Owing to the position of the sympathetic ganglia, deeply imbedded in the
tissues of the chest and abdomen, it is exceedingly difficult to subject
them to any satisfactory experiments. A few isolated facts form the
basis of all our knowledge concerning their functions. They give off
both motor and sensory filaments. The contraction of the _iris_ is one
of the most familiar examples of the action of the sympathetic system.

In the reflex actions of the nerves of special sense, the sensation is
transmitted through the cerebro-spinal system, and the motor impulse is
sent to the deep-seated muscles by the sympathetic system. Physiologists
enumerate three kinds of reflex actions, which are either purely
sympathetic, or partially influenced by the cerebro-spinal system. Dr.
Dalton describes them as follows:

_First_.--"Reflex actions taking place from the internal organs, through
the sympathetic and cerebro-spinal systems, to the voluntary muscles and
sensitive surfaces.--The convulsions of young children are often owing
to the irritation of undigested food in the intestinal canal. Attacks of
indigestion are also known to produce temporary amaurosis [blindness],
double vision, strabismus, and even hemiplegia. Nausea, and a diminished
or capricious appetite, are often prominent symptoms of early pregnancy,
induced by the peculiar condition of the uterine mucous membrane."

_Second_.--"Reflex actions taking place from the sensitive surfaces,
through the cerebro-spinal and sympathetic systems to the involuntary
muscles and secreting organs.--Imprudent exposure of the integument to
cold and wet, will often bring on a diarrhea. Mental and moral
impressions, conveyed through the special senses, will affect the
motions of the heart, and disturb the processes of digestion and