Oogenesis (o'-o-JEN-e-sis) Formation and development of female
Oophorectomy (o'-of-o-REK-to-me) Surgical removal of the ovaries.
Ophthalmic (of-THAL-mik) Pertaining to the eye.
Ophthalmologist (of'-thal-MOL-o-jist) A physician who specializes
in the diagnosis and treatment of eye disorders using drugs,
surgery, and corrective lenses.
Ophthalmology (of'-thal-MOL-o-je) The study of the structure, func-
tion, and diseases of the eye.
Opsin (OP-sin) The glycoprotein portion of a photopigment.
Opsonization (op-so-ni-ZA-shun) The action of some antibodies that
renders bacteria and other foreign cells more susceptible to
Optic (OP-tik) Refers to the eye, vision, or properties of light.
Optic chiasm (KT-azm) A crossing point of the optic (II) nerves, anteri-
or to the pituitary gland. Also called the optic chiasma (kT-AZ-ma).
Optic disc A small area of the retina containing openings through
which the axons of the ganglion cells emerge as the optic nerve
(cranial nerve II). Also called the blind spot.
Optician (op-TISH-an) A technician who fits, adjusts, and dispenses
corrective lenses on prescription of an ophthalmologist or
Optic tract A bundle of axons that carry nerve impulses from the
retina of the eye between the optic chiasm and the thalamus.
Optometrist (op-TOM-e-trist) Specialist with a doctorate degree in
optometry who is licensed to examine and test the eyes and treat
visual defects by prescribing corrective lenses.
Orbit (OR-bit) The bony, pyramidal-shaped cavity of the skull that
holds the eyeball.
Organ A structure composed of two or more different kinds of tis-
sues with a specific function and usually a recognizable shape.
Organelle (or-gan-EL) A permanent structure within a cell with char-
acteristic shape that is specialized to serve a specific function in
Organic compound (or-GAN-ik) Compound that always contains
carbon in which the atoms are held together by covalent bonds.
Examples include carbohydrates, lipids, proteins, and nucleic
acids (DNA and RNA).
Organism (OR-ga-nizm) A total living form; one individual.
Orgasm (OR-gazm) Sensory and motor events involved in ejaculation
for the male and involuntary contraction of the perineal muscles
in the female at the climax of sexual intercourse.
Orifice (OR-i-fis) Any aperture or opening.
Origin (OR-i-jin) The attachment of a muscle tendon to a stationary
bone or the end opposite the insertion.
Oropharynx (or'-o-FAR-inks) The intermediate portion of the phar-
ynx, lying posterior to the mouth and extending from the soft
palate to the hyoid bone.
Orthopedics (or'-tho-PE-diks) The branch of medicine that deals with
the preservation and restoration of the skeletal system, articula-
tions, and associated structures.
Osmoreceptor (oz'-mo-re-SEP-tor) Receptor in the hypothalamus
that is sensitive to changes in blood osmolarity and, in response
to high osmolarity (low water concentration), stimulates synthe-
sis and release of antidiuretic hormone (ADH).
Osmosis (os-MO-sis) The net movement of water molecules through
a selectively permeable membrane from an area of higher water
concentration to an area of lower water concentration until equi-
librium is reached.
Osmotic pressure The pressure required to prevent the movement
of pure water into a solution containing solutes when the solu-
tions are separated by a selectively permeable membrane.
Osseous (OS-e-us) Bony.
Ossicle (OS-si-kul) One of the small bones of the middle ear (malleus,
Ossification (os'-i-fi-KA-shun) Formation of bone. Also called osteo-
Osteoblast (OS-te-o-blast) Cell formed from an osteogenic cell that
participates in bone formation by secreting some organic compo-
nents and inorganic salts.
Osteoclast (OS-te-o-clast') A large, multinuclear cell that resorbs
(destroys) bone extracellular matrix.
Osteocyte (OS-te-o-sTt') A mature bone cell that maintains the daily
activities of bone tissue.
Osteogenic layer (os'-te-o-JEN-ik) The inner layer of the periosteum
that contains cells responsible for forming new bone during
growth and repair.
Osteology (os'-te-OL-o-je) The study of bones.
Osteon (OS-te-on) The basic unit of structure in adult compact bone,
consisting of a central (haversian) canal with its concentrically
arranged lamellae, lacunae, osteocytes, and canaliculi. Also
called an haversian system (ha-VER-shan).
Osteoporosis (os'-te-o-po-RO-sis) Age-related disorder characterized
by decreased bone mass and increased susceptibility to frac-
tures, often as a result of decreased levels of estrogens.
Otic (O-tik) Pertaining to the ear.
Otolith (O-to-lith) A particle of calcium carbonate embedded in the
otolithic membrane that functions in maintaining static equilibrium.
Otolithic membrane (o-to-LITH-ik) Thick, gelatinous, glycoprotein
layer located directly over hair cells of the macula in the saccule
and utricle of the internal ear.
Otorhinolaryngology (o'-to-rT-no-lar'-in-GOL-o-je) The branch of
medicine that deals with the diagnosis and treatment of diseases
of the ears, nose, and throat.
Oval window A small, membrane-covered opening between the
middle ear and inner ear into which the footplate of the stapes fits.
Ovarian cycle (o-VAR-e-an) A monthly series of events in the ovary
associated with the maturation of a secondary oocyte.
Ovarian follicle (FOL-i-kul) A general name for oocytes (immature
ova) in any stage of development, along with their surrounding
Ovary (O-var-e) Female gonad that produces oocytes and hormones
(estrogens, progesterone, inhibin, and relaxin).
Ovulation (ov-u-LA-shun) The rupture of a mature ovarian (graaf-
ian) follicle with discharge of a secondary oocyte into the pelvic