AXIAL SKELETON

CHAPTER OBJECTIVES

When you have completed this chapter you should be able to:

Name and identify all the bones of the axial skeleton.

Describe the major landmarks of the cranial and facial bones

Describe the foramen found in the base of the skull.

Describe and discuss the differences between the cervical, thoracic, lumbar, sacral and coccygeal vertebrae.

The axial skeleton forms the central axis of the body. It is made up of the skull, the vertebral column, the ribs and the sternum.

Skull

The bones of the skull can be divided into two categories, those that form the vault or cranium and enclose the brain, and those which form the framework of the face.

Cranial bones

There are 8 cranial bones which form a case to protect and contain the brain;

Bone	No. of Bones	Description	Important Landmarks
Frontal bone	1	The frontal bone is a single convex bone extending from the orbits to the coronal suture behind. It forms the forehead and the roof of the orbits.	Frontal eminences - smooth elevated prominences above the eyebrows. Supra-orbital margins - are two ridges which lie beneath each eyebrow. Orbital plates - two horizontal plates that form the roofs of the orbits. Frontal sinuses - are air filled cavities, lined with a mucous membrane and that lie within the frontal bone above and behind the superciliary arches.
Parietal bones	2	The two parietal bones form the bulk of the vault of the skull behind the frontal bone. Each bone is quadrilateral in shape and articulates with the frontal, occipital, temporal and sphenoid bones as well as with each other.	Grooves for meningeal vessels - the concave internal surface of the parietal bone shows grooves related to the meningeal vessels. Groove for the superior sagittal sinus - a groove present along the internal surface of the sagittal margin.
Occipital bone	1	The occipital bone forms the back and base of the skull. It is divided into squamous, lateral and basilar parts.	External occipital protuberance - an external pronounced lump on the back of the head. Foramen magnum - a large, ovoid opening in the floor of the posterior cranial fossa. Occipital condyles - located on the either side of the foramen magnum on the lateral parts of the occipital bone they articulate inferiorly with the atlas at the atlanto-occipital joints.
Temporal bone	2	Each temporal bone contributes to the base and to the lower lateral aspect of the skull. It is divided into squamous, petrous, mastoid and tympanic parts. The squamous part is thin, translucent and forms the anterior and upper part of the bone. The petrous part is the solid, wedge of bone that forms most of the posterior and inferior portions of the temporal bone. The mastoid part of the temporal bone lies below the squamous part and behind the tympanic part. The tympanic part of the temporal bone surrounds the external auditory meatus.	Zygomatic process - an arched process that projects from the lower part of the squamous part to articulate with the zygomatic bone. Mastoid process - a large prominence located immediately behind the external acoustic meatus; it gives attachment to the sternocleidomastoid muscle. External acoustic meatus - the opening into the temporal bone Styloid process - is an elongated, narrow projection of bone which passes downwards and forwards from the base of the tympanic part.
Ethmoid bone	1	The ethmoid is a single mid line bone which forms parts of the nasal septum, medial wall of the orbital cavity and the roof and lateral wall of the nose.	Cribriform plate - a thin plate of bone that forms a large part of the nasal roof. It is perforated with holes (olfactory foramina), which transmit the olfactory nerves from the nose to the olfactory bulbs. Perpendicular plate - a thin, mid line quadrilateral plate of bone, which descends vertically from the cribriform plate to form the superior part of the nasal septum. Ethmoidal labyrinths - a network of air cells which lie under the cribriform plate. Superior nasal concha - upper, thin scroll-like plate of bone that hangs down from the medial surfaces of the ethmoidal labyrinth. Middle nasal concha - lower, thin scroll-like plate of bone that hangs down from the medial surfaces of the ethmoidal labyrinth.
Sphenoid bone	1	This single bone is found in the base of the skull and is often described as being ‘butterfly-shaped’ as it consists of a central body and three paired processes.	Greater wings - two strong processes of bone, which arise from the sides of the body. Lesser wings - are two thin triangular plates, which arise from the upper and anterior parts of the body. Pituitary (hypophysial) fossa - a deep pit in the body, which houses the pituitary gland. There are usually two air sinuses within the body of the sphenoid bone which communicate with the nasal cavity. Optic canals - two holes located where the lesser wing attaches to the body; they transmit the optic nerves. Superior orbital fissures - a large fissure between the greater and lesser wings at the back of the orbit. It transports important nerves and vessels into the orbit.

SKULL VIEWED FROM THE SIDE

SKULL CUT VIEWED FROM ABOVE

Clinical Considerations

Sinusitis

The frontal sinuses within the frontal bone of the cranium are lined by mucous membrane and may become infected, causing sinusitis.

SELF-TEST

Complete the following questions before you go onto the next section:

Name the bones that make up the cranium

See if you can work out which bones articulate with each other.

Name 5 important landmarks on bones of the cranium.

Facial bones

These 14 bones do not contribute to the cranial cavity, but form the structure of the face;

Bone	Number of bones	Description	Important Landmarks
Maxilla	2	The paired maxillary bones support the teeth of the upper jaw and contribute to much of the skeleton of the upper face.	Alveolar process - extends inferiorly from the body of the maxilla and supports the teeth within bony sockets. Zygomatic process - projects laterally from the body and articulates with the zygomatic bone. Palatine process - extends horizontally to form most of the hard palate. Maxillary sinuses - are the largest of the paranasal sinuses and are situated in the bodies of the maxillary bones. It communicates with the nasal cavity through the maxillary hiatus.
Zygomatic bones	2	The two zygomatic bones form the skeleton of the cheeks and the inferior and lateral walls of the orbit.	Frontal process - forms outer margin of the orbit by articulating with the zygomatic process of the frontal bone. Temporal process - forms the prominent zygomatic arch by articulating with the zygomatic process of the temporal bone.
Mandible	1	The mandible is the only movable bone in the skull. It consists of a horizontal, horseshoe-shaped body and two vertical rami.	Body - an arch that forms the main bulk of the mandible. Alveolar ridge - forms the superior margin of the body of the mandible and houses the lower teeth. Rami - these are the flat vertical projects that ascend from the obtuse angle of the body. Coronoid - found on the top of the ramus it lies anteriorly and serves as an attachment point for temporalis. Condylar processes - found on the top of the ramus it lies posteriorly and articulates with the temporal bone at the temporomandibular joint (TMJ).
Palatine bones	2	The palatine bone forms part of the hard palate, the floor and lateral wall of the nasal cavity and the floor of the orbit.	Horizontal plate - forms the posterior portion of the hard palate. Perpendicular plate - the medial surface forms the lateral wall of the nasal cavity and articulates with the inferior nasal concha. Orbital process - projecting from the perpendicular plate it forms the posterior part of the orbital floor.
Hyoid bone	1	The horseshoe-shaped hyoid bone is situated in the upper part of the front of the neck, at the level of C3. It does not articulate with any bones but is maintained in position by the muscles, ligaments and membranes attaching to it. It gives attachment to the tongue muscles.	Body - the curved medial portion of the bone. Lesser cornu - two small superior projections from the junction between the body and the greater cornu. Greater cornu - two backward projections from the body.
Inferior nasal conchae	2	The inferior nasal conchae are curved plates of bone attached to the lateral wall of the nasal cavity covered in mucous membrane. There are also superior and middle conchae but they are part of the ethmoid bone.	It has medial and lateral surfaces. Superior border - attaches the bone to the lateral wall of the nasal cavity. Inferior border - thick and curved inwards, it lies free within the nasal cavity.
Lacrimal bones	2	The paired lacrimal bones are small, thin and rectangular, and each lie in the anterior part of the medial wall of the orbit. They house the lacrimal sac which collects tears from the eyes via the nasolacrimal duct and empties them into the nasal cavity.	Lacrimal fossa - a depression that houses the lacrimal sac.
Nasal bones	2	The two nasal bones form the upper part of the bridge of the nose.	Superior border - articulates with the frontal bone. Inferior border - is continuous with the lateral nasal cartilage.
Vomer	1	The vomer is a flat bone which forms the lower part of the septum of the nose.	Alae - a wing like projection which articulates with the sphenoid bone. Anteriorly it articulates with the septal cartilage.

SKULL VIEWED FROM THE FRONT

SELF-TEST

Complete the following questions before you go onto the next section:

Which facial bones contribute to the orbit?

Name 5 important landmarks of the mandible.

Which bone does not articulate with any other bone; what holds it in position?

Trunk

The trunk includes the thorax and abdomen and is supported by the thoracic cage and vertebral column.

Thoracic cage

Bone	Number of bones	Description	Important Landmarks
Sternum	1	The sternum is a flat bone forming the middle part of the anterior thoracic wall. It is constituted from three bones; the manubrium sterni, body and xiphoid process. Manubrium - a triangular portion on top of the body. Body - the largest part of the sternum. Xiphoid process - is the narrow irregular shaped inferior end of the sternum.	Suprasternal notch - a large indentation in the superior border of the manubrium. Articular fossae for clavicles - two indents found either side of the suprasternal notch for articulation with the clavicles. Articular facets for the 1st costal cartilages - shallow depressions either side of the manubrium for articulation with the 1st costal cartilage. Sternal angle - the junction between the manubrium and the sternal body. Facets for the costal cartilages - indents found on the lateral borders of the body for articulation with the 2nd - 7th costal cartilages.
Ribs	24	The ribs are 12 paired bones which form the curved walls of the thorax. The first 7 ribs are termed 'true ribs' and join the sternum directly via costal cartilages. The 8th to 10th pairs of ribs are joined to the sternum via the cartilage of the rib above and so termed 'false ribs'. The lowest 2 ribs are unconnected to the sternum and termed 'free-floating'. All the ribs are connected behind to the vertebral column.	Head - slightly expanded, it is found at the back of the rib and has two articular facets for articulation with the corresponding thoracic vertebrae. Shaft - the longest part of the rib, it is thin and flat with a curved convex external surface. Tubercle - located at the angle of the rib at the junction between the neck and the shaft, it has an articular part for articulation with the transverse process of the corresponding thoracic vertebrae.

Vertebral column

The vertebral column forms the axis of the trunk and is formed by 31 firmly connected irregular bones. It allows minimal movement and encases and protects the spinal cord. The vertebral column displays a series of normal curvatures when viewed from the side (in the sagittal plane); the cervical, thoracic, lumbar and pelvic curves. The cervical curve is convex forwards, the thoracic curve is concave forwards, the lumbar is convex forwards and the pelvic curve is concave forwards and downwards.

The thoracic and pelvic curves are primary curves and are present at birth; the cervical and lumbar curves are secondary curves and develop after birth. The cervical curve develops when the child is able to hold its head up and the lumbar curve when they begin to walk.

The curves give the vertebral column strength when in the upright position by distributing the weight evening and acting as a shock absorber.

NORMAL CURVES OF THE VERTEBRAL COLUMN

Clinical Considerations

Abnormal curves of the spine

The overall alignment of the spine can be altered in many conditions; degenerative, congenital or traumatic and can be severe enough to result in an abnormal curvature of the spine. Abnormal curves are described as below and their severity is measured using Cobb's Measurement Method.

All vertebrae possess similar main features, but the size and shape of them change depending on their position along the vertebral column. The vertebral body is the most anterior and largest structure of each vertebra, with the exception of the first cervical vertebra which does not have a body. Each body is separated from the bodies of adjacent vertebra by a fibrous intervertebral disc. Behind the body is a hole, the vertebral canal (foramen) for the passage of the spinal cord and meninges. Projecting posteriorly from the vertebral body are two stout pedicles, which together with the laminae and spinous process form the vertebral arch posteriorly. Between the pedicles of adjacent vertebrae are openings called intervertebral foramen; they allow the exit of the spinal nerves from the vertebral canal. The two laminae are broad, flat structures which emerge posteriorly from each pedicle; they join in the mid line to form a posterior projection, the spinous process. The transverse processes project laterally from the sides of the vertebrae at the junctions between the laminae and the pedicles. The superior articular facets project superiorly and the inferior articular facets project inferiorly from the junctions between the laminae and the pedicles. They articulate with the articular facets of the adjacent vertebrae.

Bone	Number of bones	Description	Important Landmarks
Cervical vertebrae	7	The cervical vertebrae form the neck. The 1st (atlas) and 2nd (axis) cervical vertebrae are specialised to allow the head to rotate on the neck. The atlas does not have a body; it is a ring of bone that articulates with the occipital bone of the cranium. The body of the axis has an upward projection (dens) which the atlas rotates around.	Vertebral foramen - large and triangular. Body - small and broad the bodies are separated from each other by the cervical intervertebral discs; except between the 1st and 2nd vertebrae where there is no disc. Spinous process - short bifid processes. Transverse processes - short with a transverse foramen for the passage of the vertebral arteries. Superior articular facets - they point postero-superiorly to articulate with the inferior facets of the vertebra above. Inferior articular facets - they point antero-inferiorly to articulate with the superior facets of the vertebra below.
Thoracic vertebrae	12	The thoracic vertebrae form the vertebral column of the thorax. The 12 bones articulate with the 12 ribs.	Vertebral foramen - small and round. Body - heart-shaped, they are separated from each other by the fibrous thoracic intervertebral discs. Spinous processes - long and thin. Transverse processes - large and club-like. Superior articular facets - they point postero-superiorly to articulate with the inferior facets of the vertebra above. Inferior articular facets - they point antero-inferiorly to articulate with the superior facets of the vertebra below.
Lumbar vertebra	5	The lumbar vertebrae are the largest vertebrae as they are designed to support the weight of the body.	Vertebral foramen - triangular. Body - large and kidney-shaped, they are separated from each other by the fibrous lumbar intervertebral discs. Spinous process - quadrangular. Transverse process - long and thin. Superior articular facets - they point medially and slightly posteriorly to articulate with the inferior facets of the vertebra above. Inferior articular facets - they point laterally and slightly anteriorly to articulate with the superior facets of the vertebra below.
Sacrum	5	The 5 sacral vertebrae fuse to each other to form the large triangular sacrum. It is concave anteriorly. It articulates laterally with the hip bones.	Sacral canal - triangular it is the vertebral canal of the sacrum and contains the cauda equina and the lower dural sac. Body - the bodies are fused together to form the ventral and dorsal surfaces of the sacrum. Sacral promontory - the upper border of the upper end of the 1st sacral vertebral body. Ventral and dorsal sacral foramina - 4 holes on the front and 4 holes on the back of the sacrum which communicate with the vertebral canal and transmit the sacral spinal nerves. Median sacral crest - an irregular raised ridge on the back of the sacrum; it represents the fused spinous processes of the sacrum. Alae (lateral parts) - wide above and narrower below, they represent the fused transverse processes and pedicles of the sacrum. Sacral hiatus - an inverted U-shaped opening in the posterior wall of the sacral canal.
Coccygeal vertebrae	3-5	The 3-5 coccygeal vertebrae fuse to each other to form the coccyx.	Cornu - two processes that project from the sides of the 1st coccygeal vertebra to articulate with the sacrum above.

A TYPICAL CERVICAL, THORACIC , AND LUMBAR VERTEBRA VIEWED FROM ABOVE

SELF-TEST

Complete the following questions before you go onto the next section:

Describe the important landmarks found on a typical rib.

Describe the differences between the cervical, thoracic and lumbar vertebrae.

Which cervical vertebrae are atypical; explain.

Curvature	Description
Scoliosis	Side-to-side (lateral) curvatures.
Kyphosis	Hunchback curve (forward bend) commonly found in the thoracic or thoracolumbar regions.
Lordosis	Excessive inward curve of the spine, commonly found in the lumbar region.

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