glossary G-25vocal cords A pair of elastic ligaments on either side of the
larynx wall. Air forced between them causes the cords to
vibrate and produce sounds.
water (hydrologic) cycle The movement of water from
oceans to the atmosphere, the land, and back to the ocean.
white blood cell Leukocyte; any of the macrophages, eosino-
phils, neutrophils, and other cells that are the central compo-
nents of the immune system.
white matter Of the spinal cord, major nerve tracts so named
because of the glistening myelin sheaths of their axons.
X chromosome A sex chromosome with genes that cause an
embryo to develop into a female, provided that it inherits a
pair of these.
X inactivation A compensating phenomenon in females that
“switches off” one X chromosome soon after the first cleav-
ages of the zygote.X-linked gene Any gene on an X chromosome.
X-linked recessive inheritance Recessive condition
in which the responsible, mutated gene occurs on the X
chromosome.
Y chromosome A sex chromosome with genes that cause
the embryo that inherited it to develop into a male.
yellow marrow Bone marrow that consists mainly of fat and
hence appears yellow. It can convert to red marrow and pro-
duce red blood cells if the need arises.
Y-linked gene Any gene on a Y chromosome.yolk sac One of four extraembryonic membranes. Part
becomes a site of blood cell formation and some of its cells
give rise to the forerunners of gametes.
zona pellucida A protein layer around an ovarian follicle.
zygote (zye-goat) The first cell of a new individual, formed
by the fusion of a sperm nucleus with the nucleus of an egg
(fertilization).veins Of the circulatory system, the large-diameter vessels
that lead back to the heart.
ventricle (ven-tri-kul) Of the heart, one of two chambers
from which blood is pumped out. Compare atrium.
venule Small blood vessel that receives blood from tissue
capillaries and merges into larger-diameter veins; a limited
amount of diffusion occurs across venule walls.
vertebra (plural: vertebrae) One of a series of hard bones
arranged with intervertebral disks into a backbone.
vertebrate Animal having a backbone of bony segments, the
vertebrae.
vesicle (vess-ih-kul) [L. vesicula, little bladder] One of a vari-
ety of small membrane-bound sacs in the cell cytoplasm that
function in the transport, storage, or digestion of substances
or in some other activity.
vestibular apparatus A closed system of fluid-filled canals
and sacs in the inner ear that functions in the sense of bal-
ance. Compare semicircular canals.
villus (vil-us), plural: villi. Any of several types of absorptive
structures projecting from the free surface of an epithelium.
virulence The relative ability of a pathogen to cause serious
disease.
virus A noncellular infectious agent consisting of DNA or
RNA and a protein coat; can replicate only after its genetic
material enters a host cell and takes over its metabolic
machinery.
vision Sensory reception of visual stimuli (especially light)
followed by image formation in the brain.
visual cortex Part of the brain that receives signals from the
optic nerves.
vital capacity Maximum volume of air that can move out of
the lungs after a person inhales as deeply as possible.
vitamin Any of numerous organic substances that the body
requires in small amounts for normal cell metabolism but
generally cannot synthesize for itself.
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