Skeletal System Notes

Chapter 8: The Skeletal System

Axial vs. Appendicular Skeleton

The skeletal system is divided into two main parts:
- Axial Skeleton: Includes the skull (cranium and facial bones), vertebral column, and thoracic cage (ribs and sternum).
- Appendicular Skeleton: Includes the bones of the pectoral girdle (clavicle and scapula), upper limbs (humerus, radius, ulna, carpals, metacarpals, phalanges), pelvic girdle, and lower limbs (femur, patella, tibia, fibula, tarsals, metatarsals, phalanges).

Number of Bones

The number of bones changes throughout life:
- Approximately $270$ bones at birth.
- Decreases with fusion during development.
- Typically $206$ bones in the adult skeleton.
Variations occur due to:
- Sesamoid bones: Bones embedded in tendons (e.g., patella).
- Sutural (Wormian) bones: Small bones within sutures of the skull.

Bone Markings

Bone markings include:
- Ridges, spines, bumps, depressions, canals, pores, slits, cavities, and articular surfaces.

Anatomical Features of Bones

Articulations
- Condyle: A rounded knob that articulates with another bone (e.g., occipital condyles of the skull).
- Facet: A smooth, flat, slightly concave or convex articular surface (e.g., articular facets of the vertebrae).
- Head: The prominent, expanded end of a bone, sometimes rounded (e.g., head of the femur).
Depressions
- Alveolus: A pit or socket (e.g., tooth socket).
- Fossa: A shallow, broad, or elongated basin (e.g., mandibular fossa).
- Fovea: A small pit (e.g., fovea capitis of the femur).
- Sulcus: A groove for a tendon, nerve, or blood vessel (e.g., intertubercular sulcus of the humerus).
Extensions and Projections
- Crest: A narrow ridge (e.g., iliac crest of the pelvis).
- Epicondyle: An expanded region superior to a condyle (e.g., medial epicondyle of the femur).
- Line: A slightly raised, elongated ridge (e.g., nuchal lines of the skull).
- Process: Any bony prominence (e.g., mastoid process of the skull).
- Protuberance: A bony outgrowth or protruding part (e.g., mental protuberance of the chin).
- Spine: A sharp, slender, or narrow process (e.g., mental spines of the mandible).
- Trochanter: Two massive processes unique to the femur.
- Tubercle: A small, rounded process (e.g., greater tubercle of the humerus).
- Tuberosity: A rough elevated surface (e.g., tibial tuberosity).
Passages and Cavities
- Canal: A tubular passage or tunnel in a bone (e.g., auditory canal of the skull).
- Fissure: A slit through a bone (e.g., orbital fissures behind the eye).
- Foramen: A hole through a bone, usually round (e.g., foramen magnum of the skull).
- Meatus: An opening into a canal (e.g., external acoustic meatus of the ear).
- Sinus: An air-filled space in a bone (e.g., frontal sinus of the forehead).

The Skull

The skull consists of $22$ bones joined together by sutures.
Cranial cavity encloses the brain.
Other cavities include orbits, nasal cavity, oral (buccal) cavity, middle and inner ear cavities, and paranasal sinuses.

Cranium

The cranium (braincase) protects the brain and associated sense organs.
It has two parts: calvaria (skullcap) and cranial base.
Meninges separate the brain from direct contact with bones.
There are $8$ cranial bones: 1 frontal, 2 parietal, 2 temporal, 1 occipital, 1 sphenoid, and 1 ethmoid.
Foramina are holes that allow passage for nerves and blood vessels.

Foramina of the Skull

Frontal Bone
- Supraorbital foramen or notch: Transmits the supraorbital nerve, artery, and vein.
Temporal Bone
- Carotid canal: Transmits the internal carotid artery.
- External acoustic meatus: Transmits sound waves to the eardrum.
- Internal acoustic meatus: Transmits the vestibulocochlear nerve.
- Jugular foramen: Transmits the internal jugular vein; glossopharyngeal, vagus, and accessory nerves.
Occipital Bone
- Foramen magnum: Transmits the spinal cord; accessory nerve; vertebral arteries.
- Hypoglossal canal: Transmits the hypoglossal nerve to muscles of the tongue.
Sphenoid Bone
- Foramen ovale: Transmits the mandibular division of the trigeminal nerve; accessory meningeal artery.
- Foramen rotundum: Transmits the maxillary division of the trigeminal nerve.
- Optic canal: Transmits the optic nerve; ophthalmic artery.
- Superior orbital fissure: Transmits the oculomotor, trochlear, and abducens nerves; ophthalmic division of the trigeminal nerve; ophthalmic veins.
Maxilla
- Inferior orbital fissure: Transmits the infraorbital nerve; zygomatic nerve; infraorbital vessels.
- Infraorbital foramen: Transmits the infraorbital nerve and vessels.
Mandible
- Mental foramen: Transmits the mental nerve and vessels.
- Mandibular foramen: Transmits the inferior alveolar nerves and vessels to the lower teeth.

Cranial Fossae

Base is divided into 3 basins, forming the cranial floor:
- Anterior cranial fossa: Contains the frontal lobes of the brain.
- Middle cranial fossa: Contains the temporal lobes of the brain.
- Posterior cranial fossa: Contains the cerebellum hemispheres.

Paranasal Sinuses

Paranasal sinuses include the frontal, sphenoid, ethmoid, and maxillary sinuses.
These sinuses are lined by mucous membranes, are air-filled, lighten the anterior portion of the skull, and add resonance to the voice.

Ethmoid Bone

Features include the perpendicular plate, cribriform plate, crista galli, cribriform (olfactory) foramina, and labyrinth with ethmoid cells.
Two pairs of nasal conchae (plus inferior nasal conchae) create turbulence of airflow.

Facial Bones

There are 14 facial bones.
They have no direct contact with the brain or meninges.
Support the teeth.
Give shape and individuality to the face.
Form part of the orbital and nasal cavities.
Provide attachments for muscles of facial expression and mastication.
Consist of:
- 2 maxillae
- 2 nasal bones
- 2 palatine bones
- 2 inferior nasal conchae
- 2 zygomatic bones
- 2 lacrimal bones
- 1 vomer
- 1 mandible

Mandible

The mandible is the strongest bone of the skull.
Supports the lower teeth.
Provides attachments for muscles of facial expression and mastication.
The mental symphysis is a median cartilaginous joint in the fetus.

Bones Associated with the Skull

Auditory Ossicles

There are $3$ in each middle ear: malleus, incus, and stapes.

Hyoid Bone

Does not articulate with any other bone.
Suspended from the styloid process of the skull by muscle and ligament.

The Skull in Infancy & Childhood

Fontanelles

Anterior, posterior, sphenoid (anterolateral), and mastoid (posterolateral) fontanelles.
Two frontal bones fuse by age $6$ (metopic suture).
The skull reaches adult size by $8-9$ years of age.

Vertebral Column

Functions

Supports the skull and trunk.
Allows for their movement.
Protects the spinal cord.
Absorbs stress of walking, running, and lifting.
Provides attachments for limbs, thoracic cage, and postural muscles.

Vertebral Groups

$7$ cervical vertebrae
$12$ thoracic vertebrae
$5$ lumbar vertebrae
$5$ sacral vertebrae (fused into the sacrum)
$4$ coccygeal vertebrae (fused into the coccyx)

Spinal Curvatures

S-shaped with $4$ normal curvatures:
- Cervical lordosis
- Thoracic kyphosis
- Lumbar lordosis
- Sacral (Pelvic) kyphosis
Primary curvatures (present at birth):
- Thoracic and sacral/pelvic
Secondary curvatures (develop later):
- Cervical and lumbar

Abnormal Spinal Curvatures

Scoliosis: Lateral curvature; body and arch fail to develop on one side of vertebrae.
Hyperkyphosis (hunchback): Exaggerated thoracic curvature; may be caused by osteoporosis or wrestling/weight lifting in young boys.
Hyperlordosis (swayback): Exaggerated lumbar curvature.

Structure of Vertebrae

Body (Centrum)

Spongy bone with red bone marrow.
Rough superior and inferior surfaces for attachment of intervertebral discs.

Vertebral Foramen

Forms the vertebral canal for the spinal cord.

Intervertebral Foramen

Opening between pedicles of two joined vertebrae.

Intervertebral Discs

There are $23$ intervertebral discs within the spine
First disc is between C2 & C3
Last disc is between L5 & sacrum
Pad consisting of:
- Nucleus pulposus
- Anulus fibrosus
Bind vertebrae together.
Support weight of body.
Absorb shock.
Herniated (“ruptured” or “slipped”) disc puts pressure on spinal nerves or the spinal cord.

The Cervical Vertebrae

The Cervical Vertebrae are the Smallest and lightest vertebrae, other than the coccygeals
Small body and larger vertebral foramen
Bifid (forked) spinous processes in C2 - C6
C7 vertebra prominens - long spinous process; not bifid
- Prominent bump on lower back of neck
- Convenient landmark
Transverse foramina - passage & protection for vertebral arteries & veins

Atlas (C1)

Supports the head; has no body.
Superior articular facets articulate with occipital condyles, forming the atlanto-occipital joint.
- Allows nodding of the skull, gesturing “yes.”
Inferior articular facets articulate with the axis (C2).

Axis (C2)

Features a dens or odontoid process that projects into the vertebral foramen of the atlas.
Allows rotation of the head, gesturing “no.”
The atlantoaxial joint is located between the atlas and axis.

The Thoracic Vertebrae

Spinous processes pointed and angled downward.
Body larger than cervical, but smaller than lumbar.
Inferior & superior costal facets on body for rib attachment
In most cases, ribs insert between 2 vertebra
Transverse costal facets on T1–T10 – 2nd point of articulation for ribs 1–10

The Lumbar Vertebrae

$5$ lumbar vertebrae (L1–L5)
Thick, stout body
Blunt, squarish spinous process
Superior articular processes face medially, making the lumbar region resistant to twisting movements.

The Sacrum

Forms the posterior wall of the pelvic cavity.
$5$ separate vertebrae (S1–S5) fuse by age $26$ .
$4$ pairs of sacral (pelvic) foramina allow passage of nerves and arteries into pelvic organs.
The sacral promontory on S1 supports L5.

The Coccyx

Consists of $4$ (sometimes $5$ ) small vertebrae (Co1–Co4) that fuse into a single bone by age $20-30$ .
Horns (cornua) on Co1 attach to ligaments that bind the coccyx to the sacrum.
Provides attachment for muscles of the pelvic floor.

The Thoracic Cage

Composition

Consists of thoracic vertebrae, sternum, and ribs.

Functions

Encloses and protects the lungs and heart.
Provides attachment for the pectoral girdle and upper limbs.
Protects thoracic organs, spleen, liver, and kidneys.
The costal margin is the inferior border formed by the downward arc of ribs.

The Sternum

Regions

Manubrium:
- Clavicular notches articulate with clavicles.
- Ribs attach along scalloped lateral margins.
Body (gladiolus):
- Ribs attach along scalloped lateral margins.
Xiphoid process:
- Attachment for some abdominal muscles.

The Ribs

$12$ pairs.
The Posterior (proximal) end attaches to vertebrae
The Anterior (distal) end mostly attached to sternum via costal cartilages (hyaline cartilage)

Types of Ribs

True ribs (1–7):
- Each has its own costal cartilage connecting to the sternum.
False ribs (8–12):
- Costal cartilages of ribs 8-10 join with that of rib 7.
Floating ribs (11–12):
- Articulate with bodies of T11 & T12.
- No anterior attachment to the sternum.

The Pectoral Girdle

Composition

Supports the arm and consists of two bones:
- Clavicle (collarbone)
- Scapula (shoulder blade)

Articulations

The clavicle articulates medially to the sternum and laterally to the scapula:
- Sternoclavicular joint
- Acromioclavicular joint
The scapula articulates with the humerus.

The Clavicle

Features

Sternal end: Articulates with the sternum.
Acromial end: Articulates with the acromion of the scapula.

Functions

Braces the shoulder, keeping the upper limb away from the midline of the body.
Most frequently fractured bone in the body.

The Scapula

Spine: Transverse ridge on the posterior surface.
Acromion: Articulates with the clavicle; the sole point of attachment of the scapula and upper limb to the rest of the skeleton.

The Upper Limb

$4$ regions containing a total of $30$ bones per limb:
- Brachium (arm proper): shoulder to elbow (humerus).
- Antebrachium (forearm): elbow to wrist (radius & ulna).
- Carpus (wrist): $8$ small bones arranged in $2$ rows.
- Manus (hand): $5$ metacarpals in the palm; $14$ phalanges in the fingers.

The Pelvic Girdle

Composition

Three bones:
- 2 hip (coxal) bones, also known as ossa coxae or innominate bones
- Sacrum
- Coccyx
Sacroiliac joint: Joins the hipbone to the vertebral column.
Anterior interpubic disc: Pad of fibrocartilage joins pubic bones.
Pubic symphysis: Interpubic disc & adjacent regions of pubic bone.

Functions

Supports the trunk on the lower limbs.
Protects viscera, lower colon, urinary bladder, and internal reproductive organs.

Hip Bone Formation

Each adult hip bone is formed by the fusion of $3$ bones: ilium, ischium, and pubis (pubic bone).

Features

Iliac crest
Acetabulum
Obturator foramen

The Pelvis

Greater (false) pelvis: Between the flare of the hips.
Lesser (true) pelvis: Narrower & below.
Pelvic brim: Round margin that separates the two.
Pelvic inlet: Opening circumscribed by brim.
Pelvic outlet: Lower margin of lesser pelvis.

Differences Between Male and Female Pelvis

Feature	Female	Male
Cavity of true pelvis	Broad, shallow	Narrow, deep
Pelvic inlet	Oval	Heart-shaped
Sacrum	Wider; shorter	Narrow; longer
Acetabula	Smaller; farther apart	Larger; closer
Pubic angle	Broad, more rounded, >90°	Acute, <90°
Greater sciatic notch	Wide; shallow	Narrow; deep
Coccyx	Straighter, more movable	Curved, less movable
Body of Pubis	Rectangular	Triangular

The Lower Limb

$4$ regions containing $30$ bones per limb:
- Femoral region (thigh): hip to knee region (femur & patella).
- Crural region (leg proper): knee to ankle (medial tibia and lateral fibula).
- Tarsal region (tarsus): ankle (7 tarsal bones).
- Pedal region (pes): foot (5 metatarsals; 14 phalanges in toes).

The Foot

Contains $3$ springy arches, held together by strong ligaments, to absorb stress:
- Medial longitudinal arch: From heel to hallux (big toe).
- Lateral longitudinal arch: From heel to little toe.
- Transverse arch: Across the middle of the foot.
Pes planus (flat feet): Excessive weight, repetitious stress, or congenital weakness can cause flattening of the arches.