Developmental psychology.pdf

90 The Human Organism
the remotest parts of the body; others are densely packed in countless cables of nerve
tissues. For these reasons, and because the brain's operation remains such a mystery,
this tangle of exquisitely fine fibers has justifiably been called "the great raveled knot."
This chapter is the story of our efforts to understand this tangled tissue and
its related structures, and the metaphor of the knot is used to tie things together. In a
spirit of detective work, we follow the thinking of several curious minds in the history
of psychophysiology, as they attempt to unravel this evolutionary phenomenon.
We begin with an examination of the outlying strands, called the peripheral
nervous system. Then we turn to the chief mass of this tissue, the cerebral cortex. Next,
a special probe is used to penetrate the brain's interior, providing information on the
underlying regions, called subcortical structures. We then change our focus and consider
certain parts of the body that serve largely a maintenance function, providing an
optimal environment within which the nervous systems can operate. Finally, we conclude
with a discussion of special research methods and the techniques behind them,
which have made possible these investigations of interlocking problems.
NEURAL PATHWAYS AND IMPULSES
The ancient Greeks did not recognize the brain as the fundamental organ of thought.
Aristotle decided instead that it served to cool the blood and that the mind was situated
in the heart. Plato, his teacher, correctly decided that thought arose in the head, but
for the wrong reason. He was impressed with its spherical shape, as opposed to other
body parts, rather than its anatomical features.
The idea that the nervous system underlies all human behavior has been accepted
for only three centuries, and the early thoughts about its mode of operation are
humorous by today's standards. The brain at one time was believed to be a reservoir
of various fluids, and these were pumped to the muscles through the many small tubes
now known as nerve fibers. The body parts moved whenever the fluid inflated appropriate
muscles, as could be observed when the muscles were flexed. When the nervous
tubes were destroyed, preventing the flow of fluid, the muscles necessarily became inoperative
and paralysis was the result.
By the beginning of the nineteenth century, the major nerves had been identified,
but it was thought that they were all the same type. A given nerve somehow was
responsible for sensations and also for movements. It could bring about a feeling of
warmth under the collar and at the same time a movement of the neck. How was it
possible? How could the nerves transmit both reactions simultaneously?
Peripheral Nervous System
A Scottish anatomist, Sir Charles Bell, attempted to answer this question. He doubted
that any nerve could transmit both types of impulses, and he observed in laboratory
studies of certain animals that nerves extend down the spinal cord in the front and
back. Possibly different functions were involved in these different locations.
Bell stimulated the nerve going up the back, and there was no reaction. Then
he touched a nerve in the front, and immediately the animal showed a muscular contraction.
Impressed with this result, Bell made a series of such tests and concluded that
there are two basic types of spinal nerves, each serving a different function. Not wanting
to be too bold, he wrote a short report on his findings and gave it the title An Idea
of a New Anatomy of the Brain. He distributed only a few hundred copies, largely for
the benefit of friends (Bell, 1811).