The document discusses the structure and formation of bone. It begins by introducing bone composition, including inorganic crystals like hydroxyapatite that provide strength and an organic matrix of collagen that provides flexibility. It describes the different cells involved, including osteoblasts that build bone and osteoclasts that resorb it. It then covers the classification of bone by location in the body, quality from dense to porous, shape as long, short, flat or irregular, and developmental origin as intramembranous or endochondral. The structure of long bones is outlined including the diaphysis, epiphysis, periosteum and marrow cavity. Finally, the two mechanisms of bone formation are summarized - endochondral

1. Dr. Vibhuti Singh Gopal

2. INDEX
 Introduction of bone
 Bone composition
- Composition of bone & bone matrix
- Cells of bone
 Classification of bone
- Type of bone
 Structure of bone
 Mechanism of bone formation
- Intamambranous ossification
- Endochondral ossification

4. Bone is one of the
hardest substances of
the body.
Bone is a dynamic tissue
that constantly changes
shape in relation to the
stresses placed on it.

5.  Pressures applied to bone lead to its
resorption, whereas tension applied to it
results in development of new bone.
 Applying these facts, the orthodontist is
able to remodel the bone of the dental
arches by moving and straightening the
teeth to correct malocclusion.

7. Bone Composition

8. COMPOSITION OF BONE MATRIX
1. INORGANIC CRYSTALS
2. ORGANIC MATRIX

9. INORGANIC CRYSTALS
 About 70% of the matrix of bone is made of
inorganic salts, principally salts of calcium
and phosphate.
 These salts are deposited as flat crystal plates
of hydroxyapatites,which have a variable ratio
of calcium and phosphorus.

10.  Many other ions ,particularly fluoride, magnesium,
sodium, potassium, and carbonate absorb on to the
hydroxyapatite crystals but do not themselves seem
to be incorporated into bone as crystals.
 Inorganic crystals give bone its compressional
strength. Provide much of the bone’s stiffness
and resistance to pressing or squeezing forces.

11.  Organic matrix give bone its tensile strength
 About 30% of matrix of bone made up of
organic matrix.
 90% of organic matrix is made up of collagen
fibers.
ORGANIC MATRIX

12.  The collagen fibers are arranged in lines of
tensional force for each bone and are
surrounded by a homogeneous ground
substance containing:-
 hyaluronic acid
 sialic acid combined with various proteins
and mucopolysaccharides.

13.  Collagen gives bone its characteristic
flexibility and contributes to its ability
to resist pulling and stretching forces
 With aging, collagen is lost
progressively and bone becomes more
brittle

14. Water
Bone consists of much smaller
proportion of water than other body
parts

15. Cells of Bone
 Osteoprogenitor
cells
 Osteoblasts
 Osteocytes
 Osteoclasts.

16. Osteoprogenitor Cells
 These cells are unspecialized stem cell derived
from embryonic mesenchyme .
 They are the only cells to undergo cell division;
the resulting cell develop into osteoblast.

17.  Osteogenic cells are found along the inner
portion of the periosteum,in the
endosteum,and in the canals with in bone
that contain blood vessels.
 Most active during the period of intense
bone growth.

18.  Under certain conditions of low oxygen
tension, these cells may differentiate into
chondrogenic [cartilage forming]cells.

19. OSTEOBLASTS
 Derived from
osteoprogenitor cells .
 Located on the surface of
the bone in a sheet-like
arrangement of cuboidal
to columnar cells .

20.  Osteoblasts exocytose
their secretory products,
each cell surrounds
itself with the bone
matrix.
 When this occurs, the
imprisoned cell is
referred to as an
osteocyte, and the
space it occupies is
known as a lacuna.

21.  Most of the bone matrix becomes calcified
 Osteoblasts as well as osteocytes are always
separated from the calcified substance by a
thin, noncalcified layer known as
the osteoid (uncalcified bone matrix).

22. OSTEOCYTES
 Mature bone cells derived from
osteoblasts that became trapped
in their lacunae.
 Radiating out in all directions
from the lacunae are narrow,
tunnel like spaces (canaliculi)
that contain cytoplasmic
processes of the osteocyte.

23.  Processes make contact with similar processes
of neighboring osteocytes, forming gap
junctions through which ions and small
molecules can move between the cells.
 Canaliculi also contain extracellular fluid
carrying nutrients and metabolites that nourish
the osteocytes.

24. OSTEOCLASTS
 Osteoclast are huge cells derived from the
fusion of as many as 50 monocytes and are
concentrated in the endosteum.
 On the side of the cell that faces the bone
surface, the osteoclast’s plasma membrane
is deeply folded into a ruffled border.

25.  Cell releases
powerful lysosomal
enzymes and acids
that digest the
protein and mineral
components of the
underlying bone
matrix.

26.  The breakdown of
bone extracellular
matrix is termed
resorption.
 It is part of the normal
development, growth ,
maintenance and
repair of bone.

28. Bone tissue can be classified in several ways:-
 Based on location
 Based on quality
 Based on the shape
 Texture
 Matrix
 Arrangement
 Maturity
 And developmental origin
CLASSIFICATION OF BONE

29. 1- BASED ON LOCATION
 AXIAL SKELETON:- bones of skull,
vertebral column, sternum and ribs
 APPENDICULAR SKELETON:-bone of the
pectoral girdle, pelvic girdle, and limb.
 ACRAL SKELETON:-part of the
appendicular skeleton Including bone of the
hands and feet.

30. 2-Based on quality
 Classified in to four types, expressed as
type I, II, III and VI ( D1, D2, D3, and D4)

31. D1 type bone
 D1 bone is composed of almost all
cortical bone mass located
primarily in the anterior mandible.
 Due to its density this type of bone
has fewer intrinsic blood vessels
and depends for a significant
portion of nutrient and blood
supply on the periosteum.

32. D2 type bone
 D2 bone is composed of a
thick crestal layer of
cortical bone and coarse
trabecular bone
underneath the cortical
bone.
 This type of bone can
mostly be found in the
anterior and posterior
mandible.

33. D3 type bone
 D3 bone is composed of a
porous crestal layer of
cortical bone and fine
trabecular bone
underneath the cortical
bone.
 This type of bone can
mostly be found in the
anterior and posterior
maxilla but also in the
posterior mandible

34. D4 type bone
 D4 bone is composed of
primarily fine trabecular
bone and often the
absence of cortical bone.
 This type of bone can
mostly be found in the
posterior maxilla and
poses the greatest
challenge in implant
placement.

35. 3- According to shape of bone
 Almost all bones of the body can be classified
into five main types based on shape:
1- Long
2- Short
3- Flat
4- Irregular
5- sesamoid

36. Long bone
 Long bone have
greater length than
width, consist of a
shaft and a variable
number of
extremities, and
slightly curve for
strength.

37. Long bone consist
mostly of compact
bone tissue in their
diaphyses but have
considerable
amount of spongy
bone tissue in their
epiphyses.

38.  Long bones vary in size and include those in the
1. Thigh (femur)
2. Leg (tibia and fibula)
3. Arm (humerus),
4. Forearm (ulna and radius)
5. Fingers
6. Toe (phalanges).

39. SHORT BONE
 Short bone are
somewhat cube –
shaped and nearly
equal in length and
width.
 They consist of spongy
bone tissue except at
surface,which has a
thin layer of compact
bone tissue.

40.  Example of short
bones are the
1. Carpal (wrist) bone
2. Tarsal (ankle)bones

41. FLAT BONE
 Flat bones are generally thin and composed
of two nearly parallel plates of compact
bone tissue enclosing a layer of spongy bone
tissue.

42.  Flat bones include the
1. Cranial bones, which
protect the brain;
2. The sternum and ribs,
which protect organs in
the thorax and
scapulae(shoulder
blades)

43. IRREGULAR BONES
 Irregular bones have
complex shapes.
 They vary in the spongy
and compact bone present.
 Example : vertebrae, hip
bones,and certain facial
bones.

44. SESAMOID BONES
 Sesamoid bone develop in
certain tendons where there is
considerable friction, tension
and physical stress such as
palm and soles.
 They may vary in number
from person to person, are not
always completely ossified,
and measure only in few
millimeters in diameter.

45. 4-According to texture of bone
 According to the degree of
porosity, bone can be
classified into two general
categories:
 Cortical bone (low porosity)
 Spongy or cancellous bone
(high porosity)

46. Cancellous bone Compact Bone
•Porosity •High (Low mineral content
and high collagen)
•Low (High mineral content
and low collagen)
•Structure •Honey comb •Compact
•Characteristic •Provides more flexibility but
is not as stress resistant
•Stiffer and can resist greater
stress but less flexible
•Function •Shock absorption due to its
better ability to change shape
are important
•Withstanding stress in
body areas that are subject to
higher impact loads
•Location •e.g., vertebrae •Long bones (e.g., bones of
the arms and legs)

47. 5-According to matrix arrangement
 LAMELLAR BONE:- lamellar bone is mature bone
with collegen fiber that are arranged in lamellae.

48.  WOVEN BONE:-woven bone is immature bone , in
which collegen fiber are arranged in irregular random
arrays and contain smaller amount of mineral
substance and higher proportion of osteocytes than
lamellar bone.

49. 6- According to maturity of bone
 IMMATURE BONE:-immature bone is woven bone.
 MATURE BONE:-mature bone is characteristically
lamellar bone . Almost all bone in adults are lamellar
bone.

50. 7-According to developmental origin
 INTRAMEMBRANOUS BONE:-develops from direct
transformation of condensed mesenchyme. Flat bone
are formed in this way.
 INTRACARTILAGINOUS BONE:-forms by replacing a
preformed cartilage model. Long bone are formed in
this way.

52. Structure of a long bone
 Diaphysis
 Epiphysis
 Proximal
 Distal
 Periosteum
 Medullary cavity
 Articular cartilage
 Epyphyseal plates

53. 1. DIAPHYSIS:- (GROWING
BETWEEN) is the bone’s
shaft or body- the
long,cylindrical ,main
portion of the bone
2. EPIPHYSIS-
Epiphysis are the distal
and proximal ends of
bone

54. 3- METAPHYSIS-
are the regions in a
mature bone where the
diaphysis joins the
epiphysis.
 In the growing bone
each metaphysis
includes an epiphysis
plate

55. 4- ARTICULAR CARTILAGE
is a thin layer of hyaline
cartilage covering the part
of the epiphysis where bone
form an articulation with
another bone.
 It reduce friction and
absorb shock at freely
movable joint.

56. 5- PERIOSTEUM- is a
tough sheath of dense
irregular connective
tissue that surrounds the
bone surface where it is
not covered by articular
cartilage.

57. 6-MEDULLARY CAVITY- is the
space with in the diaphysis that
contain fatty yellow bone
marrow in adults
7-ENDOSTEUM-
is a thin membrane that
lines the medullary cavity.
 It contains a single layer of
bone – forming cells and a
small amount of connective
tissue

59. MECHANISM OF BONE FORMATION
 Endochondral ossification
 Intramembranous ossification

60. Endochondral Ossification
 The replacement of cartilage by bone is called
endocondral ossification.
 Almost all bone of the body are formed in this
way, the process is best observed in long bone.

61. Endochondral Ossification
 It proceeds as follow:-
1. Development of the cartilage model.
2. Development of primary ossification center.
3. Development of secondary ossification center.

62. Endochondral Ossification
 Aggregation of mesenchymal cells Differentiation
into condroblast.
 These condroblast lay down Hyaline cartilage.
 This is surrounded by a membrane-called
perichondrium

63.  Calcification of intercellular substance.
 Nutrition is cut off leading to cell death.
Empty spaces are formed-primary areolae. .
 Blood vessel and osteogenic cells invade the
calcified cartilaginous matrix.

64.  This leaves a large empty space- secondary
areolae.
 Osteogenic cells become osteoblast arrange
themselves along the surface of these
calcified matrix.
 Ostoblasts lay down osteoid, which later
becomes calcified to form a lamellae of bone

65. INTRAMEMBRANOUS OSSIFICATION
 The flat bone of the skull and mandible are formed by
intramembranous ossification.
 The soft spots that help the fetal skull pass through the
birth canal later harden as they undergo
intramenbranous ossification

66. INTRAMEMBRANOUS OSSIFICATION
It occurs as follow:-
1- Development of ossification center
2- Calcification
3-Formation of trabeculae
4-Development of periosteum

67.  Aggregation of mesenchymal cell at the site
of bone formation.
 Some cells lay down bundles of collagen
fibers.
 Some convert to osteoblasts.
 Osteoblast secrete a gelatanious matrix
called osteoid.

68.  Calcium salts are deposited into the osteoid.
 Now osteoblast move away from lamellae &
new layer of osteoid is sectreted which also
calcified.
 Some osteoblasts gets entrapped between
two lamellae- ostocyte .

70. INDEX
 THE SKULL
 THE CRANIAL BONES
OCCIPITAL BONE
TWO PARIETAL BONE
FRONTAL BONE
TWO TEMPORAL BONE
SPHENOIDAL BONE
ETHMOIDAL BONE

71.  THE FACIAL BONES
TWO NASAL BONE
TWO LACRIMAL BONE
TWO ZYGOMATIC BONE
TWO PALATINE BONE
TWO INFERIOR NASAL CONCHA BONE
TWO MAXILLA BONE
VOMER BONE
MANDIBLE BONE

73. The Skull
 The skull is supported on the summit of the
vertebral column.
 skull consist an oval shape .
 Skull is wider in posterior than in front.
 Skull is composed of a series of flattened or
irregular bones.

74.  skull is divided into two
parts:
(1) the cranium, which
lodges and protects the
brain, consists of eight
bones
(2) the skeleton of the
face, of fourteen bones

75. Skull consist of 22
bones
OCCIPITAL
TWO PARIETAL
FRONTAL
Cranium, 8 bones
TWO TEMPORAL
SPHENOIDAL
ETHMOIDAL
TWO NASAL
TWO LACRIMAL
Face, 14 bones TWO ZYGOMATIC
TWO PALATINE
TWO INFERIOR NASAL
CONCHA
TWO MAXILLA
VOMER
MANDIBLE

77. The Cranial Bones
 OCCIPITAL BONE
 TWO PARIETAL BONE
 FRONTAL BONE
 TWO TEMPORAL BONE
 SPHENOIDAL BONE
 ETHMOIDAL BONE

78. 1.The Occipital Bone
 The occipital bone ,
situated at the back and
lower part of the cranium.
 It is trapezoid in shape .
It is pierced by a large
oval aperture the foramen
magnum through which
the cranial cavity
communicates with the
vertebral canal.

79. OSSIFICATION
 The planum occipital
of the squama is
developed in
membrane and may
remain separate
throughout life when it
constitutes
the interparietal bone.
 The rest of the bone is
develop in cartilage.

80.  The planum nuchale of
the squama is ossified
from two centers, which
appear about the
seventh week of fetal
life and soon unite to
form a single piece.
 Union of the upper and
lower portions of the
squama takes place in
the third month of
fetal life

81.  An occasional center
(Kerckring) appears in
the posterior margin of
the foramen magnum
during the fifth month.
 Each of the lateral parts
begins to ossify from a
single center during the
eighth week of fetal
life.

82.  About the fourth year the squama and the
two lateral portions unite.
 About the sixth year the bone consists of a
single piece.
 Between the eighteenth and twenty-fifth
years the occipital and sphenoid become
united, forming a single bone.

83. 2. The Parietal Bone
 The parietal bones form by their union to
the sides and roof of the cranium.
 Each bone is irregularly quadrilateral in
form, and has two surfaces, four borders,
and four angles.

84. sagittal region
occipital angle
Frontal angle
the parietal lobe
superior temporal line
occipital region
frontal margin
lower temporal line
mastoid angle
wedge angle
scaly edge

85. Ossification.—
 The parietal bone is ossified in membrane
from a single center
 which appears at the parietal eminence
about the eighth week of fetal life.
 Ossification gradually extends in a radial
manner from the center toward the margins
of the bone

86.  The angles are consequently the parts last
formed.

87. The Frontal Bone
 The frontal bone resembles a cockle-shell
in form, and consists of two portions-
1. vertical portion
2. squama
 Corresponding with the region of the
forehead and an orbital or
horizontal portion, which enters into the
formation of the roofs of the orbital and
nasal cavities.

89. Ossification —
 The frontal bone is ossified in membrane
from two primary centers
1. One for each half, which appear toward the
end of the second month of fetal life
2. One above each supraorbital margin

90.  From each of these centers ossification
extends upward to form the corresponding
half of the squama, and backward to form
the orbital plate.
 The spine is ossified from a pair
of secondary centers, on either side of the
middle line; similar centers appear in the
nasal part and zygomatic processes.

91. The Temporal Bone
 The temporal
bones are situated at
the sides and base of the
skull.
 Each consists of five
parts-
1. Squama
2. Petrous
3. Mastoid
4. Tympanic parts
5. Styloid process.

92. Ossification.-
 The temporal bone is ossified from eight centers-
1. One for the squama including the zygomatic
process.
2. One for the tympanic part.
3. Four for the petrous and mastoid parts.
4. Two for the styloid process.

93.  Just before the close of fetal life the temporal bone
consists of three principal parts:
1. The squama is ossified in membrane from a single
nucleus, which appears near the root of the
zygomatic process about the second month.

94. 2. The petromastoid part is developed from four centers-
 Proötic
 Opisthotic
 Pterotic
 epiotic
which make their appearance in the cartilaginous ear
capsule about the fifth or sixth month.

95. 3.The tympanic ring is an
incomplete circle,
 In the concavity of which is
a groove
 The tympanic sulcus for the
attachment of the
circumference of the
tympanic membrane.
 This ring expands to form the
tympanic part, and is ossified
in membrane from a single
center which appears about
the third month .

96. The Sphenoid Bone
 The sphenoid bone is situated at the base of the skull
in front of the temporals and basilar part of the
occipital.

97. Ossification.—
 Until the seventh or eighth month of fetal life the
body of the sphenoid consists of two parts –
1- One in front of the tuberculum sella the presphenoid,
with which the small wings are continuous.
2- The other, comprising the sella turcica and dorsum
sella, the postsphenoid,which are associated with the
great wings, and pterygoid processes.

98.  The greater part of the bone is ossified in
cartilage.
 There are fourteen centers in all,
 Six for the presphenoid
 Eight for the postsphenoid.

99. Presphenoid.—
 About the ninth week of fetal life an ossific center
appears for each of the small wings
(orbitosphenoids) just lateral to the optic foramen.
 The sphenoidal concha are each developed from a
center which makes its appearance about the fifth
month.
 At birth they consist of small triangular lamina,

100.  And in the third year that they become hollowed
out and cone shaped
 About the fourth year they fuse with the
labyrinths of the ethmoid
 And between the ninth and twelfth years they
unite with the sphenoid.

101. Postsphenoid.—
 The first ossific nuclei for the great wings are --
alisphenoids
 One makes its appearance in each wing between
the foramen rotundum and foramen ovale about
the eighth week.
 The orbital plate and that part of the sphenoid
which is found in the temporal fossa, as well as the
lateral pterygoid plate, are ossified in membrane .

102.  The presphenoid is
united to the
postsphenoid about the
eighth month and at
birth the bone is in three
pieces-
 A central, consisting of
the body and small wings,
and
 Two lateral, each
comprising a great wing
and pterygoid process

103. 6. Ethmoid bone
 The ethmoid bone is exceedingly light and spongy, and
cubical in shape.
 It is situated at the anterior part of the base of the
cranium, between the two orbits, at the roof of the nose,
and contributes to each of these cavities. It consists of
four parts:
 Cribriform plate
 Perpendicular plate
 Two lateral masses

104. Ossification.—
 The ethmoid is ossified in the cartilage of the
nasal capsule by three centers: - one for the
perpendicular plate, and one for each labyrinth.
 The labyrinths are first developed,
 At birth, the bone consists of the two labyrinths,
which are small and ill-developed.

105.  During the first year after birth the
perpendicular plate and crista galli begin to ossify
from a single center and are joined to the
labyrinths about the beginning of the second
year.
 The cribriform plate is ossified partly from the
perpendicular plate and partly from the labyrinths.
 The development of the ethmoidal cells begins
during fetal life.

108. The Facial Bones.
 TWO NASAL
 TWO LACRIMAL
 TWO ZYGOMATIC
 TWO PALATINE
 TWO INFERIOR NASAL CONCHA
 TWO MAXILLA
 VOMER
 MANDIBLE

109. The Nasal Bones
 The nasal bones are two
small oblong bones.
 Varying in size and form
in different individuals.
 They are placed side by
side at the middle and
upper part of the face, and
form, by their junction,
“the bridge” of the nose .

110. Ossification.—
 Each bone is ossified from one center,
 which appears at the beginning of the third
month of fetal life in the membrane
overlying the front part of the cartilaginous
nasal capsule

111. The Maxilla (Upper Jaw)
 The maxilla are the largest
bones of the face and
form upper jaw by their
union.

112.  Each assists in forming
the boundaries of three
cavities-
 The roof of the mouth
 The floor and lateral wall
of the nose
 The floor of the orbit

113. It also enters into the formation of two fossa-
 The infratemporal
 The pterygopalatine
Two fissures-
 The inferior orbital
 Pterygomaxillary.

114. Ossification.—
 The maxilla is ossified in membrane
 It is ossified from two centers only
One for the maxilla proper
One for the premaxilla
 These centers appear during the sixth week of fetal
life and unite in the beginning of the third month.
 But the suture between the two portions persists on
the palate until nearly middle life.

115.  The maxilla was formerly described as ossifying from
six centers-
 Orbitonasal
 Zygomatic
 Palatine
 Premaxillary
 Nasal
 Infravomerine

116. The Lacrimal Bone
 The lacrimal bone, the
smallest and most fragile
bone of the face, is situated
at the front part of the
medial wall of the orbit.
 It has two surfaces and four
borders

117. Ossification.—
 The lacrimal is ossified from a single center,
 Which appears about the twelfth week in the
membrane covering the cartilaginous nasal capsule

118. The Zygomatic Bone
 The zygomatic bone is small and quadrangular, and is
situated at the upper and lateral part of the face:
 It forms the prominence of the cheek.
 Part of the lateral wall
 Floor of the orbit
 Parts of the temporal and infratemporal fossa.

120. Ossification.
 The zygomatic bone is generally described as ossifying
from three centers—
 One for the malar
 Two for the orbital portion
 These appear about the eighth week and fuse about
the fifth month of fetal life.

121. The Palatine Bone
 The palatine bone is
situated at the back part
of the nasal cavity
between the maxilla and
the pterygoid process of
the sphenoid .

123.  It contributes to the walls of three cavities:
1-The floor and lateral wall of the nasal cavity
2-The roof of the mouth
3-The floor of the orbit
 It enters into the formation of two fossa-
1-The pterygopalatine
2- pterygoid fossa
 One fissure-
1- Inferior orbital fissure.

124. Ossification.—
 The palatine bone is ossified in membrane from a single center
 Which makes its appearance about the sixth or eighth week of
fetal life at the angle of junction of the two parts of the bone.
 From this point ossification spreads-
 Medialward to the horizontal part
 Downward into the pyramidal process
 Upward into the vertical part.

125. The Inferior Nasal Concha
 The inferior nasal
concha extends
horizontally along the
lateral wall of the nasal
cavity and consists of a
lamina of spongy bone.
 It has two surfaces, two
borders, and two
extremities.

126. Ossification.—
 The inferior nasal concha is ossified from a single
center, which appears about the fifth month of fetal
life in the lateral wall of the cartilaginous nasal
capsule.

127. The Vomer
 The vomer is situated in
the median plane but its
anterior portion is
frequently bent to one or
other side
 It is thin, somewhat
quadrilateral in shape,
and forms the hinder
and lower part of the
nasal septum

128. Ossification
 At an early period the septum of the nose consists of a
plate of cartilage, the ethmovomerine cartilage
 The postero-superior part of this cartilage is ossified
to form the perpendicular plate of the ethmoid
 Its antero-inferior portion persists as the septal
cartilage

129.  While the vomer is ossified in the membrane covering
its postero-inferior part.
 Two ossific centers, one on either side of the middle
line, appear about the eighth week of fetal life in this
part of the membrane, and hence the vomer consists
primarily of two lamella.
 About the third month these unite below, and thus a
deep groove is formed in which the cartilage is lodged.

130. The Mandible (Lower Jaw)
 The mandible the largest
and strongest bone of the
face, serves for the reception
of the lower teeth.
 It consists of a curved,
horizontal portion,
the body, and two
perpendicular portions, the
rami, which unite with the
ends of the body nearly at
right angles.

131. Ossification.—
 The mandible is ossified in the fibrous membrane
covering the outer surfaces of Meckel’s cartilages.
 These cartilages form the cartilaginous bar of the
mandibular arch and are two in number, a right and a
left.
 Their proximal or cranial ends are connected with the
ear capsules, and their distal extremities are joined to
one another at the symphysis by mesodermal tissue.

132.  They run forward immediately below the condyles
and then, bending downward,
 lie in a groove near the lower border of the bone
 In front of the canine tooth they incline upward to
the symphysis.

133.  Ossification takes place in the membrane covering
the outer surface of the ventral end of Meckel’s
cartilage
 And each half of the bone is formed from a single
center which appears, near the mental foramen,
 About the sixth week of fetal life.

134.  By the tenth week the portion of Meckel’s
cartilage which lies below and behind the incisor
teeth is surrounded and invaded by the membrane
bone.
 Somewhat later, accessory nuclei of cartilage make
their appearance,

135.  A wedge-shaped nucleus in the condyloid process
and extending downward through the ramus;
 A small strip along the anterior border of the
coronoid process;
 And smaller nuclei in the front part of both
alveolar walls and along the front of the lower
border of the bone.

136. The Hyoid Bone
 The hyoid bone is shaped
like a horseshoe
 And is suspended from the
tips of the styloid processes
of the temporal bones by
the stylohyoid ligaments.
 It consists of five segments-
a body, two greater
cornua, and two lesser
cornua.

138. Ossification.—
 The hyoid is ossified from six centers:-
 Two for the body,
 One for each cornu.
 Ossification commences in the greater cornua toward
the end of fetal life,
 In the body shortly afterward
 In the lesser cornua during the first or second year
after birth.

139. THANKS