HUMAN BIOLOGY

252 Chapter 13

the brain: Command Central

The forebrain is the most highly developed brain region.

It includes the cerebrum, where information is processed

and sensory input and motor responses are integrated.

The cerebrum consists of two cerebral hemispheres. A

band of nerve tracts, the corpus callosum, carries signals

between the hemispheres. The thalamus lies just below

the corpus callosum. It is mainly a “switchboard” where

incoming signals in sensory nerve tracts are relayed to

clusters of neuron cell bodies called basal nuclei and then

sent onward. The basal nuclei also process some outgoing

motor information.

Located under the thalamus, the hypothalamus is the

body’s “supercenter” for controlling homeostatic adjust-

ments in the activities of internal organs. It also helps to

govern thirst, hunger, and sexual behavior.

Cerebrospinal fluid fills spaces in the brain

and spinal cord

In addition to being shielded by its bony case, the brain is

protected by three meninges (meh-nin-jeez). These are

membranes of connective tissue layered between the skull

and the brain (Figure 13.17). Meninges cover the fragile

CNS neurons and blood vessels that service the tissue.

The leathery, outer membrane, the dura mater, is folded

double around the brain. Its upper surface attaches to

the skull. The lower surface is the brain’s outer covering

and separates its two hemispheres. A second

membrane is called the arachnoid, and the even

more delicate pia mater wraps the brain and

spinal cord. The meninges also enclose spaces

called ventricles.

Our brain and spinal cord would both be

extremely vulnerable to damage if they were

not protected by bones and meninges. Both

also contain cerebrospinal fluid, or CSF. This

transparent fluid forms from blood plasma and

n The brain is divided into three main regions, each one

containing centers that manage specific biological tasks.

The spinal cord merges with the brain, which weighs about

3 pounds (1,300 grams) in an adult. Just as the bony verte-

brae protect the spinal cord, the cranial bones of the skull,

or cranium, protect the brain.

the brain’s three main functional areas

are the hindbrain, midbrain, and forebrain

The hindbrain sits atop the spinal cord (Figure 13.16). The

portion just above the cord, the medulla oblongata

(meh-dull-uh ahb-lawn-gah-tuh), helps govern breath-

ing rhythm and the strength of heartbeats. It also controls

reflexes such as swallowing, coughing, sneezing, and vom-

iting. Above the medulla is the pons (pa h nz), which helps

regulate breathing. Pons means “bridge,” and nerve tracts

extend through it to the midbrain. The cerebellum is the

largest hindbrain region. It lies at the back of the brain and

mainly coordinates voluntary movements.

The midbrain is the smallest of the three brain regions.

It mainly relays information from sensory organs to the

forebrain. Together, the pons, medulla, and midbrain form

the brain stem. When the brain stem is damaged by a

stroke, disease, or a head injury, the results can be severe

or even fatal.

Cerebrum (the right
hemisphere, at the
longitudinal fissure
between it and the
left hemisphere)

Hypothalamus Thalamus Pineal gland

Corpus
callosum

Optic
chiasm

Midbrain

Pons

Medulla

Cerebellum

Brain

stem

F i g u r e 13.16 Animated! The brain has three major regions that are subdivided into functional areas. a Major brain areas in the right

hemisphere. B The two brain hemispheres viewed from above. (A: Mark Nielsen, Department of Biology, University of Utah; B: Dr. Colin Chumbley/Science Source)

13.7

a B

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