The Axial Skeleton

Overview of Skull Structure

The skull is a complex structure composed of 22 bones, organized into cranial and facial groups. Cranial bones protect the brain, while facial bones form the framework of the face and support sensory organs.

• Cranial bones (8): Frontal, parietal (paired), sphenoid, ethmoid, temporal (paired), occipital

• Facial bones (14): Nasal (paired), lacrimal (paired), palatine (paired), zygomatic (paired), inferior nasal concha (paired), maxilla (paired), mandible, vomer

• Sutures: Immovable joints connecting skull bones, except the mandible

Skull Cavities and Sinuses

The skull contains several cavities that house sensory organs and air-filled spaces called sinuses. These structures are essential for functions such as smell, taste, vision, hearing, and balance.

• Paranasal sinuses: Air-filled spaces in four bones surrounding the nasal cavity

• Orbits: Contain the eyeballs

• Nasal cavity: Houses olfactory receptors

• Oral cavity: Surrounds teeth and tongue

• Cranial cavity: Encloses the brain

The Fetal Skull

Unlike the adult skull, the fetal skull bones are not fused, allowing flexibility during birth and growth. Membranous areas called fontanels are present until the cranial bones complete development.

• Fontanels: Soft spots between fetal cranial bones

• Sutures: Unfused in infants, allowing skull expansion

• Development: Fontanels close by 18–24 months

Hyoid Bone

The hyoid bone is unique because it does not articulate with any other bone. It is suspended by ligaments and serves as an attachment point for muscles involved in swallowing and speech.

• Location: Near the larynx, attached to temporal bone by ligaments

• Function: Supports tongue and muscles of the neck

The Vertebral Column and Thoracic Cage

Overview of the Vertebral Column

The vertebral column, or spine, consists of 33 vertebrae classified by region and function. It provides structural support, protects the spinal cord, and allows flexibility.

• Cervical (C1–C7): Neck region

• Thoracic (T1–T12): Articulate with ribs

• Lumbar (L1–L5): Lower back, weight-bearing

• Sacral (5 fused): Form sacrum, articulate with pelvis

• Coccygeal (3–5 fused): Form coccyx, most inferior

Structure of the Vertebrae

All vertebrae share common structural features that allow for movement and protection of the spinal cord.

• Body: Main weight-bearing portion

• Vertebral foramen: Passage for spinal cord

• Pedicles and laminae: Form vertebral arch

• Articular processes: Allow articulation between vertebrae

• Transverse and spinous processes: Sites for muscle attachment

Structure of the Cervical Vertebrae

Cervical vertebrae are the smallest and are easily identified by the presence of transverse foramina. The first two cervical vertebrae, atlas (C1) and axis (C2), have unique structures.

• Transverse foramina: Allow passage of vertebral arteries

• C1 (Atlas): Lacks a body, has anterior and posterior arches

• C2 (Axis): Contains the dens (odontoid process) for head rotation

Structure of the Thoracic Vertebrae

Thoracic vertebrae are larger than cervical vertebrae and have features for rib articulation. Their spinous processes point inferiorly.

• Costal facets: Articulate with ribs

• Heart-shaped bodies: Distinctive shape

• Long spinous processes: Point downward

Structure of the Lumbar Vertebrae

Lumbar vertebrae are the largest and heaviest, designed to bear the weight of the torso. They have thick bodies and short spinous processes.

• Kidney-shaped bodies: Adapted for weight-bearing

• Flattened vertebral foramina: Triangular shape

• Thick pedicles and laminae: Provide strength

Structure of the Sacrum

The sacrum consists of five fused vertebrae forming the posterior boundary of the pelvic cavity. It contains openings for nerve passage and articulates with the pelvis and coccyx.

• Base: Articulates with L5

• Apex: Articulates with coccyx

• Sacral canal: Continuation of vertebral canal

• Sacral foramina: Passage for nerves

Structure of the Coccyx

The coccyx is formed by three to five fused vertebrae and represents the terminal end of the vertebral column. It provides attachment for ligaments and muscles.

• Coccygeal cornu: Hornlike projections

Intervertebral Discs

Intervertebral discs are fibrocartilage pads between vertebrae that absorb shock and bind the column together. Each disc has a soft nucleus pulposus and a tough anulus fibrosus.

• Nucleus pulposus: Jelly-like center, shock absorber

• Anulus fibrosus: Outer ring, binds vertebrae

The Thoracic Cage

Structure of the Thoracic Cage

The thoracic cage consists of the sternum, ribs, and thoracic vertebrae. It protects vital organs and provides attachment points for muscles involved in respiration.

• Sternum: Composed of manubrium, body, and xiphoid process

• Ribs: 12 pairs, classified as true (1–7), false (8–12), and floating (11–12)

• Costal cartilages: Connect ribs to sternum

The Appendicular Skeleton

The Pectoral Girdle

The pectoral girdle consists of the clavicle and scapula, which connect the upper limb to the axial skeleton and allow for a wide range of motion.

• Clavicle: Articulates with sternum and scapula

• Scapula: Provides attachment for muscles and articulates with humerus

Clavicle

The clavicle, or collarbone, is a slender bone that acts as a strut between the sternum and scapula. It is important for shoulder stability.

• Sternal end: Medial, articulates with manubrium

• Acromial end: Lateral, articulates with scapula

Scapula

The scapula is a flat, triangular bone with several important features for muscle attachment and articulation with the humerus.

• Acromion: Articulates with clavicle

• Coracoid process: Muscle attachment

• Glenoid cavity: Articulates with humerus

• Spine: Prominent ridge

Bones of the Forearm

The forearm consists of the radius and ulna, which are connected by an interosseous membrane. Both bones articulate with the humerus and carpal bones.

• Radius: Lateral bone

• Ulna: Medial bone

Bones of the Wrist: Carpals

The wrist contains eight carpal bones arranged in two rows. These bones allow for flexibility and movement of the hand.

• Proximal row: Scaphoid, lunate, triquetrum, pisiform

• Distal row: Trapezium, trapezoid, capitate, hamate

Bones of the Hand and Fingers: Metacarpals and Phalanges

The hand consists of five metacarpals and fourteen phalanges. Each finger has three phalanges, except the thumb, which has two.

• Metacarpals: Numbered I–V from lateral to medial

• Phalanges: Proximal, middle, distal (thumb lacks middle)

Bones of the Pelvic Girdle and Lower Limb

Pelvic Girdle Structure

The pelvic girdle is formed by two coxal bones and the sacrum. It supports the weight of the upper body and protects pelvic organs.

• Coxal bones: Each consists of ilium, ischium, and pubis

• Acetabulum: Deep socket for femur articulation

Pelvic Differences Between Genders

The pelvis differs between males and females, with the female pelvis adapted for childbirth. Key differences include width, shape, and angle of the pubic arch.

The Femur and Patella

The femur is the largest bone in the body, articulating with the pelvis at the hip joint and with the tibia at the knee. The patella is a sesamoid bone within the tendon of the thigh muscle.

• Femur: Head, greater and lesser trochanters, epicondyles, condyles

• Patella: Triangular, articulates with femur

Bones of the Leg: Tibia and Fibula

The tibia and fibula are the bones of the leg, connected by an interosseous membrane. The tibia bears weight, while the fibula provides stability.

• Tibia: Medial, weight-bearing

• Fibula: Lateral, slender

Bones of Ankle and Foot: Tarsals, Metatarsals, and Phalanges

The ankle and foot contain seven tarsal bones, five metatarsals, and fourteen phalanges. The talus articulates with the leg bones, and the calcaneus forms the heel.

• Proximal tarsals: Talus, calcaneus, navicular

• Distal tarsals: Medial, intermediate, lateral cuneiform, cuboid

• Metatarsals: Numbered I–V

• Phalanges: Great toe (hallux) has two, others have three

Study Boost: Remembering Skull Bones and Vertebrae

Mnemonic Devices

Mnemonics are useful for memorizing the names and numbers of bones in the skull and vertebral column.

• Pest Of 6: Parietal, Ethmoid, Sphenoid, Temporal, Occipital, Frontal (cranial bones)

• Virgil Is Now Making My Pet Zebra Laugh: Vomer, Inferior nasal conchae, Nasal, Mandible, Maxillae, Palatine, Zygomatic, Lacrimal (facial bones)

• Breakfast at 7, lunch at 12, dinner at 5: 7 cervical, 12 thoracic, 5 lumbar vertebrae

Visual Analogies

• Sphenoid bone: Resembles a bat with wings

• Ethmoid bone: Like an iceberg, only tip visible

• Thoracic vertebra: Looks like a giraffe's head

• Lumbar vertebra: Resembles a moose's head

Lab Activities

• Studying the bones of the skull

• Studying the bones of the vertebral column and thoracic cage

• Identifying bones in a bag

• Studying the bones of the appendicular skeleton

Compare the Vertebrae

Comparing cervical, thoracic, and lumbar vertebrae helps in identification and understanding their functional adaptations.

• Cervical: Small, transverse foramina

• Thoracic: Heart-shaped body, costal facets

• Lumbar: Large, kidney-shaped body

Summary Table: Vertebral Column Regions