Disorders Of Muscular And Skeletal System

Step 1: Understanding Myofibril Regulation

- Troponin and tropomyosin regulate muscle contraction by controlling actin-myosin interaction.
- Actin and myosin are contractile proteins, not regulatory proteins.

Step 1: Classification of Ribs:
There are 12 pairs of ribs in humans, classified based on their attachment to the sternum:

• True Ribs (Vertebrosternal): First 7 pairs ( ). They attach directly to the sternum ventrally.
• False Ribs (Vertebrochondral): Next 3 pairs ( ). They do not attach directly to the sternum but join the rib via hyaline cartilage.
• Floating Ribs (Vertebral): Last 2 pairs ( and ). They are not connected to the sternum ventrally at all; they "float" in the abdominal muscles.

Step 2: Final Answer:
The floating ribs are the and pairs.

• In the evolution of mammals from reptilian ancestors the quadrate bone of the lower jaw became the incus of the middle ear.
• The articular bone became the malleus; hence option (3) (articular) corresponds to malleus, not incus.
• Therefore the correct answer is (2) Quadrate.

The occipital bone forms the posterior wall and base of the cranium, housing the foramen magnum for spinal cord passage.