middle of the bone is a little saddle-shaped depression, the hypophyseal fossa (sella turcica) in which the pituitary gland rests. The body of the bone contains some fairly large air sinuses lined with ciliated mucous membrane with openings into the nasal cavity. The optic nerves pass through the optic foramina on their way to the brain.
