ileocecal valve: A fold of mucous membrane that guards the connection between the ileum and the cecum.

ileostomy: The surgical creation of an opening into the ileum; the opening created when the ilium is surgically attached to the abdominal wall.

ileum: The last 2.5 m of the small intestine.

ilium: The largest of the three bones whose fusion creates an os coxae.

immunity: Resistance to injuries and diseases caused by foreign compounds, toxins, or pathogens.

immunization: The production of immunity by the deliberate exposure to antigens under conditions that prevent the development of illness but stimulate the production of memory B cells.

immunodeficiency: An inability to produce normal numbers and types of antibodies and sensitized lymphocytes.

immunoglobulin: A circulating antibody.

immunosuppression: The suppression of immune responses by the administration of drugs or exposure to toxic chemicals, radiation, or infection.

implantation: The erosion of a blastocyst into the uterine wall.

impotence: The inability to obtain or maintain an erection.

inclusions: Aggregations of insoluble pigments, nutrients, or other materials in cytoplasm.

incontinence: The inability to control micturition (or defecation) voluntarily.

incus: The central auditory ossicle, situated between the malleus and the stapes in the middle ear cavity.

inducer: A stimulus that promotes the activity of a specific gene.

inexcitable: Incapable of conducting an action potential.

infarct: An area of dead cells that results from an interruption of blood flow.

infection: The invasion and colonization of body tissues by pathogens.

inferior: Below, in reference to a particular structure, with the body in the anatomical position.

inferior vena cava: The vein that carries blood from the parts of the body inferior to the heart to the right atrium.

infertility: The inability to conceive; also called sterility.

inflammation: A nonspecific defense mechanism that operates at the tissue level; characterized by swelling, redness, warmth, pain, and some loss of function.

inflation reflex: A reflex mediated by the vagus nerve (N X) that prevents overexpansion of the lungs.

infundibulum: A tapering, funnel-shaped structure; in the brain, the connection between the pituitary gland and the hypothalamus; in the uterine tube, the entrance bounded by fimbriae that receives the oocytes at ovulation.

ingestion: The introduction of materials into the digestive tract by way of the mouth.

inguinal canal: A passage through the abdominal wall that marks the path of testicular descent and that contains the testicular arteries, veins, and ductus deferens.

inguinal region: The area near the junction of the trunk and the thighs that contains the external genitalia.

inhibin: A hormone, produced by the sustentacular cells, that inhibits the pituitary secretion of follicle-stimulating hormone.

inhibitory postsynaptic potential (IPSP): A hyperpolarization of the postsynaptic membrane after the arrival of a neurotransmitter.

initial segment: The proximal portion of the axon where an action potential first appears.

injection: The forcing of fluid into a body part or organ.

inner cell mass: Cells of the blastocyst that will form the body of the embryo.

inner ear: See internal ear.

innervation: The distribution of sensory and motor nerves to a specific region or organ.

insensible perspiration: Evaporative water loss by diffusion across the epithelium of the skin or evaporation across the alveolar surfaces of the lungs.

insertion: A point of attachment of a muscle; the end that is easily movable.

inspiration: Inhalation; the movement of air into the respiratory system.

inspiratory reserve: The maximum amount of air that can be drawn into the lungs over and above the normal tidal volume.

insoluble: Incapable of dissolving in solution.

insomnia: A sleep disorder characterized by the inability to fall asleep.

insulin: A hormone secreted by the beta cells of the pancreatic islets; causes a reduction in plasma glucose concentrations.

integument: The skin.

intercalated discs: Regions where adjacent cardiocytes interlock and where gap junctions permit electrical coupling between the cells.

intercellular cement: Proteoglycans situated between adjacent epithelial cells.

intercellular fluid: See interstitial fluid.

interdigitate: To interlock.

interferons: Peptides released by virus-infected cells, especially lymphocytes, that slow viral replication and make other cells more resistant to viral infection.

interleukins: Peptides, released by activated monocytes and lymphocytes, that assist in the coordination of cell-mediated and antibody-mediated immunities.

internal capsule: The collection of afferent and efferent fibers of the white matter of the cerebral hemispheres, visible on gross dissection of the brain.

internal ear: The membranous labyrinth that contains the organs of hearing and equilibrium.

internal nares: The entrance to the nasopharynx from the nasal cavity.

internal respiration: The diffusion of gases between interstitial fluid and cytoplasm.

interneuron: An association neuron; neurons inside the central nervous system that are interposed between sensory and motor neurons.

interoceptors: Sensory receptors monitoring the functions and status of internal organs and systems.

interosseous membrane: The fibrous connective tissue membrane between the shafts of the tibia and fibula and between the radius and ulna; an example of a fibrous amphiarthrosis.

interphase: The stage in the life cycle of a cell during which the chromosomes are uncoiled and all normal cellular functions except mitosis are under way.

intersegmental reflex: A reflex that involves several segments of the spinal cord.

interstitial fluid: The fluid in the tissues that fills the spaces between cells.

interstitial growth: A form of cartilage growth through the growth, mitosis, and secretion of chondrocytes in the matrix.

interventricular foramen: The opening that permits fluid movement between the lateral and third ventricles of the brain.

intervertebral disc: A fibrocartilage pad between the bodies of successive vertebrae that absorbs shocks.

intestinal crypt: A tubular epithelial pocket that is lined by secretory cells and opens into the lumen of the digestive tract; also called intestinal gland.

intestine: The tubular organ of the digestive tract.

intracellular fluid: The cytosol.

intrafusal fibers: Muscle spindle fibers.

intramembranous ossification: The formation of bone within a connective tissue without the prior development of a cartilaginous model.

intramuscular injection: The injection of medication into the bulk of a skeletal muscle.

intraocular pressure: The hydrostatic pressure exerted by the aqueous humor of the eye.

intrapleural pressure: The pressure measured in a pleural cavity; also called intrathoracic pressure.

intrapulmonary pressure: The pressure measured in an alveolus of the lungs; also called intraalveolar pressure.

intrauterine: Within the uterus; during prenatal development.

intrinsic factor: A glycoprotein, secreted by the parietal cells of the stomach, that facilitates the intestinal absorption of vitamin B₁₂.

intrinsic pathway: A pathway of the clotting system that begins with the activation of platelets and ends with the formation of platelet thromboplastin.

inversion: A turning inward.

in vitro: Outside the body, in an artificial environment.

in vivo: In the living body.

involuntary: Not under conscious control.

ion: An atom or molecule bearing a positive or negative charge due to the donation or acceptance, respectively, of an electron.

ionic bond: A molecular bond created by the attraction between ions with opposite charges.

ionization: Dissociation; the breakdown of a molecule in solution to form ions.

ipsilateral: A reflex response that affects the same side as the stimulus.

iris: A contractile structure, made up of smooth muscle, that forms the colored portion of the eye.

ischemia: An inadequate blood supply to a region of the body.

ischium: One of the three bones whose fusion creates the os coxae.

islets of Langerhans: See pancreatic islets.

isometric contraction: A muscle contraction characterized by rising tension production but no change in length.

isotonic: A solution with an osmolarity that does not result in water movement across cell membranes.

isotonic contraction: A muscle contraction during which tension climbs and then remains stable as the muscle shortens.

isotopes: Forms of an element whose atoms contain the same number of protons but different numbers of neutrons (and thus differ in atomic weight).

isthmus: A narrow band of tissue connecting two larger masses.