Apneustic area (ap-NOO-stik) A part of the respiratory center in the
pons that sends stimulatory nerve impulses to the inspiratory
area that activate and prolong inhalation and inhibit exhalation.
Aponeurosis (ap'-o-noo-RO-sis) A sheetlike tendon joining one
muscle with another or with bone.
ap'-op-TO-sis) Programmed cell death;
a normal type of cell death that removes unneeded cells during
embryological development, regulates the number of cells in
tissues, and eliminates many potentially dangerous cells such as
cancer cells. During apoptosis, the DNA fragments, the nucleus
condenses, mitochondria cease to function, and the cytoplasm
shrinks, but the plasma membrane remains intact. Phagocytes
engulf and digest the apoptotic cells, and an inflammatory
response does not occur.
Aqueous humor (AK-we-us HO-mer) The watery fluid, similar in compo-
sition to cerebrospinal fluid, that fills the anterior cavity of the eye.
Arachnoid mater (a-RAK-noyd MA-ter) The middle of the three
meninges (coverings) of the brain and spinal cord. Also termed
Arachnoid villus (VIL-us) Berrylike tuft of the arachnoid mater that
protrudes into the superior sagittal sinus and through which
cerebrospinal fluid is reabsorbed into the bloodstream.
Areola (a-RE-o-la) Any tiny space in a tissue. The pigmented ring
around the nipple of the breast.
Arm The part of the upper limb from the shoulder to the elbow.
Arrhythmia (a-RITH-me-a) An irregular heart rhythm. Also called a
Arteriole (ar-TE-re-ol) A small, almost microscopic, artery that deliv-
ers blood to a capillary.
Arteriosclerosis (ar-te-re-o-skle-RO-sis) Group of diseases character-
ized by thickening of the walls of arteries and loss of elasticity.
Artery (AR-ter-e) A blood vessel that carries blood away from the
Arthritis (ar-THRT-tis) Inflammation of a joint.
Arthrology (ar-THROL-o-je) The study or description of joints.
Arthroscopy (ar-THROS-co-pe) A procedure for examining the inte-
rior of a joint, usually the knee, by inserting an arthroscope into
a small incision; used to determine extent of damage, remove
torn cartilage, repair cruciate ligaments, and obtain samples for
Arthrosis (ar-THRO-sis) A joint or articulation.
Articular capsule (ar-TIK-u-lar) Sleevelike structure around a synovial
joint composed of a fibrous capsule and a synovial membrane.
Articular cartilage (KAR-ti-lij) Hyaline cartilage attached to articular
Articular disc Fibrocartilage pad between articular surfaces of bones
of some synovial joints. Also called a meniscus (men-IS-kus).
Articulation (ar-tik'-u-La-shun) A joint; a point of contact between
bones, cartilage and bones, or teeth and bones.
Ascending colon (KO-lon) The part of the large intestine that passes
superiorly from the cecum to the inferior border of the liver,
where it bends at the right colic (hepatic) flexure to become the
Ascites (as-ST-tez) Abnormal accumulation of serous fluid in the
Association areas Large cortical regions on the lateral surfaces of
the occipital, parietal, and temporal lobes and on the frontal
lobes anterior to the motor areas connected by many motor
and sensory axons to other parts of the cortex. The association
areas are concerned with motor patterns, memory, concepts of
word-hearing and word-seeing, reasoning, will, judgment, and
Asthma (AZ-ma) Usually allergic reaction characterized by smooth
muscle spasms in bronchi resulting in wheezing and difficult
breathing. Also called bronchial asthma.
Astigmatism (a-STIG-ma-tizm) An irregularity of the lens or cornea of
the eye causing the image to be out of focus and producing faulty
Astrocyte (AS-tro-sTt) A neuroglial cell having a star shape that
participates in brain development and the metabolism of neu-
rotransmitters, helps form the blood-brain barrier, helps maintain
the proper balance of K+ for generation of nerve impulses, and
provides a link between neurons and blood vessels.
Ataxia (a-TAK-se-a) A lack of muscular coordination, lack of precision.
Atherosclerotic plaque (ath'-er-o-skle-RO-tic PLAK) A lesion that
results from accumulated cholesterol and smooth muscle fibers
(cells) of the tunica media of an artery; may become obstructive.
Atom Unit of matter that makes up a chemical element; consists of
a nucleus (containing positively charged protons and uncharged
neutrons) and negatively charged electrons that orbit the nucleus.
Atomic mass (weight) Average mass of all stable atoms of an ele-
ment, reflecting the relative proportion of atoms with different
Atomic number Number of protons in an atom.
Atrial fibrillation (A-tre-al fib-ri-LA-shun) Asynchronous contraction
of cardiac muscle fibers in the atria that results in the cessation of
Atrial natriuretic pentide (ANP) (na'-tre-u-RET-ik) Peptide hormone,
produced by the atria of the heart in response to stretching, that
inhibits aldosterone production and thus lowers blood pressure;
causes natriuresis, increased urinary excretion of sodium.
Atrioventricular (AV) bundle (a'-tre-o-ven-TRIK-u-lar) The part of the
conduction system of the heart that begins at the atrioventricular
(AV) node, passes through the cardiac skeleton separating the
atria and the ventricles, then extends a short distance down the
interventricular septum before splitting into right and left bundle
branches. Also called the bundle of His (HISS).
Atrioventricular (AV) node The part of the conduction system of the
heart made up of a compact mass of conducting cells located in
the septum between the two atria.
Atrioventricular (AV) valve A heart valve made up of membranous
flaps or cusps that allows blood to flow in one direction only, from
an atrium into a ventricle.
Atrium (A-tre-um) A superior chamber of the heart.
Atrophy (AT-ro-fe) Wasting away or decrease in size of a part, due to
a failure, abnormality of nutrition, or lack of use.