(a) Bicephalic ribs, each ribs has two articulating surfaces on its dorsal end hence, are called as bicephalic ribs.

(b) True ribs are the first seven pairs of ribs Dorsally these ribs are attached to the thoracic vertebrae and ventrally connected to the sternum with the help of hyaline cartilage.

(c) Floating ribs are the last two pair (11th and 12th) of ribs and are not connected ventrally to the sternum therefore, called as floating ribs.

The condition of stiff and inflammed joints is called as osteoporosis. It is an age-related disorder characterised by decreased bone mass and increased chances of fractures. Decreased levels of estrogen is a common cause for osteoporosis in females after menopause, in old aged females.

(a) More $\mathrm{Ca}^{2+}$ concentration in extracellular fluid is associated with hyperparathyroidism. It causes demineralisation, resulting in softening and bending of the bones. This condition leads to osteoporosis.

(b) Very less amount of $\mathrm{Ca}^{2+}$ in extracellular fluid is associated with hypoparathyroidism. This increases the excitability of nerves and muscles, causing cramps, sustained contraction of the muscles of larynx, face, hands and feet. This disorder is called parathyroidtetany or hypercalcemictetany.

Parathyroid hormone and calcitonin results in the fluctuation of $\mathrm{Ca}^{2+}$ level. Parathyroid Hormone (PTH) increases the $\mathrm{Ca}^{2+}$ levels in the blood. PTH acts on the bones and stimulates the process of bone resorption (dissolution/demineralisation). PTH also stimulates reabsorption of $\mathrm{Ca}^{2+}$ by the renal tubules and increases $\mathrm{Ca}^{2+}$ absorption from the digested food. Calcitonin is a 32-amino acid linear polypeptide hormone, that is produced in humans primarily by the parafollicular cells of the thyroid. It acts by reducing blood calcium $\left(\mathrm{Ca}^{2+}\right)$, levels opposing the effect of Parathyroid Hormone (PTH).

Rahul has gained weight because his muscle shape has changed. Regular exercise increase the body muscle as there is an enlargement of muscles due to increase in the amount of sarcoplasm and mitochondria and the strength he to developed led him gain the mass and size of body muscle and reduction in fat content.