Human Physiology, 14th edition (2016)

208 Chapter 8

humans, forming the two enormous cerebral hemispheres (of
the cerebrum ) that cover the diencephalon, the midbrain, and a
portion of the hindbrain. Also, notice that the CNS begins as a
hollow tube, and indeed remains hollow as the brain regions are
formed. The cavities of the brain are known as ventricles and
become filled with cerebrospinal fluid (CSF). The cavity of the
spinal cord is called the central canal, and is also filled with
CSF ( fig. 8.4 ).
Cerebrospinal fluid is formed by structures called the
choroid plexuses, which are thin structures protruding into
the ventricles. The choroid plexuses consist of a simple epi-
thelium associated with blood capillaries containing a fenes-
trated endothelium (chapter 13, section 13.6). These form CSF
by diffusion and active transport, rather than by the simple fil-
tration of blood plasma. As a result, the CSF has a constant

composition, is slightly hypertonic, and differs from plasma

in its concentration of various ions. CSF in the ventricles is

moved by pulsations of the choroid plexuses and by cilia on

the ependymal cells that line the ventricles. As a result, CSF

exits through openings in the brain stem to fill the subarach-

noid space between the pia (the membrane adhering to the

brain and spinal cord) and the middle arachnoid membrane.

The CSF is reabsorbed into venous blood across arachnoid

villi that project through the dura mater, the outer connective

tissue covering of the CNS. The average CSF volume of 140 to

200 mL is turned over 3–5 times per day, producing a normal

intracranial pressure of 5–15 mmHg.

The CNS is composed of gray and white matter, as

described in chapter 7. The gray matter, containing neuron cell

bodies and dendrites, is found in the cortex (surface layer) of

Figure 8.2 Embryonic development of the CNS. This dorsal view of a 22-day-old embryo shows transverse sections at
three levels of the developing central nervous system.

Cranial neuropore

Caudal neuropore

Neural groove

Neural crest

Waldrop

Neural crest

Neural groove

Neural crest

Neural crest

Neural canal

Neural tube

Neural groove

Wall of

yolk sac

Figure 8.3 The developmental sequence of the brain. ( a ) During the fourth week, three principal regions of the brain are
formed. ( b ) During the fifth week, a five-regioned brain develops and specific structures begin to form.

Adult derivatives of

Cavities

Cerebral

hemisphere

Thalamus

Hypothalamus

Midbrain

Lateral

ventricles

Third

ventricle

Aqueduct

Cerebellum

Medulla

oblongata

Upper

portion

Lower

portion

of fourth

ventricle

Spinal cord

Diencephalon

Mesencephalon

Metencephalon

Myelencephalon

Telencephalon

Three primary vesicles

Wall Cavity

Mesencephalon

(midbrain)

(hindbrain)

Prosencephalon

(forebrain)

Five secondary vesicles

Walls

Pons

Rhombencephalon