ejaculatory duct and urethra. The tube connects the
testes with the urethra and has thick muscles to move
the sperm down the tract.
See alsoSPERM.
vector A vector can be a bacterium or virus that does
not cause disease in humans. It is often used in geneti-
cally engineered vaccines to transport antigen-encoding
genes into the body to initiate an immune response.
Additionally, a vector can be an organism, usually an
insect, that transmits an infectious agent to its alternate
host. Examples are malaria, where the vector of the
parasite is mosquitoes, and the hosts are humans.
vegetative reproduction Asexual reproduction in
plants from vegetative parts (stems, leaves, or roots) or
from modified stems (bulbs, tubers, rhizomes, and
stolons). There is no exchange of gametes, and unless
mutations occur, each new generation of plants is
genetically identical to the parent plant.
vein A thin-walled blood vessel that carries blood to
theheart. Smaller veins called venules connect veins to
capillaries.
In plants, they are the vascular bundles in leaves that
consist of xylem and phloem. The veins are large along
the leaf midvein and petiole and get smaller as they radi-
ate out into the leaf, becoming known as veinlets.
See alsoARTERY.
ventilation Ventilation is the passage of air into and
out of the respiratory tract. Ventilation exchange (VE)
is the exchange of oxygen and carbon dioxide and
other gases during the passage of air into and out of
the respiratory passages.
ventricle The chamber of the heart that receives blood
from the atrium (chamber that collects blood returning
from the rest of the body) and contracts and pumps
blood away from the heart. There are two ventricles in
mammals and birds. The right ventricle pumps deoxy-
genated blood to the lungs via the pulmonary artery; the
left ventricle pumps oxygenated blood to the body via the
aorta. The tricuspid valve separates the right atrium and
the right ventricle and prevents backflow from the ventri-
cle to the atrium. The mitral valve separates the left atri-
um and the left ventricle and prevents backflow from the
ventricle to the atrium. The aortic valve prevents back-
flow of blood from the aorta into the left ventricle.
There are four brain ventricles. The right and left
lateral ventricles lie within the cerebral cortex and con-
nect with the very narrow third ventricle by way of
openings called interventricular foramina. This third
ventricle lies between the two halves of the thalamus
and connects to the fourth ventricle via a long, thin
tube called the cerebral aqueduct. The fourth ventricle
lies in the hindbrain, under the cerebellum and dorsal
to the pons and medulla.vernalization Arequired chilling period leading to
the breaking of dormancy in plants. Flowering can be
accelerated or induced by exposure to a long period of
near-freezing temperatures.vertebra One of the segments of bone or cartilage
composing the spinal column of vertebrates. In the
spinal column of adult humans, there are33 vertebrae:
seven cervical vertebrae in the neck, 12 thoracic verte-
brae that support the ribs, five lumbar vertebrae in the
lower back, and other fused vertebrae forming the
sacrum and the coccyx, beneath the sacrum.vertebral bodies The oval segments of bone on the
spine that support most of the axial load of the spine
and consist mostly of the thoracic and lumbar verte-
brae. The lumbar part of the spine—consisting of five
lumbar vertebral bodies that sit on top of the sacrum—
is above the coccyx, the tailbone. The lumbar spine
supports most of the weight of the body, and its verte-
bral bodies are larger than the rest. The thoracic part
of the spine consists of 12 bodies. The thoracic region
supports the cervical spine (neck), which has seven
bodies and supports the head.vertebrates Chordate animals that have a spinal col-
umn or backbone (with the exception of hagfish and
the rudimentary form of lampreys). The categoryvertebrates 339