during which there is no physical re-
sponse to further sexual stimulation.
Both men and women may experience
problems with orgasm (see ejaculation,
disorders of; orgasm, lack of).
orgasm, lack ofInability to achieve
orgasm during sexual activity. It may be
due to inhibited sexual desire (see sexual
desire, inhibited) or inability to become
aroused or maintain arousal (see frigid-
ity;impotence). In men, there may be a
problem achieving orgasm despite nor-
mal arousal (see ejaculation, disorders of).
The problem is common in women; some
may achieve orgasm through masturba-
tionbut not during sexual intercourse.
Sometimes it is due to pain during
intercourse (see intercourse, painful).
For both sexes, contributory factors
include problems with technique or in
the relationship, unfamiliarity with sex-
ual responses, psychological problems
(such as anxiety, early sexual trauma, or
inhibitions), and fear of pregnancy. Sex
therapy, relationship counselling, and
psychotherapyare sometimes helpful.
orlistatAn anti-obesity drug used with a
slimming diet to treat severe obesity.
Unlike appetite suppressants, orlistat acts
on the gastrointestinal tract, preventing
the digestion of fats by lipases (pancreat-
icenzymes). Instead of being absorbed,
the fats pass out of the body in faeces.
Side effects are gastrointestinal and
can be minimized by reducing fat in-
take. Flatulence and faecal urgency are
common. Deficiencies of fat-soluble vit-
amins may develop with prolonged use.
ornithosisA disease of birds, caused by
the microorganism CHLAMYDIA PSITTACI,
that can cause psittacosisin humans.
orphan drugsDrugs that have been
developed to treat rare conditions but
are not manufactured generally.
orphenadrineA muscle-relaxant drug
used to treat painful muscle spasm due
to soft-tissue injury and Parkinson’s dis-
ease. Side effects include a dry mouth
and blurred vision.
ORTAn abbreviation for oral rehydration
therapy. (See also rehydration therapy.)
ortho- A prefix meaning normal, correct,
or straight, as in orthopaedics, a branch
of surgery concerned with correcting dis-
orders of the bones and joints.organelleOne of various specialized
structures contained within a body cell.
organicRelated to a body organ; hav-
ing organs or an organized structure; or
related to organismsor to substances
from them. In chemistry, “organic” refers
to certain compounds that contain car-
bon. In medicine, the term indicates the
presence of disease. (See also inorganic.)
organic brain syndromeSee brain
syndrome, organic.
organismA general term for an individ-
ual animal or plant. Bacteriaand viruses
are disease-causing microorganisms.
organophosphates Highly poisonous
agricultural insecticides that are harm-
ful when absorbed through the skin, by
inhalation, or by swallowing. Among the
many possible symptoms are nausea,
vomiting,abdominal cramps, diarrhoea,
blurred vision, excessive sweating, head-
ache, confusion, and twitching. Severe
poisoning may cause breathing difficulty,
palpitations, seizures, and unconscious-
ness. If left untreated, death may result.
Treatment may include washing out the
stomach (see lavage, gastric) or removing
soiled clothing and washing contami-
nated skin. Injections of atropinemay be
given, and oxygen therapyand/or artifi-
cial ventilationmay be needed. With
rapid treatment, people may survive
doses that would otherwise have been
fatal. Long term effects of organophos-
phates in sheep dips are thought to be
responsible for debilitating illness with
neural, muscular, and mental symptoms.
orgasmIntense sensations produced
by a series of muscular contractions at
the peak of sexual excitement. Orgasm
usually lasts for about 3–10 seconds but
can last up to a minute in women.
In men, contractions of the muscles of
the inner pelvis massage seminal fluid
from the prostate glandinto the urethra,
from which it is forcefully propelled via
the urethral orifice (see ejaculation).
Orgasm in women is associated with
irregular contractions of the voluntary
muscles of the walls of the vaginaand,
in some women, of the uterus, followed
by relief of congestion in the pelvic
area. Some women experience multiple
orgasms if stimulation is continued.
Orgasm is followed by a refractory phase
ORGANELLE ORTHO-
O