Human Skeleton and Vertebral Anatomy

Overview of the Human Skeleton

• The human skeleton is categorized into two major divisions, providing support, protection, and movement for the body:

  • Axial Skeleton: Forms the central axis of the body, primarily involved in protecting internal organs and providing support.

  • Appendicular Skeleton: Comprises the limbs and their girdles, primarily involved in locomotion and manipulation of objects.

Major Components of the Skeleton

• Axial Skeleton includes:

  • Skull: Protects the brain and houses sensory organs.

  • Vertebral column: Provides central support and protects the spinal cord.

  • Thoracic cage (ribs and sternum): Protects organs in the thoracic cavity, such as the heart and lungs.

• Appendicular Skeleton includes:

  • Limbs (arms and legs): Essential for movement and interaction with the environment.

  • Pelvic girdle and pectoral girdle: Connect the limbs to the axial skeleton, providing points of attachment and facilitating movement.

Detailed Structure of the Skeleton

Axial Skeleton

• Skull consists of 22 bones, providing the framework for the head and protecting the brain:

  • Cranial Bones (8 bones): Enclose and protect the brain.

  • Frontal bone (1)

  • Parietal bone (2)

  • Temporal bone (2)

  • Occipital bone (1)

  • Sphenoid bone (1): Often called the "keystone bone" as it articulates with all other cranial bones.

  • Ethmoid bone (1): Forms part of the nasal cavity and the orbit.

  • Facial Bones (14 bones): Form the face, contain cavities for special sense organs (sight, taste, smell), provide openings for air and food passage, secure teeth, and anchor facial muscles.

  • Mandible (1): The lower jaw, the only movable bone of the skull.

  • Maxillae (2): Form the upper jaw and central facial skeleton.

  • Zygomatic (2): Cheekbones.

  • Nasal (2)

  • Lacrimal (2)

  • Palatine (2): Form the posterior part of the hard palate.

  • Vomer (1)

  • Inferior Nasal Conchae (2)

  • Sutures connecting bones of the skull: Immovable fibrous joints that ossify over time.

  • Coronal suture: Between the frontal and parietal bones.

  • Sagittal suture: Between the two parietal bones.

  • Lambdoid suture: Between the parietal and occipital bones.

  • Squamous suture: Between the parietal and temporal bones.

Sinuses

• Paranasal Sinuses (air-filled spaces adjacent to the nasal cavity and connected to it): Serve to lighten the skull, warm and humidify incoming air, and provide resonance for the voice.

  • Frontal sinus

  • Ethmoidal air cells (sinus)

  • Sphenoidal sinus

  • Maxillary sinus: The largest of the paranasal sinuses.

Hyoid Bone

• Unique features:

  • Does not articulate directly with any other bone; it is suspended by ligaments and muscles.

  • Function: Serves as a movable base for the tongue and an attachment point for muscles involved in swallowing and speech.

  • Components:

  • Body

  • Greater and lesser horns (cornua)

Vertebral Column

• Composed of $33$ individual vertebrae during development, which fuse to form $26$ bones in the adult:

  • Cervical vertebrae: 7 (in the neck region)

  • Thoracic vertebrae: 12 (in the chest region, articulate with ribs)

  • Lumbar vertebrae: 5 (in the lower back, largest and strongest)

  • Sacral vertebrae: 5 fused (form the sacrum)

  • Coccygeal vertebrae: 3-5 fused (form the coccyx or tailbone)

• Curvatures: Provide flexibility and resilience, acting as shock absorbers.

  • Cervical and lumbar are posteriorly concave (secondary curvatures, develop after birth).

  • Thoracic and sacral are posteriorly convex (primary curvatures, present at birth).

  • Clinical conditions associated with abnormal curvatures:

  • Thoracic kyphosis: Exaggerated posterior curvature of the thoracic spine (hunchback).

  • Lumbar lordosis: Exaggerated anterior curvature of the lumbar spine (swayback).

  • Scoliosis: Abnormal lateral curvature of the spine, often with a rotational component.

• Ligaments (connect vertebrae and provide stability):

  • Anterior longitudinal ligament: Broad, strong, runs down the anterior aspect of the vertebral bodies, preventing hyperextension.

  • Posterior longitudinal ligament: Narrower, weaker, runs down the posterior aspect of the vertebral bodies, preventing hyperflexion.

  • Ligamentum flavum: Connects adjacent laminae, contains elastic tissue, helps maintain upright posture and prevent abrupt stopping of flexion.

• Intervertebral Discs (cartilaginous pads between vertebrae):

  • Function as shock absorbers and allow for movement between vertebrae. They account for about $25\%$ of the height of the vertebral column.

  • Structure:

  • Inner portion: nucleus pulposus, a gel-like center that gives the disc elasticity and compressibility.

  • Outer portion: annulus fibrosus, a strong collar of fibrocartilage that limits expansion of the nucleus pulposus and binds successive vertebrae together.

Vertebral Structure

• Key features of a typical vertebra include:

  • Body (centrum): Anterior, weight-bearing region.

  • Vertebral arch: Formed by the pedicles and laminae, encloses the vertebral foramen.

  • Vertebral foramen: Opening for the spinal cord.

  • Pedicle: Connects the body to the transverse process.

  • Lamina: Connects the transverse process to the spinous process.

  • Spinous process: Posterior projection for muscle attachment.

  • Transverse process: Lateral projections for muscle attachment and articulation with ribs in thoracic region.

  • Superior and inferior articular processes: Facets for articulation with adjacent vertebrae.

Specific Vertebrae

• Atlas (C1):

  • Unique because it lacks a body and a spinous process.

  • Has large superior articular facets that articulate with the occipital condyles, allowing for the "yes" nodding motion of the head.

• Axis (C2):

  • Notable for the odontoid process (dens), a tooth-like projection extending superiorly from the body, which acts as a pivot for the rotation of the atlas and skull, allowing for the "no" shaking motion of the head.

Types of Vertebrae

• Cervical Vertebrae (C1-C7):

  • Smallest bodies, adapted for flexibility.

  • Possess transverse foramina in their transverse processes, which transmit the vertebral arteries to the brain.

  • Bifid (forked) spinous process (C2-C6).

  • Triangular shaped vertebral foramen.

• Thoracic Vertebrae (T1-T12):

  • Heart-shaped body.

  • Long, downward projecting spinous process.

  • Circular vertebral foramen.

  • Have facets (demifacets) on their bodies and transverse processes (except T11, T12) for articulation with the heads and tubercles of the ribs.

• Lumbar Vertebrae (L1-L5):

  • Largest and strongest vertebral bodies to support the greatest amount of body weight.

  • Short, hatchet-shaped spinous process.

  • Triangular shaped vertebral foramen, but larger than thoracic.

  • Articular facets directed medially and laterally, providing stability but limiting rotation.

Sacrum and Coccyx

• Sacrum:

  • A triangular bone formed by the fusion of five sacral vertebrae, articulates with the fifth lumbar vertebra superiorly and the coccyx inferiorly, and with the hip bones laterally.

  • Forms the posterior wall of the pelvis.

  • Features:

  • Sacral promontory: Anterior superior margin of the first sacral vertebra.

  • Alae: Lateral portions, formed by fusion of transverse processes.

  • Transverse lines: Indicate points of vertebral fusion.

  • Ventral and dorsal sacral foramina: Passageways for sacral nerves.

  • Median sacral crest: Formed by fusion of spinous processes.

  • Lateral sacral crests: Formed by fusion of transverse processes.

• Coccyx:

  • Commonly known as the tailbone, composed of 3-5 fused coccygeal vertebrae at the base of the vertebral column.

  • A somewhat vestigial structure, it provides slight support for pelvic organs and an attachment point for some muscles of the pelvic floor.

Bony Thorax (Thoracic Cage)

• Composed of the sternum, ribs, and thoracic vertebrae; protects the vital organs of the thoracic cavity (heart, lungs, great vessels) and provides attachment for muscles.

  • Sternum (breastbone):

  • A flat bone anterior to the heart.

  • Manubrium: Superior portion, articulates with clavicles and first two pairs of ribs.

  • Body: Middle and largest part, articulates with ribs 2-7.

  • Xiphoid process: Inferior tip, ossifies by age $40$, an attachment point for some abdominal muscles.

  • Sternal angle (angle of Louis): A palpable horizontal ridge where the manubrium and body join, helpful anatomical landmark for counting ribs.

  • Clavicular notch: Articulation point for the clavicles.

  • Ribs:

  • Total of $12$ pairs, all articulating posteriorly with the thoracic vertebrae.

  • First $7$ pairs are "true ribs" (vertebrosternal ribs): Attach directly to the sternum via their own costal cartilages.

  • Last $5$ pairs are "false ribs" (vertebrochondral & vertebral ribs):

    • Ribs 8-10 attach indirectly to the sternum by fusing their costal cartilages to the cartilage of the rib above.

    • Last $2$ pairs (ribs 11-12) are "floating ribs" (vertebral ribs): Have no anterior attachment to the sternum or other costal cartilages, they only articulate with the vertebrae.

  • Components of ribs: head (articulates with vertebral body), neck, tubercle (articulates with transverse process of vertebra), angle, shaft, costal groove (for intercostal nerves and vessels).

Appendicular Skeleton

Shoulder Girdle (Pectoral Girdle)

• The pectoral girdle includes the clavicle and the scapula, connecting the upper limbs to the axial skeleton and allowing a wide range of motion, though with less stability than the pelvic girdle.

  • Clavicle (collarbone): Acts as a brace, holding the scapula and arm away from the thorax.

  • Scapula (shoulder blade): A triangular flat bone, key for shoulder joint movement, provides attachment for many muscles.

Upper Limb

• Comprises $30$ bones per limb, highly movable for manipulation:

  • Humerus: The bone of the upper arm, articulates with the scapula at the shoulder and with the ulna and radius at the elbow.

  • Ulna: Medial bone of the forearm.

  • Radius: Lateral bone of the forearm, articulates with carpals for wrist movement.

  • Hand:

  • Carpals (8 wrist bones): Arranged in two rows, provide flexibility to the wrist.

  • Metacarpals (5 hand bones): Form the palm.

  • Phalanges (14 finger bones): Each finger has three (proximal, middle, distal), except the thumb which has two (proximal, distal).

Pelvic Girdle

• Composed of two coxal (hip) bones, which uniteanteriorly and articulate with the sacrum posteriorly. It connects the lower limbs to the axial skeleton, transmits weight to the lower limbs, and supports the visceral organs.

  • Each coxal bone is formed by the fusion of three bones: Ilium, Ischium, and Pubis.

  • Features: Acetabulum (hip socket), Obturator foramen, Pubic symphysis.

Lower Limb

• Includes $30$ bones per limb, adapted for weight-bearing and locomotion:

  • Femur: The single bone of the thigh, the longest and strongest bone in the body, articulates with the coxal bone (hip) and tibia (knee).

  • Tibia (shinbone): Larger, medial bone of the lower leg, weight-bearing.

  • Fibula: Smaller, lateral bone of the lower leg, primarily for muscle attachment, does not bear weight.

  • Patella (kneecap): A sesamoid bone embedded in the quadriceps tendon, protects the knee joint.

  • Foot:

  • Tarsals (7 ankle bones): Form the posterior half of the foot (e.g., Talus, Calcaneus).

  • Metatarsals (5 bones of the sole): Form the instep.

  • Phalanges (14 toe bones): Similar to fingers, each toe has three (proximal, middle, distal), except the great toe (hallux) which has two (proximal, distal).