Skeletal System: Axial and Appendicular Overview with Landmarks

Division and Overview of the Skeleton

• The skeleton is divided into two main parts:

  • Axial skeleton: head, neck, and torso; core of the body. It contains $80$ bones.

  • Appendicular skeleton: upper and lower limbs (arms and legs); contains $126$ bones.

• Total number of bones in the adult human skeleton: $206$.

• These counts are a practical framework for studying bone structure, landmarks, and articulation.

Bone Markings and Bony Landmarks

• Bone body: the central or main portion of a bone.

• Foramen: a round hole in a bone through which blood vessels or nerves pass. Example: foramen magnum on the occipital bone.

• Meatus: an opening or canal that leads to the outside of the body. Example: external auditory meatus (ear canal).

• Fossa: a depression in a bone that may receive a bone process or be involved in a joint.

• Notch: a V-shaped depression.

• Six common bone processes (points of attachment or articulation):

  • Head: a rounded, distinct portion of a long bone that forms a joint (e.g., head of the femur into the acetabulum of the hip bone).

  • Condyle: a rounded bump that forms a joint with a fossa (e.g., mandibular condyle into the mandibular fossa of the temporal bone).

  • Epicondyle: a projection above a condyle; typically for muscle attachment.

  • Spine: a sharp, pointed process for muscle attachment.

  • Trochanter: a large bump for muscle attachment (notably on the femur).

  • Tuberosity: a smaller bump for muscle attachment (not associated with a condyle).

• Sinuses: hollow, air-filled cavities enclosed within a bone.

  • Paranasal sinuses are around the nose.

  • Examples: frontal sinuses, maxillary sinuses, sphenoid sinuses, ethmoid sinuses.

• These markings are important for identifying skull bones in labs and for understanding their functions.

The Skull: Cranium and Facial Bones

• The skull comprises $8$ cranial bones and $14$ facial bones, plus the ear ossicles (in the middle ear).

  • Cranial bones (protect the brain):

  • Frontal

  • Parietal (two, left and right)

  • Temporal (two, left and right)

  • Occipital

  • Sphenoid

  • Ethmoid

  • Facial bones:

  • Zygomatic (two)

  • Lacrimal (two)

  • Nasal (two)

  • Maxillary (two)

  • Inferior nasal concha (two)

  • Palatine (two)

  • Mandible

  • Vomer

• Frontal bone: forms the forehead; contributes to the anterior cranial floor and the superior portion of the orbits.

• Temporal bones: located laterally on the skull; paired.

• Occipital bone: forms the posterior aspect of the skull.

• Sphenoid bone: often called the keystone of the skull; anchors other cranial bones; forms lateral walls of the cranium and orbits.

• Ethmoid bone: irregular bone between nasal and sphenoid bones; forms the medial portions of the orbits and the upper portion of the nasal septum.

• Facial bones:

  • Maxillary bones: form the upper jaw; keystone of the face; contribute to the anterior hard palate and the floors of the orbits.

  • Mandible: lower jaw; the largest and strongest facial bone; only movable bone of the skull.

  • Orbits (eye sockets): formed by seven bones — frontal, sphenoid, zygomatic, ethmoid, lacrimal, maxillary, palatine.

  • Nasal septum: midline division of the nasal cavity; formed by the perpendicular plate of the ethmoid bone (upper portion) and the vomer (lower portion); anterior septum is reinforced by hyaline cartilage.

• Notes on lab and anatomy:

  • Skull bones come up frequently in anatomy labs (skull lab and skeleton lab).

  • Understanding which bones form orbits and the nasal septum helps in clinical assessments (e.g., sinus disease, orbital fractures).

Sutures and Fontanels

• Sutures: immovable joints where skull bones come together; function to lock bones in place.

  • Squamous suture: along the top curved edge of the temporal bone between temporal and parietal bones and parts of the sphenoid.

  • Lambdoidal suture: between the parietal and occipital bones.

  • Coronal suture: between the parietal and frontal bones.

  • Sagittal suture: between the two parietal bones.

• Fontanels: soft spots in an infant's skull where ossification is not complete.

  • Function: allow passage through the birth canal and enable brain growth after birth.

  • Over time, fontanels ossify and become sutures in adulthood.

The Vertebral Column and Curves

• Vertebral counts:

  • Cervical: $7$

  • Thoracic: $12$

  • Lumbar: $5$

  • Sacrum: fusion of $5$ vertebrae

  • Coccyx: fusion of about $4{-}5$ vertebrae

• Primary (initial) curves and secondary (develop later) curves:

  • Primary curves (present at birth) are convex: thoracic and sacral/pelvic curves.

  • Secondary curves (develop after birth) are concave: cervical (head control) and lumbar (sitting/walking).

• Function of spinal curves: provide strength, balance, stability, and protection from fracture.

Ribs and the Thoracic Cage

• Ribs: $12$ pairs in total.

  • True ribs: $7$ pairs directly attached to the sternum via costal cartilage.

  • False ribs: $5$ pairs; upper three indirectly attach to the sternum (via rib 7).

  • Floating ribs: the last two pairs that are not attached to the sternum.

• Thoracic cage: sternum, ribs, and vertebral column together; protects vital organs like the lungs and heart.

The Appendicular Skeleton: Girdles

• Shoulder girdle: connects the upper limb to the axial skeleton and facilitates shoulder movement.

  • Clavicle (collarbone) in the front; scapula (shoulder blade) in the back.

  • Together they form the shoulder girdle.

• Pelvic girdle: supports weight and provides attachment for lower limbs.

  • Formed by two coxal bones (os coxae) on left and right.

  • Each os coxae is a fusion of three bones: ilium, ischium, and pubis.

  • The coxal bones + sacrum + coccyx form the bony pelvis; the broadest region of the skeleton.

• Notes on terminology and landmarks:

  • The pelvic girdle is sometimes described as consisting of the broadest bone structure in the body due to the size of the pelvis.

  • The coxal bone on each side is composed of the fused ilium, ischium, and pubis.

Upper and Lower Limbs: Bones and Function

• Upper limb anatomy:

  • Bones of the hand: carpals (wrist), metacarpals (palm), and phalanges (fingers).

  • Major function: manipulation and grasping; the opposable thumb enables fine motor control.

• Lower limb anatomy:

  • Bones of the foot: tarsals (ankle), metatarsals (soles), and phalanges (toes).

  • Major function: weight-bearing and locomotion; arches of the foot provide balance and stability.

• Arches of the foot:

  • Transverse arch: runs across the foot.

  • Medial longitudinal arch: along the inner edge of the foot.

  • Lateral longitudinal arch: along the outer edge of the foot.

  • Tendons and ligaments maintain these arches, enabling body support, stability, and balance.

Hands vs Feet: Functional Differences

• Although hand and foot bones share naming patterns (carpals/metacarpals/phalanges vs tarsals/metatarsals/phalanges), their functions differ:

  • Hands: primarily for manipulating objects and grasping; enabled by opposable thumbs.

  • Feet: primarily for supporting body weight and providing balance; arches and big toes contribute to propulsion and stability.

Sex Differences in the Skeleton: Pelvis and Beyond

• General differences between male and female skeletons:

  • Male: typically larger and heavier; pelvis is more narrow and funnel-shaped, deeper and more vertically oriented.

  • Female: pelvis is broader, shallower, and more flared to facilitate childbirth.

  • Pelvic arch angle: male < $90^ ext{o}$; female > $90^ ext{o}$, reflecting functional differences in childbirth.

Practical Takeaways and Exam Connections

• A common exam question: which bones form the shoulder girdle? Answer: clavicle and scapula.

• The hyoid bone is a single, free-floating bone in the neck (does not articulate with other bones).

• The ear ossicles (malleus, incus, stapes) amount to $6$ total bones in the middle ear.

• The skull contains $8$ cranial bones and $14$ facial bones, with the rest of the facial structure composed by the other facial landmarks.

• Chapter 9 (joints) will cover sutures as immovable joints and how joints form connections between bones.

Quick Reference: Key Numerical Facts

• Axial skeleton: $80$ bones

• Appendicular skeleton: $126$ bones

• Total: $206$ bones

• Skull bones: $8$ cranial + $14$ facial (plus $6$ ear ossicles)

• Vertebral counts: $7$ cervical, $12$ thoracic, $5$ lumbar; sacrum ($5$ fused), coccyx ($4{-}5$ fused)

• Ribs: $12$ pairs ($7$ true, $5$ false; $2$ floating)

• Pectoral girdle: clavicle + scapula

• Pelvic girdle: two coxal bones (os coxae), each formed by ilium, ischium, and pubis

• Pelvic arch angles: male < 90^ ext{o}, female > 90^ ext{o}

Closing Note

• This overview covers major and minor points from the skeletal system lectures that are likely to appear on the exam, including anatomy, landmarks, and the functional implications of bone structure and relationships. Review the lab materials for the skull and skeleton to reinforce these concepts and prepare for Chapter 9 on joints.