The Skeletal
System
Imagine for a moment that people did
not have skeletons.
What comes to mind? Probably that
each of us would be a little heap on
the floor, much like a jellyfish out of
water.
the most obvious function of the
skeleton: to support the body. it is a
framework for the body, the
skeleton is not at all like the wooden
beams that support a house.
Bones are living organs that actively
contribute to the maintenance of
the internal environment of the
body.
The skeletal system
consists of bones and
other structures that
make up the joints of
the skeleton.
The types of tissue
present are bone tissue,
cartilage, and fibrous
connective tissue,
which forms the
ligaments that connect
bone to bone.
FUNCTIONS OF THE SKELETON
1. Provides a framework that supports the
body; the muscles that are attached to
bones move the skeleton.
2. Protects some internal organs from
mechanical injury; the rib cage protects
the heart and lungs, for example.
3. Contains and protects the red bone
marrow, the primary hemopoietic
(blood-forming) tissue.
4. Provides a storage site for excess
calcium. Calcium may be removed from
bone to maintain a normal blood
calcium level, which is essential for
blood clotting and proper functioning of
muscles and nerves.
TYPES OF BONE TISSUE
Bone cells are called osteocytes, and the matrix
of bone is made of calcium salts and collagen.
The calcium salts are
- Calcium carbonate (CaCO3) and
- Calcium phosphate (Ca3(PO4)2),
Which give bone the strength required to perform its
supportive and protective functions.
Bone matrix is non-living, but it changes constantly, with
calcium that is taken from bone into the blood replaced
by calcium from the diet.
In normal circumstances, the amount of calcium that
is removed is replaced by an equal amount of calcium
deposited.
This is the function of osteocytes, to regulate the amount
of calcium that is deposited in, or removed from, the
bone matrix.
Types of bone
There are two types of bone tissue:
1- Compact bone, looks solid but is very precisely structured.
It is made of osteons or haversian systems, microscopic cylinders of bone
matrix with osteocytes in concentric rings around central haversian
canals. In the haversian canals are blood vessels; the osteocytes are in
contact with these blood vessels and with one another through
microscopic channels (canaliculi) in the matrix.
2- Spongy bone, which does look rather like a sponge with its visible
holes or cavities. Osteocytes, matrix, and blood vessels are present but are
not arranged in haversian systems.
The cavities in spongy bone often contain red bone marrow, which
produces red blood cells, platelets, and the five kinds of white blood
cells.
CLASSIFICATION OF BONES
1. Long bones—the bones of the arms, legs, hands,
and feet (but not the wrists and ankles).
The shaft of a long bone is the diaphysis, and the
ends are called epiphyses. The diaphysis is made
of compact bone and is hollow, forming a canal
within the shaft. This marrow canal (or medullary
cavity) contains yellow bone marrow, which is
mostly adipose tissue. The epiphyses are made of
spongy bone covered with a thin layer of compact
bone. Although red bone marrow is present in
the epiphyses of children’s bones, it is largely
replaced by yellow bone marrow in adult bones.
2. Short bones—the bones of the wrists and ankles.
3. Flat bones—the ribs, shoulder blades, hip bones,
and cranial bones.
4. Irregular bones—the vertebrae and facial bones.
Short, flat, and irregular bones are all made of spongy
bone covered with a thin layer of compact bone.
Red bone marrow is found within the spongy
bone.
FACTORS THAT AFFECT BONE GROWTH AND
MAINTENANCE
1. Heredity—each person has a genetic potential for
height, that is, a maximum height, with genes
inherited from both parents.
2. Nutrition—nutrients are the raw materials of which
bones are made.
- Calcium, phosphorus, and protein become part of the
bone matrix itself.
- Vitamin D is needed for the efficient absorption of
calcium and phosphorus by the small intestine.
- Vitamins A and C do not become part of bone but are
necessary for the process of bone matrix formation
(ossification).
Without these and other nutrients, bones cannot grow
properly.
3. Hormones—endocrine glands produce hormones that
stimulate specific effects in certain cells.
THE SKELETON
There are 206 bones in total, and the human skeleton has
two divisions:
1- The axial skeleton, which forms the axis of the body,
consists of the skull, vertebral column, and rib cage.
2- The appendicular skeleton, which supports the limbs.
The bones of the arms and legs and the shoulder and
pelvic girdles make up the appendicular skeleton.
- Many bones are connected to other bones across joints by
ligaments, which are strong cords or sheets of fibrous
connective tissue.
- The importance of ligaments becomes readily apparent
when a joint is sprained.
SKULL
The skull consists of 8 cranial bones and 14 facial bones. Also in
the head are three small bones in each middle ear cavity and the
hyoid bone that supports the base of the tongue.
A- The cranial bones, form the braincase (lined with the meninges)
that encloses and protects the brain, eyes, and ears. These are:
1- The frontal bone, forms the forehead and the anterior part of
the top of the skull.
2- The parietal bones (two), Parietal means “wall,” form the
posterior top and much of the side walls of the skull.
3- The temporal bones (two), on the side of the skull contains an
external auditory meatus (ear canal), a middle ear cavity, and an
inner ear labyrinth.
4- The occipital bone, forms the lower,
posterior part of the braincase.
Function: Its foramen magnum is a large
opening for the spinal cord, and the two
condyles (rounded projections) on either side
articulate with the atlas, the first cervical
vertebra, enabling movement of the head
relative to the spine.
Skull. Inferior view with mandible removed.
5- The sphenoid bone: is said to be shaped like a
bat, and the greater wing is visible on the side of the
skull between the frontal and temporal bones. The
body of the bat has a depression called the sella
turcica, which encloses the pituitary gland.
Function: Articulates with the frontal, parietal and
temporal bones.
6- The ethmoid bone has a vertical projection
called the crista galli (“rooster’s comb”) that anchors
the cranial meninges. The rest of the ethmoid bone
forms the roof and upper walls of the nasal cavities,
and the upper part of the nasal septum.
Function: Forms part of the nasal cavity and the orbits.
Main support structure of the nasal cavity
All of the joints between cranial bones are
immovable joints called sutures.
It may seem strange to refer to a joint without
movement, but the term joint (or articulation)
is used for any junction of two bones.
B- Facial bones:
1- Mandible: Known as the lower jaw bone. Also forms the chin and
sides of the face, (Largest, strongest facial bone.)
Function:
1- Bone into which the lower teeth are attached.
2- The only moveable facial bone; motion of this bone is necessary
for chewing food (the first stage of the digestion process).
3- Each side of the mandible has a condyle and a coronoid process.
The condyle articulates with the temporal bone to form the
temporomandibular joint.
2- The maxillae: are the two upper jaw bones, which also form the
anterior portion of the hard palate (roof of the mouth).
Function:
1- Into which the upper teeth are attached.
2- Each maxilla contains a maxillary sinus that drains fluid into the nasal
cavity.
3- The two nasal bones form the bridge of the nose
Function: They articulate with the frontal bone (the rest of the
nose is supported by cartilage).
4- The two lacrimal bone: each of the bone present at the
medial side of each orbit;
Function: the lacrimal canal contains the lacrimal sac, a
passageway for tears
5- The two zygomatic bones: each of them forms the point of
a cheek, and articulates with the maxilla, frontal bone, and
temporal bone.
Function: Articulates with the frontal, maxilla, sphenoid and
temporal bones.
6- The two palatine bones: are the posterior portion of
the hard palate, or form the back of the roof of the
mouth.
Function: Form the bottom of the orbitals and nasal
cavities, and also the roof of the mouth.
7- The vomer ; plow-shaped
forms the lower part of the
nasal septum; it articulates
with the ethmoid bone.
Function: Separates the nasal
cavities into left and right
sides.
8- Inferior nasal Concha:
These terms refer to any of
two thin bones that form
the sides of the nasal
cavity
Function: Form the nasal
cavities.
SINUSES
Is a sac or cavity in any organ or
tissue.
The sinuses functions are:
1- They make the skull lighter in
weight, because air is lighter than
bone.
2- They provide resonance for the
voice, meaning
more air to vibrate and thus deepen
the pitch of the voice.
1- Paranasal sinuses: air cavities in the cranial bone
located in the maxillae and frontal, sphenoid, and
ethmoid bones.
As the name paranasal suggests, they open into the
nasal cavities and are lined with ciliated epithelium
continuous with the mucosa of the nasal cavities.
We are aware of our sinuses only when they become
“stuffed up,” which means that the mucus they
produce cannot drain into the nasal cavities. This
may happen during upper respiratory infections such
as colds, or with allergies such as hay fever (is an
allergic inflammation of the nasal airways).
2- The mastoid sinuses are air
cavities in the mastoid process
of each temporal bone; they
open into the middle ear. Before
the availability of antibiotics,
middle ear infections often
caused mastoditis, infection of
these sinuses.
Within each middle ear
cavity are three auditory
bones:
the malleus, incus, and
stapes.
As part of the hearing
process , these bones
transmit vibrations from
the eardrum to the
receptors in the inner ear.
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