Webster's New World Medical Dictionary

chyme
A predigested, acidified mass of food that
passes from the stomach into the small intestine.
Ci
The abbreviation for a Curie, a unit of radioac-
tivity. See also 
Curie.
-cide
Suffix indicating killing or killer, as in bac-
tericide (a solution capable of killing bacteria).
ciliary neuralgia
See 
cluster headache.
circadian
Refers to events occurring within the
span of a full 24-hour day, as in a circadian clock.
circadian clock
An internal time-keeping system
in all organisms. Changes in the external environ-
ment, particularly in the light–dark cycle, train this
biologic clock. When environmental conditions are
constant, rhythms driven by the circadian clock fol-
low a nearly perfect 24-hour pattern. The human
circadian clock regulates many daily activities, such
as sleep and waking. When a person doesn’t follow
these natural rhythms, or when the external environ-
ment strays from its usual rhythm (as occurs in the
long nights and short days of deep winter), the cir-
cadian clock must readjust. Rapid environmental
changes and problems with circadian clock adjust-
ment are among the causes of jet lag, problems that
affect shift workers, some types of sleep disorders,
and bipolar disorders, particularly seasonal affective
disorder. Certain genes serve to set and control the
circadian clock. See also 
bipolar disorder; jet lag;
seasonal affective disorder; sleep disorder.
circinate balanitis
See 
balanitis, circinate.
circle of Willis
A critical arterial circle at the
base of the brain. The circle of Willis receives all the
blood that is pumped up the two internal carotid
arteries that come up the front of the neck. All the
principal arteries that supply the two halves of the
brain (hemispheres) branch off from the circle of
Willis.
circulation
In medicine, the movement of fluid
through the body in a regular or circuitous course.
The circulatory system, composed of the heart and
blood vessels, functions to produce circulation.
Heart failure is an example of a problem with cir-
culation.