"The density of a substance determines how much light is refracted"
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Answer
cutting or incision
for example: gastrotomy means a surgical incision to the stomach
The brain which, along with the spinal cord gathers informationand responds to changes in the environment.
The second line of defense is nonspecific resistance that destroys invaders in a generalized way without targeting specific individuals: Phagocytic cells ingest and destroy all microbes that pass into body tissues. For example macrophages are cells derived from monocytes.