The Axial Skeleton
• Forms longitudinal axis of the
body
• 80 bones
• 40% of the bones in the human
body
Axial Skeleton
• Three Regions:
1. Skull (8 cranial & 14 facial)
** bones associated with skull (6 auditory
ossicles and hyoid)
2. Vertebral column (24 vertebrae, the sacrum &
coccyx)
3. Thoracic cage (sternum & 24 ribs)
The Skull
• The bones of the skull protect the brain and guard the
entrances to the digestive & respiratory systems
• The skull (22 bones), the body’s most complex bony
structure, is formed by the cranium (8 bones) and
facial bones (14 bones)
• 6 auditory ossicles (tiny bones) are situated within the
temporal bones of the cranium (smallest bones in the
body that are contained in the middle ear space;
hammer, anvil, stirrup)
• Hyoid bone (connected to the inferior surfaces of the
temporal bones)
The Skull
• Cranium – protects the brain and is the site of attachment
for head and neck muscles
• Facial bones
– Supply the framework of the face, the sense organs, and
the teeth
– Provide openings for the passage of air and food
– Anchor the facial muscles of expression
Anatomy of the Cranium
Eight cranial bones:
1.
2.
3.
4.
5.
6.
•
•
2 parietal
2 temporal
Frontal
Occipital
Sphenoid
Ethmoid
The cranial bones enclose the cranial cavity, a fluidfilled chamber that cushions and supports the brain
Cranial bones are thin and remarkably strong for
their weight
Skull – Anterior View
Figure 7.2a
Frontal Bone
• Forms the anterior portion
of the cranium & the roof of
the orbits (eye sockets)
Parietal Bones
• Forms most of the superior and lateral aspects of the
skull
Figure 7.3a
Parietal Bones & Major Associated Sutures
• Four sutures mark the articulations of the parietal
bones
1. Coronal suture – articulation between parietal bones and
frontal bone anteriorly
2. Sagittal suture – where right and left
parietal bones meet superiorly
Parietal Bones & Major Associated Sutures
3. Lambdoid suture – where parietal bones meet the
occipital bone (posterior)
4. Squamosal or
squamous suture –
where parietal and
temporal bones meet
Occipital Bone
• Located at the back and lower part of the
cranium
Temporal Bones
Form part of both the
lateral walls of the
cranium &
zygomatic arches
Divided into four
major regions:
A. Squamous,
B. Tympanic,
C. Mastoid
D. Petrous
Figure 7.5
Sphenoid Bone
• Butterfly-shaped bone that forms part of the
floor of the cranium, unites the cranial and
facial bones, and acts as a cross brace that
strengthens the sides of the skull
• Forms the central wedge that articulates with
all other cranial bones
Ethmoid Bone
• Most deep of the skull bones; lies between the
sphenoid and nasal bones
Figure 7.7
Facial Bones
• Fourteen bones of which only the mandible
and vomer are unpaired
• The paired bones are the maxillae,
zygomatics, nasals, lacrimals, palatines, and
inferior conchae
Mandible
• The mandible (lower
jawbone) is the
strongest bone of the
face
Figure 7.8a
Maxillary Bones
• Medially fused bones that make up the upper
jaw and the central portion of the facial
skeleton (largest facial bones)
Figure 7.8b
Zygomatic Bones
• Irregularly shaped bones (cheekbones) that
form the prominences of the cheeks and the
inferolateral margins of the orbits
Other Facial Bones
• Nasal bones – thin medially fused bones that form the
bridge of the nose
• Lacrimal bones – contribute to the medial walls of the
orbit and contain a deep groove that house the tear
ducts
• Palatine bones – two bone plates that form portions
of the hard palate and contribute to the floor of each
orbit
Other Facial Bones
• Vomer – forms part of the nasal septum
• Inferior nasal conchae – paired, curved bones
in the nasal cavity that form part of the
lateral walls of the nasal cavity
Paranasal Sinuses
Figure 7.11
Hyoid Bone
• Lies just inferior to the mandible in the anterior neck
• Only bone of the
body that does not
articulate directly
with another bone
• Attachment point
for neck muscles
that raise and
lower the larynx
during swallowing
and speech
Figure 7.12
Vertebral Column
• 26 irregular bones (vertebrae)
• Provide a column of support, bearing the
weight of the head, neck, and trunk.
• Transfers weight to the appendicular skeleton
of the lower limbs
• Protects spinal cord
• Helps maintain an upright body position
• Approx. length of an adult column is 71cm
Vertebral Column
Cervical vertebrae
7 bones of the neck
Thoracic vertebrae
12 bones of the torso
Lumbar vertebrae
5 bones of the lower back
Figure 7.13
Vertebral Column
Sacrum - 5 fused vertebrae
– bone inferior to the
lumbar
– vertebrae that articulates
with the hip bones
Coccyx – 4 fused vertebrae
Figure 7.13
Disks are small shock absorbers between the
vertebrae (gel-like interior)
General Structure of Vertebrae:
1. Vertebral body (centrum) – disc-shaped,
weight-bearing region
2. Vertebral arch – composed of pedicles (walls)
and flat layers called laminae (roof)
** forms the posterior margin of each vertebral
foramen (together they form the vertebral
canal which encloses the spinal cord)
3. Articular processes– projections on each
vertebra
Cervical Vertebrae
Table 7.2
Cervical Vertebrae
• Most mammals have 7 cervical vertebrae
(giraffes, whales, mice & humans)
• Seven vertebrae (C1-C7) are the smallest,
lightest vertebrae
Cervical Vertebrae: The Atlas (C1)
– Holds up the head
– The superior surface articulates with the occipital
condyles of the skull (permits you to nod)
»Has no body and no spinous process
Cervical Vertebrae: The Axis (C2)
• The axis has a body, spine, and vertebral
arches as do other cervical vertebrae
• Articulates with the atlas to permit rotation
Figure 7.16c
Thoracic Vertebrae
• There are twelve vertebrae (T1-T12)
• Distinctive heart-shaped body (more massive than that of a
cervical vertebra)
• Each thoracic vertebra articulate with ribs
Lumbar Vertebrae
• The five lumbar vertebrae (L1-L5) are located in the
small of the back and have an enhanced weightbearing function
• Largest vertebrae
Tip: Mealtimes
Breakfast: 7 a.m. (7 cervical)
Lunch: 12 p.m. (12 thoracic)
Dinner: 5 p.m. (5 lumbar)
Sacrum and Coccyx
• The sacrum
– Consists of five fused vertebrae (S1-S5), which
shape the posterior wall of the pelvis
– Begin fusing after puberty and are completely
fused at age 25-30
– Protects reproductive, digestive, and urinary organs
– It articulates with L5 superiorly, and with the
auricular surfaces of the hip bones
Coccyx
• Coccyx (Tailbone)
– The coccyx is made up of four (in some cases
three to five) fused vertebrae that articulate
superiorly with the sacrum
– Generally begun fusing by age 26
Sacrum and Coccyx
Figure 7.18a
Sacrum and Coccyx
Figure 7.18b
Bony Thorax (Thoracic Cage)
• The thoracic cage is composed of the thoracic
vertebrae, the ribs, and the sternum
• Functions
– Forms a protective cage around the heart, lungs,
and great blood vessels
– Supports the shoulder girdles and upper limbs
– Provides attachment for many neck, back, chest,
and shoulder muscles
Bony Thorax (Thoracic Cage)
Figure 7.19a
Sternum (Breastbone)
• A dagger-shaped, flat bone that lies in the anterior
midline of the thorax
• Fusion is not complete until at least age 25 (until this
age the sternal body consist of four separate bones)
Ribs
• There are twelve pair of ribs
• All ribs attach posteriorly to the thoracic vertebrae
• The superior 7 pair (true, or vertebrosternal ribs)
attach directly to the sternum via costal cartilages
• Ribs 8-10 (false, or vertebrocondral ribs) attach
indirectly to the sternum via costal cartilage
• Ribs 11-12 (floating, or vertebral ribs) have no anterior
attachment
Ribs
Figure 7.19a