The Axial SkeletonForms longitudinal axis of the

body80 bones 40% of the bones in the

human body

Axial SkeletonThree Regions:

1. Skull (8 cranial & 14 facial)** bones associated with skull (6 auditory ossicles and hyoid)

2. Vertebral column (24 vertebrae, sacrum & coccyx)

3. Thoracic cage (sternum & 24 ribs)

Anatomy of the CraniumEight cranial bones:

1. 2 parietal2. 2 temporal3. Frontal4. Occipital5. Sphenoid6. Ethmoid

The cranial bones enclose the cranial cavity, a fluid-filled 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

Occipital BoneLocated at the

back and lower part of the cranium

Temporal Bones

Form part of both the lateral walls of the cranium & zygomatic arches

Figure 7.5

Parietal Bones & Major Associated SuturesFour 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 Sutures3. Lambdoid suture – where parietal bones meet

the occipital bone (posterior)

4. Squamosal or squamous suture – where parietal and temporal bones meet

Sphenoid BoneButterfly-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 BoneMost deep of the skull bones; lies between

the sphenoid and nasal bones

Figure 7.7

Facial BonesFourteen 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 BonesMedially fused bones that make up the upper jaw and the

central portion of the facial skeleton (largest facial bones)

Figure 7.8b

Zygomatic BonesIrregularly shaped bones (cheekbones)

that form the prominences of the cheeks and the inferolateral margins of the orbits

Other Facial BonesNasal 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 continued…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

Hyoid BoneLies 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 Column26 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 limbsProtects spinal cordHelps maintain an upright body positionApprox. length of an adult column is 71cm

Vertebral ColumnCervical vertebrae7 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

Table 7.2

Cervical VertebraeMost mammals have 7

cervical vertebrae (giraffes, whales, mice & humans)

Seven vertebrae (C1-C7) are the smallest and lightest vertebrae

Cervical Vertebrae: The Atlas (C1)

◦Holds up the head◦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 vertebraeArticulates with the atlas to permit rotation

Figure 7.16c

Thoracic VertebraeThere are twelve vertebrae (T1-T12)

Distinctive heart-shaped body (more massive than that of a cervical vertebra)

Each thoracic vertebra articulate with ribs

Lumbar VertebraeThe five lumbar vertebrae (L1-L5) are

located in the small of the back and have an enhanced weight-bearing function

Largest vertebrae

Tip: Mealtimes

Breakfast: 7 a.m. (7 cervical)

Lunch: 12 p.m. (12 thoracic)

Dinner: 5 p.m. (5 lumbar)

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

CoccyxCoccyx (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 7.18b

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

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)

RibsThere are twelve pair of ribsAll ribs attach posteriorly to the thoracic vertebraeThe superior 7 pair (true, or vertebrosternal ribs)

attach directly to the sternum via costal cartilagesRibs 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