Glossary
Apatite: The mineral fraction of bone, microscopic calcium phosphate crystals
deposited on the preexisting collagen structure of the bone, making it
hard. See also Collagen.
Axon: The prolongation of a nerve cell that carries a message, or stimulus,
away from the cell body. For example, a motor nerve cell axon carries a
contraction stimulus to a muscle. See also Dendrite, Neuron.
Base pair: An association between two of the four fundamental chemical
groups that make up all DNA and RNA molecules. Base pairs are the
smallest structures that form units of meaning in the genetic code. The
more base pairs, the larger the molecule.
Biological cycles: Changes in the activities of living things in an ebb-and-flow
pattern. Such changes occur in almost all physical aspects, including sleep-
wakefulness, hormone levels, and numbers of white cells in the blood. The
predominant pattern is approximately twenty-four hours and usually fol-
lows the lunar day closely. See also Orcadian rhythm.
Blastema: The mass of primitive, unspecialized cells that appears at the site of
an injury in animals that regenerate. Blastema cells specialize and form the
replacement part.
Circadian rhythm: The predominant biological cycle of all living things, from
Latin circa ("approximately") and dies ("a day"). See also Biological cy-
cles.
Collagen: A protein that makes up most of the fibrous connective tissue that
holds the body's parts together. Tendons, ligaments, and scar tissue are