Dotham, Ala., for his question:

WHAT IS CARTILAGE?

Cartilage, which is often called gristle, is a bluish white, rubbery tissue found in human beings and animals that have backbones. It is found at the ends of long bones, between the vertebrae of the spine and also in the ears, nose and internal respiratory passages.

Cartilage actually cushions long bones against shock and prevents them from rubbing against one another. It also makes an elastic but firm framework for the ear and respiratory passages, ensuring that these openings do not collapse.

Animals with backbones have skeletons of cartilage before they are born. Some, such as sharks, retain this skeleton throughout their lives. With others, bone gradually replaces the cartilage as the animal grows.