Incontinence (in-KON-ti-nens) Inability to retain urine, semen, or
feces through loss of sphincter control.
Indirect motor pathways Motor tracts that convey information from
the brain down the spinal cord for automatic movements, coor-
dination of body movements with visual stimuli, skeletal muscle
tone and posture, and balance. Also known as extrapyramidal
Infarction (in-FARK-shun) A localized area of necrotic tissue, pro-
duced by inadequate oxygenation of the tissue.
Infection (in-FEK-shun) Invasion and multiplication of microorgan-
isms in body tissues, which may be inapparent or characterized
by cellular injury.
Inferior (in-FER-e-or) Away from the head or toward the lower part of
a structure. Also called caudad (KAW-dad).
Inferior vena cava (IVC) (VE-na CA-va) Large vein that collects deoxy-
genated blood from parts of the body inferior to the heart and
returns it to the right atrium.
Infertility Inability to conceive or to cause conception. Also called
Inflammation (in'-fla-MA-shun) Localized, protective response to
tissue injury designed to destroy, dilute, or wall off the infecting
agent or injured tissue; characterized by redness, pain, heat,
swelling, and sometimes loss of function.
Inflation reflex Reflex that prevents overinflation of the lungs. Also
called the Hering-Breuer reflex.
Ingestion (in-JES-chun) The taking in of food, liquids, or drugs, by
Inguinal (IN-gwi-nal) Pertaining to the groin.
Inguinal canal An oblique passageway in the anterior abdominal wall
just superior and parallel to the medial half of the inguinal liga-
ment that transmits the spermatic cord and ilioinguinal nerve in
the male and round ligament of the uterus and ilioinguinal nerve
in the female.
Inhalation (in-ha-LA-shun) The act of drawing air into the lungs. Also
Inheritance The acquisition of body traits by transmission of genetic
information from parents to offspring.
Inhibin (in-HIB-in) A hormone secreted by the gonads that inhibits
release of follicle-stimulating hormone (FSH) by the anterior
Inhibiting hormone Hormone secreted by the hypothalamus that
can suppress secretion of hormones by the anterior pituitary.
Inner cell mass A region of cells of a blastocyst that differentiates
into the three primary germ layers—ectoderm, mesoderm, and
endoderm—from which all tissues and organs develop.
Inorganic compound (in'-or-GAN-ik) Compound that usually lacks
carbon, usually is small, and often contains ionic bonds. Exam-
ples include water and many acids, bases, and salts.
Insertion (in-SER-shun) The attachment of a muscle tendon to a mov-
able bone or the end opposite the origin.
Inspiratory capacity (in-SPT-ra-tor-e) Total inspiratory capacity of the
lungs; the total of tidal volume plus inspiratory reserve volume;
averages 3600 mL in males.
Inspiratory reserve volume (in-SPT-ra-tor-e) Additional inspired air
over and above tidal volume; averages 3100 mL.
Insulin (IN-soo-lin) A hormone produced by the beta cells of a
pancreatic islet (islet of Langerhans) that decreases the blood
Insulinlike growth factor (IGF) Small protein, produced by the liver
and other tissues in response to stimulation by human growth
hormone (hGH), that mediates most of the effects of human
growth hormone. Previously called somatomedin (so'-ma-to-
Integumentary (in-teg'-u-MEN-tar-e) Relating to the skin.
Intercalated disc (in-TER-ka-lat-ed) An irregular transverse thickening
of sarcolemma that contains desmosomes, which hold cardiac
muscle fibers (cells) together, and gap junctions, which aid in con-
duction of muscle action potentials from one fiber to the next.
Intercostal nerve (in'-ter-KOS-tal) A nerve supplying a muscle located
between the ribs.
Interferons (IFNs) (in'-ter-FER-ons) Antiviral proteins produced by
virus-infected host cells; induce uninfected host cells to synthe-
size proteins that inhibit viral replication and enhance phagocytic
activity of macrophages; types include alpha interferon, beta
interferon, and gamma interferon.
Internal Away from the surface of the body.
Internal capsule A large tract of projection fibers lateral to the thala-
mus that is the major connection between the cerebral cortex
and the brain stem and spinal cord; contains axons of sensory
neurons carrying auditory, visual, and somatic sensory signals to
the cerebral cortex plus axons of motor neurons descending from
the cerebral cortex to the thalamus, subthalamus, brain stem,
and spinal cord.
Internal ear The inner ear or labyrinth, lying inside the temporal
bone, containing the organs of hearing and balance.
Internal nares (NA-rez) The two openings posterior to the nasal
cavities opening into the nasopharynx. Also called the choanae
Internal respiration The exchange of respiratory gases between
blood and body cells. Also called tissue respiration.
Interneurons (in'-ter-NOO-ronz) Neurons whose axons extend only
for a short distance and contact nearby neurons in the brain, spi-
nal cord, or a ganglion; the vast majority of neurons in the body
Interoceptor (in'-ter-o-SEP-tor) Sensory receptor located in blood
vessels and viscera that provides information about the body's
Interphase (IN-ter-faz) The period of the cell cycle between cell divi-
sions, consisting of the G,-(gap or growth) phase, when the cell
is engaged in growth, metabolism, and production of substances
required for division; S-(synthesis) phase, during which chromo-
somes are replicated; and G2-phase.
Interstitial fluid (in'-ter-STISH-al) The portion of extracellular fluid
that fills the microscopic spaces between the cells of tissues;
the internal environment of the body. Also called inter cellular or