The function of
the nervous system is to recieve messages from outside the body and
after analysing them, to cause the animal to respond in an appropriate
way. The nervous system is essentially divided into two parts:
The central nervous system This consists of
the brain and spinal cord, both of which are protected by bone - the
skull in the case of the brain and the bony canal through the vertebrae
in the case of the spinal cord.
The peripheral nervous system This consists of
nerves which connect the central nervous system to the rest of the
body. If a ball is thrown for a dog to catch, messages are sent
through the sensory nerves from the light-sensitive nerve endings in
the eye to the brain. This in turn sends messages via the motor
nerves to the muscles which move the dog into the right place and open
its mouth at the correct time.
impulses are sent automatically from an organ, for example the stomach,
when it is full, and the central nervous system responds, also
automatically, to send a nerve impulse to the muscles in the organ to
make them contract and empty it. In some cases, eg the urinary
bladder, the brain can override the automatic response, as occurs in
house trained animals. In other cases, for example the filling
and emptying of the stomach, the nervous impulses which case
contraction of the muscles and dilation of the sphincter are purely
automatic and cannot be overridden consciously.