Auditory ossicle (AW-di-to-re OS-si-kul) One of the three small bones
of the middle ear called the malleus, incus, and stapes.
Auditory tube The tube that connects the middle ear with the nose
and nasopharynx region of the throat. Also called the eustachian
u-STA-ke-an) or pharyngotympanic tube.
Auscultation (aws-kul-TA-shun) Examination by listening to sounds in
Autoimmunity An immunological response against a person's own
Autolysis (aw-TOL-i-sis) Self-destruction of cells by their own lyso-
somal digestive enzymes after death or in a pathological process.
Autonomic ganglion (aw'-to-NOM-ik GANG-le-on) A cluster of cell
bodies of sympathetic or parasympathetic neurons located out-
side the central nervous system.
Autonomic nervous system (ANS) Visceral sensory (afferent) and vis-
ceral motor (efferent) neurons. Autonomic motor neurons, both
sympathetic and parasympathetic, conduct nerve impulses from
the central nervous system to smooth muscle, cardiac muscle,
and glands. So named because this part of the nervous system
was thought to be self-governing or spontaneous.
Autophagy (aw-TOF-a-je) Process by which worn-out organelles are
digested within lysosomes.
Autopsy (AW-top-se) The examination of the body after death.
Autosome (AW-to-som) Any chromosome other than the X and Y
chromosomes (sex chromosomes).
Axilla (ak-SIL-a) The small hollow beneath the arm where it joins the
body at the shoulders. Also called the armpit.
Axon (AK-son) The usually single, long process of a nerve cell that
propagates a nerve impulse toward the axon terminals.
Axon terminal Terminal branch of an axon where synaptic vesicles
undergo exocytosis to release neurotransmitter molecules.
B cell A lymphocyte that can develop into a clone of antibody-
producing plasma cells or memory cells when properly stimu-
lated by a specific antigen.
Babinski sign (ba-BIN-ske) Extension of the great toe, with or without
fanning of the other toes, in response to stimulation of the outer
margin of the sole; normal up to 18 months of age and indicative
of damage to descending motor pathways such as the corticospi-
nal tracts after that.
Back The posterior part of the body; the dorsum.
Ball-and-socket joint A synovial joint in which the rounded surface
of one bone moves within a cup-shaped depression or socket of
another bone, as in the shoulder or hip joint.
Baroreceptor (bar'-o-re-SEP-tor) Neuron capable of responding to
changes in blood, air, or fluid pressure. Also called a pressore-
Basal ganglia (GANG-gle-a) Paired clusters of gray matter deep in
each cerebral hemisphere including the globus pallidus, puta-
men, and caudate nucleus. Nearby structures that are function-
ally linked to the basal ganglia are the substantia nigra of the
midbrain and the subthalamic nuclei of the diencephalon.
Basal metabolic rate (BMR) (BA-sal met'-a-BOL-ik) The rate of
metabolism measured understandard or basal conditions (awake,
at rest, fasting).
Base (BAS) A nonacid or a proton acceptor, characterized by excess
of hydroxide ions (OH-) and a pH greater than 7. A ring-shaped,
nitrogen-containing organic molecule that is one of the com-
ponents of a nucleotide, namely, adenine, guanine, cytosine,
thymine, and uracil; also known as a nitrogenous base.
Basement membrane Thin, extracellular layer between epithelium and
connective tissue consisting of a basal lamina and a reticular lamina.
Basilar membrane (BAS-i-lar) A membrane in the cochlea of the inter-
nal ear that separates the cochlear duct from the scala tympani
and on which the spiral organ (organ of Corti) rests.
Basophil (BA-so-fil) A type of white blood cell characterized by a pale
nucleus and large granules that stain blue-purple with basic dyes.
Belly The abdomen. The gaster or prominent, fleshy part of a skel-
Beta cell (BA-ta) A type of cell in the pancreatic islets (islets of Lang-
erhans) in the pancreas that secretes the hormone insulin.
Bicuspid valve (bT-KUS-pid) Atrioventricular (AV) valve on the left
side of the heart. Also called the mitral valve.
Bilateral (bT-LAT-er-al) Pertaining to two sides of the body.
Bile (BTLj A secretion of the liver consisting of water, bile salts, bile
pigments, cholesterol, lecithin, and several ions; it emulsifies
lipids prior to their digestion.
Bilirubin (bil-e-ROO-bin) An orange pigment that is one of the end
products of hemoglobin breakdown in the hepatocytes and is
excreted as a waste material in bile.
Blastocyst (BLAS-to-sist) In the development of an embryo, a hollow
ball of cells that consists of a blastocyst cavity (the internal cav-
ity), trophoblast (outer cells), and inner cell mass.
Blastomere (BLAS-to-mer) One of the cells resulting from the cleav-
age of a fertilized ovum.
Blind spot Area in the retina at the end of the optic (II) nerve in which
there are no photoreceptors.
Blood The fluid that circulates through the heart, arteries, capillar-
ies, and veins and that constitutes the chief means of transport
within the body.
Blood-brain barrier (BBB) A barrier consisting of specialized brain
capillaries and astrocytes that prevents the passage of materials
from the blood to the cerebrospinal fluid and brain.
Blood pressure (BP) Force exerted by blood against the walls of
blood vessels due to contraction of the heart and influenced by
the elasticity of the vessel walls; clinically, a measure of the pres-
sure in arteries during ventricular systole and ventricular diastole.
S e e a ls o
mean arterial blood pressure.
Blood-testis barrier (BTB) A barrier formed by Sertoli cells that
prevents an immune response against antigens produced by
spermatogenic cells by isolating the cells from the blood.
Body cavity A space within the body that contains various internal
Body fluid Body water and its dissolved substances; constitutes
about 60% of total body mass.