Old NCERT: Structural Organisation in Animals (For NEET) | Biology Class 11 PDF Download

 Page 1


100 BIOLOGY
In the preceding chapters you came across a large variety of organisms,
both unicellular and multicellular, of the animal kingdom. In unicellular
organisms, all functions like digestion, respiration and reproduction
are performed by a single cell. In the complex body of multicellular
animals the same basic functions are carried out by different groups of
cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? In multicellular animals, a group of similar cells alongwith
intercellular substances perform a specific function. Such an organisation
is called tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.
7.1 ANIMAL TISSUES
The structure of the cells vary according to their function. Therefore, the
tissues are different and are broadly classified into four types : (i) Epithelial,
(ii) Connective, (iii) Muscular and (iv) Neural.
STRUCTURAL ORGANISATION IN
ANIMALS
CHAPTER  7
7.1 Animal Tissues
7.2 Organ and Organ
System
7.3 Earthworm
7.4 Cockroach
7.5 Frogs
2015-16(19/01/2015)
Page 2


100 BIOLOGY
In the preceding chapters you came across a large variety of organisms,
both unicellular and multicellular, of the animal kingdom. In unicellular
organisms, all functions like digestion, respiration and reproduction
are performed by a single cell. In the complex body of multicellular
animals the same basic functions are carried out by different groups of
cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? In multicellular animals, a group of similar cells alongwith
intercellular substances perform a specific function. Such an organisation
is called tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.
7.1 ANIMAL TISSUES
The structure of the cells vary according to their function. Therefore, the
tissues are different and are broadly classified into four types : (i) Epithelial,
(ii) Connective, (iii) Muscular and (iv) Neural.
STRUCTURAL ORGANISATION IN
ANIMALS
CHAPTER  7
7.1 Animal Tissues
7.2 Organ and Organ
System
7.3 Earthworm
7.4 Cockroach
7.5 Frogs
2015-16(19/01/2015)
STRUCTURAL ORGANISATION IN ANIMALS 101
7.1.1 Epithelial Tissue
We commonly refer to an epithelial tissue as epithelium (pl.: epithelia).
This tissue has a free surface, which faces either a body fluid or the outside
environment and thus provides a covering or a lining for some part of the
body. The cells are compactly packed with little intercellular matrix. There
are two types of epithelial tissues namely simple epithelium and
compound epithelium.  Simple epithelium is composed of a single layer
of cells and functions as a lining for body cavities, ducts, and tubes. The
compound epithelium consists of two or more cell layers and has protective
function as it does in our skin.
On the basis of structural modification of the cells, simple epithelium
is further divided into three types. These are (i) Squamous, (ii) Cuboidal,
(iii) Columnar (Figure 7.1).
The squamous epithelium is made of a single thin layer of flattened
cells with irregular boundaries. They are found in the walls of blood vessels
and air sacs of lungs and are involved in functions like forming a diffusion
boundary. The cuboidal epithelium is composed of a single layer of
cube-like cells. This is commonly found in ducts of glands and tubular
parts of nephrons in kidneys and its main functions are secretion and
absorption. The epithelium of proximal convoluted tubule (PCT) of
nephron in the kidney has microvilli. The columnar epithelium is
composed of a single layer of tall and slender cells. Their nuclei are located
at the base. Free surface may have microvilli. They are found in the lining
of stomach and intestine and help in secretion and absorption. If the
columnar or cuboidal cells bear cilia on their free surface they are called
ciliated epithelium (Figure 7.1d). Their function is to move particles or
mucus in a specific direction over the epithelium. They are mainly present
in the inner surface of hollow organs like bronchioles and fallopian tubes.
Figure 7.1 Simple epithelium: (a) Squamous (b) Cuboidal (c) Columnar
(d) Columnar cells bearing cilia
(a)
Flattened cell
Cube-like cell
Tall cell
(b)
(d)
(c)
2015-16(19/01/2015)
Page 3


100 BIOLOGY
In the preceding chapters you came across a large variety of organisms,
both unicellular and multicellular, of the animal kingdom. In unicellular
organisms, all functions like digestion, respiration and reproduction
are performed by a single cell. In the complex body of multicellular
animals the same basic functions are carried out by different groups of
cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? In multicellular animals, a group of similar cells alongwith
intercellular substances perform a specific function. Such an organisation
is called tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.
7.1 ANIMAL TISSUES
The structure of the cells vary according to their function. Therefore, the
tissues are different and are broadly classified into four types : (i) Epithelial,
(ii) Connective, (iii) Muscular and (iv) Neural.
STRUCTURAL ORGANISATION IN
ANIMALS
CHAPTER  7
7.1 Animal Tissues
7.2 Organ and Organ
System
7.3 Earthworm
7.4 Cockroach
7.5 Frogs
2015-16(19/01/2015)
STRUCTURAL ORGANISATION IN ANIMALS 101
7.1.1 Epithelial Tissue
We commonly refer to an epithelial tissue as epithelium (pl.: epithelia).
This tissue has a free surface, which faces either a body fluid or the outside
environment and thus provides a covering or a lining for some part of the
body. The cells are compactly packed with little intercellular matrix. There
are two types of epithelial tissues namely simple epithelium and
compound epithelium.  Simple epithelium is composed of a single layer
of cells and functions as a lining for body cavities, ducts, and tubes. The
compound epithelium consists of two or more cell layers and has protective
function as it does in our skin.
On the basis of structural modification of the cells, simple epithelium
is further divided into three types. These are (i) Squamous, (ii) Cuboidal,
(iii) Columnar (Figure 7.1).
The squamous epithelium is made of a single thin layer of flattened
cells with irregular boundaries. They are found in the walls of blood vessels
and air sacs of lungs and are involved in functions like forming a diffusion
boundary. The cuboidal epithelium is composed of a single layer of
cube-like cells. This is commonly found in ducts of glands and tubular
parts of nephrons in kidneys and its main functions are secretion and
absorption. The epithelium of proximal convoluted tubule (PCT) of
nephron in the kidney has microvilli. The columnar epithelium is
composed of a single layer of tall and slender cells. Their nuclei are located
at the base. Free surface may have microvilli. They are found in the lining
of stomach and intestine and help in secretion and absorption. If the
columnar or cuboidal cells bear cilia on their free surface they are called
ciliated epithelium (Figure 7.1d). Their function is to move particles or
mucus in a specific direction over the epithelium. They are mainly present
in the inner surface of hollow organs like bronchioles and fallopian tubes.
Figure 7.1 Simple epithelium: (a) Squamous (b) Cuboidal (c) Columnar
(d) Columnar cells bearing cilia
(a)
Flattened cell
Cube-like cell
Tall cell
(b)
(d)
(c)
2015-16(19/01/2015)
102 BIOLOGY
Some of the columnar or cuboidal cells
get specialised for secretion and are called
glandular epithelium (Figure 7.2). They
are mainly of two types: unicellular,
consisting of isolated glandular cells (goblet
cells of the alimentary canal), and
multicellular, consisting of cluster of cells
(salivary gland). On the basis of the mode of
pouring of their secretions, glands are
divided into two categories namely
exocrine and endocrine glands. Exocrine
glands secrete mucus, saliva, earwax, oil,
milk, digestive enzymes and other cell
products. These products are released
through ducts or tubes. In contrast,
endocrine glands do not have ducts. Their
products called hormones are secreted
directly into the fluid bathing the gland.
Compound epithelium is made of more
than one layer (multi-layered) of cells and thus
has a limited role in secretion and absorption
(Figure 7.3). Their main function is to provide
protection against chemical and mechanical
stresses. They cover the dry surface of the skin,
the moist surface of buccal cavity, pharynx,
inner lining of ducts of salivary glands and of
pancreatic ducts.
All cells in epithelium are held together with little intercellular material.
In nearly all animal tissues, specialised junctions provide both structural
and functional links between its individual cells. Three types of cell junctions
are found in the epithelium and other tissues. These are called as tight,
adhering and gap junctions. Tight junctions help to stop substances
from leaking across a tissue. Adhering junctions perform cementing to
keep neighbouring cells together. Gap junctions facilitate the cells to
communicate with each other by connecting the cytoplasm of adjoining
cells, for rapid transfer of ions, small molecules and sometimes big molecules.
7.1.2 Connective Tissue
Connective tissues are most abundant and widely distributed in the body
of complex animals. They are named connective tissues because of their
special function of linking and supporting other tissues/organs of the
body. They range from soft connective tissues to specialised types, which
Figure 7.2 Glandular epithelium : (a) Unicellular
(b) Multicellular
unicellular
gland
Multicelluar
gland
(a)
(b)
Figure 7.3 Compound epithelium
Multi-
layered
cells
2015-16(19/01/2015)
Page 4


100 BIOLOGY
In the preceding chapters you came across a large variety of organisms,
both unicellular and multicellular, of the animal kingdom. In unicellular
organisms, all functions like digestion, respiration and reproduction
are performed by a single cell. In the complex body of multicellular
animals the same basic functions are carried out by different groups of
cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? In multicellular animals, a group of similar cells alongwith
intercellular substances perform a specific function. Such an organisation
is called tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.
7.1 ANIMAL TISSUES
The structure of the cells vary according to their function. Therefore, the
tissues are different and are broadly classified into four types : (i) Epithelial,
(ii) Connective, (iii) Muscular and (iv) Neural.
STRUCTURAL ORGANISATION IN
ANIMALS
CHAPTER  7
7.1 Animal Tissues
7.2 Organ and Organ
System
7.3 Earthworm
7.4 Cockroach
7.5 Frogs
2015-16(19/01/2015)
STRUCTURAL ORGANISATION IN ANIMALS 101
7.1.1 Epithelial Tissue
We commonly refer to an epithelial tissue as epithelium (pl.: epithelia).
This tissue has a free surface, which faces either a body fluid or the outside
environment and thus provides a covering or a lining for some part of the
body. The cells are compactly packed with little intercellular matrix. There
are two types of epithelial tissues namely simple epithelium and
compound epithelium.  Simple epithelium is composed of a single layer
of cells and functions as a lining for body cavities, ducts, and tubes. The
compound epithelium consists of two or more cell layers and has protective
function as it does in our skin.
On the basis of structural modification of the cells, simple epithelium
is further divided into three types. These are (i) Squamous, (ii) Cuboidal,
(iii) Columnar (Figure 7.1).
The squamous epithelium is made of a single thin layer of flattened
cells with irregular boundaries. They are found in the walls of blood vessels
and air sacs of lungs and are involved in functions like forming a diffusion
boundary. The cuboidal epithelium is composed of a single layer of
cube-like cells. This is commonly found in ducts of glands and tubular
parts of nephrons in kidneys and its main functions are secretion and
absorption. The epithelium of proximal convoluted tubule (PCT) of
nephron in the kidney has microvilli. The columnar epithelium is
composed of a single layer of tall and slender cells. Their nuclei are located
at the base. Free surface may have microvilli. They are found in the lining
of stomach and intestine and help in secretion and absorption. If the
columnar or cuboidal cells bear cilia on their free surface they are called
ciliated epithelium (Figure 7.1d). Their function is to move particles or
mucus in a specific direction over the epithelium. They are mainly present
in the inner surface of hollow organs like bronchioles and fallopian tubes.
Figure 7.1 Simple epithelium: (a) Squamous (b) Cuboidal (c) Columnar
(d) Columnar cells bearing cilia
(a)
Flattened cell
Cube-like cell
Tall cell
(b)
(d)
(c)
2015-16(19/01/2015)
102 BIOLOGY
Some of the columnar or cuboidal cells
get specialised for secretion and are called
glandular epithelium (Figure 7.2). They
are mainly of two types: unicellular,
consisting of isolated glandular cells (goblet
cells of the alimentary canal), and
multicellular, consisting of cluster of cells
(salivary gland). On the basis of the mode of
pouring of their secretions, glands are
divided into two categories namely
exocrine and endocrine glands. Exocrine
glands secrete mucus, saliva, earwax, oil,
milk, digestive enzymes and other cell
products. These products are released
through ducts or tubes. In contrast,
endocrine glands do not have ducts. Their
products called hormones are secreted
directly into the fluid bathing the gland.
Compound epithelium is made of more
than one layer (multi-layered) of cells and thus
has a limited role in secretion and absorption
(Figure 7.3). Their main function is to provide
protection against chemical and mechanical
stresses. They cover the dry surface of the skin,
the moist surface of buccal cavity, pharynx,
inner lining of ducts of salivary glands and of
pancreatic ducts.
All cells in epithelium are held together with little intercellular material.
In nearly all animal tissues, specialised junctions provide both structural
and functional links between its individual cells. Three types of cell junctions
are found in the epithelium and other tissues. These are called as tight,
adhering and gap junctions. Tight junctions help to stop substances
from leaking across a tissue. Adhering junctions perform cementing to
keep neighbouring cells together. Gap junctions facilitate the cells to
communicate with each other by connecting the cytoplasm of adjoining
cells, for rapid transfer of ions, small molecules and sometimes big molecules.
7.1.2 Connective Tissue
Connective tissues are most abundant and widely distributed in the body
of complex animals. They are named connective tissues because of their
special function of linking and supporting other tissues/organs of the
body. They range from soft connective tissues to specialised types, which
Figure 7.2 Glandular epithelium : (a) Unicellular
(b) Multicellular
unicellular
gland
Multicelluar
gland
(a)
(b)
Figure 7.3 Compound epithelium
Multi-
layered
cells
2015-16(19/01/2015)
STRUCTURAL ORGANISATION IN ANIMALS 103
Fat storage
area
Nucleus
Macro-
phage
Mast
cell
Fibroblast
Collagen
fibers
Plasma
Membrane
include cartilage, bone, adipose, and blood. In all
connective tissues except blood, the cells secrete fibres of
structural proteins called collagen or elastin. The fibres
provide strength, elasticity and flexibility to the tissue.
These cells also secrete modified polysaccharides, which
accumulate between cells and fibres and act as
matrix (ground substance). Connective tissues are
classified into three types: (i) Loose connective tissue,
(ii) Dense connective tissue and (iii) Specialised
connective tissue.
Loose connective tissue has cells and fibres loosely
arranged in a semi-fluid ground substance, for example,
areolar tissue present beneath the skin (Figure 7.4). Often
it serves as a support framework for epithelium. It
contains fibroblasts (cells that produce and secrete fibres),
macrophages and mast cells. Adipose tissue is another
type of loose connective tissue located mainly beneath the
skin. The cells of this tissue are specialised to store fats.
The excess of nutrients which are not used immediately
are converted into fats and are stored in this tissue.
Fibres and fibroblasts are compactly packed in the
dense connective tissues. Orientation of fibres show a
regular or irregular pattern and are called dense regular
and dense irregular tissues. In the dense regular
connective tissues, the collagen fibres are present in rows
between many parallel bundles of fibres. Tendons, which
attach skeletal muscles to bones and ligaments which
attach one bone to another are examples of this tissue.
Dense irregular connective tissue has fibroblasts and
many fibres (mostly collagen) that are oriented differently
(Figure 7.5). This tissue is present in the skin. Cartilage,
Figure 7.4 Loose connective tissue : (a) Areolar tissue  (b) Adipose tissue
(a)
(b)
fibres
Collagen fibre
(a)
(b)
Figure 7.5 Dense connective tissue:
(a) Dense regular
(b) Dense irregular
2015-16(19/01/2015)
Page 5


100 BIOLOGY
In the preceding chapters you came across a large variety of organisms,
both unicellular and multicellular, of the animal kingdom. In unicellular
organisms, all functions like digestion, respiration and reproduction
are performed by a single cell. In the complex body of multicellular
animals the same basic functions are carried out by different groups of
cells in a well organised manner. The body of a simple organism like
Hydra is made of different types of cells and the number of cells in each
type can be in thousands. The human body is composed of billions of
cells to perform various functions. How do these cells in the body work
together? In multicellular animals, a group of similar cells alongwith
intercellular substances perform a specific function. Such an organisation
is called tissue.
You may be surprised to know that all complex animals consist of
only four basic types of tissues. These tissues are organised in specific
proportion and pattern to form an organ like stomach, lung, heart and
kidney. When two or more organs perform a common function by their
physical and/or chemical interaction, they together form organ system,
e.g., digestive system, respiratory system, etc. Cells, tissues, organs and
organ systems split up the work in a way that exhibits division of labour
and contribute to the survival of the body as a whole.
7.1 ANIMAL TISSUES
The structure of the cells vary according to their function. Therefore, the
tissues are different and are broadly classified into four types : (i) Epithelial,
(ii) Connective, (iii) Muscular and (iv) Neural.
STRUCTURAL ORGANISATION IN
ANIMALS
CHAPTER  7
7.1 Animal Tissues
7.2 Organ and Organ
System
7.3 Earthworm
7.4 Cockroach
7.5 Frogs
2015-16(19/01/2015)
STRUCTURAL ORGANISATION IN ANIMALS 101
7.1.1 Epithelial Tissue
We commonly refer to an epithelial tissue as epithelium (pl.: epithelia).
This tissue has a free surface, which faces either a body fluid or the outside
environment and thus provides a covering or a lining for some part of the
body. The cells are compactly packed with little intercellular matrix. There
are two types of epithelial tissues namely simple epithelium and
compound epithelium.  Simple epithelium is composed of a single layer
of cells and functions as a lining for body cavities, ducts, and tubes. The
compound epithelium consists of two or more cell layers and has protective
function as it does in our skin.
On the basis of structural modification of the cells, simple epithelium
is further divided into three types. These are (i) Squamous, (ii) Cuboidal,
(iii) Columnar (Figure 7.1).
The squamous epithelium is made of a single thin layer of flattened
cells with irregular boundaries. They are found in the walls of blood vessels
and air sacs of lungs and are involved in functions like forming a diffusion
boundary. The cuboidal epithelium is composed of a single layer of
cube-like cells. This is commonly found in ducts of glands and tubular
parts of nephrons in kidneys and its main functions are secretion and
absorption. The epithelium of proximal convoluted tubule (PCT) of
nephron in the kidney has microvilli. The columnar epithelium is
composed of a single layer of tall and slender cells. Their nuclei are located
at the base. Free surface may have microvilli. They are found in the lining
of stomach and intestine and help in secretion and absorption. If the
columnar or cuboidal cells bear cilia on their free surface they are called
ciliated epithelium (Figure 7.1d). Their function is to move particles or
mucus in a specific direction over the epithelium. They are mainly present
in the inner surface of hollow organs like bronchioles and fallopian tubes.
Figure 7.1 Simple epithelium: (a) Squamous (b) Cuboidal (c) Columnar
(d) Columnar cells bearing cilia
(a)
Flattened cell
Cube-like cell
Tall cell
(b)
(d)
(c)
2015-16(19/01/2015)
102 BIOLOGY
Some of the columnar or cuboidal cells
get specialised for secretion and are called
glandular epithelium (Figure 7.2). They
are mainly of two types: unicellular,
consisting of isolated glandular cells (goblet
cells of the alimentary canal), and
multicellular, consisting of cluster of cells
(salivary gland). On the basis of the mode of
pouring of their secretions, glands are
divided into two categories namely
exocrine and endocrine glands. Exocrine
glands secrete mucus, saliva, earwax, oil,
milk, digestive enzymes and other cell
products. These products are released
through ducts or tubes. In contrast,
endocrine glands do not have ducts. Their
products called hormones are secreted
directly into the fluid bathing the gland.
Compound epithelium is made of more
than one layer (multi-layered) of cells and thus
has a limited role in secretion and absorption
(Figure 7.3). Their main function is to provide
protection against chemical and mechanical
stresses. They cover the dry surface of the skin,
the moist surface of buccal cavity, pharynx,
inner lining of ducts of salivary glands and of
pancreatic ducts.
All cells in epithelium are held together with little intercellular material.
In nearly all animal tissues, specialised junctions provide both structural
and functional links between its individual cells. Three types of cell junctions
are found in the epithelium and other tissues. These are called as tight,
adhering and gap junctions. Tight junctions help to stop substances
from leaking across a tissue. Adhering junctions perform cementing to
keep neighbouring cells together. Gap junctions facilitate the cells to
communicate with each other by connecting the cytoplasm of adjoining
cells, for rapid transfer of ions, small molecules and sometimes big molecules.
7.1.2 Connective Tissue
Connective tissues are most abundant and widely distributed in the body
of complex animals. They are named connective tissues because of their
special function of linking and supporting other tissues/organs of the
body. They range from soft connective tissues to specialised types, which
Figure 7.2 Glandular epithelium : (a) Unicellular
(b) Multicellular
unicellular
gland
Multicelluar
gland
(a)
(b)
Figure 7.3 Compound epithelium
Multi-
layered
cells
2015-16(19/01/2015)
STRUCTURAL ORGANISATION IN ANIMALS 103
Fat storage
area
Nucleus
Macro-
phage
Mast
cell
Fibroblast
Collagen
fibers
Plasma
Membrane
include cartilage, bone, adipose, and blood. In all
connective tissues except blood, the cells secrete fibres of
structural proteins called collagen or elastin. The fibres
provide strength, elasticity and flexibility to the tissue.
These cells also secrete modified polysaccharides, which
accumulate between cells and fibres and act as
matrix (ground substance). Connective tissues are
classified into three types: (i) Loose connective tissue,
(ii) Dense connective tissue and (iii) Specialised
connective tissue.
Loose connective tissue has cells and fibres loosely
arranged in a semi-fluid ground substance, for example,
areolar tissue present beneath the skin (Figure 7.4). Often
it serves as a support framework for epithelium. It
contains fibroblasts (cells that produce and secrete fibres),
macrophages and mast cells. Adipose tissue is another
type of loose connective tissue located mainly beneath the
skin. The cells of this tissue are specialised to store fats.
The excess of nutrients which are not used immediately
are converted into fats and are stored in this tissue.
Fibres and fibroblasts are compactly packed in the
dense connective tissues. Orientation of fibres show a
regular or irregular pattern and are called dense regular
and dense irregular tissues. In the dense regular
connective tissues, the collagen fibres are present in rows
between many parallel bundles of fibres. Tendons, which
attach skeletal muscles to bones and ligaments which
attach one bone to another are examples of this tissue.
Dense irregular connective tissue has fibroblasts and
many fibres (mostly collagen) that are oriented differently
(Figure 7.5). This tissue is present in the skin. Cartilage,
Figure 7.4 Loose connective tissue : (a) Areolar tissue  (b) Adipose tissue
(a)
(b)
fibres
Collagen fibre
(a)
(b)
Figure 7.5 Dense connective tissue:
(a) Dense regular
(b) Dense irregular
2015-16(19/01/2015)
104 BIOLOGY
bones and blood are various types of specialised
connective tissues.
The intercellular material of cartilage is solid and pliable
and resists compression. Cells of this tissue (chondrocytes)
are enclosed in small cavities within the matrix secreted by
them (Figure 7.6a). Most of the cartilages in vertebrate
embryos are replaced by bones in adults. Cartilage is
present in the tip of nose, outer ear joints, between adjacent
bones of the vertebral column, limbs and hands in adults.
Bones have a hard and non-pliable ground substance
rich in calcium salts and collagen fibres which give bone
its strength (Figure 7.6b). It is the main tissue that provides
structural frame to the body. Bones support and protect
softer tissues and organs. The bone cells (osteocytes) are
present in the  spaces called lacunae. Limb bones, such as
the long bones of the legs, serve weight-bearing functions.
They also interact with skeletal muscles attached to them
to bring about movements. The bone marrow in some bones
is the site of production of blood cells.
Blood is a fluid connective tissue containing plasma,
red blood cells (RBC), white blood cells (WBC) and platelets
(Figure 7.6c). It is the main circulating fluid that helps in
the transport of various substances. You will learn more
about blood in Chapters 17 and 18.
7.1.3 Muscle Tissue
Each muscle is made of many long, cylindrical fibres
arranged in parallel arrays. These fibres are composed of
numerous fine fibrils, called myofibrils. Muscle fibres
contract (shorten) in response to stimulation, then relax
(lengthen) and return to their uncontracted state in a
coordinated fashion. Their action moves the body to adjust
to the changes in the environment and to maintain the
positions of the various parts of the body. In general,
muscles play an active role in all the movements of the body.
Muscles are of three types, skeletal, smooth, and cardiac.
Skeletal muscle tissue is closely attached to skeletal
bones. In a typical muscle such as the biceps, striated
(striped) skeletal muscle fibres are bundled together in a
parallel fashion (Figure 7.7a). A sheath of tough connective
tissue encloses several bundles of muscle fibres (You will
learn more about this in Chapter 20).
Platelets
WBC
RBC
(a)
(b)
(c)
Figure 7.6 Specialised connective
tissues : (a) Cartilage
(b) Bone  (c) Blood
2015-16(19/01/2015)