[LEC 6] The Skeletal System

Classification based on shape and function

Long bones, short bones, flat bones, irregular bones, and sesamoid bones

Classification of bones based on type of ossification

Cartilaginous/endochondral ossification and membranous ossification

Classification of bones based on structure

Compact bone and spongy (cancellous) bones

Long bones

* Cylinder-like, longer than it is wide


* For movement and support
* Ex. femur, tibia, fibula, metatarsals, humerus, ulna, metacarpals, phalanges

Short bones

* Cube like shape, equal in width and length


* Provide stability and support with motion
* Ex. carpals, tarsals

Flat bones

* Thin and curved


* Point of attachment for muscles
* Ex. Sternum, ribs, scapulae, cranial bones

Irregular bones

* Complex shape


* Protect internal organs, for movement, and support
* Ex. vertebrae and facial bones

Sesamoid bones

* Small and round; embedded in tendons


* Protect tendons from excessive forces
* Ex. patellae

Cartilaginous/Endochondral Ossification

* Bones are developed from cartilage models


* Ex. Short and long bones

Membranous ossification

* Bones are formed in connective tissue membrane


* Ex. Frontal and parietal bones of skull, mandible

Compact bone

* Denser, stronger of the two types of osseous tissue


* Make up outer cortex of all bones
* In immediate contact with the periosteum
* Made up of highly organized arrangement of concentric circles

Lamellae

Each osteon ring made up of collagen and calcified matrix.

Central canal (Haversian canal)

Runs down the center of each osteon, containing blood vessels, nerves, and lymphatic vessels.

Perforating canal (Volkmann’s canal)

Oriented at right angle to the central canal, where vessels and nerves branch off

Canaliculi

* Connect with other lacunae and the central canal


* Allow nutrient transport and waste removal

Spongy (Cancellous) bone

Not arranged in concentric circles but have osteocytes in lacunae

Trabecular

* Lattice like network of matrix spikes where the lacunae and osteocytes are located
* Forms along lines of stress to direct forces out to the more solid compact bone

Diaphysis

Hollow, tubular shaft that runs between the proximal and distal bone ends

Epiphysis

Wider section on each bone end filled internal with spongy bone

Medullary cavity

Space inside the diaphysis filled with yellow bone marrow in adults

Cortex (cortical bone)

Outer walls of the diaphysis composed of dense and hard compact bone

Metaphysis

Where each epiphysis meets the diaphysis

Epiphyseal line

Site of long bone elongation

Epiphyseal line

When the bone stops growing.

Articular cartilage

* Thin layer of hyaline cartilage covering the epiphysis


* Reduces friction and acts as a shock absorber

Endosteum

* Lines the inside of the bone adjacent to the medullary cavity


* Bone cells cause the bone to grow, repair, and remodel

Periosteum

* Covers the entire outer surface except around the articular cartilage


* Grow, repair, remodel bones
* Contains blood and lymph vessels along with nerves

Bone matrix

Consists of 1/3rd collagen fibers and 2/3rds calcium phosphate salt (hydroxyapatite)

Hydroxyapatite

* Adheres to surface of collagen scaffolding


* Incorporates magnesium hydroxide, fluoride, and sulfate 
* Give bones their hardness and strength 

Collagen fibers

* Gives a framework for calcification
* Gives the bone flexibility so that it can bend without being brittle

Osteoblast

* The bone cell responsible for forming new bone


* Found in the endosteum and the periosteum
* Synthesize and secrete the collagen matrix and other proteins

Osteocyte

* Primary cell of mature bone located (trapped) in a lacuna


* Maintain the mineral concentration of the matrix via the secretion of enzymes
* Communicate with each other via long cytoplasmic processes that extend through canaliculi

Articulation

* Where two bone surfaces come together


* These surfaces tend to conform to one another, such as one being rounded and the other cupped

Projection

* An area of a bone that is raised above the surface of the bone


* Attachment points for tendons and ligaments
* Their size and shape is an indication of the forces exerted through the attachment of the bone

Hole

* Allows blood vessels and nerves to enter the bone


* Size and shape reflected vessel and nerve size

Articulations

Where two bones meet

Head

Prominent rounded surface

Facet

Flat surface

Condyle

Rounded surface

Fovea

Small pit

Process

Prominence feature

Tubercle

Small, rounded process

Tuberosity

Rough surface

Sulcus

Groover

Axial skeleton

* Central axis


* 80 bones
* Serves as the attachment site for muscles of head, neck, back, and the shoulder and hip joints.

The skull

* Facial bones and cranium


* 22 bones

Orbit

Bony socket that houses the eyeball and muscles that move the eyeball or open the upper eyelid

Nasal cavity

* Divided by naval septum


* Upper portion is formed by the plate of ethmoid bone
* Lower portion is the vomer bone


* Inferior, middle, and superior nasal conchae

Zygomatic arch

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Arch that spans from the area of the cheek above the ear canal

Zygomatic bone

Forms the zygomatic arch (anterior)

Temporal bone

Forms the zygomatic arch (posterior)

Masseter

* For biting and chewing
* Arises from the zygomatic arch

Temporal fossa

Shallow space above zygomatic arch

Temporalis

* For chewing
* Inserts into the zygomatic arch through the temporal fossa

Paired bone(s) of the cranium

Paired parietal and temporal bones

Unpaired bone(s) of the cranium

Frontal, occipital, sphenoid, and ethmoid bones

Cranial cavity

Almost completely occupied by the brain

Base of the skull

The floor of the braincase; a complex area that varies in depth and has numerous openings for cranial nerves, blood vessels and the spinal cord.

Orbital roof

* Frontal bone


* Lesser wing of the sphenoid
* “FrontLess Roof”

Lateral Wall

* Greater wing of the sphenoid


* Zygomatic bone
* “Greater zygoma”

Orbital floor

* Maxilla


* Palatine bone
* Zygomatic bone
* “Floor MaPZ”

Medial wall

* Sphenoid bone


* Maxilla
* Ethmoid
* Lacrimal
* “SMEL”

Parietal Bone

Forms upper and lateral side of the skull

Sagittal suture

The right and left parietal bones joining together

Bregma

Point of intersection between the coronal and sagittal sutures.

Temporal Bone

Forms the lower lateral side of the skull

Squamous portion

Flattened, upper portion

Zygomatic process

Below the squamous and projecting anteriorly; forms the posterior portion of the zygomatic arch

Mastoid portion

Has the prominent mastoid process

Mastoid process

Locks during lock jaw

Petrous portion

House the middle and inner ears

Pterior

Weakest part of the skull

External acoustic meatus (ear canal)

Opening on the lateral side of the skull associated with the ear

Internal acoustic meatus

Opening located inside the cranial cavity that connects the middle and inner ear cavities to the bone

Mandibular fossa

Depression on the base of the skull anterior to the external acoustic meatus that forms the temporomandibular joint with the mandible (lower jaw)

Styloid process

A downward bony projection that is the attachment site for muscles and ligament supporting the hyoid bone

Occipital bone

Forms the posterior skull

External occipital protuberance

Protrusion at the posterior midline, which serves as an attachment site for ligaments of the neck

Superior nuchal line

The most superior point at which muscles of the next attach to the skull

Foramen magnum

Large opening on the base of the skull which allows the spinal cord to exit skull

Occipital condyle

On either side of the foramen magnum where they form joins with the first cervical vertebrae for nodding motion of the head.

Sphenoid bone

* A complex bone of the central skull


* Joins with almost every other bone of the skull
* Forms much of the base of the central skull

Lesser wings of the sphenoid bone.

Marks the boundary between the anterior and middle cranial fossae

Sella turcica

Located at the midline of the middle cranial fossa; Rounded depression in the floor houses the pituitary (hypophyseal) gland

Greater wings of the sphenoid bone

Extend laterally to either side away from sella turcica where they form the anterior floor of the middle cranial fossa

Medial pterygoid plate and lateral pterygoid plate

Two thin, vertically oriented bone plates on the inferior aspect of the skull.

Ethmoid bone

* Midline bone that forms part of the upper nasal cavity, upper nasal septum, and medial wall of the orbit.


* Contributes to the superior nasal concha and middle nasal concha

Crista galli

Bony projection located at the midline for attachment for the meninges

Cribiform plate

Flattened area with small openings termed olfactory foramina

Facial bones of the skull

* Form the upper and lower jaws, the nose, nasal cavity, and nasal septum, and the orbit


* 14 bones

Unpaired facial bone(s) of the skull

Vomer and mandible

Paired facial bone(s) of the skull

Maxilla, palatine, zygomatic, nasal, lacrimal, and inferior nasal conchae bones

Maxillary bone

Form the upper jaw, much of the hard palate, the medial floor of the orbit, and the lateral base of the nose

Palatine process

From each maxillary bone join together at the midline to form the anterior hard palate

Palatine bone

* Irregularly shaped bones
* Contribute to the lateral walls of the nasal cavity and the medial wall of each orbit


* Join together at the midline to form the posterior hard palate

Nasal bone

* Small bones that form the bony bridge of the nose
* Support the cartilages that form the lateral walls of the nose

Zygomatic bone

* Cheekbone


* Forms much of the lateral wall of the orbit and the lateral-inferior margins of the anterior orbital opening

Lacrimal bone

* Small bone that forms the antero-medial wall of the orbit
* Anterior portion that forms a shallow depression which drains tears at the medial corner of the eye into the nasolacrimal canal

Lacrimal fossa

Shallow depression that drains tears at the median corner of the eye into the nasolacrimal canal

Inferior nasal conchae

Formed a curved bony plate (turbinate) that projects into the nasal cavity space from the lower lateral wall