7.1: Background Information

Classification of Bones by Shape

The 206 bones that compose the adult skeleton are divided into five categories based on their shapes into long bones, short bones, flat bones, irregular bones, and sesamoid bones Figure \(\PageIndex{2}\). Their shapes and their functions are related such that each categorical shape of bone has a distinct function. For example, long bones like the femur provide leverage for movement, while flat bones like the sternum protect internal organs. Each bone contains specific markings such as foramina (holes for blood vessels and nerves), processes (projections for muscle attachment), and fossae (depressions for articulation with other bones).

Figure \(\PageIndex{2}\) illustrates the five major types of bones in the human skeleton, each labeled with a representative example:

1. Flat Bone – Represented by the sternum (breastbone), which protects internal organs and provides surface area for muscle attachment.
2. Irregular Bone – Shown as a vertebra, which has complex shapes and helps protect the spinal cord.
3. Long Bone – Illustrated by the femur (thigh bone), which supports weight and facilitates movement.
4. Sesamoid Bone – Depicted as the patella (kneecap), which is embedded in tendons and helps reduce friction.
5. Short Bones – Shown as the medial, intermediate, and lateral cuneiform bones in the foot, which provide stability and support with limited motion.

Each bone type is visually distinct and labeled to help learners understand their structure and function.

• Long Bones - A long bone is one that is cylindrical and is longer than it is wide. Long bones are found in the arms (humerus, ulna, radius) and legs (femur, tibia, fibula), as well as in the fingers (metacarpals, phalanges) and toes (metatarsals, phalanges). Long bones function as levers; they move when muscles contract.
• Short Bones - A short bone is one that is cube-like in shape, being approximately equal in length, width, and thickness. The only short bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles. Short bones provide stability and support as well as some limited motion.
• Flat Bones - Flat bones are thin, flattened, and usually curved. Most of the bones of the cranium are flat bones. Other examples include the scapulae (shoulder blades), the sternum (breastbone), and the ribs. Flat bones serve as points of attachment for muscles and often protect internal organs.
• Irregular Bones - An irregular bone is one that does not have any easily characterized shape and therefore does not fit any other classification. These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. Many facial bones, particularly the ones containing sinuses, such as the ethmoid, sphenoid, frontal, and maxillary bones, are classified as irregular bones. The zygomatic bone (cheek bone) is also in this category.
• Sesamoid Bones - A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form in tendons (the sheaths of tissue that connect bones to muscles) where a great deal of pressure is generated in a joint. Sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and placement from person to person but are typically found in tendons associated with the feet, hands, and knees. The patellae (singular:patella) are the only sesamoid bones found in common with every person.

Each Bone is an Organ - Bone Organization

As with all organs, bones are made of more than one tissue type. Bones are made of dense fibrous connective tissue, reticular tissue, blood, as well as bone (osseous) tissue. A leathery, dense fibrous connective tissue called the periosteum makes up the most superficial layer. Under the periosteum is the bulk of the bone’s mass, osseous tissue, either compact (cancellous) bone or spongy bone. Because solid bone would be too heavy, the inside of many bones is mostly hollow, the medullary cavity. The outer layer of the medullary cavity, called endosteum, contains a layer of bone forming and bone resorbing cells. The rest of the medullary cavity is filled with reticular tissue called bone marrow. There are two types of bone marrow: yellow marrow, which stores fat, and red marrow, which produces red blood cells, white blood cells, and platelets.

Figure \(\PageIndex{3}\) illustrates the structure of a long bone, such as the femur or humerus. It is labeled with both external and internal anatomical features:

• Proximal and Distal Epiphysis: The rounded ends of the bone.
• Metaphysis: The region between the epiphysis and diaphysis, where growth occurs.
• Diaphysis: The long, central shaft of the bone.
• Articular Cartilage: Smooth tissue at both ends that reduces friction in joints.
• Spongy Bone: Porous bone tissue found in the epiphysis, containing red bone marrow.
• Epiphyseal Line: A remnant of the growth plate.
• Red Bone Marrow: Found in spongy bone, responsible for blood cell production.
• Endosteum: A thin membrane lining the medullary cavity.
• Compact Bone: Dense outer layer providing strength.
• Medullary Cavity: Central cavity within the diaphysis, containing yellow bone marrow.
• Yellow Bone Marrow: Stores fat.
• Periosteum: Outer fibrous layer covering the bone.
• Nutrient Artery: Supplies blood to the bone.

When we look at bone under a microscope, we notice that there are two types of bones: compact (or cancellous) bone and spongy bone. Compact (cancellous) bone is found at the edges of bone and consists of circular, tree-trunk shaped units called osteons. Each osteon consists of rings of tissue surrounding a central canal that holds blood vessels and nerves. The tiny osteocytes, located in cavities called lacunae, are in the concentric rings called lamellae surrounding the central canal. There are also tiny radiating canals called canaliculi emanating from the central canal that deliver nutrients to the lacunae.

For an osteocyte to receive nutrients and get rid of waste, it must be serviced by a series of blood vessels. Blood vessels enter bone via the nutrient foramen where they wind their way among the bone tissue via horizontal Volkmann’s (or perforating) Canals. These horizontal canals intersect with vertical central canals that serve the center of each osteon. From there, the tiny canaliculi distribute blood from the central canal to the lacunae.

Figure \(\PageIndex{4}\) presents a microscopic cross-sectional view of compact bone tissue, highlighting its complex internal structure. Key components include:

• Compact Bone: Dense outer layer providing strength.
• Spongy Bone: Porous inner layer with trabeculae, located near the medullary cavity.
• Medullary Cavity: Central cavity containing bone marrow.
• Periosteum: Outer covering of bone with two layers:
  • Outer Fibrous Layer: Protective and structural.
  • Inner Osteogenic Layer: Contains cells for bone growth and repair.
• Osteon: The functional unit of compact bone, made up of concentric rings.
• Concentric Lamellae: Circular layers of bone matrix around the central canal.
• Interstitial Lamellae: Fills spaces between osteons.
• Circumferential Lamellae: Surrounds the entire bone structure.
• Central Canal (Haversian Canal): Contains blood vessels, lymphatic vessels, and nerves.
• Perforating Canal (Volkmann’s Canal): Connects central canals across osteons.
• Periosteal Artery and Vein: Supplies blood to the outer bone layers.

Spongy Bone Tissue Organization

Like compact bone, spongy bone, also known as cancellous bone, contains osteocytes, but they are not arranged in concentric circles. Instead, the osteocytes are found in a lattice like network of matrix spikes called trabeculae (Trabecula is singular form). The spaces in some spongy bones contain red marrow, protected by the trabeculae, where hematopoiesis occurs.

Figure \(\PageIndex{5}\) is a multi-level anatomical diagram of spongy bone structure, showing increasing magnification from whole bone to cellular detail. It is divided into three main sections:

1. Overview Section: Displays the location of spongy bone within a larger bone, adjacent to compact bone. Spongy bone appears porous and is typically found at the ends of long bones.
2. Magnified View of Trabeculae: Shows the trabeculae, which are the thin, branching structures that form the lattice-like framework of spongy bone. This section also highlights:
   • Canaliculi openings on the surface, which are small channels for nutrient exchange.
   • The endosteum, a thin membrane lining the inner surface of the trabeculae.
3. Cellular Detail View: Focuses on the microscopic components embedded in the trabeculae:
   • Lamellae: Concentric layers of bone matrix.
   • Lacuna: Small cavities housing bone cells.
   • Osteocyte: A mature bone cell residing in a lacuna.
   • Osteoclast: A large cell responsible for bone resorption.
   • Osteoblasts: Bone-forming cells aligned along the surface of new trabeculae.

The Skull (Cranium)

The skull, a critical component of the axial skeleton, is divided into the cranium and facial bones. The cranium protects the brain and houses the structures of the inner ear, while the facial bones support the features of the face and provide attachment points for muscles.

Figure \(\PageIndex{6}\) displays a labeled diagram of a human skull, highlighting two major anatomical regions:

• Cranial Bones: These form the upper portion of the skull, enclosing and protecting the brain. They include structures such as the frontal, parietal, temporal, and occipital bones.
• Facial Bones: These make up the lower portion of the skull, including the jaw, cheekbones, nasal bones, and teeth. These bones support facial structure and play roles in breathing, eating, and communication

In the adult, the skull consists of a total of 22 individual bones. Sum of 8 cranial and 14 facial bones, 21 of which are immobile. The \(22^{nd}\) bone is the mandible (lower jaw), is the only moveable bone of the skull.

Figure \(\PageIndex{7}\) is a diagram of the human skull, specifically highlighting the eight cranial bones. Each bone is labeled with its name and the number of bones present in the skull:

• Frontal (1): Located at the front of the skull, forming the forehead.
• Parietal (2): Two bones located on the sides and top of the skull.
• Temporal (2): Two bones located near the ears, forming the lower sides of the skull.
• Occipital (1): Located at the back and base of the skull.
• Sphenoid (1): A butterfly-shaped bone located at the base of the skull, behind the eyes.
• Ethmoid (1): A light, spongy bone located between the eyes, forming part of the nasal cavity.

Figure \(\PageIndex{8}\) illustrates human skull scheme highlighting the 14 facial bones in anterior view showing the paired and unpaired bones.

• Maxilla: Forms the upper jaw and floor of the nasal cavity.
• Nasal bones: Two small bones forming the bridge of the nose.
• Palatine bones: Two L-shaped bones forming part of the roof of the mouth and floor of the nasal cavity.
• Zygomatic bones: Two cheekbones that contribute to the shape of the face.
• Vomer: A single, midline bone that separates the nasal cavities.
• Lacrimal bones: Two small bones forming part of the eye socket.
• Inferior nasal conchae: Two curved bones within the nasal cavity.
• Mandible: The lower jawbone.

Sutures of the Skull

A suture is an immobile joint between adjacent bones of the skull. The narrow gap between the bones is filled with dense, fibrous connective tissue that unites the bones adding strength to the skull for brain protection.

• The coronal suture runs from side to side across the skull, within the coronal plane of section. It joins the frontal bone to the right and left parietal bones.
• The sagittal suture extends posteriorly from the coronal suture, running along the midline at the top of the skull in the sagittal plane of section. It unites the right and left parietal bones.
• On the posterior skull, the sagittal suture terminates by joining the lambdoid (lambdoidal) suture that extends downward and laterally to either side away from its junction with the sagittal suture. The lambdoid suture joins the occipital bone to the right and left parietal and temporal bones.
• The squamous suture is located on the lateral skull. It unites the temporal bone with the parietal bone.

Figure \(\PageIndex{9}\) presents a top-down view of a human skull, focusing on the cranial sutures, which are the joints between bones of the skull. Three major sutures are labeled:

• Coronal Suture: Located at the top of the skull, running horizontally from left to right. It separates the frontal bone from the parietal bones.
• Sagittal Suture: Found in the center of the skull, running vertically from front to back. It separates the left and right parietal bones.
• Lambdoidal Suture: Positioned at the bottom of the image, forming an inverted "V" shape. It separates the parietal bones from the occipital bone at the back of the skull.

Figure \(\PageIndex{10}\) presents a diagram of a human skull, highlighting three major cranial sutures, which are immovable joints connecting the bones of the skull:

• Coronal Suture: Runs horizontally across the top of the skull, separating the frontal bone from the parietal bones.
• Squamous Suture: Located on the side of the skull, this suture separates the parietal bone from the temporal bone.
• Lambdoidal Suture: Found at the back of the skull, forming an arch that separates the parietal bones from the occipital bone.

Sutures of the Skull. Sutures represent the immovable joints found between skull bones. In the superior view the coronal, sagittal and lambdoid sutures are visible. A lateral view of the skull shows the coronal, squamous, and lambdoid sutures.

Helpful Hints for Identification of Skull Bones by Touching/Palpating on the Body

• Zygomatic bone and arch - The most prominent part of your cheek is your zygomatic bone. Follow the posterior course of the zygomatic arch to its junction with your temporal bone.
• Mastoid Process - Touch the rough slight projection behind your ears.
• Temporomandibular joint - the articulation of the mandibular condyle with the mandibular fossa of the temporal bone. Palpate directly over the joint while opening and closing your jaws.
• Supraorbital foramen - this landmark lies on a vertical line with the pupil (when the eye is looking directly forward). Apply pressure along the superior orbital margin and locate the indentation.
• Mandibular angle - trace the junction of the lower border of the ramus of the mandible with the posterior border Nasal bones: two small, oblong bones that form the bridge of the nose superior to the nasal cartilage.

Vertebral Column

• The vertebral column is formed by stacking vertebrae together, made of 26 bones namely cervical, thoracic, lumbar, sacral and coccyx vertebrae. The vertebral foramina lining up to create the spinal canal that contains the spinal cord.
• The size and orientation of the articular processes determine the range of motion (ROM) at each vertebral level.

Figure \(\PageIndex{11}\) is a labeled diagram of the human vertebral column (spine) shown in two views: a back view on the left and a side view on the right. The labels identify the major spine regions, the number of vertebrae in each region, and the normal spinal curves. Key labeled parts (top to bottom):

• Cervical vertebrae (C1–C7): 7 vertebrae in the neck region; labeled as forming the cervical curve.
• Thoracic vertebrae (T1–T12): 12 vertebrae in the upper and mid-back; labeled as forming the thoracic curve.
• Lumbar vertebrae (L1–L5): 5 vertebrae in the lower back; labeled as forming the lumbar curve.
• Sacrum: shown as fused vertebrae at the base of the spine.
• Coccyx: the small terminal bone below the sacrum; also shown as fused segments.
• The diagram notes that the fused vertebrae of the sacrum and coccyx form the sacrococcygeal curve.
• An intervertebral disc label points to the cushioning discs located between adjacent vertebrae.

The side view emphasizes (spinal curves): The side view highlights the spine’s natural alternating curves from the neck through the lower back and into the fused sacrum/coccyx region.

Figure \(\PageIndex{12}\) is a labeled side view of the human vertebral column (spine). The vertebrae are grouped into regions and labeled from top to bottom: cervical, thoracic, lumbar, sacrum, and coccyx. Labels shown (top to bottom):

• Cervical vertebrae (C1–C7)
  • C1 is labeled Atlas
  • C2 is labeled Axis
  • Then C3, C4, C5, C6, C7
• Thoracic vertebrae (Th1–Th12)
  • Labeled sequentially Th1 through Th12
• Lumbar vertebrae (L1–L5)
  • Labeled sequentially L1 through L5
• Sacrum
  • Labeled Os sacrum (a fused set of vertebrae)
• Coccyx
  • Labeled Coccyx at the bottom tip of the spine

General Anatomical Features of Vertebrae

A vertebra is a single individual bone that makes up the vertebral column (spine), which is a series of connected bones that run from the base of the skull to the tip of the tailbone, primarily functioning to protect the spinal cord and provide structural support for the body; a typical vertebra consists of a body, pedicles, laminae, transverse processes, spinous process, and superior and inferior articular processes, with different regions of the spine (cervical, thoracic, lumbar, sacral, coccygeal) having distinct vertebral shapes adapted to their specific functions.

Figure \(\PageIndex{13}\) is labeled with orientation terms anterior (front) and posterior (back) to show which side faces the front of the body and which faces the back.

• The body is the large, thick, anterior portion of the vertebra. It is designed for weight-bearing and forms the front of the spinal column.
• Behind the body is the vertebral foramen, an opening that forms part of the canal that houses the spinal cord. The spinal cord is shown passing through this space.
• The vertebral arch surrounds the vertebral foramen and is labeled in parts:
  • Pedicle: the short bony segment connecting the body to the rest of the arch.
  • Lamina: the bony plate forming the posterior portion of the arch.
• Several projections are labeled, which serve as attachment points and joint surfaces:
  • Spinous process: a posterior projection you can think of as the “backward-pointing” spine projection where muscles and ligaments attach.
  • Transverse process: a lateral (side) projection where muscles attach; in some regions it also relates to rib attachment.
• Joint surfaces are labeled as facets (smooth surfaces where bones articulate):
  • Facet of superior articular process: part of the joint surface where one vertebra connects to the vertebra above.
  • Facet for head of rib: a facet indicating this vertebra region is associated with rib articulation (a thoracic feature).

Right panel: left posterolateral view of articulated vertebrae (stacked vertebrae) - this view shows multiple vertebrae connected, emphasizing how the spine is built as a column with cushioning discs and spaces for nerves.

• The intervertebral disc is shown between vertebral bodies. Its role is to cushion and allow limited movement between adjacent vertebrae.
• The spinal cord is shown running vertically through the stacked vertebral foramina, forming the spinal canal.
• The spinal nerve exiting through intervertebral foramen is labeled to show that nerves leave the spinal canal through openings between adjacent vertebrae.
• The vertebral arch is again labeled with lamina and pedicle, reinforcing that these parts form the protective ring around the spinal cord.
• Additional joint and projection labels show how vertebrae connect and stabilize:
  • Facet of superior articular process
  • Inferior articular process
  • Spinous process
  • Transverse process
  • Facet for head of rib

Features for Identification of Regional Vertebra

• Cervical Vertebrae (Neck)
  • Smallest vertebrae
  • Transverse processes with a foramen transversarium to accommodate the vertebral artery
  • Bifid (split) spinous processes
  • Prominent C1 (atlas) and C2 (axis) vertebrae with unique articulation for head movement
• Thoracic Vertebrae (Mid-back)
  • Heart-shaped vertebral body
  • Costal facets on the transverse processes and vertebral body for rib articulation
  • Long, downward-sloping spinous processes
• Lumbar Vertebrae (Lower Back)
  • Largest vertebrae with a large, thick vertebral body
  • Short, thick, and nearly horizontal spinous processes
  • Transverse processes are long and robust

Figure \(\PageIndex{14}\) contains multiple diagrams of cervical vertebrae:

1. Leftmost diagram: Lateral view of the entire spine.
2. Top middle diagram: Structure of a typical cervical vertebra, showing labeled parts such as:
   • Spinous process (bifid)
   • Vertebral foramen
   • Lamina and pedicle
   • Superior articular process
   • Transverse process and transverse foramen
   • Body and groove for spinal nerve
3. Top right diagram: Lateral view of cervical vertebrae from C1 (atlas) to C7 (vertebra prominens), highlighting:
   • Dens of axis
   • Transverse ligament
   • Inferior articular process
   • Bifid spinous process
4. Bottom left diagram: Superior view of the atlas (C1), showing:
   • Dens
   • Anterior and posterior arches
   • Superior articular facet
   • Transverse foramen and ligament
5. Bottom middle diagram: Superior view of the axis (C2), showing:
   • Dens
   • Superior articular facet
   • Transverse foramen
   • Lamina and spinous process
6. Bottom right diagram: Anterior view of the axis (C2), showing:
   • Dens
   • Body
   • Inferior articular process

Coccyx

Also known as the tailbone, the coccyx is made up of four fused vertebrae. The coccyx provides slight support for the pelvic organs and attaches to the pelvic floor muscles and ligaments. During childbirth, the coccyx moves backward to help widen the pelvic passage. The sacrum and coccyx are part of the pelvis, which is a basin-shaped structure that supports the spinal column and protects the abdominal organs. The fusion of the sacral and coccygeal vertebrae doesn't begin until around age 20 and isn't complete until middle age.

Figure \(\PageIndex{15}\) displays the sacrum and coccyx in three views:

1. Left: Full spine with the sacrum and coccyx highlighted in purple.
2. Center: Anterior view of the sacrum, labeled with:
   • Sacral promontory
   • Body of first sacral vertebra
   • Transverse ridges (sites of vertebral fusion)
   • Anterior sacral foramina
   • Apex
   • Coccyx
3. Right: Posterior view of the sacrum, labeled with:
   • Sacral canal
   • Body
   • Facet of superior articular process
   • Auricular surface
   • Lateral sacral crest
   • Median sacral crest
   • Posterior sacral foramina
   • Sacral hiatus

The Thoracic Cage

The thoracic cage (rib cage) forms the thorax (chest) portion of the body. It consists of the 12 pairs of ribs with their costal cartilages and the sternum Figure \(\PageIndex{16}\). The ribs are anchored posteriorly to the 12 thoracic vertebrae (T1 to T12). The thoracic cage protects the heart and lungs.

Figure \(\PageIndex{16}\) represents the thoracic cage which provides protection for vital organs and supports breathing. It consists of:

Main Components

• Sternum: Made up of three parts:
  • Manubrium
  • Body
  • Xiphoid process
• 12 Pairs of Ribs with costal cartilages
• Thoracic Vertebrae: Ribs are anchored posteriorly to 12 thoracic vertebrae.

Rib Classification

• True Ribs (1 to 7): Attach directly to the sternum via costal cartilage.
• False Ribs (8 to 12): Do not attach directly to the sternum.
  • Floating Ribs (11 to 12): A subset of false ribs; no anterior attachment.