W2 - Axial Skeleton Flashcards
LO1: Describe the bones of the axial skeleton: skull, vertebral column and thoracic skeleton (what is in the axial skeleton)
The axial skeleton consists of the bones of the head and trunk of the body. It is composed of three main parts: the skull (including the ossicles of the middle ear and a bone associated with the skull called the hyoid bone), the vertebral column and the thoracic skeleton. Let’s examine these three parts in more detail, starting with the skull.
2 categories of the skull
Cranial Bones
Eight bones form the cranium: unpaired frontal, occipital, sphenoid and ethmoid bones and paired temporal and parietal bones. These bones not only form the cavity for the brain but also provide attachment sites for the muscles of the head, face and neck.
Facial Bones
Fourteen bones form the facial skeleton: unpaired mandible and vomer and paired maxillae, inferior nasal conchae, zygomatic, nasal, lacrimal and palatine bones. They form the entrances for the digestive and respiratory tracts and provide attachment sites for the facial muscles.
Sutures of the skull
Except for the temporomandibular joint which unites the temporal bone and mandible, all joints of the skull are immovable and are called sutures.
- The lambdoid suture unites the occipital bone with the two parietal bones
- The coronal suture unites the frontal bone with the two parietal bones
- The sagittal suture unites the two parietal bones; it runs in the sagittal plane, hence its name
Cavities of the skull
The skull contains several prominent cavities. The largest cavity is the cranial cavity that encloses the brain. Several smaller cavities include:
Orbits
Oral cavity
Nasal cavity
Paranasal sinuses
Identifying major bones from the anterior view of the skull
Frontal Bone
The frontal bone forms the forehead.
Orbits
The orbits are formed by the articulation of multiple bones, including the maxillae, frontal, zygomatic, sphenoid, ethmoid and lacrimal bones.
Nasal Bones
The left and right nasal bones form the bony bridge of the nose.
Nasal Septum
The bony nasal septum is seen through the piriform aperture, which is the anterior opening of the bony nasal cavity (piriform means pear-shaped). The bony nasal septum is formed by the vomer and a part of the ethmoid bone called the perpendicular plate.
Maxillae
The left and right maxillae join to form the upper jaw and the lateral boundaries of the nasal cavity.
Mandible
The mandible forms the lower jaw.
Identifying major bones from the lateral view of the skull
Greater Wing of Sphenoid
This is a part of the sphenoid bone called the greater wing.
Medial Wall of the Orbit
The medial wall of the orbit is formed with contributions from the ethmoid and lacrimal bones.
Zygomatic Arch
The zygomatic arch is a prominent feature on the lateral aspect of the skull that is formed by the zygomatic and temporal bones.
Mastoid Process
On the temporal bone, below the external acoustic meatus which is the opening of the external ear, note the feature called the mastoid process. You can easily palpate this as a large bony bump found just behind the ear.
Anterior cranial fossa
Formed by part of frontal bone (orbital plate) that surrounds the cribriform plate (part of ethmoid bone - forms the bony roof of the nasal cavity)
Contains olfactory foramina (passage for olfactory nerves responsible for smell)
Posterior cranial fossae
Internal acoustic meatus: paired openings within the petrous part of the temporal bone for the facial and vestibulocochlear nerves
Jugular foramen: paired openings sitting in between the petrous part of the temporal bone and occipital bone for the glossopharyngeal, vagus and accessory nerves.
Also transmits the huge internal jugular veins (formed as continuations of the sigmoid intracranial venous sinuses)
Hypoglossal canal: paired openings on either side of foramen magnum for the hypoglossal nerve
Foramen magnum: Huge opening where the brainstem is continuous with the spinal cord
Middle cranial fossae
Optic canal: for optic nerve - vision
Superior orbital fissure: for several cranial nerves travelling into the orbit
Foramen rotundum: for one of the main branches of the trigeminal nerve - main sensor nerve of the face
Foramen ovale: for one of the main branches of the trigeminal nerve - main sensor nerve of the face
Cranial nerves
Hyoid bone
Bone of the axial skeleton found in the neck that does not articulate directly with any other bone. It is supported by ligaments and serves as an attachment site for muscles of the tongue and neck.
The vertebral column
The adult vertebral column is typically composed of 26 bones, including 24 movable vertebrae and the fused vertebrae that form the sacrum and the coccyx. The vertebral column is divided into 5 regions.
Cervical Region
Includes 7 cervical vertebrae (C1 to C7). C1 articulates with the occipital condyles of the skull, while C7 articulates with the 1st thoracic vertebra.
Thoracic Region
Includes 12 thoracic vertebrae (T1 to T12), each articulating with the ribs laterally.
Lumbar Region
Includes 5 lumbar vertebrae (L1 to L5).
Sacral Region
Includes 5 fused sacral segments (S1 to S5) to form the sacrum.
Coccygeal Region
Includes anywhere from 3 to 5 coccygeal segments to form the coccyx.
Curvatures of the vertebral column
The thoracic and sacral curvatures are convex posteriorly. These curvatures are called primary curvatures as they are present at birth. They are also called kyphoses (singular = kyphosis).
The cervical and lumbar curvatures are concave posteriorly. These curvatures are called secondary curvatures as they are formed after birth due to weight-bearing. The cervical curvature forms when a baby starts to lift its head, and the lumbar curvature forms when a toddler starts to walk. They are also called lordoses (singular = lordosis).
Distortions of the vertebral column
hyperkyphosis (hunchback)
This is an excessive outward (posterior) curvature of the vertebral column, causing hunching of the back. It commonly affects the thoracic region.
Hyperlordosis (swayback)
This is an excessive inward (anterior) curvature of the vertebral column. It commonly affects the lumbar region in pregnant women.
Scoliosis
This is a lateral (sideways) deviation of the vertebral column. It may be caused by congenital or acquired abnormalities of the vertebrae, muscles, and/or nerves.
Features of a typical vertebrae
A typical vertebra consists of a body anteriorly and a vertebral arch posteriorly, separated by a vertebral foramen.
The vertebral arch includes the pedicles, laminae and 7 processes: one spinous, two transverse, two superior articular and two inferior articular processes.
The articular process have articular facets on them to allow for articulation of adjacent vertebrae.
The articular processes of adjacent vertebrae are united by synovial joints, while the vertebral bodies of adjacent vertebrae are united by the intervertebral discs.
Types of vertebrae
Typical Cervical Vertebra
Small, oval body
Large, triangular vertebral foramen
Short, bifid spinous process
Transverse foramen in the transverse process (for vertebral artery)
Typical Thoracic Vertebra
Heart shaped body
Small vertebral foramen
Costal facets on body & transverse process (for ribs)
Long spinous process
Typical Lumbar Vertebra
Large, kidney-shaped body
Large vertebral foramen
Quadrate spinous process
Atlas
The first cervical vertebra, C1, is called the atlas and is ring-shaped. It has no body, but rather two lateral masses united by anterior and posterior arches. The lateral masses have two articular facets (superior and inferior) for articulation with the occipital condyles of the skull above and the C2 vertebra below.
Axis
The second cervical vertebra, C2, is called the axis due to the presence of the odontoid process or dens that, when articulated with the anterior arch of C1, acts as an axis for rotatory movements of the head.
Atlanto-Axial joints
Here you can see the articulated C1 and C2 vertebrae at the joint called the atlanto-axial joint. The anterior arch of C1 articulates with the dens of C2. The inferior articular facets of C1 articulate with the superior articular facets of C2. This creates an arrangement that facilitates the pivoting movements of C1 on top of C2.
The thoracic skeleton
True Ribs
The first seven ribs are called true ribs as they directly articulate with the sternum via their costal cartilages (costal refers to the ribs), while the rest are called false ribs.
False Ribs
Ribs 8 to 10 indirectly articulate with the sternum via the costal cartilages of the ribs above. The 11th and 12th ribs are called floating ribs because they do not articulate with the sternum at all.
Sternum
The sternum consists of the three fused parts called the manubrium, body and xiphoid process. Important landmarks of the sternum include the jugular (suprasternal) notch, the sternal angle at the manubriosternal joint, which is the joint between the manubrium and the body of the sternum, and the xiphisternal joint, which is the joint between the body and xiphoid process of the sternum.
Typical rib
A typical rib consists of a head with two facets, a neck, a tubercle and a shaft.
The facets on the head of the rib articulate with the facets on the lateral sides of the thoracic vertebral bodies, while the facet on the tubercle of the rib articulates with the facet on the transverse process of the numerically corresponding thoracic vertebra.
There is a costal cartilage on the anterior end of each rib for articulation with the sternum (either directly or indirectly), excluding ribs 11 and 12, as these do not articulate with the sternum.
The internal surface of the rib features the costal groove that contains the intercostal neurovascular bundle, which is a bundle of blood vessels and nerves travelling between the ribs.
