The skeletal system consists of bones and other connective tissues that provide structure and protect internal organs. Bones are composed of both organic and inorganic materials, including collagen proteins and hydroxyapatite salts. The skeletal system includes long bones in the limbs, short and flat bones, and irregular bones like those in the spine. Bones are living tissues that contain osteoblasts, osteocytes, and osteoclasts, which form, maintain, and break down bone respectively. The skeletal system provides structure, movement, protection, mineral storage, and blood cell formation.

1. SKELETAL SYSTEM
BY
SONI KUMARI SHAH

2. Skeletal System
Skeletal system is the framework of our body, consisting of bones and
other connective tissues which protects and supports the body and
internal organs.
Skeleton can be defined as hard framework of human body around
which the entire body is built.

3. Bone
Bone is dense type of connective tissue saturated with organic salts
mainly the salt of calcium such as calcium phosphate, calcium
carbonate, etc.
Bone is composed of 1/3 organic portion and 2/3 inorganic salts.
Although bones are often thought to be static or permanent, they are
highly vascular living structures that are continuously being remodeled.

4. Composition Of Bone
Bone is made up of 2 types of substances:
Bone Matrix
Bone Cells

5. Bone Matrix
The bone matrix is composed of two types of substances: organic and
inorganic substances.
Organic Substance: It is composed of protein fibers known as collagen
fibers which are embedded in the gelatinous ground substances. This forms
30-35% of bone. Collagen is very strong and gives bone slight flexibility.
Inorganic Substances: It is composed of calcium and phosphate salts,
especially hydroxyapatite. It constitute 60-70% of bone. This inorganic
matrix gives bone great hardness, but on its own would be brittle and prone
to shattering.

6. Bone Cells
These are cells within bone tissue and are responsible for makeup of
the skeleton. The cellular component of bone contributes less than 2%
of bone mass.
Types Of Bone Cells
1. Osteogenic cells
2. Osteoblasts
3. Osteocytes
4. Osteoclasts

7. Osteogenic Cells
These are derived from embryonic mesenchymal cells.
These are present in cellular layer of periosteum, endosteum and in
Haversian canals.
These are stem cells which after cell division give origin to osteoblasts.

8. Osteoblasts
These bone-forming cells are responsible for the deposition of both
inorganic salts and osteoid in bone tissue.
They are therefore present at sites where bone is growing, repairing or
remodeling, e.g.:
in the deeper layers of periosteum
in the centers of ossification of immature bone
at the ends of the diaphysis adjacent to the epiphyseal cartilages of long
bones
at the site of a fracture.
As they deposit new bone tissue around themselves, they eventually
become trapped in tiny pockets (lacunae) in the growing bone, and
differentiate into osteocytes.

9. Osteocytes
These are mature bone cells that monitor and maintain bone tissue,
and are nourished by tissue fluid in the canaliculi that radiate from the
central canals.
These are derived from osteoblasts.
They maintain its daily metabolism such as exchange of nutrients and
wastes with the blood.
They do not undergo cell division like osteoblasts.

10. Osteoclasts
These cells break down bone, releasing calcium and phosphate.
They are very large cells with up to 50 nuclei, which have formed from the
fusion of many monocytes.
The continuous remodeling of healthy bone tissue is the result of balanced
activity of the bone’s osteoblast and osteoclast populations.
Osteoclasts are found in areas of the bone where there is active growth, repair
or remodeling, e.g.:
under the periosteum, maintaining bone shape during growth and to remove
excess callus formed during healing of fractures
round the walls of the medullary canal during growth and to canalize callus
during healing.

11. Bone contains
99% of body’s Calcium
4% of body’s Potassium
35 % of body’s Sodium
50 % of body’s Magnesium
80 % of body’s Carbonate
99 % of body’s Phosphate

12. Functions Of Bones
providing the body framework
giving attachment to muscles and tendons
allowing movement of the body as a whole and of parts of the body, by forming joints
that are moved by muscles
forming the boundaries of the cranial, thoracic and pelvic cavities, and protecting the
organs they contain
haemopoiesis, the production of blood cells in red bone marrow
mineral storage, especially calcium phosphate – the mineral reservoir within bone is
essential for maintenance of blood calcium levels, which must be tightly controlled.

13. Which of the following is a function of the skeletal system?
A. haemopoiesis
B. haemostasis
C. peristalsis
D. Glycogenolysis
In which of the following bone structures do osteocytes live?
A. osteons
B. canaliculi
C. lacunae
D. Lamellae
In which structure are osteoclasts and osteoblasts found?
A. in the periosteum
B. in the haversian canals
C. in the lacunae of osteons
D. in the trabeculae of osteons

14. The functions of bones may be stated as:
A. fat storage, movement, mineral storage, protection, blood cell formation.
B. mineral storage, haemopoiesis, movement, leverage, protection.
C. blood cell formation, hormone production, movement, support, protection.
D. support, storage, movement, haemopoiesis, protection
The formula for the inorganic salts in bone is
A. NH6C3COOH
B. C6H12O6
C. Ca10(PO4)6OH2
D. CH3(CH2)7CH=CH(CH2)7COOH

15. Which of the list below is a cell that reabsorbs bone?
A. osteon
B. osteoblast
C. osteocyte
D. osteoclast
Blood cell formation (haemopoiesis) occurs in which of the following structures?
A. red marrow
B. yellow marrow
C. medullary cavity
D. epiphyseal plate

16. Classification Of Bone
On The Basis Of Shape And Size
Long bone
typical long bone
miniature long bone
modified long bone
Short bone
Flat bone
Irregular bone
Pneumatic bone
Sesamoid bone

17. Long Bones
They have elongated shaft called diaphysis and two
expanded ends called epiphysis on either side of the shaft.
The shaft has central medullary cavity.
Typical long bone: eg. humerus, radius, ulna, tibia, fumer,
fibula.
Miniature long bone: they are shorter in length and have
only one epiphysis.eg. metatarsals, metacarpals, phalanges
Modified long bone: They have either modified shaft or
ends and has no medullary cavity. Eg. Clavicle.

18. Short Bones
They are small and thick in size. They are named
according to their shape usually cuboid,
cunieform, trapezoid or scaphoid.
Examples
Carpal and tarsal bones

19. Flat Bones
They are thin with two parallel surface. They
resembles shallow plates and forms
boundaries of certain body cavities.
They are found in skull, sternum, ribs and
scapula

20. Irregular Bones
They are irregular in shape and do not fit
into any category of shape.
Examples
Vertebrae, hip bone, bones in base of skull

21. Sesamoid Bones
They resemble shape of sesame seed in size or shape.
These are bony nodules found embedded in tendons
and joints capsule. They have no periosteum and ossify
after birth.
Examples
Patella, pisiform, fabella
Functions
To resist pressure
To minimize friction
To alter the direction of pull of muscle
To maintain local circulation

22. Pneumatic Bones
They are irregular bones with large air spaces
lined by epithelium.
Example:
Maxilla, sphenoid, ethmoid
Function:
Make skull light in weight
Help in resonance of voice
Act as air conditioner chambers

23. On The Basis Of Development
Membranous Bone
Cartilaginous Bone
Membrano-cartilaginous Bone

24. Membranous Bones
These bones ossify in membrane and
are derived from mesenchymal
condensation
Examples
Bones of vault of skull, facial bones

25. Cartilaginous Bone
These bones ossify in cartilage and are
derived from performed cartilaginous
models
Examples
Bones of limbs, vertebral column, thoracic
cage

26. Membrano-cartilaginous Bone
These bones ossifies partly in membrane and
partly in cartilage.
Examples
Clavicle, mandible, occipital, temporal, sphenoid

27. On The Basis Of Structure
Macroscopic Structure
Compact Bone
Spongy Bone
Microscopic Structure
Lamellar Bone
Fibrous Bone

28. Compact Bone
It is dense in texture but extremely
porous.
It has an adaptation to bending and
twisting force.
Haversian system is present in this type
of bone.

29. Cancellous Or Spongy Bone
To the naked eye, spongy bone looks like a
honeycomb.
It is open in texture and is made up of meshwork
of trabeculae.
The spaces between the trabeculae contain red
bone marrow.
In addition, spongy bone is lighter than compact
bone, reducing the weight of the skeleton.
They have adaptation to compressive force.

30. Lamellar Bone
They are composed of thin plates of bony
tissue called lamella.
These are arranged in piles in cancellous bone
and in concentric cylinders in compact bone.

31. Fibrous Bone
They have numerous fibers present.
These are seen in fetal bone, fracture repair and in cancer bone.

32. On The Basis Of Region
Axial skeleton
Appendicular skeleton

33. Division Of Skeleton
Axial Skeleton
• Skull
• Spine
• Rib cage
Appendicular Skeleton
• Upper limb
• Lower limb
• Shoulder girdle
• Pelvic girdle

34. Which one of the following is a bone that is embedded within a tendon?
A. sphenoid
B. hyoid
C. ethmoid
D. Sesamoid
Choose the correct sentence. Compact bone contains
A. lamellae and osteocytes but no osteons.
B. trabeculae, canaliculi and osteons.
C. haversian systems and canaliculi but no osteons.
D. osteons and lamellae but no trabeculae.
Compact bone differs from spongy (cancellous) bone because compact bone:
A. does not contain osteons
B. is used to form short bones
C. contains marrow
D. has Haversian canals

35. Axial Skeleton

36. Bones Of Cranium

37. Bones Of Face

38. Ear Ossicles And Hyoid Bone

39. Bones Of Thorax

40. Bones Of Vertebral Column

41. Appendicular Skeleton

42. Bones Of Shoulder Girdle

43. Bones Of Upper Limb

44. Bones Of Pelvic Girdle

45. Bones Of Lower Limb

46. What are the bones of the fingers known as?
A. short bones
B. metacarpals
C. carpals
D. Phalanges
Which of the following comprise seven bones?
A. Cervical vertebrae
B. Carpals
C. Cranial bones
D. Lumbar vertebrae
To which bones does the word phalanges apply? Those in the
A. fingers and toes
B. wrist and ankle
C. ankle and foot
D. fingers and hand

47. Where in the skeleton is the scapula located?
A. in the axial skeleton
B. in the appendicular skeleton
C. in the carpal region
D. in the shoulder girdle
Where are the bones known as the humerus and radius located?
A. in the axial skeleton
B. in the arm
C. in the leg
D. in the arm and leg respectively
Which of the following bones is part of the cranium?
A. occipital
B. mandible
C. hyoid
D. Carpal

48. The axial skeleton groups together which sets of bones?
A. the arms and hands, the legs and feet, shoulder girdle and pelvic girdle.
B. the head, shoulder girdle, arms and hands.
C. the thoracic cage, vertebral column, shoulder girdle, the pelvic girdle, the skull and facial bones.
D. bones of the skull and face, thoracic cage and vertebral column.
The appendicular skeleton groups together which sets of bones?
A. the arms and hands, the legs and feet, shoulder girdle and pelvic girdle.
B. the head, shoulder girdle, arms and hands.
C. the thoracic cage, vertebral column, shoulder girdle, the pelvic girdle, the skull and facial bones.
D. bones of the skull and face, thoracic cage and vertebral column.
One of the following lists contains only bones in the appendicular skeleton. Which one?
A. patella, ethmoid, femur, coccyx, tibia
B. clavicle, fibula, metatarsal, phalange, radius
C. humerus, scapula, occipital, metacarpal, sternum
D. ulna, radius, phalange, mandible, coxal

49. Structure Of Young Bone/ Long Bone
These have a diaphysis (shaft) and two epiphyses
(extremities).
The diaphysis is composed mainly of compact
bone with a central medullary canal, containing
fatty yellow bone marrow.
The epiphyses consist of an outer covering of
compact bone with spongy (cancellous) bone
inside.
The diaphysis and epiphyses are separated by
epiphyseal cartilages, which ossify when growth
is complete.

50. Structure Of Young Bone/ Long Bone
Long bones are almost completely covered by a vascular membrane, the periosteum,
which has two layers.
The outer layer is tough and fibrous, and protects the bone underneath.
The inner layer contains osteoblasts and osteoclasts, the cells responsible for bone
production and breakdown, and is important in repair and remodeling of the bone.
The periosteum covers the whole bone except within joint cavities, allows attachments
of tendons and is continuous with the joint capsule.
Hyaline cartilage replaces periosteum on bone surfaces that form joints.
Thickening of a bone occurs by the deposition of new bone tissue under the
periosteum.

51. Parts Of Young Bone
Epiphysis
Diaphysis
Metaphysis
Epiphyseal plate of cartilage

52. Internal Structure Of Bone
It is made up of a large number of parallel tube
shaped units called osteons (Haversian
systems), each of which is made up of a central
canal surrounded by a series of expanding rings,
similar to the growth rings of a tree.
The central canal contains nerves, lymphatics
and blood vessels, and each central canal is
linked with neighboring canals by tunnels
running at right angles between them, called
perforating canals.
The series of cylindrical plates of bone arranged
around each central canal are called lamellae.

53. Internal Structure Of Bone
Between the adjacent lamellae of the osteon are
strings of little cavities called lacunae, in each of
which sits an osteocyte.
Lacunae communicate with each other through
a series of tiny channels called canaliculi.
Between the osteons are interstitial lamellae.

54. Surface Features Of Bone
Articulations (joints)
Condyle - a rounded knob
Facet - a slightly raised, elongated ridge
Head - the prominent expanded end of a bone

55. Surface Features Of Bone
Extensions and projections
Crest - a narrow ridge
Line - a slightly raised elongated ridge
Epicondyle - projection superior to condyle
Process - any bony prominence
Trochanter - a massive process
Spine - a sharp, narrow process
Tubercle - a small, rounded process
Tuberosity - a rough surface

56. Surface Features Of Bone
Depressions
Alveolus - a pit or socket
Fossa - a shallow, broad or elongated basin
Fovea - a small pit
Sulcus - a groove for tendon, Nerve or blood vessels

57. Surface Features Of Bone
Passages
Foramen – a hole through a bone
Canal or meatus – a tubular passage or tunnel through a bone
Fissure - a slit through a bone
Sinuses - space or cavities within a bone

58. Choose the correct sentence. Cancellous bone contains
A. lamellae and osteocytes but no trabeculae.
B. trabeculae, canaliculi and osteons.
C. haversian systems and canaliculi but no osteons.
D. trabeculae and lamellae but no osteons.
Where is the epiphyseal plate of a long bone located?
A. in the diaphysis
B. between the diaphysis and the epiphysis
C. in the epiphysis
D. in the medullary canal
Where does the increase in the length of a long bone take place? At the :
A. diaphysis ossification centres
B. epiphyseal plates
C. cartilaginous plates
D. medullary canal

59. Where are blood vessels in compact bone found?
A. in the canaliculi
B. in the periosteum
C. in the lacunae
D. in the central canal
What does the term “haversian canal” refer to in bone?
A. the larger examples of foramina.
B. a groove that receives a muscle’s tendon.
C. the centre of an osteon that contains blood capillaries.
D. the space within a long bone that contains marrow.
What is the name given to the central tunnel of an osteon that contains blood vessels?
A. canaliculus
B. endosteum
C. haversian canal
D. medullary canal

60. Which term below refers to a depression in a bone?
A. tuberosity
B. fossa
C. tubercle
D. Condyle
Which of the following is NOT a depression or cavity on a bone?
A. tuberosity
B. facet
C. meatus
D. Sinus
Which of the following is a NOT a projection from a bone surface?
A. trochanter
B. tubercle
C. trabeculum
D. tuberosity

61. Which of the following is a projection from a bone surface?
A. fossa
B. fissure
C. foramen
D. Facet
What is a “foramen”?
A. a basin-like depression serving as an articular surface.
B. a raised roughening which is a site for muscle attachment.
C. a hole through a bone for a nerve or blood vessel.
D. a sharp slender projection of bone.

62. Bone Marrow
Bone marrow is a soft, fluid tissue that occupies the medullary cavity of long
bone, the hollow space of spongy bone and large Haversian canals.
They are of two types:
Red Bone Marrow: This looks like thick blood and consists of developing
blood cells in a delicate mesh of reticular fibers.
Yellow Bone Marrow: With ageing, red bone marrow is gradually converted
to yellow bone marrow as a result of more deposition of adipose tissue in
place of haemopoietic tissue. At around age of 30, most of the medullary
cavity is filled with fat. It cannot produce blood cells but can be reverted to
red bone marrow and produce blood cells in case of severe anemia.

63. Ossification Of Bones
Ossification or osteogenesis is the process of bone formation.
It begins before birth and completes only around 25 years of age.
Long, short and irregular bones develop in the fetus from rods of
cartilage, cartilage models.
Flat bones develop from membrane models and sesamoid bones from
tendon models.

64. Types Of Ossification
Endochondral or Cartilaginous Ossification: A bone is derived from a pre-
existing similar hyaline cartilage model. This cartilage is broken down,
reorganized and hardened to form a bone.
Intramembranous Ossification: Bone formation begins with formation of
highly vascular sheet or membrane of soft tissue from mesenchyme. Its
cells enlarge and differentiate into bone cells and deposit the organic
matrix. With further calcification, it converts to bone.

65. Ossification Of Bones
During ossification, osteoblasts secrete osteoid, which gradually replaces the
initial model; then this osteoid is progressively calcified, also by osteoblast
action.
As the bone grows, the osteoblasts become trapped in the matrix of their own
making and become osteocytes.
In mature bone, a fine balance of osteoblast and osteoclast activity maintains
normal bone structure.
If osteoclast activity exceeds osteoblast activity, the bone becomes weaker.
On the other hand, if osteoblast activity outstrips osteoclast activity, the bone
becomes stronger and heavier.

66. Bone Growth and Remodeling
The ossification of cartilage into bone, or osteogenesis, begins at about the
ninth week of fetal development.
The diaphysis of long bones are formed by birth, and the epiphysis begin to
ossify at about that time.
Stages of Bone Growth and Remodeling of The Epiphyses of a Long Bone:
Creation of An Ossification Center
Osteoblasts Form Bone
Bone Length Grows
Remodeling

67. Creation of An Ossification Center
At about the ninth month, an ossification
center develops in the epiphysis.
Some cartilage cells enlarge and stimulate
ossification of surrounding cells.
The enlarged cells die, leaving small cavities.
New cartilage continues to develop.

68. Osteoblasts Form Bone
Osteoblasts begin to form bone on the
remaining cartilage, creating the trabeculae
network of cancellous bone.
Cartilage continues to form on the outer
surfaces of the epiphysis and along the upper
surface of the epiphyseal plate.

69. Bone Length Grows
Cartilage is replaced by compact bone near the
outer surfaces of the epiphysis.
Only cartilage cells on the upper surface of the
epiphyseal plate continue to multiply rapidly,
pushing the epiphysis away from the diaphysis.
This new cartilage ossifies, creating trabeculae
on the medullary side of the epiphyseal plate.

70. Remodeling
Osteoclasts produce enzymes and acids that
reduce trabeculae created by the epiphyseal
plate, thus enlarging the medullary cavity.
In the epiphysis, osteoclasts reduce bone, making
its calcium available for new osteoblasts that give
the epiphysis its adult shape and proportion.
In young adults, the epiphyseal plate completely
ossifies (closes) and becomes the epiphyseal line;
longitudinal growth of bone then ceases.

71. Factors Helping Bone Growth
Hormonal regulation of bone growth
Growth Hormone and Thyroid Hormone
Testosterone and Estrogen
Calcitonin and Parathyroid Hormone
Diet and bone
Exercise and bone

72. Blood Supply Of Bone
Long bones
• Nutrient artery
• Periosteal artery
• Epiphyseal artery
• Metaphyseal artery
Other bones
• Periosteal vessels

73. Nerve Supply Of Bone
The sensory nerve supply usually enters the bone at the same site as the
nutrient artery, and branches extensively throughout the bone.
Bone injury is, therefore, usually very painful.

74. Fractures
Fracture is the break in continuity of cortex of bone.
It can be defined as a complete or incomplete interruption in the
continuity of cortex of bone.
Causes of fracture
Direct trauma
Injury
Indirect trauma
Stress fracture due to stress and strain on the bone
Pathological fracture causes osteomyelitis, tumors, osteoporosis, etc.

75. Classification of Bone Fractures
Simple
The bone ends do not protrude through the skin
Compound
The bone ends protrude through the skin
Pathological
Fracture of a bone weakened by disease.

76. Clinical Features
Severe pain
Deformity
Muscle spasm
Oedema
Paralysis
Crepitation

77. Stages of Bone Healing
1. A haematoma (collection of clotted
blood) forms between the ends of
bone and in surrounding soft tissues.

78. 2. There follows development of acute
inflammation and accumulation of
inflammatory exudate, containing
macrophages that phagocytose the
haematoma and small dead fragments of
bone (this takes about 5 days).
Fibroblasts migrate to the site;
granulation tissue and new capillaries
develop.

79. 3. New bone forms as large numbers of
osteoblasts secrete spongy bone, which
unites the broken ends, and is protected
by an outer layer of bone and cartilage;
the new deposits of bone and cartilage is
called a callus.
Over the next few weeks, the callus
matures, and the cartilage is gradually
replaced with new bone.

80. 4. Reshaping of the bone continues
and gradually the medullary canal is
reopened through the callus (in
weeks or months).
In time the bone heals completely
with the callus tissue completely
replaced with mature compact bone.
Often the bone is thicker and stronger
at the repair site than originally, and a
second fracture is more likely to occur
at a different site.

81. Factors That Delay Healing Of Fractures
Tissue fragments between bone ends
Deficient blood supply.
Poor alignment of bone ends
Continued mobility of bone ends
Miscellaneous: These include infection, systemic illness, malnutrition,
drugs, e.g. corticosteroids and ageing.