Platelet plug Aggregation of platelets at a site where a blood vessel
is damaged that helps stop or slow blood loss.
Pleura (PLOOR-a) The serous membrane that covers the lungs and
lines the walls of the chest and the diaphragm.
Pleural cavity Small potential space between the visceral and pari-
etal pleurae.
Plexus (PLEK-sus) A network of nerves, veins, or lymphatic vessels.
Pluripotent (plu-RIP-o-tent) stem cell Immature stem cell in red bone
marrow that gives rise to precursors of all the different mature
blood cells.
Pneumotaxic area (noo-mo-TAK-sik) A part of the respiratory center
in the pons that continually sends inhibitory nerve impulses to
the inspiratory area, limiting inhalation and facilitating exhalation.
Podiatry (po-DT-a-tre) The diagnosis and treatment of foot disorders.
Polar body The smaller cell resulting from the unequal division of
primary and secondary oocytes during meiosis. The polar body
has no function and degenerates.
Polycythemia (pol'-e-sT-THE-me-a) Disorder characterized by an
above-normal hematocrit (above 55%) in which hypertension,
thrombosis, and hemorrhage can occur.
Polysaccharide (pol'-e-SAK-a-rTd) A carbohydrate in which three or
more monosaccharides are joined chemically.
Polyunsaturated fat (pol'-e-un-SACH-u-ra'-ted) A fatty acid that contains
more than one double covalent bond between its carbon atoms;
abundant in triglycerides of corn oil, safflower oil, and cottonseed oil.
Polyuria (pol'-e-U-re-a) An excessive production of urine.
Pons (PONZ) The part of the brain stem that forms a "bridge"
between the medulla oblongata and the midbrain, anterior to the
Positive feedback A feedback mechanism in which the response
enhances the original stimulus.
Postcentral gyrus A gyrus of the cerebral cortex located immediate-
ly posterior to the central sulcus; contains the primary somato-
sensory area.
Posterior (pos-TER-e-or) Nearer to or at the back of the body. Equiva-
lent to dorsal in bipeds.
Posterior column-medial lemniscus pathways (lem-NIS-kus) Sensory
pathways that carry information related to proprioception, touch
pressure, and vibration. First-order neurons project from the
spinal cord to the ipsilateral (same side) medulla in the posterior
columns. Second-order neurons project from the medulla to the
contralateral (opposite side) thalamus in the medial lemniscus.
Third-order neurons project from the thalamus to the somatosen-
sory cortex (postcentral gyrus) on the same side.
Posterior pituitary (pi-TOO-i-tar-e) Posterior lobe of the pituitary
gland. Also called the neurohypophysis (noo-ro-hT-POF-i-sis).
Posterior root The structure composed of sensory axons lying
between a spinal nerve and the dorsolateral aspect of the spinal
cord. Also called the dorsal (sensory) root.
Posterior root ganglion (GANG-gle-on) A group of cell bodies of
sensory neurons and their supporting cells located along the
posterior root of a spinal nerve. Also called a dorsal (sensory) root
Postganglionic neuron (post'-gang-le-ON-ik NOO-ron) The second
autonomic motor neuron in an autonomic pathway, having its
cell body and dendrites located in an autonomic ganglion and its
unmyelinated axon ending at cardiac muscle, smooth muscle, or
a gland.
Postsynaptic neuron (post-sin-AP-tik) The nerve cell that is activated
by the release of a neurotransmitter from another neuron and
carries nerve impulses away from the synapse.
Precapillary sphincter (SFINGK-ter) A ring of smooth muscle fibers
(cells) at the site of origin of true capillaries that regulate blood
flow into true capillaries.
Precentral gyrus (JT-rus) A gyrus of the cerebral cortex located
immediately anterior to the central sulcus; contains the primary
motor area.
Preganglionic neuron (pre'-gang-le-ON-ik) The first autonomic motor
neuron in an autonomic pathway, with its cell body and dendrites
in the brain or spinal cord and its myelinated axon ending at an
autonomic ganglion, where it synapses with a postganglionic
Pregnancy Sequence of events that normally includes fertilization,
implantation, embryonic growth, and fetal growth and terminates
in birth.
Premenstrual syndrome (PMS) Moderate to severe physical and
emotional stress ocurring late in the postovulatory phase of the
menstrual cycle and sometimes overlapping with menstruation.
Prepuce (PRE-poos) The loose-fitting skin covering the glans of the
penis and clitoris. Also called the foreskin.
Presbyopia (prez-be-O-pe-a) A loss of elasticity of the lens of the eye
due to advancing age with resulting inability to focus clearly on
near objects.
Presynaptic neuron (pre-sin-AP-tik) A neuron that propagates nerve
impulses toward a synapse.
Prevertebral ganglion (pre-VER-te-bral GANG-le-on) A cluster of cell
bodies of postganglionic sympathetic neurons anterior to the
spinal column and close to large abdominal arteries. Also called a
collateral ganglion.
Primary germ layer One of three layers of embryonic tissue, called
ectoderm, mesoderm, and endoderm, that give rise to all tissues
and organs of the body.
Primary motor area A region of the cerebral cortex in the precentral
gyrus of the frontal lobe of the cerebrum that controls specific
muscles or groups of muscles.
Primary somatosensory area (so-ma-to-SEN-so-re) A region of the
cerebral cortex posterior to the central sulcus in the postcentral
gyrus of the parietal lobe of the cerebrum that localizes exactly
the points of the body where somatic sensations originate.
Prime mover The muscle directly responsible for producing a
desired motion. Also called an agonist (AG-o-nist).
Primitive gut Embryonic structure formed from the dorsal part of the
yolk sac that gives rise to most of the gastrointestinal tract.
Proctology (prok-TOL-o-je) The branch of medicine concerned with
the rectum and its disorders.
Progeny (PROJ-e-ne) Offspring or descendants.
Glossary 543
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