There are two types of skeletal systems in animals.
(i) Exoskeleton and (ii) Endoskeleton
(i) Exoskeleton : This system is present on the surface of body. Scales in fishes, wings in birds, hair, teeth, horn in mammals are the examples of exoskeleton.
(ii) Endoskeleton : It is made of bones and muscles. Man is the well known example. It gives the shape and size to the body.
BONES
There are 206 bones in an adult man and more than 500 muscles. Skeleton is mainly divided into two parts namely :
1. Axial skeleton and 2. Appendicular skeleton. Details of these two are given below.
(1) Axial skeleton : It forms the main axis. It contains the following bones
(a) Skull : 22 bones
(b) Vertebral column : 33 bones.
Cervical-7
Thoracic-12
Lumbar-5
Sacral-5
Caudal-4
(c) Sternum : 1
(d) Ribs : 24
(2) Apendicular skeleton : It contains
(a) Pectoral girdle : 4 bones
(b) Pelvic girdle : 2 bones
(c) Forelimbs : 60 bones
Humerus-2
Radioulna-4
Carpels-16
Metacarpels-10
Phalanges_28
(d) Hindimbs : 60 bones
Femur-2
Tibio-fibula-4
Patela-2
Tarsels-14
Metatarsels-10
Phalanges-28
Grand total-206
JOINTS
Joint is the place where two or more bones contact. These are of three types—
(i) Complete or movable joints : These are of following types—
(a) Ball and socket joint : e.g., joints of pectoral girdle and humerus bone, joints of pelvic girdle and femur bone.
(b) Gliding joint : Joints of pre and post zygapophysis of the vertebrae, joints of radius and ulna.
(c) Hinge joint : Joints of elbow, knee, wrist and phalanges of hand.
(d) Pivot joint : Odontoid process of axis and the skull.
(e) Saddle joint : Carpals and metacarpals of thumb.
(ii) Incomplete Joints : Joints of some bones of pectoral and pelvic girdle, joints between vertebrae, ileum and transverse process of sacral vertabrae etc.
(iii) Immovable or fixed joints : Joints of skull bones.
SKELETAL MUSCLES
According to the movements—
(i) Extensor : Erects the organ. e.g. triceps.
(ii) Flexor : Bends a part upon other. e.g. biceps.
(iii) Abducor : Takes away a part from the main axis of body. e.g. deltoid.
(iv) Adductor : Takes nearer a part to the main axis of body. e.g. latissimus dorsi.
(v) Elevator : Elevates a part. e.g. maseter.
(vi) Depressor : Depresses a part, e.g. depressor mandibularis.
(vii) Constrictor : Constricts an aperture. e.g. anal sphincter.
(viii) Rotator : Rotates about an axis. e.g. pyriformis.
(ix) Dilator : Dilates an aperture.
(x) Pronator : Takes the hidden surface to the front.
Antagonistic muscles : Muscles which perform opposite action, e.g. biceps and triceps
Features of Skeleton
1. Exoskeleon is developed from the mesoderm of embryo.
2. Cartilage does not contain calcium and magnesium salts.
3. Turbinal bones are present in nasal chambers.
4. Only one rod like bone constitutes stemum.
5. Only one complete bone constitutes lower jaw.
6. The inter-vertebral disc is found between two successive vertebrae.
7. Centrum is not present in first cervical vertebrae.
8. 11th and 12th ribs are called floating ribs.
9. Clavicle is a membranous bone.
10. The lower jaw contains tooth sockets.
11. A cell that causes erosion of bone is called osteociast.
12. A mature bone cell is called osteocyte.
13. Portion of the skeleton to which the legs are attached is called pelvic girdle.
14. A cavity in the human skull is called sines.
15. Structure consisting of fibrous connective tissue that connects skeletal muscles to bones is called tendon.
16. Chemical crest is present on the tibia.
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