has been removed without the appearance of untoward symptoms. |
The four parathyroid glands, each one about the size of a grape seed,
which are located on the back surface of the thyroid, are essential in
maintaining the normal calcium content of the blood. The loss of all
four results in fatal tetanic convulsions, but the loss of only two is not
followed by any disturbance at all.

Margins of Safety in the Nervous System. What holds true for paired
organs also is true for various paired nerves. A pair of nerves called the
vagus nerves, or vagi, form the grand trunk line for the outflow of nerve
impulses to organs of the chest and abdomen from the cranial division
of the autonomic nervous system. If both vagi are cut, the rhythm
of respiration is completely changed, the heart beats considerably faster
than usual, and digestion is seriously upset. Yet if only one of the vagi
is cut everything goes on as usual.

The cerebrum has two hemispheres and might for that reason be called a bilateral or
paired organ, but this is not a true designation in the sense that both hemispheres have
the same functions. We know that the cerebrum is a collection of many centers, both
sensory and motor, the injury of any one of which results in some sensory or motor
disturbance in a definite area of the body. Motor centers in the left hemisphere control
movements on the right side of the body, and those in the right hemisphere attend to
muscular movements on the left. Removal or serious damage to one cerebral hemi-
sphere results in paralysis on the opposite side of the body. This does not hold true,
however, for muscles contracting on both sides simultaneously—for example, those used
in swallowing and in breathing. In the case of these muscles the motor area of one

side can take charge of the muscles of both sides and thus maintain functions that are
required for continued existence.

Nerve cells, on the whole, do not regenerate. No new ones are created
to take the place of those destroyed as a result of disease or injury or
old age, as is the case for most of the other cells in the body. Since this
is true, we might assume that the brain has a very small margin of
safety. However, we seem to have a superfluity of brain tissue. Surgery
for the removal of brain tumor has brought to light the fact that one or
both of the two frontal lobes of the cerebrum may be completely removed
without interfering with cerebral function. Even though important
nerve centers in the brain or spinal cord may be destroyed, it is some-
times possible to educate adjacent centers or to arouse dormant centers
to take up the work of the lost ones. For example, when groups of
motor nerve cells in the spinal cord have been destroyed by the virus of
poliomyelitis, new connections between adjacent centers and the muscles
affected may be established.

Factors of Safety in Unpaired Organs. In the case of unpaired organs
we often find an even greater amount of surplus tissue than in paired
organs. The internal secretion, insulin, which is manufactured by
clusters of cells embedded in the pancreas, is required for the utilization
of sugar by the body. Complete removal of the pancreas in experi-
mental animals results at once in extreme diabetes. Yet if only one
fifth of the normal pancreas is left, enough insulin is produced to supply

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