The Human Skeleton

Introduction

Osteology (the study of bones) is involved in forensics, archaeology, and paleontology.

An adult has around 206 bones, while a child has around 270 bones.

Bone Biology

Bone Composition and Function

Bone is 70-80% mineral (hydroxyapatite) and 20-30% organic (collagen). The collagen prevents the bone from becoming brittle.

Bone acts as both an organ and a tissue. As an organ, bone protects soft tissues; anchors muscles, tendons, and ligaments; and acts as levers for movement. As a tissue, bone produces blood cells and acts as a reservoir of nutrients (calcium, fat).

Bone Variation

Human bones vary widely. There are four main types of variation: ontogeny (development), sexual dimorphism, geography (population), and individual.

The human skeleton is constantly remodeling in response to stress. Wolff's Law states that bone is deposited where needed and resorbed where not needed.

Musculoskeletal System

Joints are the structure between bones. There are three main types of joints:

  1. Synovial joints are freely movable. They include ball-and-socket joints (hip), hinge joints (elbow, knee), saddle-shaped joints (thumb), and planar joints (wrist, foot arch).
  2. Cartilaginous joints are made of cartilage and allow limited movement. One example are growth plates, which temporarily connect growth centers in a single bone.
  3. Fibrous joints are immovable (cranial sutures).

Both ligaments and tendons are connective tissue made of collagen. Ligaments connect bone to bone to stabilize joints. Tendons connect muscle to bone to facilitate movement.

Cartilage is a flexible connective tissue found in joints. It is composed mostly of collagen.

Anatomy of bones

Bones can be categorized based on their shape

  1. Long bones include the arms and legs
  2. Short bones include metacarpals (hands), phalanges (fingers), and clavicles (collarbones)
  3. Flat bones include the cranial vault, scapulae, sternum (breastbone), ribs, and innominate (hipbone)
  4. Irregular bones include carpals (wrists), tarsals (ankles), and vertebrae
  5. Sesamoid bones are small, round bones embedded in tendons, like the patella (kneecap)

Bones can also be categorized based on their internal structure

  1. Compact bone is hard and makes up the outer layer of bones
  2. Spongy (cancellous) bone is porous and makes up the inner layer of bones
  3. Subchondral bone is covered by cartilage in joints

The long bone has several areas:

  1. The epiphysis (end of bone) develops from secondary ossification centers
  2. The diaphysis (shaft) develops from the primary ossification center
  3. The metaphysis is the area between the epiphysis and diaphysis where growth plates are located
  4. The medullary cavity is the hollow center of the diaphysis that contains bone marrow

The periosteum is a membrane that covers the outer surface of bones. It contains blood vessels, nerves, and cells that help with bone growth and repair.

Histology

Histology is the study of tissues at the microscopic level.

The Haversian system (osteon) is the basic structural unit of compact bone. It consists of concentric layers (lamellae) of bone tissue surrounding a central canal (Haversian canal) that contains blood vessels and nerves. Within each lamella, there are small cavities called lacunae that house bone cells (osteocytes).

There are three primary types of bone cells:

  1. Osteoblasts lay new bones and become osteocytes
  2. Osteocytes live in lacunae and maintain bone tissue
  3. Osteoclasts resorb bone

Ossification

There are two types of ossification (bone formation):

  1. In intramembranous ossification, mesenchymal tissue is directly converted into bone.
  2. In endochondral ossification, a cartilage model is first formed and then replaced by bone. This process is responsible for the formation of most bones in the body.

Primary centers of ossification appear during fetal development and form the diaphysis. Secondary centers of ossification appear after birth and form the epiphyses.

The process of bone growth occurs through two main mechanisms:

  1. The bone grows in length at the growth plates located in the metaphysis.
  2. The bone grows in width with the help of nutrient foramen, or openings in the bone that allow blood vessels to enter.

Neurocranium I

Frontal

Ectocranial

  1. Frontal eminence (original center of ossification)
  2. Temporal lines (appear when you grit your teeth)
  3. Brow ridge
  4. Supraorbital margin
  5. Supraorbital foramen or notch (passage for vessels)
  6. Glabella (between the eyebrows)
  7. Zygomatic process of the frontal (articulates with zygomatic bone)

Endocranial

  1. Sagittal sulcus
  2. Frontal crest
  3. Foramen cecum
  4. Meningeal grooves

Inferior view

  1. Ethmoidal notch (location of ethmoid bone)
  2. Lacrima fossa (location of lacrimal gland)
  3. Frontal sinuses

Variations

  1. Metopic suture

Compared to the parietals, the minengeal impressions are fainter and the endocranial surface is more undulating.

Parietal

Angles

  1. Frontal angle (90 degrees)
  2. Sphenoidal angle
  3. Occiptal angle (near the occipital bone)
  4. Mastoid angle (near the mastoid process)

Ectocranial

  1. Parietal boss
  2. Parietal foramen (near the sagittal suture)
  3. Parietal striae (away from the sagittal suture)
  4. Temporal lines (extending from the frontal bone)

Endocranial

  1. Sagittal sulcus
  2. Sigmoid sulcus (near the mastoid angle)
  3. Meningeal grooves

Variations

  1. Bregmatic bone
  2. Wormian bones

Occipital

Ectocranial

  1. External occipital protuberence
  2. Highest, superior, and inferior nuchal lines
  3. External occipital crest (leading to the foramen magnum)
  4. Foramen magnum ("big hole")
  5. Occipital condyle (bean-shaped and articulates with C1)
  6. Condylar foramen (superior to the condyles)
  7. Hypoglossal canal (inferior to the condylar foramen)

Endocranial

  1. Cruciform eminence
  2. Cerebral fossa (triangular)
  3. Cerebellar fossa (quadrangular)
  4. Sagittal sulcus (usually swings right)
  5. Transverse sulcus (opp. sagittal sulcus)
  6. Internal occipital crest (leading to the foramen magnum)
  7. Jugular notch (below the foramen magnum)
  8. Middle meningeal grooves

Variations

  1. Inca bone
  2. Apical bone

Temporal

There are four parts of the temporal bone: squama, mastoid (right behind the ear), petrous (pyramid), and tympanic (inside).

Ectocranial

  1. Zygomatic process (and articulates with zygomatic bone)
  2. Articular eminence (right below the zygomatic process)
  3. Mandibular fossa (right next to the articular eminence)
  4. Post-glenoid process (small cone behind the mandibular fossa)
  5. Suprameatal crest (root of zygomatic process above the EAM)
  6. Supramastoid crest (extension of the suprameatal crest)
  7. Parietal notch (end of the crest that is level with the zygomatic process)
  8. Mastoid process (behind the ear)
  9. Mastoid notch (behind the mastoid process)
  10. Mastoid foramen
  11. External acoustic meatus (EAM)

Endocranial (posterior view)

  1. Internal acoustic meatus (biggest hole on the inside)

Endocranial (inferior view)

  1. Styloid process (pointed projection)
  2. Styloidmastoid foramen (under the styloid process)
  3. Jugular fossa (bottom-right of the styloid process)
  4. Cartoid canal (above the styloid process)