Introduction , The Nervous System An Overall View

Position of the Nervous System in
the Body A B The central nervous system (CNS) is divided into the brain, encephalon A1, and the spinal cord SC medulla spinalis (A2). The brain in the cranial cavity is surrounded by a bony capsule; the spinal cord in the vertebral canal is enclosed by the bony vertebral column. Both are covered by meninges that enclose a cavity filled with a fluid, the cerebrospinal fluid. Thus, the CNS is The peripheral nervous system (PNS) includes the cranial nerves, which emerge through holes (foramina) in the base of the skull, and the spinal nerves, which emerge through spaces between the vertebrae (intervertebral foramina) A3 The peripheral nerves extend to muscles and skin areas. They form nerve plexuses before entering the limbs: the brachial plexus (A4) and the lumbosacral plexus (A5) in which the fibers of the spinal nerves intermingle; as a result, the nerves of the limbs contain portions of different spinal nerves (see pp. 70 and 86 . At the entry points of the afferent nerve fibers lie ganglia (A6); these are small oval bodies containing sensory neu

When describing brain structures, terms like top bottom front and back are inaccurate, because we have to distinguish between different axes of the brain (B). Owing to the upright posture of humans, the neural tube is bent; the axis of the spinal cord runs almost vertically, while the axis of the forebrain (Forel’s axis, orange) runs horizontally the axis of the lower brain divisions Meinert’s axis, violet runs obliquely. The positional terms relate to theses axes:
the anterior end of the axis is called oral or rostral (os, mouth; rostrum, beak), the posterior end is called caudal cauda, tail the underside is called basal or ventral (venter,
abdomen), and the upper side is called dorsal
dorsum, back The lower brain divisions, which merge into the spinal cord, are collectively called

the brain stem (light gray) (B7). The anterior
division is called the forebrain (gray) (B8).
The divisions of the brain stem, or encephalic
trunk, have a common structural plan (consisting
of basal plate and alar plate, like the
spinal cord, see p. 13, C). Genuine peripheral
nerves emerge from these divisions, as they
do from the spinal cord. Like the spinal cord,
they are supported by the chorda dorsalis
during embryonic development. All these
features distinguish the brain stem from the
forebrain. The subdivision chosen here
differs from the other classifications in
which the diencephalon is viewed as part of
the brain stem.
The forebrain, prosencephalon, consists of
two parts, the diencephalon and the telencephalon
or cerebrum. In the mature brain,
the telencephalon forms the two hemispheres
(cerebral hemispheres). The diencephalon
lies between the two hemispheres.
A9 Cerebellum.