fabella: A sesamoid bone commonly located in the gastrocnemius muscle, just behind the knee.

facilitated diffusion: The passive movement of a substance across a cell membrane by means of a protein carrier.

facilitated: Brought closer to threshold, as in the depolarization of a nerve cell membrane toward threshold; making the cell more sensitive to depolarizing stimuli.

falciform ligament: A sheet of mesentery that contains the ligamentum teres, the fibrous remains of the umbilical vein of the fetus.

falx: Sickle-shaped.

falx cerebri: The curving sheet of dura mater that extends between the two cerebral hemispheres; encloses the superior sagittal sinus.

fasciae: Connective tissue fibers, primarily collagenous, that form sheets or bands beneath the skin to attach, stabilize, enclose, and separate muscles and other internal organs.

fasciculus: A small bundle; usually refers to a collection of nerve axons or muscle fibers.

fatty acids: Hydrocarbon chains that end in a carboxylic acid group.

fauces: The passage from the mouth to the pharynx, bounded by the palatal arches, the soft palate, and the uvula.

febrile: Characterized by or pertaining to a fever.

feces: Waste products eliminated by the digestive tract at the anus; contains indigestible residue, bacteria, mucus, and epithelial cells.

fenestra: An opening.

fertilization: The fusion of an oocyte and a spermatozoon to form a zygote.

fetus: The developmental stage lasting from the start of the third developmental month to delivery.

fibrillation: Uncoordinated contractions of individual muscle cells that impair or prevent normal function.

fibrin: Insoluble protein fibers that form the basic framework of a blood clot.

fibrinogen: A plasma protein that is the soluble precursor of the fibrous protein fibrin.

fibrinolysis: The breakdown of the fibrin strands of a blood clot by a proteolytic enzyme.

fibroblasts: Cells of connective tissue proper that are responsible for the production of extracellular fibers and the secretion of the organic compounds of the extracellular matrix.

fibrocartilage: Cartilage containing an abundance of collagen fibers; located around the edges of joints, in the intervertebral discs, the menisci of the knee, and so on.

fibrous tunic: The outermost layer of the eye, composed of the sclera and cornea.

fibula: The lateral, slender bone of the leg.

filariasis: A condition resulting from infection by mosquito-borne parasites; can cause elephantiasis.

filiform papillae: Slender conical projections from the dorsal surface of the anterior two-thirds of the tongue.

filtrate: The fluid produced by filtration at a glomerulus in the kidney.

filtration: The movement of a fluid across a membrane whose pores restrict the passage of solutes on the basis of size.

filtration pressure: The hydrostatic pressure responsible for filtration.

filum terminale: A fibrous extension of the spinal cord, from the conus medullaris to the coccygeal ligament.

fimbriae: Fringes; the fingerlike processes that surround the entrance to the uterine tube.

fissure: An elongate groove or opening.

fistula: An abnormal passageway between two organs or from an internal organ or space to the body surface.

flaccid: Limp, soft, flabby; a muscle without muscle tone.

flagellum/flagella: An organelle that is structurally similar to a cilium but is used to propel a cell through a fluid.

flatus: Intestinal gas.

flavin adenine dinucleotide (FAD): A coenzyme important in oxidative phosphorylation; cycles between the oxidized (FADH₂) and reduced (FAD) states.

flavin adenine mononucleotide (FMN): A coenzyme important in oxidative phosphorylation; cycles between the oxidized (FMNH₂) and reduced (FMN) states.

flexion: A movement that reduces the angle between two articulating bones; the opposite of extension.

flexor: A muscle that produces flexion.

flexor reflex: A reflex contraction of the flexor muscles of a limb in response to an unpleasant stimulus.

flexure: A bending.

fluoroscope: An instrument that permits the body to be examined in real time with X-rays rather than with fixed images on photographic plates.

folia: Leaflike folds; the slender folds in the surface of the cerebellar cortex.

follicle: A small secretory sac or gland.

follicle-stimulating hormone (FSH): A hormone secreted by the anterior pituitary; stimulates oogenesis (female) and spermatogenesis (male).

folliculitis: An inflammation of a follicle, such as a hair follicle of the skin.

fontanel: A relatively soft, flexible, fibrous region between two flat bones in the developing skull; also spelled fontanelle.

foramen/foramina: An opening or passage through a bone.

forearm: The distal portion of the upper limb between the elbow and wrist.

forebrain: The cerebrum.

fornix: An arch or the space bounded by an arch; in the brain, an arching tract that connects the hippocampus with the mamillary bodies; in the eye, a slender pocket situated where the epithelium of the ocular conjunctiva folds back on itself as the palpebral conjunctiva.

fossa: A shallow depression or furrow in the surface of a bone.

fourth ventricle: An elongate ventricle of the metencephalon (pons and cerebellum) and the myelencephalon (medulla oblongata) of the brain; the roof contains a region of choroid plexus.

fovea: The portion of the retina that provides the sharpest vision because it has the highest concentration of cones; also called macula lutea.

fracture: A break or crack in a bone.

frenulum: A bridle; see lingual frenulum.

frontal plane: A sectional plane that divides the body into an anterior portion and a posterior portion; also called coronal plane.

fructose: A hexose (six-carbon simple sugar) in foods and in semen.

fundus: The base of an organ.

fungiform papillae: The mushroom-shaped papillae on the dorsal and dorsolateral surfaces of the tongue.

furuncle: A boil, resulting from the invasion and inflammation of a hair follicle or sebaceous gland.