labium/labia: Lips; the labia majora and labia minora are components of the female external genitalia.

labrum: A lip or rim.

labyrinth: A maze of passageways; the structures of the inner ear.

lacrimal gland: A tear gland on the dorsolateral surface of the eye.

lactase: An enzyme that breaks down milk proteins.

lactation: The production of milk by the mammary glands.

lacteal: A terminal lymphatic within an intestinal villus.

lactic acid: A compound produced from pyruvic acid under anaerobic conditions.

lactiferous duct: A duct draining one lobe of the mammary gland.

lactiferous sinus: An expanded portion of a lactiferous duct adjacent to the nipple of a breast.

lacuna: A small pit or cavity.

lambdoidal suture: The synarthrosis between the parietal and occipital bones of the cranium.

lamellae: Concentric layers; the concentric layers of bone within an osteon.

lamina: A thin sheet or layer.

lamina propria: The loose connective tissue that underlies a mucous epithelium and forms part of a mucous membrane.

laminectomy: The removal of the spinous processes of a vertebra to gain access and treat a herniated disc.

Langerhans cells: Cells in the epithelium of the skin and digestive tract that participate in the immune response by presenting antigens to T cells.

laparoscope: A fiber-optic instrument used to visualize the contents of the abdominopelvic cavity.

large intestine: The terminal portions of the intestinal tract, consisting of the colon, the rectum, and the anorectal canal.

laryngopharynx: The division of the pharynx that is inferior to the epiglottis and superior to the esophagus.

larynx: A complex cartilaginous structure that surrounds and protects the glottis and vocal cords; the superior margin is bound to the hyoid bone, and the inferior margin is bound to the trachea.

latent period: The time between the stimulation of a muscle and the start of the contraction phase.

lateral: Pertaining to the side.

lateral apertures: Openings in the roof of the fourth ventricle that permit the circulation of cerebrospinal fluid into the subarachnoid space.

lateral ventricle: A fluid-filled chamber within a cerebral hemisphere.

laxatives: Compounds that promote defecation through increased peristalsis or an increase in the water content and volume of feces.

lens: The transparent body that is inferior to the iris and pupil and superior to the vitreous humor.

lesion: A localized abnormality in tissue organization.

lesser omentum: A small pocket in the mesentery that connects the lesser curvature of the stomach to the liver.

leukemia: A malignant disease of the blood-forming tissues.

leukocyte: A white blood cell.

leukocytosis: Abnormally high numbers of circulating white blood cells.

leukopenia: Abnormally low numbers of circulating white blood cells.

ligament: A dense band of connective tissue fibers that attaches one bone to another.

ligamentum arteriosum: The fibrous strand in adults that is the remnant of the ductus arteriosus of the fetal stage.

ligamentum nuchae: An elastic ligament between the vertebra prominens and the occiptal bone.

ligamentum teres: The fibrous strand in the falciform ligament of adults that is the remnant of the umbilical vein of the fetal stage.

ligate: To tie off.

limbic system: The group of nuclei and centers in the cerebrum and diencephalon that are involved with emotional states, memories, and behavioral drives.

limbus: The edge of the cornea, marked by the transition from the corneal epithelium to the ocular conjunctiva.

linea alba: The tendinous band along the midline of the rectus abdominis muscle.

lingual: Pertaining to the tongue.

lingual frenulum: An epithelial fold that attaches the inferior surface of the tongue to the floor of the mouth.

lipase: A pancreatic enzyme that breaks down triglycerides.

lipemia: An elevated concentration of lipids in the blood.

lipid: An organic compound containing carbons, hydrogens, and oxygens in a ratio that does not approximate 1:2:1; includes fats, oils, and waxes.

lipofuscin: A pigment inclusion in aging nerve cells that is of unknown significance.

lipogenesis: The synthesis of lipids from nonlipid precursors.

lipoids: Prostaglandins, steroids, phospholipids, glycolipids, and so on.

lipolysis: The catabolism of lipids as a source of energy.

lipoprotein: A compound containing a relatively small lipid bound to a protein.

liver: An organ of the digestive system that has varied and vital functions, including the production of plasma proteins, the excretion of bile, the storage of energy reserves, the detoxification of poisons, and the interconversion of nutrients.

lobule: Histologically, the basic organizational unit of the liver.

local hormone: See prostaglandin.

long-term memories: Memories that persist for an extended period.

loop of Henle: The portion of the nephron that creates the concentration gradient in the renal medulla.

loose connective tissue: A loosely organized, easily distorted connective tissue that contains several fiber types, a varied population of cells, and a viscous ground substance.

lordosis: An exaggeration of the lumbar curvature.

lumbar: Pertaining to the lower back.

lumen: The central space within a duct or other internal passageway.

lungs: The paired organs of respiration, situated in the pleural cavities.

luteinizing hormone (LH): An anterior pituitary hormone that, in females, assists follicle-stimulating hormone in follicle stimulation, triggers ovulation, and promotes the maintenance and secretion of the endometrial glands; in males, stimulates spermatogenesis; also called lutropin; formerly called interstitial cell–stimulating hormone in males.

luxation: The dislocation of a joint.

lymph: The fluid contents of lymphatic vessels, similar in composition to interstitial fluid.

lymphadenopathy: The pathological enlargement of the lymph nodes.

lymphatic vessels: The vessels of the lymphatic system; also called lymphatics.

lymphedema: The swelling of peripheral tissues as a result of excessive lymph production or inadequate drainage.

lymph nodes: Lymphatic organs that monitor the composition of lymph.

lymphocyte: A cell of the lymphatic system that participates in the immune response.

lymphokines: Chemicals secreted by activated lymphocytes.

lymphopoiesis: The production of lymphocytes.

lymphotoxin: A secretion of lymphocytes that kills the target cells.

lysis: The destruction of a cell through the rupture of its cell membrane.

lysosome: An intracellular vesicle containing digestive enzymes.

lysozyme: An enzyme, present in some exocrine secretions, that has antibiotic properties.