Webster's New World Medical Dictionary

2
The outward
appearance of a natural object, which was once
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taken as a token of its special properties. For exam-
ple, this ancient doctrine of signatures led some to
conclude that the walnut, which looks something like
a tiny brain, could be used to heal brain problems;
the liverwort plant, which has a three-lobed liver-like
leaf, was useful in treating liver disease; and so on. 
SIL
Squamous intraepithelial lesion.
silver
A metal that is used in some medications
and in many natural remedies. Used in the past in
silver amalgam for filling cavities in teeth. Silver has
antibiotic properties. However, overuse of silver or
use of products containing silver by people with cer-
tain health conditions can result in silver poisoning
(argyria). See also 
argyria.
silver poisoning
See 
argyria.
Simian crease
A single transverse crease in the
palm, a minor variation associated with Down syn-
drome (trisomy 21) and a number of chromosomal
and other abnormalities. Also called a four-finger
crease; single palmar flexion crease; single upper
palm crease.
single gene disease
See 
disease, single gene.
singultus
See 
hiccup.
sinoatrial node
The heart’s natural pacemaker,
one of the major elements in the cardiac conduction
system, the system that controls the heart rate.
Abbreviated SA node. The SA node consists of a
cluster of cells that are situated in the upper part of
the wall of the heart’s right atrium, where the elec-
trical impulses are generated. An electrical signal
generated by the SA node moves from cell to cell,
down through the heart, until it reaches the atri-
oventricular (AV) node, a cluster of cells situated in
the center of the heart, between the atria and ven-
tricles. The AV node serves as a gate, slowing the
electrical current before the signal is permitted to
pass down to the ventricles. This delay ensures that
the atria have a chance to fully contract before the
ventricles are stimulated. After passing the AV node,
the electrical current travels to the ventricles along
special fibers embedded in the walls of the lower