Page 214 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy of the Nervous System / 199

communication systems of the body, the called the medulla) contains important
autonomic centers and nuclei for cranial
nervous and endocrine systems.
VetBooks.ir Mesencephalon. The mesencephalon, nerves.
or midbrain, lies between the diencephalon Ventricular System. The ventricles of
rostrally and the pons caudally. The two the brain develop from the lumen of the
cerebral peduncles and four colliculi are embryonic neural tube (Fig. 10‐10). Right
the most prominent features of the and left lateral ventricles lie within the
midbrain. respective cerebral hemispheres. They
The two cerebral peduncles, also called communicate with the midline third
crura cerebri (singular crus cerebri), are ventricle by way of two interventricular
large bundles of nerve fibers connecting foramina. The third ventricle is
the spinal cord and brainstem to the cere- surrounded by the diencephalon. It
bral hemispheres. These peduncles consist connects with the fourth ventricle by way
largely (but not exclusively) of descending of the narrow mesencephalic aqueduct
motor fiber tracts. (cerebral aqueduct) passing through the
The colliculi (corpora quadrigemina) are midbrain. The fourth ventricle, between
four small bumps (colliculus is Latin for the cerebellum above and pons and
little hill) on the dorsal side of the midbrain. medulla below, communicates with the
They consist of right and left rostral colli- subarachnoid space surrounding the CNS
culi and right and left caudal colliculi. through two lateral apertures.
The rostral colliculi coordinate certain Each ventricle features a choroid
visual reflexes, and the caudal colliculi are plexus, a tuft of capillaries that protrudes
relay nuclei for audition (hearing). into the lumen of the ventricle. The plexus
of capillaries is covered by a layer of epend-
Metencephalon. The metencephalon ymal cells that are continuous with the
includes the cerebellum dorsally and the lining membrane of the ventricles.
pons ventrally. The cerebellum features The choroid plexus is responsible for
two lateral hemispheres and a median
ridge called the vermis because of its forming the bulk of the cerebrospinal fluid
(CSF) that fills the ventricular system and
resemblance to a worm. The surface of the surrounds the CNS. A smaller contribu-
cerebellum consists of many laminae called tion to that formation is made by the
folia. In the cerebellum, like the cerebrum, ependyma lining the ventricles. CSF is
the white matter is central, and the gray continuously formed, circulated, and reab-
matter is peripheral in the cerebellar sorbed in the spaces of the nervous sys-
cortex. The cerebellum is critical to the tem; its entire volume is replaced several
accurate timing and execution of times a day.
movements; it acts to smooth and The circulation of cerebrospinal fluid
coordinate muscle activity. begins in the two lateral ventricles (where
The pons is ventral to the cerebellum, the majority is produced). It flows through
and its surface possesses visible transverse the interventricular foramina into the third
fibers that form a bridge from one hemi- ventricle, then by way of the cerebral aque-
sphere of the cerebellum to the other. Many duct into the fourth ventricle, and finally
other fiber tracts and cranial nerve nuclei through the lateral apertures into the suba-
make up the remainder of the pons.
rachnoid space, where it surrounds both
Myelencephalon. The myelencephalon the brain and spinal cord. CSF is reab-
becomes the medulla oblongata in the sorbed back into the venous blood via spe-
adult. It is the cranial continuation of the cial modifications of the meninges called
spinal cord, from which it is arbitrarily arachnoid granulations. These tiny
distinguished at the foramen magnum. structures protrude into the blood‐filled
The medulla oblongata (often simply dural sinuses (see later) and act as one‐way