Webster's New World Medical Dictionary

skin test for allergy
See 
allergy skin test.
skin test for immunity
A method of evaluating
whether a person has developed an immune
response to a certain infection. A substance is
injected into the deep layer of the skin (dermis) and
causes a reaction if the immune system recognizes
it. One of the most common skin tests is the tuber-
culin test, which reveals whether a person has been
exposed to tuberculosis. The injection is placed on
the inside of the forearm, and a positive reaction
results in a raised, firm area of skin at the injection
site within 2 days.
skull
A collection of bones that encase the brain
and give form to the head and face. These bones
include the frontal, parietal, occipital, temporal,
sphenoid, ethmoid, zygomatic, maxilla, nasal,
vomer, palatine, inferior concha, and mandible. See
also 
bones of the head.
slanted ear
See 
ear, slanted.
SLE
Systemic lupus erythematosis. See 
lupus ery-
thematosis, systemic.
sleep
The body’s rest cycle. Sleep is triggered by
a complex group of hormones that respond to cues
from the body itself and the environment. About 80
percent of sleep is dreamless and is known as non-
rapid eye movement (NREM) sleep. During NREM
sleep, the breathing and heart rate are slow and reg-
ular, the blood pressure is low, and the sleeper is
relatively still. NREM sleep is divided into four stages
of increasing depth of sleep: Level 1 sleep is a tran-
sition period between sleep and wakefulness; Level
2 sleep features significant slowing of heartbeat and
breathing and makes up about 50 percent of all
sleep; and Levels 3 and 4 (delta) sleep are marked
by very slow respiration and heartbeat. Level 4 sleep
leads to rapid eye movement (REM) sleep, also
known as Level 5 sleep. Dreams occur during three
to five periods of REM sleep each night. REM sleep
occurs at intervals of 1 to 2 hours and is variable in
length. REM sleep is characterized by irregular
breathing and heart rate and involuntary muscle
jerks. Most adults need around 8 hours of sleep on
a regular schedule to function well, although some
require less and others more. Children, particularly
teenagers, often need 9 or 10 hours for optimal
functioning. See also 
NREM sleep; REM sleep.