Skeleton systems are of following two types—
Exoskeleton : This system remains on the surface of the body such as scales in fishes; wings, claws and feathers in birds; hair, teeth, horn and nail in mammals.
Endoskeleton : This provides the main base and shape to the man. It is made up of bone and cartilage.
Bones
(a) Replacing bone or cartilaginous: In this the primary structure is made up of cartilage and then deposition of calcium salts takes place afterwards. e.g., bones of legs, vertebral column, girdles etc.
(b) Membranous bone: These are not formed from the pre-existing cartilage but undifferentiated mesenchymal cells alongwith collagen fibres, ossein proteins and calcium salts lay down an entirely different bone. They usually invest the membranes hence these are known as investing bones.
In man there are 206 bones which form a big skeleton. The skeleton is divided into—
(1) Axial skeleton : It forms the main axis. It is again divided into—
(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) Appendicular skeleton
(a) Pectoral girdle : 4 bones
(b) Pelvic girdle : 2 bones
(c) Forelimbs : 60 bones
Humerus-2, Radioulna-4, Carpals-16, Metacarpals-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 point where two or more bones contact to each other. These are of three types :
(i) Complete or perfect or movable joints : These are of following types :
(a) Ball and socket type joint : e.g., joints of pectoral girdle and humerus bone, joints of pelvic girdle and femur bone.
(b) Gliding joint : e.g. joints of pre and post zygapophysis of the vertebrae, joints of radius and ulna.
(c) Hinge joint : e.g. Joints of elbow, knee and wrist and joints of phalanges of hand.
(d) Pivot joint : e.g. Odontoid process of axis and the skull.
(e) Saddle joint : e.g. 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 vertebrae etc.
(iii) Immovable or fixed joints : Joints of skull bones.
Skeletal muscles
According to the movements—
(i) Extensor : It erects or extends the organ e.g. triceps.
(ii) Flexor : Bends a part upon other. e.g. biceps.
(iii) Abductor : 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 : A part rotates about an axis. e.g. pyriformis.
(ix) Dilator : An aperture dilates.
(x) Pronator : Takes the hidden surface to the front.
Antagonistic muscles : Muscles which do opposite action, e.g. biceps and triceps.
Salient features of skeleton
1. Endoskeleton is developed from the mesoderm of embryo.
2. Cartilages are devoid of salts of calcium and magnesium.
3. Sternum of human is made up of only one rod like bone.
4. Mammalian skull is dicondylic and tropibasic.
5. Chondrocranium is completely bony. The places of bone’s joining are called suture.
6. Nasal chambers are having turbinal bones.
7. Lower jaw is made up of only one bone dentary.
8. Foramen magnum is formed by four occipital bones viz. supra-occipital, basio-occipital and two exo-occipitals.
9. Basisphenoid is having pituitary foramen.
10. Optic foramen is present between presphenoid and orbitosphenoid.
11. Hyoid apparatus is completely bony.
12. Vertebrae are typically amphiplatian.
13. Between two successive vertebrae the inter-vertebral disc is found.
14. First cervical vertebrae or atlas is not having centrum.
15. Odontoid process of 2nd or axis vertebrae is supposed to be the centrum of first.
16. Transverse processes of typical thoracic vertebrae are having articular facets for the tubercular processes.
17. Capitular facet is present on centrum. These are called demifacets.
18. 11th and 12th pair of ribs are called floating ribs.
19. Mamillary process is present on the lumbar vertebrae.
20. Clavicle is a membranous bone.
21. Each half of pelvic girdle is called Os innominatum.
22. Bone between acetabulum and pubis is called cotyloid bone.
23. Supratrochlear foramen is present on humerus.
24. Sigmoid notch is present on the olecranon process of ulna, near the proximal end of the radius.
25. Cnemial crest is present on the tibia.
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