Skeletal System | Biology for Grade 11 PDF Download

SKELETAL SYSTEM

DEFINITION AND INTRODUCTION

The hard protective or supportive part of the animal constitute skeletal system. Study of skeleton is called Osteology or Skeletology.
Design of the skeleton depends on the animals mode of life. Skeleton of different design are needed for the aquatic or terrestrial animals.

TYPES OF SKELETON

On the basis of location skeleton is of two types-

Exoskeleton Endoskeleton

1. Exoskeleton

It is formed by secretion of the skin from epidermis.
It is always non living.
It is found in invertebrate as well as vertebrate.
It is composed of a non-living protein material called keratin or horn.

Epidermal exoskeleton
Scales of reptiles are epidermal exoskeleton.

Exception - Crocodile and Tortoise. (mesodermal in origin)

Beak, claws, horn hoof, feathers, hairs and nail are epidermal exoskeleton.

Dermal exoskeleton

Scales of fishes, crocodile and tortoise are dermal exoskeleton.

2. Endoskeleton

The endoskeleton includes hard parts present inside the body. The bone and cartilage endoskeleton. are example of
It is always living.

Some invertebrate like, corals, cuttle fish and Echinodermata also possess endoskeleton.

TOTAL NUMBER OF BONES

In adult total number of bones are - 206
Axial skeleton - 80
Appendicular skeleton - 126
In neonates total number of bones are 306
On the basis of the position of skeletal structure in the body Skeleton system is divided into 2 parts-

APPENDICULAR SKELETON

These are of 2 types-

(A) Pectoral girdle (B) Pelvic girdle

(A) Pectoral girdle
Pectoral girdle is formed of two bones clavicle and scapula.
Clavical is also called collar bone.
Scapula is also called shoulder bone or shoulder blade. It is flat and triangular structure present at the back of shoulder.

At the junction of clavical and scapula a depression is found in the scapula called glenoid cavity.

In a glenoid cavity head of humerus fits to form shoulder joint.

(B) Pelvic girdle

It is also called hip girdle.
It is composed of 3 bones, which are as follows-
Ilium - Upper side
Pubis - Inner side
Ischium - Below the pubis
Human pelvis from anterior aspect

At the junction of these 3 bones, a depression is found. Which is called acetabulum.

Femur fits in acetabulum to form hip joint.

The pelvic girdle of female are broader than male.

In each fore limb total 30 bones are found-
Upper arm - Humerus - 1
Fore arm - Radius (outer) - 2
ulna (inner)
Wrist - Carpals - 8
Palm - Metacarpals - 5
Fingers - Phalanges - 14

Phalangeal formula for human hand is 2,3,3,3,3.

The Humerus

Head : It articulates with the glenoid cavity of scapula to form shoulder joint.

Deltoid ridge : Elevated rough part on the shaft here deltoid muscle is attached.

Lower end : Articulated laterally with radius & medialy with ulna

Coronoid fossa : depression just above the anterior aspect of trochlea. It accommodated the

Coronoid process of ulna when elbow is flexed.

Olecranon fossa : It accommodate the olecranon process of ulna when Elbow is extended.

The Radius & Ulna

Radius : Its head is disc shaped, covered with hyaline cartilage. it’s superior concave surface articulates with the capitulum of humerus at the elbow joint.

Circumference of head is also articular, it fits into socket formed by the radial notch of the ulna to form radioulanar joint.

Olecranon process : Projects upwards from shaft of ulna. It is responsible for making elbow joint hinge.

Carpal Bones

Proximal Row : From lateral to medial -

Scaphoid, lunate, triquetrum, pisciform

Distal Row : Trapezium, trapezoid, capitate, Hammate. Metacarpal bones : 5 bones, numbered lateral to medial.

Phalanges : There are 14 phalanges in each hand . 3 for each finger & for the thumb. Bones of hind limb
In each hind limb total 30 bones are found.

Thigh - Femur - 1
Shank - Tibia (inner) - 2
Fibula (outer)
Ankle - Tarsals - 7
In step - Metatarsal - 5
Toes - Phalanges - 14
Knee joint - Patella ( Knee cap) - 1

Femur

Strongest heaviest and largest bone Articulates with acetabulum to form the hip joint.
Lower end of femur is widely expanded to form two large condyles, one medial & one  lateral.
Greater and lesser trochanter are rough projections to provide attachment to muscles.

Tibia

Medial & larger bone of the leg.
Upper end : Expanded from side to side of from two large condyles.

Medial condyle : Its superior surface articulates with medial condyle of femur.

Lateral condyle : Superior surface of condyle articulates with lateral condyle of femur.

Fibula

Lateral & smaller bone of the leg.
Its upper end articulates with the lateral condyle of tibia.

Tarsus

Tarsus is made of seven tarsal bones arranged in two rows

Proximal row : Talus above, Navicular in between and Calcaneum below. Tarsal bones are larger & stronger than carpal bones because they have to support & distribute body weight.

Talus is second largest tarsal bone, lies between tibia above & calcaneum below.

Calcaneum : Largest tarsal bone, forms the prominence of heal.

Distal row : Four tarsal bones lying side by side (three cuneiform and one cuboid) Meta tarsus

Made of 5 meta tarsal bones which are numbered medial to lateral.

Phalanges
14 Phalanges, 2 for great tow & 3 each for each other four toes.
As compared to Phalanges of hand these are small in size.
Phalangeal formula for human foot is 2, 3, 3, 3, 3

AXIAL SKELETON

It is present on the median longitudinal axis of the body. It consists of -
– Skull – Ribs
– Sternum – Vertebral column

SKULL

Skull with two occipital condyles is called dicondylic. Human skull is dicondylic.
Skull is formed of 29 bones.
In rabbit 53 bones are found in skull.

Skull is composed of 4 portions.
(a) Cranium (b) Face (c) Hyoid (d) Sensory capsule

(A) Cranium

It is also called brain box.
It is large and hollow round part of skull which encloses and protects the brain in the cranial cavity.
It has large opening foramen magnum at the posterior end , through which brain is continuous with spinal cord.

Cranium is composed of 8 bones, which are as follows :

These bones are joined by immovable fibrous joints called Sutures.
Coronal Suture lies between frontal and parietal.

Internal nares

Lambdoidal suture lies between parietal and occipital bones.
Sella Turcica - depression in sphenoid bone of skull that lodges pituitary body.
largest foramen - foramen magnun which is present below brain box, through it the spinal chord comes out.

(B) Face

Face is composed of 14 bones which are as follows -

Note :
Mandible is movable to allow mastication and speech.(Only Movable bone apart from ear ossicles.) In its posterior part a pair of condyle are present which fit in the cavity of temporal bone, so the lower jaw attaches with the cranium. This type of suspension is called craniostylic.

(C) Hyoid bone

It is also called tongue bone.
It is situated in the wall of the upper part of the throat, just above the larynx.

(D) Sensory capsule

Ear is surrounded by bony auditory capsules, middle ear has 3 movable ear ossicles
(i) Maleus (Modification of articular bone)
(ii) Incus (Modification of Quadrate bone)
(iii) Stapes (smallest bone of body) modification of Hyomandibular bone.

RIBS

There are 12 ribs on each side of thoracic cage First 7 which are connected throuhg cartilage to the sternum are called True Ribs.
(Vertebrosternal ribs)
Remaining 5 are False Ribs, out of these cartilage of the 8th, 9th & 10th ribs are joined to the next higher cartilage, 8th, 9th, 10th are called Vertebrochondral ribs. The anterior ends of 11th & 12th ribs are free & are called floating ribs.

Head - Has two parts. Lower part articulates with numerically corresponding vertebrae . While the upper part articulates with higher vertebrae.

Costal Cartilages

Unossified anterior parts of ribs made of hyaline cartilage. These contribute to the elasticity of thoracic wall.
Medial ends of CC of first seven ribs are directly attached to sternum.

8th, 9th & 10th CC articulate with one another. The cartilage of 11th & 12th ribs are small. Their ventral end are free and lie in the muscle of abdominal wall.

Note : Like human in rabbit also 12 pairs of ribs are found.

STERNUM

It is also called breast bone.
It is 15 cm long in man.
In embryo sternum is made up of a series of bone called sternebra. In rabbit there is 7 sternebra while in human there is 3 sternebra.

It is composed of 3 parts -

Manubrium - Upper part
Body - Middle part
(It is largest part)
Cartilaginous Xiphoid - Lower part
(Xiphisternum)
Note : Sternum is absent in fishes.

VERTEBRAL COLUMN

General facts
Presence of vertebral column is characteristic feature of vertebrates.
It is also called spinal column or backbone.
The component of vertebral column are called vertebrae. Vertebrae of man is acoelus (centrum is flat with cavity)

In between vertebrae pads are found which are composed of fibrocartilage, called intervertebral pads.
These pads or disc acts as shock absorber.
Vertebral formula of man
The vertebral formula is C7T12L5S(5)Co(4) = 33 vertebrae.
Vertebral formula of rabbit
The vertebral formula is C7T12L7S4Co16 = 46 vertebrae.

Basic structure of vertebrae

(i) A vertebrae is typically a bony ring.
(ii) The two main portion of vertebrae is disc like centrum or body and neural arch.
(iii) Within neural arch a hole is found is called vertebral foramen. The vertebral formen of all the
vertebrae when intact form a vertebral canal, that encloses the spinal cord.

Thoraclic– vertebrae (Chest region)
Cervical –vertebrae (Neck region)

Note :
Centrum of man is flat with no cavity, such centrum is called Acoelus or Amphiplatyn.

Functions of vertebral column

(i) It encloses and protects the spinal cord.
(ii) It carries the weight of the body.
(iii) It helps in upright posture and locomotion.
(iv) It helps in free movement of the head.

Groups of vertebrae

The vertebrae of vertebral column can be put in 5 groups, according to their location, which are as follows-

(I) Cervical vertebrae

Cervical vertebrae are present in neck.
They are 7 in number. In most mammal number of cervical vertebrae is 7 All cervial vertebrae have apertures in their transverse process (Foramina transversalis which form vertebraterial canal on either side for vertebral artery to pass through to supply brain & posterior spinal cord).

C₁ & C₂ = Atypical, C₃ to C₇ = typical

Atlas : - Transverse process are wing like, centrum is absent, Ring like.

Neural arch of this vertebrae is divisible in 2 parts with a ligament. In the upper part of the ligament,
Spinal cord is present. In lower part odontoid fossa is present in which odontoid process of axis is fitted to make pivot joint. (Also called as median atlanto axial joint)

On each surface of atlas a pair of articular process are present. The upper pair articulates with condyle of skull to form Rt & Lt Atlantooccipital joints.

Lower pair articulates with superior Articular process of axis to make Rt & Lt Lateral atlanto axial joints.

Axis : C₂ - Centrum is present. At anterior surface of centrum a long odontoid process is present which fits into odonotoid fossa of atlas vertebrae.

Only C₇ has demifacets where upper part of head of 1st Rib articulates.

Note : Cervical vertebrae are 7 in all mammals is despite of long or small neck.

(II)Thoracic vertebrae

Thoracic vertebrae are present in chest.

They are 12 in number.

Presence of costal demifacets helps in their identification

(III)Lumbar vertebrae

Lumbar vertebrae are present in the abdomen.

They are 5 in number.

Lumbar vertebrae are largest and heaviest.

(IV)Sacral vertebrae

They are 5 in number.

The 5 sacral vertebra fuse to form single bone.

(V) Coccygeal vertebrae

These are 4 in number.

The 4 vertebrae fuse to form curved and triangular bone, called the coccyx or tail bone.

JOINTS

Study of joints is called Arthrology.

Types of joints

Joints are mainly of 3 types-

1. Immovable or fibrous joints / Synarthroses
2. Slightly movable or cartilaginous joints / Amphiarthroses.
3. Freely movable or synovial joints/Diarthroses

Structural Classification :

(A) Fibrous joints (Immovable) : Also called as Synarthrosis.
(i) Sutures : Eg. Skull
(ii) Syndesmosis : Bones are connected by interosseus ligament e.g. inferior tibio fibular joints.
(iii) Gomphosis : (Peg & Socket) – e.g. tooth in its socket

(B) Cartilagenous joints (Slightly movable) : also called as amphiarthrosis
(i) Primary : hyaline cartilage joints (synchondrosis). After certain age the cartilage is replaced by bone e.g. joint between Epiphysis & Shaft.

(ii) Secondary : (Symphysis) fibro cartilagenous joints. Articular surface are covered by a thin layer of hyaline cartilage & united by a disc of fibro cartilage. there joint are persistant throughout life. e.g. Symphysis pubis, intervertebral joint.

(C) Freely movable or Synovial joint : also called as diarthrosis The ends of both the bones are covered by hyaline cartilage or articular cartilages.
The cartilage provides smooth and elastic surface and reduce friction and acts as shock absorber.
In between two bones a space is found called synovial space or cavity. This space provides free movement to the bone.
The synovial cavity is lined by a synovial membrane. Synovial membrane contains secretory cells. Synovial membrane secrete or fluid called synovial fluid.  Synovial fluid lubricates and nourish the joints.

Type of Synovial joints : (Most mobile) :

(i) Plain synovial or gliding joint : Permit slight gliding movement e.g., joint between articular processes of vetebrae and joints between Carpals.

(ii) Hinge joint : Movements are permitted in one plane around transverse axis e.g. elbow, ankle, intephalangeal joint.

(iii) Pivot joint : Articular surfaces comprise of

- a central bony pivot
- surrounded by osteoligamentous ring
movement are permitted in one plane around vertical axis. e.g. radioulnar joint, median atlanto axial joint.

(iv) Condylar joint : Articular surface include two distinct condyles and movement are permitted in both transverse & vertical axis. Rt & left jaw joint.

(v) Ellipsoid joint : e.g. wrist, Metacarpophalengeal joint.movement are permitted in both axis, but articular
surfaces are not in form of condyles.

(vi) Saddle joint : Articulating surface are reciprocally concavo convex. these are improperly developed ball & socket joints e.g. first carpometacarpal joint and calcaneocuboid joint.

(vii) Ball & socket joint : Here one articular surface is like a ball and other of the shape of socket and movements are possible around

infinte axises. e.g. shoulder & Hip joint.

ROLE OF MUSCLES AND BONES IN MOVEMENT

(i) Movement of an organ occurs due to the pulling of the bones caused by force generated due to contraction in muscles. Movements take place along the joints which act as fulcrum of the lever.

(ii) Three types of lever functioning can be observed in movements of human joints .

(A) Class I lever : Here Fulcrum is between effort & resistance.

Example : Joint between the first vetebra (ATLAS) and occipital

(C) Class III lever : Here effort is between resistance and fulcrum.

Example : Flexing movement of the elbow of forearm.

Fulcrum : Is the elbow joint.

Effort : Is contraction in Bicep muscle.

Resistance : Weight of distal part of hand.

SKELETAL DISORDER

(i) Dislocation : It is displacement of bone from their normal position at joint.
(ii) Slipped disc : It is displacement of intervertebral fibro-cartilage disc or vertebrae from their normal position.
(iii) Sprain : It is stretching or tearing of tendons or ligaments.
(iv) Osteitis or Osteomyelitis : It is inflammation of bone.
(v) Spondylitis : It is inflammation of one or more vertebrae.
(vi) Osteoporosis : It is excessive loss of calcium and phosphorus from the bone.
(vii) Rickets : In this disease bones of leg get curved bowlegs, It is due to deficiency of vitamin D.

CHARACTERISTICS OF FEMALE'S SKELETON

(i) Skull - Female skull is lighter in comparison to male.
(ii) Shoulders - Female shoulder's are narrower.
(iii) Sacrum - Female sacrum is shorter and wider.
(iv) Pelvis - Female pelvis is lighter and broader.
(v) Pelvic cavity - The female cavity is wider in diameter to accommodate the growing foetus during pregnancy and to facitilate the child birth.
(vi) Coccyx : Female coccyx is more movable than male coccyx.

TYPES OF BONES (ON BASIS OF SHAPE AND SIZE)

(i) Long bones
e.g. Humerus, Radius, Ulna, Tibia fibula, Femur.
(ii) Short bones
e.g. Carpals and tarsals.
(iii) Flat bones
e.g. Skull bone, sternum and ribs.
(iv) Irregular bones
e.g. Ear ossicles and vertebrae.
(v) Sesamoid bones
e.g. Patella (knee cap)., pisciform

SPECIAL POINTS

(i) The bones of a children have large amount of organic matter, so, their bones are very flexible and less likely to break. (hence they may undergo green stick fracture)
(ii) In frog tibiofibula is the longest bone.
(iii) Bones formed by ossification in cartilages is called replacing bone e.g. Humerus, femur. (cartilagenous bones)
(iv) Bones formed by ossification in the dermis and sink to get attached over the cartilages. e.g. Frontals and parietals is called investing bone of the skull.(Dermal bones or membranous bones)
(v) Bones formed by ossification in the tendons at the joint is called sesamoid bones e.g. Patella.
(vi) Skull of reptiles and birds possess one occipital condyles so the skull is called monocondylic, skull of amphibians and mammals possess two occipital condyles, so the skull is called dicondylic.
(vii) Os penis : A bone supporting the penis of bats is called os penis.
(viii) Os cordis : A bone supporting the heart of cattle is called oscordis.
(ix) The end of two bones are connected by ligaments. While a muscle atlaches with bone through tendon.
(x) Procoelous - Centrum concave anteriorly but convex posteriorly 2nd to 7th vertebrae of frog. All reptilian vertebrae are procoelous.

Amphicoelous - Centrum concave on both sides. 8th vertebrae of frog is amphicoelous.
Acoelous - Centrum remain flat. 9th vertebra of frog is acoelous.
Heterocoelous - Centrum partly convex and partly concave on both sides.

Vertebrae of birds are heterocoelous.

Coeloplatyn vertebrae - Centrum concave anteriorly but flat posteriorly.

Platycoelous vertebrae - Centrum flat anteriorly but concave posteriorly.

(xi) In Avian skull sutures remains absent.
(xii) Weberian ossicles - These are modified in vertebrae in cat and fishes. These help in sound production by connecting air bladder and internal ear.
(xiii) Like mammals amphibian skull is also dicondylic. Skull of reptiles and aves are monocondylic.

(xiv) The pelvic girdle of birds is attached to a complex structure formed by the fusion of last thoracic all lumbar and first five caudal vertebra this structure is called synsacrum.
(xv) Talus in Rabbit is called as Astragalus.
(xvi) Weberian ossicles are found in fishes.
(xvii) Fontanelles are temporary apertures present in the skull of a new born baby.
(xviii) Superior and inferior articular processes of vertebrae are also called as pre and post zygapophysis respectively.
(xix) In rabbit elbow and knee joints are also called ginglymoid joints.
(xx) Cnenial crest is a ridge found is Tibia of Rabbit.
(xxi) Trochlear notch of Rabbit is also called as sigmoid notch.
(xxii) Zygomatic arch of rabbit is formed of maxilla, squamosal and jugal bone.
(xxiii) Pelvic girdles are made of two hip bones which are also called as Innominates.
(xxiv) Dentary is a membranous bone found in the lower jaw of same vertebrates like frog.
(xxv) Jaw suspension of mammals is craniostylic.
(xxvi) Pterygoid bone is a wing shaped extension of sphenoid bone in Rabbit’s skull.
(xxvii) Movement of hands while walking occurs for balancing.
(xxviii) A small sessamoid bone Fabella often develops in the tendon of lateral haed of gastromenius muscles behind the knee joint.

DISORDERS OF BONES

1. Arthritis - It is caused by the inflammation of the joints. This of several types, e.g., rheumatoid arthritis, osteoarthritis and gouty arthritis.

(A) The Rheumatoid Arthritis
(Rheum = Watery secretion of the body) It is diagnosed by the presence of rheumatoid factor (a type of immunoglobulin gM).

It is the primary symptom of inflammation of synovial membrane.
If it is left untreated, then the membrane thickens and synovial fluid increases, exerting pressure that causes pain.
The membrane then starts secreting abnormal granules, called pannus, which after accumulating on the surface of the cartilage, cause its erosion.
As a result, the fibrous tissues are attached with the bones and become ossified, making the joints immovable.

(B) Osteoarthritis
It is a degenerative joint disease characterised by the degeneration of the articular cartilage and proliferation of new bones. Usually, afflicted joints are of spine, knees and hands.

(C) Gouty Arthritis or Gout
It is caused either due to excessive formation of uric acid, or inability to excrete it. It gets deposited in joints as monosodium salt.
Treatment – Reduction of pain and inflammation by heat treatment and physiotherapy and, in extreme cases, replacement of the damaged joints. Traditionally the toxin of honey bee is also used.

2. Osteoporosis
It is an age-dependent systemic disorder characterised by low bone mass, micro-architectural deterioration of the bone, increased fragility and proneness or susceptibility to fracture.

The elderly men and women are most susceptible.

It may occur in a pregnant woman, In individuals under prolonged treatment of cortisone, the skeleton fails to withstand the stress of the body and bones are easily fractured.
Imbalances of hormones like thyrocalcitonin, parathyroid and sex-hormones, deficiencies of calcium and vitamin D, menopause are the major causative factors.

IMPORTANT POINTS OF FROG SKELETON SYSTEM

(i) Total number of bones in frog are 150.
(ii) Total number of skull bones are 30.
(iii) Skull of frog is dicondylic.
(iv) In frog, ribs are absent.
(v) In frog, sternum is associated with pectoral girdle.
(vi) In frog, pectoral girdle is formed of episternum (cartilage), xiphisternum (cartilage) omosternum (bone), mesosternum (bone).
(vii) Half of the pelvic girdle of frog is osinnominatum. Each os-innominatum is formed of ilium, pubis and ischium.
(viii) Forelimb of frog includes humerus, radioulna, carpels, metacarpels and phalanges. Phalanges formula of hand is 02233.
(ix) Hindlimb of frog includes. Femur, Tibia fibula, Astragalus, calcaneum and bones of foot.

Note :
In frog, largest and longest bone is tibia fibula.
Digital formula for foot in frog is 22343.
Vertebral column of frog is composed of 10 vertebrae 10th vertebrae is called urostyle.