e 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
secretion. Terror, or an absorbing interest of any kind, will produce a
dilatation of the pupil, and communicate in this way a peculiarly wild
and unusual expression to the eye. Disagreeable sights or odors, or even
unpleasant occurrences, are capable of hastening or arresting the
menstrual discharge, or of inducing premature delivery."

_Third_.--"Reflex actions taking place through the sympathetic system
from one part of the body to another.--The contact of food with the
mucous membrane of the small intestine excites a peristaltic movement in
the muscular coat. The mutual action of the digestive, urinary, and
internal generative organs upon each other takes place entirely through
the medium of the sympathetic ganglia and their nerves. The variation of
the capillary circulation in different abdominal viscera, corresponding
with the state of activity or repose of their associated organs, are to
be referred to a similar nervous influence. These phenomena are not
accompanied by any consciousness on the part of the individual, nor by
any apparent intervention of the cerebro-spinal system."

* * * * *

CHAPTER XIII.

THE SPECIAL SENSES.

SIGHT.

The eye is the organ through which we perceive, by the agency of ligh