Webster's New World Medical Dictionary

aqueduct of Sylvius
A canal between the third
and fourth ventricles in the brain within the system
of four communicating cavities that are continuous
with the central canal of the spinal cord. The ventri-
cles are filled with cerebrospinal fluid, which is car-
ried by the aqueduct of Sylvius.
aqueduct of the midbrain
See 
aqueduct of
Sylvius.
arachnodactyly
A condition in which a person
has long, spider-like fingers and toes.
Arachnodactyly is a frequent finding in those with
Marfan syndrome. See also 
Marfan syndrome.
arachnophobia
An abnormal and persistent fear
of spiders. Sufferers from arachnophobia experi-
ence undue anxiety, even though they realize that the
risk of encountering a spider and being harmed by
it is small or nonexistent. They may avoid going
barefoot and may be especially alert when taking
showers or getting into and out of bed.
arbitration agreement
An arrangement in
which the patient waives the right to sue the physi-
cian and, instead, agrees to submit any dispute to
arbitration. Arbitration agreements are legal and
binding. The arguments in their favor are that, for
patients, the case can be settled faster, and more
money can go to the patient (rather than to a
lawyer). Physicians can often get a discount on their
malpractice insurance if the majority of their
patients sign such agreements.
arbovirus
A type of virus transmitted to humans
by mosquitoes and ticks. Arbovirus can cause
inflammation of the brain (encephalitis). The types
appendectomy
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of arboviral encephalitis that occur in the US
include LaCrosse, eastern equine, western equine,
and St. Louis encephalitis, all of which are transmit-
ted by mosquitoes. Another arbovirus, Powassan,
transmitted by ticks, is a cause of encephalitis in the
northern US. Many other types of arboviral
encephalitis occur throughout the world. Most are
problems only for travelers to countries where the
viruses are endemic. One, the West Nile virus, has
made a major entry into the US. It causes West Nile
encephalitis, also known as West Nile fever. See also
hemorrhagic fever, viral.