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Central Nervous System

The Central Nervous System (CNS) is composed of the brain and spinal cord. The
CNS is surrounded by bone-skull and vertebrae. Fluid and tissue also insulate the brain
and spinal cord.

Areas of the Brain

The brain is composed of three parts: the cerebrum (seat of consciousness), the
cerebellum, and the medulla oblongata (these latter two are “part of the unconscious

The medulla oblongata is closest to the spinal cord and is involved with the
regulation of heartbeat, breathing, vasoconstriction (blood pressure), and reflex centers
for vomiting, coughing, sneezing, swallowing and hiccupping. The hypothalamus
regulates homeostasis. It has regulatory areas for thirst, hunger, body temperature,
water balance and blood pressure and links the nervous system to the Endocrine
System. The midbrain and pons are also part of the unconscious brain. The thalamus
serves as a central relay point for incoming nervous messages.

The cerebellum is the second largest part of the brain, after the cerebrum. It
functions for muscle coordination and maintains normal muscle tone and posture. The
cerebellum coordinates balance.

The conscious brain includes cerebral hemispheres, which are separated by the
corpus callosum. In reptiles, birds, and mammals, the cerebrum coordinates sensory
data and motor functions. The cerebrum governs intelligence and reasoning, learning
and memory. While the cause of memory is not yet definitely known, studies on slugs
indicate learning is accompanied by a synapse decrease. Within the cell, learning
involves change in gene regulation and increased ability to secrete transmitters.

The Brain
During embryonic development, the brain first forms a tube, the anterior end

which enlarges into three hollow swellings that form the brain, and the posterior of which
develops into spinal cord. Some parts of the brain have changed little during vertebrate
evolutionary history.

Parts of the Brain as seen from the Middle of the Brain

Vertebrate evolutionary trends include:

1. Increase in brain size relative to body size.
2. Subdivision and increasing specialization of the forebrain, midbrain and hindbrain.
3. Growth is relative in size of the fore brain, especially the cerebrum, which is

associated with increasingly complex behavior in mammals.

The Brain Stem and Midbrain

The brain stem is the smallest and from an evolutionary viewpoint, the oldest and
most primitive part of the brain. The brain stem is continuous with the spinal cord, and is
composed of the parts of the hindbrain and midbrain. The medulla oblongata and pons
control heart rate, constriction of blood vessels, digestion and respiration.

The midbrain consists of connections between the hindbrain and forebrain.
Mammals use this part of the brain only for eye reflexes.

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