Chapter 7 - Skeletal System

Diaphysis- shaft of long bone; tubular; consists of a thick collar of compact bone surrounding a central medullary canal
Epiphysis- expanded end of a long bone; consists of a thin layer of compact bone surrounding spongy bone
articular cartilage- hyaline cartilage that covers the epiphyses of long bones; cushions bones and absorbs stress during bone movements
medullary cavity- canal that runs through the center of the diaphysis; in adults it contains yellow bone marrow (fat)
periosteum- double layered membrane that surrounds the diaphysis of a long bone; inner layer contains bone forming cells and bone remodeling cells; outer layer is dense fibrous connective tissue; rich in nerves, blood vessels, and lymphatics
endosteum- membrane that lines the medullary cavity and the holes of spongy bone; contains bone forming cells and bone remodeling cells.

Click here to see a long bone

Click here to see the difference between compact bone and spongy bone

Microscopic Structure of Compact Bone

At the microscopic level, the following structures can be observed in compact bone:

Osteon - structural unit of compact bone; an elongated cylinder oriented parallel to the long axis of the bone; Osteonic canals run through the middle of these cylinders; Osteonic canals contain blood vessels, and nerves.
Bone matrix- components between osteocytes; forms lamella; consists of inorganic salts, collagen fibers, and proteins.
Lamella- layers of bone surrounding osteonic canals.
Lacunae- holes in the matrix of bone that hold osteocytes
Canaliculi- hairlike canals that connect lacunae to each other and to the osteonic canal; they allow osteocytes to spread nutrients to each other.

Only compact bone is arranged microscopically into osteons; spongy bone is not arranged into osteons.

Click here to view an osteon

There are two types of bones based on how they are formed. If a bone forms through intramembranous ossification (bone formation) then it is called an intramembranous bone. If a bone forms through endochondral ossification it is called an endochondral bone.

Intramembranous Bones

Intramembranous Ossification- uses fibrous membranes as bone models. The bones start out as tough fibrous membranes but osteoblasts (bone forming cells) eventually turn the membrane to bone. Intramembranous bones are bones of the skull (except for the mandible) and the clavicles. All other bones are endochondral bones

Endochondral Bones

Endochondral Ossification- uses hyaline cartilage as bone models.

membrane surrounding cartilage model becomes vascular and osteoblasts are formed
bone collar forms around the diaphysis of the hyaline cartilage model
cartilage in the center of the diaphysis calcifies which destroys the cartilage
osteoblasts form bone in the center of the diaphysis (this is the primary ossification center

e. medullary cavity forms- osteoclasts (bone destroying cells) break down spongy bone to form the medullary cavity
f. osteoblasts form bone in the epiphyses and secondary ossification centers appear; similar to the ossification of the primary ossification center except no medullary cavity is formed

Click here to view a summary figure of endochondral ossification

Homeostasis on Bone Tissue

After bones form, osteoclasts and osteoblasts continually remodel them. Throughout life, osteoclasts break down bone matrix which is called bone resorption; osteoblasts replace lost bone matrix which is called bone deposition. These processes are controlled by hormones.

Bone Function

Bones have many functions as follows:

Support and Protection

Bones give shape to body parts such as the head, legs, arm, and trunk. Bones also support and protect soft structures in the body. For example, the bones of the cranium protect the brain.

Body Movement

Skeletal muscles attach to bone. When these skeletal muscles contract, the bones they are attached to move producing body movements.

Blood Cell Formation

The red bone marrow of bone produces new blood cells.

Storage of Inorganic Salts

Calcium phosphate is the most abundant inorganic salt stored in bone matrix. It is the primary source of calcium for all body cells.

Skeletal Organization

The skeleton is divided into an axial skeleton and an appendicular skeleton.

Axial Skeleton

The axial skeleton contains 80 bones. It includes the bones of the skull, vertebral column, hyoid bone and bony thorax. It functions to support the head, neck, and trunk and protects the brain, spinal cord, and the organs in the thorax.

Appendicular Skeleton

The appendicular skeleton includes the bones of the arms, legs, the pectoral girdle and the pelvic girdle.

Click here to view an anterior view of a skeleton

Click here to view a posterior view of a skeleton

Click here for a summary table of all the bones in the body

Skull

The bones of the skull include bones of the cranium and facial bones.

Cranium

Frontal- anterior portion of the cranium

Parietals- forms most of the superior and lateral aspects of the skull

Occipital- forms most of the posterior wall and base of the skull

Temporals- inferior to the parietal bones on the lateral aspects of the skull;

Sphenoid- butterfly-shaped bone; forms part of the base of the cranium

Ethmoid- lies between the sphenoid bone and the nasal bones; forms part of the base of the cranium

Ear ossicles- malleus, incus, stapes; within the middle ear cavities of the temporal bones

Special features on Cranial Bones include:

foramen magnum- large hole in the base of the occipital bone; allows the brain to connect to the spinal cord
occipital condyles- lateral to the foramen magnum; position where the atlas articulates with the occipital bone
zygomatic process- barlike process of a temporal bone that articulates with a zygomatic bone
external auditory meatus- external ear canal; located in temporal bones
mastoid process- anchoring site for neck muscles; on a temporal bone behind an external auditory meatus
sella turcica- saddle-like depression in the medial, superior portion of the sphenoid bone; protects the pituitary gland

Click here for an anterior view of a skull

Click here for a lateral view of a skull

Click here for a inferior view of a skull

Click here for a view of the cranial floor

Facial Skeleton

Mandible- lower jaw bone; only moveable bone in skull; anchors the lower teeth; forms the chin; largest and strongest facial bone

Maxilla- upper jaw bone; anchors the upper teeth; forms the central portion of the facial skeleton; consists of two bones fused together

Zygomatic bones- cheekbones

Nasal bones- several thin bones fused together to form the bridge or top of the nose

Palatine bones- two L-shaped bones that form the anterior portion of the palate and the posterior walls of the nasal cavity

Vomer- thin bone that forms part of the nasal septum

Click here for an anterior view of a skull

Click here for a lateral view of a skull

Infantile Skull

The skull bones of an infant are not completely formed. The "soft spots" felt in an infant's skull are actually fontanels which are tough membranes that connect the incompletely developed bones.

Vertebral Column

The vertebral column consists of 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, a sacrum, and a coccyx.

Click here for a view of the vertebral column

Typical Vertebra

A typical vertebra has the following parts:

Body- anterior thick, disc-shaped portion; bears the weight of the body

Spinous process – medial posterior projection ; attachment site for muscles that move the vertebral column and ligaments that stabilize it

Vertebral foramen- opening between the body and vertebral arch; allows the passage of the spinal cord

Transverse processes- lateral extensions from each side of vertebral arches; attachment sites for muscles that move the vertebral column and ligaments that stabilize it

Click here to see a thoracic vertebrae

Cervical Vertebrae

Cervical vertebrae- seven; smallest and lightest vertebrae; the first cervical vertebra is called the atlas and the second is called the axis

Thoracic Vertebrae

Thoracic vertebrae - twelve; articulate with the 12 pairs of ribs; have long, sharp spinous processes

Lumbar Vertebrae

Lumbar vertebrae- five; have very sturdy structures; form the "small of the back"; bear the most weight of all the vertebrae; have short, flat spinous processes

Sacrum

Sacrum- triangular in shape; consists of five fused vertebrae in adults

Coccyx

Coccyx- small triangular shaped bone consisting of 3-5 fused vertebrae in the adult; considered useless

Thoracic Cage

The thoracic cage is made of 12 pairs of ribs and the sternum.

Click here to view the thoracic cage

Ribs

All 12 pairs of ribs are attached posteriorly to thoracic vertebrae. Most ribs are also attached to structures anteriorly. Based on what ribs attach to anteriorly, they can be classified as follows:

True- first seven pairs; attach directly to the sternum through costal cartilages

False- pairs 8,9 and 10; attach to the costal cartilage of rib number 7

Floating- pairs 11 and 12; do not attach anteriorly to any structure

Sternum

The sternum forms the anteriomedial wall of the bony thorax. It articulates with the clavicles and most ribs.

Pectoral Girdle

The pectoral girdles include clavicles and scapulae. They act to attach the arm to the trunk of the body.

Click here to view a pectoral girdle

Clavicles

Clavicle- collar bone; slender in shape; articulates with the sternum and the scapula; very commonly broken bone in body

Scapulae

Scapula- thin, triangular shaped flat bones; also called shoulder blades; lie on the dorsal surface of the rib cage; articulates with the head of the humerus

Upper Limb

The upper limb or arm bones include the humerus, radius, ulna, carpals, metacarpals, and phalanges.

Click here to view bones of the arm

Humerus

Humerus- largest bone of the arm; bone of the upper arm; articulates with the scapula and radius and ulna

Radius

Radius- lateral bone of the forearm; articulates with the humerus, radius and bones of the wrist

Ulna

Ulna- medial bone of the lower arm; articulates with the humerus to form the elbow joint; it also articulates with the radius and slightly with bones of the wrist

Hand

The bones of the hand include carpals, metacarpals, and phalanges.

Carpals- wrist bones; eight marble-sized short bones; articulate with the radius and metacarpals

Metacarpals- five; form the palms of the hands; articulate with the carpals and phalanges

Phalanges- bones of the finger; 14 per hand; three for each finger and two per thumb

Pelvic Girdle

The Pelvic Girdle attaches the leg to the axial skeleton. It also protects pelvic organs and is formed by a pair of coxal bones (hip bones). The three parts of a coxal bone are as follows:

Ilium- superior part of coxal bone
Ischium- posterioinferior part of a coxal bone
Pubis- anterior part of a coxal bone

Click here to see pelvic girdle bones

Lower Limb

The bones of the lower limb or leg include the femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges.

Click here to see bones of the leg

Femur

Femur- thigh bone; largest bone in body; articulates with the hip bone, tibia and patella (knee cap)

Tibia

Tibia- medial lower leg bone; articulates with the femur, fibula, and tarsals

Fibula

Fibula- lateral lower leg bone; thinner than tibia; articulates with the tibia and tarsals

Foot

The bones of the foot include the following:

Tarsals- bones of the back of the foot; the calcaneus or heel bone is the largest tarsal; seven total; they articulate with the tibia, fibula and metatarsals

Metatarsals- five; bones of the front of the foot; articulate with the tarsals and phalanges

Phalanges- bones of the toes; fourteen total

Joints

Joints are junctions between bones. Based on structure joints can be classified as fibrous, cartilagenous, or synovial.

Fibrous Joints

Bones of fibrous joints are connected together with short fibers. Therefore, bones of this type of joint do not normally move against each other. Most fibrous joints are found between cranial bones and facial bones.

Cartilaginous Joints

Bones of cartilaginous joints are connected together with a disc of cartilage. This type of joint is slightly moveable. The joints between vertebrae are cartilaginous joints.

Synovial Joints

Bones of synovial joints are covered with hyaline cartilage. The bones are also held together by a fibrous joint capsule. The joint capsule is lined with a synovial membrane. The membrane secretes a slippery fluid called synovial fluid which allows the bones to move easily against each other. Synovial joints are freely moveable. Examples of synovial joints are the elbow, knee, shoulder, and knuckles.

Types of Joint Movements

Joint movements include the following:

Flexion - bending of a body part

Extension - straightening of a body part

Dorsiflexion - pointing the toes up

Plantarflexion - pointing the toes down

Abduction - moving a body part away from their position in the anatomical position

Adduction - moving a body part towards their position in the anatomical position

Rotation - twisting of a body part; for example, saying "no" with your head

Circumduction- moving a body part in a circle; for example, tracing a circle with your arm

Pronation - turning the palms of the hand down

Supination - turning the palms of the hand up

Inversion - turning the sole of the foot medially

Eversion - turning the sole of the foot laterally

Elevation - lifting a body part; for example, elevating the shoulders as in "shrugging"

Depression - lowering a body part; for example, lowering the shoulders

Click here to see various joint movements