are divided, only the sensation of the part to which they
are distributed is lost. Thus, if the optic nerve is divided, the
sense of vision disappears, but the motions of the eye are performed
as readily as before. But, if the spinal nerves are divided, both
sensation and motion of the part to which they lead are destroyed.

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What portions have been removed without impairing the intellect? What
is remarkable of the brain? 775. What is the influence of the brain
upon the muscles? 776. What do some physiologists assert of the
medulla oblongata? 777. What is remarkable of the nerves? Give the 1st
observation relative to the cranial nerves.

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2d. When the spinal cord is divided or compressed, as in fractures of
the spinal column, all parts below the fracture are paralyzed, though
the nerves leading to these parts may be uninjured.

3d. Again, one side of the body or one limb may become insensible, and
the power to move it, be perfectly retained; or the reverse of this
may happen--the power of motion will be lost while sensation remains.
In the former instance, the function of the posterior, or sensitive
column of the spinal cord on one side is destroyed; in the latter, the
anterior, or motor column is affected.

4th. In some cases, both sensation and motion of one side of the body
or one limb are destroyed. In such instances, both the anterior and
the posterior columns of one side of the spinal cord are diseased.

778. Vigorous and controllable muscular contraction requires a sound
and well-developed brain. If this organ is defective in these
particulars, the movements will be inefficient, and may be irregular.
The central organ of the nervous system must, likewise, be in an
active condition, to induce regular, steady, and controllable muscular
movements.

_Observations._ 1st. Persons who have suffered from apoplexy and other
severe diseases of the brain, have an involuntary trembling of the
limbs, which results from a weakened state of the nervous system.

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To the spinal nerves. What is said of the compression of the spinal
cord? Give the 3d observation relative to the spinal nerves. The 4th
observation. 778. Upon what does vigorous controllable muscular
contraction depend? What causes the involuntary trembling of the limbs
in persons who have suffered from apoplexy?

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2d. The tremor of the hand, that