Download Chapter 7: The Axial Skeleton

Chapter 7: The Axial Skeleton
I. The Axial Division of the Skeletal System, p. 206
Objective
1. Identify the bones of the axial skeleton and their functions.
•
In studying individual bones, we are concerned with their functions, including
which bones they connect or articulate with, and their structures and marks,
including muscle and ligament attachments, and openings for nerves and blood
vessels (foramina).
Figure 7-1
• The axial skeleton:
- forms the longitudinal axis of the body
- has 80 bones
•
The axial skeleton includes:
- the skull (8 cranial bones and 14 facial bones)
- bones associated with the skull (6 auditory ossicles and the hyoid bone)
- the vertebral column (24 vertebrae, the sacrum and the coccyx)
- the thoracic cage (24 ribs and the sternum)
•
Functions of the axial skeleton include:
- support and protect organs in the body cavities
- attach to muscles that support head, neck and trunk
- attach to breathing muscles
- attach to muscles of the appendicular skeleton
•
Axial bones are strong, with many ligaments, but are restricted in motion.
II. The Skull, p. 206
Objectives
1. Identify the bones of the cranium and face, and the significance of their
markings.
2. Describe the structures and functions of the nasal complex.
3. Explain the functions of paranasal sinuses.
4. Describe the differences between the skulls of infants, children and adults.
•
The skull protects the brain and entrances to respiratory and digestive systems.
Figure 7-2
• The skull has 22 bones:
- 8 cranial bones form the braincase or cranium. The cranial bones enclose
the cranial cavity, which contains the brain and associated fluids, blood vessels,
nerves and membranes.
- 14 facial bones protect and support the entrances to the digestive and
respiratory tracts. Superficial facial bones are for muscle attachment (including
the maxillary, lacrimal, nasal, zygomatic and maxillary bones, and the mandible).
The deep facial bones separate the oral and nasal cavities, and form the nasal
septum (including the palatine bones, inferior nasal conchae, and the vomer).
•
The bones of the skull also contain the sinuses, cavities which decrease the weight
of the skull and are lined with mucus membranes that help protect the entrances of
the respiratory system.
Figure 7-3
• The immovable joints of the skull are called sutures. The 4 major sutures are:
1. The lambdoid suture separates the occipital bone from the parietal bones
and may contain sutural bones.
2. The coronal suture attaches the frontal bone to the parietal bones. The
calvaria is made up of the occipital, parietal and frontal bones.
3. The sagittal suture, between the parietal bones, extends from the lambdoid
suture to the coronal suture.
4. The squamous sutures form the boundaries between temporal bones and
parietal bones.
Figure 7-4
Cranial Bones, p. 212
•
The 8 cranial bones include the occipital bone, frontal bone, sphenoid, ethmoid,
parietal bones and temporal bones.
Figure 7-5
1. The Occipital Bone
Functions:
- forms the posterior and inferior surfaces of the cranium.
Articulations:
- parietal bones
- temporal bones
- sphenoid
- first cervical vertebra (atlas)
Marks:
- external occipital protuberance
- external occipital crest (attach ligaments)
- occipital condyles (articulate with neck)
- inferior and superior nuchal lines (attach ligaments)
Foramina:
- foramen magnum (connects cranial and spinal cavities)
- jugular foramen (jugular vein)
- hypoglossal canals (hypoglossal nerves)
Figure 7-5b
2. The Parietal Bones
Functions:
- forms part of the superior and lateral surfaces of the cranium
Articulations:
- parietal bone
- occipital bone
- temporal bone
- frontal bone
- sphenoid bone
Marks:
- superior and inferior temporal lines (temporalis muscle)
- grooves for cranial blood vessels
Figure 7-6
3. The Frontal Bone
Functions:
- forms the anterior cranium and upper eyesockets
- contains frontal sinuses
Articulations:
- parietal bone
- sphenoid bone
- ethmoid bone
- nasal bone
- lacrimal bone
- maxillary bone
- zygomatic bone
- metopic suture (fusion of 2 frontal bones, disappears about age 8)
Marks:
- frontal squama (forehead)
- supraorbital margin (protects eye)
- lacrimal fossa (tear ducts)
- frontal sinuses
Foramina:
- supraorbital foramen (blood vessels of eyebrows, eyelids and
frontal sinuses)
- supraorbital notch (an incomplete supraorbital foramen)
Figure 7-7
4. The Temporal Bones
Functions:
- form part of the lateral walls of the cranium and zygomatic arches
- articulate with the mandible
- surround and protect the inner ear
- attach muscles of jaws and head
Articulations:
- zygomatic bone
- sphenoid bone
- parietal bone
- occipital bone
- mandible
Marks:
- squamous part (borders the squamous suture)
- zygomatic process (articulates with the temporal process of the
zygomatic bone to form the zygomatic arch or cheekbone)
- zygomatic process (inferior to the squamous portion)
- mandibular fossa (articulates with the mandible)
- mastoid process (muscle attachment) contains mastoid air cells
connected to the middle ear cavity
- styloid process (attach tendons and ligaments of the hyoid,
tongue and pharynx)
- petrous part (encloses structures of the inner ear)
- auditory ossicles (3 tiny bones in the tympanic cavity, or middle
ear, transfer sound from the tympanic membrane or eardrum, to the
inner ear)
Foramina:
- carotid canal (internal carotid artery)
- foramen lacerum (carotid and small arteries, hyaline cartilage,
auditory tube)
- external acoustic canal (ends at the tympanic membrane)
- stylomastoid foramen (facial nerve)
- internal acoustic canal (blood vessels and nerves of the inner ear,
facial nerve)
Figure 7-8
5. The Sphenoid
Functions:
- forms part of the floor of the cranium
- unites cranial and facial bones
- strengthens the sides of the skull
- contains the sphenoidal sinuses
Articulations:
- ethmoid
- frontal bone
- occipital bone
- parietal bone
- temporal bone
- palatine bones
- zygomatic bones
- maxillary bones
- vomer
Marks:
- sphenoid body (central axis of the sphenoid)
- sella turcica (saddle-shaped enclosure on the superior surface of
the body)
- hypophyseal fossa (depression within the sella turcica, holds the
pituitary gland)
- sphenoidal sinuses (either side of the body, inferior to the sella
turcica)
- lesser wings (anterior to the sella turcica)
- greater wings (form part of the cranial floor, sphenoidal spine and
posterior wall of the orbit)
- pterygoid processes (form pterygoid plates which attach muscles
of the lower jaw and soft palate)
Foramina:
- optic canals (optic nerves)
- superior orbital fissure (blood vessels and nerves of the orbit)
- foramen rotundum (blood vessels and nerves of the face)
- foramen ovale (blood vessels and nerves of the face)
- foramen spinosum (blood vessels and nerves of the jaws)
Figure 7-9
6. The Ethmoid
Functions:
- forms the anteromedial floor of the cranium
- forms the roof of the nasal cavity
- forms part of the nasal septum and medial orbital wall
- contains ethmoidal air cells (sinuses)
Articulations:
- frontal bone
- sphenoid
- nasal bone
- lacrimal bone
- palatine bone
- maxillary bones
- inferior nasal conchae
- vomer
Marks:
1. The cribriform plate (floor of the cranium and roof of the nasal
cavity):
- contains the crista galli (bony ridge attached to the falx
cerebri, a membrane that stabilizes the brain)
2. The 2 lateral masses:
- contain the ethmoidal labyrinth (interconnected ethmoidal
air cells open to nasal cavities)
- superior nasal conchae (projections)
- middle nasal conchae (projections)
3. The perpendicular plate (part of the nasal septum)
Foramina:
- olfactory foramina in the cribriform plate (olfactory nerves)
Facial Bones, p. 217
Figure 7-10
1. The Maxillary Bones, the largest facial bones
Functions:
- support the upper teeth
- form the inferior orbital rim
- form the lateral margins of the external nares
- form the upper jaw and hard palate
- contain maxillary sinuses (the largest sinuses)
Articulations:
- frontal bones
- ethmoid
- with other maxillary bone
- with all other facial bones except the mandible
Marks:
- orbital rim (protects the eye and orbit)
- anterior nasal spine (attaches the cartilaginous anterior nasal
septum)
- alveolar processes (border the mouth, support upper teeth)
- palatine processes (form the hard palate or roof of mouth)
- contain maxillary sinuses (to lighten bone)
- nasolacrimal canal (protects the lacrimal sac and nasolacrimal
duct)
Foramina:
- infraorbital foramen (a major sensory nerve reaches the brain via
the foramen rotundum of the sphenoid)
- inferior orbital fissure (cranial nerves and blood vessels)
2. The Palatine Bones
Functions:
- form the posterior portion of the hard palate
- contribute to the floors of the orbits
Articulations:
- with one another
- maxillary bones
- sphenoid
- ethmoid
- inferior nasal conchae
- vomer
Marks:
The palatine bones have 2 parts in an L shape:
- the horizontal plate (posterior part of the hard palate)
- the perpendicular plate (from the horizontal plate to the orbital
process of the orbit floor, which contains a small sinus open to the
sphenoidal sinus)
Foramina:
- many in the lateral portion of the horizontal plate (small blood
vessels and nerves of the roof of the mouth)
Figure 7-11
3. The Nasal Bones
Functions:
- support the bridge of the nose (superior)
- connect to cartilages that support the distal part of the nose
(cartilages extend to the external nares or entrances to the nasal
cavity)
Articulations:
- with one another
- the ethmoid
- frontal bones
- maxillary bones
4. The Vomer
Functions:
- forms the inferior portion of the bony nasal septum
Articulations:
- sphenoid
- ethmoid
- palatine bones
- maxillary bones
- the cartilaginous part of the nasal septum
5. The Inferior Nasal Conchae
Functions:
- create air turbulence in the nasal cavity
- increase the epithelial surface area to warm and humidify inhaled
air
Articulations:
- ethmoid
- maxillary bones
- palatine bones
- lacrimal bones
6. The Zygomatic Bones
Functions:
- contribute to the rim and lateral wall of the orbit
- form part of the zygomatic arch
Articulations:
- sphenoid
- frontal bone
- temporal bones
- maxillary bones
Marks:
- temporal process (meets the zygomatic process of the temporal
bone)
Foramina:
- zygomaticofacial foramen (sensory nerves of cheeks)
7. The Lacrimal Bones (the smallest facial bones)
Functions:
- form part of the medial wall of the orbit
Articulations:
- frontal bone
- maxillary bones
- the ethmoid
Marks:
- lacrimal sulcus (location of the lacrimal sac, leads to the
nasolacrimal canal between the orbit and the nasal cavity)
Figure 7-12
8. The Mandible
Functions:
- forms the lower jaw
Articulations:
- mandibular fossae of the temporal bones
Marks:
- body of the mandible (horizontal portion)
- alveolar processes (support the lower teeth)
- mental protuberance (attaches several facial muscles)
- a depression on the medial surface for the submandibular
salivary gland
- mylohyoid line for insertion of the mylohyoid muscle or floor of
the mouth
- ramus (ascending from the mandibular angle on either side)
•
On each ramus:
1. The condylar process articulates with the temporal bone at the
temporomandibular joint.
2. The coronoid process is the insertion point for the temporalis
muscle (closes the jaws).
3. The mandibular notch separates the condylar and coronoid
processes.
Foramina:
- mental foramina (sensory nerves of lips and chin)
- mandibular foramen (the entrance to the mandibular canal, a
passageway for blood vessels and nerves of the lower teeth)
9. The Hyoid Bone
Functions:
- supports the larynx
- attaches muscles of the larynx, pharynx, and tongue
Articulations:
- connect the lesser horns to the styloid processes of the temporal
bones
Marks:
- body of the hyoid (attaches muscles of the larynx, tongue, and
pharynx)
- greater horns or greater cornua (help support the larynx, attach to
muscles of the tongue)
- lesser horns or lesser cornua (attach to the stylohyoid ligaments
which support the hyoid and larynx)
Table 7-1: Summary: The Foramina and Fissures of the Skull, p. 220
The Orbital and Nasal Complexes, p. 220
Figure 7-13
• The eye sockets (orbits) which contain the eyes are formed of portions of 7 cranial
and facial bones called the orbital complex:
- frontal bone (roof)
- maxillary bone (floor)
- maxillary, lacrimal and ethmoid bones (orbital rim and medial wall)
- sphenoid and palatine bones
Figure 7-14
• The bones of the nasal cavities and paranasal sinuses make up the nasal complex:
- frontal bone, sphenoid and ethmoid (superior wall of nasal cavities)
- maxillary, lacrimal, ethmoid and inferior nasal conchae (lateral walls of
nasal cavities)
- maxillary and nasal bones (bridge of nose)
•
The paranasal sinuses are air-filled chambers connected to the nasal cavities.
Paranasal sinuses lighten skull bones and provide extensive areas of mucous
epithelium, which flushes the nasal cavities with mucus and protects the entrance
of the respiratory tract. Cilia sweep the mucus and trapped particles back into the
throat, where it is swallowed.
Figure 7-15
The Skulls of Infants and Children, p. 222
•
During development, the skull grows rapidly and has many ossification centers.
At birth, fusion is not complete; there are 2 frontal bones, 4 occipital bones and
several sphenoid and temporal elements. The bones are connected by 4 areas of
fibrous connective tissue called fontanels, which allow the skull to flex during
birth:
- the large anterior fontanel (soft spot) at the intersection of frontal,
sagittal and coronal sutures
- the occipital fontanel between the lambdoid and sagittal sutures
- the sphenoidal fontanels between the squamous sutures and the coronal
suture
- the mastoid fontanel between the squamous sutures and the lambdoid
suture
•
The occipital, sphenoidal and mastoid fontanels disappear soon after birth, but the
anterior fontanel persists for nearly 2 years.
•
The skulls of infants and adults differ in proportion as well as size. Most skull
growth occurs before age 5, making the child’s skull relatively large compared to
the rest of the skeleton.
III. The Vertebral Column, p. 224
Objectives
1. Identify the curvatures of the spinal column and their functions.
2. Identify the vertebral regions, and describe the structures and functions of each
vertebral group.
•
The spine or vertebral column protects the spinal cord and supports the head and
body.
Figure 7-16
• The vertebral column consists of 26 bones (24 vertebrae, the sacrum and the
coccyx).
•
The vertebral column is divided into 5 regions:
- cervical (C)
- thoracic (T)
- lumbar (L)
- sacral (S)
- coccygeal (Co)
•
•
•
•
The neck consists of 7 cervical vertebrae.
The upper back has 12 thoracic vertebrae, each articulated with one or more pairs
of ribs.
The lower back consists of 5 lumbar vertebrae.
The fifth lumbar vertebra articulates with the sacrum, which articulates with the
coccyx.
Spinal Curvature, p. 224
•
The spine is configured in 4 spinal curves:
- cervical curve
- thoracic curve
- lumbar curve
- sacral curve
•
The thoracic and sacral curves are present during fetal development, and are
called primary curves or accommodation curves because they accommodate the
internal organs.
•
The lumbar and cervical curves appear after birth, and are called secondary curves
or compensation curves because they shift body weight to allow upright posture.
Vertebral Anatomy, p. 225
Figure 7-17
• Each vertebra consists of 3 basic parts:
1. The vertebral body (centrum) transfers weight along the spine.
2. The vertebral arch (posterior margin of the vertebral foramen) includes:
- pedicles (walls of the vertebral arch)
- laminae (roof of the vertebral arch)
- spinous process (projection where vertebral laminae fuse)
- transverse process (projection where laminae join pedicles)
3. The articular processes (lateral projections at the junction between
laminae and pedicles, with smooth surfaces called articular facets on the
articular faces) are 2 types:
- superior articular process (articulates with the inferior articular
process of the superior vertebra)
- inferior articular process (articulates with the inferior vertebra)
•
Together, the vertebral foraminae form the vertebral canal which encloses the
spinal cord.
•
Vertebral bodies are separated by pads of fibrocartilage called intervertebral discs.
•
Gaps between the pedicles of adjacent vertebrae form intervertebral foramina
which allow nerves to run to and from the spinal cord.
Vertebral Regions, p. 226
Figure 7-16 (repeat)
• Vertebrae are numbered by region, from top to bottom: C1 articulates with the
skull, L5 with the sacrum. Vertebrae of each region have characteristics
determined by their functions.
Figure 7-18
1. Cervical Vertebrae (C1-7):
- The body of a cervical vertebra is small compared to the vertebral
foramen.
- The spinal cord within the vertebral foramen is largest near the brain, but
the bodies of the vertebrae can be small because they only have to support
the head.
- The superior surface is concave, and slopes from posterior to anterior.
- C1 (atlas) has no spinous process, and others have short spinous
processes.
- The tip of each spinous process is notched (bifid)
- The transverse processes are fused to costal processes which encircle
transverse foramina that protect vertebral arteries and veins.
- The Atlas (C1) articulates with the occiptal condyles of the skull:
1. C1 has no body or spinous process.
2. C1 has a large, round foramen bounded by anterior and posterior
arches.
- The Axis (C2) supports the atlas:
1. Axis and atlas bodies fuse during development to form the dens.
2. Important muscles of the head and neck attach to the heavy
spinous process of C2.
- The Vertebra Prominens (C7) transitions to the thoracic vertebrae:
1. C7 has a long spinous process with a broad tubercle.
2. C7 has large transverse processes.
3. The ligamentum nuchae (elastic ligament) extends from C7 to
skull.
o A dislocation of the cervical vertebrae is called whiplash.
Figure 7-19
2. Thoracic Vertebrae (T1-12):
- The body is heart shaped, and larger than those of cervical vertebrae.
- The vertebral foramen is smaller.
- Spinous processes are long and slender.
- T10-12 transition to lumbar vertebrae.
- The dorsolateral surfaces of the body have costal facets which articulate
with the heads of ribs.
- T1-8 each articulate with 2 pairs of ribs (superior and inferior costal
facets).
- T9-11 each articulate with 1 pair of ribs.
- T1-10 have transverse costal facets on thick transverse processes for rib
articulation.
- Ribs at T1-10 contact both costal and transverse costal facets.
Figure 7-20
3. Lumbar Vertebrae (L1-5):
- Are the largest vertebrae.
- The body is thicker than thoracic vertebrae, and oval-shaped.
- Do not have costal or transverse costal facets.
- Have slender transverse processes that project dorsolateraly.
- The vertebral foramen is triangular.
- The superior articular processes face up and in.
- The inferior articular processes face down and out.
- The spinous process is short and heavy, for attachment of lower back
muscles.
Table 7-2 compares the structural and functional differences between cervical, thoracic,
and lumbar vertebrae.
Figure 7-21
4. The Sacrum
- The adult sacrum consists of 5 fused sacral vertebrae.
- The sacrum fuses between puberty and ages 25-30, leaving transverse
lines.
- The sacrum is curved, more so in males than in females.
- Protects reproductive, urinary and digestive organs.
- Attaches the axial skeleton to the pelvic girdle of the appendicular
skeleton.
- Attaches broad muscles that move the thigh.
- The sacral canal replaces the vertebral canal.
- The fused spinous processes of sacral vertebrae form the median sacral
crest.
- The sacral cornua is the horn-shape formed by the laminae of the fifth
sacral vertebra, which do not meet at the midline.
- The ridges of the sacral cornua form an opening at the inferior end of the
sacral canal called the sacral hiatus, which is covered by connective
tissues.
- Four pairs of sacral foramina open to either side of the median sacral
crest.
- The fused transverse processes form the lateral sacral crest, which
attaches to the muscles of lower back and hip.
- The thick, flattened auricular surface articulates with the pelvic girdle
(sacroiliac joint).
- The rough sacral tuberosity attaches ligaments of the sacroiliac joint.
- The sacrum is divided into regions:
- The base is the broad superior surface.
- At either side of the base, the wings of the sacral ala attach
muscles.
- At the center of the base is the sacral promontory.
- The narrow inferior portion is the apex, which articulates with the
coccyx.
5. The Coccyx
- The mature coccyx consists of 3 to 5 fused coccygeal vertebrae.
- The coccyx attaches ligaments and a constricting muscle of the anus.
- The first 2 coccygeal vertebrae have transverse processes and unfused
vertebral arches.
- The laminae of the first coccygeal vertebra form the coccygeal cornua.
IV. The Thoracic Cage, p.231
Objective
1. Explain the significance of articulations between thoracic vertebrae and ribs, and
between ribs and sternum.
Figure 7-22
• The thoracic cage is the skeleton of the chest, supporting the thoracic cavity. It
consists of the thoracic vertebrae, ribs and sternum (breastbone). The ribs and
sternum form the rib cage.
•
The thoracic cage has two main functions:
1. To protect the organs of the thoracic cavity, including the heart, lungs and
thymus.
2. To attach muscles for respiration, the vertebral column, and pectoral girdle
and upper limbs.
The Ribs, p. 233
Figure 7-23
• Ribs (costae) are 12 pairs of long, curved, flat bones extending from the thoracic
vertebrae. They are divided into 2 types, true ribs and false ribs.
•
Ribs 1-7 are true ribs (vertebrosternal ribs), connected to the sternum by costal
cartilages.
•
Ribs 8-12 are false ribs, because they do not attach directly to the sternum. False
ribs are divided into 2 types:
- vertebrochondral ribs (ribs 8-10) which fuse together and merge with the
cartilage of rib 7 before reaching the sternum.
- floating or vertebral ribs (ribs 11-12) which connect only to the vertebrae
and have no connection with the sternum
♣ Structures of the ribs include:
1. The head (capitulum):
- at the vertebral end of the rib
- has superior and inferior articular facets
2. The neck:
- the short area between the head and the tubercle
3. The tubercle (tuberculum):
- a small dorsal elevation
- has an auricular facet that contacts the facet of its thoracic vertebra
(at T1-10 only)
4. The tubercular body (shaft):
- attaches muscles of the pectoral girdle and trunk
- attaches to the intercostal muscles which move the ribs
♣ Ribs are flexible, mobile, and can absorb shock. Rib movements (breathing) affect
the width and depth of the thoracic cage, changing its volume.
The Sternum, p. 234
♣ The sternum, a flat bone in the midline of the thoracic wall, has 3 parts:
1. The manubrium:
- the superior portion of the sternum, is broad and triangular
- articulates with the collarbones (clavicles) and cartilages of the
first rib pair
- has a jugular notch, a shallow indentation on the manubrium,
between the clavicular articulations
2. The sternal body:
- is tongue-shaped
- attaches to the manubrium
- attaches to the costal cartilages of ribs 2-7
3. The xiphoid process:
- is the smallest part of the sternum
- attaches to the sternal body
- attaches to the diaphragm and rectus abdominis muscles
•
The developing sternal body consists of 4 bones that complete fusion about age
25, leaving transverse lines. The xiphoid process is the last part to fuse and can be
easily broken away.
Key
•
The axial skeleton protects the brain, spinal cord and visceral organs of the chest.
Vertebrae conduct body weight to the lower limbs. Lower vertebrae are larger and
stronger because they bear more weight.