THE
SKELETAL
SYSTEM
A. FUNCTIONS OF BONE AND THE SKELETAL SYSTEM
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The skeletal system performs the following functions:
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Support
Protection
Assisting in movement
Mineral homeostasis
Production of blood cells
Triglyceride storage
B. TYPES OF BONES
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Bones can be classified on the basis of shape and location into four main types. These are LONG,
SHORT, FLAT, or IRREGULAR.
LONG BONES have a greater length than width and a variable number of ends. Examples include the
femur (thighbone) and humerus (arm bone).
SHORT BONES are somewhat cubed-shaped and nearly equal in length and width. Examples include
most wrist and anklebones.
FLAT BONES are thin and comprised of two or more or less parallel plates of compact tissue enclosing
spongy bone.
IRREGULAR BONES have complex shapes and include vertebrae and some facial bones.
C. STRUCTURE OF BONE
1.
The typical anatomy of a long bone includes the following:
 DIAPHYSIS- shaft or long cylindrical, main portion of the bone
 EPIPHYSES- the distal and proximal ends of a long bone
 METAPHYSES- the areas where the diaphyses and epiphyses meet
 ARTICULAR CARTILAGE- a thin layer of hyaline cartilage covering the epiphyseal ends at an
articulation point with another bone
 PERIOSTEUM- the tough, dense irregular connective tissue on the outer surface of the bone
 MEDULLARY (MARROW) CAVITY- space within the diaphysis; it contains fatty yellow bone marrow
in the
adult
 ENDOSTEUM- thin membrane lining of the medullary canal which contains bone-forming cells
D. HISTOLOGY OF BONE
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Structurally, several types of connective tissue are involved in the skeletal system. These include
cartilage, dense connective tissue, and osseous tissue.
Bone is connective tissue with a great deal of extracellular substance ( MATRIX) and sparsely distributed
cells. The extracellular substance consists of calcium, phosphate salts, and collagenous fibers. The
collagenous fibers form the framework around which the salts will crystallize.
The types of cells found in bone tissue are OSTEOGENIC CELL, STEOBLASTS, OSTEOCYTES, and
OSTEOCLASTS.
The typical large bone has several parts. The microscopic structure is easier to understand if one is
first familiar with its gross structure.
Bone is not solid. It contains a SOLID REGION (COMPACT), and a more POROUS REGION (SPONGY).
The structural unit of compact bone is the OSTEON or HAVERSIAN SYSTEM. It is characterized by a
concentric ring structure or layer ( LAMELLAE), a central (HAVERSIAN) CANAL which is surrounded by
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interspersed layers of matrix, and osteocytes in small spaces (LACUNAE), from which radiate tiny
channels (CANALICULI).
The structural unit of cancellous bone is the TRABECULA, an irregular latticework of thin columns of
bone. Contained in the trabeculae are osteocytes and red bone marrow.
E. COMPACT BONE TISSUE
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Compact bone tissue contains few spaces and forms the external layer of all bones.
Compact bone tissue provides protection and support and aids in the stress of weight placed upon it.
NUTRIENT VESSELS penetrate compact bone through PERFORATING CANALS and are connected to those
of the CENTRAL (HAVERSIAN) CANALS.
The central canals run lengthwise and are surrounded by concentric LAMELLAE.
The lamellae contain small spaces called LACUNAE which contain the OSTEOCYTES.
Radiating from the lacunae are the CANALICULI which contain the slender process of the ostocytes.
These interconnect all of the lacunae to the central canal.
The sum of the central canal, surrounding lamellae, lacunae, osteocytes, and canaliculi comprise the
HAVERSIAN SYSTEM or OSTEON.
F.
SPONGY BONE TISSUE
1.
Spongy bone tissue does not contain true osteons. It is composed of an irregular latticework of thin
plates of bone called TRABECULAE in which the microscopic spaces are filled with RED BONE MARROW.
Spongy bone tissue comprises the bulk of all bones in the body and the spongy bone tissue of the
hipbones, ribs, breastbone, backbones, and some of the epiphyseal ends of long bones, is the site of red
bone marrow and hemopoiesis in the adult.
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G. BONE FORMATION: OSSIFICATION
1. Bone formation, or OSSIFICATION, begins with OSTEOGENIC CELLS. These arise from mesenchymal
embryonic connective tissue.
2. The osteogenic cells give rise to OSTEOBLASTS, which form BONE.
3. Bone formation takes place in the embryo, within mesenchyme arranged in sheetlike layers that
resemble membranes (intramembranous ossification), or in hyaline cartilage (endochondral
ossification).
4. The SURFACE SKULL BONES and MANDIBLE are formed by INTRAMEMBRANOUS OSSIFICATION
5. The OTHER BONES, including the LOWER SKULL BONES, are formed by ENDOCHONDRAL OSSIFICATION.
6. The cartilage that exists between the epiphysis and diaphysis is the epiphyseal plate. It permits growth
in length until adulthood.
7. When the ossification process is complete, the cartilage in the epiphyseal plate ossifies and forms the
epiphyseal line.
H. FACTORS AFFECTING BONE GROWTH
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Bone growth is under control of the HUMAN GROWTH HORMONE (hGH) from the anterior pituitary
gland as well as ovarian and testicular sex hormones.
VITAMINS A, C, and D is also necessary for the absorption of calcium ions from the digestive tract into
the blood, proper bone mineralization, calcium removal from the bones, and reabsorption of calcium
from potential urine via kidney tubules.
Adequate minerals, such as calcium, phosphorus, and magnesium are essential for proper bone growth.
Growth and remodeling of bone is also dependent several hormones and upon weight-bearing exercise.
I.
BONE’S ROLE IN CALCIUM HOMEOSTASIS
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Bone stores 99% of the total amount of calcium present in the body.
Calcium becomes available to other tissue when bone is broken down during remodeling.
This is controlled by CALCITONIN (CT), PARATHYROID HORMONES (PTH), and VITAMIN D.
PTH activates the ostoclasts and decreases bone density by increasing serum calcium levels.
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J.
EXERCISE AND BONES
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When placed under mechanical stress, bone tissue becomes stronger. Absence of mechanical stress
weakens bone.
The important mechanical stresses result from the pull of skeletal muscles and the pull of gravity.
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K. DIVISIONS OF THE SKELETAL SYSTEM
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The adult human skeleton consists of 206 bones in two principal divisions
The axial skeleton is composed of 80 bones.
The appendicular skeleton consists of 126 bones.
L. AXIAL SKELETON: SKULL
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The SKULL rests atop the vertebral column and consists of the CRANIAL BONES (8) and FACIAL BONES
(14).
The CRANIAL BONES are the FRONTAL bone (1), PARIETAL bones (2), TEMPORAL bones (2), OCCIPITAL
bone (1), SPHENOID bone (1), and ETHMOID bone (1).
The FACIAL bones are the NASAL bones (2), MAXILLA (2), ZYGOMATIC bones (2), MANDIBLE (1),
LACRIMAL (2), PALATINE bones (2), INFERIOR NASAL CONCHAE (2), and the VOMER (1).
M. SUTURES
1.
Sutures are immovable joints in an adult found only in the skull bones. They are represented by the
following:
 CORONAL SUTURE – unites the frontal bone with the two parietal bones
 SAGITTAL SUTURE – unites the two parietal bones
 LAMBDOIDAL SUTURE – unites the occipital bones with the two parietal bones
 SQUAMOUS SUTURE – unites the two parietal bones with the temporal bone
N. PARANASAL SINUSES AND FONTANELS
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Certain skull bones contain paranasal sinuses, which serve as resonating chambers, and producing
mucus.
Fontanels are membrane-filled spaces found between cranial bones. They enable the fetal skull to
compress during labor and delivery.
O. AXIAL SKELETON: HYOID BONE
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The HYOID bone is unique in that it does not articulate with any other bone in the body. It acts as an
attachment sites for several muscles and ligaments of the tongue, neck, and pharynx.
P. AXIAL SKELETON: VERTEBRAL COLUMN
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The VERTEBRAL COLUMN is composed of 26 bones, distributed into five regions.
The CERVICAL REGION in the neck region contains 7 bones. The THORACIC REGION contains 12 bones.
The LUMBAR region contains 5 bones. The SACRAL region contains 5 bones fused into one. The
COCCYGEAL region can contain up to 4 bones fused into one.
There are FOUR CURVES found in the normal vertebral column. They aid to increase strength, maintain
balance, absorb shocks, and prevent fracturing of the vertebral column bones.
The THORACIC and SACRAL CURVES are primary curves, remnants of the fetus’s single anteriorly
concave curve.
The CERVICAL and LUMBAR CURVES are anteriorly convex. These secondary curves develop as a child
begins to hold the head up (cervical curve) and assumes an upright position (lumbar curve)
respectively.
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Between each vertebrae there is a disc of fibroelastic cartilage called the INTERVERTEBRAL DISC, which
serves to act as a shock absorber.
The typical vertebra can be characterized by specific components. These are a BODY, VERTEBRAL
ARCH (PEDICLE & LAMINA), SEVEN PROCESSES. Some exceptions do occur in the cervical and sacral
regions.
Q. AXIAL SKELETON: THORAX
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The THORACIC CAGE consists of the STERNUM, RIBS (and associated costal cartilages), and the BODIES
OF THE THORACIC VERTEBRAE. It protects the organs in the thoracic and abdominal cavity.
The STERNUM is composed of three major areas. These are the MANUBRIUM, BODY, and the XIPHOID
process.
Twelve pairs of RIBS make up the thoracic cavity. They attach posterior to the thoracic vertebrae.
The FIRST SEVEN PAIRS of RIBS are attached directly to the sternum via hyaline cartilage and are called
the TRUE RIBS.
The remaining FIVE PAIRS OF RIBS are referred to as the FALSE RIBS because their costal cartilages do
not directly attach to the sternum.
The ELEVENTH and TWELFTH FALSE RIBS are also called the FLOATING RIBS because their anterior ends
to not attach at all.
R. APPENDICULAR SKELETON
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There are 126 bones of the appendicular skeleton.
The APPENDICULAR SKELETON consists of the PECTORAL (SHOULDER) GIRDLES, UPPER LIMBS, PELVIC
(HIP) GIRDLE, and the LOWER LIMBS.
S.
APPENDICULAR SKELETON: PECTORAL GIRDLES
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The PECTORAL or SHOULDER GIRDLES attach the bones of the upper limbs to the axial skeleton. Each of
the two pectoral girdles consists of two bones: a CLAVICLE and a SCAPULA.
The CLAVICLE articulates with the sternum at the sternoclavicular joint.
The SCAPULA articulates with the clavicle and the humerus.
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T. APPENDICULAR SKELETON: UPPER LIMB
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Each upper extremity consists of 30 bones (60 total). This includes the HUMERUS, RADIUS, ULNA,
CARPALS, METACARPALS and PHALANGES.
The HUMERUS, or arm bone, is the longest and the largest bone in the upper extremity.
The forearm contains the RADIUS and the ULNA, which lie parallel to one another. The ulna is medial to
the radius. The radius and ulna articulate with the humerus and distally with the carpus (wrist).
Eight CARPAL bones comprise each wrist and are bound together by ligaments.
Distal to the carpal bones are five METACARPAL bones contained in the palm of each hand.
A total of 14 PHALANGES comprise the five digits of each hand. Each digit is comprised of a proximal,
medial, and distal phalanx except the thumb, which lacks a medial phalanx.
U. APPENDICULAR SKELETON: PELVIC GIRDLE
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The PELVIC GIRDLE consists of two hip (COXAL) bones and provides a strong and stable support for the
lower extremities on which the weight of the body is carried.
Together with the sacrum and coccyx, the two coxal bones form the PELVIS.
The PELVIS is anatomically subdivided into the false pelvis, and the true pelvis.
Each coxal bone is composed of three separate bones at birth. These are the ILIUM, ISCHIUM, and
PUBIS. These bones fuse at the ACETABULUM.
The two coxal bones articulate posteriorly with the sacrum and are fused by fibrocartilage anteriorly at
a region called the symphysis pubis.
V. APPENDICULAR SKELETON: LOWER LIMB
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The lower extremities are composed of 30 bones (60 in total).
The FEMUR, or thighbone, is the longest and heaviest bone in the body and lies distal to the pelvic
girdle.
Distal to the femur is the PATELLA, kneecap, which lies anterior to the knee joint.
Distal to the patella are the TIBIA and FIBULA. They are parallel bones in the lower leg. The TIBIA is
larger and medial and bears the major portion of weight in the leg.
The TARSUS is a collective term for the seven bones of the ankle called the tarsals. The two largest
ones, the talus and the calcaneus, are located on the posterior part of the foot. These bones articulate
with the tibia and the fibula.
The METATARSUS consists of five metatarsal bones and are analogous to the metacarpals of the palm of
the hand.
The PHALANGES of the foot resemble those of the hand in both number and arrangement. All are
comprised of three bones except for the big toe which consists of two bones.
W. ARCHES OF THE FOOT
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The bones of the foot are arranged in two arches that enable the foot to support the weight of the body,
provide an ideal distribution of body weight over the hard and soft tissues of the foot, and provide
leverage while walking.
The TWO ARCHES are the LONGITUDINAL ARCH and the TRANSVERSE ARCH.
The arches are flexible.
The LONGITUDINAL ARCH runs from the front to the back of the foot, and consists of a medial and
lateral part.
The TRANSVERSE ARCH is formed by the navicular, third cuneiform, and the proximal end of the fifth
metatarsal.
The bones comprising the arches are maintained in position by the ligaments and tendons of the foot.
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COMPARISON: FEMALE AND MALE SKELETONS
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Male bones are generally larger and heavier than those of the female. The male joint surfaces also tend
to be larger.
Muscle attachment points are more well-defined in the bones of the male than in the female due to the
larger size of muscles in the male.
A number of anatomical differences exist between the male and female pelvic girdles. This is due to
the larger spatial considerations needed in the female pelvic girdle to accommodate childbirth.
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Y. AGING AND THE SKELETAL SYSTEM
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Aging results in the bones becoming more brittle and its loss of mass.
It begins after the age of 30 in the female and accelerates around age 45, as the estrogen levels
decrease.
In the male, calcium loss does not generally begin until after the age of 60.