Chapter 7: The Axial Skeleton

I. The Axial Division of the Skeletal System, p. 206

Objective1. Identify the bones of the axial skeleton and their functions.

• In studying individual bones, we are concerned with their functions, includingwhich bones they connect or articulate with, and their structures and marks,including muscle and ligament attachments, and openings for nerves and bloodvessels (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

Objectives1. 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 andrespiratory tracts. Superficial facial bones are for muscle attachment (includingthe maxillary, lacrimal, nasal, zygomatic and maxillary bones, and the mandible).The deep facial bones separate the oral and nasal cavities, and form the nasalseptum (including the palatine bones, inferior nasal conchae, and the vomer).

• The bones of the skull also contain the sinuses, cavities which decrease the weightof the skull and are lined with mucus membranes that help protect the entrances ofthe 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 bonesand may contain sutural bones.

2. The coronal suture attaches the frontal bone to the parietal bones. Thecalvaria is made up of the occipital, parietal and frontal bones.

3. The sagittal suture, between the parietal bones, extends from the lambdoidsuture to the coronal suture.

4. The squamous sutures form the boundaries between temporal bones andparietal bones.

Figure 7-4Cranial Bones, p. 212

• The 8 cranial bones include the occipital bone, frontal bone, sphenoid, ethmoid,parietal bones and temporal bones.

Figure 7-51. 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 7-5b2. 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 7-63. 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 7-74. 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 thezygomatic 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 cellsconnected 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 middleear, transfer sound from the tympanic membrane or eardrum, to theinner 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 7-85. 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 ofthe body)- hypophyseal fossa (depression within the sella turcica, holds thepituitary gland)- sphenoidal sinuses (either side of the body, inferior to the sellaturcica)- lesser wings (anterior to the sella turcica)- greater wings (form part of the cranial floor, sphenoidal spine andposterior wall of the orbit)- pterygoid processes (form pterygoid plates which attach musclesof 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 7-96. 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 nasalcavity):

- contains the crista galli (bony ridge attached to the falxcerebri, a membrane that stabilizes the brain)

2. The 2 lateral masses:- contain the ethmoidal labyrinth (interconnected ethmoidalair 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 7-101. 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 nasalseptum)- 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 nasolacrimalduct)

Foramina:- infraorbital foramen (a major sensory nerve reaches the brain viathe foramen rotundum of the sphenoid)- inferior orbital fissure (cranial nerves and blood vessels)

2. The Palatine BonesFunctions:

- 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 orbitalprocess of the orbit floor, which contains a small sinus open to thesphenoidal sinus)

Foramina:- many in the lateral portion of the horizontal plate (small bloodvessels and nerves of the roof of the mouth)

Figure 7-113. 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 nasalcavity)

Articulations:- with one another- the ethmoid- frontal bones- maxillary bones

4. The VomerFunctions:

- forms the inferior portion of the bony nasal septumArticulations:

- sphenoid- ethmoid- palatine bones- maxillary bones- the cartilaginous part of the nasal septum

5. The Inferior Nasal ConchaeFunctions:

- create air turbulence in the nasal cavity- increase the epithelial surface area to warm and humidify inhaled

airArticulations:

- ethmoid- maxillary bones- palatine bones- lacrimal bones

6. The Zygomatic BonesFunctions:

- 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 temporalbone)

Foramina:- zygomaticofacial foramen (sensory nerves of cheeks)

7. The Lacrimal Bones (the smallest facial bones)Functions:

- form part of the medial wall of the orbitArticulations:

- frontal bone- maxillary bones- the ethmoid

Marks:- lacrimal sulcus (location of the lacrimal sac, leads to thenasolacrimal canal between the orbit and the nasal cavity)

Figure 7-128. 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 submandibularsalivary gland- mylohyoid line for insertion of the mylohyoid muscle or floor ofthe mouth- ramus (ascending from the mandibular angle on either side)

• On each ramus:1. The condylar process articulates with the temporal bone at thetemporomandibular joint.

2. The coronoid process is the insertion point for the temporalismuscle (closes the jaws).3. The mandibular notch separates the condylar and coronoidprocesses.

Foramina:- mental foramina (sensory nerves of lips and chin)- mandibular foramen (the entrance to the mandibular canal, apassageway for blood vessels and nerves of the lower teeth)

9. The Hyoid BoneFunctions:

- supports the larynx- attaches muscles of the larynx, pharynx, and tongue

Articulations:- connect the lesser horns to the styloid processes of the temporalbones

Marks:- body of the hyoid (attaches muscles of the larynx, tongue, andpharynx)- greater horns or greater cornua (help support the larynx, attach tomuscles of the tongue)- lesser horns or lesser cornua (attach to the stylohyoid ligamentswhich 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 ofnasal 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 mucousepithelium, which flushes the nasal cavities with mucus and protects the entranceof the respiratory tract. Cilia sweep the mucus and trapped particles back into thethroat, where it is swallowed.

Figure 7-15The 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 andseveral sphenoid and temporal elements. The bones are connected by 4 areas offibrous connective tissue called fontanels, which allow the skull to flex duringbirth:

- 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 coronalsuture- the mastoid fontanel between the squamous sutures and the lambdoidsuture

• The occipital, sphenoidal and mastoid fontanels disappear soon after birth, but theanterior fontanel persists for nearly 2 years.

• The skulls of infants and adults differ in proportion as well as size. Most skullgrowth occurs before age 5, making the child’s skull relatively large compared tothe rest of the skeleton.

III. The Vertebral Column, p. 224

Objectives1. 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 andbody.

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 arecalled primary curves or accommodation curves because they accommodate theinternal organs.

• The lumbar and cervical curves appear after birth, and are called secondary curvesor 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 betweenlaminae and pedicles, with smooth surfaces called articular facets on thearticular faces) are 2 types:

- superior articular process (articulates with the inferior articularprocess 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 foraminawhich 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 characteristicsdetermined by their functions.

Figure 7-181. Cervical Vertebrae (C1-7):

- The body of a cervical vertebra is small compared to the vertebralforamen.- The spinal cord within the vertebral foramen is largest near the brain, butthe bodies of the vertebrae can be small because they only have to supportthe head.- The superior surface is concave, and slopes from posterior to anterior.- C1 (atlas) has no spinous process, and others have short spinousprocesses.- The tip of each spinous process is notched (bifid)- The transverse processes are fused to costal processes which encircletransverse 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 posteriorarches.

- 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 heavyspinous 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 toskull.

o A dislocation of the cervical vertebrae is called whiplash.

Figure 7-192. 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 articulatewith the heads of ribs.- T1-8 each articulate with 2 pairs of ribs (superior and inferior costalfacets).- T9-11 each articulate with 1 pair of ribs.- T1-10 have transverse costal facets on thick transverse processes for ribarticulation.- Ribs at T1-10 contact both costal and transverse costal facets.

Figure 7-203. 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 backmuscles.

Table 7-2 compares the structural and functional differences between cervical, thoracic,and lumbar vertebrae.

Figure 7-214. The Sacrum

- The adult sacrum consists of 5 fused sacral vertebrae.- The sacrum fuses between puberty and ages 25-30, leaving transverselines.- 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 appendicularskeleton.- Attaches broad muscles that move the thigh.- The sacral canal replaces the vertebral canal.- The fused spinous processes of sacral vertebrae form the median sacralcrest.- The sacral cornua is the horn-shape formed by the laminae of the fifthsacral vertebra, which do not meet at the midline.- The ridges of the sacral cornua form an opening at the inferior end of thesacral canal called the sacral hiatus, which is covered by connectivetissues.- Four pairs of sacral foramina open to either side of the median sacral

crest.- The fused transverse processes form the lateral sacral crest, whichattaches 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 attachmuscles.- At the center of the base is the sacral promontory.- The narrow inferior portion is the apex, which articulates with thecoccyx.

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 unfusedvertebral arches.- The laminae of the first coccygeal vertebra form the coccygeal cornua.

IV. The Thoracic Cage, p.231

Objective1. 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 andsternum 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. Falseribs are divided into 2 types:

- vertebrochondral ribs (ribs 8-10) which fuse together and merge with thecartilage of rib 7 before reaching the sternum.- floating or vertebral ribs (ribs 11-12) which connect only to the vertebraeand 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) affectthe 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 articulations2. 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 age25, 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 andstronger because they bear more weight.