Webster's New World Medical Dictionary

ultraviolet C
See 
ultraviolet radiation.
ultraviolet radiation
Invisible rays that are part
of the energy that comes from the sun. Abbreviated
UV. UV radiation is made up of three types of rays:
ultraviolet A (UVA), ultraviolet B (UVB), and ultravi-
olet C (UVC). UVC is the most dangerous type of UV
light in its potential to harm life on earth, but it can-
not penetrate the earth’s protective ozone layer. UVA
and UVB do penetrate the ozone layer and reach
earth. UVA is weaker than UVB and less likely to
cause sunburn, but it passes farther into the skin.
Both UVA and UVB cause melanoma and other types
of skin cancer; therefore, it is recommended that
people use sunscreens that block both UVA and UVB
radiation. The light from tanning lamps is like that
from the sun and contains both UVA and UVB. Using
tanning lights poses a major long-term risk of skin
cancer. Electric arc lamps can also generate UV light
to enable motion-picture projectors to show
movies. Although UV light can damage health, it can
also maintain or improve health. When UV light
strikes human skin, it triggers the production of
vitamin D, which promotes the growth of bones and
teeth. See also 
basal cell carcinoma; cancer, skin;
squamous cell carcinoma; melanoma; sun-
screen.
umbilical cord
The cord that connects the
developing fetus with the placenta while the fetus 
is in the uterus. The umbilical arteries and vein 
run within this cord. The umbilical cord is clamped
and cut at birth, and its residual tip forms the 
bellybutton.
umbilical duct
See 
yolk stalk.
umbilicus
See 
bellybutton.
unconscious 1
Interruption of awareness of
oneself and one’s surroundings; the lack of the abil-
ity to notice or respond to stimuli in the environ-
ment. A person may become unconscious due to
oxygen deprivation, shock, injury, or use of central
nervous system depressants such as alcohol and
drugs.