both these defects existed from birth; so that
the sum of his intelligence was conveyed by the touch, smell, and
taste, or in other words, his mind was exclusively composed of the
perceptions he derived from these senses. This case will be more
particularly noticed in a subsequent chapter. The alterations which take
place in the state of our perceptions from a morbid cause, are generally
known. Thus a person labouring under a catarrh, will be unable to detect
the odours which certain substances communicate in a healthy condition
of his olfactory organ. In fever excited by a disordered stomach, the
taste will become vitiated, and the partial obstruction of the ear by
accumulated wax, will impress him with the bubbling of a pot, the
singing of birds, or the ringing of bells.

The same law that produces fatigue in a muscle from exertion, appears
to obtain in the organs of sense. If they be excited by their
appropriate stimuli too violently, or for a too long continuance,
fatigue or languor is produced, their percipience is diminished, or
confusedly conveyed; and they require a period of rest for their
refreshment.

As we advance in our enquiries into the nature of perception, it will be
evident that we cannot long continue to treat of it as a simple act, or
as a distinct faculty. The organs by which we obtain our different
perceptions are not insulated parts, but communicate with a substance,
termed the brain, and which is continued through the vertebral column.
The ultimate expansion of a nerve of sense, has been termed its sentient
or percipient extremity; and where it is united to the brain, its
sensorial insertion. If we were to divide the optic nerve where it
passes into the foramen, taking care to leave the apparatus of the eye
uninjured, the visual organ would be deprived of its function, and the
person or animal would be completely blind of that eye; so that a
communication with the brain is necessary for the purpose or act of
perception. As therefore the union of the nerve with the brain is
indispensably necessary for the purpose or act of perception, we are
naturally led to inquire into the properties of this substance, termed
the brain. Before we proceed to this part of the subject, it will be
proper to notice a fact which is of frequent occurrence. In amputations
of the thigh, at the moment the femoral nerve is divided, it often
occurs that a pain is distinctly felt in the toes; and after the limb
has been r