Anatomy: The Skull

• The skeletal system includes bones, cartilage, joints, and ligaments
• These components make up about 20% of a person's body mass.

Axial Skeleton

• The axial skeleton consists of the skull, vertebral column, and thoracic cage.

Skull

• The skull is made up of 22 different bones
• It is divided into cranial bones that protect the brain, and facial bones that provide structure to the face.
• Most skull bones are flat bones, connected by serrated lines called sutures in the cranium.
• The cranium has a vault and a base, divided into anterior, middle, and posterior cranial fossae, which form the cranial cavity.
• The skull also contains ear, nasal cavities, and orbits for the eyes.
• There are eight cranial bones:
• Frontal bone
• Two parietal bones
• Occipital bone
• Two temporal bones
• Sphenoid bone
• Ethmoid bone.
• Cranial bones are connected by specific sutures:
• Coronal
• Sagittal
• Lambdoid
• Squamous
• Occipitomastoid sutures.
• Foramina, such as the foramen magnum, allow nerves, arteries, and veins to pass through the skull.
• There are fourteen facial bones
• Mandible (lower jawbone)
• Maxillary bones (upper jaw and part of the face)
• Two zygomatic bones (cheekbones)
• Nasal bones (bridge of the nose)
• Lacrimal bones
• Palatine bones
• Vomer
• Inferior nasal conchae.
• The hyoid bone, while not part of the skull, sits below the mandible and doesn't connect to any other bone.

Vertebral Column

• The vertebral column, or spine, has 26 irregular bones forming a flexible, curvy S-shape.
• It supports everything from the skull to the pelvis.
• The spine is divided into five sections:
• Cervical vertebrae: the first seven
• Thoracic vertebrae: the next twelve
• Lumbar vertebrae: the remaining five.
• Vertebrae increase in size going down the spine to support more weight.
• Below the vertebrae are the sacrum (five fused vertebrae) and the coccyx (tailbone), made of a few tiny fused vertebrae.
• Key ligaments include the anterior and posterior longitudinal ligaments, which run along the column from neck to sacrum.
• Shorter ligaments connect adjacent vertebrae, along with intervertebral discs.
• Intervertebral discs are cushiony pads with a nucleus pulposus (elastic part) surrounded by an anulus fibrosus (collagen-rich).
• Discs act as shock absorbers between vertebrae.
• Individual vertebrae have a body and a vertebral arch.
• The vertebral foramen is the hole through which the spinal cord passes.
• The vertebral arch is made of two pedicles and two laminae, which project various processes.
• Processes include the spinous process, two transverse processes, and superior/inferior articular processes.
• Cervical vertebrae have a short spinous process, a large vertebral foramen, and transverse foramina for vertebral arteries.
• Thoracic vertebrae have a long, downward-pointing spinous process and demifacets for rib connections.
• Lumbar vertebrae have short, thick pedicles and laminae.

Thoracic Cage

• The thoracic cage includes the sternum and the ribs, along with costal cartilage.
• The sternum is a flat bone in the middle of the thorax, made of three fused bones: manubrium, body, and xiphoid process.
• There are twelve pairs of ribs projecting from the vertebrae
• The first seven pairs attach directly to the sternum via costal cartilage and are called true ribs
• The next five pairs are false ribs
• Three attach to the sternum indirectly via cartilage from ribs above
• The last two are floating ribs, which do not attach to the sternum.
• Ribs are flat bones that get longer from pairs one to seven, then shorter from eight to twelve.

Appendicular Skeleton

• The appendicular skeleton mainly consists of the limbs

Pectoral Girdle

• The pectoral girdle is made of the clavicle (collarbone) and scapula (shoulder blade).
• It provides structure to the shoulder and attaches the upper limbs to the axial skeleton.
• The clavicle has a sternal end attaching to the manubrium, and an acromial end joining the scapula.
• The scapula is a thin, flat, roughly triangular bone with superior, medial (vertebral), and lateral (axillary) borders.

Upper Limb

• The upper limb consists of the arm, forearm, and hand.
• The arm contains the humerus: a typical long bone.
• Key features of the humerus include the greater and lesser tubercles, radial groove, medial and lateral epicondyles, radial and coronoid fossa, trochlea, and capitulum.
• The forearm has two bones: the radius and the ulna, connected by the interosseous membrane.
• The ulna is slightly longer, with its olecranon and coronoid process.
• The radius goes from wide to thin, with a thin head, the radial tuberosity, and a radial styloid process.
• The carpus (wrist) is made of eight short bones called carpals:
• Scaphoid
• Lunate
• Triquetrum
• Pisiform
• Trapezium
• Trapezoid
• Capitate
• Hamate
• The metacarpals (palm of the hand) are five bones numbered one through five, from thumb to pinky.
• There are fourteen phalanges (finger bones) per hand
• Three per finger (distal, middle, and proximal phalanges)
• Except the thumb, which has two (no middle phalanx).

Pelvic Girdle

• The pelvic girdle attaches the lower limbs to the axial skeleton, providing more stability and less mobility than the pectoral girdle.
• This girdle includes the sacrum and two hip bones.
• Hip bones are made of three bones at birth (ilium, ischium, and pubis) that fuse into one by adulthood.

Lower Limb

• The lower limb includes very thick bones.
• The thigh contains the femur, the largest bone in the body.
• Key features of the femur include the head (with fovea capitis), greater and lesser trochanter, intertrochanteric crest, gluteal tuberosity, linea aspera, medial and lateral condyles and epicondyles, intercondylar fossa, and patella.
• The leg like the forearm contains two bones: the tibia and the fibula, connected by an interosseous membrane.
• In the larger tibia, key features include the medial and lateral condyles, intercondylar eminence, tibial tuberosity, anterior border, medial malleolus, and fibular notch.
• The fibula is much thinner, with its head and lateral malleolus.
• Foot anatomy is similar to the hand.
• The tarsus is made of seven bones called tarsals.
• The two biggest tarsals, the talus and calcaneus, make up the ankle.
• Other tarsals include the cuboid, navicular, and medial, intermediate, and lateral cuneiform bones.
• The metatarsus has five long metatarsals, numbered one through five.
• There are fourteen phalanges (toe bones), three per toe, except two for the big toe (hallux).