The Axial Skeleton

Divisions of the Skeleton Axial skeleton – skull, vertebrae, and bony thorax Appendicular skeleton – bones of the arms and legs, including their associated girdles

The Axial Skeleton Figure 5.6a

The Axial Skeleton Figure 5.6b

The Skull Two sets of bones Bones are joined by sutures Cranium Facial bones Bones are joined by sutures Only the mandible is attached by a freely movable joint Consists of 22 bones interlocked along sutures (all except the mandible) 8 bones make up the cranium 14 bones make up the facial skeleton Mandible = lower jawbone; only movable bone held to the cranium by ligaments Orbit of the eye is formed by cranial and facial bones

The Cranium Encloses and protects the brain Surface provides attachments for muscles involved in chewing and head movements Does not include the mandible

Sinuses Air-filled cavities of the cranium Lined with mucous membranes All connected by passageways to the nasal cavity Function to reduce the weight of the skull and increase voice intensity and resonance

Paranasal Sinuses Hollow portions of bones surrounding the nasal cavity Functions of paranasal sinuses Lighten the skull Give resonance and amplification to voice

Paranasal Sinuses Figure 5.10a

Paranasal Sinuses Figure 5.10b

Cranial Bones Frontal bone Parietal bones (2) Occipital bone Temporal bones (2) Sphenoid bone Ethmoid bone

Human Skull, Lateral View Figure 5.7

Frontal Bone Features Frontal sinuses Develops in 2 parts that grow together by 5-6 years old

Parietal Bone Features Sagittal suture Coronal suture

Occipital Bone Features Lambdoidal suture Joins parietal bones to the occipital Foramen magnum ”Large Hole” Occipital condyles Rest on the first vertebra

Human Skull, Inferior View Figure 5.9

Temporal Bone Features Squamosal suture temporal to parietal External auditory meatus ear canal Mandibular fossae joint with mandible Mastoid process neck muscle attachment Contains mastoid sinuses Styloid process below EAM, anchors muscles of tongue and pharynx Zygomatic process helps form cheek prominence

Sphenoid Bone Features Wedged between several other bones in anterior portion of the cranium 2 winglike structures extend laterally toward each side of the skull Sella turcica – houses pituitary gland Sphenoidal sinuses

Ethmoid Bone Features Located in front of the sphenoid bone Consists of 2 masses on each side of the nasal cavity Cribriform plates Holey areas that allow nerve fibers from the nose to reach the brain. Superior and middle nasal conchae support mucous membranes of the nose Ethmoid sinuses Crista galli triangular process that projects upward from the cribriform plates; place of attachment for membranes around the brain

Human Skull, Superior View Figure 5.8

Facial Skeleton 13 immovable bones + the mandible Provides attachments for muscles for facial expressions and jaw movements

Bones of the Facial Skeleton Maxillary bones (2) Palatine bones (2) Zygomatic bones (2) Lacrimal bones (2) Nasal bones (2) Vomer bone Inferior nasal conchae (2) Mandible

Maxillary Bone Features Form upper jaw All other immovable facial bones articulate with them Hard palate Sockets of upper teeth Maxillary sinuses Palatine processes where the maxillary bones meet

Palatine Bone Features L-shaped bones behind the maxillae Posterior hard palate

Zygomatic Bone Prominences of the cheeks Commonly called check bones

Lacrimal Bone Features Thin, scalelike bones between the ethmoid bone and maxillae in the medial walls of the orbits Groove in anterior portion provides pathway for tears

Nasal Bone Features Form the bridge of the nose

Vomer Bone Features Forms most of the boney nasal septum Vomer means “plow” and refers to the shape of the bone.

Inferior Nasal Conchae Features Scroll-shaped bones attached to lateral walls of the nasal cavity Support mucous membranes of the nose

Mandible Features Only free moving bone of the face Largest and strongest bone of the face Parts of the Mandible Ramus attachment for large chewing muscles Mandibular condyle articulates with mandibular fossae of temporal bones Alveolar border houses lower tooth sockets Mandibular foramen carries nerves and blood vessels to the lower teeth; dental injection site Mental foramen – carries branches of nerves and blood vessels of the mandibular foramen

The Hyoid Bone The only bone that does not articulate with another bone Serves as a moveable base for the tongue Aids in swallowing and speech

The Fetal Skull The fetal skull is large compared to the infant’s total body length Fontanels (flexible) fibrous membranes connecting the cranial bones Allow the brain to grow Convert to bone within 24 months after birth

The Vertebral Column Each vertebrae is given a name according to its location There are 24 single vertebral bones separated by intervertebral discs Seven cervical vertebrae are in the neck Twelve thoracic vertebrae are in the chest region Five lumbar vertebrae are associated with the lower back

The Vertebral Column Nine vertebrae fuse to form two composite bones Sacrum Coccyx

Functions of the Vertebral Column Supports the head and trunk Permits movement Protects the spinal cord which passes through the vertebral canal

The Vertebral Column The spine has a normal curvature Primary curvatures are the spinal curvatures of the thoracic and sacral regions Present from birth Secondary curvatures are the spinal curvatures of the cervical and lumbar regions Develop after birth

Typical Vertebra Body Intervertebral disks Pedicles Laminae thick, drum-shaped, anterior portion of bone Intervertebral disks cushion and soften forces caused by movements Pedicles 2 short stalks that project posteriorly from each vertebral body Laminae 2 plates that arise from pedicles to fuse and form the spinous process Transverse processes between the pedicles and laminae; project laterally and posteriorly

Typical Vertebra continued… Vertebral arch formed by the pedicles, laminae, and spinous process; around the vertebral foramen Vertebral foramen opening through which the spinal cord passes Intervertebral foramina passageways for spinal nerves; between adjacent vertebrae

Cervical Vertebrae 7 vertebrae Make up the neck region Smallest vertebrae Denser bone tissues than the other regions Distinctive because they have transverse foramina (passageways for arteries leading to the brain) Spinous processes are uniquely forked (C2-C6) C7 = vertebrae prominens; spinous process is longer and protrudes beyond the other cervical vertebrae

Atlas C1 Supports the head Has no body or spine Consists of a bony ring with 2 transverse processes Facets – kidney-shaped areas on the superior surface that articulate with the occipital condyles

Axis C2 Vertebrae associated with hangman’s fracture Dens – tooth-like process that projects upward and lies in the ring of the atlas As the head is turned from side to side, the atlas pivots around the dens.

Thoracic Vertebrae 12 in number Larger than cervical vertebrae Long, pointed spinous process which slopes downward Facets on sides of vertebral body articulate with the ribs Bodies of the vertebrae increase in size from T3 down can bear an increasing load of body weight

Lumbar Vertebrae 5 in number Located in the small of the back Larger, stronger, and support more weight than the others Transverse processes project posteriorly at sharp angles Short, thick spinous processes are nearly horizontal

Sacrum Triangular structure at the base of the vertebral column 5 vertebrae fuse to form the sacrum between 18-30 years of age Fused spinous processes form a ridge of tubercles called the median sacral crest Dorsal sacral foramina – openings to the sides of the tubercles through which nerves and blood vessels pass Sacral canal – formed from vertebral foramina and opens at the sacral hiatus

Coccyx Tailbone Lowest part of the vertebral column Made of 4 vertebrae that fuse by the 25th year Acts as a shock absorber when sitting

Vertebral Column Disorders Ruptured/herniated disk outer layers of the intervertebral disk are broken and the central mass of the disk is squeezed out from extra pressure, pressing on the spinal cord and spinal nerves pain, numbness, loss of muscular function

Curvature Disorders of the Spine Kyphosis – hunchback; exaggerated thoracic curvature Scoliosis – abnormal lateral curvature Lordosis – swayback; exaggerated lumbar curvature

Scoliosis Repair

Thoracic Cage Includes ribs, thoracic vertebrae, sternum, and costal cartilages Supports the shoulder girdle and upper limbs Protects viscera Plays a role in breathing

Ribs 12 pair – one pair for each vertebra True ribs – 1st 7 rib pairs; join the sternum directly by costal cartilages (hyaline) False ribs – bottom 5 rib pairs; do not join the sternum directly Cartilages of the upper 3 false ribs join the cartilage of the 7th rib Floating ribs – last 2 rib pairs; no attachment to the sternum

Sternum Breastbone Develops in 3 parts: Manubrium – articulates with clavicles at clavicular notches Body – fuses to manubrium at middle age at the sternal angle Xiphoid process – begins as cartilage, slowly ossifies, and fuses to the body at middle age Red bone marrow in sternum produces RBCs into adulthood