Animal Tissue, Types of Animal Tissue

Table of Contents

Animal Tissue Types of Animal Tissue

In unicellular organisms the body consists of a single cell which performs all the functions of life viz. nutrition, respiration, excretion, metabolism, reproduction etc. In multicellular organisms (metazoans), the body is formed of large number of cells which become specialised to perform different functions. For example, muscle cells help in locomotion, nerve cells for conduction of impulses etc. To carry on these functions, the cells become morphologically different. This is known as cell differentiation. As different cells perform different functions of life unlike the unicellular organisms this phenomenon is termed as division of labour. In a multicellular body with higher degrees of differentiation, specialization and division of labour, cells group together to form tissues which take up specific functions of the body. Hence, an assemblage of the specific types or cels having same origin, structure, function and mode of development constitute a tissue. A particular type of tissues serves a particular function. Some tissues bring about movements or the Day parts and help in locomotion, some secrete juices, some conduct information and coordinate various activities of the body, etc.

different tissues collectively make an organ to serve a specific purpose. For example stomach is an organ to algest 10u, Kaney is an organ meant for the removal of metabolic wastes.

An organ system consists of Varlous organs which carry out major functions of the body. digestive system consists of mouth, buccal cavity, pharynx, oesophagus, stomac For e liver, pancreas, rectum and anus; respiratory System consists of external nostril, interna nostril, epiglottis, trachea, bronchi, lungs etc. The other organ systems of the body are a nostril, epiorent organs. All these systems are performing a major function of the Do formed by different organs. All these systems are performing a major function of the body organ systems of the body are also Various of these organ systems constitute the body.

ANIMAL TISSUE

The major organ systems with their main functions in a body are as follows

1. Integumentary System. It consists of skin and its derivatives. It protects the body external injury. body from
2. Skeletal System. It is formed of bones and cartilages. It supports and protects the body. It maintains shape and size of the body.
3. Muscular System. It consists of muscles which help in body movements and locomotion.
4. Digestive System. It is formed of mouth, buccal cavity, pharynx, oesophagus, small intestine, large intestine, liver and pancreas. It helns in digestion and absorption oi food.
5. Circulatory System. It consists of heart, blood vessels (arteries, veins, capillaries). Transportation of food and oxygen is carried out by this system to different parts of the boay. It transports metabolic wastes to excretory organs for excretion.
6. Respiratory System. It consists of various parts involved in the exchange of gases with the atmosphere. This system includes nostril, wind pipe, bronchial tubes and lungs. It helps in gaseous exchange in the organism.
7. Excretory System. Kidneys, ureters, urinary bladder and urethra together form this system. Its primary function is the removal of metabolic wastes from the body.
8. Endocrine System. It consists of ductless or endocrine glands which secrete hormones. The hormones control and coordinate the various body activities like growth, metabolism, reproduction, behaviour etc.
9. Nervous System. It consists of brain, spinal cord, nerves and sense organs. It coordinates the various activities of different organs of the organism. It serves as a link between our body with the outer environment.
10. Reproductive System. It consists of sex organs like testes in male and ovary in female. The sex organs produce sex cells and carry out reproduction to produce offsprings for the continuance of race. lgane

Animal Tissues

Tissue is defined as a group of structurally similar cells having common embryonic origin and performing same function or functions. The term tissue was introduced by a French anatomist Francois Xavier Bichat (also known as Father of Histology). The study of tissue is called Histology was coined by Mayer (1819). The tissues of two or more types unite to form larger functional units, called organs. The organs and associated structures form organ systems. The organ systems together constitute the living body of multicellular animals.

Based on the location and function, the animal tissues are classified into four types. They are……

(1) epithelial tissue
(2) connective tissue
(3) muscular tissue
(4) nervous tissue.

STRUCTURE AND FUNCTION OF EPITHELIAL TISSUE

STRUCTURE

The term epithelium was given by Dutch anatomist Raysch in 18th century. The epithella tissue has the following features
(a) The cells are many and closely placed without intercellular space and intercelular substance (matrix).
(b) The cells rest upon a noncellular gelatinous basement membrane. cells
(c) formed The cells of glycoprotein6 together by a cementing substance (secreted by the cells) formed of glycoprotein. activities.
(e) The cell membranes body surface and organs and lines the body cavities
(d) The epithelium forms covering of body surface and organs and lines the other by desmosomes or interdigitatisnt cells are firmly connected to each oher Dy
(e) The cell membranes of adjacent cells are firmly connected to each desmosomes or interdigitations.
(Epithelial cells possess the power of regeneration.
(g) Epithelium originates from all three germinal layers of the embryo (ectoderm,mesoderm and endoderm).

Funcations

(a) It protects the underlying tissue from dehydration and mechanical or chemical injury,
b) It helps in absorption, excretion, secretion, respiration and perception.

TYPES OF EPITHELIUM

Simple Epithelium

It consists of a single layer of cells. It is of following types

1. Squamous Epithelium

It is formed of a single layer of flattened or scale-like (squama-scale) polygonal cells. The cells are closely fitted like the tiles on a mosaic floor and hence known as pavement epithelium.

It is also known as Tessalated epithelium due to its wavy appearance.

ANIMAL TISSUE

Occurrence It forms the lining of blood vessel and lymphatic vessel and is called andothelium. It lines the boay cavity and is known as peritoneum. It forms outer covering of visceral organs esothelium). It lines Bowman’s capsules of nephron, alveoli in lungs, is lahrinth of internal ear, lens of eyes, eye lid, taste bud, mesentery. The outer most layer of frog’s skin is also formed of squamous epithelium.
Function. It helps in protection, absorption, ultra filtration and gaseous exchange.

2. Cuboidal Epithelium

The cells are cube like with central rounded nuclei.
Occurrence. Acini of thyroid gland, pancreas, thymus, liver, sweat gland, convoluted part of uriniferous tubule, bronchi of lungs, internal ear lining, lining of germinal layer, ciliary body of eye are lined by cuboidal epithelium.

Function. It performs the function of secretion, storage of glycogen, excretion.

3. Columnar Epithelium
   The cells are long and pillar like their height exceedss
   their width. The nuclei of the cells are generally elongated
   and lie in the basal part.

intestinal Occurrence. and gastric Mucosa glands layer and of vas alimentary deferens canal are lined from

4. Ciliated Epithelium
   When the cuboidal or columnar epithelium bear cilia (hair like protoplasmic processes) at their free end they form ciliated epithelium.

Unicellular and multicellular glands.

Occurrence. The cuboidal ciliated epithelium occur in the neck region of uriniferous tubules where it keeps the urine moving. The columnar ciliated epithelium lines oviduct,
retina of eye and buccopharyngeal cavity of frog.
Function. Conduction of mucus or some other fluid is their main function.

Glandular Epithelium

The cells of columnar epithelium are modified to secrete some substance forming glandular epithelium.
It is of two types: unicellular and multicellular.

A. Unicellular Glands. These are formed of single cells which secrete mucus. Such cells are called goblet cells.
Occurrence. It occurs in the mucosa layer of stomach, intestine and rectum.

B. Multicellular Glands. These are formed of many cells. These glands are formed by inpushing of epithelium. Their classification is based on their structure, nature of secretion and presence or absence of duct.

Classification Based on Structure of the Gland

Multicellular glands are classified into 3 types based on the shape of the secretory unit They are tubular gland, acinous gland and alveolar gland.

(A) Tubular Glands. The secretory unit is tube like. It is of three types:

(i) Simple Tubular Glands. They are in the form of a simple tubes, found in the large
intestine.
(ii) Simple Coiled Tubular Glands. The glandular part (bottom part) is coiled as in sweat glands (sudorufic glands) of skin.
(iii)Compound Tubular. These are branched tubular glands found in fundic gland (gastric gland) of stomach.

(B) Acinous Gland. Secretory units are rounded.

(i) Simple Acinous Gland. Unbranched as in cutaneous glands of frog’s skin.
(ii) Compound Acinous Gland. Branched e.g., liver, some gastric glands.

(C) Alveolar Glands. Secretory units flask-shaped.

(i) Simple Alveolar Glands. Unbranchede.g., some sebaceous glands.
(ii) Compound Alveolar Glands. Branched e.g salivary glands of mammnals. pancreas.

(D) Tubulo-alveolar Glands. It is mixed type.e.g, mammary gland, Bartholin’s gland and Cowper’s gland.

Based on Nature of Secretion

A. Serous Glands. Secretes clear, watery fluide.g., sweat gland, intestinal glands a. parotid glands. Mucous Glands. Secretes mucus (mixture of mucinogen and water) which is

B. proteinaceous slimy substance. e.g. goblet cells, cardiac and pyloric glands of stomach.

C. Mixed Glands. Secretes both serous and mucus e.g., most gastric glands, submandibular (submaxillary) salivary glands, pancreas.

Based on the Presence or Absence of Ducts.

A Exocrine Glands. Having ducte,g., Liver.

B. Endocrine Glands. Without ducte.g, pituitary, thyroid, adrenal, parathyroid.

Based on Mode of Secretion

A. Holocrine Glands. An entire cell, when filled with secretory product, is discharged and dies, e,8, Sebaceous or oil secretory gland in mammals.

B. Apocrine Glands. Secretory product, when collected in the apical part, it breaks off Causing partial damage to the cel, e.g mammary glands.

C. Epicrine or Merocrine Glands. Secretory product, after collected in the gland cells diffuse across cell membrane without causing damage to the cell, e.g., sweat glands,
salivary glands and goblet cells.

Sensory Epithelium

It is composed of modified columnar cells having sensory hairs at their tips and sensory nerve fibres at base.
Occurrence. Retina of eye, nasal epithelium (Schneiderian membrane) lining of internal ear and mucous membrane of tongue.

Function. Reception of stimuli of different kinds. kerate

Germinal Epithelium

It is modified cuboidal epithelium. Its cells undergo repeated division to produce gamereS. Occurrence. It lines seminiferous tubules of testis and ovary Function. Produce sex cells (sperms and ova).

Pseudostratified Epithelium

It consists of two kinds of cells, tall and small cells, arranged in a single layer but gives the false appearance of being multilayered. Its cells are mostly ciliated.
Occurrence. Nasal cavities, trachea and bronchi, vasa deferentia, epididymis of testis, male urethra. Function. Like ciliated columnar epithelium it conducts mucus and other fluids.

Pigmented Epithelium

The cells of this epithelium contain pigments. It forms basal layer of retina of eye.

Compound Epithelium

Compound or Stratified Epithelium composed of more than one layer of cells (bi-or multilaminar) and occur at places where protection is the main function. The basal layer or germinative layer is a living layer of cells undergoing repeated mitosis to replace cells on the surface, lost through wear and tear. It is of following types

(i) Stratified Squamous.

It is found in the epidermis of skin, lining of buccal cavity, pharynx, oesophagus and vagina only. Epidermis of skin
is keratinized due to deposition of a fibrous protein called keratin to make it water proof where as other parts are nonkeratinized.

(ii) Stratified Cuboidal.

It is found in conjunctiva of eyes, lining of ducts of sweat glands mammary glands and female urethra.

(iii) Stratified Columnar.

It is found in some parts of larynx, epiglottis, ducts of large glands (e.8, parotid and mammary glands)

(iv) Transitional Epithelium.

It is found in the wall of urinary bladder, ureters and upper part of urethra. It is remarkably adapted for mechanical changes due to contraction and distension. There is no basement membrane. 1he cells lie on the connective tissue,

CONNECTIVE TISSUE STRUCTURE

The Connective tissue is the binding and supporting tissue having following features

1. it connects other tissues and organs in the body.
2. t he cells are few in number, scattered having drage intercellular spaces filled with intercenular Substance or matrix.
3. The matrix is non-living, homogenous, containing fibres. It is mainly composed of protein Combinations of carbohydrate and glycoproteins and proteoglycans).
4. Basement membrance is absent.
5. from embrvonic connective tissue called mesenchyme (this term was used by Hertwig in 1883). The mesenchyme develops from embryonic mesoderm.

Functions

1.it binds different tissues, organs and act as a packing material.

2. It stores fat (adipose tissue).
3. lt torms a supporting frame work (skeleton) of the body. It is called supporting tissue.
4. It protects the body against germs and toxins. ConnectIve Tissue Proper

(a) Areolar Tissue. Areolar tissue is the most abundant tissue in the body forming frame for all body organs. It acts as packing material between organs. It consists of two types of work ares and generally tour types of cells. The two types of fibres are

(a) white collagen fibres
(b) yellow elastic fibres.

The four types of cells are..

(i) Fibroblasts
(ii) Histiocytes or Macrophages
(iv) Lymphoid cells or Lymphocytes.
(iii) Mast cell

Fibroblasts. These are most numerous and largest among the four types of cells. These are flattened cells secreting fibres. Old and inactive fibroblasts are called as fibrocytes. These cells play important role in healing of wounds and growth and repair of tissues.

Histiocytes or Macrophages. These are next to fibroblasts in size and in number. They are iregular and phagocytic. They destroy dead cells and bacteria killing infection.

Mast cell. These are comparatively small and roughly spherical cells having granular cytoplasm and big nucleus. They secrete three active substances: heparin (an anticoagulant preventing blood clotting within normal blood vessels), serotaonin (a protein that acts as a vasoconstrictor (It constricts blood vessel to arrest bleeding and io increase blood pressure) and histamine (also a protein that acts as a vasodilaior. It widens blood vessel).

Lymphoid Cells or Lymphocytes. These are the smallest, less numerous and irregular cells resembling the lymphocytes of blood and lymph. They form and carry antibodies.

Besides these 4 types of cells plasma cells (synthesize antibodies) and adipose or fat storing cells are also present. As stated earlier the fibres are of two types: white collagen fibers and yellow elastic fibres. White fibres consist of fibrils arranged to form bundles. They are white in colour and made of a protein called collagen which changes to gelatin when boiled in Water. The yellow fibers are fewer in number, elastic, branched. They never form bundles and Fun straight. They are pale yellow and formed of a protein called elastin.

Occurrence. It is the most abundant tissue occurring in mesenteries, subcutaneous tissues covering around visceral organs.

Functions. It binds visceral organs together. Through the areolar tissue exchange between blood and tissue cells takes place. It protects the body against inflection and helps in maintaintining water balance.

(b) Adipose Tissue. It is a modified areolar tissue specialised to store fat. It containg a large number of fat storing cells called adipocytes and few fibres. In a fat storing cell the oil or fat globule occupy the large central part of the cell pushing the nucleus and cytoplasm to pose . periphery. There are two kinds of adipose tissue; White adipose tissue (b) Brown adipose tissue. White adipose cells contain a single large late droplet whereas brown adipose
cells contain many small fat droplets. The brown colour of fat is due to high concentration of iron, containing cytochrome pigments. It contains many mitochondria and produce more ENERGY than white fat (twenty times more than white fat).

Occurrence. Brown fat occurs largely in new born mammals (5-6% of the body weight rats and other rodents. White fat are found beneath skin of mammals, blubber (thick layer of under the skin) of whale and elephant, hump of camel, around kidney of vertebrate and yellow bone marrow and fat bodies of frogs.

Function. The adipose tissue contains reserve food, insulates the body against heat lose (thermo-insulator), protects visceral organs against shock and injury, protects joints agains friction. The brown fat yield more energy for which new -born babies do not shiver durining winter.

(C) White Fibrous Tissue. it is composed of bundles of thick collagen fibres with less number of fibroblasts. It is represented by tendon (which connects skeletal muscles to bones and most ligament (which connects bone to bone).

Occurrence. Urinary bladder, dermis of skin (form leather through tanning) and wall of intestine.

Function. Mechanical.
(d) Yellow Elastic Tissue. It is formed mostly of yellow elastic fibres which run parallel. The fibroblasts are scattered among fibres.

Occurrence. Certain Ligaments, wall
of blood vessels and lungs.

Function. Because of its elasticity it provides flexibility to joints.

(e) Reticular Tissue. It is formed of starshaped reticular cells. From these cells large number of protoplasmic processes arise and join to form a net work. The
network of fibres is made of a protein called reticulin. The spaces of the network are filled with lymph.

Occurrence. It forms the supporting frame work of lymph glands, spleen and bone marrow tonsil, thymus.

Function. It supports lymph glands, spleen and bone marrow.

(f) Pigmented Tissue. It consists of large branched (stellate) cells called chromaduced or melanophores containing pigment granules or melanin. The melan in pigments are proud from cells called melanocytes. The pigments may be black, brown, yellow, white and blue.

Occurrence. Dermis of skin, iris and choroid layer of eye.

Function. This tissue in skin protects internal organs from the injurious effects of sunlight.

(g) Mucous Connective Tissue. It is mostly found in embryos and formed of a gelatinous substance called Wharton’s jelly. It is found in umblical cord of foetus, cock’s comb, vitreous body of eye ball.

Skeletal tissue

The skeletal tissues in vertebrates occur in three forms: chordal tissue in notochord, cartilage and bone. They form endoskeleton, protect internal organs, provide surfaces for muscle attachment, give shape to body and act as levers for movement of limbs.

(a) Notochord. It is formed of chordal tissue which have chordal cells. The chrodal cells have big vacuoles filled with a fluid by which the cells are kept turgid. Such vacuolated cells are surrounded by a single layer of peripheral cells. Around the peripheral cells has an inner elastic sheath and an outer elastic sheath.

Occurrence. Notochord is found in all chordates. In adult vertebrates it is replaced by vertebral column whereas it is very much present in embryo.

Function. It forms the supporting axial skeleton in the chordate body.

(b) Cartilage. Cartilage is a flexible, elastic skeletal tissue which can withstand stress and strain. It consists of cartilage cells or chondrocytes (= chondriocytes) which lie within fluid filled spaces called lacunae (sing. lacuna). The matrix or ground substance is a transparent, homogenous substance called chondrin. It is formed of a glycoprotein called chondromucoid. The cartilage is covered externally by a white fibrous connective tissue called perichondrium. The food and OXygen are supplied by means of blood vessels to nerichondrium from where they diffuse through matrix to reach the chondrocytes. Below perichondrium are cartilage-forming cells called chondroblasts Chondrioblasts) which divide repeatedly adding new chondrocytes. The chondrocytes secrete chondrin or matrix. In the matrix the collagen fibres are present which give flexibility to it. The cartilage is of following three types.

(i) Hyaline Cartilage It is glassy bluish- white in colour. Its matrix is homogenous, fibreless and transluscent.

Occurrence. It forms embryonic skeleton. In adult it is found at the end of limb bone, ribs, in nasal septum,
larynx, tracheal rings and hyoid apparatus of frog.

(ii) Fibrous Cartilage It contains dense and thick bundles of collagen fibres in the matrix making it toughest among the four types of cartilages.

Occurrence. Intervertebral disc of mammals, pubic symphysis of pelvic girdle. (in Elastic Cartilage. It contains yellow elastin fibers in the matrix which provide elasticity to it. It elastic cartilage. is firm, flexible and pliable.

Occurrence. Ear pinna, tip of the nose of mammals, eustachian tube, epiglottis.

(iv) Calcified Cartilage. The matrix contains calcium salts (calcium carbonate) as a result of which it is inelastic and hard. It is a modified hyaline cartilage.

Occurrence. Suprascapula of pectoral girdle and pubis of pelvic girdle of frog.

(c) BONE (COLOURED PLATE-). Bone forms the major part of the vetebrate endoskeleton. It Is hard, highly calcified, rigid connective tissue. The matrix is dense and hard. About 70% of dry weight of it is formed of inorganic salts (mainly calcium phosphate and calcium carbonate and traces of and fluoride calcium magnesium chloride) and the remaining 30% is formed of protein called ossein. The bone cells are callea osteo-cytes. They are spider shaped and of having a number protoplasmic processes called canaliculi. By means of canaliculi the neighbouring Osteocytes exchange material in between them. The osteocytes are present inside fluid filled cavities called lacunae.

The bone of mammals is different from that of frog and other vertebrates In course of evolution the mammals have attained larger body size and hence, their bones are thicker and stronger. The osteocytes are arranged concentrically around the Haversian canal and such canals open into the marrow cavity. The Heversian canal and lamellae (concentric rings of bony plates) form Haversian system. The longitudinally placed Haversian

canals are connected by means of transverse canals called Volkmann’s canal. The Haversian system provides a mean of distribution of nutrients and oxygen to deeply seated mammalian bone cells.

The bones are of following types based on their structure and modifications:

(i) Spongy Bone (= Cancellous bone).
It consists of small pieces of bones having
spaces between them which contain red bone marroW

Occurrence. Expanded ends of long bone.

(ii) Compact Bone. It is hard and solid without air space.

Occurrance. Shaft (middle part) of long boes.

(iii) Replacing or Cartilage Bone. The bone which replaces pre-existing cartilage by ossification.

Occurrence. Sternum, limb bones, hyoid, vertibrae.

(iv) Membrane Bone. (= investing bone) Bones are not formed from pre-existing cartilage but are laid
down by the ossification of dermis of skin.

Occurrence. Skull bones, phalanges, clavicle.

(v) Sesamoid Bone. When tendon and ligaments are ossified they form sesmoid bone.

Occurrence. Patella or knee cap, cotyloid bone of pelvic girdle of mammal.

Differences Between Cartilage and Bone.

Cartilage

1.it is formed of cartilage cells or chondrocytes.
2.The matrix is composed mostly of a protein called chondrin.
3.It forms minor part of the endoskeleton of vertebrate body.
4.Since it is flexible it gives elasticity to different organs.

5. Marrow cavity absent.
6. It is not provided with blood vessels (non-vascular).
   7.Cartilage is surrounded by perichondrium.

Bone

1. It is composed of bone cells or osteocytes.
2. Matrix is formed of only 30% of protein (ossein) and 70% inorganic salts.
3. It forms major part of endoskeleton.
4. It is rigid giving definite shape to the body and protecting vital organs.
5. Marrow cavity present.
6. It is richly supplied with blood vessels (vascular).
7. Surrounding bone periosteum is present.

Differences Between Tendon and Ligament.

Tendon

1. It connects bone to striated muscles.
2. It is mainly formed of white collagen fibres.
3. In between the bundles of collagen fibers flat tendon cells present.

Ligament

1. It connects bone to bone at joints and hold them in position.
2. It is mostly formed of yellow elastic fibres
3. The ligament cells lie between the fibres

Differences Between EPITHELIAL and CONNECTIVE TISSUE

Epithelial Tissue

1. It consists of large number of cells with- out intercellular space and matrix.
2. Basement membrane present.
3. Epithelium forms the covering of the body, organs and line the cavities and organs.
4. The constituent cells are held firmly by plasmodesmata and interdigitations.
5. It arises from all the three germinal layers of the embryo (ectoderm, mesoderm and endoderm).

Connective Tissue

1. It is composed of few cells having large intercellular space and matrix.
2. Basement membrane absent.
3. It connects various tissues and organs and forms packing material.
4. These are absent.
5. It originates from the embryonic mesoderm.

BLOOD (AS FLUID CONNECTIVE TISSUE)

Blood and lymph together constitute fluid connective tissue. They have a fluid matri called plasma. The cells or corpuscles float freely in the plasma. There are no fibres in plasma. Unlike other types of connective tissue, the plasma is not secreted by the corpuscle Hence, blood and lymph are not considered as true connective tissue.

BLOOD. 💉🩸

Blood is a red coloured fluid tissue that circulates in our body. It is a slightly alkaline fluid having the plH 7.3-7.5, Arterial blood is more alkaline than the venous blood. It is heavier than water (specific gravity 1.055-1.060) and a viscous liquid. It constitutes 7% of the total body weight of man. Its volume is about 6-7 litres in an adult human body. Blood consist of two components

(a) a liquid part or plasma,
(b) a solid part or corpuscles or blood cells.

PLASMA

It is a straw coloured or faint yellow coloured fluid. It forms about 60% of the total volume of blood and about 50% of the blood weight. The plasma is composed of the following constituents:

(a) Water 90% to 92%
(b) Inorganic Constituents- about 0.9%. These include chlorides, bicarbonates, phosphates and sulphates of sodium, potassium, calcium, magnesium and iron. Chlorides and carbonates of sodium are most abundant.

(c) Organic Constituents- about 8.1%

(i) Plasma Proteins or Serum Proteins. Total amount of plasma proteins are about 6.8% of plasma. These proteins contribute to maintain blood viscosity, blood osmotic pressure and blood volume. These plasma proteins include serum (derived globulins, from serum the albumins, fibrinogen and prothrombin, Serum globulins (derived from the spnocytes) help in osmoregulation, transport of proteins and other substances, and torm an essential part of immunity system (gamma globulins of nnanogiODulins). These act as antibodies and destroy toxic substances, viruses and Dalild. orum albumin constitutes the major part of the total plasma protein and is responsible for the osmotic pressure of the blood. Fibrinogen and prothrombin (7%) are necessary for the coagulation of blood.

(ii) Nutrients. In addition to different plasma proteins, certain other organic substances such as glucose, neutral fats and lipids, lactic acid, pyruvic acid, cholesterol, and different enzymes and hormones are also present in the plasma. Glucose is the main nurve substance present in the plasma. Normal glucose or blood sugar level is 80- 100 mg per 100 ml 12 hours after meal. Its concentration is highest one and half hours after a carbohydrate-rich meal and is about 100 mg per 100 ml. Fasting sugar level is about 6 mg or below. If blood sugar level exceeds 180 mg per 100 ml consistently, then glucose starts appearing in the urine. This condition is known as glucosauria. Hyperglycemia indicates blood glucose level above 160 mg while hypoglycemia indicates blood sugar level below 80 mg.
Cholesterol level in human blood normally ranges from 150 to 180 mg per 100 ml of plasma. Cholesterol is used for the synthesis of new cell membranes, Vitamin D, bile salts and steroid hormones. Too much eating of saturated fats such as butter. ghee and vanaspati increases cholesterol level in blood because these are rich in saturated fatty acids. Cholesterol and its esters are insoluble in water and so are deposited on the inner walls of arteries and veins. This leads to blocking of blood vessels supplied to heart causing heart trouble and rise of blood pressure.

(iii) Waste Products. Plasma also contains excretory substances such as urca, uric acid, a100nia, creatine and creatinine etc. Normally plasma contains 17-30 mg of urea per 100 ml of blood. Renal or cardiac failure may reduce the elimination ofurea from the body which ultimately leads to rise in the blood urea level. This condition is
known as uremia.
(iv) Hormones, enzymes, antigens and antibodies are also present in plasma.
(v) Gases. Oxygen, carbon dioxide and nitrogen are found to be present in dissolved
condition in plasma.

functions of Plasma Proteins

1. They serve as a source of proteins for the tissue cells.
2. These proteins may be utilized 1or the formation of other cellular proteins.
3. Plasma proteins maintain the acid-base equilibrium of the blood.
4. .Albumin and globulin maintain osmotic pressure of blood by retaining water. A fall in plasma protein leads to the escape of excessive volumes of water from blood to tissue causing Swelling of feet oedema (accumulation water in the tissue).
5. Globulin is responsible for the immunity of the body and fibrinogen is essential for blood coagulation.
6. Plasma proteins help to distibution heat all over the body and also conducting heat to SKin for its loss to atmmosphere.

Differences between plasma and serum

Plasma

(a) It is the fluid part of b.ood in which blood ce:ls float.
(b) It has fibrinogen.
(c) It helps in blood clotting
(d) It flows within closed blood vessels.
(e) Quantitatively it is more.

Serum

(a) It is also a blood fluid which is formed after clotting.
(b) No fibrinogen.
(c) It does not help in blood clotting due the absence of fibrinogen.
(d) It oozes at the site of injury.
(e) It is less.

BLOOD CORPUSCLES

The cellular components of blood include three type Or cells or corpuscles which constitute about 40% of the total blood. They form the livin8 part of a blood. These three types of corpuscles present in blood are : erythrocytes or red blood puscles, leucocytes or white blood corpuscles and blood platelets.

[I] ERYTHROCYTES or RED BLOOD CORPUSCLES (R.B.C.)

(Coloured Plate II)

The mature human erythrocyte is a circular, biconcave, non nucleated disc. It measures about 7-84 (lu = 106m) in diameter and 2u in thickness near the rim. In the mature mammalia erythrocyte cellular organelles including the nucleus (it is present in immature R.B.C mitochondria, endoplasmic reticulum etc. are absent. Thus, most part of the cell is beino Occupied by haemoglobin. The size and shape of erythrocytes vary in different groups o vertebrates. In ishes, amphibians, reptiles and birds erythrocytes are generally nucleated oval and biconvex. But in mammals (except camelandllama) the erythrocytes are non-nucleate biconcave and circular. Musk deer (a mammal) has smallest R.B.Cs. and the amphibians have the largest R.B.CS. In elephant the size of the R.B.C. is slightly larger than that of man (9.0u in diameter). The number (normal erythrocyte count) of erythrocytes is slightly lower in women (4.5 to 5 million per cubic millimetre of blood) than in men (5 to 5.5 million per cubic millimetre causes pernicious anaemia. Erythrocytes have a tendency to adhere together by their concave of blood). The number of R.B.Cs. is more in babies and persons living on hills. Increase in umber of R.B. Cs. count (during muscular exercise or stay athigh attitude) is called polycythemia
nd decrease in number of R.B.Cs. is called anaemia. Deficiency of vitamin B12 particularly and surface like stacks or piles of coins. This is known as Rouleaux formation. in tissues called haemopoietic tissues and the process of their formation is called haemopoiesis.

The average life span of an erythrocyte in man is about 120 days. R.B.Cs. are manufactured In the growing toetus, the haemopoietic tissues are liver and spleen. The production of RBC is stimulated by a hormone called “erytropoietin”. But following birth of the child, they are mainly produced in the red bone marrow present within the long bones. Splecn is the organ which stores excess production of erythrocytes and thus rightly called as the “blood bank’ of the body. The old and worn out erythrocytes are destroyed in liver and spleen. However, spleen is the major site of disposal of the disintegrated RBCs. and also known as graveyard of R.B.Cs. The haemoglobin ol RBC is decomposed into hacm and globin. The iron part or hacm is again utilised in the tormauon ot new R.B.Cs. However, some part of hacm is converted into bile piginent or bilirubin in liver and excreted out.

The R.B.Cs. are produced by different organs in different stages of development. In the early human embryo they are produced from the yolk sac and later from liver and spleen. After birth the erythrocytes develop from the red bone marrow and it continues to be the haemopoietic tissue upto the adulthood. The steps of erythropoiesis (duration 72 hours) are given below:

Stem cells or Myeloblast (Red bone marrow)

• Proerythroblasts Erythroblast

-Normoblasts Reticulocytes Erythrocytes

Erythrocytes contain a red coloured respiratory pigment called haemoglobin. I laemoglobin is an iron containing conjugated protein having two distinct components, the hacmeand globulin. The haeme is iron containingcomponent (tour haene groups present) and globulin is a protein. The molecule of haemoglobin is forned of foiur polypeptide chains, each provided with a hacme. One molecule of haemoglobin containing four atoms of iron can carry four molecules of oxygen.
The molecular weight ofhaemoglobin is about 68,000. Human blood normally contains 14.5 gm of haemoglobin per 100 ml of blood.

Haemoglobin also occurs in the blood of some molluscs and annelids in solution in plasma. Prawn, crab and some molluscs contain a blue copper-containing respiratory pigment called haemocyanin. In some annelids, another iron-porplhyrin protein pigment called chlorocrurin is present in plasma for the transport of oxygen.

HAEMATOCRIT VALUE

Haematocrit value or packed cell volume is the percentage of erythrocytes present in the whole blood. I he haematocrit value of normal male is 45 which means there are 45 ml of blood cells in 55 ml ofplasma. In emale this value isa bit lower, about 40% of the whole blood. It is estimated by centriluging the blood at a high speed.

Erythrocyte Sedimentation Rate (ESR)

ESR value is very useful in diagnosing varous a1seases. 1o determine this value, a same of blood is taken and to it a crystal or potasi Oxilate 1s added to prevent coauulatin blood. This blood is kept in a long narrow graduated Wintrob’s tube. It is left a on for of about 30 minutes or more: It will be seen that erytnrocytes gradually settle down red for and a clear plasma is left in the supernatant. T he rate at which the ervthro eo gravity about 30 minute so the supernatant. The rate at which the erythrocytes settle down i.e Known as erythrocyte sedimentation rate.

[II] LEUCOCYTES OR WHITE BLOOD CORPUSCLES (W.B.C.)

white blood corpuscles are large, colourless, nucleated amocboid cells and are much ess numerous than erythrocytes (one cubic millimetre of blood contains 7000 to 8000 W.B.C and the ratio of W.B.C. R.B.C. is approximately 1:600). 1he number of W.B.Cs. is higher in children and also during pathological conditions. The increase in number of leuocytes is called leucocytosis and a decrease is called leucopenia. The leucocytes exhibit phagocytosis cen eating) and amocboid movement. They are the mobile lorce or the Iiving Dody and form the defece system. Their life span is short. The new W.B.Cs. are formed (leucopoiesis) in red bone naTOw, lymph nodes, spleen and thymus. the worn out and damaged cells are destroyed in blood, liver and lymph.

                       Types of Leucocytes Leucocytes
Granulocytes                                                  Agranulocytes

The leucocytes are classified into two groups, basing on the shape of nucleus, the presence or absence of granules in cytoplasn and the dyes accepted by the granules.

(A) Granulocytes
(B) Agranulocytes.

(A) Granulocytes. These leucocytes contain granules in cytoplasm and lobed nucleus. So they are called as polymorpho-nuclear leucocytes. It is of three types.

(a) Neutrophil. It contains a nucleus which is many lobed. The cytoplasmic granules take neatral stains. The neutrophils constitute about 67% to 70% of total leucocytes.

Function. These are phagocytes and kill foreign bacteria.

(b) Basophil. The nucleus is usually ‘S’ shaped and bilobed. The cytoplasmic granules accept basic dyes. 1These are fecbly motile. Basopłhils constitute 0.5% to 1% ofthe total leucocytes.

Function: These are non-phagocytic, but engulf cabon and dust particles.

(c)Acidophil or Eosinophil. These are large cells with bi-or-trilobed nucleus. The cytoplasmic granules are comnparatively large and are stained with acidic dyes. They constitute 3% of the total leucocytes

Function. Eosinophils neutralise the toxins produced by bacteria. It engulfs the particles which are formed by the reaction of antigens and antibodies.

Origin. In the embryo, all types of leucocytes develop from liver and spleen. But in adul stage, they are developed in bone marrow, spleen and lymphatic glands.

Agranulocytes or Iymphocytes. These are a few non-specific leucocytes. Their cytoplasm is without granules and contain rounded nucleus. fasiheriex ootsndrrsn

Origin. These are produced fronm liver and spleen in embryonic stage. But in adult they are manulactured from the spleen, lymjphatic glands and bone marrow. These are of two types

1. Lymphocytes. These are again divided into two varieties.

(a) Small Lymphocytes. The diameter of these Iymphocytes is about 8u. It has a large central nucleus. it torms about 25% of the total W.B.Cs.
(b) Large Lymphocytes. The breadth is about 11u. The nucleus is very large in size, constituting about 3% of the total leucocytes.

Function. The lymphocytes are found more in the lymph and are concerned with the nrocess of immunity. They give rise to antibodies and antitoxin and facilitate wound-healing.

2. Monocytes. These are largest leucocytes. The width is about 15u. They constitute about 1.5% of the total leucocytes. The nucleus is large; but oval in shape; and excentric in position. They are manufactured in lymphatic glands but are degenerated in lymph nodes. These are
   phagocytie in nature.

Function of W.B.C.

(1) These are mainly phagocytic in nature. They ingest and kill living foreign particles.
(2) They produce antigen and antibodies.
(3)They act as mobile bodyguard. They can pass through thin walled blood capillaries, such movement is called “diapedesis” and can ingest foreign bodies.

4. They act as scavengers as they remove the dead cells.
5. Theyfiave the power to absorb fat from intestine and carry it to blood.

Differences between Erythrocytes R.B.C and LEUCOCYTES W.B.C

R.B.C

1. They are smaller in size (7-8 u), circular, biconcave and non-nucleated.

2. They are red in colour due to the presence of haemoglobin.
3. They float passively in the plasma and are immobile.
4. They transport gases, oxygen and carbondioxide.
5. They are present large in number in the blood (4-5 million per ml.)
6. They are produced in the red bone marrow (haemopoetic tissue).
7. They have an average life span of 1207. days.

W.B.C

1. They are larger (about 15 u), amoeboid and nucleated.
2. They are colourless due to the absence of haemoglobin.
3. They can move about like amoeba and chase the disease causing organisms (diapedesis).
4. They form the defence system of the body.
5. They are less numerous (7000-8000 per ml.). Their number increases at the time of infection.
6. They are produced in lymph nodes, spleen and thymus. Life span of W.B.C. is much less (few days). BLOOD PLATELETS

These are small (2-44 in diameter), rounded, colourless, biconvex and non-nucleated structures. These are pieces of certain giant cells called megakaryocytes present in the bone marrow. Their number is about 2.5-3.0 lakhs per cubic millimetre of blood and they have an average life span of about 10-12 days. These play an important role in coagulation of blood.

Thrombocytes or spindle cells occur in the blood of other groups of vertebrates except mammals. They are spindle shaped cells with spherical or oval nucleus and granular cytoplasm. Their function is similar to those of platelets.

Composition of Blood. figure is important.

Function of Blood

1. Transport of Oxygen. The erythrocytes contain haemoglobin that combines with oxygen loosely to form oxyhaemoglobin in lungs from where it is transported to the tissues of the body for the oxidation of food and release of energy.

2. Disposal of Carbon Dioxide. Carbon dioxide produced by the tissues as a result of oxidation of food is trar.sported by the blood plasma and also by haemoglobin to the lungs from where it is removed.
3. Transport of Nutrients. After digestion of food, the soluble food (glucose, amino acide etc.) are carried by the blood to the tissue cells for utilisation.
4. Transport of Excretory Matter. Nitrogenous metabolic wastes like anmonia, urea, uric acid of body are transported by blood to the excretory organs Iroin where they are excreted
5. Distribution of Hormones and Vitamins. Blood transports hormones from the endocrine glands and vitamins to the specific place of action in the body,
6. Maintenance of pli. The plasma proteins of blood acts as a buffer and thus maintains the płl of the blood.
7. Protection Against Diseases. The leucocytes of blood engulf the disease causing microorganisms by phagocytosis and produce antibodies thus protecting the body from diseases.
8. Prevention of Blood Loss. Blood coagulates at the site of injury preventing excessive loss of it from body.
9. Regulation of Body Temperature. Blood maintains the body temperature by carrying heat from heat producing organs (like muscles) to those parts from where heat is lost (e.g, lungs through expiration). The circulation of blood from superficial parts such as skin to deeper parts (e.g. heart) and vice versa distributes heat evenly.
10. Wound Healing. Blood transports necessary materials for healing the wounds.
11. Regulation of Water Balance. Plasma supplies water to the tissues and also takes away excess water from it formed during metabolic processes.
12. The pressure of blood helps in various processes like erection of penis, nipple etc. mammals.

LYMPH

lymph is a colourless or slightly, yellowish, alkaline fluid found in lymphatie contains large number of leucocytes mostly lymphocytes) ranging from 500 to vessels. 1t content 75000 per cubic millimetre. In lymph protein contents are low and it contains less fibinogen than plasma.

The composition of lymph is very much like that of the blood plasma except that the average protein concentration is comparatively less. It is formed from the blood when blood flows through the capillaries during which most part of the plasma and lymphocytes filters into the tissue spaces. It carries some oxygen and the final product of digestion to be supplied to the tissue fluid.

Functions of LYMPH

1. Lymph receives oxygen and nutrients from blood and hands over to the tissues and returns excretory matter from tissue to blood, thus acting as a’middle man’ between blood and the tissue.
2. The lymphocytes and monocytes present in lymph form defence system of body
3. The digested fats are absorbed by the lymph present in lymphatic vessel in the intestinal wall.
4. It is responsible for the maintenance of the volume of tissue fluid.

Differences between Blood and Lymph

Blood

1. The colour of the blood is red due to the presence of R.B.G. (Haemoglobin of RBC impart red colour).
2. The plasma contains R.B.C., W.B.C. and platelets. a
3. It contains many diffusible and non-diffusiblel nroteins like albumin, globin, fibrinogen etc.
4. It has more capacity to clot.
5. It contains more oxygen and digested food materials.
6. It contains normal amount of CO% and metabolic wastes.
7. It flows through regular blood vessels which have definite walls.

Lymph

1. Lymph is colourless due to the absence of RBC.
2. It is constituted of plasma and few leucocytes
3. It has only few diffusible proteins.
4. It clots to some extent.
5. It contains less amount of oxygen and digested food.
6. It contains more amounts of C02 and metabolic wastes.
7. It flows through intercellular spaces as lymph ve:sels have no definite walls.

Difference between Single Blood Circulation and Double blood Circulation

Single blood circulation

(i) Heart has one auricle and one ventricle.
(ii) Blood flows only once through the heart.
(iii) Only deoxygenated blood flows through he heart.
(iv) Pressure of blood
(v) it-is comparatively less efficient.
(vi) Example-Heart of fishes.

Double blood circulation

(i) Heart has four chambers with two auricles and two ventricles.
(ii) Blood circulates twice through the heart.
(iii) Both oxygenated and deoxygenated blood flows through the heart.
(iv) Pressure of blood is high.
(v) It is more efficient.
(vi) Example-Heart of Birds and Man,

MUSCULAR TISSUE

STRUCTURE

The muscular tissue or muscles are responsible for the movement of body, body Darm and organs. About 40% to 50% of our body is formed of muscles. The study of muscles is know as sarcology or myology. The muscular tissues have the following features

(a) Muscular tissue is formed of greatly elongated and highly contractile muscle cell. called muscle fibres.
(6) The muscle fibres contain contractile elements called myofilaments or myofibrils.

Functions

(a) lt brings about movements of body, parts of the body and organs
(b) By muscular actions it produces body heat.
(c) It contracts heart and blood vessels.
(d) Muscles are the agents of brain.

Types of Muscles

Muscles are classified on the basis of their structure and functions into 3 types.

1. Striated or striped or skeletal muscles
2. Unstriated or unstriped muscles
3. Cardiac – muscles Striated or Strpied Muscles

The striated muscles form the flesh of the body. These are mostly attached to the skeleton and hence known as skeletal muscles.

Occurrence. These muscles are mostly attached to the bones.

Histology

The striated muscles are formed of large number of long, unbranched muscle cells or muscle fibres. The length of fibre vary from 1.0 to 40 mm and the breadth from 0.01 mm to 0.1 mm. Each muscle fibre is enclosed in a transparent cell membrane called sarcolemma.A single muscle fibre is surrounded by a sheath of connective tissue known as endomycium.Bundle of muscle fibres form fasciculi which are covered by a sheath of connective tissue known asperimycium. Several fasciculi in a muscle is covered by a sheath of connective tissue called epimycium.

Microscopic Structure

A muscle tibre is a cylindrical cell being covered over by its cell membrane or sarcolemma. Its cytoplasm is known assarcoplasmwhich contains many nuclei. Such multinucleate condition is known as syncytium. The sarcoplasm contains a number of thread like contractile elements known as myofibrils. Each myofibril is about 1u in diameter. It is differentiated into alternate segments or bands of light and dark colour.

Electron microscopic structure of myofibril

Each myofibril contains nearly 300 light and dark bands in one millimeter. When studied under polarised light, the dark band is known as anisotropic or A-band and the light band is known as is otropicor I-band, appear alternately on myofibril. The I-band is bisected by a thin line, called as Z-line or Krause’s membrane. The portion enclosed by two adjacent Z-lines of a myofibril, is contractile unit or considered as sarcomere. It extends 2 to 3u in length. The
central portion of A-band has a H-band or Hensen’s disc which is lighter in colour.In the centre of II-band, there is a narrow dark line, called as M-line or Mesophragm, where the myosin filaments are thickened. The two darker bands of A-band are named as 0-band. On both sides of Z-line of I-band, there are thin, darker lines, called N-line

Electron micrography reveals that the darker bands possess myosin filaments which are nearly 100 A” in diameter. The lighter band contains actin filament of 50A° diameter. Both these protein filaments form my of ilaments.

Function. These muscles contract or relax according to the will of the anirnal. So, these are called as voluntary muscles. These are mostly found, attached to bones. These muscles get fatigued and need rest. They help in locomotion, movement of limbs, maintaining balance and
body posture of the animal.

Unstriated or Unstriped Muscles

The visceral muscles are known as plain, non-striated, sinooth and involuntary muscles. The transverse striations of striated muscles are absent. The contraction and relaxation of this muscle is not controlled by wvill.

Occurrence. These muscles are present in almost all visceral organs, like alimentary canal, blood vessel, respiratory, urino-genital and lymphatic systems etc.

Histology. The smooth muscle fibres are elongated, spindle-shaped or fusiform with tapering both ends. The length of each fibre varies from 20u to 500u and the breadth is at the central widest part. These muscles are so arranged that the thick middle portion of one is apposed by the thin-ends of the other. The muscle fibre is enclosed by a fine reticulin
sheath.

Microscopic Structure. Each muscle fibre has homogenous sarcoplasm. The elongated, oval nucleus contain delicate uniform chromatin netwoik and two or more nucleoli. The sarcoplasm also contains sarcoplasmic reticulum, free ribosomes and a small Golgiapparatus. The remaining sarcoplasm is occupied by myofilaments. The actin and myosin are present chemically.

Function. These muscles are called involuntary, as they work without the will of the
animals. They are innervated by sympathetic nerves.

Cardiac Muscles

These muscles contract rhythmically and automatically these special muscles work non stop till death.

Distribution. These muscles form the muscular body of heart. These are also found in small amounts in the ends and beginning of great vessel connected with the heart.

Histology. The cardiac muscle fibres are separated from each other by endomysium (connective tissue covering) along with blood vessels and lymphatics. These muscle fibres are not only straight, cylindrical cells, but with short cylindrical branches, coming in contact with that of adjacent fibres. It appears as a syncytium under low power microscope. Electron micrograph reveals presence of intercalated discs between the cells. These discs appear as dark lines.

MicroscopićStructure. The cardiac muscle fibre contains abundant cytoplasm covered by sarcolemma. The centrally placed nucleus is elongated one. A small Golgi apparatus is present at one pole. The sarcoplasm contains more glycogen and a few lipid droplets. The mitochondria are more numerous and are arranged in between myofilaments, The patterns of ‘A’, T, ‘Z, M and H-bands are identical with striated muscles.

Function. The cardiac muscles are special involuntary muscle which rhythmically contract and relax. lt is controlled by autonomic nervous system. It is immune to tetanus and fatigue

NERVOUS TISSUE

The various tissues are not controlled directly by the environment, but by the nervous system, which regulates the performance of the whole animal. The nervous system comprises of nervous tissues which are highly specialised tissues for reception, discharge of stimull and transmission.

the structural and functional unit of nervous tissue is the neuron or nerve cell with their processes, called nerve fibres.

Structure of Neuron The neuron consists of a cell body drawn out to forn many processes.

Distribution. Neurons are present in the brain, spinal cord and in the ganglia of cranlal and spina nerves and of autonomic nervous system. A neuron consists of the following structures

(A) Cyton or perikaryon or cell body
(B) Cell processes:
it’s two types
(a) Dendron, or Receptive process.
(b) Axon or discharging process.

(A) CYTON

the cyton is a cell, containing neuroplasm or cytoplasm, covered by cell membrane. The cytoplasm contains mitochondria and Golgi bodies in their typical form. The neuroplasm contains hirm. protoplasnmic threads, called neurofibrils. These neurofibrils are also present in cell processes. The cyton and the branching dendrons contain small granules, called Nissl’s granules. IT 1s absent in axon. The cytoplasm also contains pigment granules and a centiole. A large, spherical nucleus is present centrally.

(B) CELL PROCESSES

The neuro plasm is drawn out to form cell processes, such as

(a) Dendrons
(b) Axon.

(a)Dendron. These are short processes of cyton containing Nissl’s granules and neurofibrils. These dendrons form branchings, called as dendrites. These atferent processes conduct impulses into the neurons.
(b) Axon. This is the largest process of the cyton which does not contain Nissl’s granules. These are efferent processes, which carry impulses from the cyton. The axon may give off slender lateral branches, known as collateral fibres. There are numerous mitOchondria at the tip of axon.

(I) Nerve Fibres. The nerve fibres are of two types.

(i) Myelinated or Medullated or White Nerve Fibre. Myelinated nerve fibres are present in brain, spinal cord, cranial and spinal nerves. In these fibres, the axon is covere by a tubular myelin sheath. The cytoplasm or axoplasm is covered by axolemma or neur lemma. Below the axolemma, flattened nucleated Schwann cells are present. 1ne myelin sheath is made up of lipoprotein and forms a spiral envelop around the axon.

In between Schwann cells, there are interruptions, called as Nodes of Ranvier. These are present in the peripheral nerve fibres. The distance between two nodes of Ranvier is called as internode. At each node the Schwanns cells form a thin band, known as cementing dise. The myelin sheath, in the internode, may show a variable number of oblique clefts or myelin cleft

(ii) Non-myelinated or Non-medullated or Gray Nerve Fibres. These smaller fibers are wIthout myelin sheaths. 1he axon is surrounded by syneytial sheath of Schwann cells with scattered nuclei. The autonomic ganglia possesses these types of axons.

(II) Synapse. It is a gap at the junction between two neurons, called as synaptic cleft.
(III) Auxillary Tissues of Nervous System. The neurons of nervous system are supported by ependymal cells and neuroglial cells like astrocytes, oli godendria (both present in grey and white matter) and microglia. Function. The nerve fibres which carry impulses from receptor organs, like skin, to central nervous system are known as afferent or sensory nerves. In motor or effector nerve fibres
impulses are carried from central nervous system to effector organs.

TYPES OF NEURONS

The neurons are classified according to their number of processes into three types.
(a) Unipolar Neurons. This neuron possesses a single axon. Dendrons are absent. These neurons occur mostly in embryos.
(b) Bipolar Neuron. This neuron possesses a single axon and a single dendron. These are mostly found in retina of eye, cochlea and olfactory neur0-epithelium.
(c) Multipolar Neuron. This is the common type of neuron which has a single axon and many dendrons.

These are found in brain and spinal cord.

HISTOLOGICAL STRUCTURE OF NERVE

In a transverse section, a nerve is found to be composed of a large number of bundles nerve fibres or fasciculi. Individual nerve fibres are held together by loose connective tissue covering, called endoneurium. Bundles of several nerve fibres are enclosed in a collagenous sheath, called perineurium. Several bundles along with blood vessels and lymphatic vessels are covered by epineurium.

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