The Skeleton, Joints and Disorders: Quick Revision

A brief note on The Skeleton, Joints and Disorders. Read it before a test. Try each question at the end before you open its answer.

The skeleton

• Whole skeleton: 206 bones. Axial: 80 bones. Appendicular: 126 bones.
• Bone: hard matrix with calcium salts. Cartilage: pliable matrix with chondroitin salts.
• Skull: 8 cranial and 14 facial bones. The hyoid is 1 bone. The ear ossicles are 6 bones: malleus, incus and stapes in each middle ear. Two occipital condyles.
• Vertebral column (NCERT): 26 units. Cervical 7, thoracic 12, lumbar 5, sacral 1 and coccygeal 1. The atlas is the first.
• Ribs: 12 pairs. True: 7 pairs. Vertebrochondral (false): 3 pairs. Floating: 2 pairs. Ribs are bicephalic.
• Each limb: 30 bones. Fore limb: humerus, radius, ulna, 8 carpals, 5 metacarpals, 14 phalanges. Hind limb: femur, tibia, fibula, patella, 7 tarsals, 5 metatarsals, 14 phalanges.
• Pectoral girdle: clavicle and scapula in each half. The glenoid cavity receives the humerus. Pelvic girdle: two coxal bones (ilium, ischium, pubis). The acetabulum receives the femur.

Joints

• Fibrous: no movement (skull sutures). Cartilaginous: limited (between vertebrae). Synovial: much movement.
• Synovial: ball and socket (shoulder), hinge (knee), pivot (atlas and axis), gliding (carpals), saddle (thumb).

Disorders

• Myasthenia gravis: autoimmune, neuromuscular junction, fatigue, weakening, paralysis.
• Muscular dystrophy: progressive degeneration of skeletal muscle, mostly genetic.
• Tetany: rapid spasms, low Ca$^{2+}$ in the body fluid.
• Arthritis: inflamed joints. Osteoporosis: low bone mass, more fractures, low oestrogen. Gout: uric acid crystals in the joints.

A worked example

How many bones are in the appendicular skeleton?

1. Four limbs: $4 \times 30 = 120$ bones.
2. Girdles: $2 \times 2 + 2 \times 1 = 6$ bones.
3. Total: $120 + 6 = 126$ bones, which is $206 - 80$.

Practice with new numbers

A program to try

The program adds the bones of the appendicular skeleton and of the whole skeleton.

Mistakes to avoid

• Using 33 vertebrae in an NCERT question. NCERT gives 26 units.
• Mixing the false ribs and the floating ribs. The false ribs join the seventh rib. The floating ribs are free.
• Mixing the carpals and the tarsals. The wrist has 8, and the ankle has 7.
• Saying that tetany comes from too much calcium. The level is low.

Check yourself

1. How many bones are in the axial skeleton?

2. Which joint is a hinge, and which is a pivot?

3. What causes gout?

4. What is a common cause of osteoporosis?

Sources

• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapters 6 to 9.
• NCERT (2023). Biology, Textbook for Class XI, chapter 17.